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: 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 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 data set contains raw datafiles from a Neptune Plus MC-ICP-MS mass spectrometer in order to obtain molybdenum isotopes and isotope ratios (100Mo, 98Mo, 95Mo). The ICP-MS data was collected in 2020 on 18 samples from core E-8X, and the sample spacing varied greatly based on the intervals of interest: ~35 cm during the pre Paleocene-Eocene Therman Maximum (PETM), ~35 cm during the main phase and recovery phases, and ~3 cm around the carbon isotope excursion (CIE). The attached file includes a raw data compilation of molybdenum isotope ratios which are utilised to decribe bottom water redox changes and water circulation in the central North Sea during the Paleocene-Eocene Thermal Maximum.

Description: Sedimentary marine core E-8X (Longitude 4.98° Latitude 55.63°) was originally recovered in the Danish Sector of the North Sea by Maersk Oil & Gas, and the well was spudded and completed in 1994. The resulting sedimentary cores are stored at GEUS (Geological Survey of Denmark and Greenland). The section of core E-8X that was used for this study is 6 m long (from 2027 to 2021 m composite depth) and encompasses the Lista Formation (uppermost Ve Member and Bue Member) and the Sele Formation (lower part), corresponding to the uppermost Paleocene and lowermost Eocene. The core was sampled and analysed between 2016 and 2020 to reconstruct paleoenvironmental changes during the Paleocene-Eocene Thermal Maximum (PETM), including carbon cycle disruption, bottom water redox changes and water circulation. This collection comprises ẟ13Corg and total organic carbon (TOC) data, relative abundances of various oxides via conventional and portable XRF analysis, clay mineralogy, SEM investigations on pyrite framboid diameters, molybdenum isotopes data and total surfur concentrations.

Description: Continental margin sediments have been identified as the dominant sink in the marine budget of cadmium (Cd). The isotopic composition of this important output flux is, however, unknown. Here we present, with measurements on the Argentine continental margin, the first observational constraints on the isotopic composition of Cd in modern marine oxic and sub-oxic sediments. We identify two main removal mechanisms of Cd; in organic material, and by sulfide formation. Surface margin sediments (0–0.5 cm), with dissolved O2 below detection from ∼0.5 cm, are isotopically lighter than overlying oxygenated waters. A mass balance for these surface sediments indicates that Cd is present dominantly as organically-bound particulate Cd. In sub-surface sediments, Cd concentrations increase in the zone of nitrate reduction, and attain similar isotopic compositions as the water that overlies the sediment (i.e. ∼0.35‰ in deep waters). These observations are consistent with a downward diffusive flux of seawater Cd and redox-driven quantitative removal of that Cd during sulfide precipitation. In combination, these two routes of Cd removal lead to burial of isotopically light organic Cd in margin sub-oxic sediments that enables the global isotopic Cd budget to be balanced.