A partition coefficient for copper (D Cu ) in foraminiferal calcite has been determined by culturing individuals of two benthic species under controlled laboratory conditions. The partition coefficient of a trace element (TE) is an emperically determined relation between the TE/Ca ratio in seawater and the TE/Ca ratio in foraminiferal calcite and has been established for many divalent cations. Despite its potential to act as a tracer of human-induced, heavy metal pollution, data is not yet available for copper. Since partition coefficients are usually a function of multiple factors (seawater temperature, pH, salinity, metabolic activity of the organism, etc.), we chose to analyze calcite from specimens cultured under controlled laboratory conditions. They were subjected to different concentrations of Cu 2+ (0.1-20 µmol/l) and constant temperature (10 and 20 • C), seawater salinity and pH. We monitored the growth of new calcite in specimens of the temperate, shallow-water foraminifer Ammonia tepida and in the tropical, symbiont-bearing Heterostegina depressa. Newly formed chambers were analyzed for Cu/Ca ratios by laser ablation-ICP-MS. The estimated partition coefficient (0.1-0.4) was constant to within experimental error over a large range of (Cu/Ca) seawater ratios and was remarkably similar for both species. Neither did the presence or absence of symbionts affect the D Cu , nor did we find a significant effect of temperature or salinity on Cu-uptake.

Predicting which marine systems are close to abrupt transitions into oxygen-deficient conditions ("anoxia") is notoriously hard but important-as rising temperatures and coastal eutrophication drive many marine systems toward such tipping points. Rapid oxic-to-anoxic transitions occurred regularly within the eastern Mediterranean Sea on (multi)centennial time scales, and hence, its sedimentary archive allows exploring statistical methods that can indicate approaching tipping points. The here presented high-resolution reconstructions of past oxygen dynamics in the Mediterranean Sea reveal that early-warning signals in these deoxygenation time series occurred long before fast transitions to anoxia. These statistical indicators (i.e., rise in autocorrelation and variance) are hallmarks of so-called critical slowing down, signaling a steady loss of resilience of the oxygenated state as the system approaches a tipping point. Hence, even without precise knowledge of the mechanisms involved, early-warning signals for widespread anoxia in marine systems are recognizable using an appropriate statistical approach. Plain Language Summary As a result of rising temperatures and input of excess nutrients to coastal regions, today's oceans and seas are losing oxygen. In the geological past, large-scale anoxic events have occasionally triggered mass extinction events, and the current loss of oxygen in the marine realm has increased worries about such events happening again. Predicting these rapid transitions to anoxia has been nearly impossible until now. By studying past large-scale anoxic events in the Mediterranean Sea, we show, however, that before a fast transition to anoxia, there are early-warning signals recognizable in deoxygenation time series. These early-warning signals make it potentially possible to develop an approach to forecast rapid transitions to anoxia in areas sensitive for fast deoxygenation.

No abstract

Abstract. Over the last decades, sea surface temperature (SST) reconstructions based on the Mg∕Ca of foraminiferal calcite have frequently been used in combination with the δ18O signal from the same material to provide estimates of the δ18O of water (δ18Ow), a proxy for global ice volume and sea surface salinity (SSS). However, because of error propagation from one step to the next, better calibrations are required to increase the accuracy and robustness of existing isotope and element to temperature proxy relationships. Towards that goal, we determined Mg∕Ca, Sr∕Ca and the oxygen isotopic composition of Trilobatus sacculifer (previously referenced as Globigerinoides sacculifer) collected from surface waters (0–10 m) along a north–south transect in the eastern basin of the tropical and subtropical Atlantic Ocean. We established a new paleotemperature calibration based on Mg∕Ca and on the combination of Mg∕Ca and Sr∕Ca. Subsequently, a sensitivity analysis was performed in which one, two or three different equations were considered. Results indicate that foraminiferal Mg∕Ca allows for an accurate reconstruction of surface water temperature. Combining equations, δ18Ow can be reconstructed with a precision of about ± 0.5 ‰. However, the best possible salinity reconstruction based on locally calibrated equations only allowed for a reconstruction with an uncertainty of ± 2.49. This was confirmed by a Monte Carlo simulation, applied to test successive reconstructions in an “ideal case” in which explanatory variables are known. This simulation shows that from a purely statistical point of view, successive reconstructions involving Mg∕Ca and δ18Oc preclude salinity reconstructions with a precision better than ± 1.69 and hardly better than ± 2.65 due to error propagation. Nevertheless, a direct linear fit to reconstruct salinity based on the same measured variables (Mg∕Ca and δ18Oc) was established. This direct reconstruction of salinity led to a much better estimation of salinity (± 0.26) than the successive reconstructions.

Abstract. Most planktonic foraminifera migrate vertically through the water column during life, meeting a range of depth-related conditions as they grow and calcify. For reconstructing past ocean conditions from geochemical signals recorded in their shells it is therefore necessary to know vertical habitat preferences. Species with a shallow habitat and limited vertical migration will reflect conditions of the surface mixed layer and short- and meso-scale (i.e. seasonal) perturbations therein. Species spanning a wider range of depth habitats, however, will contain a more heterogeneous, intra-specimen variability (i.g. Mg/Ca and δ18O), which is less for species calcifying below the seasonal mixed layer (SML). Here we present results on single-chamber Mg/Ca combined with single shell δ18O and δ13C of surface water Globigerinoides ruber, the thermocline-dwelling Neogloboquadrina dutertrei and Pulleniatina obliquiloculata and the deep dweller Globorotalia scitula from the Mozambique Channel. Species-specific Mg/Ca, δ13C and δ18O data combined with a depth-resolved mass balance model confirm distinctive migration and calcification patterns for each species as a function of hydrography. Whereas single specimen δ18O not always reveal changes in depth habitat related to hydrography (i.g. temperature), measured Mg/Ca of the last chambers can only be explained by active migration in response to changes in temperature stratification. Since species show different responses to changes in hydrography, their shell chemistry can be used to reconstruct different components of the past ocean climate system such as seasonality and depth stratification. Here we present combined single-specimen δ18O and single-chamber Mg/Ca measurements for different species, providing a composite of thermocline and sub-thermocline conditions. These results allow for species-specific reconstruction of calcification depths, using a mass balance model, of four species of planktonic foraminifera. This shows that the single chamber Mg/Ca and single test δ18O are in agreement with each other and in line with the changes in hydrography induced by eddies. Whereas single chamber Mg/Ca are most affected eddy frequency, seasonality is reflected more clearly in single test δ18O.

Abstract. Over the last few decades, a suite of inorganic proxies based on foraminiferal calcite have been developed, some of which are now widely used for palaeoenvironmental reconstructions. Studies of foraminiferal shell chemistry have largely focused on cations and oxyanions, while much less is known about the incorporation of anions. The halogens fluoride and chloride are conservative in the ocean, which makes them candidates for reconstructing palaeoceanographic parameters. However, their potential as a palaeoproxy has hardly been explored, and fundamental insight into their incorporation is required. Here we used nanoscale secondary ion mass spectrometry (NanoSIMS) to investigate, for the first time, the distribution of Cl and F within shell walls of four benthic species of foraminifera. In the rotaliid species Ammonia tepida and Amphistegina lessonii, Cl and F were distributed highly heterogeneously within the shell walls, forming bands that were co-located with the bands observed in the distribution of phosphorus (significant positive correlation of both Cl and F with P; p<0.01). In the miliolid species Sorites marginalis and Archaias angulatus, the distribution of Cl and F was much more homogeneous without discernible bands. In these species, Cl and P were spatially positively correlated (p<0.01), whereas no correlation was observed between Cl and F or between F and P. Additionally, their F content was about an order of magnitude higher than in the rotaliid species. The high variance in the Cl and F content in the studied foraminifera specimens could not be attributed to environmental parameters. Based on these findings, we suggest that Cl and F are predominately associated with organic linings in the rotaliid species. We further propose that Cl may be incorporated as a solid solution of chlorapatite or may be associated with organic molecules in the calcite in the miliolid species. The high F content and the lack of a correlation between Cl and F or P in the miliolid foraminifera suggest a fundamentally different incorporation mechanism. Overall, our data clearly show that the calcification pathway employed by the studied foraminifera governs the incorporation and distribution of Cl, F, P, and other elements in their calcite shells.

Abstract. Thermokarst lakes play an important role in permafrost environments by warming and insulating the underlying permafrost. As a result, thaw bulbs of unfrozen ground (taliks) are formed. Since these taliks remain perennially thawed, they are zones of increased degradation where microbial activity and geochemical processes can lead to increased greenhouse gas emissions from thermokarst lakes. It is not well understood though to what extent the organic carbon (OC) in different talik layers below thermokarst lakes is affected by degradation. Here, we present two transects of short sediment cores from two thermokarst lakes on the Arctic Coastal Plain of Alaska. Based on their physiochemical properties, two main talik layers were identified. A “lake sediment” is identified at the top with low density, sand, and silicon content but high porosity. Underneath, a “taberite” (former permafrost soil) of high sediment density and rich in sand but with lower porosity is identified. Loss on ignition (LOI) measurements show that the organic matter (OM) content in the lake sediment of 28±3 wt % (1σ, n=23) is considerably higher than in the underlying taberite soil with 8±6 wt % (1σ, n=35), but dissolved organic carbon (DOC) leaches from both layers in high concentrations: 40±14 mg L−1 (1σ, n=22) and 60±14 mg L−1 (1σ, n=20). Stable carbon isotope analysis of the porewater DOC (δ13CDOC) showed a relatively wide range of values from −30.74 ‰ to −27.11 ‰ with a mean of -28.57±0.92 ‰ (1σ, n=21) in the lake sediment, compared to a relatively narrow range of −27.58 ‰ to −26.76 ‰ with a mean of -27.59±0.83 ‰ (1σ, n=21) in the taberite soil (one outlier at −30.74 ‰). The opposite was observed in the soil organic carbon (SOC), with a narrow δ13CSOC range from −29.15 ‰ to −27.72 ‰ in the lake sediment (-28.56±0.36 ‰, 1σ, n=23) in comparison to a wider δ13CSOC range from −27.72 ‰ to −25.55 ‰ in the underlying taberite soil (-26.84±0.81 ‰, 1σ, n=21). The wider range of porewater δ13CDOC values in the lake sediment compared to the taberite soil, but narrower range of comparative δ13CSOC, along with the δ13C-shift from δ13CSOC to δ13CDOC indicates increased stable carbon isotope fractionation due to ongoing processes in the lake sediment. Increased degradation of the OC in the lake sediment relative to the underlying taberite is the most likely explanation for these differences in δ13CDOC values. As thermokarst lakes can be important greenhouse gas sources in the Arctic, it is important to better understand the degree of degradation in the individual talik layers as an indicator for their potential in greenhouse gas release, especially, as predicted warming of the Arctic in the coming decades will likely increase the number and extent (horizontal and vertical) of thermokarst lake taliks.

Abstract. Over the last decades, sea surface temperature (SST) reconstructions based on the Mg/Ca of foraminiferal calcite have frequently been used in combination with the δ18O signal from the same material, to provide estimates of δ18O of the water (δ18Ow), a proxy for global ice volume and sea surface salinity (SSS). However, because of error propagation from one step to the next, better calibrations are required to increase accuracy and robustness of existing isotope and element to temperature proxy-relationships. Towards that goal, we determined Mg/Ca, Sr/Ca and the oxygen isotopic composition of Trilobatus sacculifer (previously referenced as Globigerinoides sacculifer), collected from surface waters (0–10 m), along a North-South transect in the eastern basin of the tropical/subtropical Atlantic Ocean. We established a new paleo-temperature calibration based on Mg/Ca, and on the combination of Mg/Ca and Sr/Ca. Subsequently, a sensitivity analysis was performed in which, one, two, or three different equations were considered. Results indicate that foraminiferal Mg/Ca allow for an accurate reconstruction of surface water temperature. Combining equations, δ18Ow can be reconstructed with a precision of about ± 0.5 ‰. However, the best possible salinity reconstruction based on locally calibrated equations, only allowed reconstruction with an uncertainty of ± 2.49. This was confirmed by a Monte Carlo simulation, applied to test successive reconstructions in an ideal case, where explanatory variables are known. This simulation shows that from a pure statistical point of view, successive reconstructions involving Mg/Ca and δ18Oc preclude salinity reconstruction with a precision better than ± 1.69 and hardly better than ± 2.65, due to error propagation. Nevertheless, a direct linear fit to reconstruct salinity based on the same measured variables (Mg/Ca and δ18Oc) was established. This direct reconstruction of salinity lead to a much better estimation of salinity (± 0.26) than the successive reconstructions.