From bedrock to burial: the evolution of particulate organic carbon across coupled watershed-continental margin systems

Deltas sequester nearly half of the organic carbon (OC) buried in the marine environment. The composition of the buried organic matter reflects both watershed and seabed processes. A conceptual model is presented that describes the evolution of particulate organic carbon (POC) as it travels from its terrestrial source to its burial at sea. Alterations to the POC occur primarily in bioactive reservoirs, such as soils and the surface mixed layer (SML) of the seabed, where new organic matter can be added and older material degraded. Bypassing or rapid passage through the reservoirs is a key parameter because it avoids change. The Eel River of northern California and the Amazon River systems illustrate the importance of reservoir transit time and storage in determining the character of POC delivered to the continental margin. The Eel exemplifies a bypass system. Mass-wasting processes on land deliver unaltered bedrock along with OC derived from extant vegetation directly to the river channel without significant storage in soils. Rapid burial on the shelf occurs as a result of flood events. As a consequence, the buried material appears to be a simple mixture of carbon derived from kerogen (bedrock C), and modern terrestrial and marine sources. This is predicted to be a characteristic of the many similar short rivers on active margins that supply >40% of the fluvial sediment to the world's ocean. Extensive storage and processing of OC in lowland soils is a characteristic of the large Amazon watershed. Upland POC compositions are either overprinted or replaced by lowland sources. Upon delivery to the shelf, over half of the riverine POC is lost as a result of residence in sediment layers that are periodically reworked over time scales of days to months. The addition of fresh reactive marine OC, exposure to oxygen, and the regeneration of metal oxidants during resuspension events fuel the oxidation of the riverine organic matter. The nature of the watershed-shelf processes likely produce a complex mixture of organics possessing a continuum of ages and reactivities. The model illustrates the need to develop tools to measure residence times of particles in the various reservoirs so that the behavior of POC can be calibrated as it moves through a sedimentary system. The ultimate goal is to be able to use the organic geochemistry of soils and sediments to quantitatively infer the history of processes that determine both the composition and amount of POC present in different depositional environments.