ALBERT

Description: AGU considers only original scientific contributions that have not been accepted or published elsewhere and are not under consideration by another publisher. A contribution is considered previously published if its data and conclusions are offered for sale or are generally available in other ways to the public. Regardless of the original publication medium, including print, magnetic tape, or microform, such contributions are not eligible for republication in AGU journals or books.

Description: The seasonal variation of the intrusion of the Philippine Sea Water into the South China Sea was studied by analyzing the historical hydrographic station data in the northern South China Sea and the Philippine Sea. Water masses at 150, 200, and 250 m were classified by discriminant analysis according to their temperature-salinity characteristics. At each depth, most water in the study region was classified into two groups representing the Philippine Sea Water and the South China Sea Water, respectively. The geographic distribution of water masses in the South China Sea shows that the Philippine Sea Water was present along the continental margin south of China between October and January. A westward current in the northern South China Sea in winter was inferred from the distribution of the intrusion water.

Description: Characteristics of water masses were analyzed to study the Kuroshio intrusion into the sea southwest of Taiwan. Hydrographic data were obtained from CTD (conductivity, temperature, and depth) casts during two cruises in May and August 1986. In May, remnants of water intruding from the Kuroshio were found on the continental slope south of the Penghu Channel. By August, these were replaced by water from the South China Sea. During this period, water from the Kuroshio also appeared near the southern tip of Taiwan. The intrusion current reached a depth of at least 500 m and was probably part of a cyclonic circulation in the northern South China Sea. The results support the hypothesis of a seasonal pattern of the intrusion process: intrusion of water from the Kuroshio begins in late summer, intensifies in winter, and ceases by late spring when South China Sea waters again enter this region.

Description: Multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central Aleutian Ridge. Since at least late Miocene time this sector of the ridge has been obliquely underthrust 30° west of orthogonal convergence by the northwestward converging Pacific plate at a rate of 80–90 km/m.y. Our data indicate that prior to late Eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits; the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits. Beginning in latest Miocene or earliest Pliocene time, a zone of outer-arc structural highs and a forearc basin began to form. Formation of these companion intraarc structures may be linked to the late Neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the Aleutian Trench. Initial structures of the zone of outer-arc highs formed as the thickening wedge underran, compressively deformed, and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust. Forearc basin strata ponded arcward of the elevating zone of outer-arc highs. However, most younger structures of the zone of outer-arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge. Oblique convergence created a major right-lateral shear zone (the Hawley Ridge shear zone) that longitudinally disrupted the zone of outer-arc highs, truncating the seaward flank of the forearc basin and shearing the southern limb of Hawley Ridge, an exceptionally large antiformal outer-arc high structure. Slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes. Uplift of Hawley Ridge may be related to the thickening of the arc massif by westward directed basement duplexes. In addition, the forearc is disrupted by structures transverse to the margin that occur where unusually high-stress accumulations have resulted in the rupture of repeated great earthquakes. It is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central Aleutian Ridge. Great structural complexity, including the close juxtaposition of coeval structures recording compression, extension, differential vertical movements, and strike-slip displacement, should be expected, even within areas of generally kindred tectonostratigraphic terranes.

Description: Several geochemical anomalies were observed before the Haichen, Longling, Tangshan, and Songpan earthquakes and their strong aftershocks. They included changes in groundwater radon levels; chemical composition of the groundwater (concentration of Ca++, Mg++, Cl−, SO4= and HCO3− ions); conductivity; and dissolved gases such as H2, CO2, etc. In addition, anomalous changes in water color and quality were observed before these large earthquakes. Before some events gases escaped from the surface, and there were reports of "ground odors" being smelled by local residents. The large amount of radon data can be grouped into long-term and short-term anomalies. The long-term anomalies have a radon emission build up time of from a few months to more than a year. The short-term anomalies have durations from a few hours or less to a few months.

Description: A non-Fickian physico-chemical model for electrolyte transport in high-ionic strength systems is developed and tested with laboratory experiments with copper sulfate as an example electrolyte. The new model is based on irreversible thermodynamics and uses measured mutual diffusion coefficients, varying with concentration. Compared to a traditional Fickian model, the new model predicts less diffusion and asymmetric diffusion profiles. Laboratory experiments show diffusion rates even smaller than those predicted by our non-Fickian model, suggesting that there are additional, unaccounted for processes retarding diffusion. Ionic diffusion rates may be a limiting factor in transporting salts whose effect on fluid density will in turn significantly affect the flow regime. These findings have important implications for understanding and predicting solute transport in geologic settings where dense, saline solutions occur.

Description: Until reliable procedures have been developed to preserve the phosphorus contained in particulate matter captured by in situ pumps and sediment traps and until these procedures are applied over a wide range of locations and depths in the sea, indirect methods will have to be used to determine the C/P ratio in marine detritus. We have taken two such approaches: (1) the use of C/N ratios for particulates captured in the upper thermocline in conjunction with 02/P and N/P ratios obtained from deconvolutions of ocean chemical data and (2) regression along isopycnals in the deep‐sea waters free of fossil fuel CO2. While neither approach yields a definitive answer, both suggest that a value of 127 carbon atoms per phosphorus atom would be a more appropriate interim value than that of 106 adopted long ago by A. C. Redfield and his associates.

Description: Axial volcanic ridges (AVRs) are found on most slow-spreading mid-ocean ridges and are thought to be the main locus of volcanism there. In this study we present high-resolution mapping of a typical, well-defined AVR on the Mid-Atlantic Ridge at 45°N. The AVR is characterized by “hummocky terrain,” composed typically of hummocks with pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of “flat seafloor,” composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are of a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50 ka time frame as the flat seafloor.

Description: The Norwegian Atlantic Current (NwAC) and its eddy field are examined using data from surface drifters. The data set used spans nearly 20 years, from June 1991 to December 2009. The results are largely consistent with previous estimates, which were based on data from the first decade only. With our new data set, statistical analysis of the mean fields can be calculated with larger confidence. The two branches of the NwAC, one over the continental slope and a second further offshore, are clearly captured. The Norwegian Coastal Current is also resolved. In addition, we observe a semipermanent anticylonic eddy in the Lofoten Basin, a feature seen previously in hydrography and in models. The eddy kinetic energy (EKE) is intensified along the path of the NwAC, with the largest values occurring in the Lofoten Basin. The strongest currents, exceeding 100 cm s−1, occur west of Lofoten. Lateral diffusivities were computed in five domains and ranged from 1–5 × 107 cm2 s−1. The Lagrangian integral time and space scales are 1–2 days and 7–23 km, respectively. The data set allows studies of seasonal and interannual variations as well. The strongest seasonal signal is in the NwAC itself, as the mean flow strengthens by approximately 20% in winter. The EKE and diffusivities on the other hand do not exhibit consistent seasonality in the sampled regions. There are no consistent indications of changes in either the mean or fluctuating surface velocities between the 1990s and 2000s.

Description: Samples collected at hourly intervals on May 18–19, 1980, at three sites 200 km downwind from Mount St. Helens, have made possible a detailed reconstruction of the conditions that contribute to the compositional heterogeneity of mineral and glass components observed in distal tephra layers. The air fall tephra deposited at the sites during the first 7 hours of the May 18 eruption is mostly coarse grained, microlite-rich, nonjuvenile glass and feldspar. Grain-size maxima in this initial tephra can be related to the cataclysmic blast at 0832 and a subsequent pulse of the eruption at 1200. Juvenile, microlite-free glass increases in relative abundance at the sampling sites beginning at about 1900. Such a change between nonjuvenile and juvenile tephra can be related to a 5-km increase in column height associated with the last major pulse of the eruption which occurred at 1700 at the volcano. Electron microprobe study of both microlite-rich and microlite-free pumice in the time series samples reveals significant compositional differences. Interstitial glass in nonjuvenile pumice deposited during the first few hours at the sampling sites is enriched in SiO2 and K2O and depleted in TiO2, FeO*, and MgO relative to juvenile glass. By comparison, major element composition of the least evolved juvenile glass sampled during the last several hours of the eruption displays a slight trend toward less evolved composition. Least squares calculations suggest that the more evolved character of the nonjuvenile glass can be explained by greater fractional crystallization brought about by enhanced cooling in a cryptodome prior to eruption, whereas the temporal changes observed in juvenile glass composition during the last several hours of the eruption suggest the presence of a small, slightly zoned magma chamber at depth. Electron microprobe study of glass-coated ilmenites, magnetites, and plagioclases provides the following estimates of the physical conditions in this reservoir: 865°±50°C, PH2O = 2.2 kbar and -log ƒO2 = 11.7. Analyses of bulk pumice, glass and selected mineral phases from May 25, June 12, July 22, and October 16–18 pumices erupted from Mount St. Helens indicate that the bulk pumice (magma) compositions have become slightly more andesitic with time, while mineral and co-existing glass compositions have changed significantly in post-May 18 eruptions with both being more highly evolved than those associated with the May 18 eruption. An application of the magnetite-ilmenite geothermometer to June 12 and July 22 samples indicates temperatures of 919°±30°C and 930°±50°C, respectively. Least squares calculations suggest that such evolved post-May 18 glass and mineral phases can be derived by fractional crystallization of a magma composition like bulk May 18 pumice into approximately 50% crystals and 50% residual liquid. Such partitioning between crystals and residual liquid appears to have occurred on the scale of centimeters and is interpreted as a consequence of accelerated crystallization under reduced water pressure.

Description: What is the relationship between volcanic eruptions and climate change? More than 200 years after the connection was first proposed, it remains a thorny question. This article provides a brief historical overview of the problem and a review of the various data bases used in evaluating volcanic events and associated climatic change. We use the term “climate” to describe changes in the atmosphere over wide regions for periods of several months and longer. We use “weather” to describe shorter-term, variable atmospheric fluctuations experienced over more restricted areas. We appraise the present state of knowledge and highlight some pitfalls involved in using available information. Cautiously, we suggest future avenues for study, including the possibility of “volcanic winters,” or severe eruption-induced coolings.

Description: The Stratigraphie record from both deep-sea and shallow-water depositional environments Indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaC02 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off-ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of C02 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric C02. A significant fraction of this atmospheric C02 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess C02 may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian-Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted from a warm global climate and increased marine transgression as well as by the higher C02.

Description: The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large-scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.

Description: Magnetic lineation mapping in the western central Pacific has revealed a pair of opposite-sensed, fanned lineation patterns that define the accretionary boundaries of the fossil Magellan microplate. This tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations. The entire evolutionary history of the Magellan microplate is well constrained to a 9-m.y. period in the Early Cretaceous by synchronous spreading patterns and associated geologic data. During this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km×600 km with spreading centers on two opposing sides and transform faults on the other two sides. The lifetime and size of the Magellan microplate are somewhat longer and larger, respectively, than presently active microplates on the East Pacific Rise. However, these modern structures are still evolving and growing, and the tectonic behavior of the modern and Cretaceous systems appears to be similar. Study of both active and fossilized microplates should provide additional insights on their common tectonic histories. In particular, we show that the Magellan Trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history. The Magellan Trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the Pacific, Farallon, and Phoenix plates at Ml0N time. Microplate activity ceased when the microplate became welded to the Pacific plate at M9 time.

Description: Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Description: A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high-velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg-leg multiples generated from with a low-velocity zone between the second and third upper crustal high-velocity layers. The third high-velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low-velocity zone that produces a seismic shadow wherein zone peg-leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high- and low-velocity layers. Synthetic seismogram modeling indicates that a low-velocity zone with transitional upper and lower boundaries generates peg-leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long-offset (90-225 km) data. This may be due to a lower crustal waveguide whose top is the high-velocity (7.6 km/s) layer and whose base is the Moho. A deep (~54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg-leg multiples have been interpreted on some long-range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg-leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide-angle reflection profiles than previously recognized.

Description: Grain‐size‐dependent flow mechanisms tend to be favored over dislocation creep at low differential stresses and can potentially influence the rheology of low‐stress, low‐strain rate environments such as those of planetary interiors. We experimentally investigated the effect of reduced grain size on the solid‐state flow of water ice I, a principal component of the asthenospheres of many icy moons of the outer solar system, using techniques new to studies of this deformation regime. We fabricated fully dense ice samples of approximate grain size 2±1 μm by transforming “standard” ice I samples of 250±50 μm grain size to the higher‐pressure phase ice II, deforming them in the ice II field, and then rapidly releasing the pressure deep into the ice I stability field. At T≤200 K, slow growth and rapid nucleation of ice I combine to produce a fine grain size. Constant‐strain rate deformation tests conducted on these samples show that deformation rates are less stress sensitive than for standard ice and that the fine‐grained material is markedly weaker than standard ice, particularly during the transient approach to steady state deformation. Scanning electron microscope examination of the deformed fine‐grained ice samples revealed an unusual microstructure dominated by platelike grains that grew normal to the compression direction, with c axes preferentially oriented parallel to compression. In samples tested at T≥220 K the elongation of the grains is so pronounced that the samples appear finely banded, with aspect ratios of grains approaching 50:1. The anisotropic growth of these crystallographically oriented neoblasts likely contributes to progressive work hardening observed during the transient stage of deformation. We have also documented remarkably similar microstructural development and weak mechanical behavior in fine‐grained ice samples partially transformed and deformed in the ice II field.

Description: We present a conceptual model of fluid circulation in a ridge flank hydrothermal system, the Mariana Mounds. The model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep-sea research vessel Alvin in small, meter-sized mounds situated on a local topographic high. These mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow. We have interpreted the chemical data with time-dependent transport-reaction models to estimate pore water velocities. In the mounds themselves pore water velocities reach several meters per year to kilometers per year. Within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year. A larger area of low heat flow surrounds these heat flow and topographic highs, with upwelling pore water velocities less than 2 cm/yr. In some nearby cores, downwelling of bottom seawater is evident but at speeds less than 2 cm/yr. Downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement. We conclude that the principal source of seawater recharge to basement is where basement outcrops exist, most likely a scarp about 2–4 km to the east and southeast of the study area.

Description: During a 10-year study more than 2,000 phytoplankton samples were collected from the entire coast of Peru and analyzed. In general, diatoms were the most abundant group of organisms in all seasons. Predominant species were Rhizosolenia delicatula, Skeletonema costatum Thalassiosira subtilis, Thalassionema nitzschioides and several species of the genus Chaetooeros. Dinoflagellates and flagellates were observed frequently during summer. The mean distribution of the phytoplankton concentration during the 10 years shows the existence of several centers with higher cell densities along the coast, coinciding with the areas of more intense and persistent upwelling. Four major centers have been identified: Pimentel (˜6°S), Chimbote (˜9°S), Callao (˜12°S), and Tambo de Mora-Pisco (˜15°S); and two minor centers, Talara (˜4°S) and Ilo (˜17°S). The relative importance of each center seems to change according to the season. The highest phytoplankton concentration tended to be in the northern part of the coast during fall and winter and in the south through spring and summer.

Description: Methane is a potent greenhouse gas and large-scale rapid release of methane from hydrate may have contributed to past abrupt climate change inferred from the geological record. The discovery in 2008 of over 250 plumes of methane gas escaping from the seabed of the West Svalbard continental margin at ~400 m water depth (mwd) suggests that hydrate is dissociating in the present-day Arctic. Here we model the dynamic response of hydrate-bearing sediments over a period of 2300 years and investigate ocean warming as a possible cause for present-day and likely future dissociation of hydrate, within 350–800 mwd, west of Svalbard. Future temperatures are given by two climate models, HadGEM2 and CCSM4, and scenarios, Representative Concentration Pathways (RCPs) 8.5 and 2.6. Our results suggest that over the next three centuries 5.3–29 Gg yr−1 of methane may be released to the Arctic Ocean on the West Svalbard margin.

Description: Aftershocks of shallow earthquakes larger than magnitude 7 in the Aleutians, southern Alaska, southeast Alaska, and offshore British Columbia from 1920 to 1970 were relocated by computer in an attempt to delineate the rupture zones of large earthquakes. Plate tectonic theory indicates that gaps in activity for large earthquakes for the past 10's to 100's of years are likely sites of future large earthquakes. Three prominent gaps of this type are delineated: one in southeast Alaska; another in southern Alaska near the epicenters of the great earthquakes of 1899 and 1900; and one in the far western Aleutians. These gaps deserve high priority for study and instrumentation. Large earthquakes appear to be much more regular than smaller shocks in their distributions with respect to space, time, and size. Aftershock zones of events since 1930 that are larger than magnitude 7.8 are longer than 250 km and those less than 7.5 are shorter than 125 km. The rupture zones of events that occurred before 1930 could not be delineated from aftershock locations. Aftershock zones of large earthquakes tend to abut without significant overlap even for rupture zones as long as 1200 km. Nearly the entire Alaska-Aleutian zone from 145°W to 171°E has broken since 1938 in a series of large earthquakes. The rupture zones of five large events appear to form a space-time sequence that progressed from 155°W in 1938 to 171°E in 1965. This sequence is much like the well-known westward progression of activity since 1939 along the North Anatolian fault. Shocks with long rupture zones tend to occur along those parts of the Alaska-Aleutian zone that are relatively simple tectonically. The ends of many aftershock zones of large earthquakes are located at the intersection of major transverse features with the Aleutian arc. Large earthquakes rarely, if ever, reoccur along the same part of a fault zone in less than several tens of years, i.e. within a time less than that for substantial strain accumulation. Events of comparable magnitude that occur soon after some great earthquakes usually involve rupture in a region adjacent to but different from that of the main shock. The March 30, 1965, earthquake of magnitude 7.5, which involved normal faulting in the Aleutian trench, appears to have been triggered by thrust faulting along the adjacent inner margin of the trench in the magnitude 7.9 earthquake of February 4, 1965. Large events of the thrust type are commonly followed within ten years by events involving normal faulting in the adjacent part of the trench. Estimates of average displacements and of the repeat times of great earthquakes from measurements of 20-sec surface waves are systematically too small and do not agree with the meager historic record of great shocks. Other estimates of repeat times vary from 30 to 850 years, but neither of these extremes appears to be typical. The aftershock zone of the April 1, 1946, Aleutian earthquake, which generated one of the largest and most widespread seismic sea waves in the Pacific during this century, was very small. A large displacement of the ocean floor may be responsible for the generation of the large sea wave. An average displacement of 2.4 to 4.1 meters was calculated from amplitudes of 100-sec waves.

Description: We measured the respiratory isotope effect ϵresp for seven representative unicellular marine organisms. The bacterium Pseudomonas halodurans, the diatom Phaeodactylum tricornutum, the phytoflagellates Cryptomonas baltica and Dunaliella tertiolecta, the heterotrophic flagellates Paraphysomonas imperforata and Bodo sp., and the ciliate Uronema sp. exhibit ϵresp values in the range 14-26‰. We also measured ϵresp for three metazoans. The ϵresp for the copepod Acartia tonsa ranged from 17 to 25‰, while two larger organisms, the mollusk Mercenaria mercenaria and the salmon Salmo salmar, respire with a smaller ϵresp of 5-10‰. The average respiratory isotope effect of the dominant marine respirers (the bacteria, microalgae and zooplankton) is about 20 ± 3‰. An ϵresp of this magnitude supports the hypothesis that the photosynthesis-respiration cycle is responsible for the 23.5‰ enrichment in the δ18O ratio of atmospheric O2 relative to seawater (the Dole effect). The large value and high variability in the average ϵresp limits the usefulness of a proposed method using the δ18O of naturally fractionated dissolved O2 in seawater as a tracer of primary production in the oligotrophic ocean.

Description: We review the current understanding of the Dole effect (the observed difference between the δ18O of atmospheric O2 and that of seawater) and its causes, extend the record of variations in the Dole effect back to 130 kyr before present using data on the δ18O of O2 obtained from studying the Vostok ice core (Sowers et al., 1993), and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present-day Dole effect, +20.8‰, is considerably lower than the observed value, +23.5‰, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05‰ lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only ±0.2‰, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical term reflects variations in low-latitude land hydrology and productivity or possibly variability in low-latitude oceanic productivity.

Description: During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3-km-wide and 15-km-long region in Vidal Valley, roughly parallel to a segment of one of the near-vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray-tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2-km depth near the Whipple Mountains to a depth of 5-km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near-vertical profiles in the Whipple Mountains. (2) On the near-vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near-surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near-vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi-three-dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near-vertical reflection profiles to be related to the mid-Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models.

Description: Much attention has been paid, in recent years, to the potential application of the Ce anomaly, measured in various marine phases, as a paleoceanographic indicator of widespread marine anoxia. In this paper we present and discuss results from recent studies of present‐day rare earth element (REE) distributions (and hence Ce anomaly distributions) in the marine environment which are particularly pertinent to paleoceanography. Subsequently, we review and discuss the validity of the recent literature in which Ce anomalies have been employed as paleoredox indicators.

Description: On Syros, high‐pressure metamorphism affects a lithological pile that is composed of, from base to top: (1) the Komito‐Vari granitic basement, (2) a margin sedimentary sequence that is predominantly made of marbles and schists (the Pyrgos and Kastri units), and (3) the Kambos metaophiolitic mélange. The tectonic history occurred in three main stages. During the first stage, in the mid‐Eocene, the Kambos oceanic unit was thrust southward on top of the sedimentary pile. Top‐to‐the‐south‐southwest ductile senses of shear are synchronous with prograde high‐pressure metamorphism and associated with this thrusting event. The second stage corresponds to a top‐to‐the‐northeast ductile shear that affects the whole metamorphic pile and is synchronous with the metamorphic retrogression from eclogite to greenschist facies. However, the Kambos oceanic unit remained partly undeformed, as shown by significant volumes containing undeformed lawsonite pseudomorphs. No major extensional detachment related to this exhumation event outcrops on the island. The localized semibrittle to brittle deformation of the third stage is associated with the postmetamorphic development of (1) a ramp‐flat extensional system at the island scale, whose southward minimum displacement is estimated at approximately 7 km, and (2) two sets of steeply dipping strike‐slip faults with a normal component, trending either east–west or around north–south, indicating that the mean stretching and shortening directions are trending NNE–SSW and ESE–WNW, respectively. This sequence of major tectonic events and their relationship to metamorphism are interpreted within the framework of the subduction of the Pindos Ocean and then of the Adria continental passive margin.

Description: We present a method for objectively characterizing a swath of digitally sampled seafloor topography. Our method analyzes the distribution of surface slopes by compiling surface-normal vectors into a two-dimensional histogram using an equal-area projection. The direction of maximum variance (first principal axis) of the histogram is used to determine the azimuth of lineations in the topography, and the variance is used as a measure of seafloor roughness. We apply the method to short sections of Sea Beam swath data and find that the histogram parameters are effective in describing the behavior of the topography. In particular, similar patterns are observed for a sequence of histograms derived from data collected over the Mendocino and the Surveyor fracture zones in the northeast Pacific. Because the method does not require any data modification and is suitable for irregularly-shaped sample regions, it lends itself to real-time analysis.

Description: Abundances of 12 species of planktonic foraminifera collected in two plankton tows from the east tropical Atlantic are compared to the chlorophyll content and the temperature of the sea water from which they were collected. As expected from previous work in the tropics, all dominant tropical species occur in greatest abundance within the photic zone. Many species occur in greatest abundances in the seasonal thermocline in association with the maximum chlorophyll concentration, while a few algal symbiont-bearing species occur in greatest abundance in the mixed layer. The δ18O measurements of planktonic foraminifera shells from core top sediment samples confirm the vertical stratification within the photic zone that is suggested by the relationship between hydrography and abundances found in the plankton tows and found in the statistical study by Ravelo et al. [1990]. Comparison between the measured δ18O values of planktonic foraminifera with the predicted δ18O profiles of the overlying water column at three core locations indicate that species abundances in the sediment record the seasonally integrated conditions of the photic zone and suggests that the abundance of a species in the sediment depends on whether the preferred ecological conditions of that species may be found within the photic zone of the overlying water column sometime during the year. Species which calcify below the photic zone have only trace relative abundances. Finally, it appears that the total range of δ18O values of the dominant species approximates the predicted annual δ18O of calcite range in the upper 80 m of the water column.

Description: Pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the Juan de Fuca Ridge as part of FlankFlux 90, a study of hydrothermal circulation through mid-ocean ridge flanks. Seismic reflection and heat flow surveys (Davis et al., 1992a) indicate that the three areas differ in sediment thickness, basement topography, abundance of outcrops, basement temperature, and fraction of heat lost by advection versus conduction. Area 1 is on 0.6 Ma crust with nearly continuous basement outcrop, area 2 is on 1.3 Ma crust over the first buried ridge parallel to the present ridge axis, and area 3 is on 3.5–3.8 Ma crust over two axis-parallel buried ridges that penetrate the sediment cover in three locations. Each area includes a hydrothermal system in which seawater flows into basement, reacts with crustal basalt, and then exits basement either through the sediment or directly into the overlying water column. As constrained by concentrations of sulfate and lithium in the pore waters, at least some seawater enters basement in all three areas without reacting fully with the overlying sediment, even where no outcrops are known nearby. Speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity, which behave conservatively in these areas, to numerical time-dependent transport models. The estimated velocities range from 〈0.1 to 7.4 cm/yr; faster flows probably occur but were not sampled. Upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness. The correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment. We have documented pore water upwelling through sediment up to 100 m thick. We estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick, corresponding to a heat flow of 0.44 W/m2 in area 2 and 0.3 W/m2 in area 3. Concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next. Both species remain at the bottom seawater concentration in area 1, where basement is cooled to 〈10°C at the base of the sediments mainly by advection. The concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2.5 mmol/kg at about 90°C in area 3. The transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement, as the uppermost basement is heated from 〈10° to 40–50°C. Chlorinity of the basement fluid increases above the present-day bottom seawater concentration in areas 2 and 3 and in nearly all other mid-ocean ridge flanks studied to date, as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period. While chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems, it reaches a maximum in area 2 at only 40°C, probably because alteration there occurs at a lower water/rock ratio than elsewhere. For all mid-ocean ridge flanks studied to date, the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age.

Description: The Pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations. One of the clearest examples of such a reorganization is documented in the Mathematician Seamounts region, where a distinctive geomorphology and well-developed magnetic anomalies are present. This reorganization involved a short-lived microplate between the failing Mathematician Ridge and a new propagating spreading center: the East Pacific Rise. It produced a transfer of a fragment of lithosphere from the Farallon to the Pacific plate, and also created a number of landforms and magnetic patterns, within and on the margins of the captured fragment; these make up the Mathematician paleoplate. In many cases, two sides of a microplate are active spreading ridges. A microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge. We define a paleoplate as the area of the seafloor, from the axis of a failed rift to the boundary of resumed, full spreading. It includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading, prior to complete abandonment of the failed rift. The Mathematician paleoplate has the following boundaries and components from west to east: the axis of the Mathematician failed rift, the fragment of captured Farallon plate, a complex rift initiation site at the Moctezuma Trough, a zone of slow spreading, and an as yet ill-defined eastern boundary where dual spreading stopped and full spreading resumed. The northern boundary of the paleoplate is the Rivera fracture zone; its southeastern boundary a now-inactive transform fault, the West O'Gorman fracture zone. In this case, as well as in other more poorly documented ones, relict landforms and magnetic patterns are carried on the aging lithosphere, away from the spreading ridge, recording a former geometry.

Description: Methane carbon isotopic composition ranged from −76.9 to −62.6‰ in a tidal freshwater estuary (the White Oak River, North Carolina, United States) with site specific seasonal variations ranging from 6 to 10‰. During warmer months, tidally induced bubble ebullition actively transported this methane to the atmosphere. At two sites, these seasonally varying fluxes ranged from 1.2 ± 0.3 to 1.3 ± 0.3 mol CH4 m−2yr−1 (19.2 to 20.8 g CH4m−2yr−1), with flux-weighted average isotopic compositions at two sites of −66.3 ± 0.4 and −69.5 ± 0.6‰. The carbon isotopic composition of naturally released bubbles was shown to be indistinguishable from the sedimentary methane bubble reservoir at three sites, leading to the conclusion that isotopic fractionation did not occur during the ebullition of methane. The hypothesis was developed that ebullitive methane fluxes are depleted in 13CH4 relative to fluxes transported via molecular diffusion or through plants, as zones of 13C enriching microbial methane oxidation are bypassed.

Description: An examination of 311 intraplate earthquakes in the Australian plate portion of the Pacific Ocean basin reported from 1918 to 1990 reveals that only 113 events are reliably intraplate, with most of the rest relocating to active trenches and transforms. The non-random distribution of the reliably intraplate events gives insight into the tectonic stresses present. The central Tasman Sea is mostly aseismic except for a swarm of activity at the predicted site of the Tasmantid hot spot. To the north, the broad regions of the Coral Sea, South Fiji Basin and Lord Howe Rise show very little intraplate seismicity, yet the narrow Norfolk Ridge and Three Kings Rise, caught between the double convergence of the New Hebrides and Tonga subduction zones, support many more earthquakes. High levels of intraplate seismicity in the southern Tasman Sea adjacent to the Macquarie Ridge Complex (MRC) indicate that this region may be undergoing internal deformation due to the unusual nature of the Australia-Pacific plate boundary. Additional support exists in the form of intraplate focal mechanisms similar to those at the plate boundary and a set of parallel gravity rolls which are observed in recent geoid maps. Some aftershocks of the Mw = 8.2 Macquarie Ridge earthquake of 1989 occurred in a fracture zone west of the Macquarie Ridge Complex [Das, 1992], but we have found several earthquakes from as early as 1924 which relocate to this feature, suggesting that its reactivation may be more significant than previously thought. This reactivation of a fossil fracture zone may be the result of the increasing amount of oblique convergence between the Australia and Pacific plates at the Macquarie Ridge Complex, formerly a spreading center, and the stresses associated with subducting recently formed Australian ocean crust beneath the older Pacific plate.

Description: The eastern Mediterranean transient (EMT) was caused by a combination of high‐salinity waters intruding into the Aegean Sea and the two particularly strong winters of 1991–1992 and 1992–1993. The approach in this chapter is to search for specific signatures in the historic hydrographic observations, which date back to 1910. To deal with the problem that up into the 1950s the data not only are of limited precision but also have gaps of about 20 years, it is advantageous to consider the fact that the evolution of the actual EMT is rather well documented over a similar time span. The chapter begins by outlining the characteristics of the current EMT. Thereafter, a selection of suitable hydrographic observations among the available historic data is provided to compare these with signatures expected from the evolution of the actual EMT.

Description: Dissolved and atmospheric nitrous oxide (N2O) were measured on the legs 3 and 5 of the R/V Meteor cruise 32 in the Arabian Sea. A cruise track along 65°E was followed during both the intermonsoon (May 1995) and the southwest (SW) monsoon (July/August 1995) periods. During the second leg the coastal and open ocean upwelling regions off the Arabian Peninsula were also investigated. Mean N2O saturations for the oceanic regions of the Arabian Sea were in the range of 99–103% during the intermonsoon and 103–230% during the SW monsoon. Computed annual emissions of 0.8–1.5 Tg N2O for the Arabian Sea are considerably higher than previous estimates, indicating that the role of upwelling regions, such as the Arabian Sea, may be more important than previously assumed in global budgets of oceanic N2O emissions.

Description: The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to ɛ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2 μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region.

Description: Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.

Description: Understanding the pre-anthropogenic Pb cycle of central North Pacific deep water has attracted a lot of attention in recent years, partly because of its unique geographical location in that it is a remote gyre system characterized by high dust fluxes and sluggish overturning circulation. However, the factors controlling Pb isotope evolution in this area over the Cenozoic are still controversial and various mechanisms have been proposed in previous studies. Here we report new Pb and Nd isotope time series of four ferromanganese crusts (two from the western Pacific near the Mariana arc and the other two from the central Pacific). Together with previously published records, we discuss for the first time the significance of a persistent and systematic Pb isotopic provinciality recorded by central North Pacific crusts over the Cenozoic. We propose that globally well mixed stratosphere volcanic aerosols could contribute Pb but have not been the major factors controlling the Pb isotope distribution in the central North Pacific over time. Island arc input (and probably enhanced hydrothermal input between about 45 and 20 Ma) likely controlled the Pb isotope provinciality and evolution prior to ~20 Ma, when coeval Pb isotope records in different crusts showed large differences and atmospheric silicate dust flux was extremely low. After the Eocene, in particular after 20 Ma, Asian dust input has become an isotopically resolvable source, while island arc-derived Pb has remained important to balance the dust input and to produce the observed Pb isotope distribution in the central North Pacific during this period.

Description: A realistic primitive-equation model of the Southern Ocean at eddying spatial resolution is used to examine the effect of ocean-surface-velocity dependence of the wind stress on the strength of near-inertial oscillations. Accounting for the ocean-surface-velocity dependence of the wind stress leads to a large reduction of wind-induced near-inertial energy of approximately 40 percent and of wind power input into the near-inertial frequency band of approximately 20 percent. A large part of this reduction can be explained by the leading-order modification to the wind stress if the ocean-surface velocity is included. The strength of the reduction is shown to be modulated by the inverse of the ocean-surface-mixed-layer depth. We conclude that the effect of surface-velocity dependence of the wind stress should be taken into account when estimating the wind-power input into the near-inertial frequency band and when estimating near-inertial energy levels in the ocean due to wind forcing.

Description: This study presents a new estimate of the oceanic anthropogenic CO2(Cant) sink over the industrial era (1780 to present), from assimilation of potential temperature, salinity, radiocarbon, and CFC-11 observations in a global steady state ocean circulation inverse model (OCIM). This study differs from previous data-based estimates of the oceanic Cant sink in that dynamical constraints on ocean circulation are accounted for, and the ocean circulation is explicitly modeled, allowing the calculation of oceanic Cant storage, air-sea fluxes, and transports in a consistent manner. The resulting uncertainty of the OCIM-estimated Cant uptake, transport, and storage is significantly smaller than the comparable uncertainty from purely data-based or model-based estimates. The OCIM-estimated oceanic Cant storage is 160–166 PgC in 2012, and the oceanic Cant uptake rate averaged over the period 2000–2010 is 2.6 PgC yr−1 or about 30% of current anthropogenic CO2 emissions. This result implies a residual (primarily terrestrial) Cant sink of about 1.6 PgC yr−1 for the same period. The Southern Ocean is the primary conduit for Cant entering the ocean, taking up about 1.1 PgC yr−1 in 2012, which represents about 40% of the contemporary oceanic Cant uptake. It is suggested that the most significant source of remaining uncertainty in the oceanic Cant sink is due to potential variability in the ocean circulation over the industrial era.

Description: Sedimentation rates (corrected for compaction) from along the passive continental margin of Africa between the Equatorial Fracture Zone and Somalia are used to compare the rates of subsidence of the continental crust since early Mesozoic time. Three distinctive subsidence histories can be identified which correspond with basinal areas that have different structural styles: rifted (west coast), sheared (Equatorial and Agulhas fracture zones) and sunk (zones of vertical tectonics in eastern Africa). A comparison of subsidence rates with other tensional margins (NE USA and the North Sea) and a consideration of the plate tectonic history of the African margins leads to the proposal of a geo and thermodynamic model that takes cognizance of the worldwide mid-Cretaceous rheological discontinuity between taphrogenic and epeirogenic basin formation recognized by Kent, and the more generally accepted, purely plate tectonic driven model of margin subsidence. The new suggestion involves a lower Mesozoic worldwide rise in the geothermal gradient in the lithosphere which produces metamorphism of the base of the continental crust and initiates taphrogenesis along lineaments throughout Gondwanaland. A lowering of the geothermal gradient in the lower Cretaceous produces a switch to epeirogenic subsidence, driven solely by sediment loading and thermal contraction, by Aptian/Albian times. The thermal event facilitated continental separation, and sea floor spreading commenced locally at various times along the active taphrogenic belts. Local thermal and tectonic aberrations associated with this phenomenon over print onto the worldwide pattern of marginal basin subsidence. A further rise in the geothermal gradient may have been responsible for renewed taphrogenesis in eastern Africa in Tertiary times.

Description: The renewal of the deep water of the East Atlantic and its large-scale internal circulation are studied on the basis of the distributions of potential temperature, silicate, ΣCO2, and 14C. An isopycnal multibox model including advection, mixing, and sources and sinks is set up and described. Tracer data are input for the model, and balance equations for the various properties for the boxes of the model serve as constraints for the determination of water fluxes, mixing coefficients, and source parameters. Extremal values for various model parameters that are consistent with the tracer data (satisfy the balance equations within the estimated tolerances) are calculated by linear programming techniques. 14C data are seen to be valuable in determining absolute flow rates. Model results confirm the importance of the Romanche Facture Zone for the renewal of east Atlantic deep water. Eastward flows through the Romanche Fracture Zone were found to be between 2.6 and 5.1 Sv. Flows through the Vema Fracture Zone amount to at most 20% of the Romanche Fracture Zone inflow. Contributions of Antarctic Bottom Water at the southern end of the East Atlantic (Walvis Ridge) and of Iceland Scotland Overflow Water at the northern end are very small (〈 5% of equatorial inflow). Diapycnal mixing coefficients are between 1 and 10 cm2/s, and values for the dissolution rates of silicate and carbon are in the expected range.

Description: The regions containing the two zonal currents of the subtropical gyre in the eastern North Atlantic, the Azores Current and the North Equatorial Current (NEC), have quite different physical characteristics. Associated with the Azores Current are strong horizontal thermohaline gradients that can be located easily both at the surface and at depth with temperature data alone, thus making satellite IR imagery and expendable bathythermograph profiles suitable for observing it. During winter, the surface expression of the Azores Current is often found to the north of the strongest subsurface gradients. In contrast to the Azores Current and to the central water mass boundary just to the south, the NEC has relatively weak horizontal temperature and salinity gradients, requiring density information in order to identify it. There is no clear surface manifestation found with the NEC. Common to both currents, though, is that each transports O(8 Sv) in the upper 800 m of the ocean near 27°W, with the largest velocities being in the upper 400 m.

Description: Large diurnal sea surface warming exceeding 1°C is common in the western North Atlantic Ocean and is often of large horizontal extent. These events correlate closely with very light winds and high insolation. In the area investigated, 17°–40°N and 55°–80°W, the largest warming is found in the western portion of the ridge associated with the Azores-Bermuda high, where the lowest wind speeds are observed. The distribution of warming events shows that the largest number occur between June and August, when insolation is highest and percent cloud cover and wind speed are low. The most probable latitude of warming events moves north from approximately 25°N in spring to near 30°N in summer, a shift similar to that seen in the minimum of the climatological winds. Local areas have a probability as high as 30% for diurnal warming in excess of 1°C in the summer. The net heat flux into the ocean, calculated by using monthly mean values for low latitudes in the summer, excluding diurnal warming events, is biased consistently high by as much as 5 W/m2 relative to the same values calculated with warming events included.

Description: Stratospheric ozone depletion and emission of greenhouse gases lead to a trend of the southern annular mode (SAM) toward its high-index polarity. The positive phase of the SAM is characterized by stronger than usual westerly winds that induce changes in the physical carbon transport. Changes in the natural carbon budget of the upper 100 m of the Southern Ocean in response to a positive SAM phase are explored with a coupled ecosystem-general circulation model and regression analysis. Previously overlooked processes that are important for the upper ocean carbon budget during a positive SAM period are identified, namely, export production and downward transport of carbon north of the polar front (PF) as large as the upwelling in the south. The limiting micronutrient iron is brought into the surface layer by upwelling and stimulates phytoplankton growth and export production but only in summer. This leads to a drawdown of carbon and less summertime outgassing (or more uptake) of natural CO2. In winter, biological mechanisms are inactive, and the surface ocean equilibrates with the atmosphere by releasing CO2. In the annual mean, the upper ocean region south of the PF loses more carbon by additional export production than by the release of CO2 into the atmosphere, highlighting the role of the biological carbon pump in response to a positive SAM event.

Description: We combined data sets of measured sedimentary calcium carbonate (CaCO3) and satellite-derived pelagic primary production to parameterize the relation between CaCO3 content on the Antarctic shelves and primary production in the overlying water column. CaCO3 content predicted in this way was in good agreement with the measured data. The parameterization was then used to chart CaCO3 content on the Antarctic shelves all around the Antarctic, using the satellite-derived primary production. The total inventory of CaCO3 in the bioturbated layer of Antarctic shelf sediments was estimated to be 0.5 Pg C. This quantity is comparable to the total CO2 uptake by the Southern Ocean in only one to a few years (dependent on the uptake estimate and area considered), indicating that the dissolution of these carbonates will neither delay ocean acidification in this area nor augment the Southern Ocean CO2 uptake capacity.

Description: The evolution of ridge-hotspot systems is not well understood. In this investigation, satellite-derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge-hotspot interaction at four sparsely explored systems in the Southern Ocean. These systems illustrate three different stages of ridge-hotspot interaction in which a migrating spreading center approaches a hotspot (Pacific-Antarctic/Louisville), passes over or is captured by the hotspot (Mid-Atlantic/Shona-Discovery), and ultimately migrates away from the hotspot (Southeast Indian/Kerguelen). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge-hotspot systems. A simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered. The azimuths of the aseismic ridges cannot be explained by plate kinematics alone; they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure. The features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets.

Description: We have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles. Regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models; we avoided those data where purely conductive heat flow suggested temperatures more than about 1000°C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity σ versus 1/T demonstrate that even low-conductivity layers (LCL) have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits. In addition, technically active regions show systematically higher conductivities than do shield areas. Because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions, these materials probably account for the relatively higher conductivities of rocks in situ in the crust; free water in amounts of 0.01–0.1% in fracture porosity could explain crustal conductivities. Other possibilities are graphite, hydrated minerals in rare instances, and sulfur in combination with other volatiles. As most of the temperatures are less than 700°C, partial melting seems likely only in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high-conductivity layers (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's. Because the differences between HCL's and LCL's are independent of temperature, we must invoke more than temperature increases as a cause for large conductivity increases; increased fluid concentration in situ seems a probable cause for enhanced conductivities in HCL's. From the point of view of these observations, it does not matter whether the fluids are in communication with the surface or trapped at lithostatic pressures.

Description: Heat flow in the Imperial Valley and adjacent crystalline rocks is very high (∼140 mW m−2). Gravity and seismic studies suggest the crust is about 23.5 km thick with the lower half composed of gabbro and the upper fourth composed of low-density sediments. Conduction through such a crust resting directly on asthenosphere would give the observed heat flow if there were no extension or sedimentation. However, both processes must have been active, as the Imperial Valley is part of the Salton Trough, a pull-apart sedimentary basin that evolved over the past 4 or 5 m.y. To investigate the interrelations of these factors, we consider a one-dimensional model of basin formation in which the lower crustal gabbro and upper crustal sediments accumulated simultaneously as the crust extended and sedimentation kept pace with isostatic subsidence. For parameters appropriate for the Salton Trough, increasing the extension rate has little effect on surface heat flow because it increases effects of heating by intrusion and cooling by sedimentation in a compensating manner; it does, however, result in progressively increasing lower crustal temperatures. Analytical results suggest that the average extensional strain rate during formation of the trough was ∼20–50%/m.y. (∼1014 s−1); slower rates are inadequate to account for the present composition of the crust, and faster rates would probably cause massive crustal melting. To achieve the differential velocities of the Pacific plate at one end of the trough and North American plate at the other with this strain rate, extension must have, on the average, been distributed (or shifted about) over a spreading region ∼150 km wide. This is about 10 times wider than the present zone of active seismicity, suggesting that the seismic pattern is ephemeral on the time scale for the trough's formation. Narrow spreading zones are typical where sustained spreading is compensated by basaltic intrusion to form the thin oceanic crust, but where such spreading occurs in thicker continental crust, broader zones of distributed extension (with smaller strain rates) may be required for heat balance. The Salton Trough model suggests that distributed extension can be associated with substantial magmatic additions to the crust; their effect on crustal buoyancy has important implications for the relation between crustal extension and subsidence.

Description: The distributions and transports of deepwater masses at the western boundary in the tropical Atlantic off Brazil have been studied on three surveys along 35 degrees W and 5 degrees S and one at 10 degrees S. Transports are obtained from direct measurements of the velocity fields (Pegasus profiling system and lowered acoustic Doppler current profiler) and from geostrophic computations. Using chlorofluoromethane (CFM) and hydrographic distributions, four water masses could be identified forming the North Atlantic Deep Water (NADW) system. Two of these have a high CFM content, the ''shallow upper NADW'' (SUNADW) and the ''overflow lower NADW'' (OLNADW). These exhibit the highest velocity signals at 35 degrees W, where distinct flow cores seem to exist; most of the southeastward flow of the SUNADW (centered around 1600 m) occurs 320 km offshore between 3 degrees 09'S and 1 degrees 50'S (9.7 +/- 3.3 Sv); farther north in that section, a highly variable reversing flow is found in a second velocity maximum. The transport of OLNADW (centered around 3800 m) of 4.6 +/- 2.6 Sv is guided by the Parnaiba Ridge at 1 degrees 45'S, 35 degrees W. The water masses located between the two CFM maxima, the Labrador Sea Water (LSW) and the LNADW old water mass (LNADW-old), did not show any persistent flow features, however, a rather constant transport of 11.1 +/- 2.6 Sv was observed for these two layers. The total southeastward flow of the NADW at 35 degrees W showed a transport of 26.8 +/- 7.0 Sv, if one neglects the reversing SUNADW north of 1 degrees 50'S. At 5 degrees S the flow of all deepwater masses shows vertically aligned cores; the main southward transport occurred near the coast (19.5 +/- 5.3 Sv). The boundary current is limited offshore by a flow reversal, present in all three surveys, but located at different longitudes. At 10 degrees S a southward transport of 4.7 Sv was observed in November 1992. However, the section extended only to 32 degrees 30'W, so that probably a significant part of the flow has been missed. An important result is the large transport variability between single cruises as well as variability of the spatial distribution of the flow at 35 degrees W, which could lead to large uncertainties in the interpretation of single cruise observations. Despite these uncertainties we suggest a circulation pattern of the various deepwater masses near the equator by combining our mean transport estimates with other observations.

Description: During March 1994 a survey of the western boundary of the tropical Atlantic, between 10 degrees N and 10 degrees S, was carried out by conductivity-temperature-depth and current profiling using shipboard and lowered acoustic Doppler current profilers. In the near-surface layer, above sigma. = 24.5, the inflow into the boundary regime came dominantly from low latitudes; out of the 14 Sv that crossed the equator in the upper part of the North Brazil Current (NBC), only 2 Sv originated from south of 5 degrees S, while 12 Sv came in from the east at 1 degrees-5 degrees S with the South Equatorial Current (SEC). After crossing the equator near 44 degrees W, only a minor fraction of the near-surface NBC retroflected eastward, while a net through flow of about 12 Sv above sigma. = 24.5 continued northwestward along the boundary, By contrast, in the isopycnal range sigma. = 24.5-26.8 encompassing the Equatorial Undercurrent (EUC), the source waters of the equatorial circulation were dominantly of higher-latitude South Atlantic origin. While only 3 Sv of eastern equatorial water entered the region through the SEC at 3 degrees-5 degrees S, there was an inflow of 10 Sv of South Atlantic water in the North Brazil Undercurrent (NBUC) along the South American coast that originated south of 10 degrees S, The transport of 14 Sv arriving at the equator along the boundary in the undercurrent layer was almost entirely retroflected into the EUC with only marginal northern water additions along its path to 35 degrees W. The off-equatorial undercurrents in the upper thermocline, the South and North Equatorial Undercurrents carried only small transports across 35 degrees W, of 5 Sv and 3 Sv, respectively, dominantly supplied out of SEC recirculation rather than out of the boundary current. Still deeper, three zonal undercurrents were observed: the westward-flowing Equatorial Intermediate Current (EIC) in the depth range 200-900 m below the EUC, and two off-equatorial eastward undercurrents, the Northern and Southern Intermediate Countercurrents (NICC, SICC) at 400-1000 m and 1 degrees-3 degrees latitude. In the lower part of the NBUC there was an Antarctic Intermediate Water (AAIW) inflow along the coast of 6 Sv, and there was a clear connection at the AAIW level to the SICC by low salinities and high oxygens and a weaker suggestion also that some supply of the NICC might be through AAIW out of the deep NBUC.

Description: Petrological, geochemical, and geophysical gradients along the SE volcanic zone in Iceland imply systematic variations in melting and crystallization conditions and in magma supply and eruption rates. At the southern tip of the zone, in Vestmannaeyjar, alkali basalt magmas are generated by small degrees of melting under a thick lithosphere. Farther north, in the Hekla-Katla region, greater degrees of melting result in the generation of transitional basalt magmas. Magma supply rates exceed eruption rates, and melts begin to accumulate at the base of the crust, as indicated by magnetotelluric evidence. Uniform rare earth element patterns in the Hekla-Katla basalts may be explained by homogenization in the melt accumulation zone or by uniform melting conditions. Infrequent replenishment of magma reservoirs in this region leads to mixing of compositionally diverse magmas and, consequently, to basalts with diverse phenocryst compositions and textures. Even farther north, in central Iceland, the melting anomaly associated with the SE zone has developed to the same degree as it has beneath the SW axial rift zone, leading to similar magmatic conditions. High magma supply rates and low cooling rates inhibit fractionation and lead to the eruption of voluminous olivine tholeiites. In these areas a broad spectrum of melt compositions is generated by variable degrees of melting over a wide depth range. The compositional diversity, e.g., in large ion lithophile element enrichment, is masked somewhat by reequilibration and mixing of melts on ascent and in the melt accummulation zone. Compositional diversity may be preserved, however, in the melt accummulation zone in a lateral direction away from the rift axis since distal parts of the melt zone are fed only by melts segregating at greater depths. The variations in magmatic conditions along the SE zone, which are analogous to those inferred along propagating rifts, may be related to a mantle blob that ascended beneath central Iceland 2–3 m.y. ago, spread out laterally and triggered a southward propagating rift.

Description: The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ∼15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches.

Description: Reducing the large volume of TIROS-N series advanced very high resolution radiometer-derived data to a practical size for application to regional physcial oceanographic studies is a formidable task. Such data exist on a global basis for January 1979 to the present at approximately 4-km resolution (global area coverage data, ≈2 passes per day) and in selected areas at high resolution (local area coverage and high-resolution picture transmission data, at ≈1-km resolution) for the same period. An approach that has been successful for a number of studies off the east coast of the United States divided the processing into two procedures: preprocessing and data reduction. The preprocessing procedure can reduce the data volume per satellite pass by over 98% for full-resolution data or by ≈84% for the lower-resolution data while the number of passes remains unchanged. The output of the preprocessing procedure for the examples presented is a set of sea surface temperature (SST) fields of 512 × 1024 pixels covering a region of approximately 2000 × 4000 km. In the data reduction procedure the number of SST fields (beginning with one per satellite pass) is generally reduced to a number manageable from the analyst's perspective (of the order of one SST field per day). This is done in most of the applications presented by compositing the data into 1- or 2-day groups. The phenomena readily addressed by such procedures are the mean position of the Gulf Stream, the envelope of Gulf Stream meandering, cold core Gulf Stream ring trajectories, statistics on diurnal warming, and the region and period of 18°C water formation. The flexibility of this approach to regional oceanographic problems will certainly extend the list of applications quickly.

Description: Correlations between particulate organic carbon (POC) and mineral fluxes in the deep ocean have inspired the inclusion of “ballast effect” parameterizations in carbon cycle models. A recent study demonstrated regional variability in the effect of ballast minerals on the flux of POC in the deep ocean. We have undertaken a similar analysis of shallow export data from the Arctic, Atlantic, and Southern Oceans. Mineral ballasting is of greatest importance in the high-latitude North Atlantic, where 60% of the POC flux is associated with ballast minerals. This fraction drops to around 40% in the Southern Ocean. The remainder of the export flux is not associated with minerals, and this unballasted fraction thus often dominates the export flux. The proportion of mineral-associated POC flux often scales with regional variation in export efficiency (the proportion of primary production that is exported). However, local discrepancies suggest that regional differences in ecology also impact the magnitude of surface export. We propose that POC export will not respond equally across all high-latitude regions to possible future changes in ballast availability.

Description: The role of biominerals in driving carbon export from the surface ocean is unclear. We compiled surface particulate organic carbon (POC), and mineral ballast export fluxes from 55 different locations in the Atlantic and Southern Oceans. Substantial surface POC export accompanied by negligible mineral export was recorded implying that association with mineral phases is not a precondition for organic export to occur. The proportion of non-mineral associated sinking POC ranged from 0 to 80% and was highest in areas previously shown to be dominated by diatoms. This is consistent with previous estimates showing that transfer efficiency in such regions is low. However we propose that, rather than the low transfer efficiency arising from diatom blooms being inherently characterized by poorly packaged aggregates which are efficiently exported but which disintegrate readily in mid water, it is due to such environments having very high levels of unballasted organic C export.

Description: A numerical algorithm based on Fermat's Principle was developed to simulate the propagation of Global Positioning System (GPS) radio signals in the refractivity field of a numerical weather model. The unique in the proposed algorithm is that the ray-trajectory automatically involves the location of the ground-based receiver and the satellite, i.e. the posed two-point boundary value problem is solved by an implicit finite difference scheme. This feature of the algorithm allows the fast and accurate computation of the signal travel-time delay, referred to as Slant Total Delay (STD), between a satellite and a ground-based receiver. We provide a technical description of the algorithm and estimate the uncertainty of STDs due to simplifying assumptions in the algorithm and due to the uncertainty of the refractivity field. In a first application, we compare STDs retrieved from GPS phase-observations at the German Research Centre for Geosciences Potsdam (GFZ STDs) with STDs derived from the European Center for Medium-Range Weather Forecasts analyses (ECMWF STDs). The statistical comparison for one month (August 2007) for a large and continuously operating network of ground-based receivers in Germany indicates good agreement between GFZ STDs and ECMWF STDs; the standard deviation is 0.5% and the mean deviation is 0.1%.

Description: In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (〈1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

Description: The importance of the Gulf Stream Extension region in climate and seasonal prediction research is being increasingly recognised. Here we use satellite-derived eddy momentum fluxes to drive a shallow water model for the North Atlantic Ocean that includes the realistic ocean bottom topography. The results show that the eddy momentum fluxes can drive significant transport, sufficient to explain the observed increase in transport of the Gulf Stream following its separation from the coast at Cape Hatteras, as well as the observed recirculation gyres. The model also captures recirculating gyres seen in the mean sea surface height field within the North Atlantic Current system east of the Grand Banks of Newfoundland, including a representation of the Mann Eddy.

Description: A decade of weak convection in the Labrador Sea associated with decreasing water mass transformation, in combination with advective and eddy fluxes into the convection area, caused significant warming of the deep waters in both the central Labrador Sea and boundary current system along the Labrador shelf break. The connection to the export of Deep Water was studied based on moored current meter stations between 1997 and 2009 at the exit of the Labrador Sea, near the shelf break at 5˚3N. More than 100 year -long current meter records spanning the full water column have been analyzed with respect to high frequency variability, decaying from the surface to the bottom layer, and for the annual mean flow, showing intra- to interannual variability but no detectable decadal trend in the strength of the deep and near-bottom flow out of the Labrador Sea.

Description: Recent work shows that multichannel seismic (MCS) systems provide detailed information on the oceans' finestructure. The aim of this paper is to analyze if high order numerical algorithms are suitable to accurately model the extremely weak wavefield scattered by the oceans' finestructures. For this purpose, we generate synthetic shot records along a coincident seismic and oceanographic profile acquired across a Mediterranean salt lens in the Gulf of Cadiz. We apply a 2D finite-difference time-domain propagation model, together with second-order Complex Frequency Shifted Perfectly Matched Layers at the numerical boundaries, using as reference a realistic sound speed map with the lateral resolution of the seismic data. We show that our numerical propagator creates an acoustical image of the ocean finestructures including the salt lens that reproduces with outstanding detail the real acquired one

Description: Recently seismic reflection methods have been successfully applied to oceanographic issues. Here, we present a new approach, combining XBT and CTD surveys with seismic observations, to visualize long sections with a resolution down to a few meters. The challenge to a full investigation of mixing processes has been the tremendous span of spatial scales ranging from hundreds of kilometers to centimeters. Traditional hydrographic observations could only resolve the large scale effects by measuring temperature and salinity profiles at discrete locations typically several kilometers apart, whereas dedicated localized measurements allowed investigation of the ocean fine structure at the other end of the spatial spectrum. The intermediate scales have in contrast been difficult to observe systematically. Here we present temperature and salinity data inverted from seismic observations that cover the intermediate scales and provide a new approach to image mesoscale processes and allow the investigation of their dynamics at unprecedented resolution.

Description: Nitrogen (N) fixation by specialized microorganisms (diazotrophs) influences global plankton productivity because it provides the ocean with most of its bio-available N. However, its global rate and large-scale spatial distribution is still regarded with considerable uncertainty. Here we use a global ocean nitrogen isotope model, in comparison with δ15NO3− observations, to constrain the pattern of N2 fixation across the Pacific Ocean. N2 fixation introduces isotopically light atmospheric N2 from to the ocean (δ15N = 0‰) relative to the oceanic average near 5‰, which makes nitrogen isotopes suitable to infer patterns of N2 fixation. Including atmospheric iron limitation of diazotrophy in the model shifts the pattern of simulated N2 fixation from the South Pacific to the North Pacific and from the East Pacific westward. These changes considerably improve the agreement with meridional transects of available δ15NO3− observations, as well as excess P (PO43− − NO3−/16), suggesting that atmospheric iron deposition is indeed important for N fixation in the Pacific Ocean. This study highlights the potential for using δ15N observations and model simulations to constrain patterns and rates of N fixation in the ocean.

Description: The Atlantic‐Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and d18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

Description: The stratospheric climate and variability from simulations of sixteen chemistryclimate models is evaluated. On average the polar night jet is well reproduced though its variability is less well reproduced with a large spread between models. Polar temperature biases are less than 5 K except in the Southern Hemisphere (SH) lower stratosphere in spring. The accumulated area of low temperatures responsible for polar stratospheric cloud formation is accurately reproduced for the Antarctic but underestimated for the Arctic. The shape and position of the polar vortex is well simulated, as is the tropical upwelling in the lower stratosphere. There is a wide model spread in the frequency of major sudden stratospheric warnings (SSWs), late biases in the breakup of the SH vortex, and a weak annual cycle in the zonal wind in the tropical upper stratosphere. Quantitatively, “metrics” indicate a wide spread in model performance for most diagnostics with systematic biases in many, and poorer performance in the SH than in the Northern Hemisphere (NH). Correlations were found in the SH between errors in the final warming, polar temperatures, the leading mode of variability, and jet strength, and in the NH between errors in polar temperatures, frequency of major SSWs, and jet strength. Models with a stronger QBO have stronger tropical upwelling and a colder NH vortex. Both the qualitative and quantitative analysis indicate a number of common and long‐standing model problems, particularly related to the simulation of the SH and stratospheric variability.

Description: The phosphorus budget of the pre-human modern ocean is constrained applying the most recent estimates of the natural riverine, eolian, and ice-rafted input fluxes, the phosphorus burial in marine sediments, and the hydrothermal removal of dissolved phosphate from the deep ocean. This review of current flux estimates indicates that the phosphorus budget of the ocean is unbalanced since the accumulation of phosphorus in marine sediments and altered oceanic crust exceeds the continental input of particulate and dissolved phosphorus. The phosphorus mass balance is further tested considering the dissolved phosphate distribution in the deep water column, the marine export production of particulate organic matter, rain rates of phosphorus to the seafloor, benthic dissolved phosphate fluxes, and the organic carbon to phosphorus ratios in marine particles. These independent data confirm that the phosphate and phosphorus budgets were not at steadystate in the pre-human global ocean. The ocean is losing dissolved phosphate at a rate of ≥ 11.6 x 1010 mol yr-1 corresponding to a decline in the phosphate inventory of ≥ 4.5 % kyr-1. Benthic data show that phosphate is preferentially retained in pelagic deep-sea sediments where extended oxygen exposure times favor the degradation of particulate organic matter and the up-take of phosphate in manganese and iron oxides and hydroxides. Enhanced C : P regeneration ratios observed in the deep water column (〉400 m water depth) probably reflect the preferential burial of phosphorus in pelagic sediments. Excess phosphate is released from continental margin sediments deposited in low-oxygen environments. The dissolved oxygen threshold value for the enhanced release of dissolved phosphate is ~20 μM. Benthic phosphate fluxes increase drastically when oxygen concentrations fall below this value.

Description: The decay kinetics of superoxide (O2−) reacting with organic matter was examined in oligotrophic waters at, and nearby, the TENATSO ocean observatory adjacent to the Cape Verde archipelago. Superoxide is the short-lived primary photochemical product of colored dissolved organic matter (CDOM) photolysis and also reacts with CDOM or trace metals (Cu, Fe) to form H2O2. In the present work we focused our investigations on reactions between CDOM and superoxide. O2− decay kinetics experiments were performed by adding KO2 to diethylenetriaminepentaacetic acid (DTPA) amended seawater and utilizing an established chemiluminescence technique for the detection of O2− at nM levels. In Cape Verdean waters we found a significant reactivity of superoxide with CDOM with maximal rates adjacent to the chlorophyll maximum, presumably from production of new CDOM from bacteria/phytoplankton. This work highlights a poorly understood process which impacts on the biogeochemical cycling of CDOM and trace metals in the open ocean.

Description: Geophysical datasets sensitive to different physical parameters can be used to improve resolution of Earth's internal structure. Herein, we jointly invert long-period magnetotelluric (MT) data and surface-wave dispersion curves. Our approach is based on a joint inversion using a genetic algorithm for a one-dimensional (1-D) isotropic structure, which we extend to 1-D anisotropic media. We apply our new anisotropic joint inversion to datasets from Central Germany demonstrating the capacity of our joint inversion algorithm to establish a 1-D anisotropic model that fits MT and seismic datasets simultaneously and providing new information regarding the deep structure in Central Germany. The lithosphere/asthenosphere boundary is found at approx. 84 km depth and two main anisotropic layers with coincident most conductive/seismic fast-axis direction are resolved at lower crustal and asthenospheric depths. We also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients.

Description: A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (〈200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.

Description: Reconstructions of the spatial pattern of recent multi-decadal sea level trends in the Indian Ocean (IO) indicate a zonally-extended band in the southern tropics where sea level has substantially fallen between the 1960s and 1990s; the decline is consistent with the observed subsurface cooling associated with a shoaling thermocline in this region. Here the origin and spatio-temporal characteristics of these trends are elucidated by a sequence of ocean model simulations. Whereas interannual variability in the southwestern tropical IO appears mainly governed by IO atmospheric forcing, longer term changes in the south tropical IO involve a strong contribution from the western Pacific via wave transmission of thermocline anomalies through the Indonesian Archipelago, and their subsequent westward propagation by baroclinic Rossby waves. The late 20th-century IO subsurface cooling trend reversed in the 1990s, reflecting the major regime shift in the tropical Pacific easterlies associated with the Pacific Decadal Oscillation.

Description: The modification of the exchange flow in a deep southern hemisphere passage, resembling the Vema Channel, by frictionally induced secondary circulation is investigated numerically. The hydrostatic primitive equation model is a two-dimensional version of the sigma-coordinate Princeton Ocean Model. The time dependent response of a stratified along-channel flow, forced by barotropic or baroclinic pressure gradients, is examined. Near the bottom, where the along-channel now is retarded, there is cross-channel Ekman nux that is associated with downwelling on the eastern side and upwelling on the western side of the channel. In the presence of stratification the cross-channel flow rearranges the density structure, which in turn acts on the along-channel velocity via the thermal wind relation. Eventually the cross-isobath Ekman flux is shut down. In the case of baroclinically driven flow of Antarctic Bottom Water through the Vema Channel the model reproduces the observed shape of the deep temperature profiles and their cross-channel asymmetry. The model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that;was proposed in earlier studies. It explains the extremely thick bottom boundary layers in the center and on the western slope of the channel. The deep thermocline is spread out in the west and sharpened in the east, and the coldest water is found on the eastern side of the deep trough; The modified density field reduces the along-channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel.

Description: A theoretical model of CO2aq-dependent phytoplankton carbon isotope fractionation (єp) and abundance (δ13Corg) is compared to observed isotopic trends with temperature and [CO2aq] in the ocean. It is shown that the model's δ13Corg response to surface ocean temperature and to [CO2aq] can simulate observed trends when the other independent model variables (phytoplankton cell growth rate, cell size, cell membrane CO2 permeability, and enzymatic isotope fractionation) are held at realistic, constant values. The possible contribution made by each of these variables to the residual scatter in δ13Corg about its trends with temperature and [CO2aq] is quantified, thus estimating a maximum isotopic sensitivity to changes in each of these variables. The model response to growth rate and especially cell size, however, appears to be unrealistically high. This may occur because the net isotopic effects of such factors may be attenuated through dependent and isotopically offsetting variations among variables. The model's indicated sensitivity to such factors as CO2 permeability, enzymatic fractionation, cell size, and cell surface area/volume provides mechanisms whereby changes in species composition can play a significant role in affecting observed variations in oceanic δ13Corg. Overall, the model is consistent with earlier suggestions that marine δ13Corg and єp variability can be explained by carbon isotope fractionation evoked by CO2aq-dependent phytoplankton. This has important implications for interpreting carbon isotopic variability encountered in plankton and their organic constituents in the present-day ocean and in the marine sedimentary record.

Description: We determined methane (CH4) emissions in a field enclosure experiment in a littoral freshwater marsh under the influence of experimentally simulated warming and enhanced nitrogen deposition. Methane emissions by ebullition from the marsh composed of Phragmites australis were measured with funnel traps deployed in a series of enclosures for two 3 week periods. Diffusive fluxes were estimated on the basis of measured CH4 concentrations and application of Fick's law. Neither diffusive nor ebullitive fluxes of methane were significantly affected by warming or nitrate enrichment, possibly because variability both within and among replicate experimental enclosures was high. Average emission rates resulted primarily from ebullition (0.2–30.3 mmol CH4 m−2 d−1), which were 4 orders of magnitude higher than estimated diffusive fluxes and were of similar importance as the coarsely estimated advective methane transport through plants. Significant correlations between dissolved oxygen and dissolved methane and ebullition flux suggest that methane release from the sediment might feed back positively on methane production by reducing dissolved oxygen in the water column and oxygen flux into the sediment. Nitrate may have a similar effect. Extrapolation of our limited data indicates that total methane fluxes from vegetated littoral zones of temperate lakes may contribute 0.5%–7% of the global natural CH4 emissions. These results emphasize the importance of freshwater marshes as sources of methane emissions to the atmosphere, even when they occupy only relatively small littoral areas. More detailed investigations are clearly needed to assess whether global warming and nitrogen deposition can have climate feedbacks by altering methane fluxes from these wetlands.

Description: We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region. This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991]. In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard. The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean. The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean. The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre. The equatorial part of the South Atlantic shows several depth-dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy-permitting resolution. The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near-surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC. The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary.

Description: We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E.

Description: The δ13C of dissolved inorganic carbon was measured on samples collected at 49°N in the northeast Atlantic in January 1994. Deeper than 2000 m, δ13C exhibits the same negative correlation versus dissolved phosphate that is observed elsewhere in the deep Atlantic. Upward from 2000 m to about 600 m, δ13C shifts to values more negative than expected from the correlation with nutrients at depth, which is likely due to penetration of anthropogenic CO2. From these data, the profile of the anthropogenic δ13C decrease is calculated by using either dissolved phosphate or apparent oxygen utilization as a proxy for the preanthropogenic δ13C distribution. The shape of the anthropogenic anomaly profile derived from phosphate is similar to that of the increase in dissolved inorganic carbon derived by others in the same area. The reconstruction from oxygen utilization results in a lower estimate of the anthropogenic δ13C decrease in the upper water column, and the vertical anomaly profile is less similar to that of the dissolved inorganic carbon increase. A 13C budget for the atmosphere, ocean, and terrestrial biosphere indicates that within the range of probable ocean CO2 uptake the ratio of δ13C to inorganic carbon change should be mostly influenced by the 13C inventory change of the biosphere. However, the uncertainty in the ratio we derive prevents a strong contraint on the size of the exchangeable biosphere.

Description: The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 10(6)m(3)s(-1)), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.

Description: Four World Ocean Circulation Experiment (WOCE) repeat cruises (October 1990 to March 1994) in the tropical Atlantic off Brazil are used to study the spatial and temporal evolution of the chlorofluorocarbon (CFC) (components CFC-11 and CFC-12) and tritium signal in the upper North Atlantic Deep Water (NADW). Its shallowest part, located in the tropical Atlantic around 1600-m depth, is the shallow upper North Atlantic Deep Water (SUNADW). It is characterized by a distinct tracer maximum, which is presumably received through winter time convection in the subpolar North Atlantic. Here we discuss the tracer fields and the temporal evolution of the tracer signal of the SUNADW in the tropical Atlantic along two meridional sections at 44 degrees and 35 degrees W and two zonal sections at 5 degrees and 10 degrees S off Brazil. The spatial and temporal development of the tracer field in the tropical Atlantic as well as the correlation with hydrographic parameters show that the temporal tracer change being due to the arrival of "younger" water is disturbed by other processes. In particular, the impact of variable mixing and spreading pathways on the observed tracer variability in the SUNADW is evident in the observations.

Description: The zonal circulation south of Sri Lanka is an important link for the exchange of water between the Bay of Bengal and the Arabian Sea. Results from a first array of three moorings along 80 degrees 30'E north of 4 degrees 10'N from January .1991 to March 1992 were used to investigate the Monsoon Current regime [Schott et al., 1994]. Measurements from a second array of six current meter moorings are presented here. This array was deployed along 80 degrees 30'E between 45'S and 5 degrees N from July 1993 to September 1994 to investigate the annual cycle and interannual variability of the equatorial currents at this longitude. Both sets of moorings contribute to the Indian Ocean current meter array ICM8 of the World Ocean Circulation Experiment. The semiannual equatorial jet (EJ) was showing a large seasonal asymmetry, reaching a monthly mean eastward transport of 35 Sv (1 Sv = 1 x 10(6) m(3) s(-1)) in November 1993, but just 5 Sv in May 1994. The Equatorial Undercurrent (EUC) had a maximum transport of 17 Sv in March to April 1994. Unexpectedly, compared to previous observations and model studies, the EUC was reappearing again in August 1994 at more than 10 Sv transport and was still flowing when the moorings were recovered. In addition, monthly mean ship drifts near the equator are evaluated to support the interpretation of the moored observations. Interannual variability of the EJ in our measurements and ship drift data appears to be related to the variability of the zonal winds and Southern Oscillation Index. The output of a global numerical model (Parallel Ocean Climate Model) driven by the winds for 1993/1994 is used to connect our observations to the larger scale. The model reproduces the EJ asymmetry and shows the existence of the EUC and its reappearance during summer 1994.

Description: The partial pressure of CO2 (pCO2) was measured during the 1995 South-West Monsoon in the Arabian Sea. The Arabian Sea was characterized throughout by a moderate supersaturation of 12–30 µatm. The stable atmospheric pCO2 level was around 345 µatm. An extreme supersaturation was found in areas of coastal upwelling off the Omani coast with pCO2 peak values in surface waters of 750 µatm. Such two-fold saturation (218%) is rarely found elsewhere in open ocean environments. We also encountered cold upwelled water 300 nm off the Omani coast in the region of Ekman pumping, which was also characterized by a strongly elevated seawater pCO2 of up to 525 µatm. Due to the strong monsoonal wind forcing the Arabian Sea as a whole and the areas of upwelling in particular represent a significant source of atmospheric CO2 with flux densities from around 2 mmol m−2 d−1 in the open ocean to 119 mmol m−2 d−1 in coastal upwelling. Local air masses passing the area of coastal upwelling showed increasing CO2 concentrations, which are consistent with such strong emissions.

Description: The deep water of the western Mediterranean Sea is known to have become warmer and saltier since about the 1950s. The causes of these changes have, however, not yet been sactisfactorily determined. Previous studies speculated on decreasing precipitation, greenhouse warming and/or anthropogenic reduction of the freshwater flux into the eastern Mediterranean. Here we report on results from a new oceanographic database of the western Mediterranean Sea together with determinations of longterm changes of the fresh water budget. We analyzed temperature and salinity data of the past 40 years to detect deviations from the longterm average. Certain areas and depth ranges are showing increases in temperature or salinity some of which have been found earlier and some which are new. From the regional and vertical distribution we conclude that the observed increases of temperature and salinity in the western Mediterranean Sea are caused both by changes in atmospheric conditions as described by the NAO‐index and by the regulation of Spanish rivers.

Description: We review what is known about the convective process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convective and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are described, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection, great progress in our description and understanding of the processes at work is being made.

Description: We examine the propagation of low-frequency electromagnetic (EM) waves in the coastal ocean produced by controlled or motional impressed sources. Four important modes are the direct, up-over-down, down-over-up, and “beach” modes. The analyses of these modes are complicated by the varying bathymetry in the coastal region. We derive criteria to determine (1) which modes are important for given parameters; (2) a “matched phase” condition describing both when the up-over-down and down-over-up modes interfere constructively in the shallow zone and when the beach mode becomes important; and (3) a low-frequency cutoff, below which the EM fields are not sensitive to the details of the coastal geometry. We verify the theoretically derived criteria with numerical examples and finally discuss the importance of our results in designing navigation and communications applications for subsurface vehicles and instruments.

Description: The sensitivity of the hydrological cycle to changes in orbital forcing and atmospheric greenhouse gas (GHG) concentrations is assessed using a fully coupled atmosphere-ocean-sea ice general circulation model (Kiel Climate Model). An orbitally-induced intensification of the summer monsoon circulation during the Holocene and Eemian drives enhanced water vapor advection into the Northern Hemisphere, thereby enhancing the rate of water vapor changes by about 30% relative to the rate given by the Clausius-Clapeyron Equation, assuming constant relative humidity. Orbitally-induced changes in hemispheric-mean precipitation are fully attributed to inter-hemispheric water vapor exchange in contrast to a GHG forced warming, where enhanced precipitation is caused by increased both the moisture advection and evaporation. When considering the future climate on millennial time scales, both forcings combined are expected to exert a strong effect.

Description: We present results from a seismic refraction and wide-angle experiment surveying an oceanic core complex on the Mid-Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un-roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

Description: The Mid-Atlantic Ridge south of the equator is a key region for many aspects of spreading axis studies, from biogeography to ridge-hotspot interaction. Despite this, the ridge axis had, until 2004, seen little systematic study. Repeated trips to the area since then have mapped and explored some 900 km of ridge length, from 2° to 14°S. The result is complete bathymetric and side-scan coverage of the axial region and the discovery and characterization of the first hydrothermal vents south of the equator. Such multisegment detailed and interdisciplinary coverage allows us to formulate a general model for the interplay between volcanism, tectonics, and hydrothermalism on a slow spreading ridge. The model defines three basic types of ridge morphology with specific hydrothermal characteristics: (a) a deep, tectonically dominated rift valley where hydrothermalism is seldom associated with volcanism and much more likely confined to long-lived bounding faults; (b) a shallower, segment-center bulge where a combination of repeated magmatic activity and tectonism results in repeated, possibly temporally overlapping periods of hydrothermal activity on the ridge axis; and (c) a very shallow axis beneath which temperatures in all but the uppermost crust are so high that deformation is ductile, inhibiting the formation of high-porosity deep fractures and severely depressing hydrothermal circulation. This model is used together with satellitederived predicted bathymetry to provide forecasts of the best places to look for hydrothermal sites in the remaining unexplored regions of the South Atlantic.

Description: The Sahara Slide is a giant submarine landslide on the northwest African continental margin. The landslide is located on the open continental slope offshore arid Western Sahara, with a headwall at a water depth of ∼2000 m. High primary productivity in surface waters drives accumulation of thick fine-grained pelagic/hemipelagic sediment sequences in the slide source area. Rare but large-scale slope failures, such as the Sahara Slide that remobilized approximately 600 km3 of sediment, are characteristic of this sedimentological setting. Seismic profiles collected from the slide scar reveal a stepped profile with two 100 m high headwalls, suggesting that the slide occurred retrogressively as a slab-type failure. Sediment cores recovered from the slide deposit provide new insights into the process by which the slide eroded and entrained a volcaniclastic sand layer. When this layer was entrained at the base of the slide it became fluidized and resulted in low apparent friction, facilitating the exceptionally long runout of ∼900 km. The slide location appears to be controlled by the buried headwall of an older slope failure, and we suggest that the cause of the slide relates to differential sedimentation rates and compaction across these scarps, leading to local increases of pore pressure. Sediment cores yield a date of 50–60 ka for the main slide event, a period of global sea level rise which may have contributed to pore pressure buildup. The link with sea level rising is consistent with other submarine landslides on this margin, drawing attention to this potential hazard during global warming.

Description: The supply of oxygen-rich water to the oxygen minimum zones (OMZs) of the eastern North and South Pacific via zonal tropical currents is investigated using shipboard acoustic Doppler current profiler and hydrographic section data. Near the equator, the Equatorial Undercurrent (EUC), Northern and Southern Subsurface Countercurrents (SCCs), and the Northern and Southern Intermediate Countercurrents (ICCs) all carry water that is oxygen richer than adjacent westward flows, thereby providing a net oxygen supply to the eastern Pacific OMZs. The synoptic velocity-weighted oxygen concentration difference between eastward and westward flows is typically 10–50 μmol kg−1. Subthermocline zonal oxygen fluxes reflect decreasing oxygen concentrations of the EUC, the SCCs, and the ICCs as they flow eastward. Approximately 30 year time series in well-sampled regions of the equatorial Pacific show oxygen content decreasing as rapidly as −0.55 μmol kg−1 yr−1 in the major oxygen supply paths of the OMZs for a 200–700 m layer and similar trends for a density layer spanning roughly these depths. This finding is in gross agreement with climate models, which generally predict expanding OMZs.

Description: Near the western boundary of the tropical North Atlantic, where the North Brazil Current (NBC) retroflects into the North Equatorial Countercurrent, large anticyclonic rings are shed. After separating from the retroflection region, the so-called NBC rings travel northwestward along the Brazilian coast, until they reach the island chain of the Lesser Antilles and disintegrate. These rings contribute substantially to the upper limb return flow of the Atlantic Meridional Overturning Circulation by carrying South Atlantic Water into the northern subtropical gyre. Their relevance for the northward transport of South Atlantic Water depends on the frequency of their generation as well as on their horizontal and vertical structure. The ring shedding and propagation and the complex interaction of the rings with the Lesser Antilles are investigated in the inline equation Family of Linked Atlantic Model Experiments (FLAME) model. The ring properties simulated in FLAME reach the upper limit of the observed rings in diameter and agree with recent observations on seasonal variability, which indicates a maximum shedding during the first half of the year. When the rings reach the shallow topography of the Lesser Antilles, they are trapped by the island triangle of St. Lucia, Barbados and Tobago and interact with the island chain. The model provides a resolution that is capable of resolving the complex topographic conditions at the islands and illuminates various possible fates for the water contained in the rings. It also reproduces laboratory experiments that indicate that both cyclones and anticyclones are formed after a ring passes through a topographic gap. Trajectories of artificial floats, which were inserted into the modeled velocity field, are used to investigate the pathways of the ring cores and their fate after they encounter the Lesser Antilles. The majority of the floats entered the Caribbean, while the northward Atlantic pathway was found to be of minor importance. No prominent pathway was found east of Barbados, where a ring could avoid the interaction with the islands and migrate toward the northern Lesser Antilles undisturbed.

Description: A joint research effort is currently focused on the oceanic region south of Africa—the gateway for the exchange of mass, heat, and salt between the Indian and Atlantic Oceans (Figure lb). The name of this collaboration, KAPEX, stands for Cape of Good Hope Experiments, Kap der guten Hoffnung Experimente, or Kaap die Goeie Hoop Eksperimente in the three languages of the participating scientists. This is the first time that scientists are using acoustically tracked floats extensively in ocean regions surrounding southern Africa to measure ocean flow patterns. At the tip of Africa, the Agulhas Current from the Indian Ocean interacts with the South Atlantic Current, contributing to the northwestward flowing Benguela Current, which transports water, heat, and salt to the subtropical and subequatorial South Atlantic (Figure la). This transport increases the heat and salinity of the North Atlantic, preconditioning it for the formation of the global thermohaline circulation cell, a driving force of the world climate [Gordon etal., 1992]. Our objective in the KAPEX is to trace the flow of intermediate water around southern Africa by the Agulhas, Benguela, and South Atlantic Current systems and to answer key questions about the inter-oceanic intermediate circulation.

Description: The output of the global eddy‐resolving ¼° ocean model of Semtner/Chervin (run by the Naval Postgraduate School, Monterey, California) has been used to study the oceanic temperature and heat flux in the Indian Ocean. The meridional heat flux in the northern Indian Ocean is at the low end of the observed values. A vertical overturning cell in the upper 500 m is the main contributor to the annual mean meridional heat flux across 5°S, whereas the horizontal gyre circulation, confined to the upper 500 m, dominates north of the equator. The change of monsoon winds is manifested in a reversal of the meridional circulation throughout the whole water column. The most notable result is a strong linear relationship of the meridional temperature flux and the zonal wind stress component north of 20°S. The model's Pacific‐Indian Ocean throughflow across the section at 120°E accounts for −8.8±5.1 Sv (1 Sv≡106 m3 s−1). A strong interannual variability during the model run of 3 years shows a maximum range of 12 Sv in January/February and a minimum during March through June. The inflow from the Pacific into the Indian Ocean results in a total annual mean temperature flux of −0.9 PW (1 PW≡1015 W). In the model the temperature flux from the Pacific through the Indian Ocean to the south dominates in comparison with the input of solar heat from the northern Indian Ocean.

Description: From geostrophic calculations the exchange of deep water from the Somali into the Arabian Basin through the Owen Fracture Zone has been estimated to be about 2 Sv, with a seasonal modulation of the same magnitude. After leaving the Fracture Zone, the flow bifurcates into a northern and a southern branch, each closely following the slope of the Carlsberg Ridge. The weaker vertical gradients of the hydrographic properties in the deep Arabian Basin are consistent with enhanced vertical mixing at the rugged topography over the Carlsberg Ridge.

Description: The Southern Hemisphere winter stratosphere exhibits prominent traveling planetary-scale Rossby waves, which generally are not able to induce Stratospheric Sudden Warmings. A series of runs of a simplified general circulation model is presented, aimed at better understanding the generation of these waves. While the generation of planetary-scale traveling waves through the interaction of synoptic-scale waves is observed in a control run, when the model is truncated to permit only waves with zonal wave number 1 or 2, the long waves are found to increase in strength, leading to a considerably more active stratosphere including Sudden Warmings comparable in strength to Northern Hemisphere winter. This finding suggests that the role of tropospheric synoptic eddies is two-fold: while generating a weak planetary-scale wave flux into the stratosphere, their main effect is to suppress baroclinic instability of planetary-scale waves by stabilizing the tropospheric mean state.

Description: Replacement dolomitization by seawater has been modeled in order to quantify the Sr-isotope signature in Cenozoic dolomites as a function of precursor mineralogy and 87Sr/86Sr ratio, reaction stoichiometry and 87Sr/86Sr ratio of the dolomitizing fluids. High Sr carbonates, such as aragonite, may introduce a significant precursor memory into an otherwise seawater dominated Sr-isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved. Dolomitization of low Sr carbonates (i.e. low-Mg calcite) are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction. Therefore, by comparison with the Sr-isotope evolution curve of seawater, the- 87Sr/86Sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition. The implications for this approach have been applied to data obtained from a dolomitized core from Little Bahama Bank, Bahamas. Two periods of dolomitization are recognized, one in the early Late Miocene involving Middle Miocene or older rocks, and a second one around 2.4 Ma ago affecting early Pliocene carbonates.