The effect of anthropogenically induced sediment disturbances on carbon mineralisation pathways in coastal sediments

Coastal waters have a shallow water column and support high primary production, which implies that underlying sediments receive high amounts of fresh, reactive organic matter. This translates into high rates of organic matter mineralisation and as a result, thermodynamically favourable electron acceptors (oxygen, nitrate, metal oxides) are rapidly depleted within the first few millimetres to centimetres. This makes organoclastic sulphate reduction and methanogenesis the most important mineralisation pathways in coastal sediments, leading to a large production of reduced sulphide and methane. At the same time, the rapid depletion of oxygen allows for the preservation and burial of organic carbon. This makes these sediments an essential carbon sink that couples the long- and short-term carbon cycle. Frequent trawling or dredging in coastal waters directly disturbs this carbon sink and thus could leave an anthropogenic fingerprint on the global carbon cycle. To date, the magnitude of this impact has however not been quantified.

Here, we provide field data that were collected before and after an anthropogenically influenced seafloor homogenisation event. This dataset was subsequently analysed by reactive-transport modelling. Our results reveal that anthropogenic disturbance of the seafloor strongly alters the respiration pathways of organic carbon mineralisation, and show the large time scale variation over which these pathways recover. This event induced a reset of the geochemical cycling within the upper seafloor, which led to an alternating sequence of electron acceptors, following the thermodynamic energy gain. It takes > 1 year for the mineralisation pathways to return to the steady state, while the total carbon mineralisation rate was transiently increased. Our results provide a clear illustration of how human activities can inadvertently change the coastal carbon cycle.