A new Li/Mg paleothermometer from pteropod shells.

Pteropods are promising proxy archives for paleoceanographic reconstructions but remain underexplored. Previous oxygen isotope analyses indicated that Heliconoides inflatus calcifies at shallow depths (50-75 m), suggesting its potential to record surface-ocean conditions. Here, we evaluate for the first time the applicability of Li/Mg thermometry in pteropod shells, a temperature proxy widely used in coral studies. We show that, in addition to recording environmental variability through shell carbon and oxygen isotopic composition, H. inflatus shells record upper-water temperature conditions, with Li/Mg ratios showing a robust relationship with ambient temperatures at sampling locations. Li/Mg ratios decrease exponentially with increasing temperature, enabling temperature reconstructions with an average precision of ± 1-2 °C. Pteropod shells (species H. inflatus, formerly known as Limacina inflata) were collected along a latitudinal transect in the Atlantic Ocean (31° N to 38° S), spanning a surface-water temperature gradient of approximately 15 °C. The global distribution and abundance of this annual species in sediments makes it a suitable alternative for paleo-surface temperature reconstruction. The utility of pteropod shells extends further to seasonal paleotemperature reconstruction, as seasonal temperature variability is captured in the elemental ratios from the embryonic to later (adult) sections of the shell. These results, in combination with a basin-scale distribution, make the Li/Mg thermometer in pteropods an exciting new tool in paleoceanography and further cement this group as new proxy archives.