  Phosphorus is an essential element for life, and the phosphorous cycle is
widely believed to be a key factor limiting the extent of Earth's biosphere and
its impact on remotely detectable features of Earth's atmospheric chemistry.
Continental weathering is conventionally considered to be the only source of
bioavailable phosphorus to the marine biosphere, with submarine hydrothermal
processes acting as a phosphorus sink. Here, we use a novel 29Si tracer
technique to demonstrate that alteration of submarine basalt under anoxic
conditions leads to significant soluble phosphorus release, with an estimated
ratio between phosphorus release and CO2 consumption (P/CO2) of 3.99+/-1.03
umol/mmol. This ratio is comparable to that of modern rivers, suggesting that
submarine weathering under anoxic conditions is potentially a significant
source of bioavailable phosphorus to planetary oceans and that volatile-rich
Earth-like planets lacking exposed continents could develop robust biospheres
capable of sustaining remotely detectable atmospheric biosignatures.
