Award: OCE-1851309

The burial of terrestrial organic matter in marine sediments sequesters atmospheric C, thereby modulating the long-term carbon cycle and climate. Yet, the drivers of terrestrial organic carbon preservation in marine sediments remain poorly understood, hampering faithful modeling of the long-term global carbon cycle. Rivers export terrestrial organic carbon to the coastal ocean, where it is believed to be efficiently incinerated, in spite of its purported recalcitrant nature. This conundrum has characterized our understanding of terrestrial organic carbon cycling in the ocean and resulted in low estimates of global terrestrial organic carbon burial rates. The results of this project show that organic carbon delivered by the Amazon River - the largest river on Earth - to the Atlantic Ocean encapsulates a broad range of reactivities and that its refractory subset is selectively preserved in marine sediments. Along the sediment transport pathway, mineral-bound soil organic carbon cycling over millennial timescales is selectively preserved, while labile young organic carbon is respired and progressively replaced by marine organic carbon. Detailled spatial investigations of organic carbon composition reveal that most labile organic carbon is rapidly oxidised in the viscinity of the freshwater - seawater interface. Combining elemental, isotopic, molecular and thermal analysis, we estimate that 45% of organic carbon exported by the Amazon gets preserved in coastal marine sediments, a proportion roughly equivalent to that of mineral-bound millennial organic carbon in the Amazon River. This challenges the current paradigm of efficient and non-selective incineration of terrestrial organic carbon in the coastal Ocean and suggests that the global terrestrial organic carbon burial flux has been underestimated. The results of this project thus transform our mechanistic understanding of the fate of terrestrial organic carbon in the ocean and show that it can be modelled by assessing its degradation potential, e.g. via thermal analysis. Furthermore, our results suggest that environmental conditions that favor the stabilization of soil organic carbon over millennial timescales - particularly through the formation of organo-mineral associations - enhance terrestrial organic carbon burial, thereby linking silicate weathering and organic carbon burial through common feedback mechanisms.

Last Modified: 02/09/2026
Modified by: Valier Galy