MIDDLE EOCENE WARMING ON SEYMOUR ISLAND, ANTARCTICA: CONTINENTAL SHELF PALEOTEMPERATURES RECORDED IN MOLLUSCAN CARBONATES

The Eocene represents a period of climate transition from global warmth to progressive cooling culminating in the initiation of Antarctic glaciation. Recent work based on fine fraction carbonate from Southern Ocean sites suggests that, rather than a monotonic decrease in temperature, this interval was punctuated by a brief episode of warming during the middle Eocene (Bohaty and Zachos, 2003). Here, we examine bivalves from the continental margin sequence preserved on Seymour Island, Antarctica, to test whether this episode of warming is manifested in coeval shallow shelf settings and therefore may have affected shelf biota.

Two bivalve taxa, Cucullaea and Eurhomalea, were collected from a stratigraphic sequence representing Telms 2 thorough 7 (Sadler, 1988) of the La Meseta Formation. ⁸⁷Sr/⁸⁶Sr analysis of skeletal aragonite preserved in 43 bivalves combined with stratigraphic relations allows construction of a coherent chronostratigraphy, despite the limitations of an undulating Eocene marine ⁸⁷Sr/⁸⁶Sr curve. Within this temporal context, δ¹⁸O of shell aragonite shows decreasing temperatures through much of the early to middle Eocene, with values of 0 to +0.5‰ VPDB. In Eurhomalea, this trend is punctuated by a brief interval between 40 and 43 Ma (lower Telm 6) where δ¹⁸O decreases to -1.0‰, suggesting short-lived warmth. Taxonomic diversity decreases markedly, and Cucullaea, previously interpreted as a cooler-water taxon, is locally absent. Temperatures decrease sharply again in upper Telm 6 and lower Telm 7 (36 to 41 Ma) with δ¹⁸O values from +0.8 to +1.4‰. Upper Telm 7, which yields ages within a range of 34 to 37 Ma, exhibits a modest decrease to values of +0.8‰.

The progression in δ¹⁸O observed within the Eocene succession at Seymour Island suggests an interval of warming that may coincide with the Middle Eocene Climatic Optimum (MECO) observed in open ocean settings. Similarly, the youngest Eocene sediments preserved in the upper Telm 7 may record a brief episode of late Eocene warmth recognized by Vonhof et al. 2000 just prior to the onset of glaciation in the early Oligocene. Such findings in shelfal environments support interpretations that these warming events were indeed of regional significance and affirm the use of shallow shelf carbonates as potential records for paleoclimate reconstruction.