Radiocarbon dating on cold-water corals, grain size, Corg, and benthic foraminfera assemblage of two sediment cores from the Ionian Sea

Continuous sedimentary records from an eastern Mediterranean cold-water coral ecosystem thriving in intermediate water depths (~600 m) reveal a temporary extinction of cold-water corals during the Early to Mid Holocene from 11.4-5.9 cal kyr BP. Benthic foraminiferal assemblage analysis shows low-oxygen conditions of 2 ml l**-1 during the same period, compared to bottom-water oxygen values of 4-5 ml l**-1 before and after the coral-free interval. The timing of the corals' demise coincides with the sapropel S1 event, during which the deep eastern Mediterranean basin turned anoxic. Our results show that during the sapropel S1 event low oxygen conditions extended to the rather shallow depths of our study site in the Ionian Sea and caused the cold-water corals temporary extinction. This first evidence for the sensitivity of cold-water corals to low oceanic oxygen contents suggests that the projected expansion of tropical oxygen minimum zones resulting from global change will threaten cold-water coral ecosystems in low latitudes in the same way that ocean acidification will do in the higher latitudes.

Stable oxygen and carbon isotopes and sediment geochemistry of sediment core GeoB12616-4

The Indian Ocean is an important component of the global thermohaline circulation system, as its western boundary currents feed the Agulhas Current, an integral part of the Atlantic meridional overturning circulation. However, Indian Ocean intermediate to deep-water variability on glacial-interglacial timescales is still a matter of debate. Here we provide stable carbon and oxygen isotopes and sediment elemental compositions of a sediment core from the edge of the Somali Basin. We demonstrate that throughout the past 600 kyr the intermediate western Indian Ocean was primarily bathed by Southern Ocean sourced Upper Circumpolar Deep Water (UCDW). This Southern Ocean sourced water mass enters the Somali Basin via the Amirante Passage or the Mozambique Channel and represents a downstream equivalent of South Atlantic UCDW. We cannot clearly account for the shortterm passage of Red Sea Water (RSW) at 1500 m water depth along the African continental margin, as previously suggested, on glacial-interglacial timescales.

Late Cretaceous-Early Neogene oxygen and carbon isotope record of the Indian Ocean

Stable isotopic data of calcareous nannofossil, monogeneric and monospecific planktic and benthic foraminifera from five Indian Ocean DSDP sites (212, 217, 220, 237, and 253), leads to the following paleoclimatic and paleoceanographic conclusions: - The latest Cretaceous oxygen isotopic record implies a cooling (3-4°C) during the Maastrichtian. At the Cretaceous/Tertiary boundary only a minor warming (about 2°C) has been recorded. The parallel delta13C decrease of more than 1? indicates a significant decrease in productivity. - During the latest Paleocene a positive delta13C excursion was detected in Sites 217 and 237. This transient enrichment in delta13C may be due to productivity changes on continents and/or a change in the storage rate of organic matter in marginal basins or shelf areas. - The most striking feature in the oxygen isotopic record is noted at the Early/Middle Eocene transition. The shift towards more positive values (which were probably enhanced to a certain extent by a preceding diagenetic alteration) delineates a dramatic climatic deterioration at high and mid latitudes during the earlier Tertiary. - Near the Eocene/Oligocene boundary a cooling is evident within the latest Eocene interval. During the earliest Oligocene time a hiatus at Sites 217 and 253 partially obscures the climatic record. - Several climatic fluctuations have been noted during the Oligocene: a cooling at the base of Zone NP 23, a warming at the top of Zone NP 23 through NP 24, and a cooling during Zone NP 25. - The Miocene oxygen isotopic record is dominated by changes in surface and bottom water environments during Zone NN5. The decreasing and then increasing delta18O values, together with the subsequent steepening of the vertical delta18O gradient, point towards major climatic instabilities. These events coincide with the Mid-Miocene build-up of Antarctic ice-sheets. During the latest Miocene to the earliest Pliocene the delta18O record of planktic foraminifera indicates a significant warming of the Indian Ocean at mid-latitudes. - The delta13C record during the Oligocene and Miocene reveals several cycles (delta13C enrichments: NP 24, NN2, NN5, NN9, and base NN 11) which are most likely related to changes in storage rates of organic matter and biological productivity due to climatic changes and transgression/regression cycles. In addition, changes in the circulation patterns may also have influenced the carbon isotopic record.

Age model, isotope analyses, geochemical measurements and grain size analyses of sediment core GeoB9508-5 from the Senegal River Mouth

The influence of the large-scale ocean circulation on Sahel rainfall is elusive because of the shortness of the observational record. We reconstructed the history of eolian and fluvial sedimentation on the continental slope off Senegal during the past 57,000 years. Our data show that abrupt onsets of arid conditions in the West African Sahel were linked to cold North Atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with Heinrich Stadials. Climate modeling suggests that this drying is induced by a southward shift of the West African monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric African Easterly Jet.

Astronomically modulated Neogene sediment records of ODP Site 184-1148

General global cooling over the Neogene has been modulated by changes in Earth's orbital parameters. Investigations of deep-sea sediment sequences show that various orbital cycles can dominate climate records for different latitudes or for different time intervals. However, a comprehensive understanding of astronomical imprints over the entire Neogene has been elusive because of the general absence of long, continuous records extending beyond the Pliocene. We present benthic foraminiferal d18O and d13C records over the past 23 Ma at Ocean Drilling Program Site 1148 in the northern South China Sea and construct an astronomically tuned timescale (TJ08) for these records based on natural gamma radiation and color reflectance data at this site. Our results show that a 41 ka cycle has dominated sediment records at this location over the Neogene, displaying a linear response to orbital forcing. A 100 ka cycle has also been significant. However, it is correlated nonlinearly with Earth's orbital variations at the 100 ka band. The sediment records also display a prominent 405 ka cycle. Although this cycle was coherent with orbital forcing during the Oligocene and the early Miocene, it was not coherent with Earth's orbital variations at the 405 ka band over the whole Neogene. Amplification of Northern Hemisphere and Southern Hemisphere glaciation since the middle Miocene may be responsible for this change in sedimentary response. Our benthic foraminifera d18O and d13C records further exhibit amplitude variations with longer periods of 600, 1000, 1200, and 2400 ka. Apparently, these cycles are nonlinear responses to insolation forcing.

Benthic foraminifera in sapropels of the eastern Mediterranean Sea

High-resolution benthic foraminiferal and geochemical investigations were carried out across sapropels S5 and S6 from two sediment cores in the Levantine Sea to evaluate the impact of climatic and environmental changes on benthic ecosystems during times of sapropel formation. The faunal successions indicate that eutrophication and/or oxygen reduction started several thousand years prior to the onset of sapropel formation, suggesting an early response of the bathyal ecosystems to climatic changes. Severest oxygen depletions appear in the early phases of sapropel formation. The initial reduction of deep-water ventilation is caused by a warming and fresh water-induced stratification of Eastern Mediterranean surface waters. During the late phase of S5 formation improved oxygenation is restricted to middle bathyal ecosystems, indicating that at least some formation of subsurface water took place. During S6 formation oxygen depletions and eutrophication were less severe and more variable than during S5 formation. Estimated oxygen contents were low dysoxic at middle bathyal to anoxic at lower bathyal depths during the early phase of S6 formation but never dropped to anoxic values in its late phase. The high benthic ecosystem variability during S6 formation suggests that water column stratification at deep-water formation sites was in a very unstable mode and susceptible to minor temperature fluctuations at a millennial time-scale.

Analytical results from sediment core MD03-2698

It is well established that orbital scale sea-level changes generated larger transport of sediments into the deep-sea during the last glacial maximum than the Holocene. However, the response of sedimentary processes to abrupt millennial-scale climate variability is rather unknown. Frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the Lisbon-Setúbal canyons, within a chronostratigraphy based on radiometric ages, oxygen isotopes and paleomagnetic key global anomalies. We found that: 1) Higher frequency of turbidites concurred with Northern Hemisphere coldest temperatures (Greenland Stadials [GS], including Heinrich [H] events). But more than that, an escalating frequency of turbidites starts with the onset of global sea-level rising (and warming in Antarctica) and culminates during H events, at the time when rising is still in its early-mid stage, and the Atlantic Meridional Overturning Circulation (AMOC) is re-starting. This short time span coincides with maximum gradients of ocean surface and bottom temperatures between GS and Antarctic warmings (Antarctic Isotope Maximum; AIM 17, 14, 12, 8, 4, 2) and rapid sea-level rises. 2) Trigger of turbidity currents is not the only sedimentary process responding to millennial variability; land-detrital carbonate (with a very negative bulk d18O signature) enters the deep-sea by density-driven slope lateral advection, accordingly during GS. 3) Possible mechanisms to create slope instability on the Portuguese continental margin are sea-level variations as small as 20 m, and slope friction by rapid deep and intermediate re-accommodation of water masses circulation. 4) Common forcing mechanisms appear to drive slope instability at both millennial and orbital scales.

Stable isotope ratios and organic carbon accumulation rates of late Pleistocene sediments of ODP Hole 117-724C

Stable isotope records of coexisting benthic foraminifers Uvigerina spp. and Cibicidoides spp. and planktonic G. ruber (white variety) from Site 724 are used to study the late Pleistocene evolution of surface and intermediate water hydrography (593 m water depth) at the Oman Margin. Glacial-interglacial d18O amplitudes recorded by the benthic foraminifers are reduced when compared to the estimated mean ocean changes of d18Oseawater . Epibenthic d13C remains at its modern level or is increased during glacial times. This implies that Red Sea outflow waters which are enriched in d18Oseawater and d13C (Sum CO2) have been replaced during glacial periods by intermediate waters still positive in d13C (Sum CO2) but more negative in d18Oseawater. Glacial-interglacial amplitudes of the planktonic d18O record exceed those of the mean ocean d18Oseawater variation and imply decreased surface water temperatures (SST) during glacial times. Throughout most of the records these cooling events correlate with enhanced rates of carbon accumulation. However, both negative (colder) SST and positive Corg accumulation rate anomalies do not correlate with potential physical upwelling maxima as inferred from the orbital monsoon index. This is in conflict with the established hypothesis that upwelling in the estern Arabia Sea should be strongest during maxima of the southwest monsoon.

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.

Stable isotope record and trace element ratios of Eocene and Oligocene foraminifers from ODP Site 120-748

Stable carbon and oxygen isotope analyses were conducted on well-preserved planktonic and benthic foraminifers from a continuous middle Eocene to Oligocene sequence at Ocean Drilling Program (ODP) Site 748 on the Kerguelen Plateau. Benthic foraminifer d18O values show a 1.0 per mil increase through the middle and upper Eocene, followed by a rapid 1.2 per mil increase in the lowermost Oligocene (35.5 Ma). Surface-dwelling planktonic foraminifer d18O values increase in the lowermost Oligocene, but only by 0.6 per mil whereas intermediate-depth planktonic foraminifers show an increase of about l.0 per mil. Benthic foraminifer d13C values increase by 0.9 per mil in the lowermost Oligocene at precisely the same time as the large d18O increase, whereas planktonic foraminifer d13C values show little or no change. Site 748 oxygen isotope and paleontological records suggest that southern Indian Ocean surface and intermediate waters underwent significant cooling from the early to late Eocene. The rapid 1.2 per mil oxygen isotope increase recorded by benthic foraminifers just above the Eocene/Oligocene boundary represents the ubiquitous early Oligocene d18O event. The shift here is unique, however, as it coincided with the sudden appearance of ice-rafted debris (IRD), providing the first direct link between Antarctic glacial activity and the earliest Oligocene d18O increase. The d18O increase caused by the ice-volume change in the early Oligocene is constrained by (1) related changes in the planktonic to benthic foraminifer d18O gradient at Site 748 and (2) comparisons of late Eocene and early Oligocene planktonic foraminifer d18Ovalues from various latitudes. Both of these records indicate that 0.3 per mil to 0.4 per mil of the early Oligocene d18O increase was ice-volume related.

Oxygen and Carbon stable isotope ratios for Site U1334

The Oligocene-Miocene transition (OMT) (~23 Ma) is interpreted as a transient global cooling event, associated with a large-scale Antarctic ice sheet expansion. Here we present a 2.23 Myr long high-resolution (~3 kyr) benthic foraminiferal oxygen and carbon isotope (d18O and d13C) record from Integrated Ocean Drilling Program Site U1334 (eastern equatorial Pacific Ocean), covering the interval from 21.91 to 24.14 Ma. To date, five other high-resolution benthic foraminiferal stable isotope stratigraphies across this time interval have been published, showing a ~1 per mil increase in benthic foraminiferal d18O across the OMT. However, these records are still few and spatially limited and no clear understanding exists of the global versus local imprints. We show that trends and the amplitudes of change are similar at Site U1334 as in other high-resolution stable isotope records, suggesting that these represent global deep water signals. We create a benthic foraminiferal stable isotope stack across the OMT by combining Site U1334 with records from ODP Sites 926, 929, 1090, 1264, and 1218 to best approximate the global signal. We find that isotopic gradients between sites indicate interbasinal and intrabasinal variabilities in deep water masses and, in particular, note an offset between the equatorial Atlantic and the equatorial Pacific, suggesting that a distinct temperature gradient was present during the OMT between these deep water masses at low latitudes. A convergence in the d18O values between infaunal and epifaunal species occurs between 22.8 and 23.2 Ma, associated with the maximum d18O excursion at the OMT, suggesting climatic changes associated with the OMT had an effect on interspecies offsets of benthic foraminifera. Our data indicate a maximum glacioeustatic sea level change of ~50 m across the OMT.

Benthic foraminifera stable isotope data from the South Pacific sediment cores PS75/059-2 and PS75/056-1

Investigating the inter-basin deep water exchange between the Pacific and Atlantic Oceans over glacial-interglacial climate cycles is important for understanding circum-Antarctic Southern Ocean circulation changes and their impact on the global Meridional Overturning Circulation. We use benthic foraminiferal d13C records from the southern East Pacific Rise to characterize the d13C composition of Circumpolar Deep Water in the South Pacific, prior to its transit through the Drake Passage into the South Atlantic. A comparison with published South Atlantic deep water records from the northern Cape Basin suggests a continuous water mass exchange throughout the past 500 ka. Almost identical glacial-interglacial d13C variations imply a common deep water evolution in both basins suggesting persistent Circumpolar Deep Water exchange and homogenization. By contrast, deeper abyssal waters occupying the more southern Cape Basin and the southernmost South Atlantic have lower d13C values during most, but not all, stadial periods. We conclude that these values represent the influence of a more southern water mass, perhaps AABW. During many interglacials and some glacial periods, the gradient between Circumpolar Deep Water and the deeper southern Cape Basin bottom water disappears suggesting either no presence of AABW or indistinguishable d13C values of both water masses.

Neogene benthic foraminiferal community and organic carbon at DSDP Holes 95-612 and 95-613

Quantitative study of benthic foraminifers from the upper Miocene to lower Pliocene section at Site 612 (1404 m present water depth) and the Pliocene section at Site 613 (2323 m present water depth) shows no evidence of widespread downslope transport of shallow-water biofacies or reworking of older material in the greater than 150 µm size fraction. In contrast, upper Miocene sediments from Site 604 (2364 m present water depth) show extensive reworking and downslope transport. At Site 612, benthic foraminifers show a succession from an upper Miocene Bolivina alata-Nonionella sp. biofacies, to an uppermost Miocene Bulimina alazanensis biofacies, to a lower Pliocene Cassidulina reflexa biofacies, to an upper Pliocene Melonis barleeanum-Islandiella laevigata biofacies. Evidence suggests that the Pliocene biofacies are in situ, although they could have been transported downslope from the upper-middle bathyal zone. At Site 613, Uvigerina peregrina dominated the "middle" Pliocene, while Globocassidulina subglobosa was dominant in the early and late Pliocene. High abundances of U. peregrina at Site 613 are associated with high values of sedimentary organic carbon.

Early Cenozoic Atlantic and Pacific benthic foraminiferal isotopes from 12 sites

Oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions, with those of benthic foraminifera providing information on the deep oceans. Reconstructions are complicated by interspecies isotopic offsets that result from microhabitat preferences (carbonate precipitation in isotopically distinct environments) and vital effects (species-specific metabolic variation in isotopic fractionation). We provide correction factors for early Cenozoic benthic foraminifera commonly used for isotopic measurements (Cibicidoides spp., Nuttallides truempyi, Oridorsalis spp., Stensioina beccariiformis, Hanzawaia ammophila, and Bulimina spp.), showing that most yield reliable isotopic proxies of environmental change. The statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies. Interspecies isotopic offsets appear to have changed through the Cenozoic, either (1) as a result of evolutionary changes or (2) as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies. Regardless of the reason, the assumption that isotopic offsets have remained constant through the Cenozoic has introduced an 1-2°C uncertainty into deep sea paleotemperature calculations. In addition, we compare multiple species isotopic data from a western North Atlantic section that includes the Paleocene-Eocene thermal maximum to determine the most reliable isotopic indicator for this event. We propose that Oridorsalis spp. was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time; it may be the best recorder at other locations and for other extreme events also.

Paleogene benthic foraminifera from the Kerguelen Plateau

Benthic foraminifers were studied from lower Paleocene through upper Oligocene sections from Sites 747 and 748. The composition of the benthic foraminifer species suggests a middle to lower bathyal (600-2000 m) paleodepth during the Neogene and a probable upper abyssal (2000-3000 m) paleodepth during the Paleocene at Site 747. Site 748 is thought to have remained at middle to lower bathyal paleodepths throughout the Cenozoic. Principal component analysis distinguished four major benthic foraminifer assemblages: (1) a Paleocene Stensioina beccariiformis assemblage at Sites 747 and 748, (2) an early Eocene Nuttallides truempyi assemblage at lower bathyal Site 747, (3) an early through middle Eocene Stilostomella-Lenticulina assemblage at middle bathyal Site 748, and (4) a latest Eocene through Oligocene Cibicidoides-Astrononion pusillum assemblage at both sites. Major benthic foraminifer changes, as indicated by the principal components and first and last appearances, occurred at or close to the Paleocene/Eocene boundary, and in the late Eocene close to the middle/late Eocene boundary.