Western Mediterranean Sea Paleothermometry Over the Last Glacial Cycle Based on the Novel RI‐OH Index

Abstract

RI‐OH (ring index of hydroxylated tetraethers) has recently been proposed to reconstruct paleotemperatures in middle‐ to low‐latitude marginal seas. However, RI‐OH has barely been tested in marginal seas under substantial terrigenous inputs. Here we analyze tetraether lipids in two adjacent marine cores from the Gulf of Lions. We then test for the first time the RI‐OH paleothermometer from 160 to 9 ka BP in the western Mediterranean Sea. While terrigenous inputs prevent TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) from behaving as a paleothermometer, RI‐OH is generally consistent with other paleothermometric proxies. RI‐OH also responds systematically and coherently to glacial‐interglacial transitions as well as to abrupt climatic events. The average difference between RI‐OH temperatures and November–May UK′37 (C37 ketone unsaturation ratio) temperatures is −2.0 °C with a standard error of 0.4 °C based on 249 RI‐OH‐UK′37 comparisons. This systematic difference suggests that hydroxylated tetraethers and alkenones record different temperatures, for instance, winter and/or subsurface temperatures for RI‐OH. Another source of bias could be linked to the available RI‐OH‐temperature calibration, which clearly needs more work at the global and regional scales, notably for semienclosed basins such as the Mediterranean Sea. Nevertheless, our RI‐OH‐based interglacial‐glacial anomalies are of 10 °C, a value within the high end of anomalies from previously published temperature records in the western Mediterranean Sea (from 3 to 13 °C). The RI‐OH‐based temperature anomalies also confirm the regional differences and seasonal contrasts in interglacial‐glacial anomalies produced by models.

The Novel Hydroxylated Tetraether Index RI‐OH′ as a Sea Surface Temperature Proxy for the 160‐45 ka BP Period Off the Iberian Margin

“…RI-OH versus U K ′ 37 relationships (r values of 0.44-0.66 for the 160-45 ka BP period) are also less linear than RI-OH′ versus U K ′ 37 relationships ( Figures S6a-S6c and S6g-S6i; Section 5.1). Our results differ with those reported by Davtian et al (2019), as both RI-OH′ and RI-OH systematically and coherently respond to wellknown climatic events in the Gulf of Lions. Following their global core top compilation, Lü et al (2015) found the smallest RI-OH′ and largest RI-OH residuals at the lowest SSTs and recommended to use RI-OH′ rather than RI-OH for polar oceans.…”

“…We identified and integrated OH-GDGT peaks as described by Davtian et al (2019) and also integrated the same peaks in the m/z 1,318.3, 1,316.3, and 1,314.3 scans. We confirm that the late eluting peaks identified by Davtian et al (2019) in the m/z 1,300.3, 1,298.3, and 1,296.3 scans correspond to abundant in-source fragmentation products of OH-GDGTs (e.g., Liu et al, 2012). We thus quantified OH-GDGTs as [M + H-18] + ions rather than as [M+H] + ions (protonated molecular ions) to calculate RI-OH and RI-OH′ values.…”

“…The Iberian Margin is located in an open ocean setting with limited terrigenous inputs, high sedimentation rates, and successful U K ′ 37 and TEX 86 paleothermometries (Bard et al, 2000;Darfeuil et al, 2016;Martrat et al, 2007;Pailler & Bard, 2002). The Iberian Margin thus constitutes a much more favorable sedimentary setting to validate the novel RI-OH′ and RI-OH paleothermometers than the Gulf of Lions (Davtian et al, 2019). Contrary to Davtian et al (2019) and similarly to other multiproxy studies (e.g., Fietz et al, 2013Fietz et al, , 2016Fietz et al, , 2020de Bar et al, 2018de Bar et al, , 2019Jonas et al, 2017;Lattaud et al, 2018;Lopes dos Santos et al, 2013;Smith et al, 2020), we studied all organic proxies using a single set of lipid extracts to facilitate comparisons between novel and established paleothermometers.…”

“…The Iberian Margin thus constitutes a much more favorable sedimentary setting to validate the novel RI-OH′ and RI-OH paleothermometers than the Gulf of Lions (Davtian et al, 2019). Contrary to Davtian et al (2019) and similarly to other multiproxy studies (e.g., Fietz et al, 2013Fietz et al, , 2016Fietz et al, , 2020de Bar et al, 2018de Bar et al, , 2019Jonas et al, 2017;Lattaud et al, 2018;Lopes dos Santos et al, 2013;Smith et al, 2020), we studied all organic proxies using a single set of lipid extracts to facilitate comparisons between novel and established paleothermometers. The deep sea study sites also offer an opportunity to assess whether, in the Iberian Margin, Mediterranean deepwater archeal communities bias RI-OH′ and RI-OH to the same extent as TEX 86 , a bias that is likely insignificant at the shallow sea study site of Davtian et al (2019).…”

Shut down of the South American summer monsoon during the penultimate glacial

“…brGDGts temperature curve. Mean annual air temperature (MAAT) is based on the analysis of branched glycerol dialkyl glycerol tetraethers (brGDGTs) following the method described by Hopmans et al 70 and adapted at CEREGE by Davtian et al 71,72 . A total of 92 sub-samples from CO14, each weighing between 0.1 to 0.9 grams (2020) 10:6275 | https://doi.org/10.1038/s41598-020-62888-x www.nature.com/scientificreports www.nature.com/scientificreports/ were freeze-dried, ground and extracted using an accelerated solvent extraction system (ASE350 Dionex-Thermo Fisher) at 120 °C, 100 bars and a mixture of dichloromethane and methanol (9 :1, v/v).…”