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The resolution at which foraminiferal stable isotopes are applied in paleo-environmental studies is ever increasing, resulting in continuous sampling of sediment cores. The resolution of such continuously sampled records depends on the rate of sedimentation of foraminiferal shells in its relation to the intensity of bioturbation. Bioturbation essentially mixes sediment layers of different age, altering the primary climate signal, thereby impacting the accuracy of both the timing and magnitude of reconstructed climate changes. A new approach to assess and correct the impact of bioturbation is investigated here, based on the δ18O of individual specimens of planktonic foraminifera Globorotalia inflata from a series of boxcore samples in the Eastern North Atlantic. Average δ18O values decrease southward from 1.62 to 1.07‰ with the exception of site T86-11 (1.35‰). The δ18O distribution of each station can be fitted with a uni- to polymodal distribution. A nonunimodal distribution strongly suggests admixing of bioturbated individuals. Quantification of these distributions allows deconvolving the original and bioturbated signals and subsequently provides a correction for bioturbation. © 2013. American Geophysical Union. All Rights Reserved

Vertical distribution and diurnal migration of atlantid heteropods

© Inter-Research 201 Understanding the vertical distribution and migratory behaviour of shelled holoplanktonic gastropods is essential in determining the environmental conditions to which they are exposed. This is increasingly important in understanding the effects of ocean acidification and climate change. Here we investigated the vertical distribution of atlantid heteropods by collating data from publications and collections and using the oxygen isotope (? 18 O) composition of single aragonitic shells. Data from publications and collections show 2 patterns of migration behaviour: small species that reside in shallow water at all times, and larger species that make diurnal migrations from the surface at night to deep waters during the daytime. The ? 18 O data show that all species analysed (n = 16) calcify their shells close to the deep chlorophyll maximum. This was within the upper 110 m of the ocean for 15 species, and down to 146 m for a single species. These findings confirm that many atlantid species are exposed to large environmental variations over a diurnal cycle and may already be well adapted to face ocean changes. However, all species analysed rely on aragonite supersaturated waters in the upper < 150 m of the ocean to produce their shells, a region that is projected to undergo the earliest and greatest changes in response to increased anthropogenic CO 2

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

Decadal-centennial scale monsoon variations in the Arabian Sea during the Early Holocene

An essential prerequisite for the prediction of future climate change due to anthropogenic input is an understanding of the natural processes that control Earth's climate on timescales comparable to human-lifespan. The Early Holocene period was chosen to study the natural climate variability in a warm interval when solar insolation was at its maximum. The monsoonal system of the Tropics is highly sensitive to seasonal variations in solar insolation, and consequently marine sediments from the region are a potential monitor of past climate change. Here we show that during the Early Holocene period rapid

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

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

About one third of the anthropogenic carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt; 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 &lt;i&gt;Globigerinoides ruber&lt;/i&gt; 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 &lt;sup&gt;14&lt;/sup&gt;C and &amp;delta;&lt;sup&gt;13&lt;/sup&gt;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 (&amp;delta;&lt;sup&gt;13&lt;/sup&gt;C and &amp;delta;&lt;sup&gt;18&lt;/sup&gt;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 &lt;sup&gt;14&lt;/sup&gt;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

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

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

Effect of supra-lysoclinal dissolution on oxygen isotopes and Mg/Ca in planktonic foraminifera

Mg/Ca in planktonic foraminiferal tests is now well established as a temperature proxy for surface and near-surface waters. The advantage of using Mg/Ca coupled with oxygen isotope ratio measured on the same foraminiferal sample provides the possibility to determine seawater d18O. There has been concern, however, about the post-mortem processes (e.g., dissolution) affecting foraminiferal Mg/Ca and d18O. We document the effect of dissolution on both Mg/Ca and d18O of several planktonic foraminiferal species along two depth transects (a) ~450-4000 m off Somalia and (b) ~1500-4500 m from Ontong Java Plateau. The planktonic foraminiferal species, Globigerinoides ruber, Globigerinoides sacculifer, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei and Globorotalia tumida were analysed for Mg/Ca and d18O from core-top samples along the depth transects within a narrow size range (e.g., 250-300, 355-425 um). Mg/Ca and d18O in shells of surface dwelling planktonic foraminiferal species (e.g., G. ruber and G. sacculifer) show a small decrease while Mg/Ca and d18O of the deeper dwelling species (e.g., N. dutertrei and G. tumid) show more pronounced effect of supralysoclinal dissolution. The effect of supralysoclinal dissolution has an insignificant effect on the reconstruction of seawater d18O based on surface dwelling planktonic species compared to the deeper dwelling species