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.

Trace and minor elements incorporated in foraminiferal shells are among the most used proxies for reconstructing past environmental conditions. A prominent issue concerning these proxies is that the inter-specimen variability in element composition is often considerably larger than the variability associated with the environmental conditions for which the proxy is used. Within a shell of an individual specimen the trace and minor elements are distributed in the form of bands of higher and lower concentrations. It has been hypothesized that differences in specimen-specific element banding patterns cause the inter-specimen and inter-species variability observed in average element composition, thereby reducing the reliability of proxies. To test this hypothesis, we compared spatial distributions of Mg, Na, Sr, K, S, P and N within chamber walls of two benthic foraminiferal species ( Amphistegina lessonii and Ammonia tepida ) with largely different average Mg content. For both species the selected specimens were grown at different temperatures and salinities to additionally assess how these parameters influence the element concentrations within the shell wall. Our results show that Mg, Na, Sr and K are co-located within shells, and occur in bands that coincide with organic linings but extend further into the calcite lamella. Changes in temperature or salinity modulate the element-banding pattern as a whole, with peak and trough heights co-varying rather than independently affected by these two environmental parameters. This means that independent changes in peak or trough height do not explain differences in average El/Ca between specimens. These results are used to evaluate and synthesize models of underlying mechanisms responsible for trace and minor element partitioning during calcification in foraminifera.

20 Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence 21 of sulphide near the sediment-water interface and can thereby record historic bottom-water 22 oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling 23 and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine 24 basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-25 MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. 26 These oscillations and high sediment Mo concentrations (up to ~130 ppm) are attributed to 27 deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of 28 molybdate (MoO4 2-) to the pore water upon reduction of these Mn-oxides, and subsequent 29 sequestration of Mo. The latter process only occurs in summer when sulphide concentrations 30 near the sediment-water interface are elevated. We hypothesise that cable bacteria enhance the 31 seasonality in sediment Mo records by contributing to remobilisation of Mo as MoO4 2during 32 oxic periods and by enhancing the pool of Mn-oxides in the system by dissolving Mn-33 carbonates. A sediment record that spans the past ~45 years indicates that sediment Mo 34 concentrations have increased over the past decades, despite less frequent occurrences of anoxia 35in the bottom waters based on oxygen measurements from water column monitoring. We 36 suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate 37 reduction and sulphide production in the surface sediment as a result of increased input of 38 organic matter into the basin from the adjacent North Sea since 1999, and an associated 39 enhanced "Mn refluxing" in the marine lake in summer. 40 93 refluxing" is thought to contribute to the high Mo burial fluxes in environments with weakly 94 sulphidic bottom waters (Algeo and Lyons, 2006). To our knowledge, there are no detailed 95 seasonal studies of the dynamics of Mo and Mn in both pore waters and sediments of hypoxic 96 systems to confirm the suggested seasonality in coupled Mn-Mo cycling. 97 Recently, it was discovered that sulphide-oxidising cable bacteria (Nielsen et al., 2010; 98 Pfeffer et al., 2012) may dissolve Fe-sulphides and Mn-carbonates in surface sediments of 99 seasonally-hypoxic systems. Consequently, these bacteria actively contribute to the formation 100 of an oxidised, Fe-and Mn-oxide rich surface layer in winter and spring (Seitaj et al., 2015; 101 Sulu-Gambari et al., 2016b; Sulu-Gambari et al., 2016a). In contrast, sulphur oxidising 102 Beggiatoaceae, present in autumn, had a more limited effect on the formation of Fe-and Mn-103 oxides in the surface sediment (Seitaj et al., 2015; Sulu-Gambari et al., 2016b; Sulu-Gambari 104 et al., 2016a). Due to the coupling of Mn, Fe, S and Mo cycles in hypoxic systems, we 105 hypothesise that the activity of cable bacteria may also be of relevance to the sedimentary 106 dynamics of Mo. More specifical...

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Hydrogen isotope ratios of long-chain alkenones (d 2 HC37) correlate with water isotope ratios and salinity, albeit with varying degrees of biological fractionation between alkenones and water. These differences in fractionation are the result of environmental and species related effects, which in some cases have consequences for the magnitude of the d 2 HC37 response per unit increase in salinity. Earlier culture experiments have focused on constraining hydrogen isotope fractionation factor a in non-calcifying strains of Emiliania huxleyi. Here we studied isotopic fractionation in a calcifying strain of E. huxleyi and show that although absolute fractionation is different, the response to changes 2 in salinity and alkalinity is similar to those of non-calcifying species. This suggests that calcification does not alter the d 2 HC37 response to salinity significantly.