The value of Iberian Margin paleotemperature records with a novel organic proxy to revisit the bipolar seesaw model

The last glacial cycle provides the opportunity to investigate large changes in the Atlantic Meridional Overturning Circulation (AMOC) beyond the small fluctuations evidenced from modern measurements. Paleotemperature records from Greenland and the North Atlantic, including the Iberian Margin, show an abrupt variability, called Dansgaard–Oeschger (DO) events, which is associated with abrupt changes of the AMOC. These DO events also have Southern Hemisphere counterparts via the thermal bipolar seesaw, a concept describing the meridional heat transport leading to asynchronous temperature changes between both hemispheres. However, temperature records from the North Atlantic, notably the Iberian Margin, show more pronounced DO cooling events during massive releases of icebergs known as Heinrich (H) events, contrary to ice-core–based temperature records from Greenland.

We present high-resolution temperature records over the last 160 kyr using several independent organic proxies (e.g., RI-OH′, TEX₈₆, and U^K′₃₇) from three deep-sea sediment cores located in a north-south transect along the Iberian Margin (cores MD99-2331, MD95-2040, and MD95-2042). Over the 160–45 ka BP period, the recent RI-OH′ proxy yields faithful temperature records along the Iberian Margin in comparison with established paleotemperature proxies (e.g., TEX₈₆ and U^K′₃₇; Davtian et al., 2021 Paleoceano. Paleoclim. https://doi.org/10.1029/2020PA004077). In the southern Iberian Margin (core MD95-2042), the RI-OH′ and U^K′₃₇ proxies faithfully reflect the contrasting DO cooling amplitudes with and without H events over the last glacial cycle (Davtian et al., 2021; Davtian and Bard, 2023 PNAS https://doi.org/10.1073/pnas.2209558120).

We also revisit the thermal bipolar seesaw model using two independent temperature records (RI-OH′ and U^K′₃₇) from the southern Iberian Margin (core MD95-2042; Davtian and Bard, 2023). We show that temperature records from the southern Iberian Margin better support the classical thermal bipolar seesaw model than do ice-core–based temperature records from Greenland. We also introduce an extended thermal bipolar seesaw model that considers the contrasting DO cooling amplitudes with and without H events in the southern Iberian Margin, and a Bipolar Seesaw Index to distinguish DO cooling events with and without H events. Our data-model comparison emphasizes the role of the thermal bipolar seesaw in the abrupt temperature variability of both hemispheres with a clear enhancement during DO cooling events with H events, implying a relationship that is more complex than a simple flip-flop between two climate states linked to a tipping point threshold.