SEASONALITY CHANGES ACROSS THE EOCENE-OLIGOCENE GREENHOUSE TO ICEHOUSE TRANSITION

Stable isotopic analyses the planktic foraminifera Subbotina eocaena support seasonality changes during the Eocene-Oligocene greenhouse/icehouse transition. Whereas atmospheric pCO₂ and ocean gateways are proposed as ultimate causes for the transition, seasonally has been suggested as both a proximal trigger for ice build up and the direct cause of correlative extinctions. Testing seasonality hypotheses, though, is difficult. Accretionally secreted materials like bivalve shells and otoliths provide only short times series, and these fossils are not present in deep sea cores that are the source of much of the deep time paleoclimatic data. To estimate changes in seasonality in deep sea cores, we applied a technique exploiting the short life span of foraminifera and the thoroughly mixed nature of many pelagic oozes. Because planktic foraminifera live for about a month and build shells whose δ¹⁸O values vary as a function of the temperature of the water in which they grew, and because specimens from many centuries are mixed in a typical 5 cm³, deep-sea sample, δ¹⁸O ratios of individual specimens can be treated as a proxy for the temperature of a randomly selected month within the time interval represented by the sample. Thus, the standard deviation of δ¹⁸O values among many specimens within a sample provides proxy for equability and is a metric that can be compared among samples.

In our study sites, averaged isotopic values within samples reproduce expected trends well supporting the fidelity of the results. No consistent seasonal signal is apparent at a mid-latitude site (DSDP Site 763), but at a relatively high latitude site (DSDP Site 592), seasonality drops markedly before and through an increase in δ¹⁸O values (Oi1) that marks the first major build up of Antarctic ice. This result is consistent with the proposition that decreased seasonality on Antarctica contributed to the initiation of Antarctic glaciation. Continuing analyses will extend the Site 592 record above the level of Oi1.