ALBERT

Description: Pliensbachian and Toarcian Ostracoda first described by Exton (Geological Paper, Carleton University, Ottawa, 79: 1–104 1979) from the Lusitanian Basin, west-central Portugal have been re-examined. As a result, a greater diversity in the Ostracoda (80 species) is now recognized. Two species are newly described (Eucytherura zambujalensis sp. nov., Ektyphocythere mediodepressa sp. nov.) from the marls and calcareous shales of the Maria Pares Hill section near the village of Zambujal. Poor preservation precludes a complete taxonomic review of the present material. Five ostracod zones are proposed; Gammacythere ubiquita–Ogmoconchella gruendeli Zone, Poly cope cerasia–Polycope cincinnata Zone, Liasina lanceolata–Ogmoconcha convexa Zone, Bairdiacypris rectangularis–Kinkelinella sermoisensis Zone, and Cytherella toarcensis-Kinkelinella costata Zone. Although the ostracod assemblages possess strong similarities to those described from Northwest Europe, some of the Zambujal assemblages are dominated by the genus Polycope. A marked faunal turnover, in association with the extinction of the Metacopina occurs in the lower Subzone of the tenuicostatum Zone of Lower Toarcian age. These faunal events are discussed in relation to changing environmental conditions.

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: 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: 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 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: 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 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: 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: We reconstructed late Quaternary deep (3000–4100 m) and intermediate depth (1000–2500 m) paleoceanographic history of the Eurasian Basin, Arctic Ocean from ostracode assemblages in cores from the Lomonosov Ridge, Gakkel Ridge, Yermak Plateau, Morris Jesup Rise, and Amundsen and Makarov Basins obtained during the 1991 Polarstern cruise. Modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant, relatively species-rich benthic ostracode assemblages, in part, reflecting the influence of high organic productivity and inflowing Atlantic water. In contrast, deep Arctic Eurasian basin assemblages have low abundance and low diversity and are dominated by Krithe and Cytheropteron reflecting faunal exchange with the Greenland Sea via the Fram Strait. Major faunal changes occurred in the Arctic during the last glacial/interglacial transition and the Holocene. Low-abundance, low-diversity assemblages from the Lomonosov and Gakkel Ridges in the Eurasian Basin from the last glacial period have modern analogs in cold, low-salinity, low-nutrient Greenland Sea deep water; glacial assemblages from the deep Nansen and Amundsen Basins have modern analogs in the deep Canada Basin. During Termination 1 at intermediate depths, diversity and abundance increased coincident with increased biogenic sediment, reflecting increased organic productivity, reduced sea-ice, and enhanced inflowing North Atlantic water. During deglaciation deep Nansen Basin assemblages were similar to those living today in the deep Greenland Sea, perhaps reflecting deepwater exchange via the Fram Strait. In the central Arctic, early Holocene faunas indicate weaker North Atlantic water inflow at middepths immediately following Termination 1, about 8500–7000 year B.P., followed by a period of strong Canada Basin water overflow across the Lomonosov Ridge into the Morris Jesup Rise area and central Arctic Ocean. Modern perennial sea-ice cover evolved over the last 4000–5000 years. Late Quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold, deepwater of Greenland Sea origin and warmer and middepth Atlantic water to the Eurasian Basin through the Fram Strait, and export of Arctic Ocean deepwater.

Description: Stable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.

Description: Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (〉8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bölling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell.

Description: Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Description: Stable oxygen and-carbon isotope and sedimentological-paleontological investigations supported by accelerator mass spectrometry (14)C datings were carried out on cores from north of 85 degrees N in the eastern central Arctic Ocean. Significant changes in accumulation rates, provenance of ice-rafted debris (IRD), and planktic productivity over the past 80,000 years are documented. During peak glacials, i.e., oxygen isotope stages 4 and 2, the Arctic Ocean was covered by sea ice with decreased seasonal variation, limiting planktic productivity and bulk sedimentation rates. In early stage 3 and during Termination I, major deglaciations of the circum-Arctic regions caused lowered salinities and poor oxygenation of central Arctic surface waters. A meltwater spike and an associated IRD peak dated to similar to 14-12 (14)C ka can be traced over the southern Eurasian Basin of the Arctic Ocean. This event was associated with the early and rapid deglaciation of the marine-based Barents Sea Ice Sheet. A separate Termination Ib meltwater event is most conspicuous in the central Arctic and is associated with characteristic dolomitic carbonate IRD. This lithology suggests an origin of glacial ice from northern Canada and northern Greenland where lower Paleozoic platform carbonates crop extensively out.

Description: The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

Description: The 14 Ma caldera-forming composite ignimbrite P1 on Gran Canaria (Canary Islands) represents the first voluminous eruption of highly differentiated magmas on top of the basaltic Miocene shield volcano. Compositional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed during eruption: (1) Crystal-poor to highly phyric rhyolite (∼10 km3), (2) sodic trachyandesite through mafic to evolved trachyte (∼6 km3), (3) Na-poor trachyandesite (〈1 km3), and (4) basalt zoned from 5.2 to 4.3 wt % MgO (∼26 km3). P1 basalt is composed of two compositionally zoned magma batches, B2 basalt and B3 basalt. B3 basalt is derived from a mantle source depleted in incompatible trace elements compared to the shield basalt source. Basaltic magmas were stored in a reservoir probably underplating the crust, in which zoned B2 basaltic magma formed by mixing of “enriched” (shield) and “depleted” (B3) mafic melts and subsequent crystal fractionation. Evolved magmas formed in a shallow crustal chamber, whereas intermediate magmas formed at both levels. Abundant pyroxenitic to gabbroid cumulates in P1 support crystal fractionation as the major differentiation process. On the basis of major and trace element modeling, we infer two contemporaneous fractional crystallization series: series I from “enriched” shield basalt through Na-poor trachyandesite to rhyolite, and series II from “depleted” P1 basalt through sodic trachyandesite to trachyte. Series II rocks were significantly modified by selective contamination involving feldspar (Na, K, Ba, Eu, Sr), zircon (Zr) and apatite (P, Y, rare earth elements) components; apatite contamination also affected series I Na-poor trachyandesite. Substantial sodium introduction into sodic trachyandesite is the main reason for the different major element evolution of the two series, whereas their different parentage is mainly reflected in the high field strength trace elements. Selective element contamination involved not only rapidly but also slowly diffusing elements as well as different saturation conditions. Contamination processes thus variably involved differential diffusion, partial dissolution of minerals, partial melt migration, and trace mineral incorporation. Magma mixing between trachyte and rhyolite during their simultaneous crystallization in the P1 magma chamber is documented by mutual mineral inclusions but had little effect on the compositional evolution of both magmas. Fe-Ti oxide thermometry yields magmatic temperatures of around 850°C for crystal-poor through crystal-rich rhyolite, ∼815°C for trachyte and ∼850°–900°C for the trachyandesitic magmas. High 1160°C for the basalt magma suggest its intrusion into the P1 magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandesite support their residence along the walls of the vertically and laterally zoned P1 magma chamber. The complex magmatic evolution of P1 reflects the transient state of Gran Canaria's mantle source composition and magma plumbing system during the change from basaltic to silicic volcanism. Our results for P1 characterize processes operating during this important transition, which also occurs on other volcanic ocean islands.

Description: Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher-order, positive-definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.

Description: Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) are the two major sources for the ventilation of the permanent thermocline in the Indian Ocean. ICW is formed by subduction in the region of negative wind stress curl, while SAMW is formed by convective overturning at the Subantarctic Front. SAMW contributes to the depth range of ICW, but is not easily identified, because most hydrographic properties (temperature, salinity and nutrients) of SAMW do not differ much from those of ICW. This study identifies ICW and SAMW in a zonal section near 32°S and evaluates the relative importance of convection vs. subduction for the ventilation process. Oxygen and nutrient data from the eastern part (50–114°E) of WOCE section 15 are used with temperature and salinity to determine water mass fractions of subducted ICW and of SAMW from water mass mixing analysis. The individual age fields of the two water mass components are then derived from a combination of the fractions obtained with a linear oxygen/CFC mixing model. Unlike earlier studies, which derive an uncalibrated apparent age, our results express water mass age in true units of time (years). The core of the SAMW near 114°E is about 5 years old, while the core of the subducted ICW (at 60–80°E) shows an increase of age with depth, in agreement with the subduction process. ICW moves eastward with the South Indian Current, reaching an age of 35 years at 114°E. SAMW spreads westward against the mean flow through turbulent diffusion, reaching an age of 25 years at 50°E.

Description: Phytoplankton processes in subantarctic (SA) waters southeast of New Zealand were studied during austral autumn and spring 1997. Chlorophyll a (0.2–0.3 μg L−1) and primary production (350–650 mg C m−2 d−1) were dominated by cells 〈2 μm (cyanobacteria) in both seasons. The photochemical efficiency of photosystem II (Fυ/Fm) of cells was low (0.3), indicating physiological stress. Dissolved Fe (DFe) levels in surface waters were subnanomolar, and the molecular marker flavodoxin indicated that cells were iron stressed. In contrast, Subtropical Convergence (STC) and subtropical waters had higher algal biomass/production levels, particularly in spring. In these waters, DFe levels were 〉1 nmol kg−1, there was little evidence of Fe-stressed algal populations, and Fυ/Fm approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with caution. In vitro Fe enrichment incubations in SA waters resulted in a switch from flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6-day experiment, iron-mediated increases in chlorophyll a (in particular, increases in large diatoms) were of similar magnitude to those observed in a concurrent Si/Fe enrichment; ambient silicate levels were 4 μM. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, resulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In contrast, during summer, as mean irradiance increases and silicate levels decrease, Fe limitation, Fe/Si colimitation, or silicate limitation may determine diatom growth.

Description: Recent studies have indicated that drifting Arctic sea ice plays an important role in the redistribution of sediments and contaminants. Here we present a method to reconstruct the backward trajectory of sea ice from its sampling location in the Eurasian Arctic to its possible site of origin on the shelf, based on historical drift data from the International Arctic Buoy Program. This method is verified by showing that origins derived from the backward trajectories are generally consistent with other indicators, such as comparison of the predicted backward trajectories with known buoy drifts and matching the clay mineralogy of sediments sampled from the sea ice with that of the seafloor in the predicted shelf source regions. The trajectories are then used to identify regions where sediment‐laden ice is exported to the Transpolar Drift Stream: from the New Siberian Islands and the Central Kara Plateau. Calculation of forward trajectories shows that the Kara Sea is a major contributor of ice to the Barents Sea and the southern limb of the Transpolar Drift Stream.

Description: Two seasonal hydrographic data sets, including temperature, salinity, dissolved oxygen, and nutrients, are used in a mixing model which combines cluster analysis with optimum multiparameter analysis to determine the spreading and mixing of the thermocline waters in the Indian Ocean. The mixing model comprises a system of four major source water masses, which were identified in the thermocline through cluster analysis. They are Indian Central Water (ICW), North Indian Central Water (NICW) interpreted as aged ICW, Australasian Mediterranean Water (AAMW), and Red Sea Water (RSW)/Persian Gulf Water (PGW). The mixing ratios of these water masses are quantified and mapped on four isopycnal surfaces which span the thermocline from 150 to 600 m in the northern Indian Ocean, on two meridional sections along 60°E and 90°E, and on two zonal sections along 10°S and 6°N. The mixing ratios and pathways of the thermocline water masses show large seasonal variations, particularly in the upper 400–500 m of the thermocline. The most prominent signal of seasonal variation occurs in the Somali Current, the western boundary current, which appears only during the SW (summer) monsoon. The northward spreading of ICW into the equatorial and northern Indian Ocean is by way of the Somali Current centered at 300–400 m on the σθ=26.7 isopycnal surface during the summer monsoon and of the Equatorial Countercurrent during the NE (winter) monsoon. More ICW carried into the northern Indian Ocean during the summer monsoon is seen clearly in the zonal section along 6°N. NICW spreads southward through the western Indian Ocean and is stronger during the winter monsoon. AAMW appears in both seasons but is slightly stronger during the summer in the upper thermocline. The westward flow of AAMW is by way of the South Equatorial Current and slightly bends to the north on the σθ=26.7 isopycnal surface during the summer monsoon, indicative of its contribution to the western boundary current. Outflow of RSW/PGW seems effectively blocked by the continuation of strong northward jet of the Somali Current along the western Arabian Sea during the summer, giving a rather small contribution of only up to 20% in the Arabian Sea. A schematic summer and winter thermocline circulation emerges from this study. Both hydrography and water ‐ mass mixing ratios suggest that the contribution of the water from the South Indian Ocean and from the Indo‐Pacific through flow controls the circulation and ventilation in the western boundary region during the summer. However, during the winter the water is carried into the eastern boundary by the Equatorial Countercurrent and leaks into the eastern Bay of Bengal, from where the water is advected into the northwestern Indian Ocean by the North Equatorial Current. The so‐called East Madagascar Current as a southward flow occurs only during the summer, as is suggested by both hydrography and water‐mass mixing patterns from this paper. During the winter (austral summer) the current seems reversal to a northward flow along east of Madagascar, somewhat symmetrical to the Somali Current in the north.

Description: The variability of the ice volume flux into the northeast Atlantic is investigated with an optimized dynamic-thermodynamic sea ice model using 40 years (1958–1997) of atmospheric forcing fields derived from the NCEP/NCAR reanalysis project. The simulated sea ice export from the Arctic exhibits considerable interannual to decadal variability and is primarily a linear response to sea level pressure anomalies over Greenland and over the Barents and Kara Seas. Our model results suggest that ice export anomalies such as in 1968 which supposedly caused the so-called “Great Salinity Anomaly” in the northern North Atlantic are not unique but rather frequent events as part of the variability of the Arctic climate system.

Description: A three-dimensional Monte Carlo transfer model for polarized radiation is developed and used to study three-dimensional (3-D) effects of raining clouds on the microwave brightness temperature. The backward method is combined with the forward method to treat polarization correctly within the cloud. In comparison with horizontally homogeneous clouds, two effects are observed: First, brightness temperatures from clouds are reduced in the 3-D case due to net leakage of radiation from the sidewalls of the cloud. Second, radiation which is emitted by the warm cloud and then reflected from the water surface increases the brightness temperatures of the cloud-free areas in the vicinity of the cloud. Both effects compete with each other, leading to either lower or higher overall brightness temperatures, depending on the geometry of the cloud, the satellite viewing angle, the coverage, and the position of the cloud within the field of view (FOV) of the satellite. At 37 GHz, for example, up to 10 K differences can occur for a cloud of 50% coverage. Finite homogeneous raining clouds matching the size of the FOV of the satellite show a similar relationship between rain rates and brightness temperatures (TB) as horizontally infinite clouds. Namely, an increase of TB with increasing rain rates at low rain rates, due to emission effects, is followed by a decrease due to temperature and scattering effects. For small horizontal cloud diameter, however, the 3-D brightness temperatures may show a second maximum due to the decrease of the leakage effect with increasing rain rates. At nadir, 3-D brightness temperatures are always lower than the 1-D values with differences up to 20 K for a cloud of 5-km vertical extent and a base of 1 × 1 km. To quantify the 3-D effects for more realistic cloud structures, we used results of a three-dimensional dynamic cloud model as input for the radiative transfer codes. The same 3-D effects are obtained, but the differences between 1-D and 3-D modeling are smaller. In general, most of the differences between the 1-D and 3-D results for off-nadir view angles are pure geometry effects, which can be accounted for in part by a modified 1-D model.

Description: Extensive investigations of sedimentary barium were performed in the southern South Atlantic in order to assess the reliability of the barium signal in Antarctic sediments as a proxy for paleoproductivity. Maximum accumulation rates of excess barium were calculated for the Antarctic zone south of the polar front where silica accumulates at high rates. The correspondence between barium and opal supports the applicability of barium as a proxy for productivity. Within the Antarctic zone north of today's average sea ice maximum, interglacial vertical rain rates of excess barium are high, with a maximum occurring during the last deglaciation and early Holocene and during oxygen isotope chronozone 5.5. During these periods, the maximum silica accumulation was supposedly located south of the polar front. Glacial paleoproductivity, instead, was low within the Antarctic zone. North of the polar front, significantly higher barium accumulation occurs during glacial times. The vertical rain rates, however, are as high as in the glacial Antarctic zone. Therefore there was no evidence for an increased productivity in the glacial Southern Ocean.

Description: The evolution of the Black Sea's salinity after the opening of the Bosporus about 7500 years ago is investigated using a simple two-box model. The model consists of watermass and salt conservation equations, and allows for changes in halocline depth. The paleoceanographic box model is forced by present-day Mediterranean inflow and outflow, and atmospheric forcings. Analytic solutions for the evolution of the box volumes are given. Model salinities reach 90% of their the present-day values in both boxes about 2,500 years after the opening of the Bosporus. The evolution of the salinities is shown to be almost independent of the evolution of the box volumes, and the results are compared with the existing paleoceanographic proxy records.

Description: Recent hydrographic sections and high-quality historical data sets are used to determine geostrophic currents at subtropical latitudes in the western basin of the South Atlantic. Levels of no motion are determined from water mass information and a mass balance constraint to obtain the transport field of North Atlantic Deep Water (NADW) in this region. The incoming NADW transport of about 20 Sv from the north at 19 degrees S appears to be balanced by only one third of this transport leaving in the south and two thirds leaving to the east or northeast at the Mid-Atlantic Ridge. A simple model is proposed to determine the cause of the NADW branching. It is shown that potential vorticity preservation in the presence of topographic changes leads to a similar flow pattern as observed, with branching near the Vitoria-Trindade-Ridge and also an eastward turning of the southward western boundary current at about 28 degrees S, the latitude where a balance of planetary vorticity change and stretching can be expected.

Description: A spectrum of halogenated hydrocarbon compounds in marine air masses were surveyed over an area in the western Pacific between 43°N, 150°E and 4°N, 113°E in September 1994. The ship's track between northern Japan and Singapore traversed three climatic zones of the northern hemisphere. Recently polluted air, clean marine air derived from the central Pacific Ocean from different latitudes, and marine air from the Indonesian archipelago were collected. Tetrachloroethene and trichloroethene of anthropogenic origin, brominated halocarbons as tribromomethane, dibromochloromethane and bromodichloromethane of anthropogenic and natural sources, and other trace gases were measured in the air samples. Very sparse data on the distribution of these compounds exist for the western Pacific atmosphere. The distribution patterns of the compounds were related to synoptic-scale meteorology, spatial conditions, and origin of the air masses. Anthropogenic and natural sources for both chlorinated and brominated substances were identified. Tetrachloroethene and trichloroethene concentrations and their ratios identify anthropogenic sources. Their mixing ratios were quite low compared to previously published data. They are in agreement with expected low concentrations of photochemically active substances during autumn, with an overall decrease in concentrations toward lower latitudes, and with a decrease of emissions during recent years. Strong evidence for a natural source of trichloroethene was discovered in the tropical region. The concentrations of naturally released brominated species were high compared to other measurements over the Pacific. Gradients toward the coasts and elevated concentrations in air masses influenced by coastal emissions point to significant coastal sources of these compounds. The trace gas composition of anthropogenic and natural compounds clearly identified the air masses which were traversed during the cruise.

Description: The temporal variability of the greater Agulhas Current system has important climatological consequences. Some recent results have suggested that this variability contains a large seasonal component, due to changes in the circulation at latitudes poleward of Madagascar only. A model simulation shows that the contribution of Tropical Surface Water to Agulhas Current waters, via the Mozambique Channel, also has a distinct seasonal characteristic that is brought about by the seasonal wind stress over the tropical Indian Ocean. This simulated flow through the Channel contributes substantially to the seasonality of the Agulhas Current. This model result is shown to be not inconsistent with available hydrographic observations.

Description: Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one-half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low-potential-vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.

Description: A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic δ18O in TS diagrams in conjunction with equilibrium δc fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (〈 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic δ13C by 0.7–1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C starts to decrease 1.5–2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5–3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic δ13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core δ18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.

Description: Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough.

Description: Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one-half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low-potential-vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.

Description: The potential for using sclerosponges, marine organisms that secrete a hard calcerous skeleton, as paleoclimatic indicators has attracted the interest of a number of scientists. Sclerosponges are composed mainly of calcium carbonate and they are very long lived. Variations in their skeletal chemistry contain proxy information regarding their environment and that information has the potential to augment, if not supplant, data from scleractinian corals in interpreting past water temperature, salinity, and productivity over periods of 100s to 1000s of years. Sclerosponges, or calcified demosponges, contain aragonite or calcite and a small amount of siliceous material. Lang et al. [1975] report that these sponges grow within a reef framework, under coral talus in the shallower parts of a reef less than 55 m deep and on steep surfaces of the fore-reef between 55 and 145 m deep. The largest and most conspicuous of the sclerosponges described by those authors is Ceratoporella nicholsoni (Figure 1), which is reported to attain a diameter in excess of 1 m. These sponges are similar in growth habit to many massive vanities of scleractinian corals, the live sponge inhabiting the upper portion of the skeleton, while the lower portion of the skeleton is essentially dead.

Description: Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior

Description: In this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger-scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves.

Description: A weakly nonhydrostatic, two-layer numerical model based on the Boussinesq equations is presented which is capable of describing, among others, the generation and propagation of nonlinear weakly dispersive internal waves in the Strait of Gibraltar. The model depends on one space coordinate only, but it retains several features of a fully three-dimensional model by including a realistic bottom profile, a variable channel width, and a trapezoidal channel cross section. The nonlinear primitive Boussinesq equations include horizontal diffusion, bottom friction, and friction between the two water layers. The model is driven by a height difference of the mean interface depth between the Atlantic and the Mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport. The model presented in this paper describes (1) the mean and tidal flow in the Strait of Gibraltar, (2) the variation of the depth of the interface during a tidal cycle, (3) the generation of strong depressions of the interface at the western sides of the Spartel Sill and the Camarinal Sill, (4) the generation of strong eastward propagating internal bores, and (5) their disintegration into trains of internal solitary waves. The surface convergence patterns associated with depressions of the interface at the Camarinal Sill, internal bores, and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar (SAR) images of the first European Remote Sensing Satellite ERS 1. In total, 155 ERS 1 SAR scenes from 94 satellite overflights over the Strait of Gibraltar, which were acquired in the period from January 1992 to March 1995, have been analyzed. It is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the Strait of Gibraltar as well as the observed east-west asymmetry of the internal wave field.

Description: From August 11 to 22, 1993, a conductivity‐temperature‐depth/acoustic Doppler current profiler survey was carried out in the Somali‐Socotra region to investigate currents and transports associated with the Great Whirl and Socotra Gyre circulation during the height of the summer monsoon. The monsoon circulation was confined to the upper 300 m depth, with intense surface currents up to 2.2 m s−1 in the Great Whirl and up to 1.4 m s−1 in the Socotra Gyre. Deeper‐reaching flow was found in the northwestern part of the Somali Basin and in the passage between the shelf of Somalia and Abd al Kuri. The Great Whirl transport was 58 Sv, of which nearly 25% were due to ageostrophic flow components. The northern part of the Great Whirl thereby appeared as a closed circulation cell in which the offshore transport was balanced by a southward transport of the same magnitude. Upwelled water was advected from the cold wedge of the upwelling regime at the Somali coast along the edge of the gyre. The water in the center of the gyre had the characteristics of Indian Equatorial Water (IEW). The Socotra Gyre carried 23 Sv of modified Arabian Sea Water (ASW). With the transports in the two anticyclonic gyres nearly balanced, the exchange of water masses between the Somali Basin, west of the Carlsberg Ridge, and the Arabian Sea occurred in two areas; about 16 Sv of warm and saline surface water of southern offshore origin entered the northern Somali Basin within a 120‐km‐wide swift current between the Great Whirl and the Socotra Gyre. The other key region for the exchange of water masses was the passage between Somalia and Abd al Kuri. There, the total northward transport was 13 Sv, with contributions of IEW, of upwelled water close to the surface, and ASW underneath.

Description: The meridional oceanic transports of dissolved inorganic carbon and oxygen were calculated using six transoceanic sections occupied in the South Atlantic between 11 degrees S and 30 degrees S. The total dissolved inorganic carbon (TCO2) data were interpolated onto conductivity-temperature-depth data to obtain a high-resolution data set, and Ekman, depth-dependent and depth-independent components of the transport were estimated. Uncertainties in the depth-independent velocity distribution were reduced using an inverse model. The inorganic carbon transport between 11 degrees S and 30 degrees S was southward, decreased slightly toward the south, and was -2150 +/- 200 kmol s(-1) (-0.81 +/- 0.08 Gt C yr(-1)) at 20 degrees S. This estimate includes the contribution of net mass transport required to balance the salt transport through Bering Strait. Anthropogenic CO2 concentrations were estimated for the sections. The meridional transport of anthropogenic CO2 was northward, increased toward the north, and was 430 kmol s(-1) (0.16 Gt C yr(-1)) at 20 degrees S. The calculations imply net southward inorganic carbon transport of 2580 kmol s(-1) (1 Gt C yr(-1)) during preindustrial times. The slight contemporary convergence of inorganic carbon between 10 degrees S and 30 degrees S is balanced by storage of anthropogenic CO2 and a sea-to-air flux implying little local divergence of the organic carbon transport. During the preindustrial era, there was significant regional convergence of both inorganic carbon and oxygen, consistent with a sea-to-air gas flux driven by warming. The northward transport of anthropogenic CO2 carried by the meridional overturning circulation represents an important source for anthropogenic CO2 currently being stored within the North Atlantic Ocean.

Description: The Theoretical and Experimental Tomography in the Sea Experiment (THETIS 1) took place in the Gulf of Lion to observe the evolution of the temperature field and the process of deep convection during the 1991–1992 winter. The temperature measurements consist of moored sensors, conductivity‐temperature‐depth and expendable bathythermograph surveys, and acoustic tomography. Because of this diverse data set and since the field evolves rather fast, the analysis uses a unified framework, based on estimation theory and implementing a Kaiman filter. The resolution and the errors associated with the model are systematically estimated. Temperature is a good tracer of water masses. The time‐evolving three‐dimensional view of the field resulting from the analysis shows the details of the three classical convection phases: preconditioning, vigourous convection, and relaxation. In all phases, there is strong spatial nonuniformity, with mesoscale activity, short timescales, and sporadic evidence of advective events (surface capping, intrusions of Levantine Intermediate Water (LIW)). Deep convection, reaching 1500 m, was observed in late February; by late April the field had not yet returned to its initial conditions (strong deficit of LIW). Comparison with available atmospheric flux data shows that advection acts to delay the occurence of convection and confirms the essential role of buoyancy fluxes. For this winter, the deep mixing results in an injection of anomalously warm water (ΔT≈0.03°) to a depth of 1500 m, compatible with the deep warming previously reported.

Description: Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and are used to assess the magnitude of seasonal variations of methane fluxes from the ocean to the atmosphere in this area. Methane concentrations northeast of Sakhalin were observed to range from 385 nmol L−1 under the ice cover in winter to 6 nmol L−1 in the icefree midsummer season. The magnitude of supersaturations indicates that this part of the Okhotsk Sea is a significant source for atmospheric methane. From the seasonal variation of the supersaturations in the surface waters it is evident that the air-sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse into the atmosphere of the order of 560 mol km−2 d−1 can be expected when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km−2 d−1 which is of the order of the average annual flux in the survey area. The magnitude of the seasonal CH4 flux variation northeast of Sakhalin corresponds to an amount of 7.3 × 105 g km−2 whereby 74% or 5.4 × 105 g km−2 are supplied to the atmosphere between April and July. For the whole Sea of Okhotsk the annual methane flux is roughly 0.13 × 1012 g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long-term data of atmospheric methane which are recorded at the same latitude adjacent to areas with seasonal ice cover show a regional methane pulse between April and July. The large-scale level of atmospheric methane in the northern hemisphere undergoes an amplitudinal variation of about 25 parts per billion by volume (ppbv) which translates into approximately 36 Tg. Thus the estimated 0.6 Tg of ice-induced methane dynamics in northern latitudes can hardly explain this seasonal signal. However, the effects of seasonal ice cover on pulsed release of methane appear strong enough to contribute, in concert with other seasonal sources, to characteristic short-term wobbles in the atmospheric methane budget which are observed between 50°N and 60°N.

Description: We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air-sea exchange process, an annual global sea-to-air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates.

Description: A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement.

Description: Numerical experiments with a medium‐resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large‐scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin‐scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so‐called “warm water path” and “cold water path” in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.

Description: The penetration of anthropogenic or “excess” CO2 into the North Atlantic Ocean was studied along WOCE‐WHP section A2 from 49°N/11°W to 43°N/49°W using hydrographic data obtained during the METEOR cruise 30–2 in October/November 1994. A backcalculation technique based on measurements of temperature, salinity, oxygen, alkalinity, and total dissolved inorganic carbon was applied to identify the excess CO2. Everywhere along the transect surface water contained almost its full component of anthropogenic CO2 ( ∼62 μmol kg−1). Furthermore, anthropogenic CO2 has penetrated through the entire water column in the western basin of the North Atlantic Ocean. Even in the deepest waters (5000 m) of the western basin a mean value of 10.4 μmol kg−1 excess CO2 was calculated. The maximum penetration depth of excess CO2 in the eastern basin of the North Atlantic Ocean was ∼3500 m with values falling below 5 μmol kg−1 in greater depths. These results compare well with distributions of carbontetrachloride. They are also in agreement with the current understanding of the role of the “global ocean conveyor belt” for the uptake of anthropogenic CO2 into the deep ocean.

Description: The response of the Atlantic Ocean to North Atlantic Oscillation (NAO)-like wind forcing was investigated using an ocean-only general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the interannual to multi-decadal frequency response of the ocean to a winter wind anomaly pattern. Overall, the strength of the SST response increased slightly with longer forcing periods. In the subpolar gyre, however, the model showed a broad response maximum in the decadal band (12-16 years).

Description: Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.

Description: We report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the Cleft and Vance segments of the Juan de Fuca Ridge. The data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers. Areas of reduced crustal electrical resistivities, with values approaching that of seawater, are seen beneath the neovolcanic zones of both active spreading centers. We interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot, young oceanic crust.

Description: Vertical gradient electromagnetic sounding (VGS) on the Endeavour segment of Juan de Fuca mid‐ocean ridge reveals the presence of a 2D ridge‐parallel, conductivity anomaly. If the anomaly is caused mainly by melt in a conventional upper mantle upwelling zone alone, then the conductivity of the zone is about 0.6 S/m. The corresponding Archie's law melt fraction exceeds 0.10. A significantly lower melt fraction requires a sheet‐like, well interconnected melt. Upwelling zone conductivity can be reduced by a third if the anomaly is broadened and a crustal conductor is added to the model.

Description: Analysis of multiple climate simulations shows much of the midlatitude Pacific decadal variability to be composed of two simultaneously occurring elements: One is a stochastically driven, passive ocean response to the atmosphere while the other is oscillatory and represents a coupled mode of the ocean‐atmosphere system. ENSO processes are not required to explain the origins of the decadal variability. The stochastic variability is driven by random variations in wind stress and heat flux associated with internal atmospheric variability but amplified by a factor of 2 by interactions with the ocean. We also found a coupled mode of the ocean‐atmosphere system, characterized by a significant power spectral peak near 1 cycle/20 years in the region of the midlatitude North Pacific and Kuroshio Extension. Ocean dynamics appear to play a critical role in this coupled air/sea mode.

Description: The coupling on decadal time scales of the mid‐latitude and tropical Pacific via an oceanic ‘bridge’ in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model. Results indicate that in the tropics decadal anomalies of isopycnal depth are forced by Ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid‐latitudes of either hemisphere. In the coupled model, temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere, but are lag correlated between southern hemisphere mid‐ and low‐latitudes. However, anomaly magnitudes on the equator are small. These results suggest that the oceanic ‘bridge’ to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial Pacific. Coupling to the southern mid‐latitudes via temperature anomalies on isopycnals remains an intriguing possibility.

Description: Analysis of global climate model simulations and observations suggest decadal, midlatitude changes in and over the North Pacific cause decadal modulation of the El Niño‐Southern Oscillation. This coupling between the two geographic regions is via atmospheric, not oceanographic, teleconnections. In essence, large scale changes in the circulation of the atmosphere over the Pacific Basin, while largest in midlatitudes, have a significant projection onto the wind field overlying the equatorial regions. These low frequency wind changes precondition the mean state of the thermocline in the equatorial ocean to produce prolonged periods of enhanced or reduced ENSO activity. The midlatitude variability that drives equatorial impacts is of stochastic origin and, although the magnitude of the signal is enhanced by ocean processes, likely unpredictable.

Description: A hierarchy of El Niño-Southern Oscillation (ENSO) prediction schemes has been developed during the Tropical Ocean-Global Atmosphere (TOGA) program which includes statistical schemes and physical models. The statistical models are, in general, based on linear statistical techniques and can be classified into models which use atmospheric (sea level pressure or surface wind) or oceanic (sea surface temperature or a measure of upper ocean heat content) quantities or a combination of oceanic and atmospheric quantities as predictors. The physical models consist of coupled ocean-atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the “shallow water” type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast in predicting typical indices of ENSO on lead times of 6 to 12 months. The TOGA program can be regarded as a success from this perspective. However, despite the demonstrated predictability, little is known about ENSO predictability limits and the predictability of phenomena outside the tropical Pacific. Furthermore, the predictability of anomalous features known to be associated with ENSO (e.g., Indian monsoon and Sahel rainfall, southern African drought, and off-equatorial sea surface temperature) needs to be addressed in more detail. As well, the relative importance of different physical mechanisms (in the ocean or atmosphere) has yet to be established. A seasonal dependence in predictability is seen in many models, but the processes responsible for it are not fully understood, and its meaning is still a matter of scientific discussion. Likewise, a marked decadal variation in skill is observed, and the reasons for this are still under investigation. Finally, the different prediction models yield similar skills, although they are initialized quite differently. The reasons for these differences are also unclear.

Description: Open‐ocean deep‐water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large‐scale models? If so, is it important that the mixing occurs over a finite time tmix, or would instantaneous mixing produce the same effect on large‐scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends on tmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume‐resolving simulations of open‐ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value for tmix is explored by setting tmix = 12 hours in some experiments, in accordance with scaling considerations, and tmix = 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the water column over a time tmix.

Description: A radiative transfer model to compute brightness temperatures in the microwave frequency range for polar regions including sea ice, open ocean, and atmosphere has been developed and applied to sensitivity studies and retrieval algorithm development. The radiative transfer within sea ice is incorporated according to the “many layer strong fluctuation theory” of Stogryn [1986, 1987] and T. Grenfell [Winebrenner et al., 1992]. The reflectivity of the open water is computed with the three-scale model of Schrader [1995]. Both surface models supply the bistatic scattering coefficients, which define the lower boundary for the atmospheric model. The atmospheric model computes the gaseous absorption by the Liebe et al. [1993] model. Scattering by hydrometeors is determined by Mie or Rayleigh theory. Simulated brightness temperatures have been compared with special sensor microwave imager (SSM/I) observations. The comparison exhibits shortcomings of the ice model for 37 GHz. Applying a simple ad hoc correction at this frequency gives consistent comparison results within the range of observational accuracy. The simulated brightness temperatures show the strong influence of clouds and variations of wind speed over the open ocean, which will affect the sea ice retrieval even for an ice-covered ocean. Simulated brightness temperatures have been used to train a neural network algorithm for the total sea ice concentration, which accounts for these effects. Sea ice concentrations sensed from the SSM/I data using the network and the NASA sea ice algorithm show systematic differences in dependence on cloudiness.

Description: In the North Atlantic we define H-0 as a Heinrich-like event which occurred during the Younger Dryas chron. On the SE Baffin shelf prior to 11 ka, surface water productivity was reasonably high, as measured by the numbers of diatom and planktic foraminifera per gram, but an abrupt increase in detrital carbonate (DC-0 event) (from approximately 15% up to 50% carbonate by weight) occurred at 11 ± 14C ka and continued to circa 10 ka. These deposits, 2–6 m thick, are dominated by detrital calcite and silt- and clay-sized sediments. During this event (DC-0/H-0), ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over Labrador-Ungava. As a consequence, the pattern of ice-rafted debris and sediment provenance shown by H-O in the North Atlantic is different from that during H-1 (14.5 ka) or H-2 (20 ka) when the ice sheet extended along the axis of Hudson Strait and may have reached the shelf break; for example, there is no concrete evidence for DC-O is cores on the floor of the Labrador Sea due east of Hudson Strait (HU75-55,-56), but H-O has been noted in cores off Newfoundland and west of Ireland. A coeval carbonate event to DC-0, but this one dominated by dolomite, occurs in HU82-SU5 on the west side of Davis Strait with a source either from northern Baffin Bay or Cumberland Sound. Although other sources for North Atlantic detrital carbonate cannot be totally excluded, our evidence suggests that H-0 represents the expression of glaciological instability of the Laurentide Ice Sheet within the general region of Hudson Strait and probably to the north (Cumberland Sound and northernmost Baffin Bay). There is one younger DC event, dated circa 8.4 ka, present in sediments along the Labrador margin and in Hudson Strait, which represents the final collapse of the ice sheet within Hudson Strait and Hudson Bay.

Description: The availability of CaCO3 and ³He content results for core TT 13-72 from 4.3 km depth in the equatorial Pacific [Marcantonio et al., 1996] allows the magnitude of the excess (i.e., over ambient) CaCO3 dissolution at the onset of marine glacial stages 10, 8, and 6 to be estimated. These three events are remarkably similar; during each an integrated loss of about 28 g CaCO3 per cm² occurred. While the magnitude of this loss is consistent with that expected from the interglacial to glacial pH shifts reconstructed based on boron isotope measurements on benthic foraminifera [Sanyal et al., 1995], measurements at a number of other locations and water depths will be required before this approach can be used to evaluate the global toll of these dissolution events.

Description: Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (〉8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bölling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell.

Description: Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and are used to assess the magnitude of seasonal variations of methane fluxes from the ocean to the atmosphere in this area. Methane concentrations northeast of Sakhalin were observed to range from 385 nmol L−1 under the ice cover in winter to 6 nmol L−1 in the icefree midsummer season. The magnitude of supersaturations indicates that this part of the Okhotsk Sea is a significant source for atmospheric methane. From the seasonal variation of the supersaturations in the surface waters it is evident that the air-sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse into the atmosphere of the order of 560 mol km−2 d−1 can be expected when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km−2 d−1 which is of the order of the average annual flux in the survey area. The magnitude of the seasonal CH4 flux variation northeast of Sakhalin corresponds to an amount of 7.3 × 105 g km−2 whereby 74% or 5.4 × 105 g km−2 are supplied to the atmosphere between April and July. For the whole Sea of Okhotsk the annual methane flux is roughly 0.13 × 1012 g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long-term data of atmospheric methane which are recorded at the same latitude adjacent to areas with seasonal ice cover show a regional methane pulse between April and July. The large-scale level of atmospheric methane in the northern hemisphere undergoes an amplitudinal variation of about 25 parts per billion by volume (ppbv) which translates into approximately 36 Tg. Thus the estimated 0.6 Tg of ice-induced methane dynamics in northern latitudes can hardly explain this seasonal signal. However, the effects of seasonal ice cover on pulsed release of methane appear strong enough to contribute, in concert with other seasonal sources, to characteristic short-term wobbles in the atmospheric methane budget which are observed between 50°N and 60°N.

Description: Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough.

Description: This study documents the biological signatures impressed upon the sedimentary record underlying both the 5°N upwelling system of the Somali Current and the equatorial area of the Somali Basin out of the upwelling influence. The evolution of these two distinct hydrographic systems is compared for the last 160 kyr. Correspondence and cluster analyses are performed on combined radiolarian and planktonic foraminiferal quantitative data in order to study the changes of the planktonic assemblages through time and space. The Upwelling Radiolarian Index (URI) is used as a productivity proxy. The water temperature and hydrographic structure of the upper water masses appear to be the major factors controlling the distribution patterns of the fauna. The relative abundances of three groups of foraminifera, cold water form (dextral N. pachyderma), mixed layer dwellers (G. trilobus, G. ruber, G. sacculifer, G. conglobatus, and G. glutinata), and thermocline dwellers (G. menardii, G. tumida, N. dutertrei, G. crassaformis, and P. obliquiloculata), follow distinct evolutionary patterns at the two sites during the last 160 kyr. At the equatorial site (core MD 85668), downcore fluctuations in the relative abundances of the three groups are closely related to the glacial/interglacial cyclicity and provide some insights into the interpretation of hydrographic changes. The dominance of the mixed layer foraminifera at the transition intervals between isotope stages 6/5 and 2/1, combined with weak URI values, is thought to reflect the reorganization of the oceanographic circulation. These short-term events (with a duration of 〈 5000 year) could be related to the rapid inflow of oxygen-depleted water through the Indonesian straits as a result of sea level rise during deglaciation. Underneath the 5°N gyre (core MD 85674), the response to global climatic changes is overprinted by the regional effect of the Somalian upwelling, which has been persistent over the last 160 kyr.

Description: Deep convection is important in forming the dense water masses that lie below the ocean's surface and feed the global thermohaline circulation system. But the exact role that deep convection plays in these processes is a subject of much debate. Now, for the first time, a pulse-like temperature signal, produced when water generated by convection drains into a deep boundary current, has apparently been detected in the Mediterranean. This observation provides clues to the mechanisms by which the dense water escapes the convection region and makes its journey. While up to 50% of the newly formed water could incorporated into the deep boundary current this way, no increase in its transport was observed.

Description: During the multidisciplinary ‘NEW92’ cruise of the United States Coast Guard Cutter (USCGC) Polar Sea to the recurrent Northeast Water (NEW) Polynya (77–81°N, 6–17°W; July–August 1992), total dissolved inorganic carbon and total alkalinity in the water column were measured with high precision to determine the quantitative impact of biological processes on the regional air-sea flux of carbon. Biological processes depleted the total inorganic carbon of summer surface waters by up to 2 mol C m−2 or about 3%. On a regional basis this depletion correlated with depth-integrated values of chlorophyll a, particulate organic carbon, and the inorganic nitrogen deficit. Replacement of this carbon through exchange with the atmosphere was stalled owing to the low wind speeds during the month of the cruise, although model calculations indicate that the depletion could be replenished by a few weeks of strong winds before ice forms in the autumn. These measurements and observations allowed formulation of a new hypothesis whereby seasonally ice-covered regions like the NEW Polynya promote a unique biologically and physically mediated “rectification” of the typical (ice free, low latitude) seasonal cycle of air-sea CO2 flux. The resulting carbon sink is consistent with other productivity estimates and represents an export of biologically cycled carbon either to local sediments or offshore. If this scenario is representative of seasonally ice-covered Arctic shelves, then the rectification process could provide a small, negative feedback to excess atmospheric CO2.

Description: The advection of sea ice and associated freshwater/salt fluxes in the Weddell Sea in 1986 and 1987 are investigated with a large‐scale dynamic‐thermodynamic sea ice model. The model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the Weddell Sea ice. The skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30‐day sea ice drift. A large number of sensitivity studies show how simulated sea ice transports and associated freshwater/salt fluxes respond to variations in physical parameterizations. The model reproduces the observed ice drift well, provided ice dynamics parameters are set to appropriate values. Optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters. The optimized model yields a mean northward sea ice volume export out of the southern Weddell Sea of 1693 km3 in 1986 and 2339 km3 in 1987. This shows the important role of sea ice transport for the freshwater budget of the Weddell Sea and gives an indication of its high interannual variability.

Description: Submersible investigations employing heat flow measurements (12 stations), sampling and imagery of the two relict high-temperature hydrothermal zones of the TAG field, the Alvin and Mir sulfide zones, elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies. Values of heat flow in the Mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers. This observation supports the hypothesis that intrusions at volcanic centers adjacent to the high-temperature hydrothermal zones supply the heat to drive hydrothermal activity. The chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field-wide high-temperature hydrothermal events recurring at intervals of tens of thousands of years, while activity at individual zones may recur at intervals of hundreds to thousands of years. A sequence of hydrothermal deposits ranges to at least 140,000 years ago from massive sulfides forming at the active mound, to recrystallization of sulfides in the active and relict zones, to pyritization of an inactive mound in the Alvin zone; low-temperature mineral phases precipitate before, during and after the sulfides.

Description: The occurrence of CF4 and SF6 in natural fluorites (CaF2) is reported. Contents are found to be typically 200–2000 ppt-mass for CF4 and 50–100 ppt-mass for SF6. CF4 and SF6 were also detected in granites where fluorite is an accessory mineral. CF4 and SF6 thus possess a large reservoir in the continental crust that by degassing can sustain natural atmospheric background levels of 40 ppt-mole and up to 0.01 ppt-mole, respectively. The article also discusses how natural SF6 may cause interference in hydrological tracer studies.

Description: We compare estimates of the anthropogenic CO2 content of seawater samples from the subpolar North Atlantic Ocean calculated on the basis of a back-calculation technique with measurements of the chlorofluorocarbon CFC-11. Estimated anthropogenic CO2 concentrations are in the range 10–80 µmol kg-1, while CFC-11 concentrations cover the full range from below detection limit to 〉 5 pmol kg-1 in waters at atmospheric equilibrium. The majority of the data points show a linear correlation between anthropogenic CO2 concentrations and CFC-11 saturation, which can only be explained by the strongly advective nature of the North Atlantic Ocean. Only deep eastern basin samples deviate from this general observation in that they show still significant concentrations of anthropogenic CO2 where CFC-11 is no longer detectable. In order to remove the influence of the Revelle factor reflected in the anthropogenic CO2 concentrations we have calculated 'excess' pCO2, showing an even tighter linear correlation with atmospheric equilibrium concentrations of CFC-11.

Description: The standard geometric optics (GO) technique predicts that the phase function for large nonspherical particles with parallel plane facets (e.g., hexagonal ice crystals) should have an infinitesimally narrow δ-function transmission peak caused by rays twice transmitted (refracted) in exactly the forward scattering direction. However, exact T-matrix computations and physical considerations based on the Kirchhoff approximation suggest that this peak is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. This convolution can be performed with a simple procedure which supplements the standard ray-tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.

Description: Current measurements from three moored arrays on the Brazilian continental slope between 20 degrees S and 28 degrees S are investigated for the existence and strength of western boundary currents from near the surface down to the North Atlantic Deep Water. The Brazil Current is found to deepen southward from 100 m to more than 670 m and to strengthen its volume transport to 16.2 x 10(6) m(3)/s. Antarctic Intermediate Water is transported in a well-developed boundary current southward at 28 degrees S and northward north of Cabo Frio (24 degrees S). This result supports earlier suggestions derived from the analysis of hydrographic data that Antarctic Intermediate Water enters the Brazil Basin from the east and bifurcates as it meets the continental break off Brazil. North Atlantic Deep Water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis.

Description: This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified ΔC* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6±2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1±1 Pg C for the same area. Approximately 6.7±1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ±3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61–0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.

Description: During May - August, 1997, the distributions of dissolved methane and CCl3F (CFC11) were measured in the Atlantic between 50° and 60°N. In surface waters throughout the region, methane was observed to be close to equilibrium with the atmospheric mixing ratio, implying that surface ocean methane is tracking its atmospheric history in regions of North Atlantic Deep Water formation. Despite the different atmospheric history and ocean chemistry of CH4 and CFC11, their spatial distribution patterns in the water column are remarkably similar. One-dimensional distributions have been simulated with an advection-diffusion model forced by the atmospheric histories. The results suggest that the similar patterns result from the increasing input of CH4 and CFC11 to newly formed deep waters over time, combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale.

Description: Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over‐steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane‐ or sulfide‐dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5±0.7 L m−2 d−1 for tectonics‐induced water flow [Wallmann et al., 1997b]. A geophysical estimate of dewatering from the same area [von Huene et al., 1997] based on sediment porosity reduction shows a fluid loss of 0.02 L m−2 d−1 for a 5.5 km wide converged segment near the deformation front. Our video‐guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m−2 d−1 is estimated from the biogeochemical approach. The two estimates for tectonics‐induced water flow from the accretionary prism are in surprisingly good agreement.

Description: A research cruise has documented changes in rift tectonics, volcanism, and hydrothermalism along the least studied and most enigmatic sector of a crustal complex in the southwest Pacific Ocean. Results from the longitudinal transect are expected to provide insight into processes involving the Kermadec arc-Havre backarc (KAHB) system, a continuum from oceanic spreading to continental rifting at a convergent plate boundary KAHB forms the central sector of an active, 2000-km arc-backarc complex between Tonga and New Zealand (Figure 1). The expedition also engaged in the first comprehensive survey of submarine vents in the Taupo Volcanic Zone (TVZ) at the south end of the KAHB system. Identified in the off-shore segment of TVZ were three major hydrothermal vent areas associated with late Quaternary fault structures. Data from the expedition and from other recent research in the same area addressed questions concerning the type of hydrothermal venting, magmatic heterogeneity along and across KAHB, the style of backarc rifting, and tectonic and magmatic consequences of anomalous terranes colliding with the subduction margin.

Description: The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26°N, Mid-Atlantic Ridge, is −200 m in diameter, exhibits 50 m of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flow and seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363°C fluid from a chimney cluster on the top of the mound and deposition of a high ƒS2-ƒO2 mineral assemblage that reflects low concentrations of H2S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, Sb, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on-going remobilization, zone refinement, and significant modification of high-temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus-type ore bodies.

Description: Global analyses of monthly sea surface temperature (SST) anomalies from 1856 to 1991 are produced using three statistically based methods: optimal smoothing (OS), the Kaiman filter (KF) and optimal interpolation (OI). Each of these is accompanied by estimates of the error covariance of the analyzed fields. The spatial covariance function these methods require is estimated from the available data; the timemarching model is a first‐order autoregressive model again estimated from data. The data input for the analyses are monthly anomalies from the United Kingdom Meteorological Office historical sea surface temperature data set (MOHSST5) [Parker et al., 1994] of the Global Ocean Surface Temperature Atlas (GOSTA) [Bottomley et al., 1990]. These analyses are compared with each other, with GOSTA, and with an analysis generated by projection (P) onto a set of empirical orthogonal functions (as in Smith et al. [1996]). In theory, the quality of the analyses should rank in the order OS, KF, OI, P, and GOSTA. It is found that the first four give comparable results in the data‐rich periods (1951–1991), but at times when data is sparse the first three differ significantly from P and GOSTA. At these times the latter two often have extreme and fluctuating values, prima facie evidence of error. The statistical schemes are also verified against data not used in any of the analyses (proxy records derived from corals and air temperature records from coastal and island stations). We also present evidence that the analysis error estimates are indeed indicative of the quality of the products. At most times the OS and KF products are close to the OI product, but at times of especially poor coverage their use of information from other times is advantageous. The methods appear to reconstruct the major features of the global SST field from very sparse data. Comparison with other indications of the El Niño‐Southern Oscillation cycle show that the analyses provide usable information on interannual variability as far back as the 1860s.

Description: Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43‐year (1950–1992) record of SST [Smith et al, 1996] and other data derived from the Comprehensive Ocean‐Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre [1992]. Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is correlated with Pacific El Niño‐Southern Oscillation (ENSO) variability in several regions. The major region affected is the North Atlantic area of NE trades west of 40°W along 10°N–20°N and extending into the Caribbean. There, about 50–80% of the anomalous SST variability is associated with the Pacific ENSO, with Atlantic warmings occurring 4–5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO‐related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in question, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North Atlantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the development of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15°–30°S.

Description: The seasonal evolution of anomalous interhemispheric sea surface temperature (SST) gradients in the tropical Atlantic from January to April is studied by composites of the 10 warmest (warm) and 10 coldest (cold) Januaries during 1948–1993 in the equatorial Pacific using Comprehensive Ocean‐Atmosphere Data Set ship observations. In the warm as compared with the cold years, an anomalous weakening of the northward SST gradient develops, mainly due to anomalous warming in the tropical North Atlantic. This stems from the combination of three forcings all related to the weakened North Atlantic trade winds during Pacific warm events. Most important are the reduced latent heat flux in much of the tropical North Atlantic and anomalous downwelling equatorward of 20°N, with a further contribution from increased net radiation resulting from the reduced cloudiness due to the diminished convergence in the downstream portion of the North Atlantic trades. In response to the development of warm anomalies in the tropical North Atlantic during January–March, the cross‐equatorial northward winds accelerate to April, and this leads south of the equator not only to Ekman downwelling, enhanced divergence, reduced cloudiness, and increased net radiation, but also to enhanced wind speed and evaporation. The result is a modest anomalous warming in the western tropical South Atlantic. Increased air temperature over the entire basin, presumably due to atmospheric advection from the Pacific, has little effect on the Atlantic SST pattern. The anomalous interhemispheric SST gradient, controlled primarily by the warm anomaly in the North Atlantic, has a pivotal role to play in steering the late boreal winter atmospheric circulation in the tropical Atlantic sector and thus regional climate anomalies.

Description: Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.

Description: Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87Sr/86Sr (0.7025–0.7029) and 206Pb/204Pb (18.80–19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030–0.7035), low 206Pb/204Pb (17.99–18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with an Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma.