No abstract

Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical cycling in the oceans. Chromium isotopes are used as a proxy to infer changes in past oceanic redox state. Chromium isotopes in carbonates, including the prime proxy carrier foraminifera, were initially thought to record the seawater composition during crystallisation. However, the uptake of Cr into foraminiferal tests and carbonates is still poorly understood and recent studies question this assumption. We assess whether Cr in foraminiferal calcite is taken up during biomineralisation, has a postdepositional origin or is a combination of the two. Laser Ablation-MC-ICP-MS analyses and NanoSIMS imaging of individual tests were used to characterise the distribution of Cr in both planktic and benthic foraminifera. Foraminifera in sediment core-top samples have up to two orders of magnitude more Cr than sediment trap, plankton net, and culture samples. In cultured specimens, Cr is incorporated in foraminiferal tests at low concentrations (0.04-0.13 ppm) with a distribution coefficient of ~250 ± 43 (2SE) which is an upper estimate due to substantial loss of dissolved Cr during the experiment. Part of the Cr signal in sedimentary foraminifera may be primary, but this primary signal is likely often overprinted by the uptake of Cr from bottom and pore waters. In sediment samples, there is no significant isotopic offset between individual species and bulk foraminiferal calcite from the same size fraction. The >500 µm fraction has a heavier isotopic composition than the smaller 250-500 µm fraction with an offset of-0.3 to-0.5‰ due to an increase in surface area to volume. We propose that Cr in foraminifera is predominantly post-depositional and records bottom/pore water signals. This is contrary to current interpretations of the foraminiferal Cr isotope proxy as a surface seawater redox proxy.

The greenhouse world of the mid-Cretaceous (~94 Ma) was punctuated by an episode of abrupt climatic upheaval: Oceanic Anoxic Event 2. High-resolution climate records reveal considerable changes in temperature, carbon cycling, and ocean chemistry during this climatic perturbation. In particular, an interval of cooling has been detected in the English Chalk on the basis of an invasive boreal fauna and bulk oxygen-isotope excursions registered during the early stages of Oceanic Anoxic Event 2-a phenomenon known as the Plenus Cold Event, which has tentatively been correlated with climatic shifts worldwide. Here we present new high-resolution neodymium-, carbon-, and oxygen-isotope data, as well as elemental chromium concentrations and cerium anomalies, from the English Chalk exposed at Dover, UK, which we evaluate in the context of >400 records from across the globe. A negative carbon-isotope excursion that correlates with the original "Plenus Cold Event" is consistently expressed worldwide, and CO 2 proxy records, where available, indicate a rise and subsequent fall in CO 2 over the Plenus interval. However, variability in the timing and expression of cooling at different sites suggests that, although sea-surface paleotemperatures may reflect a response to global CO 2 change, local processes likely played a dominant role at many sites. Variability in the timing and expression of changes in water mass character, and problems in determining the driver of observed proxy changes, suggest that no single simple mechanism can link the carbon cycle to oceanography during the Plenus interval and other factors including upwelling and circulation patterns were locally important. As such, it is proposed that the Plenus carbon-isotope event is a more reliable stratigraphic marker to identify the Plenus interval, rather than any climatic shifts that may have been overprinted by local effects.

Over the past several decades, oxygen minimum zones have rapidly expanded due to rising temperatures raising concerns about the impacts of future climate change. One way to better understand the drivers behind this expansion is to evaluate the links between climate and seawater deoxygenation in the past especially in times of geologically abrupt climate change such as the Palaeocene-Eocene Thermal Maximum (PETM), a well-characterized period of rapid warming~56 Ma. We have developed and applied the novel redox proxies of foraminiferal Cr isotopes (δ 53 Cr) and Ce anomalies (Ce/Ce*) to assess changes in paleoredox conditions arising from changes in oxygen availability. Both δ 53 Cr and Cr concentrations decrease notably over the PETM at intermediate to upper abyssal water depths, indicative of widespread reductions in dissolved oxygen concentrations. An apparent correlation between the sizes of δ 53 Cr and benthic δ 18 O excursions during the PETM suggests temperature is one of the main controlling factors of deoxygenation in the open ocean. Ocean Drilling Program Sites 1210 in the Pacific and 1263 in the Southeast Atlantic suggest that deoxygenation is associated with warming and circulation changes, as supported by Ce/Ce* data. Our geochemical data are supported by simulations from an intermediate complexity climate model (cGENIE), which show that during the PETM anoxia was mostly restricted to the Tethys Sea, while hypoxia was more widespread as a result of increasing atmospheric CO 2 (from 1 to 6 times preindustrial values).

Abstract. Planktonic foraminifera are widely used in biostratigraphic, palaeoceanographic and evolutionary studies, but the strength of many study conclusions could be weakened if taxonomic identifications are not reproducible by different workers. In this study, to assess the relative importance of a range of possible reasons for among-worker disagreement in identification, 100 specimens of 26 species of macroperforate planktonic foraminifera were selected from a core-top site in the subtropical Pacific Ocean. Twenty-three scientists at different career stages – including some with only a few days experience of planktonic foraminifera – were asked to identify each specimen to species level, and to indicate their confidence in each identification. The participants were provided with a species list and had access to additional reference materials. We use generalised linear mixed-effects models to test the relevance of three sets of factors in identification accuracy: participant-level characteristics (including experience), species-level characteristics (including a participant's knowledge of the species) and specimen-level characteristics (size, confidence in identification). The 19 less experienced scientists achieve a median accuracy of 57 %, which rises to 75 % for specimens they are confident in. For the 4 most experienced participants, overall accuracy is 79 %, rising to 93 % when they are confident. To obtain maximum comparability and ease of analysis, everyone used a standard microscope with only 35× magnification, and each specimen was studied in isolation. Consequently, these data provide a lower limit for an estimate of consistency. Importantly, participants could largely predict whether their identifications were correct or incorrect: their own assessments of specimen-level confidence and of their previous knowledge of species concepts were the strongest predictors of accuracy.