Corinne Sonzogni

Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea

Abstract Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles. Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somalia and Oman, which are characterized by large seasonal sea surface temperature (SST) and particle flux changes. By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2°C). For both cores we have reconstructed SST variations by means of the unsaturation ratio of C37 alkenones, which is compared with the δ18O records established on planktonic foraminifera. In general, the SST records follow the δ18O variations, with an SST maximum during oxygen isotope stage 5.5 (the Last Interglacial at about 120–130 kyr) and a broad SST minimum during isotope stage 4 and 3.3 (approximately 40–50 kyr). The SST difference between the Holocene and the Last Glacial Maximum (LGM) is of the order of 2°C. In both cores the SSTs during isotope stage 6 are distinctly higher by 1–2°C than the cold SST minima during the last glacial cycle (LGM and stage 3). To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, together with a productivity index based on coccolith species (Florisphaera profunda relative abundance). Within each core, there is a general agreement between the different palaeoproductivity proxies. In the southeastern Arabian Sea (core MD900963), glacial stages correspond to relatively high productivity, whereas warm interstadials coincide with low productivity. All time series of productivity proxies are dominated by a cyclicity of about 21–23 kyr, which corresponds to the insolation precessional cycle. A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone. By contrast, the records are more complicated in the upwelling region of the western Arabian Sea (core TY93-929/P). This is partly due to large changes in the sedimentation rates, which were higher during specific periods (isotope stages 6, 5.4, 5.2, 3 and 2). Unlike core MD900963, no simple relationship emerges from the comparison between the δ18O stratigraphy and productivity records. The greater complexity observed for core TY93-929/P could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons, the SW monsoon being enhanced during interglacial periods, whereas the NE monsoon was increased during glacial intervals. A similar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics.

Core-top calibration of the alkenone index vs sea surface temperature in the Indian Ocean

The alkenone index (UK′37) has been calibrated against sea surface temperature (SST) using a suite of 54 surface sediments from the Indian Ocean (20°N to 45°S), spanning temperatures between 6 and 29°C (0.2 < UK′37 < 1.0). A computerized oceanographic atlas has been used to calculate modern SST for the coring sites. We have constructed two sets of UK′37−SST calibrations for various water depths between 10 and 50m. The first set is based on the comparison of UK′37 with annual mean temperatures for all cores. In the second set, we assume that UK′37 values in surface sediments are linked to seasonal productivity patterns, and UK′37 is compared with mean temperatures at times of yearly production maximum. In both calibrations, the UK′37 values are strongly correlated with SST, with a best fit obtained using temperatures from 10 m and seasonal productivity patterns. The regression line is similar to that found with laboratory cultures of E. huxleyi, and with other calibrations based on recent sediments from other oceans. Above 24°C the slope of our UK′37−SST relationship is smaller (approximately 0.020 UK′37 unit °C−1) than the general slope (approximately 0.033).

Precision of the current methods to measure the alkenone proxy U(37)(K ') and absolute alkenone abundance in sediments: Results of an interlaboratory comparison study

[1]xa0Measurements of the U37K′ index and the absolute abundance of alkenones in marine sediments are increasingly used in paleoceanographic research as proxies of past sea surface temperature and haptophyte (mainly coccolith-bearing species) primary productivity, respectively. An important aspect of these studies is to be able to compare reliably data obtained by different laboratories from a wide variety of locations. Hence the intercomparability of data produced by the research community is essential. Here we report results from an anonymous interlaboratory comparison study involving 24 of the leading laboratories that carry out alkenone measurements worldwide. The majority of laboratories produce data that are intercomparable within the considered confidence limits. For the measurement of alkenone concentrations, however, there are systematic biases between laboratories, which might be related to the techniques employed to quantify the components. The maximum difference between any two laboratories for any two single measurements of U37K′ in sediments is estimated, with a probability of 95%, to be <2.1°C. In addition, the overall within-laboratory precision for the U37K′ temperature estimates is estimated to be <1.6°C (95% probability). Similarly, from the analyses of alkenone concentrations the interlaboratory reproducibility is estimated at 32%, and the repeatability is estimated at 24%. The former is compared to a theoretical estimate of reproducibility and found to be excessively high. Hence there is certainly scope and a demonstrable need to improve reproducibility and repeatability of U37K′ and especially alkenone quantification data across the community of scientists involved in alkenone research.

A multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr

foraminiferal species assemblages, and Uk37 0 (1� –2� C). These estimates are consistent with estimates from other WPWP cores, thereby suggesting that the discrepancy is due to proxy compatibility rather than differences in cores qualitity. Postdepositional dissolution above the lysocline might have altered the Mg/Ca-based temperature estimates in our site, but this effect is insufficient to resolve discrepancies between Mg/Ca in G. ruber and the other proxies. We suggest that the lower estimates obtained from Mg/Ca in G. sacculifer, faunal transfer functions, and Uk37 0 might reflect subsurface temperature changes rather than strict surface estimates. Accounting for potential artefacts, including dissolution and bioturbation, we suggest that the glacial WPWP SST was about 2.5� ±0 .7 � C cooler than during the Holocene, whereas the subsurface/upper thermocline temperature change was only about 1.8� ±0 .7� C. Interpreting variations in d 18 OSW in terms of salinity changes suggests a possibly slight decrease in surface salinity at the site of MD97-2138 during the LGM. Though LGM freshening in MD97-2138 is not robust to postdeposition dissolution effects, this inferred freshening appears to be a general feature of the western equatorial Pacific.

Northwest Africa 5958: A weakly altered CM‐related ungrouped chondrite, not a CI3

Northwest Africa (NWA) 5958 is a carbonaceous chondrite found in Morocco in 2009. Preliminary chemical and isotopic data leading to its initial classification as C3.0 ungrouped have prompted us to conduct a multitechnique study of this meteorite and present a general description here. The petrography and chemistry of NWA 5958 is most similar to a CM chondrite, with a low degree of aqueous alteration, apparently under oxidizing conditions, and evidence of a second, limited alteration episode manifested by alteration fronts. The oxygen isotopic composition, with Delta ` O-17 = -4.3 parts per thousand, is more O-16-rich than all CM chondrites, indicating, along with other compositional arguments, a separate parent body of origin. We suggest that NWA 5958 be reclassified as an ungrouped carbonaceous chondrite related to the CM group.