ALBERT

Description: 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.

Description: Submarine permafrost is more vulnerable to thawing than permafrost on land. Besides increased heat transfer from the ocean water, the penetration of salt lowers the freezing temperature and accelerates permafrost degradation. This data set provides sediment temperatures and pore water chemistry from two submarine permafrost cores from the Laptev Sea on the East Siberian Arctic Shelf which inundated about 540 and 2500 years ago. These data are published in partnership with a paper by Magritz et al., that traces how bacterial communities develop depending on duration of the marine influence and pore water chemistry. Magritz et al. (2017) show that submarine permafrost is a source of microbial life deep below the seafloor where it forms an unusual, non-steady state habitat. Pore water chemistry revealed different pore water units that reflected stages of permafrost thaw. Millennia after inundation by sea water, bacteria stratify into communities in permafrost, marine-affected permafrost, and seabed sediments. In contrast to pore water chemistry, the development of bacterial community structure, diversity and abundance in submarine permafrost appear site-specific, suggesting that both sedimentation and permafrost thaw histories strongly affect bacteria. Finally, highest total cell counts, DNA concentrations and bacterial gene copy numbers were observed in the ice-bonded unaffected permafrost unit of the longer inundated core, suggesting that permafrost bacterial communities exposed to submarine conditions proliferate millennia after warming.