Oxygen isotope/salinity relationship in the Northern Indian Ocean

International audienceWe analyze the surface •5•80-salinity relationships of the Bay of Bengal and the Arabian Sea, in the northern Indian Ocean, known for their contrasting hydrological conditions. New measurements of these tracers show a very low •5•80-salinity slope associated with the strong dilution in the Bay of Bengal, but a slope more typical of this latitude in the Arabian Sea. Although this region is marked by a complex monsoonal regime, numerical modeling using a box model and a general circulation model is able to capture the •5•SO-salinity slope and its geographical variation. Both models clearly show that the low •5•SO-salinity slope is due to the evaporation-minus-precipitation balance, with an important contribution of the continental runoff in the Bay of Bengal. Although the low value of these slopes (-0.25) makes past salinity reconstructions uncertain, insight into the Last Glacial Maximum conditions shows a probable stability of these slopes and limited error on paleosalinity

Corrigendum to Preface "Climate change: from the geological past to the uncertain future – a symposium honouring André Berger" published in Clim. Past, 5, 707–711, 2009

No abstract available

Planktic foraminiferal shell thinning in the Arabian Sea due to anthropogenic ocean acidification?

About one third of the anthropogenic carbon dioxide (CO<sub>2</sub>) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO<sub>2</sub> invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO<sub>2</sub> emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer <i>Globigerinoides ruber</i> in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on <sup>14</sup>C and δ<sup>13</sup>C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (δ<sup>13</sup>C and δ<sup>18</sup>O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the <sup>14</sup>C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens

Multidecadal variations in the early Holocene outflow of Red Sea Water into the Arabian Sea

We present Holocene stable oxygen isotope data from the deep Arabian Sea off Somalia at a decadal time resolution as a proxy for the history of intermediate/upper deep water. These data show an overall δ18O reduction by 0.5‰ between 10 and ~6.5 kyr B.P. superimposed upon short-term δ18O variations at a decadal-centennial timescale. The amplitude of the decadal variations is 0.3‰ prior, and up to 0.6‰ subsequent, to ~8.1 kyr B.P. We conclude from modeling experiments that the short-term δ18O variations between 10 and ~6.5 kyr B.P. most likely document changes in the evaporation-precipitation balance in the central Red Sea. Changes in water temperature and salinity cause the outflowing Red Sea Water to settle roughly 800 m deeper than today

Holocene oscillations in temperature and salinity of the surface subpolar North Atlantic

The Atlantic meridional overturning circulation (AMOC) transports warm salty surface waters to high latitudes, where they cool, sink and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm northwestern European climate, and acts as a control on the global climate. Past climate fluctuations during the Holocene epoch (~11,700 years ago to the present) have been linked with changes in North Atlantic Ocean circulation. The behaviour of the surface flowing salty water that helped drive overturning during past climatic changes is, however, not well known. Here we investigate the temperature and salinity changes of a substantial surface inflow to a region of deep-water formation throughout the Holocene. We find that the inflow has undergone millennial-scale variations in temperature and salinity (~3.5 °C and ~1.5 practical salinity units, respectively) most probably controlled by subpolar gyre dynamics. The temperature and salinity variations correlate with previously reported periods of rapid climate change. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Model studies predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. Our data suggest that such a negative feedback mechanism may have operated during past intervals of climate change

Sr/Ca in multiple species of planktonic foraminifera: implications for reconstructions of seawater Sr/CA

Sr/Ca ratios were measured on eight species of planktonic foraminifera from a core top transect of the North Atlantic. Five of the species (Globigerinoides ruber, Globigerina bulloides, Neogloboquadrina pachyderma, Globigerinoides sacculifer, Globigerinella siphonifera) show remarkably little within-species variability in Sr/Ca (1.2 ± 0.1%). Three globorotaliid species (Globorotalia hirsuta, Globorotalia inflata, Globorotalia truncatulinoides) show somewhat greater variability (3–7%). Interspecies variations are of ?10%. The variability in globorotaliid Sr/Ca is explained either by a greater temperature sensitivity than for other species or by depth-dependent dissolution. Other than this effect, the overwhelming control on foraminiferal Sr/Ca appears to be seawater Sr/Ca. In contrast, time series Sr/Ca records of four species from a North Atlantic sediment core over a glacial-interglacial cycle show very significant differences. N. pachyderma shows large-amplitude changes in Sr/Ca (9.7%), in phase with ?18O, whereas G. ruber shows the smallest amplitude changes (2.6%) out of phase with ?18O and most similar to model predictions for temporal changes in seawater Sr/Ca. Records of G. inflata and G. bulloides are intermediate (amplitude changes of 3.3–4.7%) and out of phase with ?18O. The lack of coherence in the time series records between the species shows that factors in additional to changing seawater Sr/Ca must affect planktonic foraminiferal Sr/Ca. <br/

Stable isotope "vital effects" in coccolith calcite

Uncertainties about the origin of the many disequilibrium or 'vital effects' in a variety of calcifying organisms, and whether these effects are constant or variable, have hampered paleoceanographic application of carbon and oxygen isotopic ratios. Unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology. Culture of eight species of coccolithophorids, a dominant marine phytoplankton group, reveals a 5‰ array of disequilibrium or 'vital effects' in both the carbon and oxygen isotopic composition of coccolith calcite. In moderate light and nutrient-replete cultures, oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species. However, when growth rates of a single species are increased or decreased by higher or lower light levels,