GLOBAL PATTERNS OF THE FATE OF TERRESTRIAL ORGANIC CARBON IN THE MARINE ENVIRONMENT

The fate of continentally-derived particulate organic carbon (POC) delivered to oceans by rivers is a critical aspect of global biogeochemical cycles and Earth surface evolution. POC fate is dependent on both intrinsic characteristics (molecular structure, matrix) and the environmental conditions to which fluvial POC is subjected. The intrinsic character of the exported riverine POC is a function of watershed. Small, mountainous rivers, which are commonly aligned along tectonically active margins, frequently deliver suspended loads derived from fine-grained sedimentary rocks that contain fossil OC. The fossil C is replaced with aged soil OC in the largest passive margin systems, such as the Amazon. Three distinct patterns of POC burial in the seabed are evident on continental margins supplied by rivers: (1) high-energy, mobile muds with enhanced oxygen exposure and efficient metabolite exchange have very low preservation of both terrestrial and marine POC (e.g., Amazon subaqueous delta); (2) low-energy facies with extreme accumulation rates have high POC preservation (e.g., Ganges-Brahmaputra); and (3) small, mountainous river systems that sustain moderate to average accumulation rates but deliver a large fraction of low-reactivity, fossil POC during episodic events have the highest preservation efficiencies. The global patterns of terrestrial POC preservation reflect broadly different roles for passive and active margin systems in the sedimentary C cycle.