Changes in benthic ecosystems and ocean circulation in the Southeast Atlantic across Eocene Thermal Maximum 2

S M Jennions,D Lunt,A Ridgwell,E Thomas,D N Schmidt

doi:10.1002/2015pa002821

Abstract

Eocene Thermal Maximum 2 (ETM2) occurred ~1.8 Myr after the Paleocene-Eocene Thermal Maximum (PETM) and, like the PETM, was characterized by a negative carbon isotope excursion and warming. We combined benthic foraminiferal and sedimentological records for Southeast Atlantic Sites 1263 (1500 m paleodepth) and 1262 (3600 m paleodepth) to show that benthic foraminiferal diversity and accumulation rates declined more precipitously and severely at the shallower site during peak ETM2. As the sites are in close proximity, differences in surface productivity cannot have caused this differential effect. Instead, we infer that changes in ocean circulation across ETM2 may have produced more pronounced warming at intermediate depths (Site 1263). The effects of warming include increased metabolic rates, a decrease in effective food supply and increased deoxygenation, thus potentially explaining the more severe benthic impacts at Site 1263. In response, bioturbation may have decreased more at Site 1263 than at Site 1262, differentially affecting bulk carbonate records. We use a sediment-enabled Earth system model to test whether a reduction in bioturbation and/or the likely reduced carbonate saturation of more poorly ventilated waters can explain the more extreme excursion in bulk δ13C and sharper transition in wt % CaCO3 at Site 1263. We find that both enhanced acidification and reduced bioturbation during the ETM2 peak are needed to account for the observed features. Our combined ecological and modeling analysis illustrates the potential role of ocean circulation changes in amplifying local environmental changes and driving temporary, but drastic, loss of benthic biodiversity and abundance.

Highlights

We focus here on Eocene Thermal Maximum 2 (ETM2, previously described as H1), which occurred at ~53.7 Ma, i.e., about 1.8 Myr after the Palaeocene-Eocene Thermal Maximum (PETM) [Stap et al, 2010a; Westerhold et al, 2012; Littler et al, 2014]
We suggest that the Site 1263 phenomena: (1) a sharp excursion in wt % CaCO3 together with bulk carbonate δ13C and δ18O that constitutes the ETM2 horizon and (2) temporary exclusion of benthic foraminifera are causally linked, via the impact of changes in the benthic foraminiferal contribution to bioturbation [Grosse, 2002]
The benthic foraminiferal ecosystem was perturbed in response to environmental change during the ETM2, with a decrease in abundance, diversity, and assemblage change at both sites

Summary

Introduction

Potential environmental impacts of increasing atmospheric CO2 concentrations include warming, increased intensity of the hydrological cycle, and nutrient influx into the oceans, ocean stratification, ocean acidification, and increased hypoxia [Caldeira and Wickett, 2003; Hutchins et al, 2007; Solomon et al, 2009; Coma et al, 2009; Keeling et al, 2010; Durack et al, 2012; Pörtner et al, 2014], any or all of which may affect organisms and ecosystems. The response of species and ecosystems to changing environments has been, and continues to be, tested in mostly single-driver laboratory experiments, producing short-term, species-specific, and mainly physiological information [e.g., Kroeker et al, 2010; Pörtner et al, 2014]. Such experiments are valuable but reflect neither the complexity of the natural environment nor the adaptability of organisms on long time scales. A series of global warming and carbon release events (“hyperthermals”) of variable intensity, occurring superimposed upon gradually rising global temperatures during the early to mid-Palaeogene [Thomas and Zachos, 2000; Cramer et al, 2003; Lourens et al, 2005; Sluijs et al, 2007a], provide us with the potential for just such a test

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