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• Additional insights on I/Ca ratios in planktic foraminifera with oxygen concentrations. • A modern-like pacific ocean oxygen deficient zone was established in the eastern tropical pacific by 8 – 7 ma. • Expansion of oxygen depleted waters driven by a combination of tectonic changes influencing ventilation and the biological carbon pump. The Pacific Ocean hosts the largest expanse of oxygen depleted waters owing to a combination of factors influencing oxygen supply and consumption, with consequences for biogeochemical cycling. However, understanding of the long-term evolution of the Pacific Oxygen Deficient Zone remains poorly constrained. Here we apply the Iodine to Calcium (I/Ca) ratio in planktic foraminifera, in combination with the absence/presence of Globorotaloides hexagonus , from four tropical Pacific Ocean sites to reconstruct oceanic oxygen across the late Cenozoic. To validate the application of I/Ca, we supplement existing I/Ca calibration datasets by expanding their spatial coverage with additional core-top measurements. Our downcore results, combined with other lines of evidence, indicate the emergence and establishment of low oxygen waters from the late Miocene-Pliocene. The decline in Pacific Ocean oxygen accompanies large-scale climate and tectonic changes and likely impacted marine carbon cycling. [ABSTRACT FROM AUTHOR]

Understanding the Indian Summer Monsoon (ISM) behaviour during the late Pleistocene has been largely based on the wind-driven upwelling records from the Arabian Sea. However, it remains unclear the extent to which these records can also be used to infer a concomitant signal of monsoon rainfall, or how the two ISM components, rainfall and wind, are linked on millennial timescales. In order to isolate a primary signal of ISM rainfall, we exploit two deep sea sediment cores from the northern Bay of Bengal (Site U1446) and Andaman Sea (Site U1448), both situated proximal to the South Asian continent, and thus ideally situated for capturing ISM rainfall and fluvial runoff. By comparing our multi-proxy ISM rainfall and runoff records with published ISM wind-driven records from the Arabian Sea, we observe pronounced decoupling of the rainfall and wind components of the ISM across Marine Isotope Stage 5/6 (∼140–70 thousand years ago). We reveal that the relative dominance of barometric dynamics (wind) and the thermodynamic (rainfall) components of the monsoon shifts with changes in background climate state. This finding constitutes an important consideration for the interpretation of past monsoon reconstructions. By comparing our new ISM rainfall records with high latitude climate records, we show that moisture export from low-latitudes, via the monsoon, could have preconditioned the high latitudes for ice sheet growth during glacial inceptions. • Indian Summer Monsoon rainfall and wind (de)coupling is sensitive to climate state. • Multi-site, multi-proxy approach reveals interplay of regional and local climate processes impacting marginal settings. • Indian Summer Monsoon may have contributed to glacial inception during Marine Isotope Stage 5. [ABSTRACT FROM AUTHOR]

Zinc and Cd isotope compositions are presented for a comprehensive suite of terrestrial rocks to constrain the extent of Zn and Cd isotope fractionation during igneous processes and better define the δ66Zn and δ114Cd values of the silicate Earth (the δ values denote per mille deviations of 66Zn/64Zn from JMC Lyon Zn and of 114Cd/110Cd from NIST SRM 3108 Cd). Analyses of spinel lherzolites provide a bulk silicate Earth (BSE) δ114Cd BSE value of –0.06 ± 0.03 ‰ (2SD). For Zn, the peridotite data of the current and previous studies define a mean δ66Zn BSE = 0.20 ± 0.05 ‰ (2SD). Komatiite analyses of this and published investigations yield similar mean values, which suggests that the Zn and Cd isotope compositions of the mantle remained fairly constant since the Archean. Data for loess provide upper continental crust compositions of δ114Cd = 0.03 ± 0.10 ‰ and δ66Zn = 0.23 ± 0.07 ‰. The Zn isotope and abundance data for peridotites and oceanic basalts are in accord with the previous observation of a mantle array, with basalts having higher Zn concentrations and δ66Zn values than the peridotites. To a first order, this reflects slightly incompatible behaviour of Zn during mantle melting and melt differentiation with associated enrichment of heavy Zn isotopes in the melt phase. Cadmium is marginally more incompatible than Zn during igneous processes and the oceanic basalts also display a minor enrichment of heavy Cd isotopes relative to peridotites. However, secondary processes produce significant Cd isotope variability in both mantle melts and peridotites, obscuring the primary igneous array. The δ66Zn BSE estimates of this and previous studies resemble the Zn isotope compositions of CV and CO carbonaceous and some enstatite chondrites. In contrast, the BSE has a lower δ114Cd BSE value than enstatite and carbonaceous chondrites. This implies that the Cd isotope composition of the BSE was either fractionated during accretion or that Earth's Cd inventory was not exclusively acquired from material related to carbonaceous and enstatite chondrites. Importantly, delivery of Zn and Cd to the BSE solely by CI and CM chondrites is not in accord with the meteorite and terrestrial stable isotope data of these elements. [ABSTRACT FROM AUTHOR]

The vigorous eastward flow of the Antarctic Circumpolar Current (ACC) connects all major ocean basins and plays a prominent role in the transport of heat, carbon and nutrients around the globe. However, the establishment of a deep circumpolar flow, similar to the present-day ACC, remains controversial thereby obscuring our understanding of its climatic impact. Deciphering the chemical composition of Circumpolar Deep Water (CDW) within the ACC can provide critical insights about its development and evolution. Here we present new fossil fish teeth/bone debris neodymium isotope (ε Nd) records from Deep Sea Drilling Project (DSDP) Sites 278 and 274 in the southwest Pacific Ocean, with the aim to trace changes in deep water masses across the Tasmanian Gateway between the early Oligocene and early Miocene (~ 33–22 Ma). Site 274 provides the first Nd isotope record proximal to the Ross Sea during the Oligocene (33.5–23.4 Ma). Its Nd isotope composition shows excursions to very radiogenic values, ε Nd(t) = −3.1 and ε Nd(t)= − 3.7, at 33.5 Ma and 23.8 Ma, respectively, in response to major steps in Antarctic ice sheet expansion. A shift to lower, more unradiogenic ε Nd(t) values between 29.7 and 29.1 Ma is linked to an increased influence of proto-CDW upwelling at the site. In contrast, the Nd isotope record from Site 278 in the southern Emerald Basin shows little variability (ε Nd(t) = −6.0 to −6.7) throughout the Oligocene and early Miocene (30.9–21.8 Ma). Comparison with published data north of the ACC path, demonstrates the presence of two deep water masses in the South Pacific prior to the inferred onset of the ACC (33–30 Ma), one occupying depths between ~2500 and 3000 m (ε Nd(t)= ~ −3 to −5) and a deep/bottom water mass (> 3000 m) with a more unradiogenic Nd isotope composition (ε Nd(t)= ~ −6). Site 278 located close to the proto-polar front (proto-PF) indicates that following the inferred onset of the ACC, deep waters bathing the southern Emerald Basin remained more radiogenic in the Southwest Pacific compared to sites along the proto-PF in the South Atlantic and Indian Ocean (ε Nd(t)= ~ −8.1). This indicates a provinciality in Nd isotope compositions of deep waters along the proto-PF across the Tasmanian Gateway. Our data are incompatible with the existence of a modern-like homogenous (lateral and vertical) Nd isotope composition of CDW along the main flow path of the ACC in all oceanic basins in the Oligocene to early Miocene. We attribute distinct Nd isotope compositions of deep waters across the Tasmanian Gateway to reflect a less deep reaching and weaker ACC (proto-ACC) than today. Our findings suggest that the modern strong and deep-reaching ACC flow must have been developed at a later point in the Neogene. • Distinct Nd isotope compositions of proto-CDW persisted on both sides of the Tasmanian Gateway until at least 22 Ma. • A modern-like strong, deep-reaching Antarctic Circumpolar Current (ACC) flow did not develop before 22 Ma. • Major steps in Antarctic ice expansion in the early and late Oligocene are reflected in seawater chemistry off the Ross Sea. [ABSTRACT FROM AUTHOR]