Foraminiferal Mg/Ca and Mn/Ca ratios across the Eocene-Oligocene transition

Constraining the magnitude of high-latitude temperature change across the Eocene-Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (d18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2-3 °C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal d18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical d18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2-3 °C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal d18O shift indicates that seawater d18O values increased by 0.6 ± 0.15 per mil across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60-130% modern East Antarctic ice sheet volume.

Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (PML B92/11), 2000

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO2 concentrations (1.1-53.5 micromol L-1). The carbon isotope fractionation of POC (POC) varied by ca. 7 per mil and was positively correlated with aqueous CO2 concentration [CO2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of POC and in the slope of the relationship of POC with growth rate and [CO2aq]. Also, a significantly greater offset was obtained between the delta13C of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8 per mil). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in 13C by 2.3 per mil to 4.1 per mil, phytol was depleted in 13C by 1.9 per mil, and the major sterol 24-methylcholesta-5,22E-dien-3beta-ol was depleted in 13C by 8.5 per mil. This large spread of delta13C values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have delta13C values differing by just a few per mil. Increases in [CO2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U37k- used for reconstructions of paleo sea surface temperature.

Ogasawara winter Sr/Ca and U/Ca data of coral core OGA-02-1 and PDO reconstruction (coral-geoduck index)

The Pacific Decadal Oscillation (PDO), the leading mode of sea surface temperature (SST) anomalies in the extratropical North Pacific Ocean, has widespread impacts on precipitation in the Americas and marine fisheries in the North Pacific. However, marine proxy records with a temporal resolution that resolves interannual to interdecadal SST variability in the extratropical North Pacific are extremely rare. Here we demonstrate that the winter Sr/Ca and U/Ca records of an annually-banded reef coral from the Ogasawara Islands in the western subtropical North Pacific are significantly correlated with the instrumental winter PDO index over the last century. The reconstruction of the PDO is further improved by combining the coral data with an existing eastern mid-latitude North Pacific growth ring record of geoduck clams. The spatial correlations of this combined index with global climate fields suggest that SST proxy records from these locations provide potential for PDO reconstructions further back in time.