Water mass stability reconstructions from greenhouse (Eocene) to icehouse (Oligocene) for the northern Gulf Coast continental shelf (USA)

Abstract

[1] Shallow water mass characteristics such as temperature and density profile play a critical role in the climate system. We have developed a new method by which to reconstruct the ancient shallow water mass stability on the continental shelf using oxygen isotope variation within mollusc shells and fish otoliths and applied the method to an important interval in Earth history, the most recent transition from Greenhouse (Eocene) to Icehouse (Oligocene) climate modes. We define the slope of summer temperature (density) versus the seasonal range in temperature (density) as an indicator of water mass stability. In addition, extrapolation of the regression to zero seasonality is a proxy for temperature at the bottom of the seasonal thermocline (TBST). During the greenhouse world (the early Eocene and middle Eocene) the water mass plot shows an unstable water mass, agreeing with previous planktonic foraminiferal studies showing that temperature gradients at this time were much smaller than at present. During the middle to late Eocene transition, a substantial increase in water mass stability occurred. Significant cooling ($5°C) of the TBST at this transition indicates that the greater cooling of deeper water relative to surface water caused the increase in water mass stability. The changes in water column structure at this transition were the most likely cause of a major extinction of planktonic foraminifera from warm to cold water taxa. The late Eocene T-DT profile is very similar to modern profiles, suggesting that shallow water mass structure became similar to that of the modern Gulf Coastal shelf by the late Eocene. At the Eocene/ Oligocene (E/O) boundary, no major change in water mass structure is identified. This agrees with the observation that no major extinction of planktonic foraminifera is found at the E/O boundary.

The early Eocene equable climate problem revisited

“…Correspondence is excellent, even in mid-to-high latitude regions that have previously been challenging (Shellito et al, 2003). The main remaining discrepancy is at the Puryear-Buchanan site (37.1 • N, 70.7 • W, see table), where the model produces a CMM of nearly ∼25 • C whereas the data are ∼16 • C. This may not be too serious given that there is wide uncertainty in the proxy data estimate (Greenwood and Wing, 1995) and other nearby localities, albeit from marine proxies, show winter temperatures of 21.6-24.3 • C (Kobashi et al, 2004). The model is clearly capable of matching both the general qualitative pattern of a frost-free early Eocene (Fig.…”

“…But, more fundamentally, hot (MAT much above 28 • C), wet conditions, with vegetation, have no modern analogue and empirical relationships may break down. We have every reason to think that mean MAT in the tropics was above 30 • C in the early Eocene, but just how much hotter is an unanswered question (Huber, 2008;Jaramillo et al, 2010;Kobashi et al, 2004). We will not be able to address the true warmth of the tropical to subtropical regions with great rigor in this study because of a paucity of terrestrial proxies and the ambiguity of interpreting such records under non-analogue conditions.…”

Stable Isotopes from Marine Shells, Ancient Environments, and Human Subsistence on Middle Holocene Santa Rosa Island, California, USA

“…To convert calcite measurements to temperatures, the equation of Epstein et al (1953) (modified by Craig 1965; see also Glassow et al 1994) was used as it was developed using the Haliotis genus. For the aragonite (exterior) samples from the Venus clam, the equation of Grossman and Ku (1986), later modified by Kobashi et al (2004) was used:…”

Strontium to calcium ratios in the marine gastropod Conus ermineus: Growth rate effects and temperature calibration

“…Fewer studies, however, have investigated the isotope and trace metal composition of gastropods and their paleoenvironmental applications [e.g., Buchardt and Fritz, 1978;Rosenthal and Katz, 1989;Purton et al, 1999;Andreasson and Schmitz, 1998;Tripati and Zachos, 2002]. Among the latter are Kobashi et al [2001], Kobashi and Grossman [2003], and Kobashi et al [2004], who have examined isotopic records of fossil and modern Conus ermineus shells collected from different sites in the Gulf of Mexico. They note a good correlation between d 18 O and temperature, which suggests that d 18 O closely traces seasonal variations in seawater temperature.…”