Comment on cp-2021-92

doi:10.5194/cp-2021-92-rc2

Abstract

TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, the calibration of TEX86 to SST is controversial because its correlation to SST is not significantly stronger than that to depth-integrated surface to subsurface temperatures. Because GDGTs are not exclusively produced in and exported from the surface ocean, sedimentary GDGTs may contain a depth-integrated signal that is sensitive to local subsurface temperature variability, which can only be proved in downcore studies. Here, we present a 15 Myr TEX86 record from ODP Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial-interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk’37 and Mg/Ca), is not primarily explained by glacial-interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that the TEX86 record represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100–350 m, we estimate that east equatorial Atlantic SST cooled by ~4.5 °C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as an Antarctic Intermediate Water (AAIW) proxy and evaluate climatological leads and lags around the Pliocene M2 glacial (~3.3 Ma). Our record, combined with published information, suggests that the M2 glacial was marked by AAIW cooling during an austral summer insolation minimum, and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion.

Highlights

Accurate tropical sea-surface temperature (SST) reconstructions are needed to assess oceanic heat distribution and oceanatmosphere circulation in warmer-than-present climates, such as during the Pliocene
The cooling trend seems to end in the late Pleistocene, where TEX86H-SST records a ~4°C warming between 0.5 Ma and present, with a core top TEX86H-SST of 24.0°C
Several lines of evidence show that the TEX86 record at Site 959 in the eastern equatorial Atlantic is best explained as a depthintegrated signal that is substantially affected by temperature variability below the thermocline

Summary

Introduction

Accurate tropical sea-surface temperature (SST) reconstructions are needed to assess oceanic heat distribution and oceanatmosphere circulation in warmer-than-present climates, such as during the Pliocene. The most frequently used proxies for past SSTs are Mg/Ca (foraminifer calcite), Uk’37 (alkenones) and TEX86 (glycerol dialkyl glycerol tetraethers; GDGTs). Mg/Ca paleothermometry requires corrections for the Mg/Ca ratio of seawater and diagenesis (Dekens et al, 2002; 40 Evans et al, 2016), and the Uk'37 proxy is insensitive to temperatures above ~28°C (Müller et al, 1998). TEX86 is potentially one of the best proxies to reconstruct SSTs above 28°C, but one critical concern is that GDGTs are produced by marine Thaumarchaeota throughout the water column and often dominantly below the mixed layer (Ingalls et al, 2006; Kim et al, 2015; Lengger et al, 2014; Shah et al, 2008)

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