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Abstract Estimating dissolved oxygen (O2) concentrations in seawater during the Neoproterozoic is central to testing hypotheses about the role of O2 in animal evolution. Here we apply the thallium (Tl) isotope redox proxy to samples stratigraphically below the ca. 810-million-year-old (Ma) Bitter Springs Carbon Isotope Excursion and spanning the interval between the two Snowball Earth glaciations (ca. 662–650 Ma) to constrain the evolution of Neoproterozoic bottom water redox conditions. Thallium isotopes can be used to reconstruct the global extent of oxygenated oceanic bottom waters because the primary control on seawater Tl isotope compositions (ε205Tl) over million-year time scales is changes in the amount of 205Tl removal by Mn oxides on the seafloor. Samples spanning an ~20-m.y. period preceding the Bitter Springs excursion from the Tonian Reefal Assemblage (n = 18/30) yield ε205Tlauth values lower than global oceanic inputs (ε205Tl ~–2±), with some samples approaching the modern seawater ε205Tl value of –6±. These sustained low ε205Tlauth values require enhanced burial of Mn oxides elsewhere on the seafloor, which we interpret as evidence for the oxygenation of the deep ocean in the Tonian. In contrast, the majority of samples from the Cryogenian Hay Creek Group (n = 13/16) yield ε205Tlauth values similar to global oceanic inputs, suggesting that the deep ocean was not ventilated at this time. This indicates that Earth’s deep ocean was not gradually oxygenated throughout the Neoproterozoic, but rather experienced intervals of increased and decreased O2 concentrations. 

ABSTRACT The geologically rapid appearance of most extant animal groups in the Cambrian fossil record is often linked to enhanced ocean oxygenation. However, conflicting reconstructions of the Cambrian redox landscape make it difficult to determine the extent of ocean oxygenation during this significant biotic event, particularly regarding the redox state of the global deep ocean. In this study, we present authigenic thallium isotope compositions (ε²⁰⁵Tl_auth) for two shale sequences from South China (Qingjiang and Weng'an) that span the Cambrian Stage 2–3 boundary to the appearance of the Qingjiang biota, approximately 521–518 million years ago (Ma), a timeframe that chronicles a particularly rapid interval of metazoan diversification and radiation in the broader Cambrian explosion. If this event occurred amid modern‐like extents of global ocean oxygenation, we would expect a significant increase in the global extent of seafloor Mn‐oxide burial to drive lower ε²⁰⁵Tl_authvalues near the modern open‐ocean composition of −6‱. Instead, we observe broadly stable ε²⁰⁵Tl_authvalues of around −3 to −4‱ in both studied sections. The lack of any significant Tl isotope shifts in our dataset argues against a short‐term global ocean oxygenation event and suggests the global deep ocean was not characterized by modern extents of oxygenation 521–518 Ma. We reinterpret contemporaneous near‐modern Mo and U isotope compositions to signal a relatively minor increase in marine oxygenation, likely limited to the continental shelves. However, ε²⁰⁵Tl_authlower than the average isotopic composition of approximately −2‱ in Ediacaran shales suggests a shift to comparatively better‐oxygenated conditions sometime between ~555 Ma and 521 Ma. If diversification at this time was linked to increased ocean oxygen levels, these changes were likely more dominant in the relatively shallow‐water settings of continental shelves most densely populated by Cambrian animals and were incapable of dramatically altering seawater Tl isotope mass balance through seafloor Mn‐oxide burial. 

Green sulfur bacteria biomarkers that indicate euxinia within the photic zone sometimes co-occur with evidence of contradictory depositional redox conditions, such as oxygen-requiring fossils or bioturbation. Intermittent euxinia may explain this apparent contradiction, and recent studies of modern environments show that green sulfur bacteria dwell in transiently euxinic settings. As a result, new approaches are needed to distinguish green sulfur bacteria biomarkers indicative of persistent versus intermittent euxina in ancient sedimentary samples, which this study addresses by investigating how the distribution of isorenieratene derivatives relates to depositional redox conditions. To accomplish this objective, this study focuses on two drill cores through the Upper Cretaceous Mowry Shale and the Eagle Ford Group. These drill cores are comparably thermally immature according to vitrinite reflectance data, and these formations in these cores capture a spectrum of depositional redox conditions according to multiple organic and inorganic proxies, including newly acquired iron speciation and kerogen organic sulfur data. The results presented here reveal that higher molecular weight diagenetic products of isorenieratene are preferentially preserved under persistent euxinia compared to intermittently euxinic intervals that contain isorenieratene derivatives that are shifted to lower molecular weights. Further, the total inventory of aromatic carotenoid diagenetic products contains features that can be used to identify green sulfur bacteria biomarkers from reworked petrogenic sources. Accordingly, the diagenetic fate of isorenieratene and the distribution of its diagenetic products distinguish persistent versus intermittent euxinia, which can be used to sharpen our evaluation of euxinia in the geologic record. 

The Early Paleozoic radiation of diverse animal life is commonly connected to a well-ventilated global ocean. Yet the oxygenation history of Paleozoic deep oceans remains debated. Using thallium (Tl) isotope ratios in deep-marine mudrocks, we reconstruct the history of deep marine oxygenation from ~485 to 380 million years ago. Thallium isotopes can track bottom water oxygenation indirectly through their sensitivity to seafloor Mn oxide burial. We apply Tl isotopes to a global set of mudrocks, placing a particular focus on the Road River Group of Yukon, Canada. Our data reveal an oscillatory pattern in seawater Tl isotope ratios and, in turn, a dynamic ocean ventilation history. A long-lived deep ocean oxygenation episode is identified between ~405 and 386 million years ago. These short-term dynamics are superimposed on a muted positive ocean oxygenation trend over the entire Early and Middle Paleozoic. Sustained O₂accumulation in global marine bottom waters occurred sometime after ~380 million years ago according to our dataset. 

Accurately reconstructing original Total Organic Carbon (TOC) in thermally mature rocks is essential for the correct application of geochemical proxies and understanding organic carbon burial through time. To reconstruct original TOC using empirical methods, it is vital to have an accurate estimate of the original Hydrogen Index (HI). The two most common methods are estimating original HI using kerogen type or using average HI values from immature rocks elsewhere in the basin. This study tests the ability to use inorganic geochemical data to reconstruct original HI using the Upper Cretaceous-Paleogene Moreno Formation from the San Joaquin Basin, California, USA as a case study. The study utilized cores from the Moreno Formation that are thermally immature, thus preserving original HI values, and that span a range in initial HI. First, inorganic geochemical data were produced (elemental abundances and iron speciation) for samples previously analyzed for organic geochemistry. These data suggest that bottom water conditions during deposition of the Moreno Formation were ferruginous (anoxic and non-sulfidic), without development of sustained euxinia (anoxic and sulfidic). Next, a random forest machine learning analysis was implemented to analyze which inorganic geochemical variables best predict HI in the Moreno Formation. The most important proxies were those for detrital input (Ti, Th), marine export productivity (Cu, Ni), and redox proxies for suboxic conditions (Se, Cr, iron speciation). Finally, the random forest framework was used to predict HI values for three main study cores based on their inorganic geochemistry. These predictions were compared stratigraphically and statistically against the measured values and the kerogen type and average HI methods for reconstructing HI and show this new method has better predictive power than approaches based on single values. This indicates strong promise for using inorganic geochemistry, which is relatively immune to thermal maturation, to reconstruct organic geochemical parameters that are modified during burial and diagenetic process. 

Geochemical data from ancient marine sediments are crucial for studying palaeo-environments, palaeo-climates, and elemental cycles. With increased accessibility to geochemical data, many databases have emerged. However, there remains a need for a more comprehensive database that focuses on deep-time marine sediment records. Here, we introduce the Deep-Time Marine Sedimentary Element Database (DM-SED). The DM-SED has been built upon the Sedimentary Geochemistry and Paleoenvironments Project (SGP) database with a new compilation of 34 874 data entries from 433 studies, totalling 63 627 entries. The DM-SED contains 2 522 255 discrete marine sedimentary data points, including major and trace elements and some stable isotopes. It includes 9207 entries from the Precambrian and 54 420 entries from the Phanerozoic, thus providing significant references for reconstructing deep-time Earth system evolution. The data files described in this paper are available at https://doi.org/10.5281/zenodo.14771859 (Lai et al., 2025). 

It is clear from modern analogue studies that O2-deficient conditions favor preservation of organic matter (OM) in fine-grained sedimentary rocks (black shales). It is also clear that appreciable productivity and OM flux to the sediment are required to establish and maintain these conditions. However, debates regarding redox controls on OM accumulation in black shales have mainly focused on oxic versus anoxic conditions, and the implications of different anoxic redox states remain unexplored. Here, we present detailed multi-proxy sedimentary geochemical studies of major Paleozoic and Mesozoic North American black shale units to elucidate their depositional redox conditions. This is the first broad-scale study to use a consistent geochemical methodology and to incorporate data from Fe-speciation – presently the only redox proxy able to clearly distinguish anoxic depositional conditions as ferruginous (H2S-limited) or euxinic (H2S-replete, Fe-limited). These data are coupled with total organic carbon (TOC), programmed pyrolysis, and redox-sensitive trace element proxies, with almost all measurements analyzed using the same geochemical methodology. Consistent with expectations based on previous geochemical and paleontological/ichnological studies, these analyses demonstrate that the study units were almost exclusively deposited under anoxic bottom waters. These analyses also demonstrate that there is wide variance in the prevalence of euxinic versus ferruginous conditions, with many North American black shale units deposited under predominantly ferruginous or oscillatory conditions. TOC is significantly higher under euxinic bottom waters in analyses of both preserved (present day) TOC and reconstructed initial TOC values, although sediments deposited under both redox states do have economically viable TOC content. While this correlation does not reveal the mechanism behind higher organic enrichment in euxinic environments, which may be different in different basins, it does open new research avenues regarding resource exploration and the biogeochemistry of ancient reducing environments.