The novel hydroxylated tetraether index RI-OH′ as a sea surface temperature proxy for the period 160–50 ka BP off the Iberian Margin

The stable oxygen isotope ratio (δ¹⁸O) of planktic foraminifers, the C₃₇ ketone unsaturation ratio (U^K′₃₇) and the TetraEther indeX of tetraethers consisting of 86 carbon atoms (TEX₈₆) are three well-known examples of paleothermometric proxies. These established proxies are in the realism phase of the Elderfield proxy curve (2002 Geochim. Cosmochim. Acta 66 Suppl. 1, 213, DOI: 10.1016/S0016-7037(02)01009-8), which means that their advantages and shortcomings are relatively well evidenced, though not fully understood. By contrast, the Ring Index of hydroxylated tetraethers (RI-OH′) is an example of novel paleothermometer. RI-OH′ is still in the optimism phase, so its potential in paleothermometry remains to be further explored.

Here, we present new high-resolution temperature records over the interval 160–50 ka BP using four organic proxies (RI-OH′, RI-OH, TEX₈₆ and U^K′₃₇) from three deep-sea sediment cores located in a north-south transect along the Iberian Margin. RI-OH′, RI-OH and TEX₈₆ are based on LC-MS analyses of individual tetraethers with a two-column HPLC and improved mass spectrometric method. We analyzed all organic proxies in the same organic extracts to optimize proxy-proxy comparisons and phase relationship studies.

Our main results strengthen the optimism concerning the novel RI-OH′ proxy for five reasons. 1/, the only existing global core-top calibration to date allows to reconstruct realistic sea surface temperature (SST) from RI-OH′ in comparison to those derived from U^K′₃₇ and TEX₈₆. 2/, RI-OH′ allows to establish plausible latitudinal temperature gradients, which are reasonably coherent with those based on U^K′₃₇ and TEX₈₆. 3/, RI-OH′ records resemble those from established paleothermometers, especially U^K′₃₇ and δ¹⁸O of planktic foraminifers that better reflect SST than does TEX₈₆. 4/, RI-OH′ responds to Dansgaard-Oeschger and Heinrich events as expected for North Atlantic SST proxies, which supports a direct relationship with Greenland temperature records. 5/, the outputs of a bipolar seesaw model forced with the RI-OH′ record are well correlated with Antarctic paleotemperatures as expected from theoretical considerations.

Overall, our main findings support a continued interest on the novel hydroxylated tetraether paleothermometer RI-OH′ so that it can progress along the Elderfield proxy curve. This work complements our first promising attempt based on a RI-OH record for a shallow core from the western Mediterranean Sea, located in a complex sedimentary setting much less favorable than the Iberian Margin (Davtian et al., 2019 Paleoceanography and Paleoclimatology 34, 616–634, DOI: 10.1029/2018PA003452).