Abstract:
The Eocene-Oligocene tranistion is marked by a pronounced 1.5% decrease in foraminiferal delta (super 18) O termed Oi-1, reflecting ice-volume expansion and bottom-water cooling. To assess temperature change at ODP Site 744A (Kerguelen Plateau), the temperature-dependent Mg/Ca ratios of Krithe ostracodes were determined by ICP-MS. Krithe Mg/Ca shows no net trend through Oi-1, indicating that the delta (super 18) shift largely reflects major Antarctic ice-sheet development. Major deep-ocean ostracode faunal abundance, richness, and evenness changes coincide with Oi-1 at ODP Sites 744A and 689B (Maud Rise). Following Oi-1, Site 744A faunal composition shifted slightly back towards initial compositions, whereas Site 689B faunas remained relartively distinct. The Oi-1-coincident faunal changes include both uniform and reversed. Together, these faunal patterns imply a common response to increased Oi-1 surface productivity and a disparate response to either subtle productivity intensity differences or circulation changes. At Site 744A, morphometric examination of seven Algulhasina quadrata instar stages show a strong positive correlation with biogenic opal, indicating that increased organic carbon delivery after Oi-1 allowed increased instar size through sequestering of greater metabolic reserves. (modif. auth. abstr.)