Find in Library

<p>TEX<span class="inline-formula">₈₆</span> 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, GDGTs are not exclusively produced in and exported from the mixed layer, so sedimentary GDGTs may contain a depth-integrated signal that is also sensitive to local subsurface temperature variability. In addition, the correlation between TEX<span class="inline-formula">₈₆</span> and SST is not significantly stronger than that to depth-integrated mixed-layer to subsurface temperatures. The calibration of TEX<span class="inline-formula">₈₆</span> to SST is therefore controversial. Here we assess the influence of subsurface temperature variability on TEX<span class="inline-formula">₈₆</span> using a downcore approach. We present a 15 Myr TEX<span class="inline-formula">₈₆</span> record from Ocean Drilling Program Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX<span class="inline-formula">₈₆</span> variability. Relatively high GDGT<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>[</mo><mn mathvariant="normal">2</mn><mo>/</mo><mn mathvariant="normal">3</mn><mo>]</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a434af86a172f0f2eb2fce7187afb234"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-18-1947-2022-ie00001.svg" width="27pt" height="14pt" src="cp-18-1947-2022-ie00001.png"/></svg:svg></span></span> ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (<span class="inline-formula">&gt;200</span> m) waters. Moreover, late Pliocene TEX<span class="inline-formula">₈₆</span> variability is highly sensitive to glacial–interglacial cyclicity, as is also recorded by benthic <span class="inline-formula"><i>δ</i>¹⁸</span>O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (U<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">37</mn><mrow><mi mathvariant="normal">k</mi><mo>′</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="11pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="51867edd2c8ffcf6ba814c43ace02b89"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-18-1947-2022-ie00002.svg" width="11pt" height="17pt" src="cp-18-1947-2022-ie00002.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ba1c13ec1a2f7c9d61dc1e30484e1e0a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-18-1947-2022-ie00003.svg" width="37pt" height="14pt" src="cp-18-1947-2022-ie00003.png"/></svg:svg></span></span>) is not primarily explained by glacial–interglacial cyclicity. Combined, these observations are best explained by TEX<span class="inline-formula">₈₆</span> sensitivity to sub-thermocline temperature variability. We conclude that TEX<span class="inline-formula">₈₆</span> represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible with the present-day depth distribution of Thaumarchaeota and with the GDGT<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>[</mo><mn mathvariant="normal">2</mn><mo>/</mo><mn mathvariant="normal">3</mn><mo>]</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3cf2a577ef84e1acd0e4119573c2eff8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-18-1947-2022-ie00004.svg" width="27pt" height="14pt" src="cp-18-1947-2022-ie00004.png"/></svg:svg></span></span> distribution in core tops. The depth-integrated TEX<span class="inline-formula">₈₆</span> 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 and 350 m, we estimate that east equatorial Atlantic SST cooled by <span class="inline-formula">∼5</span> <span class="inline-formula">^∘</span>C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX<span class="inline-formula">₈₆</span> record as a proxy for South Atlantic Central Water (SACW), which originates from surface waters in the South Atlantic Gyre and mixes at depth with Antarctic Intermediate Water (AAIW). Leads and lags around the Pliocene M2 glacial (<span class="inline-formula">∼3.3</span> Ma) in our record, combined with published information, suggest that the M2 glacial was marked by SACW cooling during an austral summer insolation minimum and that decreasing CO<span class="inline-formula">₂</span> levels were a feedback, not the initiator, of glacial expansion.</p>