Quantitative palaeotemperature estimates based on fossil chydorid ephippia: calibration and validation of a novel method for northern lakes

To fully utilize palaeolimnological archives for tracking past environmental changes, methodological advances are necessary. We present a novel method to quantitatively reconstruct past summer air temperatures based on fossil chydorid (Cladocera: Chydoridae) ephippia assemblages. We examined a 76‐lake training set from Finland for chydorid ephippia distribution to develop a transfer function for summer air temperatures. Subsequently, we tested its usability in downcore reconstructions by inferring Holocene temperatures from Lake Arapisto in southern Finland. Significant climate‐related variation was found in the distribution and abundance of chydorid ephippia in the training set. A transfer function for palaeotemperature reconstructions was developed using the partial least squares technique, which showed favourable performance statistics, enabling the test use of the model in a downcore reconstruction. The ephippia assemblages were responding to the Holocene climate development in the Lake Arapisto core. The ephippium‐inferred temperatures correlated closely with the previously available independent palaeoclimate data from Lake Arapisto and from an adjacent lake, although the early Holocene samples had poor modern analogues in the training set. The use of fossil chydorid ephippia in palaeoclimatological reconstructions is a promising tool and it can contribute as a valuable addition for multiproxy assessments of past climate changes.