ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: The biosynthesis of Pep5, a lanthionine-containing antimicrobial peptide, is directed by the 20-kbp plasmid pED503. We identified a 7.9-kbp DNA-fragment within this plasmid which covers the information for Pep5 synthesis in the homologous host Staphylococcus epidermidis 5 which has been cured of pED503. This fragment contained, in addition to the previously described structural gene pepA and the immunity gene pepl [Reis, M., Eschbach-Bludau, M., Iglesias-Wind, M. I., Kupke, T. & Sahl, H.-G. (1994) Appl. Env. Microbiol. 60, 2876–2883], a gene pepT coding for a translocator of the ABC transporter family, a gene pepP coding for a serine protease and two genes pepB and pepC coding for putative modification enzymes; the gene arrangement is pepTIAPBC. We analyzed the biosynthetic genes with respect to their function in Pep5 biosynthesis. Deletion of PepT reduced Pep5 production to about 10%, indicating that it can be partially replaced by other host-encoded translocators. Inactivation of PepP by site-directed mutagenesis of the active-site His residue resulted in production of incorrectly processed Pep5 fragments with strongly reduced antimicrobial activity. Deletion of pepB and pepC leads to accumulation of Pep5 prepeptide in the cells without excretion of processed peptide. A pepC-deletion clone did not excrete correctly matured Pep5 but it did produce fragments from which serine and threonine were absent. Only one of these fragments contained a single lanthionine residue out of three expected while the remaining, unmodified cysteine residues could be detected by reaction with Ellman's reagent. These results demonstrate that PepC is a thioether-forming protein and strongly suggest that PepB is responsible for dehydration of serine and threonine

Beschreibung: 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.

Beschreibung: We have prevously shown that a marine chlorophyte expressed flavodoxin under iron limitation but not under other nutrient stress conditions. Here we use polyclonal antiserum raised against the diatom Phaeodactylum tricornutum Bohlin to show that a similar response is observed in this species. Using our antibody, western blotting techniques, and standard colorimetric detection (4‐chloro‐1‐naphthol), we can detect at least a 25–50‐fold increase in flavodoxin in iron‐depleted compared to iron‐replete cells. In iron‐limited batch cultures of P. tricornulum, flavodoxin accumulation was inversely proportional to growth rate and was not detectable in cultures containing initially more than 750 nm of iron. We demonstrated that the accumulation of flavodoxin under iron stress is widespread among marine diatoms and that it may be possible to use the presence or absence of flavodoxin in natural marine diatom assemblages to detect iron limitation. However, our polyclonal antisera appears to be specific for diatoms and did not cross‐react with Synechococcus sp., Micromonas pusilla (Butcher) Manton et Parke, Dunaliella tertiolecta Butcher, Chlorella sp., Emiliania huxleii (Lohm.) Hay et Parke, or Isochrysis galbana Parke. A reverse bioassay experiment was conducted with natural phytoplankton assemblages containing mainly diatoms from Long Island Sound and in shelf waters near Cape Hatteras, two areas not suspected to be iron‐limited. Although flavodoxin was not detected in situ in these areas, natural populations of diatoms driven into iron limitation expres.sed flavodoxin. Flavodoxin was detected in mats of the diatom Rhizosolenia castracanei Cleve collected from the Equatorial Pacific during a JGOFS cruise in 1992, consistent with the hypothesis that iron may be limiting in this high‐nutrient, low‐chlorophyll region.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Seismic data are usually acquired and processed for imaging reflections. This paper describes a method of processing seismic data for imaging discontinuities (e.g., faults and stratigraphic features). One application of this nontraditional process is a 3-D volume, or cube, of coherence coefficients within which faults are revealed as numerically separated surfaces. Figure 1 compares a traditional 3-D reflection amplitude time slice with the results of the new method. To our knowledge, this is the first published method of revealing fault surfaces within a 3-D volume for which no fault reflections have been recorded.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: This paper presents quantitative analyses at the community level of the spatial dispersion of 44 species of pelagic birds in the South Atlantic Ocean. I have examined the extent to which assemblages of pelagic bird species are influenced by the combined influences of the abundances of other species and the physical structure of the pelagic environment. Results are based upon strip transects collected from oceanographic research vessels during the four consecutive austral summers of 1982–1986. The distributions of most seabird species were statistically independent of the distributions of other species. Species assemblages, however, did occur and were correlated with the physical structure of the ocean. The study area was divided into four zones, each defined by a distinctive assemblage of birds. Physical oceanography proved to be important in determining the species composition of each zone. These relationships were apparent despite the potentially confounding influence of large numbers of immatures and non‐breeding adults.

Beschreibung: 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.

Beschreibung: The diet of the white‐chinned petrel at Bird Island, South Georgia was studied during chickrearing in 1986 by quantitative analysis (by weight, frequency of occurrence and number of individuals) of regurgitated or lavaged adult stomach contents. Antarctic krill comprised over 90% of prey items and formed 47% of the diet by weight; fish and squid occurred in 67% and 35% of samples and formed 33% and 19% of the diet by weight, respectively. Decapods, amphipods and salps occurred in a few samples. The fish were mainly lanternfish (Myctophidae) of eight species (chiefly Electrona and Gymnoscopelus, forming 80% by number and 52% by mass of fish prey) and the nototheniid Patagonotothen guntheri (14% by number and 35% by mass). Of squid taken, the ommastrephid Martialia hyadesi comprised 57% by number and 52% by mass and the gonatid Gonatus antarcticus 14% by number and 42% by mass. These dietary data confirm white‐chinned petrel as the most important avian consumer of fish and squid at South Georgia (and the third most important consumer of krill). In 1986 the nototheniid fish were probably obtained via commercial fishing operations but the myctophids and squid were probably live‐caught, most likely at night. Meal size increased rapidly until chicks were three weeks old and then remained constant until the chicks were within 10 days of fledging, when it decreased. Meal delivery rate was high (one per day) for young chicks (1–10 days old) and thereafter fluctuated between 0.56 and 0.88 meals per day until close to fledging, when it was halved. These provisioning rates, and the proportion of krill in the diet, are higher than those recorded previously at South Georgia and Indian Ocean sites, probably reflecting high local availability of krill at South Georgia in 1986. In many respects, white‐chinned petrels at South Georgia are intermediate ecologically between prions and albatrosses, although specialized in their extensive consumption of myctophids. Because krill and all the main fish prey are currently the targets of substantial commercial fishing and the main squid prey (Martialia) is a potential target, the ecological role and status of white‐chinned petrels are of additional importance.

Beschreibung: Oxides of Fe and Mn in soils are capable of sorbing large amounts of trace metal ions and can therefore be important in controlling trace metal concentrations in soil solution, and hence trace metal bioavailability in soils. There is, however, relatively little information on the rates of desorption of trace metals from oxide materials or on the factors affecting desorption rates. The objective of this study was to examine the kinetics of desorption of Cd and Co from two Fe oxides, goethite and ferrihydrite, and from two Mn oxides, hausmannite and cryptomelane. The concentrations of Cd and Co specifically sorbed by the oxides at pH 6.0 were greater for the Mn oxides than for the Fe oxides. The metals were also much less readily desorbed from the Mn than the Fe oxides and, in general, Cd was more readily desorbed than Co from all four oxides. Increasing the initial sorption period from 1 to 15 wk substantially decreased the proportion of sorbed Cd or Co subsequently desorbed from goethite, with a similar but much smaller effect also observed with the Mn oxides. Desorption kinetics for both Cd and Co were found to be described well by assuming either the occurrence of two simultaneous first-order desorption reactions, or by a continuous distribution of reaction sites, distributed lognormally with respect to desorption first-order rate constant. With increasing initial sorption period, the parameters obtained from fitting either type of kinetic equation to the experimental data could be interpreted as indicating a movement of metal ions to sites with slower desorption reactions.

Beschreibung: An analysis of a large drifting buoy data set is presented. The objective is to obtain a self‐contained description of the properties of the near‐surface circulation (drogue depth 100 m) in the central North Atlantic Ocean, independent of hydrographic data. A necessary preanalysis step was the removal of all data from undrogued buoys from the data set. The physical parameters of the circulation were deduced by averaging the remaining data in 2°×3° boxes. The minimum amount of data which is necessary to get statistically stable results was determined by an empirical quality criterion. All important mean currents in the investigation area are reproduced by the near‐surface mean velocity field. A separation of the mean velocity field into a nondivergent and an irrational part shows that the flow field is almost nondivergent. The distribution of eddy kinetic energy is concentrated along the mean currents and provides the largest part of the total kinetic energy, but there are regional variations. Energy from inertial movements dominates the high‐frequency part of the kinetic energy. Its distribution is very patchy. The analysis of the Reynolds stress terms shows an energy transfer from the eddy field to the mean circulation in the vicinity of the North Atlantic Current.

Beschreibung: The role of sea ice in preconditioning the mixed layers of the central Greenland Sea for deep convection is investigated, with particular emphasis on the formation of the “Nordbukta.” The opening of the ice free bay in late January 1989 indicated that the upper layer was well preconditioned for deep convection which reached down to 1500 m depth in March 1989. We propose that the ice free bay occurred due to diminishing new ice formation without extensive ice melt. A key process is wind‐driven ice drift to the southwest, as observed by upward looking acoustic Doppler current profilers, which will alter the upper ocean freshwater budget when an ice volume gradient along the ice‐drift direction exists. We investigated the importance and effects of such an ice‐drift‐induced freshwater loss on upper ocean properties using an ice‐ocean mixed‐layer model. Observed temperature and salt profiles from December 1988 served as initial conditions, and the model was integrated over the winter season. Given the one‐dimensional physics and climatological surface fluxes, the model was not able to produce a reasonable ice and mixed‐layer evolution. However, allowing ice drift to reduce the local ice thickness improved the ice‐ocean model performance dramatically. An average ice export of 5–8 mm d−1 was needed to be consistent with the observed evolution of mixed‐layer properties and ice cover. Using the same fluxes and ice export, but initial conditions from the “Is Odden” region, yielded ice cover throughout the winter over a shallow mixed layer, both of which are consistent with the observations from the Odden region.

Beschreibung: Nutrient and oxygen distributions were measured during a hydrographic survey of the Northeast Water Polynya off the northeast coast of Greenland (77–81°N, 6–17°W) during July–August 1992 and were interpreted in the context of satellite imagery of the region. Satellite imagery revealed a convoluted plume of cold water flowing along isobaths from underneath fast ice in the southwestern portion of the polynya toward the northeast. This plume carried relatively high nutrient and low oxygen inventories. Nitrate to phosphate ratios were low in the polar water, consistent with an ultimate source of this water mass in the Pacific sector of the Arctic Ocean. It is hypothesized that the low N:P Arctic outflow might be the cause of nitrate limitation along the east coast of America as far as Cape Hatteras. Gradients of both nutrients and oxygen inventories in the euphotic zone were observed along and across the axis of mean flow within the polynya and are shown to be due to net production of organic matter. On the basis of these spatial gradients of nitrate and oxygen, an assumed along-axis current velocity of 10 cm s−1, and the observed relationships of biologically removed inorganic carbon with nitrate and oxygen, the net organic matter production was estimated to be 40–60 mmol(C) m−2 d−1. This represents the organic carbon available for export from the polynya euphotic zone. Nutrient-deficient and oxygen-rich waters were observed merging with the southward flowing East Greenland Current, suggestive of possible export, however, the ultimate fate of organic carbon produced within the polynya requires further study.

Beschreibung: Three lakes of concentrated brine were discovered in the seabed of the Mediterranean Ridge, southwest of Crete in late 1993 and early 1994. These lakes have a distinctive appearance on side-scan sonar images with no backscattering from the lake surface and enhanced backscattering at their edges (Figure 1). Measurements with conductivity-temperature-depth (CTD) and heat flow probe show in one case a near-isothermal temperature for the lake water and in another thermal layering. The chemistry of the brines varies in each lake and shows that the brines are derived from selective solution of different parts of the Messinian evaporites that lie beneath the seafloor. Flow of seawater through the sediments containing the Messinian evaporites appears to be required to form the lakes.

Beschreibung: A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post‐Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present‐day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond‐hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction‐related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post‐Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.