The ostracod clumped-isotope thermometer: A new tool to quantify continental climate change.

Ostracod shells are small aquatic crustaceans (0.3 - 5 mm) capable of recording climatic and environmental changes at high-resolution in sedimentary archives of modern and ancient lakes. Their stable low-Mg calcite shell mineralogy makes them ideally suited for targeted geochemical analyses. Therefore, ostracods represent the best candidate to develop a new carbonate clumped isotope (∆₄₇) lacustrine paleothermometer that disentangles and quantifies the effects of global climate changes at regional scale. To establish the relationship between ∆₄₇ and the temperature for ostracod shells, three different species were collected in monitored environments at 4°C and 12°C and one was cultivated in the lab at 23°C. Our results show a linear regression between ostracod-∆₄₇ and calcification temperature that is in agreement with previous published calibrations. This implies that ostracods are an equally-good recorder of (paleo)temperatures as other carbonaceous micro- or macrofossils from the marine realm. Moreover, we report the absence of a consistent offset between the species Eucypris virens and Bradleystrandesia fuscata coming from the same environment and precipitated at the same temperature. This observation suggests the absence of a vital effect at the genus and species level. Samples from shallow Lake Trasimeno (Italy) cover the last 50000 years and confirm the ability of the ostracod clumped-isotope thermometer as well as the absence of vital effect in the fossil record. The new paleothermometer identifies warmer/colder and humid/dryer conditions during Greenland Interstadial and Greenland Stadial/Heinrich events respectively.

These findings show that the ostracod-∆₄₇ thermometer has several advantages that makes it an attractive tool for scientific drilling: (i) It is independent of ostracod species and geography. Hence, one can also use endemic species.  (ii) It is applicable throughout geological time, as extinct species can be used. (iii) Temperature reconstructions for all environments where ostracods live are within reach. We emphasize that also high-diversity lacustrine environments are suitable for ∆₄₇ analysis, by mixing shells of different species together. This is of particular importance when working with small samples size from sediment cores.

The establishment of this new lacustrine proxy enables precise paleoclimatic reconstructions from different climate belts. It opens the door to new high-resolution continental paleoclimate and paleoenvironmental reconstructions and therefore has the potential to be a key tool in future lacustrine drilling in the ICDP framework.