Paulina Tamez‐Hidalgo

The turnover rate of microbial cells drops steeply by increasing depth in the seabed, in accordance with the decreasing rate of organic matter mineralization. Bacterial endospore formation and germination may take place concurrently, yet their coupling to the mineralization rates during burial is poorly understood. We used high‐resolution ¹⁴ C‐dating of coring sites on the Peru Margin to show that the upper 8.5 m of sediment had been lost due to landslides at one site, thereby exposing 13,500‐yr old sediment. Dynamic transport‐reaction modeling of sulfate and methane showed that these landslides took place several thousand years ago. We discovered that microbiological and biogeochemical data from the two sites could be combined to deconvolute the effect of sediment age on microbial turnover and organic matter mineralization. A D : L amino acid racemization model showed that the mean turnover time of both microbial biomass and bacterial endospores was a few hundred years. Detailed chrono‐stratigraphy combined with microbiological and geochemical data revealed a 10,000‐fold span of anaerobic mineralization rates across 25,000 yr of sediment age. The mean power consumption per cell of the subsurface microbial community could theoretically enable the calculated production by Firmicutes of one endospore per cell per 140–1400 years, thereby maintaining a constant stock of 10 ⁶ –10 ⁷ endospores cm ⁻³ over thousands of years. These results suggest that bacterial endospores continue to be produced and to germinate deep down in the subseafloor.