Stable isotope record and lithic fragments distribution in sediments of MIS9-14 of the North Atlantic

Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9-14 (300-540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north - south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.

Sedimentology on 19 cores from the South China Sea

Based on the study of 10 sediment cores and 40 core-top samples from the South China Sea (SCS) we obtained proxy records of past changes in East Asian monsoon climate on millennial to bidecadal time scales over the last 220,000 years. Climate proxies such as global sea level, estimates of paleotemperature, salinity, and nutrients in surface water, ventilation of deep water, paleowind strength, freshwater lids, fluvial and/or eolian sediment supply, and sediment winnowing on the sea floor were derived from planktonic and benthic stable-isotope records, the distribution of siliciclastic grain sizes, planktonic foraminifera species, and the UK37 biomarker index. Four cores were AMS-14C-dated. Two different regimes of monsoon circulation dominated the SCS over the last two glacial cycles, being linked to the minima and maxima of Northern Hemisphere solar insolation. (1) Glacial stages led to a stable estuarine circulation and a strong O2-minimum layer via a closure of the Borneo sea strait. Strong northeast monsoon and cool surface water occurred during winter, in part fed by an inflow from the north tip of Luzon. In contrast, summer temperatures were as high as during interglacials, hence the seasonality was strong. Low wetness in subtropical South China was opposed to large river input from the emerged Sunda shelf, serving as glacial refuge for tropical forest. (2) Interglacials were marked by a strong inflow of warm water via the Borneo sea strait, intense upwelling southeast of Vietnam and continental wetness in China during summer, weaker northeast monsoon and high sea-surface temperatures during winter, i.e. low seasonality. On top of the long-term variations we found millennial- to centennial-scale cold and dry, warm and humid spells during the Holocene, glacial Terminations I and II, and Stage 3. The spells were coeval with published variations in the Indian monsoon and probably, with the cold Heinrich and warm Dansgaard-Oeschger events recorded in Greenland ice cores, thus suggesting global climatic teleconnections. Holocene oscillations in the runoff from South China centered around periodicities of 775 years, ascribed to subharmonics of the 1500-year cycle in oceanic thermohaline circulation. 102/84-year cycles are tentatively assigned to the Gleissberg period of solar activity. Phase relationships among various monsoon proxies near the onset of Termination IA suggest that summer-monsoon rains and fluvial runoff from South China had already intensified right after the last glacial maximum (LGM) insolation minimum, coeval with the start of Antarctic ice melt, prior to the d18O signals of global sea-level rise. Vice versa, the strength of winter-monsoon winds decreased in short centennial steps only 3000-4000 years later, along with the melt of glacial ice sheets in the Northern Hemisphere.

Stable carbon and oxygen isotope record of Cibicidoides from DSDP/ODP holes

High-resolution delta18O records from the equatorial Pacific (site 503B), equatorial Atlantic (site 665A), and North Atlantic (site 606A) based on the benthic foraminifera Cibicidoides wuellerstorfi show the 2.4 Ma onset of major northern hemispheric glaciation to be a package of three events occurring at 2.39, 2.35, and 2.31 Ma in which a periodicity of about 40 kyr is evident. The amplitude of the signals at the three sites indicates that these events were 1/2 to 2/3 the size of the latest Quaternary glaciation and also indicates cooling of northern source bottom water by 2.7°-4.1°C relative to southern source water during glaciations. Carbon isotopes indicate that southern source waters were less oxygenated than in the Quaternary and that there was reduced production of northern source water during glacial intervals. The dominant presence of southern source water in the eastern basin of the equatorial Atlantic, regardless of climatic cycles, throughout the late Pliocene indicates a greater influence of these waters relative to northern source waters in the late Pliocene ocean.

Pliocene-Pleistocene stable oxygen isotope record of Cibicidoides wuellerstorfi from the North Atlantic

We generated benthic isotope records from Ocean Drilling Program (ODP) site 981 on the Feni drift (2173 m water depth) and from ODP site 983 on the Gardar drift (1983 m water depth) to examine the interaction between North Atlantic Deep Water (NADW) and Glacial North Atlantic Intermediate Water (GNAIW) formation from 2.0 to 1.4 Ma. We find NADW at both sites during interglacial periods, and a mix of NADW and Southern Ocean water at the Feini drift during most glacial periods. Prior to 1.7 Ma we find no evidence ofr GNAIW at the Gardar drift site. Instead, glacial Gardar drift delta13C values are as low or lower than values for all other sites in the North Atlantic and reflect continued glacial overflow from the Nordic seas. After 1.7 Ma Gardar drift delta13C values increase and suggest that there was GNAIW at the Gardar drift site during some glacial intervals. Overall, we find that NADW and GNAIW production changed around 1.7 Ma in concert with changes in sea surface temperature and salinity and in the Earth's obliquity cycle.

Authigenic uranium, mass accumulation rates, benthic d13C and SST during the last interglacial period of ODP Site 177-1094

Southern Ocean sediments reveal a spike in authigenic uranium 127,000 years ago, within the last interglacial, reflecting decreased oxygenation of deep water by Antarctic Bottom Water (AABW). Unlike ice age reductions in AABW, the interglacial stagnation event appears decoupled from open ocean conditions and may have resulted from coastal freshening due to mass loss from the Antarctic ice sheet. AABW reduction coincided with increased North Atlantic Deep Water (NADW) formation, and the subsequent reinvigoration in AABW coincided with reduced NADW formation. Thus, alternation of deep water formation between the Antarctic and the North Atlantic, believed to characterize ice ages, apparently also occurs in warm climates.

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.

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 record of foraminifera from ODP Hole 114-704A

Isotopic and sedimentologic data from Ocean Drilling Program hole 704A suggest that isotopic stages 7, 9, and 11 were marked by unusually strong interglacial conditions in surface waters of the southern ocean. During interglacial stages 9 and 11, warm surface waters penetrated far poleward and may have led to destabilization of the West Antarctic Ice Sheet. In contrast, the strongest glacial conditions in surface waters of the subantarctic South Atlantic occurred during oxygen isotopic stage 12. Comparisons of benthic carbon isotopic gradients between sites located in the North Atlantic, southern ocean, and Pacific indicate that the production of upper North Atlantic Deep Water (uNADW) was strongest during stages 7,9, and 11 and weakest during stage 12, These results suggest a possible link between the flux of uNADW and paleoceanographic change in the southern ocean and support the traditional NADW-Antarctic connection whereby increased NADW leads to warming of the southern ocean.

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.

Oxygen- and carbon-isotopes of Globigerina bulloides and Neogloboquadrina pachyderma at DSDP Holes 80-548 and 80-549A

Oxygen- and carbon-isotope analyses have been performed on the Quaternary planktonic foraminifers of Sites 548 and 549 (DSDP Leg 80) to investigate major water mass changes that occurred in the northeastern Atlantic at different glacial-interglacial cycles and to compare them with the well-defined picture of 18,000 yr. ago. Oxygen-isotope stratigraphy also provides a chronological framework for the more important data on the fauna and flora. Although bioturbation and sedimentary gaps obliterate the climatic and stratigraphic record, general trends in the oceanographic history can be deduced from the isotopic data. Isotopic stratigraphy has tentatively been delineated down to isotopic Stage 16 at Site 548 and in Hole 549A. This stratigraphy fits well with that deduced from benthic foraminiferal d18O changes and with bioclimatic zonations based on foraminiferal associations at Site 549. Variations in the geographic extension and in the flux of the Gulf Stream subtropical waters are inferred from both d18O and d13C changes. Maximal fluxes occurred during the late Pliocene. Northward extension of subtropical waters increased through the various interglacial phases of the early Pleistocene and decreased through the late Pleistocene interglacial phases. Conversely, glacial maxima were more intense after Stage 16. Isotopic Stages 12 and 16 mark times of important change in water mass circulation. Oxygen- and carbon-isotope analyses have been performed on the Quaternary planktonic foraminifers of Sites 548 and 549 (DSDP Leg 80) to investigate major water mass changes that occurred in the northeastern Atlantic at different glacial-interglacial cycles and to compare them with the well-defined picture of 18,000 yr. ago. Oxygen-isotope stratigraphy also provides a chronological framework for the more important data on the fauna and flora. Although bioturbation and sedimentary gaps obliterate the climatic and stratigraphic record, general trends in the oceanographic history can be deduced from the isotopic data. Isotopic stratigraphy has tentatively been delineated down to isotopic Stage 16 at Site 548 and in Hole 549A. This stratigraphy fits well with that deduced from benthic foraminiferal d18O changes and with bioclimatic zonations based on foraminiferal associations at Site 549. Variations in the geographic extension and in the flux of the Gulf Stream subtropical waters are inferred from both d18O and d13C changes. Maximal fluxes occurred during the late Pliocene. Northward extension of subtropical waters increased through the various interglacial phases of the early Pleistocene and decreased through the late Pleistocene interglacial phases. Conversely, glacial maxima were more intense after Stage 16. Isotopic Stages 12 and 16 mark times of important change in water mass circulation.

Stable isotope record of the Okhotsk Sea

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000-4000 m. Holocene (core-top) benthic foraminiferal d18O and d13C data are calibrated to modern hydrographic properties through measurements of d13C of SumCO2 and d18O of seawater. Cibicidoides stable isotope ratios are close to the d13C and equilibrium d18O of seawater, whereas Uvigerina d18O and d13C are variably offset from Cibicidoides. Glacial maximum d13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32 per mil to account for the secular change in d13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ~2 km. However, at shallower depths d13C gradually increases by as much as 1 per mil above the modern value. Furthermore, above 2 km the benthic d18O decreases by ~0.5 per mil. These results are consistent with a benthic front at ~2 km in the North Pacific (see Herguera et al., 1992), but they differ from interpretations based on trace metal data which indicate a source of nutrient-depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient-rich as today.

Mid-Pleistocene planktonic foraminiferal composition and IRD content in ODP Sites 162-980 and 162-984

We investigated surface and deep ocean variability in the subpolar North Atlantic from 1000 to 500 thousand years ago (ka) based on two Ocean Drilling Program (ODP) sites, Feni drift site 980 (55°29'N, 14°42'W) and Bjorn drift site 984 (61°25'N, 24°04'W). Benthic foraminiferal stable isotope data, planktic foraminiferal faunas, ice-rafted debris data, and faunally based sea-surface temperature estimates help test the hypothesis that oceanographic changes in the North Atlantic region were associated with the onset of the 100-kyr world during the mid-Pleistocene revolution. Based on percentage of Neogloboquadrina pachyderma (s) records from both sites, surface waters during interglacials and glacials were cooler in the mid-Pleistocene than during marine isotope stages (MIS) 5 and 6. In particular, interglaciations at Bjorn drift site 984 were significantly cooler. Faunal evidence suggests that the interglacial Arctic front shifted from a position between the two sites to a position northwest of Bjorn drift site 984 after ca. 610 ka. As during the late Pleistocene, we find faunal evidence for lagging surface warmth at most of the glacial initiations during the mid-Pleistocene. Each initiation is associated with high benthic d13C values that are maintained into the succeeding glaciation, which we term "lagging NADW production." These findings indicate that lagging warmth and lagging NADW production are robust features of the regional climate system that persist in the middle to late Pleistocene.

Stable isotope record of Cibicidoides spp. from Late Miocene sediments of the Southern Ocean

Deepwater circulation plays an important role in climate modulation through its redistribution of heat and salt and its control of atmospheric CO2. Oppo and Fairbanks (1987, doi:10.1016/0012-821X(87)90183-X) showed that the Southern Ocean is an excellent monitor of deepwater circulation changes for two reasons: (1) the Southern Ocean is a mixing reservoir for incoming North Atlantic Deep Water and recirculated water from the Pacific and Indian oceans; and (2) the nutrient/delta13C tracers of deepwater are not significantly changed by surficial processes within the Southern Ocean. We can extend these principles to the late Miocene because tectonic changes in the Oligocene and early and middle Miocene developed near-modern basinal configurations. However, on these time scales, changes in the oceanic carbon reservoir and mean ocean nutrient levels also affect the delta13C differences between ocean basins. From 9.8 to 9.3 Ma, Southern Ocean delta13C values oscillated between high North Atlantic values and low Pacific values. The Southern Ocean recorded delta13C values similar to Pacific values from 9.2 to 8.9 Ma, reflecting a low contribution of Northern Component Water (NCW). The delta13C differences between the NCW and Pacific Outflow Water (POW) end-members were low from 8.9 to 8.0 Ma, making it difficult to discern circulation patterns. NCW production may have completely shutdown at 8.6 Ma, allowing Southern Component Water (SCW) to fill the North Atlantic and causing the delta13C values in the North Atlantic, Pacific, and Southern oceans to converge. Deepwater delta13C patterns resembling the modern distributions evolved by 7.0 Ma: delta13C values were near 1.0 per mil in the North Atlantic; 0.0 per mil in the Pacific; and 0.5 per mil in the Southern Ocean. Development of near-modern delta13C distributions by 7.0 Ma resulted not only from an increase in NCW flux but also from an increase in deepwater nutrient levels. Both of these processes increased the delta13C difference between the North Atlantic and Pacific oceans. Deepwater circulation patterns similar to today's operated as early as 9.8 Ma, but were masked by the lower nutrient/delta13C differences. During the late Miocene, 'interglacial' intervals prevailed during intervals of NCW production, while 'glacial' intervals occurred during low NCW production.

Stable isotope record of Cibicidoides spp. from early and middle Miocene sediments

The middle Miocene delta18O increase represents a fundamental change in the ocean-atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ~20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high delta13C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep-waters warmed in the early Miocene by ~3°C (?20 to 16 Ma) and cooled by a similar amount during the middle Miocene delta18O increase (14.8 to 12.6 Ma), corresponding to the increase (?20 Ma) and subsequent decrease (~16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 per mil), relatively rapid (~0.5 m.y.) delta18O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long-term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene delta18O shift.