Fig. 1

Concentration of d- and l-Aspartic acid in bulk sediment of the Middle Pleistocene marine Kon’kovaya suite (a); Measured d/l Asp in bulk sediment and separated cells (left) and the predicted d/l Asp calculated from Asp racemization rate, the geological ages of each depth of the permafrost (26 kyr for 3.4 m sample and 100 kyr for 5.8 and 14.8 samples for conservative estimation) and an assumed constant paleotemperature of − 7.7 °C. The equation (y = 94.213x - 8.2957) with high R² (0.996) was deduced from the linear regression of the increasing d/l Asp with depth in the bulk sediment. The error bars represent the standard deviation from three biological replicates