How deep do signs of life extend into Earth's subsurface? A new compilation of data from sedimentary basins suggests that certain biosignatures remain detectable through the entire top kilometer of sediment, according to a study by McIntosh and colleagues published in AGU Advances and highlighted in Eos.
The researchers found that the carbon isotope composition of carbon dioxide and methane, along with the gravity (inverse density) of oil, are particularly strong indicators of biotic activity. These signals show up most clearly where microbial cells are abundant and where the subsurface structure allows surface waters to infiltrate. In contrast, in units with small pores that restrict water flow, microbial activity and its associated biosignatures are limited, even at shallow depths.
At depths greater than one kilometer, such low-permeability conditions often prevail, and the biosignatures become less robust. The authors infer that significant microbial activity is needed for these signals to leave a durable record. This finding extends estimates of how deeply life's signatures can propagate, with implications for the recycling of organic carbon in the subsurface and for how geologists interpret past microbial activity from the rock record.