Li/Mg systematics in scleractinian corals: Calibration of the thermometer

We show that the Li/Mg systematics of a large suite of aragonitic coral skeletons, representing a wide range of species inhabiting disparate environments, provides a robust proxy for ambient seawater temperature. The corals encompass both zooxanthellate and azooxanthellate species (Acropora sp., Porites sp., Cladocora caespitosa, Lophelia pertusa, Madrepora oculata and Flabellum impensum) collected from shallow, intermediate, and deep-water habitats, as well as specimens cultured in tanks under temperature-controlled conditions. The Li/Mg ratios observed in corals from these diverse tropical, temperate, and deep-water environments are shown to be highly correlated with temperature, giving an exponential temperature relationship of: Li/Mg (mmol/mol) = 5.41 exp (−0.049 * T) (r² = 0.975, n = 49). Based on the standard error of the Li/Mg versus temperature correlation, we obtain a typical precision of ±0.9 °C for the wide range of species analysed, similar or better than that of other less robust coral temperature proxies such as Sr/Ca ratios.The robustness and species independent character of the Li/Mg temperature proxy is shown to be the result of the normalization of Li to Mg, effectively eliminating the precipitation efficiency component such that temperature remains as the main controller of coral Li/Mg compositions. This is inferred from analysis of corresponding Li/Ca and Mg/Ca ratios with both ratios showing strong microstructure-related co-variations between the fibrous aragonite and centres of calcification, a characteristic that we attribute to varying physiological controls on growth rate. Furthermore, Li/Ca ratios show an offset between more rapidly growing zooxanthellate and azooxanthellate corals, and hence only an approximately inverse relationship to seawater temperature. Mg/Ca ratios show very strong physiological controls on growth rate but no significant dependence with temperature, except possibly for Acropora sp. and Porites sp. A strong positive correlation is nevertheless found between Li/Ca and Mg/Ca ratios at similar temperatures, indicating that both Li and Mg are subject to control by similar growth mechanisms, specifically the mass fraction of aragonite precipitated during calcification, which is shown to be consistent with a Rayleigh-based elemental fractionation model.The highly coherent array defined by Li/Mg versus temperature is thus largely independent of coral calcification mechanisms, with the strong temperature dependence reflecting the greater sensitivity of the KdLi/Ca partition coefficient relative to KdMg/Ca. Accordingly, Li/Mg ratios exhibit a highly coherent exponential correlation with temperature, thereby providing a more robust tool for reconstructing paleo-seawater temperatures

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems

Development of a mechanistic understanding of elements incorporation into biogenic calcite (foraminifera)

The elemental composition of biogenic carbonates has become, in the last decade, an important tool to reconstruct past oceanic conditions. My thesis aimed to optimise these reconstructions by developing a mechanistic understanding of the biotic processes that control divalent cation concentration into calcitic foraminiferal tests. On timescales much shorter than their residence time, the elements strontium and magnesium occur in seawater with nearly constant ratios to calcium. Variation in Sr/Ca and Mg/Ca in foraminiferal tests can then be explained as a function of environmental parameters that control their incorporation into the tests. For foraminifera, temperature appears to be the dominant parameter controlling the incorporation of Mg, however, a better understanding of the possible impact of other parameters such as pH or [CO32-], and salinity is needed to increase the accuracy of element ratio proxies. Culture studies carried out under controlled physico-chemical conditions can rule out microhabitat effects and provide the best opportunity to deconvolve potential environmental effects. However, laboratory experiments with benthic foraminifera are long-lasting as their life cycle is typically longer than that of other unicellular marine calcifier, and therefore fastidious. Analytical improvement allows to determine elemental concentration on a single chamber. The latter is crucial in the investigation of benthic foraminifera since the growth of new chambers under laboratory conditions do not face the difficulties associated with experiments involving reproduction. The addition of chambers can be recognized by the incorporation of the fluorescent dye calcein. In order to use this experimental technique, the potential impact of calcein on the elemental composition of calcium carbonate (both biotic and abiotic) was investigated. In that aim, specimens of the shallow water benthic foraminifera Ammonia tepida were cultured in the presence and absence of calcein (15°C, salinity 33), and Mg and Sr in newly formed chambers were analysed. Additionally, the impact of calcein on Mg and Sr incorporation in inorganically precipitated calcium carbonate crystals was quantified. Results show that presence of calcein does not significantly impact the incorporation of Mg and Sr into biologically and inorganically precipitated calcium carbonate (publication I). In the light of results from publication I, several culture experiments were performed in order to quantify in isolation the effect of temperature and salinity on Mg and Sr incorporation into benthic foraminifera calcite. Individuals of Ammonia tepida were cultured under three different salinities (20, 33 and 40) and two different temperatures (10 and 15°C). Weights and elemental composition were determined. Results indicate that both Mg and Sr incorporation are enhanced with increasing temperatures and increasing salinity (salinity increase of 2 results in enhanced Mg incorporation equivalent to 1°C temperature increase). However, the temperature dependency for Sr disappears when the distribution factor DSr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. Alternatively, Mg incorporation appeared independent of calcification rate. (publication II). To assess the sensitivity of benthic foraminifera to changing carbon dioxide levels and subsequent alteration in seawater carbonate chemistry, a second series of experiment were performed. Individuals of Ammonia tepida were maintained under two concentrations of atmospheric CO2 (120 and 2000 ppm) and two temperatures (10°C and 15°C). Shell weights and elemental compositions were determined. Results indicate that shell weights decrease with decreasing [CO32-], and increase with decreasing temperature. Changes in [CO32-] or total dissolved inorganic carbon do not affect the Mg partition coefficient. On the contrary, Sr incorporation is enhanced under increasing [CO32-] (publication III). Finally, Mg/Ca, Sr/Ca ratio and oxygen isotope composition of Globigerinoides sacculifer specimens collected in surface waters (0-10m) along an eastern transect of the tropical Atlantic Ocean were measured, in order to test and further calibrate geochemical proxies based on planktonic foraminifera species. This allowed for establishment of (1) a new calibration for the paleo-temperature reconstruction equation considering Mg/Ca but also Sr/Ca ratios and, (2) a new calibration for the oxygen isotopes paleo-temperature equation for surface waters collected G. sacculifer specimens. Subsequently, several paleo-reconstruction scenarios were tested in which, one, two, or three reconstruction equations were considered. Results indicate that foraminiferal Mg/Ca ratios allow for an accurate reconstruction of surface water temperature. In contrast, δ18Ow can only be reconstructed within a precision of about ±0.5. No reliable salinity reconstruction could be performed in this study (publication VI)

Impact of seawater pCO2 on calcification and Mg/Ca and Sr/Ca ratios in benthic foraminifera calcite: results from culturing experiments with Ammonia tepida

Evidence of increasing concentrations of dissolved carbon dioxide, especially in the surface ocean and its associated impacts on calcifying organisms, is accumulating. Among these organisms, benthic and planktonic foraminifera are responsible for a large amount of the globally precipitated calcium carbonate. Hence, their response to an acidifying ocean may have important consequences for future inorganic carbon cycling. To assess the sensitivity of benthic foraminifera to changing carbon dioxide levels and subsequent alteration in seawater carbonate chemistry, we cultured specimens of the shallow water species Ammonia tepida at two concentrations of atmospheric CO2 (230 and 1900 ppmv) and two temperatures (10 °C and 15 °C). Shell weights and elemental compositions were determined. Impact of high and low pCO2 on elemental composition are compared with results of a previous experiment were specimens were grown under ambient conditions (380 ppvm, no shell weight measurements of specimen grown under ambient conditions are, however, available). Results indicate that shell weights decrease with decreasing [CO32−], although calcification was observed even in the presence of calcium carbonate under-saturation, and also decrease with increasing temperature. Thus both warming and ocean acidification may act to decrease shell weights in the future. Changes in [CO32−] or total dissolved inorganic carbon do not affect the Mg distribution coefficient. On the contrary, Sr incorporation is enhanced under increasing [CO32−]. Implications of these results for the paleoceanographic application of foraminifera are discussed

La mémoire des océans

International audienc

Combined use of foraminifera Mg/Ca and Sr/Ca concentrations to strengthen temperature reconstructions over geological timescales.

International audienc

Combined use of foraminifera Mg/Ca and Sr/Ca concentrations to strengthen temperature reconstructions over geological timescales.

International audienceTo overcome the lack of long-term instrumental records, paleo-reconstructions using geochemical signatures preserved in the carbonate shells of foraminifera, provide a unique opportunity to reconstruct changes in seawater environmental parameters over time and evaluate the validity of climate change scenarios. High resolution paleoceanographic studies can provide new insights about Paleoclimatic perturbations allowing better prediction of biotic responses to modern warming and acidification. In this study, we will present calibrations using modern foraminifera sampled from surface seawater of the Atlantic Ocean, where all environmental parameters were recorded. Mg and Sr concentrations were determined using LA-ICP-MS, while the oxygen isotope composition of the same tests as used for the elemental analyses was subsequently measured by IRMS. Our results show that combining Mg and Sr data to compute temperature improve temperature reconstruction most probably by accounting for the impact of salinity and/or carbonate chemistry.To further test this hypothesis a new set of temperature data covering the Paleogene was extracted from δ18O, porosity, and combined Mg/Ca and Sr/Ca data, measured on Subbotina linaperta, from three localities: South and North Atlantic, and North Pacific. This foraminifera multi-proxies approach allowed to determine a continuous Paleogene sub-surface temperature. For the first time, the results of the three proxies are consistent with each other and highlight the existence of a latitudinal gradient over the Paleogene, punctuated by short term variations. Hence, we emphasize here: 1) the combined use of Mg/Ca and Sr/Ca concentrations within temperature equation reconstruction to strengthen paleoenvironmental reconstructions on geological timescales, and 2) the utility of porosity as a complementary ecological proxy, to either discard or complement geochemical data