Reply on RC1

Abstract

Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge as the commonly used bulk OM parameters do not reach the required level of source- and process-specific information. Here, we analysed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi and Atchafalaya Rivers (MAR), as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil-, fluvial, higher plant, and marine produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of soil and fluvial OC near the Mississippi River mouth (BIT = 0.6, FC32 1,15 >50 %), which rapidly decrease further away from the river mouth (BIT <0.1, FC32 1,15 <20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore, and reveal that marine producers (diatoms, dinoflagellates, coccolithophorids) have different spatial distributions, indicating their preferential niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean, suggests that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic/phototrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our multi-disciplinary approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental shelf and open ocean.