Productivity response of calcareous nannoplankton to Eocene Thermal Maximum 2 (ETM2)

M Dedert,P Ziveri,N Shimizu,D Kroon,H M Stoll,K Kanamaru

doi:10.5194/cp-8-977-2012

Abstract

Abstract. The Early Eocene Thermal Maximum 2 (ETM2) at ~53.7 Ma is one of multiple hyperthermal events that followed the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The negative carbon excursion and deep ocean carbonate dissolution which occurred during the event imply that a substantial amount (103 Gt) of carbon (C) was added to the ocean-atmosphere system, consequently increasing atmospheric CO2(pCO2). This makes the event relevant to the current scenario of anthropogenic CO2 additions and global change. Resulting changes in ocean stratification and pH, as well as changes in exogenic cycles which supply nutrients to the ocean, may have affected the productivity of marine phytoplankton, especially calcifying phytoplankton. Changes in productivity, in turn, may affect the rate of sequestration of excess CO2 in the deep ocean and sediments. In order to reconstruct the productivity response by calcareous nannoplankton to ETM2 in the South Atlantic (Site 1265) and North Pacific (Site 1209), we employ the coccolith Sr/Ca productivity proxy with analysis of well-preserved picked monogeneric populations by ion probe supplemented by analysis of various size fractions of nannofossil sediments by ICP-AES. The former technique of measuring Sr/Ca in selected nannofossil populations using the ion probe circumvents possible contamination with secondary calcite. Avoiding such contamination is important for an accurate interpretation of the nannoplankton productivity record, since diagenetic processes can bias the productivity signal, as we demonstrate for Sr/Ca measurements in the fine (<20 μm) and other size fractions obtained from bulk sediments from Site 1265. At this site, the paleoproductivity signal as reconstructed from the Sr/Ca appears to be governed by cyclic changes, possibly orbital forcing, resulting in a 20–30% variability in Sr/Ca in dominant genera as obtained by ion probe. The ~13 to 21% increase in Sr/Ca above the cyclic background conditions as measured by ion probe in dominating genera may result from a slightly elevated productivity during ETM2. This high productivity phase is probably the result of enhanced nutrient supply either from land or from upwelling. The ion probe results show that calcareous nannoplankton productivity was not reduced by environmental conditions accompanying ETM2 at Site 1265, but imply an overall sustained productivity and potentially a small productivity increase during the extreme climatic conditions of ETM2 in this portion of the South Atlantic. However, in the open oceanic setting of Site 1209, a significant decrease in dominant genera Sr/Ca is observed, indicating reduced productivity.

Highlights

The present rapid increase of greenhouse gases in the atmosphere has led to an increased interest in similar transient warming events that occurred in the geological past
Eocene Thermal Maximum 2 (ETM2) interval (Fig. 3a), which is followed by a return to former Sr/Ca in Coccolithus a few centimeters below the onset of the Carbon Isotope Excursion (CIE) (Fig. 3b; as measured in the fine bulk fraction (
Chiasmolithus does not show a significant response in the ETM2 interval, except for a small increase prior to the Elmo horizon that coincides with the increase in Coccolithus and Toweius

Summary

Introduction

The present rapid increase of greenhouse gases in the atmosphere has led to an increased interest in similar transient warming events that occurred in the geological past. Negative C-isotope excursions and dissolution of deep-sea carbonates in large areas of the ocean, which together suggest addition of carbon (C) to the atmosphere and transient increases in atmospheric CO2 (Cramer et al, 2003). All events show transient warming believed to result from the increased CO2, as well as biotic responses such as a decrease in benthic foraminifera species richness. We focus here on one of these later hyperthermals, the Eocene Thermal Maximum 2 (ETM2) at ∼53.7 Ma, first recognized by Lourens et al (2005) on the Walvis Ridge, South Atlantic. The ETM2 at this location is characterized by a negative Carbon Isotope Excursion (CIE) of ∼1,5 ‰ in bulk carbonates and a low carbonate content interval (Elmo horizon) resulting from shoaling of the lysocline

Methods

Results

Discussion

Conclusion