Late Paleocene–early Eocene Arctic Ocean sea surface temperatures: reassessing biomarker paleothermometry at Lomonosov Ridge

Appy Sluijs,Joost Frieling,Stefan Schouten,Francien Peterse,Francesca Sangiorgi,Klaas G J Nierop,Gordon N Inglis

doi:10.5194/cp-16-2381-2020

Abstract

Abstract. A series of papers published shortly after the Integrated Ocean Drilling Program Arctic Coring Expedition (ACEX, 2004) on Lomonosov Ridge indicated remarkably high early Eocene sea surface temperatures (SSTs; ca. 23 to 27 ∘C) and land air temperatures (ca. 17 to 25 ∘C) based on the distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (isoGDGT and brGDGT) lipids, respectively. Here, we revisit these results using recent analytical developments – which have led to improved temperature calibrations and the discovery of new temperature-sensitive glycerol monoalkyl glycerol tetraethers (GMGTs) – and currently available proxy constraints. The isoGDGT assemblages support temperature as the dominant variable controlling TEX86 values for most samples. However, contributions of isoGDGTs from land, which we characterize in detail, complicate TEX86 paleothermometry in the late Paleocene and part of the interval between the Paleocene–Eocene Thermal Maximum (PETM; ∼ 56 Ma) and the Eocene Thermal Maximum 2 (ETM2; ∼ 54 Ma). Background early Eocene SSTs generally exceeded 20 ∘C, with peak warmth during the PETM (∼ 26 ∘C) and ETM2 (∼ 27 ∘C). We find abundant branched GMGTs, likely dominantly marine in origin, and their distribution responds to environmental change. Further modern work is required to test to what extent temperature and other environmental factors determine their distribution. Published Arctic vegetation reconstructions indicate coldest-month mean continental air temperatures of 6–13 ∘C, which reinforces the question of whether TEX86-derived SSTs in the Paleogene Arctic are skewed towards the summer season. The exact meaning of TEX86 in the Paleogene Arctic thus remains a fundamental issue, and it is one that limits our assessment of the performance of fully coupled climate models under greenhouse conditions.

Highlights

The Eocene epoch (56 to 34 million years ago; Ma) has long been characterized by warm climates
This fraction was analyzed by high-performance liquid chromatography (HPLC) and atmospheric pressure chemical ionization–mass spectrometry using an Agilent 1260 Infinity series HPLC system coupled to an Agilent 6130 singlequadrupole mass spectrometer at Utrecht University following Hopmans et al (2016) to measure the abundance of GDGTs
The new GDGT distributions (Table S1 in the Supplement) are consistent with the TEX86 and branched and isoprenoid tetraether (BIT) index data generated over a decade ago using the older analytical HPLC setup (Hopmans et al, 2000; Hopmans et al, 2016) (Fig. 3)

Summary

Introduction

The Eocene epoch (56 to 34 million years ago; Ma) has long been characterized by warm climates. The earliest signs of a balmy Eocene Arctic region – fossil leaves of numerous plant species – were documented 150 years ago (Heer, 1869). Subsequent findings identified palms, baobab and mangroves, indicating the growth of temperate rainforests and year-round frost-free conditions in the Eocene Arctic region (Schweitzer, 1980; Greenwood and Wing, 1995; Suan et al, 2017; Willard et al, 2019). Fossils of animals, including varanid lizards, tortoises and alligators, indicate warm Arctic climates (Dawson et al, 1976; Estes and Hutchinson, 1980). Paleobotanists have focused on Arctic plant fossils and significantly refined their paleoclimatological interpretation towards estimates of precipitation as well as seasonal and mean annual temperature (e.g., Uhl et al, 2007; Greenwood et al, 2010; Eberle and Greenwood, 2012; Suan et al, 2017; Willard et al, 2019)

Methods

Results

Discussion

Conclusion