Alkenones and sea surface reconstruction of sediments from, the Arabian Sea

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 delta18O records established on planktonic foraminifera. In general, the SST records follow the delta18O 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 delta18O 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.

Stable carbon and oxygen isotope ratios of planktonic foraminifera from the equatorial Atlantic

Carbon isotopic records of nutrient-depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pCO2 levels. The best records of nutrient-depleted delta13C are obtained from planktonic foraminifera living in the thick mixed layers of the western equatorial and tropical Atlantic Ocean. We have produced a composite, stacked Globigerinoides sacculifer delta13C record from the equatorial Atlantic, which exhibits significant spectral power at the 100,000- and 41,000-year Milankovitch periods, but no power at the 23,000-year period. Similar to the record presented by Shackleton and Pisias [1985], surface-deep ocean Delta delta13C produced with the G. sacculifer record leads the delta18O ice volume record. However, the glacial-interglacial amplitudes of Delta delta13C differ between our record and Shackleton and Pisias [1985] record. Although large changes in Delta delta13C occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface-deep Delta delta13C at glacial maxima (18O stage 2, late stage 6, and late stage 8) was only about 0.2? greater than during the subsequent interglacial. Our results imply that nutrient-driven pCO2 changes account for about one third of the pCO2 decrease observed in ice cores, and consequently, Delta delta13C should not be used as a proxy pCO2 index. Enough variance in the ice core pCO2 records remains to be explained that conclusions about pCO2 and ice volume phase relationships should also be reexamined. As much as 40 ppm pCO2 change still has not been accounted for by models of past physics and chemistry of the ocean.

Stable isotope record and aklenonen index of MIS 11 sediments

The interglacial known as Marine Isotope Stage 11 has been proposed to be analogous to the Holocene, owing to similarities in the amplitudes of orbital forcing. It has been difficult to compare the periods, however, because of the long duration of Stage 11 and a lack of detailed knowledge of any extreme climate events that may have occurred. Here we use the distinctive phasing between seasurface temperatures and the oxygen-isotope records of benthic foraminifera in the southeast Atlantic Ocean to stratigraphically align the Holocene interglacial with the first half of the Marine Isotope Stage 11 interglacial optimum. This alignment suggests that the second half of Marine Isotope Stage 11 should not be used as a reference for 'pre-anthropogenic' greenhouse-gas emissions. By compiling benthic carbon-isotope records from sites in the Atlantic Ocean on a single timescale, we also find that meridional overturning circulation strengthened about 415,000 years ago, at a time of high orbital obliquity. We propose that this mechanism transported heat to the high northern latitudes, inhibiting significant ice-sheet build-up and prolonging interglacial conditions. We suggest that this mechanism may have also prolonged other interglacial periods throughout the past 800,000 years.

Percentages of sand-sized material and biogenic components in sediment core GIK12392-1

The paleo-oceanography of the southeastern North Atlantic Ocean during the last 150,000 yr has been studied using biogenous and terrigenous components of hemipelagic sediments sampled close to the northwest African continental margin. Variations of oxygen isotope ratios in shells of benthic calcareous foraminifers in two cores allow the assignment of absolute ages to these cores (in the best case at 1000 yr increments). The uncorrected bulk sedimentation rates of the longest core range from 3.4 to 7.6 cm/ 1000 yr during Interglacial conditions, and from 6.5 to 9.9 cm/1000 yr during Glacial conditions; all other cores have given results of the same order of magnitude, but with generally increasing values towards the continental edge. The distribution of sediment components allow us to make inferences about paleo-oceanographic changes in this region. Frequencies of biogenic components from benthic organisms, oxygen isotope ratios measured in benthic calcareous foraminiferal shells, the total carbonate contents of the sediment and distributions of biogenic components from planktonic organisms often fluctuate in concert. However, all fluctuations which can be attributed to changes of the bottom water masses (North Atlantic Deep Water) seem to precede by several thousand years those which can be linked to changes of the surface water mass distributions or to changes of the climate over the neighboring land masses. Late Quaternary planktonic foraminiferal assemblages in the cores from the northwest African continental margin can be defined satisfactorily in the way that distributions of assemblages found in sediment surface samples from the northeast Atlantic Ocean have been explained. The distributions of assemblages in the northwest African cores can also be used to estimate past sea surface temperatures and salinities. The downcore record of these estimates reveals two warm periods during the last 150,000 yr, the lower one corresponding to the oxygen isotope stage 5 e (equivalent to the Eemian proper in Europe), the upper one to the younger half of the Holocene. Winter surface water temperatures during oxygen isotope stages 6, 4, 3, and 2 are remarkably constant in most cores, while summer sea surface temperatures during stage 3 reach values comparable to those of the warm periods during the Late Holocene and Eemian. Estimated winter sea surface temperatures range from > 16 °C to < 11°C, the summer sea surface temperatures from > 22 °C to < 15 °C during the last 150,000 yr. Estimates of the winter sea surface salinities fluctuate between 36.6? and 35.5?, the higher values being restricted to the warm periods since the penultimate Glacial. Estimates for sea surface temperatures and salinities for two cores from the center of today's coastal upwelling region show less pronounced fluctuations than the record of the open ocean cores in the case of the station 12379 off Cape Barbas, more pronounced in the case of station 12328 off Cape Blanc. Seasonal differences between winter and summer sea surface temperatures derived from the estimated temperatures are today more pronounced in the boundary region of the ocean to the continent than further away from the continent. The differences are generally higher during warm climatic periods of the last 150,000 yr than during cooler ones. The abundance of terrigenous grains in the coarse fractions generally decreases with increasing distance from the continental edge, and also from south to north. The dominant portion of the terrigenous detritus is carried out into the ocean during the relatively cool climatic periods (stage 6, 4, later part of stage 3, stage 2 and oldest part of stage 1). The enhanced precision of dating combined with the stratigraphic resolution of these high deposition rate cores make it clear that the peaks of the terrigenous input off this part of the northwest African continental margin occur simultaneously with times of rapid sea level fluctuations resulting from large volume changes of the large Glacial ice sheets.

Oxygen and carbon isotopic compositions of foraminifers at DSDP Hole 72-517

Oxygen and carbon isotope analyses were performed on monospecific or mixed-species samples of benthic foraminifers, as well as on the planktonic species Globigerinoides ruber from a 24-m hydraulic piston core raised on the western flank of the Rio Grande Rise, at DSDP Site 517 (30°56.81'S and 38°02.47'W, water depth 2963 m) in the southwestern Atlantic. This site is presently located in the core of North Atlantic Deep Water (NADW). This is the first long isotopic record of Quaternary benthic foraminifers; it displays at least 30 isotopic stages, 25 of them readily correlated with the standard sequence of Pacific Core V28-239. The depths of both the Bruhnes/Matuyama boundary and the Jaramillo Event based on oxygen isotope stratigraphy agree well with paleomagnetic results. Quaternary faunal data from this part of the Atlantic are dated through isotopic stratigraphy and partially contradict data previously published by Williams and Ledbetter (1979). There was a substantial increase in the size of the earth's major ice sheets culminating at Stage 22 and corresponding to a l per mil progressive increase of d18O maximal values. Further, ice volume-induced isotopic changes were not identical for different glacial cycles. Oxygen and carbon isotope analyses of benthic foraminifers show that during Pleistocene glacial episodes, NADW was cooler than today and that Mediterranean outflow might still have contributed to the NADW sources. The comparison of coiling ratio changes of Globorotalia truncatulinoides with planktonic and benthic oxygen isotope records shows that there might have been southward excursions of the Brazil Current during the Pleistocene, perhaps related to Antarctic surface water surges. The question of the location of NADW sources during glacial maxima remains open.

Seawater carbonate chemistry, mass, length and gene expression of Sepia officinalis during experiments, 2011

The specific transporters involved in maintenance of blood pH homeostasis in cephalopod molluscs have not been identified to date. Using in situ hybridization and immuno histochemical methods, we demonstrate that Na+/K+-ATPase (soNKA), a V-type H+-ATPase (soV-HA), and Na+/HCO3- cotransporter (soNBC) are co-localized in NKA-rich cells in the gills of Sepia officinalis. mRNA expression patterns of these transporters and selected metabolic genes were examined in response to moderately elevated seawater pCO2 (0.16 and 0.35 kPa) over a time-course of six weeks in different ontogenetic stages. The applied CO2 concentrations are relevant for ocean acidification scenarios projected for the coming decades. We determined strong expression changes in late stage embryos and hatchlings, with one to three log2-fold reductions in soNKA, soNBCe, socCAII and COX. In contrast, no hypercapnia induced changes in mRNA expression were observed in juveniles during both short- and long-term exposure. However a transiently increased demand of ion regulatory demand was evident during the initial acclimation reaction to elevated seawater pCO2. Gill Na+/K+-ATPase activity and protein concentration were increased by approximately 15% in during short (2-11 day), but not long term (42 day) exposure. Our findings support the hypothesis that the energy budget of adult cephalopods is not significantly compromised during long-term exposure to moderate environmental hypercapnia. However, the down regulation of ion-regulatory and metabolic genes in late stage embryos, taken together with a significant reduction in somatic growth, indicates that cephalopod early life stages are challenged by elevated seawater pCO2.