High resolution δ18O seasonality record in a French Eemian tufa stromatolite (Caours, Somme Basin)

Recent and sub-recent laminated tufa stromatolites can contain high-resolution δ18O records of in-stream temperature change. Fossil tufa stromatolites are therefore key targets for reconstructing terrestrial palaeoclimatic, but so far only a few examples have been published. In this research, we studied a 2.5-cm-radius tufa stromatolite from the Eemian of the Somme Basin, Northern France. We show (1) that high-resolution sampling of fossil laminated tufas is highly reproducible. We demonstrate (2) that within the limitations of the δ18O method, NW European Eemian seasonality was essentially similar to the present day. Perhaps most important, we show (3) that precise observations from thin section that match the position of lamina boundaries with the position of reversals in direction of δ18O values are a record of the style or intensity of seasonality. In this French tufa stromatolite, abrupt δ18O reversals do not coincide with sharp lamina boundaries; rather, the lamina boundaries occur where δ18O values are either gradually decreasing or increasing. We interpret this to be a record of abrupt changes in the growth rate, accelerating in spring or early summer and decelerating in late autumn or early winter. This contrasts with a published montane record from the Eemian of Greece, where lamina boundaries and δ18O reversals do coincide, interpreted as more extreme seasonality caused by cessations in growth due to summer aridity or winter cold. We thus propose this criterion as a method to help identify regional styles of seasonality in tufa stromatolites.