Stable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from the Holocene and the Last Glacial Maximum

We measured oxygen isotopes and Mg/Ca ratios in the surface-dwelling planktonic foraminifer Globigerinoides ruber (white s.s.) and the thermocline dweller Pulleniatina obliquiloculata to investigate upper ocean spatial variability in the Indo-Pacific Warm Pool (IPWP). We focused on three critical time intervals: the Last Glacial Maximum (LGM; 18-21.5 ka), the early Holocene (8-9 ka), and the late Holocene (0-2 ka). Our records from 24 stations in the South China Sea, Timor Sea, Indonesian seas, and western Pacific indicate overall dry and cool conditions in the IPWP during the LGM with a low thermal gradient between surface and thermocline waters. During the early Holocene, sea surface temperatures increased by ~3°C over the entire region, indicating intensification of the IPWP. However, in the eastern Indian Ocean (Timor Sea), the thermocline gradually shoaled from the LGM to early Holocene, reflecting intensification of the subsurface Indonesian Throughflow (ITF). Increased surface salinity in the South China Sea during the Holocene appears related to northward displacement of the monsoonal rain belt over the Asian continent together with enhanced influx of saltier Pacific surface water through the Luzon Strait and freshwater export through the Java Sea. Opening of the freshwater portal through the Java Sea in the early Holocene led to a change in the vertical structure of the ITF from surface- to thermocline-dominated flow and to substantial freshening of Timor Sea thermocline waters.

Ice-rafted debris record in the Prydz Bay

Ice-rafted debris mass accumulation rates (IRD MAR) at a drill site on the Antarctic continental margin are investigated to evaluate the linkages between East Antarctic Ice Sheet extent and Southern Ocean temperatures in the early to mid-Pliocene. ODP Site 1165 is within 400 km of the Antarctic coastline and in the direct pathway of icebergs released by the Amery Ice Shelf. The Amery Ice Shelf is the largest ice shelf in East Antarctica and it buttresses the Lambert Glacier drainage system, which accounts for 14% of the outflow from the East Antarctic Ice Sheet. IRD MAR were low during peak Southern Ocean warming in the early Pliocene. After a brief precursor, a tenfold increase in IRD MAR at 3.3 Ma marks the termination of the early Pliocene ice sheet minimum, coincident with the M2 glacial. For the mid-Pliocene, a strong correlation exists between the high-amplitude signal in the LR04 benthic stack and IRD MAR, suggesting linkages between East Antarctic ice extent, global ice volume and deep-water temperatures. The IRD record at Site 1165 provides evidence of greater sensitivity of the Lambert Glacier-Amery Ice Shelf system to Southern Ocean warming than is currently predicted by ice sheet models, which may relate to uncertainties in the understanding of ocean heat uptake, poleward heat transport and ice sheet-ocean interactions.

Calcium carbonate, sedimentation and mass accumulation rates at DSDP Site 85-572

At Site 572, located at 1°N, 114° W (3903 m water depth), we recovered a continuous hydraulic piston cored section of upper Miocene to upper Pleistocene pelagic sediments. The sediment is composed of biogenic carbonate and silica with nonbiogenic material as a minor component. Detailed analysis of the calcium carbonate content shows that the degree of variability in carbonate deposition apparently changed markedly between the late Miocene and Pliocene at this equatorial Pacific site. During this interval carbonate mass accumulation rates decreased from 2.6 to 0.8 g/cm**2 per 10**3 yr. If we assume that variations in CaCO3 content reflect changes in the degree of dissolution, then the detailed carbonate analysis would suggest that the degree of variability in carbonate deposition decreases by a factor of 5 as the dominant wavelength of variations increases significantly. However, if the variability in carbonate concentration is described in terms of changes in mean mass accumulation, calculations then suggest that relatively small changes in noncarbonate rates may be important in controlling the observed carbonate records. In addition, the analysis suggests that the degree of variability observed in pelagic carbonate data may in part reflect total accumulation rates. Intervals with high sedimentation rates show lower amplitude variations in concentration than intervals with lower sedimentation rates for the same degree of change in the carbonate accumulation rate.

Neogene ice-rafted debris record of ODP Hole 120-751A

One of the primary objectives of Leg 120 was to obtain a high-resolution Neogene stratigraphic section from the Kerguelen Plateau. Site 751, located in the central part of the Raggatt Basin on the Southern Kerguelen Plateau in 1633.8 m of water (57°43.56'S; 79°48.89'E), was selected as the dedicated Neogene site for this objective. High-resolution sampling at Site 751 was used to delineate in detail the Neogene ice-rafted debris (IRD) occurrences on the Kerguelen Plateau. The oldest IRD found at Site 751 was approximately 9.9 Ma, and it was not until approximately 8.5 Ma that significant concentrations of IRD were detected. The first major IRD event at this site occurred in the uppermost Miocene between 6.0 and 5.5 Ma. During this time period, a general climatic cooling and glacial expansion occurred on Antarctica. The late Miocene IRD event was followed by a continuous episode of elevated IRD deposition in the lowermost Pliocene between 4.5 and 4.1 Ma. The 0.4-m.y. duration and the timing of the early Pliocene IRD event on the Kerguelen Plateau corresponds with IRD fluxes observed on the Falkland Plateau and in the Weddell Abyssal Plain. This correspondence of data indicates that a major global climatic event occurred during the early Pliocene. The East Antarctic Ice Sheet may have experienced deglaciation between 4.5 and 4.1 Ma and, as a result, released large volumes of sediment-laden ice into the Southern Ocean.

Mid-Pleistocene benthic foraminiferal record in the Southwest Pacific

Benthic foraminiferal faunas from three bathyal sequences provide a proxy record of oceanographic changes through the mid-Pleistocene transition (MPT) on either side of the Subtropical Front (STF), east of New Zealand. Canonical correspondence analyses show that factors related to water depth, latitude and climate cycles were more significant than oceanographic factors in determining changes in faunal assemblage composition over the last 1 Ma. Even so, mid-Pleistocene faunal changes are recognizable and can be linked to inferred palaeoceanographic causes. North of the largely stationary STF the faunas were less variable than to the south, perhaps reflecting the less extreme glacial-interglacial fluctuations in the overlying Subtropical Surface Water. Prior to Marine Isotope Stage (MIS) 21 and after MIS 15, the northern faunas had fairly constant composition, but during most of the MPT faunal composition fluctuated in response to climate-related food-supply variations. Faunal changes through the MPT suggest increasing food supply and decreasing dissolved bottom oxygen. South of the STF, beneath Subantarctic Surface Water, mid-Pleistocene faunas exhibited strong glacial-interglacial fluctuations, inferred to be due to higher interglacial nutrient supply and lower oxygen levels. The most dramatic faunal change in the south occurred at the end of the MPT (MIS 17- 12). with an acme of Abditodentrix pseudothalmanni, possibly reflecting higher carbon flux and lower bottom oxygen. This study suggests that the mid-Pleistocene decline and extinction of a group of elongate, cylindrical deep-sea foraminifera may have been related to decreased bottom oxygen concentrations as aresult of slower deep-water currents.

Mineralogical and grain size analysis of ODP Hole 119-745B from the Southern Ocean

The upper Miocene to Pleistocene sediments recovered at ODP Sites 745 and 746 in the Australian-Antarctic Basin are characterized by cyclic facies changes. Sedimentological investigations of a detailed Quaternary section reveal that facies A is dominated by a high content of siliceous microfossils, a relatively low terrigenous sediment content, an ice-rafted component, low concentrations of fine sediment particles, and a relatively high smectite content. This facies corresponds to interglacial sedimentary conditions. Facies B, in contrast, is characteristic of glacial conditions and is dominated by a large amount of terrigenous material and a smaller opaline component. There is also a prominent ice-rafted component. The microfossils commonly are reworked and broken. The clay mineral assemblages show higher proportions of glacially derived illite and chlorite. A combination of four different processes, attributed to glacial-interglacial cycles, was responsible for the cyclic facies changes during Quaternary time: transport by gravity, ice, and current and changes in primary productivity. Of great importance was the movement of the grounding line of the ice shelves, which directly influenced the intensity of ice rafting and of gravitational sediment transport to the deep sea. The extension of the ice shelves was also responsible for the generation of cold and erosive Antarctic Bottom Water, which controlled the grain-size distribution, particularly of the fine fraction, in the investigated area.

Sedimentology and stable isotope record of DSDP Hole 68-503B

Samples from the upper portion of a cyclic pelagic carbonate sediment sequence in Deep-Sea Drilling Project (DSDP) hole 503B (4.0°N, 95.6°W) are the first group to be analyzed for paleoceanographic and paleoclimatic proxy-indicators of ice volume, deep ocean and surface water circulation, and atmospheric circulation in order to resolve the complex origin of the cyclicity. Temporal resolution is taken from the delta18O time scale, most other parameters are calculated in terms of their mass flux to the seafloor. CaCO3 percent in the sediments fluctuates in the well-known Pacific pattern and is higher during glacial times. The fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2.5 to 4. The longer organic carbon record shows flux maxima during both glacial and interglacial times. The accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and/or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods. Eolian dust fluxes are greater during interglacial periods by factors of 2 to 5, indicating that eolian source regions in central and northern South America were more arid during interglacial periods. The record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds. The eolian data suggest more intense atmospheric circulation during interglacial periods, opposite to the anticipated results. We interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503B during glaciations.

A reconstruction of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the glacial inception

The reconstruction of the stable carbon isotope evolution in atmospheric CO2 (d13Catm ), as archived in Antarctic ice cores, bears the potential to disentangle the contributions of the different carbon cycle fluxes causing past CO2 variations. Here we present a new record of d13Catm before, during and after the Marine Isotope Stage 5.5 (155 000 to 105 000 years BP). The record was derived with a well established sublimation method using ice from the EPICA Dome C (EDC) and the Talos Dome ice cores in East Antarctica. We find a 0.4 permil shift to heavier values between the mean d13Catm level in the Penultimate (~ 140 000 years BP) and Last Glacial Maximum (~ 22 000 years BP), which can be explained by either (i) changes in the isotopic composition or (ii) intensity of the carbon input fluxes to the combined ocean/atmosphere carbon reservoir or (iii) by long-term peat buildup. Our isotopic data suggest that the carbon cycle evolution along Termination II and the subsequent interglacial was controlled by essentially the same processes as during the last 24 000 years, but with different phasing and magnitudes. Furthermore, a 5000 years lag in the CO2 decline relative to EDC temperatures is confirmed during the glacial inception at the end of MIS 5.5 (120 000 years BP). Based on our isotopic data this lag can be explained by terrestrial carbon release and carbonate compensation.

Late Pliocene carbonate mineralogy of ODP Hole 133-818B

Shedding of shallow carbonate material toward the deep slopes and basin floors is clearly tied to the position of the carbonate bank tops relative to the photic zone. The onset of bank shedding in periplatform sediments can record either the flooding of the bank tops within the photic zone during a rise in sea level following a period of exposure, referred to in the literature as the "highstand shedding" scenario, or the reentry of the bank tops into the photic zone during a lowering of sea level following a period of drowning, referred to as the "lowstand shedding" scenario. Results from Leg 133 post-cruise research on the Pliocene sequences, drilled in six sites within different slope settings of the Queensland Plateau, seem to point out that the latter "lowstand shedding" scenario can be applied to this particular carbonate system. At the Queensland Plateau sites, the early Pliocene (5.2-3.5 Ma) and the earliest part of the late Pliocene (3.5-2.9 Ma) age sequences were characterized, especially in the ôdeepö Sites 811 and 817, by pelagic sediments (foraminifers and coccoliths) and by typically pelagic sedimentation rates not exceeding 20 mm/k.y. The earliest part of the late Pliocene age section was characterized by well-developed hardgrounds in the "shallow" Sites 812 and 814 and by normal pelagic sediments mixed with reworked phosphatized planktonic foraminifers in Site 813. Finally, the early part of the late Pliocene (2.9-2.4 Ma) section was characterized by high sedimentation rates, related to the shedding and admixture into the pelagic sediments of bank-derived materials. These bank-derived materials consist of either diagenetically unaltered fine aragonite with traces of dolomite in Site 818 or micritic calcite resulting from seafloor and/or shallow burial alteration in the deepest Sites 817 and 811. The highest sedimentation rates (163 mm/k.y.) were recorded in Site 818, drilled nearest the modern carbonate bank of Tregrosse Reef. The sedimentation rates decrease with increasing distance from Tregrosse Reef - 120 mm/k.y. in Site 817 and 47.5 mm/k.y. in Site 811. The initial appearance of fine aragonite in Site 818, corresponding to the transition from pelagic to periplatform sedimentation rates, has been dated at 2.9 Ma. This Pliocene sediment pattern on the Queensland Plateau is different from the pattern observed in sediments from two earlier ODP legs (i.e., Leg 101 in the Bahamas and in Leg 115 in the Maldives), where aragonite-rich sediments, characterized by high periplatform sedimentation rates, were observed in the lower Pliocene section (5.2-3.5 Ma), whereas the upper Pliocene (3.5-1.6 Ma) sediments are more pelagic in nature and are characterized by low sedimentation rates or major hiatuses. These Pliocene periplatform sequences in the Bahamas and in the Maldives and late Quaternary age periplatform sequences worldwide have pointed out that "highstand shedding" was the typical response of carbonate platforms to fluctuations in sea level, just opposite to a "lowstand shedding" response to sea-level fluctuations, typical of siliciclastic shelves. Assuming that the envelope of Haq et al.'s (1987) sea-level curve, showing a well-defined lowering of sea level between 3.5 and 2.9 Ma, can also be applied to the southwest Pacific Ocean, based on a high-resolution Pliocene d18O record from the Ontong Java Plateau recently published by Jansen et al. (1993, doi:10.2973/odp.proc.sr.130.028.1993), the Pliocene periplatform sequences on the Queensland Plateau would have recorded the reentry of the bank tops into the photic zone during a general lowering of sea level, following an interval characterized by high sea level, during which the shallow carbonate system on the Queensland Plateau was drowned. The early Pliocene age (5.2-3.5 Ma) sediments deposited on the Queensland Plateau, an established interval of eustatic sea-level highstand, are typically pelagic in character. In addition, relatively cold surface temperatures (estimated to have ranged from 18° to 20°C by Isern et al. [this volume]) might have also stressed the reefs during early Pliocene time and contributed to the drowning of the Queensland Plateau carbonate system during the late Miocene and early Pliocene. Differential and relatively high subsidence rates, inferred by variations in paleodepth of water (based upon benthic foraminifer assemblages; Katz and Miller, this volume) may also have influenced the drowning of the carbonate bank tops on the Queensland Plateau during the late Miocene and early Pliocene. The sediments of early late Pliocene age (2.9-2.4 Ma), a well-established interval of lowering of sea level, are clearly periplatform and cyclic in nature. High-frequency (~40 k.y.) aragonite cycles, well-developed between 2.9 and 2.45 Ma, correlate with the planktonic high-resolution Pliocene d18O record from the Ontong Java Plateau, a good sea-level proxy (Jansen et al., in press). Contrary to late Quaternary age aragonite cycles from the Bahamas, the Nicaragua Rise, the Maldives, and the Queensland Plateau, the late Pliocene aragonite cycles in Hole 818B display high levels of aragonite during glacial stages and, therefore, lowstands of sea level. In addition, sediments deposited during the earliest part of the late Pliocene (3.5-2.9 Ma), transition between the early Pliocene highstand and the late Pliocene lowering in sea level, have recorded the first evidence of a fall in sea level, by (1) the occurrence of synchronous submarine hardgrounds in the two shallowest sites (Sites 812 and 814), (2) the deposition of reworked material from the shallower part of the slope into the intermediate Sites 813 and 818, and (3) the deposition of pelagic sediments in the deepest Sites 817 and 817. In summary, contrary to previous findings, the Pliocene periplatform sediments on the Queensland Plateau appear to have recorded a regional shedding of shallow carbonate bank tops during an interval of sea-level lowering, a good illustration of the "carbonate lowstand shedding" scenario, occurring during the reentry of previously drowned carbonate bank tops into the photic zone related to a decrease in sea level.

Pliocene-Plistocene deposition of carbonate and organic carbon in ODP Hole 159-959C

During Ocean Drilling Program (ODP) Leg 159, four sites (Sites 959-962) were drilled along a depth transect on the Côte d'Ivoire/Ghana Transform Margin. In this study, the Pliocene-Pleistocene history of carbonate and organic carbon accumulation at Hole 959C is reconstructed for the eastern equatorial Atlantic off the Ivory Coast/Ghana based on bulk carbonate, sand fraction, organic carbon, and other organic geochemical records (d13Corg, marine organic matter percentages derived from organic petrology, hydrogen index, C/N). Pliocene-Pleistocene sedimentation off the Ivory Coast/Ghana was strongly affected by low mean sedimentation rates, which are attributed to persistently enhanced bottom-water velocities related to the steep topography of the transform margin. Sand fraction and bulk carbonate records reveal typical glacial/interglacial cycles, preserved, however, with low time resolution. Intermediate carbonate accumulation rates observed throughout the Pliocene-Pleistocene suggest intense winnowing and sediment redistribution superimposed by terrigenous dilution. 'Atlantic-type' sand and carbonate cycles, consistent with records from pelagic areas of the eastern equatorial Atlantic, are encountered at Hole 959C prior to about 0.9 Ma. Total organic carbon (TOC) records are frequently inversely correlated to carbonate contents, indicating mainly productivity-driven carbonate dissolution related to changes in paleoproductivity. During Stages 22-24, 20, 16, 12, 8, and 4, sand and carbonate records reveal a 'Pacific-type' pattern, showing elevated contents during glacials commonly in conjunction with enhanced TOC records. Formation of 'Pacific-type' patterns off the Ivory Coast/Ghana is attributed to drastically increased bottom-water intensities along the transform margin in accordance with results reported from the Walvis Ridge area. Short-term glacial/interglacial changes in paleoproductivity off the Ivory Coast/Ghana are to some extend recognizable during glacials prior to 1.7 Ma and interglacial Stages 21, 19, 13, 9, and 1. Enhanced coastal upwelling during interglacials is attributed to local paleoclimatic and oceanographic conditions off the Ivory Coast/Ghana. Quantitative estimates of marine organic carbon based on organic petrologic and d13Corg records reveal an offset in concentration ranging from 15% to 60%. Highest variabilities of both records are recorded since ~0.9 Ma. Discrepancies between the isotopic and microscopic records are attributed to an admixture of C4 plant debris approaching the eastern equatorial Atlantic via atmospheric dust. Terrestrial organic material likely originated from the grass-savannah-covered Sahel zone in central Africa. Estimated C4 plant concentrations and accumulation rates range from 10% to 37% and from almost zero to 0.006 g/cm**2/k.y., respectively. The strongest eolian supply to the northern Gulf of Guinea is indicated between 1.9 and 1.68 Ma and during glacial isotopic Stages 22-24, 20, 14, and 12. The presence of grass-type plant debris is further supported by organic petrologic studies, which reveal well-preserved cell tissues of vascular plants or tube-shaped, elongated terrestrial macerals showing different levels of oxidation.

Organic carbon accumulation in the equatorial Atlantic

Organic geochemical records of the last 940 kyr are presented for equatorial Atlantic Ocean Drilling Program (ODP) sites 663 and 664 and discussed with regard to the development of ocean productivity and African paleoclimate. Proportions of marine and terrigenous organic matter (OM) are estimated from elemental, pyrolytic, isotopic, and petrologic data. Spectral analyses reveal a strong power at the eccentricity and obliquity band, indicating a close response of tropical organic sedimentation to the climatic evolution at high latitudes. The orbital covariance of organic carbon with biogenous opal and terrigenous records favor that glacially enhanced dust supply and surface water mixing were primary controls for deposition of organic carbon. Wind-borne supply of terrigenous OM contributes 26 to 55% and 0 to 39% to the bulk OM based on microscopic and isotopic records, respectively. Admixture of C4 plant matter was approximated to contribute up to 16% to the bulk organic fraction during peak glacial conditions.

Geochemistry of sediment cores of the Southern Ocean

We examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic Cd inventory, thereby decoupling Cd from PO4. We suggest that the precipitation of Cd-sulfide in suboxic sediments is the single largest sink in the oceanic Cd budget and that the accumulation of authigenic Cd and U is tightly coupled to the organic carbon flux into the seafloor. Sediments from the Subantarctic Ocean and the Cape Basin (South Atlantic), where oxic conditions currently prevail, show high accumulation rates of authigenic Cd and U during glacial intervals associated with increased accumulation of organic carbon. These elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon. A third core, overlain by Circumpolar Deep Water (CPDW) as are the other two cores but located south of the Antarctic Polar Front, shows an approximately inverse pattern to the Subantarctic record. The contrasting patterns to the north and south of the Antarctic Polar Front suggest that higher accumulation rates of Cd and U in Subantarctic sediments were driven primarily by increased productivity. This proposal is consistent with the hypothesis of glacial stage northward migration of the Antarctic Polar Front and its associated belt of high siliceous productivity. However, the increase in authigenic Cd and U glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts, suggesting greater partitioning of organic carbon into the sediments during glacial intervals. Lower oxygen content of CPDW and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes. Higher glacial productivity in the Cape Basin record very likely reflects enhanced coastal up-welling in response to increased wind speeds. We suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum. However, our calculations suggest low sensitivity of seawater Cd concentrations to glacial doubling of the extent of reducing sediments. The model suggests that during the last 250 kyr seawater Cd concentrations fluctuated only slightly, between high levels (about 0.66 nmol/kg) on glacial initiations and reaching lowest values (about 0.57 nmol/kg) during glacial maxima. The estimated 5% lower Cd content at the last glacial maximum relative to modern levels (0.60 nmol/kg) cannot explain the discordance between Cd and delta13C, such as observed in the Southern Ocean. This low sensitivity is consistent with foraminiferal data, suggesting minimal change in the glacial Cd mean oceanic content.

Analysis of macerals in sediment core 159-959C

Organic petrologic and geochemical analyses were performed on modern and Quaternary organic carbon-poor deep sea sediments from the Equatorial Atlantic. The study area covers depositional settings from the West African margin (ODP Site 959) through the Equatorial Divergence (ODP Site 663) to the pelagic Equatorial Atlantic. Response of organic matter (OM) deposition to Quaternary climatic cycles is discussed for ODP Sites 959 and 663. The results are finally compared to a concept established for fossil deep sea environments [Littke and Sachsenhofer, 1994 doi:10.1021/ef00048a041]. Organic geochemical results obtained from Equatorial Atlantic deep sea deposits provide new aspects on the distribution of sedimentary OM in response to continental distance, atmospheric and oceanographic circulation, and depositional processes controlling sedimentation under modern and past glacial-interglacial conditions. The inventory of macerals in deep sea deposits is limited due to mechanical breakdown of particles, degree of oxidation, and selective remineralization of labile (mostly marine) OM. Nevertheless, organic petrology has a great potential for paleoenvironmental studies, especially as a proxy to assess quantitative information on the relative abundance of marine vs. terrigenous OM. Discrepancies between quantitative data obtained from microscopic and isotopic (delta13Corg) analyses were observed depending on the stratigraphic level and depositional setting. Strongest offset between both records was found close to the continent and during glacial periods, suggesting a coupling with wind-born terrigenous OM from central Africa. Since African dust source areas are covered by C4 grass plants, supply of isotopically heavy OM is assumed to have caused the difference between microscopic and isotopic records.