Assessing divergent SST behavior during the last 21 ka derived from alkenones andG. ruber-Mg/Ca in the equatorial Pacific

Abstract

Equatorial Pacific SST reconstructions derived from Mg/Ca ratios in planktonic foraminifera Globigerinoides ruber and from alkenone‐producing coccolithophorids record different trends throughout the Holocene and the last deglaciation. We set forth the hypothesis that their diverging behavior may be related to different seasonal sensitivities which result from the annually varying production rates of alkenone‐producing coccolithophorids and of G. ruber. Using a series of transient paleoclimate model simulations forced with the time‐varying forcing history over the last 21 ka, a good qualitative agreement is found between simulated boreal winter temperatures and alkenone‐SST reconstructions as well as between simulated boreal summer temperatures and reconstructed Mg/Ca‐based SST variations. Pronounced features in the reconstructions that can be readily explained by the conjectured seasonal biases include the mismatch in middle‐to‐late Holocene temperature trends and the different onsets of deglacial climate change in the eastern equatorial Pacific. The analysis presented here further suggests that through combinations of Mg/Ca and alkenone SST reconstructions information can be gained on annual mean temperature changes and the amplitude of the seasonal cycle in SST. Our study concludes by discussing potential weaknesses of the proposed model‐derived seasonal bias interpretation of tropical Pacific SST proxies in terms of present‐day core‐top data, sediment trap studies, and satellite‐based observations of chlorophyll.

Greening of the Sahara suppressed ENSO activity during the mid-Holocene

“…Nevertheless, our modelled MH seasonal SST gradients do reveal a close match between the proxy records and the modelled SST gradient during boreal winter/early spring. It is possible that some proxy-based reconstructions of equatorial Pacific SSTs could reflect specific seasons rather than annual means, as suggested by previous studies515253. For example, cooling of the warmer months and warming of the colder months in the equatorial Pacific during the MH may have shifted the preferred growing season relative to today.…”

The climate response of the Indo‐Pacific warm pool to glacial sea level

“…In contrast, alkenone data from the western Indian Ocean suggest a more modest cooling of 1–2 K [ Sonzogni et al , ; Dahl and Oppo , ]. This could be indicative of a stronger gradient; however, it should be noted that these changes are within error of most proxy measurements and are subject to proxy‐specific uncertainties, such as seasonal biases [ Timmermann et al , ], and therefore require further investigation to confirm.…”

Glacial cooling as inferred from marine temperature proxies TEXH86 and UK′37

“…While all the foregoing estimates are closer to our U K 37 estimate, a tropical LGM cooling of 3-4 • C, comparable in magnitude to that of the TEX H 86 , is not unprecedented -especially as estimated from single site reconstructions. A cooling estimate of 3-4 • C was previously reported for the west Pacific warm pool, based on a consortium of proxies, including the clumped isotopes of planktic foraminifera and coccoliths (Tripati et al, 2014), the Mg/Ca ratio of planktic foraminifera (De Garidel-Thoron et al, 2007), and the coral Sr/Ca (Beck et al, 1992). However, it is unclear whether these estimates are site-and/or region-specific.…”