Paleogene cooling (55-30 MA) as inferred from oxygen isotope variation within mollusc shells

Abstract

Paleogene cooling (c. 50-30 Ma) started sometime in the early-middle Eocene. This was a time when high-latitude and deep-sea temperatures were significantly warmer than today. This cooling culminated during the earliest Oligocene marked by the sudden appearance of a major continental glacier on Antarctica. We examine this cooling trend by analyzing oxygen isotope variation within mollusc shells from the Gulf Coastal Plain of the southern U.S. Our records show a secular cooling trend of mean annual temperature (MAT) in the Mississippi Embayment from an early Eocene tropical climate (26-27 ⁰C), with a seasonal temperature range (seasonality) of ~6 ⁰C, to an Oligocene paratropical climate (22-23⁰C) with an seasonality of ~8 ⁰C. These temperature records agree well with terrestrial climate proxies. This secular cooling trend, combined with sea-level change, was likely one of the major causes of molluscan turnover in the Mississippi Embayment to cool-tolerant taxa along the Paleogene cooling. Winter temperatures steadily decreased from the middle Eocene to early Oligocene. This contrasts with the sudden winter cooling at Eocene-Oligocene boundary proposed by Ivany et al. (2000). We examined seasonal temperature distribution of the modern marine shelf of the present northern U.S. Gulf Coast. A deeper water temperature model fits well with isotopic temperature profiles derived from fossils shells of the Red Bluff and Yazoo Formations shells, consistent with the paleobathymetry estimates inferred from independent proxies. This reveals that depth effect is one of the major factors controlling seasonality recorded in mollusc shells, resulting in decreasing MAT estimates when temperature stratification exists as in the present ocean. Warm Eocene low-latitude temperatures derived from molluscan oxygen isotope data agree with computer modeling results incorporating higher greenhouse gas concentrations. This supports the contention that the major reason for warm earth climate is elevated concentration of the greenhouse gases, giving a new insight for future climate response to anthropogenic CO₂ increase.