Understanding the causal mechanisms of past marine deoxygenation is critical to predicting the long-term Earth systems response to climate change. However, the processes and events preceding widespread carbon burial coincident with oceanic anoxic events remain poorly constrained. Here, we report a comprehensive biomarker inventory enveloping Oceanic Anoxic Event 2 that captures microbial communities spanning epipelagic to benthic environments in the southern proto-North Atlantic Ocean. We identify an abrupt, sustained increase in primary productivity that predates Oceanic Anoxic Event 2 by ∼220 ± 4 thousand years, well before other geochemical proxies register biogeochemical perturbations. During the event, recurrent photic zone euxinia triggered a major marine microbial reorganization accompanied by a decrease in primary production. These findings highlight how organic carbon burial drivers operated along a continuum in concert with microbial ecological changes, with antecedent, localized increases in primary production destabilizing carbon cycling and promoting the progressive marine deoxygenation leading to Oceanic Anoxic Event 2.

Concentrations of chemical constituents in water are given in either milligram per liter (mg/L), microgram per liter (µg/L), or nanogram per liter (ng/L). viii Abbreviations 9Cl-PF3ONS 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate 11Cl-PF3OUdS 11-chloroeicosafluoro-3-oxaundecane-1-sulfonate 4:2 FTS 4:2 fluorotelomersulfonate 6:2 FTS 6:2 fluorotelomersulfonate 8:2 FTS 8:2 fluorotelomersulfonate ADONA 4,8-dioxa-3H-perfluorononanoate CEC contaminants of emerging concern DO dissolved oxygen DoD U.S. Department of Defense EPA U.S. Environmental Protection Agency FASA perfluoroalkane sulfonamides HA health advisory HFPO-DA perfluoro-2-propoxypropanoate MCL maximum contaminant level N-EtFOSAA n-ethylperfluorooctanesulfonamidoacetate N-MeFOSAA n-methylperfluorooctanesulfonamidoacetate NFM National Field Manual NWQL National Water Quality Laboratory PFAA perfluoroalkyl acids PFAS per-and polyfluoroalkyl substances PFBA perfluorobutanoate PFBS perfluorobutanesulfonate PFCA perfluoroalkyl carboxylic acid PFDA perfluorodecanoate PFDS perfluorodecanesulfonate PFDoDA perfluorododecanoate PFHpA perfluoroheptanoate PFHpS perfluoroheptanesulfonate PFHxA perfluorohexanoate PFHxS perfluorohexanesulfonate PFNA perfluorononanoate PFNS perfluorononanesulfonate PFOA perfluorooctanoate PFOS perfluorooctanesulfonate ix PFOSA perfluorooctanesulfonamide PFPeA perfluoropentanoate PFPeS perfluoropentanesulfonate PFSA perfluoroalkyl sulfonic acid PFTeDA perfluorotetradecanoate PFTrDA perfluorotridecanoate

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Studying tropical hydroclimate and productivity change in the past is critical for understanding global climate dynamics. Northwest Australia is an ideal location for investigating Australian monsoon dynamics, the variability of the Indonesian Throughflow (ITF), and their impact on past productivity and warm pool evolution, which remain poorly understood during the 40 kyr world in the mid-early Pleistocene. In this study, we present multi-proxy records from International Ocean Discovery Program (IODP) Site U1483 in the Timor Sea spanning the last 2000 ka, including orbitally-resolved records from the 40 kyr world between 2000 and 1300 ka. Our results suggest that northwest Australia underwent a step of increased aridification and that productivity in the Timor Sea declined during the transition from ~ 1700 to ~ 1400 ka. We attribute this aridification to the reduced moisture supply to this region caused by the ITF restriction and warm pool contraction. We ascribe the declined productivity to a decrease in the nutrient supply of the Pacific source water associated with global nutrient redistribution. At orbital timescale, multiple mechanisms, including sea level changes, monsoon, and the Intertropical Convergence Zone (ITCZ) dynamics, and variations in the ITF and Walker circulation could control variations of productivity and terrigenous input in the Timor Sea during the 40 kyr world. Our bulk nitrogen and benthic carbon isotope records suggest a strong coupling to biogeochemical changes in the Pacific during this period. This research contributes to a better understanding of tropical hydroclimate and productivity changes during the 40 kyr world.

Tocopherols serve a critical role as antioxidants inhibiting lipid peroxidation in photosynthetic organisms, yet are seldom used in geobiological investigations. The ubiquity of tocopherols in all photosynthetic lifeforms is often cited as an impediment to any diagnostic paleoenvironmental potential, while the inability to readily analyze these compounds via conventional methods, such as gas chromatography–mass spectrometry, further diminishes the capacity to serve as useful ‘biomarkers’. Here, we analyzed an exceptionally preserved black shale sequence from the Demerara Rise that spans Oceanic Anoxic Event 2 (OAE‐2) to reexamine the significance of tocopherols and associated derivatives (i.e. tocol derivatives) in ancient sediments. Tocol derivatives were analyzed via liquid chromatography–quadrupole time‐of‐flight–mass spectrometry and included tocopherols, a methyltrimethyltridecylchroman, and the first reported detection of tocopherol quinones and methylphytylbenzoquinones in the geologic record. Strong correlations between tocol derivatives were observed over the studied interval. Tocol derivative concentrations and ratios, which normalized tocopherols to potential derivatives, revealed absolute and relative increases in tocopherols as exclusive features of OAE‐2 that can be explained by two possible mechanisms related to tocopherol production and preservation. The development of photic zone euxinia during OAE‐2 likely forced an upward migration of oxygenic photoautotrophs, increasing oxidative stress that elicited heightened tocopherol biosynthesis. However, shoaling euxinic conditions may have simultaneously acted to enhance tocopherol preservation given the relatively high lability of tocopherols in the water column. Both scenarios could produce the observed stratigraphic distribution of tocol derivatives in this study, although the elevated tocopherol concentrations that define OAE‐2 at the Demerara Rise are primarily attributed to enhanced tocopherol production by shoaling phytoplanktonic communities. Thus, the occurrence of tocopherols and associated derivatives in sediments and rocks of marine origin is likely indicative of shallow‐water anoxia, tracking the phytoplanktonic response to the abiotic stresses associated with vertical fluctuations in pelagic redox.