Abstract:
The Eocene-Oligocene interval represents a transition from global "greenhouse" to "icehouse"conditions related to drawdown of atmospheric carbon dioxide and tectonic isolation of Antarctica. Benson et al. (1985), using 1-Myr sampling intervals, documented the global deep-sea ostracode response to this transition to be decreased abundance, increased diversity, and decreased provinciality. To better understand the tempo and mode of ostracode response in the Southern Ocean, an important locale for thermohaline circulation, high-resolution (20-50 kyr) faunal analyses were conducted at ODP Sites 744 (Kerguelen Plateau) and 689 (Maud Rise). The 2.5 Myr study interval brackets the major positive step in oxygen isotopes (Oi-1; approximately 34.4 Ma), which records significant cryospheric growth and deep-water cooling.At Site 744, ostracode abundance and rarefied richness is greatest prior to Oi-1, decreases rapidly at its onset, and then increases towards initial values through two Oi-1 relative maxima. In contrast, Site 689 abundance and rarefied richness are lower overall, with relative maxima from approximately 34.5-34.0 Mya, during the first Oi-1 relative maxima, and after the second Oi-1 relative maxima. Site 744 evenness is generally lower and less variable than at Site 689, but increases during Oi-1. Cluster and principal component analyses demonstrate that Site 744 faunal changes were transient, with pre- and post-Oi-1 faunas slightly more similar to one another than to the Oi-1 fauna, while at Site 689 the Oi-1 faunal shift persisted through the remainder of the study interval. At both sites, Cytherella, Bairdoppilata, and Cytherelloidea disappear at the onset of Oi-1, except for rare, single occurrences. Unlike deposit-feeding podocopid taxa, these filter-feeding platycopids can thrive under dysoxic conditions through efficient water circulation over the ventral body surface. Actinocythereis and Trachyleberis are common throughout Site 744, but at Site 689 are absent after Oi-1. Bradleya is also common throughout Site 744, but at Site 689 is present after Oi-1. Henryhowella, predominant in modern Antarctic Intermediate Water, significantly increases in abundance at Site 744 after Oi-1. These faunal changes do not correspond with surface productivity (i.e., biogenic opal) and likely reflect synchronous changes from warmer, lower-oxygen to cooler, higher-oxygen bottom water conditions.