Deep thermohaline Circulation in the low-latitude Atlantic during the Last Glacial

Present-day low-latitude eastern and western Atlantic basins are geochemically distinct below the sill depth of the Mid-Atlantic Ridge. While Antarctic Bottom Water (AABW) circulates freely in the western Atlantic, flow into the eastern Atlantic is restricted below 4 km which results in filling the abyssal depths of this basin with water of geochemical similarity to nutrient depleted North Atlantic Deep Water. Using carbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Results indicate that deep eastern and western Atlantic basins became geochemically identical during the last glacial. This was achieved by shoaling of the upper surface of AABW above the sill depth of the Mid-Atlantic Ridge, which allowed bottom waters in both basins to be filled with the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene-glacial delta13C-PO4 shifts in this basin are in Redfield proportions, unlike the disproportionate Holocene-glacial delta13C-PO4 shifts observed in the Southern Ocean. By examining the composition of deep and intermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the delta13C-PO4 deepwater mixing line during glacial times. Evidence from high-latitude planktonic data suggests that the change in gradient of the deepwater mixing line was brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled delta13C-PO4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low-latitude glacial Atlantic were composed of a mixture of northern and southern source waters in a ratio of 1:3.

Revised age models and foraminiferal records obtained for the Last Interglacial period in six marine sediment cores

The dataset contains the revised age models and foraminiferal records obtained for the Last Interglacial period in six marine sediment cores: - the Southern Ocean core MD02-2488 (age model, sea surface temperatures, benthic d18O and d13C for the period 136-108 ka), - the North Atlantic core MD95-2042 (age model, planktic d18O, benthic d18O and d13C for the period 135-110 ka), - the North Atlantic core ODP 980 (age model, planktic d18O, sea surface temperatures, seawater d18O, benthic d18O and d13C, ice-rafted detritus for the period 135-110 ka), - the North Atlantic core CH69-K09 (age model, planktic d18O, sea surface temperatures, seawater d18O, benthic d18O and d13C, ice-rafted detritus for the period 135-110 ka), - the Norwegian Sea core MD95-2010 (age model, percentage of Neogloboquadrina pachyderma sinistral, sea surface temperatures, benthic d18O, ice-rafted detritus for the period 134-110 ka), - the Labrador Sea core EW9302-JPC2 (age model, percentage of Neogloboquadrina pachyderma sinistral, sea surface temperatures, benthic d18O for the period 134-110 ka).

Stable isotope ratios and abundances of selected foraminifera taxa from ODP Leg 172 sites

This data report describes the results of post-Leg 172 sampling of Sites 1054, 1055, and 1063 for two purposes: to investigate the climatic significance of red-colored intervals in the hemipelagic sediments cored during Leg 172 and to better understand the stratigraphy and chronology of Carolina Slope Sites 1054 and 1055. Gravity cores collected from the Carolina Slope on site survey cruise Knorr 140/2 show very high rates of sedimentation during the Holocene and lower rates during the last glacial maximum (LGM). Because of the high rates, many of the sediments in the recovered cores never reached the LGM. In other cores, it is possible that deglacial oscillations have been mistaken for the LGM. Although radiocarbon dating could solve that problem, some of the gravity cores are at or very close to the Ocean Drilling Program (ODP) sites, and it is useful to compare the isotope stratigraphies among them before proceeding with dating. Furthermore, some of the site survey cores have red-colored intervals and others do not, even though there is some indication they are time equivalent. Either the stratigraphy is wrong, diagenesis has affected the color of the sediment, or red sediment is carried to some sites but not to others that differ in depth by only a few hundred meters.

Stable carbon and oxygen isotope ratios of foraminifera in surface sediments of the South China Sea

The relationship between planktonic and benthic foraminiferal stable-isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modern surface and Last Glacial Maximum (LGM) samples from the South China Sea (SCS). The harmonic variation of benthic delta18O in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic delta18O variations. Planktonic delta18O fluctuates with sea surface temperature (SST) and salinity (SSS). The N-S temperature gradient results in planktonic delta18O decreasing from the northeast to the south. Cool, saline waters driven by the winter monsoon are interpreted to have been responsible for the high delta18O values in the northeast SCS. The East Asian monsoons not only bring nutrients into the South China Sea and maintain high nutrient concentration levels at the southwestern and northeastern ends, which cause depleted delta13C both in planktonic (surface) and benthic (bottom) samples but also reduce planktonic/benthic delta18O differences. The distribution of delta18O and delta13C in the surface and LGM samples are strikingly similar, indicating that the impact of SST and SSS has been maintained, and nutrient inputs, mainly from the northeastern and southwestern ends, have been controlled by monsoons since the LGM. Comparisons of the modern and LGM delta18O indicate a difference of about 3.6 °C in bottom-water temperature and a large surface-to-bottom temperature gradient during the LGM as compared to today.

Paleoclimate reconstructions from sediment core MD01-2378

On the basis of the radiocarbon (14C) plateau-tuning method a new age model for Timor Sea Core MD01-2378 was established. It revealed a precise centennial-scale phasing of climate events in the ocean, cryo-, and atmosphere during the last deglacial and provides important new insights into causal linkages controlling events of global climate change. At Site MD01-2378 reservoir ages of surface waters dropped from 1600 yr prior to 20 cal ka to 250-500 yr after 18.8 cal ka. This evidence was crucial for generating a high-resolution age model for deglacial events in the Indo-Pacific Warm Pool. Sea-surface temperatures (SST) started to change near 18.8 cal ka, that is ~500 yr after the start of, presumably northern hemispheric, deglacial melt and sea level rise as shown by the benthic foraminiferal oxygen isotope ratio (d18O). However, the SST rise occurred 500-1000 yr prior to the onset of deglacial Antarctic warming and the first major rise in atmospheric carbon dioxide at about 18 ka. The increase in SST may partly reflect reduced seasonal upwelling of cold subsurface waters along the eastern margin of the Indian Ocean, which is reflected by a doubling of the thermal gradient between the sea surface and the thermocline, a halving of chlorin productivity from 19 to 18.5 cal ka, and in particular, by the strong decrease in surface water reservoir ages. Two significant increases in deglacial Timor Sea surface salinities from 19-18.5 and 15.5-14.5 cal ka, may partly reflect the deglacial increase in the distance of local river mouths, partly an inter-hemispheric millennial-scale see-saw in tropical monsoon intensity, possibly linked to a deglacial increase in the dominance of Pacific El Niño regimes over Heinrich stadial 1.

Benthic and planktonic foraminifera in the Japan Sea

Biostratigraphy and paleoenvironmental history of deep and surficial waters of the Japan Sea are addressed using sequences recovered from the floor of the backarc basin. The study is divided into two parts: (1) foraminifer biostratigraphy and paleoenvironmental assessment of sedimentary sequences recovered from above igneous basement at the four sites and (2) detailed planktonic foraminifer paleoenvironmental analysis of Quaternary and Pliocene sequences from Sites 794 and 797 in the Yamato Basin. A total of 253 samples were examined for the foraminifer biostratigraphy and 325 samples for the detailed paleoenvironmental study of Quaternary and Pliocene sequences. Low abundance and sporadic occurrence of foraminifers limited interpretation of results. Foraminifer-bearing intervals were correlated where possible to diatom and calcareous nannofossil zonations, and the sequences were successfully assigned to the foraminifer zonation of Matsunaga. Unfortunately, extensive barren intervals and sporadic occurrences of planktonic foraminifers prevented zonation of Quaternary and Pliocene intervals, although some interesting conclusions about paleoenvironment were possible and are listed below. A sequence of Neogene (sensu lato) paleoenvironmental events were identified: (1) deepening of the Yamato basins to middle bathyal depths by the early to middle Miocene, an event contemporaneous with the age of some deep basins known from uplifted sections adjacent to the Japan Basin; (2) cooling of the Japan Sea in the early middle Miocene; (3) oxygenation of deep waters in the late Miocene; (4) further cooling of surficial water masses between the Olduvai Subchron and the Brunhes/Matuyama Boundary; and (5) extermination of lower middle bathyal faunas and replacement by upper middle bathyal faunas near the base of the Quaternary.

Isotope ratios of osmiun, carbon and oxygen of Miocene sediments and benthic foraminifera

Seawater 187Os/188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os/188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os/188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os/188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187Os/188Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87Sr/86Sr ratios at 16 Ma. A large increase of 1? in benthic foraminiferal delta18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os/188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os/188Os ratios and also nearly constant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187Os/188Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187Os/188Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187Os/188Os variations may correlate inversely with short-term benthic foraminiferal delta18O and bulk carbonate delta13C variations that reflect glacioeustatic events.

Stable isotope record of ODP Sites 108-658 and 108-659

Ocean Drilling Program Site 658, cored below a major upwelling cell offshore Cap Blanc, contains a largely undisturbed hemipelagic sediment section spanning the Brunhes Chron and the early Quaternary and late Pliocene. The companion Site 659 recovered a complete and undisturbed Neogene profile further offshore that serves as a nonupwelling pelagic reference section. Oxygen and carbon isotope ratios in benthic (C. wuellerstorfi and in part Uvigerina sp.) and planktonic foraminifers (G. inflata) provide a climatic record of high resolution for the Brunhes Chron. At Site 658 the record extends back to the early Pleistocene and late Pliocene. The standard oxygen isotope record of the last 730,000 yr is markedly refined by a well-documented high-frequency variation (e.g., by a new "aborted" ice age at stage 13.2 and by Younger-Dryas style climatic setbacks during most terminations). In the late Pliocene, the numerical oxygen isotope stage taxonomy was extended back to stage 137 about 3.3 Ma ago. In comparison with published records, stage 114 at 2.7 Ma represents the first major glaciation event, when 18O was short-term enriched up to a middle Pleistocene glacial d18O level. About 3.17 Ma ago (stage 133), the interglacial oxygen isotope values of C. wuellerstorfi started to increase by 0.5 per mil until 2.7 Ma and then remained largely constant until the Holocene. Based on the d13C difference between C. wuellerstorfi and G. inflata, the dissolved CO2 in the ambient bottom water of Site 658 was dominated by the flux of particulate carbon from the overlying upwelling cell during the last 630,000 yr. In contrast, the advection of (upper) North Atlantic Bottom Water dominated in the control of the local CO2 content during the early Pleistocene and late Pliocene.

Assamblages and cluster analysis from DSDP Hole 22-214 of the eastern Indian Ocean

Multivariate analyses of latest Pliocene through Holocene benthic foraminifera from 61 samples from Deep-Sea Drilling Project (DSDP) Site 214, eastem Indian Ocean were carried out. The 46 highest ranked species were used in R-mode factor analysis which has enabled to the identification of three environmentally significant assemblages at Site 214. Assemblage 1 is characterized by Uvigerina hispido-costata, Osangularia culter , Gavelinopsis lobatulus, Cibicides wuellerstorfi and Karreriella baccata as principal species. This assemblage is inferred to reflect high-energy, well-oxygenated and probably low-organic carbon deep-sea environment at Site 214. Assemblage 2 is defined principally by Globocassidulina pacifica and U. proboscidea and is considered to indicate an organic carbon-rich environment which resulted from high surface productivity irrespective of dissolved oxygen content. Assemblage 3 is marked by Oridorsalis umbonatus, Textularia lythostrota, Hoeglundina elegans, Pyrgo murrhina, and Pullenia quinqueloba as principal species. This assemblage is inferred to indicate a low-organic carbon environment with high pore water oxygen concentration leading to better preservation of deep-sea sediments.

Oxygen- and carbon-isotopes at DSDP Leg 73 Holes

We established a composite oxygen- and carbon-isotope stratigraphy for the Pliocene in the central South Atlantic. Monospecific samples of benthic and planktonic foraminifers from pelagic sediments from DSDP Sites 519, 521, 522, and 523 were analyzed isotopically. The resulting benthic oxygen-isotope stratigraphy allowed three paleoclimatic periods in the Pliocene to be distinguished. During the early Pliocene (5.2-3.3 Ma), low-amplitude climatic changes prevailed in a world that was less glaciated than during the Pleistocene. A net increase in global ice volume is documented in a 0.5 permil positive shift in the average 18O composition of the benthic foraminifers at 3.2 Ma. The middle Pliocene (3.3-2.5 Ma) is not only characterized by a more widespread glaciation of the Southern and Northern hemispheres but also by more drastic isotopic differences between glacial and interglacial times. A minor shift in the average 18O composition of the benthic foraminifers marks the beginning of the late Pliocene-early Pleistocene climatic period (2.5-1.1 Ma). Alternating cold and warm climate is documented in both the oxygen-isotope record and in the pelagic sediments. During cold periods, sediments with a lower CaCO3 content indicate more corrosive bottom-water conditions. More negative 13C signals in the benthic foraminifers from these sediments suggest that the Antarctic Bottom Water current was intensified in glacial times. The oxygen-isotope composition of the measured planktonic foraminifers suggests that the surface water in this part of the South Atlantic remained relatively warm during the growth of the Pliocene glaciers.

Age determination, planktic foraminifera and stable istopes of sediment core MSM05/5_712-2

A sediment core from the West Spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last ~9 ka in the eastern Fram Strait. Our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern Fram Strait occurred stepwise, in response to the postglacial sea-level rise and the related onset of modern sea-ice production on the shallow Siberian shelves. The late Early and Mid Holocene interval (9 to 5 ka) was generally characterized by relatively unstable conditions. High abundance of the subpolar planktic foraminifer species Turborotalita quinqueloba implies strong intensity of Atlantic Water (AW) inflow with high productivity and/or high AW temperatures, resulting in a strong heat flux to the Arctic. A series of short-lived cooling events (8.2, 6.9. and 6.1 ka) occurred superimposed on the warm late Early and Mid Holocene conditions. Our proxy data imply that simultaneous to the complete postglacial flooding of Arctic shallow shelves and the initiation of modern sea-ice production, strong advance of polar waters initiated modern oceanographic conditions in the eastern Fram Strait at ~5.2 ka. The Late Holocene was marked by the dominance of the polar planktic foraminifer species Neogloboquadrina pachyderma, a significant expansion of sea ice/icebergs, and strong stratification of the water column. Although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface Atlantic Water after 3 ka, a relatively stable cold-water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the Neoglacial period.

Sedimentology, foraminiferal assemblage, and isotope record of the Agulhas Bank region

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

Stable isotope ratios and foraminiferal abundance of sediment cores from the Western Iberian Margin

Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500-600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean-climate responses on similar and even faster time scales.

Response of deep-sea benthic foraminifera to Late Quaternary climate changes, southeast Indian Ocean, offshore Western Australia

The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122-760A, ODP 122-762B, BMR96GC21 and RC9-150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The delta13C trends of Cibicidoides wuellerstorfi, and differences between the delta13C trends of planktics (Globigerinoides sacculifer) and benthics (C. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in delta13C, and strong delta13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, delta13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated by Uvigerina peregrina, another dominated by U. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and of Epistominella exigua and Bulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum delta13C values of C. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.