Geologic-hydrologic setting and surface sedimentology in the Persian Gulf

1. Morphology and sedimentation The deepest parts of the Persian Gulf lie off the Iranian coast. Several swells separate the Persian Gulf into the Western Basin, the Central Basin and the Strait of Hormuz, which leads without noticeable morphological interruption onto the Biaban Shelf; the latter gradually drops off towards the continental slope, which itself has a strongly subdivided morphology. The sediment distribution in the Western Basin runs parallel to the basin's axis to a depth of 50 -60 m. This is caused by the shallow and uniform slope of the Iranian coast into the Western Basin, by clear exposure of the area to the Shamal-Winds and by tidal currents parallel to the basin's axis. Most other parameters also show isolines parallel to the coast line. Data from the sediment analyses show a net transport which extends out along the Central Swell: coarse fraction > 63 µ, total carbonate content, carbonate in fine fractions < 2 µ, 2-6 µ and 20-63 µ, calcite-aragonite ratios in the fine fractions 2-6 µ and 20-63 µ and quartz-dolomite ratios in fine fraction 2-6 µ. At least the uppermost 10-40 m of this sediment is late Holocene. This implies sedimentation rates of several meters per 1000 years. The slope from the Iranian coast into the Central Basin (max. depth 100 m) is generally steeper, with interspersed islands and flats. Both facts tend to disturb a sediment dustribition parallel to the basin's axis over extensive areas and may preclude any such trend from being detected by the methods and sample net used. The spatial distribution of the coarse fraction, however, seems to indicate sediment transport at greater water depths perpendicular to the basin's long axis and along the steepest gradients well into the Central Basin. The flats of the Central Basin have a sediment cover distinctly different from those of the deeper basin areas. Characteristic parameters are the extremely high percentages of coarse grained sediments, total content of carbonate CO2 over 40, low total organic carbon content, (however values are high if calculated on the basis of the < 63 µ fraction), low total N-content, and low C/N ratios. These characteristics probably result from the absence of any terrigenous material being brought in as well as from exposure to wave action. Finest terrigenous material is deposited in the innermost protected part of the Hormuz Bay. In the deep channel cut into the Biaban Shelf which carries the Persian Gulf out-flow water to the Indian Ocean, no fine grained sediment is deposited as shown by grain size data. 2. Geographic settings and sedimentation Flat lands border the Arabian coast of the Persian Gulf except for the Oman region. The high and steep Zagros Mountains form the Iranian coastline. Flat topography in combination with generally low precipitation precludes fluviatile sediment being added from the South. Inorganic and biogenic carbonates accumulating under low sedimentation rates are dominant on the shallow Arabic Shelf and the slopes into the Western and Central Basins. The fluviatile sediment brought in from the Iranian side, however decisively determine the composition of the Holocene sediment cover in the Persian Gulf and on the Biaban Shelf. Holocene sediments extend 20-30 km seaward into the Western Basin and about 25 km on to the Biaban Shelf. As mentioned before, sedimentation rates are of several meters/1000 years. The rocks exposed in the hinterland influence the sediments. According to our data the Redbeds of the Zagros Mountains determine the colour of the very fine grained sediments near the Iranian Coast of the Persian Gulf. To the West of Hormuz, addition of carbonate minerals is particularly high. Dolomite and protodolomite, deposited only in this area, as well as palygorskite, have proven to be excellent trace minerals. To the East of Hormuz, the supply of terrigenous carbonates is considerably lower. Clay minerals appear to bring in inorganically bound nitrogen thus lowering the C/N ratio in these sediments especially off river mouths. 3. Climate and sedimentation The Persian Gulf is located in a climatically arid region. This directly affects sedimentation through increased wind action and the infrequent but heavy rainfalls which cause flash floods. Such flash floods could be responsible for transporting sedheats into the Central Basin in a direction perpendicular to the Gulf's axis. Eolian influx is difficult to asses from our data; however, it probably is of minor importance from the Iranian side and may add, at the most, a few centimeters of fine sediment per 1000 years. 4. Hydrology and sedimentation High water temperatures favor inorganic carbonate precipitation in southern margin of the Gulf, and probably on the flats, as well as biogenic carbonate production in general. High evaporation plus low water inflow through rivers and precipitation cause a circulation pattern that is typical for epicontinental seas within the arid climate region. Surface water flows in from the adjoining ocean, in this case the Indian Ocean and sinks to the bottom of the Persian Gulf mainly in the northern part of the Western Basin, on the "Mesopotamischer Flachschelf" ard probably in the area of the "Arabischer Flachschelf". This sinking water continually rejuvenates the bottom out-flow water. The inflowing surface water from the Indian Ocean brings organic matter into the Persian Gulf, additional nutrients are added by the "fresh" upwelling waters of the Gulf of Oman. Both nutrients and organic matter diminish very rapidly as the water moves into the Persian Gulf. This depletion of nutrients and organic matter is the reasonfor generally low organic carbon contents of the Persian Gulf sediments. The Central Swell represents a distinct boundary, to the west of which the organic carbon content are lower than to the east when sediment samples of similar grain size distribution are compared. The outflow carries well oxygenated water over the bottom of the Persian Gulf and the resulting oxidation further decreases the content of organic matter. In the Masandam-Channel and in the Biaban-Shelf channel, the outflowing water prevents deposition of fine material and transports sediment particles well beyond the shelf margin. The outflowing water remains at a depth of 200-300 m depending on its density and releases ist suspending sediment load to the ocean floor, irrespectative of the bottom morphology. This is reflected in several parameters in which the sediments from beneath the outflow differ from nearby sediments not affected by the outflowing water. High carbonate content of total samples and of the individual size fraction as well as high aragonite and dolomite contents of individual size fractions characterize the sediment beneath the outflowing water. The tidal currents, which avt more or less parallel to the Gulf's axis, favor mixing of the water masses, they rework sediments at velocities reported here. This fact enlarges to a certain degree the extent of our interfaces which are based on only a few sample points (Persian Gulf and Biaban Shelf one sample per 620 km**2, continental slope one sample per 1000 km**2). The water on the continental slope shows and oxygen minimum at 200-1200 m which favors preservation of organically-bound carbon in the sediment. The low pH-values may even permit dissolution of carbonate minerals.

Organic geochemistry of Cretaceous black shales

Organic-carbon-rich 'black shales' and adjacent organic-carbon-poor rocks from three different Cretaceous settings encountered during ODP Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich marlstone turbidites, large amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian/Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deep-water setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deep-water anoxia may have been involved.

Sediment descriptions, stage, calcium carbonate, organic carbon, delta 13C and geolipids at DSDP Hole 93-603B

Organic matter contents of black shales from the Cretaceous Hatteras and Blake-Bahama formations have been compared to those from surrounding organic-poor strata using C/N ratios, d13C values, and distributions of extractable and nonsolvent-extractable, long-chain hydrocarbons, acids, and alcohols. The proportion of marine and land-derived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbon-poor relative to their organic-carbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this outer continental rise location under generally oxygenated bottom-water conditions.

General descriptions, organic contents and geolipid ratios at DSDP Leg 72 Holes

Organic matter in sediments from Sites 515, 516, and 517 reflects a history of low marine productivity and of oxygenated bottom waters in the western South Atlantic since the Pliocene. Organic carbon contents are low, averaging 0.26% of sediment weight. Distributions of n-alkanes, n-alkanols, and n-alkanoic acids show evidence of microbial reworking, and n-alkanes contain important terrigenous contributions, presumably of eolian origin.

Planktic foraminiferal distribution and paleotemperature reconstruction for the last 560 kyr of ODP Hole 165-999A from the Carribean Sea

Faunal analyses of planktonic foraminifera and upper-water temperature reconstructions with the modern analog technique are studied and compared to themagnetic susceptibility and gamma ray logs of ODP Core 999A (western Caribbean) for the past 560 kyr in order to explore changes in paleoceanographic conditions in the western Caribbean Sea. Long-term trends in the percentage abundance of planktonic foraminifera inODP Core 999Asuggest two hydrographic scenarios: before and after 480 ka.High percentage abundances of Neogloboquadrina pachyderma and Globorotalia inflata, low abundances of Globorotalia menardii and Globorotalia truncatulinoides, low diversity, and sea-surface temperatures (SST) under 24 °C are typical characteristics occurring from 480 to 560 ka. These characteristics suggest a "shallow" well-oxygenated upper thermocline and the influx of nutrients by either seasonal upwelling plumes and/or eddy-mediated entrainment. The second scenario occurred after 480 ka, and it is characterized by high and fluctuating percentage abundances of Neogloboquadrina dutertrei, G. truncatulinoides, G. menardii, Globigerinita glutinata, Globigerinella siphonifera, and Globigerinoides ruber; a declining trend in diversity; and large SSTs. These characteristics suggest a steady change from conditions characterized by a "shallow" thermocline and chlorophyll maximum to conditions characterized by a "deep" thermocline (mainly during glacial stages) and by more oligotrophic conditions. The influence of the subtropical North Atlantic on the upper thermocline was apparently larger during glacial stages, thus favoring a deepening of the thermocline, an increase in sea-surface salinity, and a dramatic reduction of nutrients in the Guajira upwelling system. During interglacial stages, the influx of nutrients from the Magdalena River is stronger, thus resulting in a deep chlorophyll maximumand a fresher upper ocean. The eddy entrainment of nutrients is the probable mechanism responsible of transport from the Guajira upwelling and Magdalena River plumes into ODP 999A site.

Geochemistry and diatom abundances in sediment trap samples of Effingham Inlet, Canada

Sediment traps were deployed inside the anoxic inner basin of Effingham Inlet and at the oxygenated mouth of the inlet from May 1999 to September 2000 in a pilot study to determine the annual depositional cycle and impact of the 1999-2000 La Niña event within a western Canadian inlet facing the open Pacific Ocean. Total mass flux, geochemical parameters (carbon, nitrogen, opal, major and minor element contents, and stable isotope ratios) and diatom assemblages were determined and compared with meteorological and oceanographic data. Deposition was seasonal, with coarser grained terrestrial components and benthic diatoms settling in the autumn and winter, coincident with the rainy season. Marine sedimentary components and abundant pelagic diatoms were coincident with coastal upwelling in the spring and summer. Despite the seasonal differences in deposition, the typical temperate-zone Thalassiosira-Skeletonema-Chaetoceros bloom succession was muted. A July 1999 total mass flux peak and an increase in biogenous components coincided with a rare bottom-water oxygen renewal event in the inlet. Likewise, there were cooler-than-average sea surface temperatures (SSTs) just outside the inlet, and unusually high abundances of a previously undescribed cool-water marine diatom (Fragilariopsis pacifica sp. nov.) within the inlet. Each of these occurrences likely reflects a response to the strong La Niña that followed the year after the strongest-ever recorded El Niño event of 1997-1998. By the autumn of 1999, SSTs had returned to average, and F. pacifica had all but disappeared from the remaining trap record, indicating that oceanographic conditions had returned to normal. Oxygenation events were not witnessed in the inlet in the years before or after 1999, suggesting that a rare oceanographic and climatic event was captured by this sediment trap time series. The data from this record can therefore be used as a benchmark for identifying anomalous environmental conditions on this coast.

Quaternary through early Eocene benthic foraminifera off Peru

Stratigraphic assemblages of Quaternary through early Eocene benthic foraminifers were recovered among 10 Peru margin drill sites. Various hiatuses and intervals barren in foraminifers characterize the sections, but numerous samples contain abundant, well-preserved benthic foraminifers. Bathymetry of the extant species and California-based estimates of the paleobathymetry of the extinct species permit recognition of Quaternary sea-level fluctuations between shelf and upper bathyal depths that produced vertical migrations of oxygenated and low-oxygen habitats at the six shallow sites. Assemblages from lower-slope sites at about 9° and 11°S indicate a general subsidence of the continental margin from shelf or upper bathyal depths in Eocene time to the present lower bathyal depths. Data from 11°S suggest a major part of this subsidence occurred in late Oligocene to early Miocene time. Downslope-transported shelf specimens, particularly the small biserial species, Bolivina costata and B. vaughani, are major contributors to these lower bathyal assemblages from the middle Miocene through Quaternary time.

Biogeochemical investigation of cold seep sediments in the Eastern Mediterranean Sea

Cold seep ecosystems are highly productive, fragmented ecosystems of the deep-sea floor. They form worldwide where methane reaches the surface seafloor, and are characterized by rich chemosynthetic communities fueled by the microbial utilization of hydrocarbons. Here we investigated with in situ (benthic chamber, microprofiler) and ex situ (pore water constituents, turnover rates of sulfate and methane, prokaryote abundance) techniques reduced sites from three different seep ecosystems in the Eastern Mediterranean deep-sea. At all three cold seep systems, the Amon Mud Volcano, Amsterdam Mud Volcano and the Nile Deep Sea Fan Pockmark area, we observed and sampled patches of highly reduced, methane-seeping sulfidic sediments which were separated by tens to hundreds of (kilo)meters with non-reduced oxygenated seafloor areas. All investigated seep sites were characterized by gassy, sulfidic sediments of blackish color, of which some were overgrown with thiotrophic bacterial mats. Fluxes of methane and oxygen, as well as sulfate reduction rates varied between the different sites.

Bromine and iodine concentrations in sediments and pore fluids of ODP Leg 112

At the Peruvian convergent margin, two distinct pore fluid regimes are recognized from differences in their Cl- concentrations. The slope pore fluids are characterized by low Cl- concentrations, but elevated Br- and I- concentrations due to biogenic production. The shelf pore fluids exhibit elevated Cl- and Br- concentrations due to diffusive mixing with an evaporitic brine. In the slope pore fluids, the Br-, I-, and NH4+ concentrations are elevated following bacterial decomposition of organic matter, but the I- concentrations are in excess of those expected based on mass balance calculations using NH4+ and Br- concentrations. The slope sediment organic matter, which is enriched in iodine from oxidationreduction processes at the oxygenated sediment-water interface, is responsible for this enrichment. The increases in dissolved I- and the I- enrichments relative to NH4+ and Br- correlate well with sedimentation rates because of differential trapping following regeneration. The pore-fluid I-/Br- ratios suggest that membrane ion fiitration is not a major cause of the decreases in Cl- concentrations. Other possible sources for low Cl- water, including meteoric water, clathrate dissociation, and/or mineral dehydration reactions, imply that the diluting component of the slope low-Cl- fluids has flowed at least 1 km through the sediment. The low bottom-water oxygenation in the shelf is responsible for the low (if any) enrichment of iodine in the shelf sediments. Fluctuations in bottom-water oxygen concentrations in the past, however, may be responsible for the observed variations in the sediment I/Br ratios. Comparison of Na+/Cl- and Br-/Cl- molar ratios in the pore fluids shows that the shelf high-Cl- fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution. This dense brine, located below the sediment sections drilled, appears to have flowed a distance >500 km through the sediment.

Petrology and carbon and oxygen stable isotopic composition of macrofossils and sediments from the Blake-Bahama Formation, DSDP Hole 93-603B

Strata that record the evolutionary history of the North American continental margin in a region that serves as the basin margin interface between allochthonous sedimentation from the continent and pelagic sedimentation from the oceanic realm were recovered at Deep Sea Drilling Project Site 603, on the lower continental rise. The lowermost unit recovered at this site is composed of upper Berriasian-Aptian interbedded laminated limestone and bioturbated limestone with sandstone to claystone turbidites. This unit can be correlated with the Blake-Bahama Formation in the western North Atlantic. Studies of the laminated and bioturbated limestones were used to determine the depositional environment. Geochemical and petrographic studies suggest that the laminated limestones were deposited from the suspended particulate loads of the nepheloid layer associated with weak bottom-current activity as well as moderate to poorly oxygenated bottom-water conditions. Fragments of macrofossils are also found in the Blake-Bahama Formation drilled at Site 603. Twelve specimens and their host sediment were analyzed for their carbon and oxygen isotopic composition. The macrofossil samples chosen for analysis consist of nine samples of Inoceramus, two ammonite aptychi, and one belemnite sample. Depletion in 18O is observed in recrystallized specimens. The ammonite aptychi have been diagenetically altered and/or exhibit evidence of isotopic fractionation by the organism. Oxygen isotope paleotemperatures obtained from five well-preserved specimens - four of Inoceramus and one of a belemnite - suggest that bottom-water temperatures in the North Atlantic Basin during the Early Cretaceous were very warm, at least 11°C.

Geochemical analysis of nine sections from the Nama Group, Namibia

The first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but a precise spatial and temporal reconstruction of their evolution has not been resolved. Here we consider the evolution of ocean chemistry from ~550 to ~541 Ma across shelf-to-basin transects in the Zaris and Witputs Sub-Basins of the Nama Group, Namibia. New carbon isotope data capture the final stages of the Shuram/Wonoka deep negative C-isotope excursion, and these are complemented with a reconstruction of water column redox dynamics utilising Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these inter-basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation. The strongly negative d13C values in the lower parts of the sections reflect both a secular, global change in the C-isotopic composition of Ediacaran seawater, as well as the influence of 'local' basinal effects as shown by the most negative d13C values occurring in the transition from distal to proximal ramp settings. Critical, though, is that the transition to positive d13C values postdates the appearance of calcified metazoans, indicating that the onset of biomineralization did not occur under post-excursion conditions. Significantly, we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive d13C that marks the end of the Shuram/Wonoka excursion. Thus, if the C isotope trend reflects the transition to global-scale oxygenation in the aftermath of the oxidation of a large-scale, isotopically light organic carbon pool, it was not sufficient to fully oxygenate the deep ocean. Both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner- and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.

Stable carbon and oxygen isotopes from samples of the Anaximander seamounts in the eastern Mediterranean Sea

Porous seep-carbonates are exposed at mud volcanoes in the eastern Mediterranean Sea. The 13C-depleted aragonitic carbonates formed as a consequence of the anaerobic oxidation of methane in a shallow sub-surface environment. Besides the macroscopically visible cavernous fabric, extensive carbonate corrosion was revealed by detailed analysis. After erosion of the background sediments, the carbonates became exposed to oxygenated bottom waters that are periodically influenced by the release of methane and upward diffusion of hydrogen sulphide. We suggest that carbonate corrosion resulted from acidity locally produced by aerobic oxidation of methane and hydrogen sulphide in the otherwise, with respect to aragonite, oversaturated bottom waters. Although it remains to be tested whether the mechanisms of carbonate dissolution suggested herein are valid, this study reveals that a better estimate of the significance of corrosion is required to assess the amount of methane-derived carbon that is permanently fixed in seep-carbonates.

Benthic foraminifera of the central Indian Ocean

Late Oligocene to late Pliocene vertical water-mass stratification along depth traverses in the northern Indian Ocean is depicted in this paper by benthic foraminifer index faunas. During most of this time, benthic faunas indicate well-oxygenated, bottom-water conditions at all depths except under the southern Indian upwelling and in the Pliocene in the southern Arabian Sea. Faunas suggest the initiation of lower oxygen conditions at intermediate depths in the northern Indian Ocean beginning in Oligocene Zone P21a. Lower oxygen conditions intensified during primary productivity pulses, possibly related to increased upwelling vigor, in the latest Oligocene and throughout most of the late middle through late Miocene. During times of elevated primary production, there may be more oxygen flux into sedimentary pore waters and the shallow infaunal habitat may become more oxygenated. One criterion for locating the source of "new" water masses is vertical homogeneity of benthic foraminifer indexes for well-oxygenated water masses from intermediate through abyssal depths. In the northern Mascarene Basin, this type of faunal homogeneity with depth corroborates the proposal that the northern Indian Ocean was an area of sinking well-oxygenated waters through most of the Miocene before Zone N17. Oxygenated, possibly "new" intermediate-water masses in the low- to middle-latitude Mascarene and Central Indian basins first developed in the late Oligocene. These well-oxygenated waters were probably more fertile than the Antarctic Intermediate Waters (AAIW) that cover intermediate depths in these areas today. Production of intermediate waters more similar to modern AAIW is indicated by the sparse benthic population of epifaunal rotaloid species in the northern Mascarene Basin during middle Miocene Zone N9 and from early through late Pliocene time. Deep-water characteristics are more difficult to interpret because of the extensive redeposition at the deeper sites. Redeposited intermediate, rather than shallow, water fossils and erosion from north to south in the Mascarene Basin are incompatible with the sluggish circulation from south to north through the western Indian Ocean basins today. Such erosion could result from the vigorous sinking of an intermediate-depth water mass of northern origin. Before late Oligocene Zone P22, benthic faunas indicate a twofold subdivision of the troposphere, with the boundary between upper and lower well-oxygenated water masses located from 2500-3000 mbsl. No characteristic bottom-water fauna developed before the end of late Oligocene Zone P22. Deep and abyssal benthic indexes suggest the development of water masses similar to those of the present day in the latest Miocene. Faunas containing deep-water benthic indexes, including the uvigerinids, suggestive of a water mass similar to modern Indian Deep Water (IDW), appeared during the late Miocene in the northern Mascarene and Central Indian basins. In the early Pliocene, this deep-water fauna was found only in the Central Indian Basin, whereas a fauna typical of modern Antarctic Bottom Water (AABW) spread through deep waters at 2800 mbsl in the Mascarene Basin. By late Pliocene Zone N21, however, deep-water faunas similar to their modern analogs were developed in both the eastern and western basins. Abyssal faunas, studied only in the Mascarene Basin, show more or less similarity to those under modern AABW. Bottom-water faunas containing Nuttallides umbonifera or Epistominella exiguua were first differentiated at the end of Zone P22, then appeared episodically during the early Miocene. These AABW-type faunas reappeared and migrated updepth into deep waters during the glacial episodes at the end of the Miocene and at the beginning of the Pliocene. By late Pliocene Zone N21, however, a bottom-water fauna similar to that under eastern Indian Bottom Water (IBW) developed in the Mascarene Basin. Modern bottom-water characteristics of the Mascarene Basin must have developed after ZoneN21.

Geochemistry on sapropel deposition in the eastern Mediterranean from cores TTR-GL94 and M25/4-KL11

The geochemistry of the youngest Mediterranean sapropel layer suggests changes in productivity and water column oxygen conditions during sapropel deposition. The Ba-enriched interval is broader than the organic-carbon-rich interval of this sapropel. We suggest that the Ba-enriched horizon records the original thickness of the sapropel prior to subsequent partial oxidation. The main carrier of Ba is barite, as microcrystals (0.5-5 µm ) having a morphology characteristic of marine barite, particularly abundant beneath high productivity regions. Ba concentrations do not change at the sapropel layer oxidation front and diagenetic barite crystals are absent, thus the Ba-enriched layer reflects original oceanic conditions of increased biological productivity during sapropel deposition and not diagenetic Ba remobilization. Paleoredox indicators point to restricted oxygenated bottom water but not to fully anoxic conditions. Detrital elements within this layer indicate a lower eolian terrigenous input, enhanced humidity, and increased precipitation/runoff, thus likely higher nutrient supply.

Neogene benthic foraminifers from the Kerguelen Plateau

Benthic foraminifers were studied quantitatively in 120 lower Miocene through upper Pleistocene samples from Ocean Drilling Program Site 747 (Central Kerguelen Plateau) and Sites 748 and 751 (Southern Kerguelen Plateau). These sites are situated on an 450-km-long, north-south transect between 54°49'S and 58°26'S at present water depths between 1696 and 1288 m. Principal component analysis on the census data of the most abundant 92 taxa helped to identify 8 benthic foraminifer assemblages. These benthic foraminifer assemblages were compared with Holocene faunas from southern high latitudes to reconstruct paleoenvironmental conditions. Middle lower Miocene sediments are characterized by a Uvigerina hispidocostata assemblage, indicating high paleoproductivity and/or not well-ventilated bottom water. From late early to late middle Miocene time, the Southern Kerguelen Plateau was bathed by a young, well-oxygenated, and carbonate-aggressive water mass, as indicated by a Nuttallides umbonifer-dominated benthic foraminifer assemblage. During late middle Miocene time, an Astrononion pusillum assemblage took over for only about 1 m.y., probably indicating the first injection of an aged water mass, similar to the North Atlantic Deep Water (NADW), into a developing circumpolar current system. Around the middle to late Miocene boundary, the fauna again became dominated by N. umbonifer. After the last appearance of N. umbonifer, reestablishment of the A. pusillum assemblage from the early late through at least the late late Miocene, indicated the established influence of a NADW-like water mass. The latest Miocene through middle late Pliocene benthic foraminifer assemblage was characterized by Epistominella exigua and strong carbonate dissolution, indicating very high biosiliceous production, and this in turn may indicate the formation and paleoposition of an Antarctic Polar Frontal Zone. From the late late Pliocene, a Trifarina angulosa assemblage (indicative today of sandy substrate and vigorous bottom currents) strongly dominated the fauna up to the late Pleistocene, when Bulimina aculeata (indicative today of calm sedimentation with high organic matter fluxes) became an important and partly dominating constituent of the fauna. This is interpreted as the faunal response to the decreased winnowing force (bottom current velocities) of the Antarctic Circumpolar Current during periods of global climatic amelioration and raised sea level.