Residence Times and Legacy of Biogenic Carbon in Ocean Reservoirs

Quantifying the sequestration potential of biologically driven carbon fluxes in the ocean depends critically on residence times - how long carbon remains stored in reservoirs before being re-exposed to the atmosphere. Simple mass balance provides estimates for many of the major ocean biogenic carbon reservoirs. For vegetated coastal ecosystems (mangroves, sea grass meadows, salt marshes) that globally store 20 to 40 PgC, this is 200 to 500 years, while for the biological carbon pump, a reservoir of about 2000 PgC, it is between 200 to 800 years. Over these time scales respective reservoirs reach equilibrium if left undisturbed. Importantly, near equilibrium of ocean reservoirs during the Holocene can be inferred from the near steady atmospheric concentrations during this period. The degradation of habitats and the over-exploitation of living marine resources particularly in the last 75 years have tipped these natural processes out of balance, to the extent where many are now net emitters of legacy carbon back to the atmosphere. The analysis exposes a conflict between how sequestration is reported in oceanographic literature and how it is understood with regards durable carbon capture and storage. Nature-based solutions can be sought to address parts of the climate crisis, by improving ecosystem health and biodiversity, but are unlikely to provide solutions to carbon management on a scale commensurate with anthropogenic emissions. The best we can do is to limit net emissions by restoring what we can, and to ensure that future practices do not further tip ocean carbon reservoirs out of balance. Significance StatementMarine animals and plants maintain large pools of carbon in the ocean and coastal areas that have been laid down by generations past. This legacy carbon is continuously being recycled on time scales of 100s of years. Left undisturbed, as they were for most of the last 10000 years, these carbon pools tend to equilibrium; flux in equals flux out. Human activities such as over fishing and coastal construction, particularly in the past 75 years, have tipped these natural cycles out of balance to the extent where many pools are now net emitters of carbon. Conservation and restoration of marine habitats can bring these cycles back into balance but cannot be counted as offsetting fossil fuel emissions.