Pore water chemistry, grain sizes and sediment temperature of 4 sediment cores from submarine permafrost at Mamontov Klyk Cape, Laptev Sea shelf

The mobilization of carbon in degrading permafrost is a long-term process and an important feedback upon climate change. Under submarine conditions substantial permafrost warming occurs millennia before permafrost thaws, potentially stimulating microbial communities. How microbial community composition and abundance responded to millennial-scale permafrost warming remains, however, unkown. We measured the in situ development of bacterial community composition and abundance together with temperature, salinity and pore water chemistry along an onshore-offshore transect on the Siberian Arctic Shelf. Samples derived from ice-bonded terrestrial permafrost comparable in age and sedimentation history that had been warming by more than 10 °C over the last 2500 years. Bacterial assemblages identified through amplicon sequencing correlated only weakly with temperature but strongly with pore water stable isotope signatures. They showed a significant spatial variation. Bacterial 16S rRNA gene copies quantified through qPCR negatively correlated with rising temperature, while both gene copies and total cell counts negatively correlated with increasing pore water salinity. Correlations of microbial community composition and abundance to stable isotope signatures and pore water salinity imply that they still mainly reflect the sedimentation history. On time-scales of centuries, permafrost warming coincided with decreasing microbial abundances, whereas millennia after inundation, microbial cell abundance was similar to onshore permafrost. We suggest that, as long as permafrost remains frozen the effect of warming alone on the permafrost-carbon-feedback is marginally even on time-scales of millennia because it has an overall low-level effect on microbial community composition and abundance.