Sea surface temperature calculated from UK37 and UK'37 in the Pacific sector of the Southern Ocean

In spite of the important role played by the Southern Ocean in global climate, the few existing paleoceanographic records in the east Pacific sector do not extend beyond one glacial-interglacial cycle, hindering circumpolar comparison of past sea surface temperature (SST) evolution in the Southern Ocean. Here we present three alkenone-based Pleistocene SST records from the subantarctic and subtropical Pacific. We use a regional core top calibration data set to constrain the choice of calibrations for paleo SST estimation. Our core top data confirm that the alkenone-based UK37 and UK'37 values correlate linearly with the SST, in a similar fashion as the most commonly used laboratory culture-based calibrations even at low temperatures (down to ~1°C), rendering these calibrations appropriate for application in the subantarctic Pacific. However, these alkenone indices yield diverging temporal trends in the Pleistocene SST records. On the basis of the better agreement with d18O records and other SST records in the subantarctic Southern Ocean, we propose that the UK37 is a better index for SST reconstruction in this region than the more commonly used UK'37 index. The UK37-derived SST records suggest glacial cooling of ~8°C and ~4°C in the subantarctic and subtropical Pacific, respectively. Such extent of subantarctic glacial cooling is comparable to that in other sectors of the Southern Ocean, indicating a uniform circumpolar cooling during the Pleistocene. Furthermore, our SST records also imply massive equatorward migrations of the Antarctic Circumpolar Current (ACC) frontal systems and an enhanced transport of ACC water to lower latitudes during glacials by the Peru-Chile Current.

Stable oxygen and carbon isotope ratios of the planktonic foraminifera Pulleniatina obliquiloculata and the benthic foraminifera Uvigerina excellens of ODP Site 117-723 in the Arabian Sea (Table 1)

Site 723 is located in a water depth of 808 m at the center of the oxygen minimum zone and the middle part of the main thermocline on the Oman Margin. Oxygen isotope curves of planktonic delta18OP and benthic delta18OB can be traced back continuously to Stage 23 with high resolution measurements. A tentative correlation to Stage 53 has been tried using oxygen isotope stratigraphy. The amplitudes of the fluctuations of the benthic delta18OB curve are small, compared with the planktonic delta18OP curve. The delays of benthic oxygen isotopes delta18OB related to the planktonic delta18OP appear in the transgressive stages. Carbon isotopes of benthic delta13CB and planktonic delta13CP generally show an inverse correlation with oxygen isotope values delta18OB and delta18OB and delta18OP, however, the changes of delta13C are more gradual than those of delta18O during transgressive stages in spite of the synchronized changes of delta13C with those of delta18O during regressive stages. The difference of oxygen isotope between benthic and planktonic foraminifers represents the degree of pushing up the thermocline by upwelling, and the difference of carbon isotope represents the relative amount of upwelling Sigma[CO2] to the biological uptake in the surface water. These isotopic differences can be used as indicators of upwelling and show strong upwelling in the interglacial and weak upwelling in the glacial stages. The organic carbon content is correlated with the isotopic upwelling indicators, and higher content is correlated with the isotopic upwelling indicators and higher content appears in the interglacial stages. The calculated rate of sedimentation based on oxygen isotope stratigraphy in glacial stages is significantly high, two to four times that of interglacial stages, and the absolute flux of fluvial sediments with variability of lithofacies increased in the glacial stage. The present glacial-interglacial cycle with the fluctuation of upwelling relating to the southwest monsoon can be traced back to Stage 8, 250 ka. From Stage 8 to 12, 250-450 ka, the upwelling indicator of oxygen isotope difference did not show such distinct cyclicity. For Stages 12-15, 450-600 ka, the upwelling can be estimated as strong as in interglacial stage of the present cycles, with slightly weak upwelling in the glacial stage. This upwelling and climate can be traced back to the late Pliocene. The strongest upwelling can be estimated in the Pliocene-Pleistocene time by the isotopic indicators and the high organic carbon content.