Sea surface temperature estimation for the Iberian Margin

Centennial climate variability over the last ice age exhibits clear bipolar behavior. High-resolution analyses of marine sediment cores from the Iberian margin trace a number of associated changes simultaneously. Proxies of sea surface temperature and water mass distribution, as well as relative biomarker content, demonstrate that this typical north-south coupling was pervasive for the cold phases of climate during the past 420,000 years. Cold episodes after relatively warm and largely ice-free periods occurred when the predominance of deep water formation changed from northern to southern sources. These results reinforce the connection between rapid climate changes at Mediterranean latitudes and century-to-millennial variability in northern and southern polar regions.

Biogenic opal estimation of deep-sea sediment cores from the Scotia Sea

We present biogenic opal flux records from two deep-sea sites in the Scotia Sea (MD07-3133 and MD07-3134) at decadal-scale resolution, covering the last glacial cycle. Besides conventional and time-consuming biogenic opal measuring methods, we introduce new biogenic opal estimation methods derived from sediment colour b*, wet bulk density, Si/Ti-count ratio, and Fourier transform infrared spectroscopy (FTIRS). All methods capture the biogenic opal amplitude, however, FTIRS - a novel method for marine sediment - yields the most reliable results. 230Th normalization data show strong differences in sediment focusing with intensified sediment focusing during glacial times. At MD07-3134 230Th normalized biogenic opal fluxes vary between 0.2 and 2.5 g/cm2/kyr. Our biogenic opal flux records indicate bioproductivity changes in the Southern Ocean, strongly influenced by sea ice distribution and also summer sea surface temperature changes. South of the Antarctic Polar Front, lowest bioproductivity occurred during the Last Glacial Maximum when upwelling of mid-depth water was reduced and sea ice cover intensified. Around 17 ka, bioproductivity increased abruptly, corresponding to rising atmospheric CO2 contents and decreasing seasonal sea ice coverage.

Estimation of sea surface temperatures and salinity of ODP Site 177-1089 (Appendix A)

ODP Site 1089 is optimally located in order to monitor the occurrence of maxima in Agulhas heat and salt spillage from the Indian to the Atlantic Ocean. Radiolarian-based paleotemperature transfer functions allowed to reconstruct the climatic history for the last 450 kyr at this location. A warm sea surface temperature anomaly during Marine Isotope Stage (MIS) 10 was recognized and traced to other oceanic records along the surface branch of the global thermohaline (THC) circulation system, and is particularly marked at locations where a strong interaction between oceanic and atmospheric overturning cells and fronts occurs. This anomaly is absent in the Vostok ice core deuterium, and in oceanic records from the Antarctic Zone. However, it is present in the deuterium excess record from the Vostok ice core, interpreted as reflecting the temperature at the moisture source site for the snow precipitated at Vostok Station. As atmospheric models predict a subtropical Indian source for such moisture, this provides the necessary teleconnection between East Antarctica and ODP Site 1089, as the subtropical Indian is also the source area of the Agulhas Current, the main climate agent at our study location. The presence of the MIS 10 anomaly in the delta13C foraminiferal records from the same core supports its connection to oceanic mechanisms, linking stronger Agulhas spillover intensity to increased productivity in the study area. We suggest, in analogy to modern oceanographic observations, this to be a consequence of a shallow nutricline, induced by eddy mixing and baroclinic tide generation, which are in turn connected to the flow geometry, and intensity, of the Agulhas Current as it flows past the Agulhas Bank. We interpret the intensified inflow of Agulhas Current to the South Atlantic as responding to the switch between lower and higher amplitude in the insolation forcing in the Agulhas Current source area. This would result in higher SSTs in the Cape Basin during the glacial MIS 10, due to the release into the South Atlantic of the heat previously accumulating in the subtropical and equatorial Indian and Pacific Ocean. If our explanation for the MIS 10 anomaly in terms of an insolation variability switch is correct, we might expect that a future Agulhas SSST anomaly event will further delay the onset of next glacial age. In fact, the insolation forcing conditions for the Holocene (the current interglacial) are very similar to those present during MIS 11 (the interglacial preceding MIS 10), as both periods are characterized by a low insolation variability for the Agulhas Current source area. Natural climatic variability will force the Earth system in the same direction as the anthropogenic global warming trend, and will thus lead to even warmer than expected global temperatures in the near future.

Sea surface temperature estimation of sediment core 86014-12PC51

A recently developed technique for determining past sea surface temperatures (SST), based on an analysis of the unsaturation ratio of long chain C37 methyl alkenones produced by Prymnesiophyceae phytoplankton (U37 k' ), has been applied to an upper Quaternary sediment core from the equatorial Atlantic. U37 k' temperature estimates were compared to those obtained from delta18O of the planktonic foraminifer Globigerinoides sacculifer and of planktonic foraminiferal assemblages for the last glacial cycle. The alkenone method showed 1.8°C cooling at the last glacial maximum, about 1/2 to 1/3 of the decrease shown by the isotopic method (6.3°C) and foraminiferal modern analogue technique estimates for the warm season (3.8°C). Warm season foraminiferal assemblage estimates based on transfer functions are out of phase with the other estimates, showing a 1.4°C drop at the last glacial maximum with an additional 0.9°C drop in the deglaciation. Increased alkenone abundances, total organic carbon percentage and foraminiferal accumulation rates in the last glaciation indicate an increase in productivity of as much as 4 times over present day. These changes are thought to be due to increased upwelling caused by enhanced winds during the glaciation. If U37 k' estimates are correct, as much as 50-70% (up to 4.5°C) of estimated delta18O and modern analogue temperature changes in the last glaciation may have been due to changes in thermocline depth, whereas transfer functions seem more strongly influenced by seasonality changes. This indicates these estimates may be influenced as strongly by other factors as they are by SST, which in the equatorial Atlantic was only reduced slightly in the last glaciation.

(Table 2) Age determination of sediment cores from the Portuguese Margin

We demonstrate that changes in the behavior of the Mediterranean Outflow Water (MOW) prior to and through the last deglaciation played an important role in promoting Meridional Overturning Circulation (MOC). Estimation of past MOW salt and heat fluxes indicates that they gradually increased through the last deglaciation. Between 17.5 and 14.6 thousand years ago (ka B.P., where B.P. references year 1950), net evaporation from the Mediterranean exported sufficient fresh water from the North Atlantic catchment to cause an average salinity increase of 0.5 psu throughout the upper 2000 m of the entire North Atlantic to the north of 25°N. Combined with rapid intensification and shoaling of the MOW plume, which we identify around 15-14.5 ka B.P., this deglacial MOW-related salt accumulation preconditioned the North Atlantic for abrupt resumption of the MOC at 14.6 ka B.P.