ALBERT

Description: The decline in dissolved oxygen in global oceans (ocean deoxygenation) is a potential consequence of global warming which may have important impacts on ocean biogeochemistry and marine ecosystems. Current climate models do not agree on the trajectory of future deoxygenation on different timescales, in part due to uncertainties in the complex, linked effects of changes in ocean circulation, productivity and organic matter respiration. More (semi-)quantitative reconstructions of oceanic oxygen levels over the Pleistocene glacial cycles may provide a critical test of our mechanistic understanding of the response of oceanic oxygenation to climate change. Even the most promising proxies for bottom water oxygen (BWO) have limitations, which calls for new proxy development and a multi-proxy compilation to evaluate glacial ocean oxygenation. We use Holocene benthic foraminifera to explore I/Ca in Cibicidoides spp. as a BWO proxy. We propose that low I/Ca (e.g., 〈3 μmol/mol) in conjunction with benthic foraminiferal carbon isotope gradients and/or the surface pore area percentages in foraminiferal tests (e.g., 〉15%) may provide semi-quantitative estimates of low BWO in past oceans (e.g., 〈∼50 μmol/kg). We present I/Ca records in five cores and a global compilation of multiproxy data, indicating that bottom waters were generally less-oxygenated during glacial periods, with low O2 waters (〈∼50 μmol/kg) occupying some parts of the Atlantic and Pacific Oceans. Water mass ventilation and circulation may have been important in deoxygenation of the glacial deep Pacific and South Atlantic, whereas enhanced remineralization of organic matter may have had a greater impact on reducing the oxygen content of the interior Atlantic Ocean.

Description: During the early Eocene, a series of short-term global warming events ("hyperthermals”) occurred in response to the rapid release of carbon into the oceans and atmosphere. In order to investigate the response of ocean redox to global warming, we have determined the molybdenum isotope compositions (d98/95Mo) of samples spanning one such hyperthermal (Eocene Thermal Maximum 2 (ETM-2, 54.1 Ma)), from Integrated Ocean Drilling Program Expedition 302 Site M0004A in the Arctic Ocean. The highest d98/95Mo in our sample set (2.00 ± 0.11 per mil) corresponds to the development of local euxinia at Site M0004A during the peak of ETM-2, which we interpret as recording the global seawater d98/95Mo at that time. The ETM-2 seawater d98/95Mo is indistinguishable from a recent estimate of seawater d98/95Mo from an earlier hyperthermal (Paleocene Eocene Thermal Maximum (PETM, 55.9 Ma), d98/95Mo = 2.08 ± 0.11 per mil). We argue that the similarity in seawater d98/95Mo during ETM-2 and the PETM was caused by the development of transient euxinia in the Arctic Ocean during each hyperthermal that allowed sediments accumulating in this basin to capture the long-term d98/95Mo of early Eocene seawater. Our new data therefore place a minimum constraint on the magnitude of transient global seafloor deoxygenation during early Eocene hyperthermals.

Description: This dataset consists of molybdenum isotope compositions and concentrations of marlstones and limestones of the Eagle Ford Group, South Texas. The data are Late Cretaceous in age, and span several millions of years before and after Oceanic Anoxic Event 2 (~94 Ma). The data were measured on samples taken from two research cores, Iona-1 and Innes-1.

Description: Geochemical data from Ocean Drilling Program Site 1138A, core 69, spanning the Cenomanian–Turonian boundary interval (~94 Ma). The data were generated from marine sediment in the archive half of the core, which was opened and sampled for this project following approval from the IODP curatorial advisory board. The data consist of: (i) Bulk organic geochemistry (total organic carbon, hydrogen and oxygen indexes, total inorganic carbon) measured by Rock Eval pyrolysis and coulormat titration; (ii) Bulk organic carbon isotopes, measured on decarbonated sediments; (iii) Bulk sediment elemental concentrations measured by ICP-MS; (iv) Bulk sediment molybdenum isotopes, measured by MC-ICP-MS; (v) n-alkane compound-specific carbon isotopes measured by GCMS-IRMS; (vi) Maceral assemblage compositions, measured by visual petrography.

Description: The development of large ice-sheets across the Northern Hemisphere during the late Pliocene and the emergence of the glacial-interglacial cycles that punctuate the Quaternary mark a significant threshold in Earth's climate history. Although a number of different mechanisms have been proposed to initiate this cooling and the onset of major Northern Hemisphere glaciation, reductions in atmospheric concentrations of CO2 likely played a key role. The emergence of a stratified (halocline) water column in the subarctic north-west Pacific Ocean at 2.73 Ma has often been interpreted as an event which would have limited oceanic ventilation of CO2 to the atmosphere, thereby helping to cool the global climate system. Here, diatom carbon isotopes (δ13Cdiatom) are used to reconstruct changes in regional carbon dynamics through this interval. Results show that the development of a salinity stratification did not fundamental alter the net oceanic/atmospheric flux of CO2 in the subarctic north-west Pacific Ocean through the late Pliocene/early Quaternary. These results provide further insights into the long-term controls on global carbon cycling and the role of the subarctic Pacific Ocean in instigating global climatic changes.

Description: The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope (δ238UCAU) data to reconstruct the evolution of global seawater δ238U, and hence quantify the expansion of anoxic U sinks on a global scale. This dataset contains trace element and U isotope data (238U/235U, expressed as δ238U) for the carbonate fraction of three well studied PETM sites; Site 865 (equatorial Pacific), Site 401 (Bay of Biscay) and Site 690 (Walvis Ridge). Samples are carbonate rich pelagic sediments, with a mixture of carbonate nannofossils, foraminifera and detrital clays. Bulk samples were selectively leached for the carbonate fraction using 1M ammonium acetate (pH 5) at room temperature for 24hrs. Trace element concentrations were measured on a Thermo-Finnigan Element XR and reported normalized to Ca. Uranium was purified by ion exchange chromatography and isotopes measured on a Thermo-Finnigan Neptune Plus. Uranium isotopes are reported as δ238U, where CRM-145 = 0‰. Sites 690 and 401 both show elevated U/Ca and δ238UCAU during the PETM and recovery interval, indicative of locally reducing conditions. By contrast, Site 865 records the global seawater δ238U and shows no resolvable change across the PETM. The lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. In the related publication we use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia to cover 〈2% of seafloor area.

Description: The interglacial known as Marine Isotope Stage 11 has been proposed to be analogous to the Holocene, owing to similarities in the amplitudes of orbital forcing. It has been difficult to compare the periods, however, because of the long duration of Stage 11 and a lack of detailed knowledge of any extreme climate events that may have occurred. Here we use the distinctive phasing between seasurface temperatures and the oxygen-isotope records of benthic foraminifera in the southeast Atlantic Ocean to stratigraphically align the Holocene interglacial with the first half of the Marine Isotope Stage 11 interglacial optimum. This alignment suggests that the second half of Marine Isotope Stage 11 should not be used as a reference for 'pre-anthropogenic' greenhouse-gas emissions. By compiling benthic carbon-isotope records from sites in the Atlantic Ocean on a single timescale, we also find that meridional overturning circulation strengthened about 415,000 years ago, at a time of high orbital obliquity. We propose that this mechanism transported heat to the high northern latitudes, inhibiting significant ice-sheet build-up and prolonging interglacial conditions. We suggest that this mechanism may have also prolonged other interglacial periods throughout the past 800,000 years.

Description: Variations in the strength of coastal upwelling in the South East Atlantic Ocean and summer monsoonal rains over South Africa are controlled by the regional atmospheric circulation regime. Although information about these parameters exists for the last glacial period, little detailed information exists for older time periods. New information from ODP Site 1085 for Marine Isotope Stages (MIS) 12-10 shows that glacial-interglacial productivity trends linked to upwelling variability followed a pattern similar to the last glacial cycle, with maximums shortly before glacial maxima, and minimums shortly before glacial terminations. During the MIS-11/10 transition, several periodic oscillations in productivity and monsoonal proxies are best explained by southwards shifts in the southern sub-tropical high-pressure cells followed by abrupt northwards shifts. Comparison to coeval sea-surface temperature measurements suggests that these monsoonal cycles were tightly coupled to anti-phased hemispheric climate change, with an intensified summer monsoon during periods of Northern (Southern) Hemisphere cooling (warming). The timing of these events suggests a pacing by insolation over precession periods. A lack of similar regional circulation shifts during the MIS-13/12 transition is likely due to the large equatorwards shift in the tropical convection zone that occurred during this extreme glaciation.

Description: There is much uncertainty surrounding the mechanisms that forced the abrupt climate fluctuations found in many palaeoclimate records during Marine Isotope Stage (MIS)-3. One of the processes thought to be involved in these events is the Atlantic Meridional Overturning Circulation (MOC), which exhibited large changes in its dominant mode throughout the last glacial period. Giant piston core MD95-2006 from the northeast Atlantic Ocean records a suite of palaeoceanographic proxies related to the activity of both surface and deep water masses through a period of MIS-3 when abrupt climate fluctuations were extremely pronounced. A two-stage progression of surface water warming during interstadial warm events is proposed, with initial warming related to the northward advection of a thin warm surface layer within the North Atlantic Current, which only extended into deeper surface layers as the interstadial progressed. Benthic foraminifera isotope data also show millennial-scale oscillations but of a different structure to the abrupt surface water changes. These changes are argued to partly be related to the influence of low-salinity deepwater brines. The influence of deepwater brines over the site of MD95-2006 reached a maximum at times of rapid warming of surface waters. This observation supports the suggestion that brine formation may have helped to destabilize the accumulation of warm, saline surface waters at low latitudes, helping to force the MOC into a warm mode of operation. The contribution of deepwater brines relative to other mechanisms proposed to alter the state of the MOC needs to be examined further in future studies.