Sea surface temperature estimates and alkenone C37:4 abundances in ODP Site 145-882 and 177-1090

The cold upwelling 'tongue' of the eastern equatorial Pacific is a central energetic feature of the ocean, dominating both the mean state and temporal variability of climate in the tropics and beyond. Recent evidence for the development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline. We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis. In addition, we show that sub-Antarctic climate changed in its response to Earth's orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.

Radiogenic Hf isotopic composition of continental eolian dust in Pacific Ocean sediments

The inorganic silicate fraction extracted from bulk pelagic sediments from the North Pacific Ocean is eolian dust. It monitors the composition of continental crust exposed to erosion in Asia. 176Lu/177Hf ratios of modern dust are subchondritic between 0.011 and 0.016 but slightly elevated with respect to immature sediments. Modern dust samples display a large range in Hf isotopic composition (IC), -4.70 < epsilon-Hf < +16.45, which encompasses that observed for the time series of DSDP cores 885/886 and piston core LL44-GPC3 extending back to the late Cretaceous. Hafnium and neodymium isotopic results are consistent with a dominantly binary mixture of dust contributed from island arc volcanic material and dust from central Asia. The Hf-Nd isotopic correlation for all modern dust samples, epsilon-Hf= =0.78 epsilon-Nd = +5.66 (n =22, R**2 =0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in epsilon-Hf of Asian dust exceeds that predicted on the basis of corresponding epsilon-Nd values (34.76 epsilon-Hf < +2.5; -10.96< epsilon-Nd <-10.1). This is attributed to: (1) the fixing of an important unradiogenic fraction of Hf in zircons, balanced by radiogenic Hf that is mobile in the erosional cycle, (2) the elevated Lu/Hf ratio in chemical sediments which, given time, results in a Hf signature that is radiogenic compared with Hf expected from its corresponding Nd isotopic components, and (3) the possibility that diagenetic resetting of marine sediments may incorporate a significant radiogenic Hf component into diagenetically grown minerals such as illite. Together, these processes may explain the variability and more radiogenic character of Hf isotopes when compared to the Nd isotopic signatures of Asian dust. The Hf-Nd isotope time series of eolian dust are consistent with the results of modern dust except two samples that have extremely radiogenic Hf for their Nd (epsilon-Hf =+8.6 and +10.3, epsilon-Nd =39.5 and 39.8). These data may point to a source contribution of dust unresolved by Nd and Pb isotopes. The Hf IC of eolian dust input to the oceans may be more variable and more radiogenic than previously anticipated. The Hf signature of Pacific seawater, however, has varied little over the past 20 Myr, especially across the drastic increase of eolian dust flux from Asia around 3.5 Ma. Therefore, continental contributions to seawater Hf appear to be riverine rather than eolian. Current predictions regarding the relative proportions of source components to seawater Hf must account for the presence of a variable and radiogenic continental component. Data on the IC and flux of river-dissolved Hf to the oceans are urgently required to better estimate contributions to seawater Hf. This then would permit the use of Hf isotopes as a monitor of past changes in erosion.

(Table A1) Diatom isotope measurements from ODP Hole 145-882A

In comparison to other sectors of the marine system, the palaeoceanography of the subarctic North Pacific Ocean is poorly constrained. New diatom isotope records of d13C, d18O, d30Si (d13Cdiatom, d18Odiatom, and d30Sidiatom) are presented alongside existing geochemical and isotope records to document changes in photic zone conditions, including nutrient supply and the efficiency of the soft-tissue biological pump, between Marine Isotope Stage (MIS) 4 and MIS 5e. Peaks in opal productivity in MIS 5b/c and MIS 5e are both associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone. Whereas the MIS 5e peak is associated with low rates of nutrient utilisation, the MIS 5b/c peak is associated with significantly higher rates of nutrient utilisation. Both peaks, together with other smaller increases in productivity in MIS 4 and 5a, culminate with a significant increase in freshwater input which strengthens/re-establishes the halocline and limits further upwelling of sub-surface waters to the photic zone. Whilst d30Sidiatom and previously published records of diatom d15N (d15Ndiatom) (Brunelle et al., 2007, 2010) show similar trends until the latter half of MIS 5a, the records become anti-correlated after this juncture and into MIS 4, suggesting a possible change in photic zone state such as may occur with a shift to iron or silicon limitation.

Stable oxygen isotope ratios of diatoms and relative species abundances in ODP Site 145-882

Recent research has increasingly advocated a role for the North Pacific Ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record. Here a diatom d18O record is presented from Ocean Drilling Program Site 882 over the Pliocene/Quaternary boundary from 2.73 Ma to 2.52 Ma (MIS G6-MIS 99). Large changes in d18Odiatom of c. 4 per mil from 2.73 Ma onwards are documented to occur on a timeframe broadly coinciding with glacial-interglacial cycles. These changes are primarily attributed to large scale inputs of meltwater from glacials surrounding the North Pacific Basin and the Bering Sea. Despite these inputs and associated change in surface water salinity, on the basis of existing opal and UK37 temperature data and new modelled water column densities, no evidence exists to suggests a removal of the halocline stratification or a resumption of the high productivity system similar to that which prevailed prior to 2.73 Ma. The permanence of the halocline suggests that the region played a key role in driving global climatic changes over the early glacial-interglacial cycles that followed the onset of major Northern Hemisphere Glaciation by inhibiting deep water upwelling and ventilation of CO2 to the atmosphere.