Abstract:
The ecology and evolution of deep-ocean organisms are strongly affected by overlying surface productivity and ambient bottom-water conditions. During the Eocene-Oligocene transition, a transient but extreme climate event termed Oi-1 is characterized by significant cryospheric growth, ocean-atmosphere cooling, and increases in surface productivity at high southern-latitudes. The impact of these environmental changes on the deep-ocean benthos was examined through morphometric analysis of the ostracode Algulhasina quadratica at ODP Site 744 on Kerguelen Plateau in the Southern Ocean. For each stratigraphic sample (n = 151) of the approximately 4. 2 Myr study interval, all complete left valves were measured for length, height, area, and outline. Discrete clustering of the entire sample population in bivariate length:height space allowed assignment of individuals to seven instar populations (A-6 through Adult). Stratigraphic size variation within each instar population was compared by product-moment correlation to coeval paleoenvironmental proxies of benthic foraminiferal oxygen and carbon isotopes, weight % biogenic opal, and weight % sand. Valve size in well-represented instar populations (i.e., A-4 through A-1) is strongly significantly correlated to weight % biogenic opal, with correlation coefficients approximately 2-8 times that of other paleoenvironmental proxies. Valve size variation within instar populations appears isometric, with no evidence for changes in epidermal cell abundance based on reticulation or changes in valve shape based on eigenshape analysis. This strong significant positive correlation of ostracode valve size with weight % biogenic opal has several implications. First, the correlation is consistent with the hypothesis that weight % biogenic opal is a useful proxy for surface export of labile organic matter. Second, the correlation supports an ecophysiological model wherein greater food availability increased metabolic reserves during intermolt intervals, and thereby fostered greater growth during intramolt intervals of high energy expenditure and non-feeding. Third, increased surface export of organic carbon appears to have "won the battle" over any related decrease in bottom-water carbonate saturation to produce generally larger individuals. Fourth, valve size changes appear ecophenotypic, with no evidence for evolution through the Oi-1 event.