Benthic respiration and physical oceanography on two contrasting continental margins in the western Indian Ocean: the Yemen-Somali upwelling region and the margin off Kenya

During the Netherlands Indian Ocean Project (NIOP, 1992-1993) sediment community oxygen consumption (SCOC) was measured on two continental margins in the Indian Ocean with different productivity: the productive upwelling region off Yemen-Somalia and the supposedly less productive Kenyan margin, which lacks upwelling. The two margins also differ in terms of river input (Kenya) and the more severe oxygen minimum in the Arabian Sea. Simultaneously with SCOC, distributions of benthic biomass and phytodetritus were studied. Our expectation was that benthic processes in the upwelling margin of the Arabian Sea would be relatively enhanced as a result of the higher productivity. On the Kenyan margin, SCOC (range 1-36 mmol/m**2/d) showed a clear decrease with increasing water depth, and little temporal variation was detected between June and December. Highest SCOC values of this study were recorded at 50 m depth off Kenya, with a maximum of 36 mmol/m**2/d in the northernmost part. On the margin off Yemen-Somalia, SCOC was on average lower and showed little downslope variation, 1.8-5.7 mmol/m**2/d, notably during upwelling, when the zone between 70 and 1700 m was covered with low O2 water (10-50 µM). After cessation of upwelling, SCOC at 60 m depth off Yemen increased from 5.7 to 17.6 mmol/m**2/d concurrently with an increase of the near-bottom O2 concentration (from 11 to 153 µM), suggesting a close coupling between SCOC and O2 concentration. This was demonstrated in shipboard cores in which the O2 concentration in the overlying water was raised after the cores were first incubated under in situ conditions (17 µM O2). This induced an immediate and pronounced increase of SCOC. Conversely, at deeper stations permanently within the oxygen minimum zone (OMZ), SCOC showed little variation between monsoon periods. Hence, organic carbon degradation in sediments on a large part of the Yemen slope appears hampered by the oxygen deficiency of the overlying water. Macrofauna biomass and the pooled biomass of smaller organisms, estimated by the nucleic acid content of the sediment, had comparable ranges in the two areas in spite of more severe suboxic conditions in the Arabian Sea. At the Kenyan shelf, benthic fauna (macro- and meiofauna) largely followed the spatial pattern of SCOC, i.e. high values on the northern shelf-upper slope and a downslope decrease. On the Yemen-Somali margin the macrofauna distribution was more erratic. Nucleic acids displayed no clear downslope trend on either margin owing to depressed values in the OMZ, perhaps because of adverse effects of low O2 on small organisms (meiofauna and microbes). Phytodetritus distributions were different on the two margins. Whereas pigment levels decreased downslope along the Kenya margin, the upper slope off Yemen (800 m) had a distinct accumulation of mainly refractory carotenoid pigments, suggesting preservation under low 02. Because the accumulations of Corg and pigments on the Yemen slope overlap only partly, we infer a selective deposition and preservation of labile particles on the upper slope, whereas refractory material undergoes further transport downslope.

Benthic foraminiferal assemblages of the Barents Sea

Foraminiferal assemblages were studied in northern Barents Sea core ASV 880 along with oxygen and carbon isotope measurements in planktonic (N. pachyderma sin.) and benthic (E. clavatum) species. AMS C-14 measurements performed on molluscs Yoldiella spp. show that this core provides a detailed and undisturbed record of Holocene climatic changes over the last 10000 calendar years. Surface and deep waters were very cold (<0°C) at the beginning of the Holocene. C. reniforme dominated the highly diverse benthic foraminiferal assemblage. From 10 to 7.8 cal. ka BP, a warming trend culminated in a temperature optimum, which developed between 7.8 and 6.8 cal. ka BP. During this optimum, the input of Atlantic water to the Barents Sea reached its maximum. The Atlantic water mass invaded the whole Franz Victoria Trough and was present from subsurface to the bottom. No bottom water, which would form through rejection of brine during winter, was present at the core depth (388 m). The water stratification was therefore greatly reduced as compared to the present. An increase in percentage of I. helenae/norcrossi points to long seasonal ice-free conditions. The temperature optimum ended rather abruptly, with the return of cold polar waters into the trough within a few centuries. This was accompanied by a dramatic reduction of the abundance of C. reniforme. During the upper Holocene, the more opportunistic species E. clavatum became progressively dominant and the water column was more stratified. Deep water in Franz Victoria Trough contained a significant amount of cold Barents Sea bottom water as it does today, while subsurface water warmed progressively until about 3.7 cal. ka BP and reached temperatures similar to those of today. These long-term climatic changes were cut by several cold events of short duration, in particular one in the middle of the temperature optimum and another, which coincides most probably with the 8.2 ka BP cold event. Both long- and short-term climatic changes in the Barents Sea are associated with changes in the flow of Atlantic waters and the oceanic conveyor belt.

Stable carbon and oxygen isotope ratios for different test sizes of live benthic foraminifera from the Arabian Sea

We determined the stable oxygen and carbon isotopic composition of live (Rose Bengal stained) tests belonging to different size classes of two benthic foraminiferal species from the Pakistan continental margin. Samples were taken at 2 sites, with water depth of about 135 and 275 m, corresponding to the upper boundary and upper part of the core region of the oxygen minimum zone (OMZ). For Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata, delta13C and delta18O values increased significantly with increasing test size. In the case of U. ex gr. U. semiornata, delta13C increased linearly by about 0.105 per mil for each 100-µm increment in test size, whereas delta18O increased by 0.02 to 0.06 per mil per 100 µm increment. For B. aff. B. dilatata the relationship between test size and stable isotopic composition is better described by logarithmic equations. A strong positive linear correlation is observed between delta18O and delta13C values of both taxa, with a constant ratio of delta18O and delta13C values close to 2:1. This suggests that the strong ontogenetic effect is mainly caused by kinetic isotope fractionation during CO2 uptake. Our data underline the necessity to base longer delta18O and delta13C isotope records derived from benthic foraminifera on size windows of 100 µm or less. This is already common practice in down-core isotopic studies of planktonic foraminifera.

(Table 3) Oxygen isotope ratios of benthic foraminifera from DSDP Hole 80-548

Presently, the intermediate depths of the North Atlantic Ocean are occupied by a great lens of warm, saline water whose source is the Mediterranean Sea. This water flows both westward and northward, finally entering the Norwegian Sea where it may contribute to the formation of North Atlantic Deep Water. The Late Neogene history of Mediterranean Outflow in the Atlantic can be monitored at DSDP-IPOD Site 548 on the continental slope Southwest of Ireland using benthic Foraminifera oxygen isotope values. Isotopic data from 154 samples indicate that Mediterranean water was absent from the mid-depth North Atlantic from 3.4 to 3.2 Ma ago. However, at about 2.9 Ma ago the isotopic values at Site 548 diverge from those recorded from the deep North Atlantic and they can be interpreted to indicate the appearance of a new water mass, possibly Mediterranean water, in the North Atlantic water column. This appearance may be related to climatic changes that occurred around the Mediterranean Basin at about 2.9 Ma ago. The analysis of 189 samples for grain-size distributions shows that a significant increase in the silt-size fraction occurs at the same level that isotopic analysis indicates a change in bottom waters at Site 548. The grainsize data support the hypothesis that mid-depth water-mass changes occurred at about 2.9 Ma ago.

Mg/Ca ratios of Cenozoic benthic foraminifera from the Indian and South Atlantic Ocean

We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (delta18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal delta18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. (2002, doi:10.1126/science.287.5451.269) and the Neogene record from the Southern Ocean of Billups and Schrag (2002, doi:10.1029/2000PA000567), we obtain three, almost complete representations of the delta18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the delta18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (~ 49-40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding delta18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among delta18Osw records is similar and similar to longer-term trends in the benthic foraminiferal delta18O record. An exception occurs during the Pliocene when delta18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal delta18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum delta18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters (Schrag et al., 1996, doi:10.1126/science.272.5270.1930) further highlighting the value of the new Mg/Ca calibrations of Martin et al. (2002, doi:10.1016/S0012-821X(02)00472-7) and Lear et al. (2002, doi:10.1016/S0016-7037(02)00941-9) applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Benthic foraminifers of the Barrow Group, Northwest Shelf, Australia

Micropaleontological analysis of the Barrow Group of Sites 762 and 763 has been undertaken with a view to determining the stratigraphic age and depositional environment of the unit. The stratigraphic age of the unit is Berriasian-Valanginian at both sites, in line with palynological findings. The unit is interpreted as having been deposited in a marine deltaic environment. Paleobathymetry at Site 763 (proximal) and Site 762 (distal) is interpreted as having been of the order of 100 m and 200-500 m, respectively. Paleontological evidence for the presence of deep-water submarine fans at Site 763 is lacking. The paleobathymetric significance of the observed variations in the benthic foraminiferal populations at Site 763 remains unclear.

Chemical composition of waters, suspended matter, bottom sediments, and benthic organisms along a section from the Ob River estuary (Obskaya Guba) to the central Kara Sea

Biogeochemical behavior of a group of heavy metals and metalloids in water (including their dissolved and suspended particulate forms), bottom sediments, and zoobenthos was studied in the Ob River estuary (Obskaya Guba) - Kara Sea section on the basis of data obtained during Cruise 54 of R/V Akademik Mstislav Keldysh in September-October 2007. Changes in ratios of dissolved and particulate forms of Fe, Mn, Zn, Cu, Pb, Cd, and As were shown, as well as growth of adsorbed fractions of the elements in near-bottom suspended matter under mixing of riverine and marine waters. Features of chemical element accumulation in typical benthic organisms of the Obskaya Guba and the Kara Sea were revealed, and their concentrating factors were calculated based on specific conditions of the environment. It was shown that shells of bivalves possessing higher biomass compared to other groups of organisms in the Obskaya Guba play an important role in deposition of heavy metals. In the Obskaya Guba mollusks accumulate Cd and Pb at the background level, whereas Cu and Zn contents appear to be higher than the background level.

Benthic foraminifers, isotopes and varimax factors at DSDP Holes 72-517 and 72-518

Pliocene changes in the vertical water mass structure of the western South Atlantic are inferred from changes in benthic foraminiferal assemblages and stable isotopes from DSDP Holes 516A, 517, and 518. Factor analysis of 34 samples from Site 518 reveals three distinct benthic foraminiferal assemblages that have been associated with specific subsurface water masses in the modern ocean. These include a Nuttalides umbonifera assemblage (Factor 1) associated with Antarctic Bottom Water (AABW), a Globocassidulina subglobosa-Uvigerina peregrina assemblage (Factor 2) associated with Circumpolar Deep Water (CPDW), and an Oridorsalis umbonatus-Epistominella exigua assemblage associated with North Atlantic Deep Water (NADW). Bathymetric gradients in d13C between Holes 516A (1313 m), 517 (2963 m), and 518 (3944 m) are calculated whenever possible to monitor the degree of similarity and/or difference in the apparent oxygen utilization (AOU) of water masses located at these depths during the Pliocene. Changes in bathymetric d13C gradients coupled with benthic foraminiferal assemblages record fundamental changes in the vertical water mass structure of the Vema Channel during the Pliocene from 4.1 to 2.7 Ma. At Site 518, the interval from 4.1 to 3.6 Ma is dominated by the N. umbonifera (Factor 1) and O. umbonatus-E. exigua (Factor 3) assemblages. The d13C gradient between Holes 518 (3944 m) and 516A (1313 m) undergoes rapid oscillations during this interval though no permanent increase in the gradient is observed. However, d13C values at Site 518 are clearly lighter during this interval. These conditions may be related to increased bottom water activity associated with the re-establishment of the West Antarctic Ice Sheet in the late Gilbert Chron (-4.2 to 3.6 Ma) (Osborn et al., 1982). The interval from 3.6 to 3.2 Ma is marked by a dominance of the G. subglobosa-U. peregrina (Factor 2) assemblage and lack of a strong d13C gradient between Holes 518 (3944 m) and 516A (1313 m). We suggest that shallow circumpolar waters expanded to depths of a least 3944 m (Site 518) during this time. The most profound faunal and isotopic change occurs at 3.2 Ma, and is marked by dominance of the N. umbonifera (Factor 1) and O. umbonatus-E. exigua (Factor 3) assemblages, a 1.1 per mil enrichment in d18O, and a large negative increase in the d13C gradient between Holes 518 and 516A. These changes at Site 518 record the vertical displacement of circumpolar waters by AABW and NADW. This change in vertical water mass structure at 3.2 Ma was probably related to a global cooling event and/or final closure of the Central American seaway. A comparison of the present-day d13C structure of the Vema Channel with a reconstruction between 3.2 and 2.7 Ma indicates that circulation patterns during this late Pliocene interval were similar to those of the modern western South Atlantic.

(Table S3) Benthic foraminiferal Mg/Ca, Li/Ca and stable isotope ratios of ODP Site 122-761

The sensitivities of benthic foraminiferal Mg/Ca and Li/Ca to bottom water temperature and carbonate saturation state have recently been assessed. Here we present a new approach that uses paired Mg/Ca and Li/Ca records to calculate simultaneous changes in temperature and saturation state. Using previously published records, we first use this approach to document a cooling of deep ocean waters associated with the establishment of the Antarctic ice sheet at the Eocene-Oligocene climate transition. We then apply this approach to new records of the Middle Miocene Climate Transition from ODP Site 761 to estimate variations in bottom water temperature and the oxygen isotopic composition of seawater. We estimate that the oxygen isotopic composition of seawater varied by ~1 per mil between the deglacial extreme of the Miocene Climatic Optimum and the glacial maximum following the Middle Miocene Climate Transition, indicating large amplitude variations in ice volume. However, the longer-term change between 15.3 and 12.5 Ma is marked by a ~1°C cooling of deep waters, and an increase in the oxygen isotopic composition of seawater of ~0.6 per mil. We find that bottom water saturation state increased in the lead up to the Middle Miocene Climate Transition and decreased shortly after. This supports decreasing pCO2 as a driver for global cooling and ice sheet expansion, in agreement with existing boron isotope and leaf stomatal index CO2 records but in contrast to the published alkenone CO2 records.

Planktic and benthic foraminifreral isotope data for ODP Site 207-1258

The mid-Cretaceous is widely considered the archetypal ice-free greenhouse interval in Earth history, with a thermal maximum around Cenomanian-Turonian boundary time (ca. 90 Ma). However, contemporaneous glaciations have been hypothesized based on sequence stratigraphic evidence for rapid sea-level oscillation and oxygen isotope excursions in records generated from carbonates of questionable preservation and/or of low resolution. We present new oxygen isotope records for the mid-Cenomanian Demerara Rise that are of much higher resolution than previously available, taken from both planktic and benthic foraminifers, and utilizing only extremely well preserved glassy foraminifers. Our records show no evidence of glaciation, calling into question the hypothesized ice sheets and rendering the origin of inferred rapid sea-level oscillations enigmatic. Simple mass-balance calculations demonstrate that this Cretaceous sea-level paradox is unlikely to be explained by hidden ice sheets existing below the limit of d18O detection.

Mg/Ca-temperature proxy in benthic foraminifera from the Florida Strait

Over the past decade, the ratio of Mg to Ca in foraminiferal tests has emerged as a valuable paleotemperature proxy. However, large uncertainties remain in the relationships between benthic foraminiferal Mg/Ca and temperature. Mg/Ca was measured in benthic foraminifera from 31 high-quality multicore tops collected in the Florida Straits, spanning a temperature range of 5.8° to 18.6°C. New calibrations are presented for Uvigerina peregrina, Planulina ariminensis, Planulina foveolata, and Hoeglundina elegans. The Mg/Ca values and temperature sensitivities vary among species, but all species exhibit a positive correlation that decreases in slope at higher temperatures. The decrease in the sensitivity of Mg/Ca to temperature may potentially be explained by Mg/Ca suppression at high carbonate ion concentrations. It is suggested that a carbonate ion influence on Mg/Ca may be adjusted for by dividing Mg/Ca by Li/Ca. The Mg/Li ratio displays stronger correlations to temperature, with up to 90% of variance explained, than Mg/Ca alone. These new calibrations are tested on several Last Glacial Maximum (LGM) samples from the Florida Straits. LGM temperatures reconstructed from Mg/Ca and Mg/Li are generally more scattered than core top measurements and may be contaminated by high-Mg overgrowths. The potential for Mg/Ca and Mg/Li as temperature proxies warrants further testing.