ACTIVE VERSUS PASSIVE MARGIN SYSTEMS
The contrasting active- and passive-margin, serial transport-reaction systems. Particulate material experiences more cycles of deposition-rest and reaction-remobilization on passive margins relative to active margins because of the longer path from upland source and burial in the seabed. As a consequence (as shown on the bottom part of the figure), original organic carbon source signals are transmitted through the system with greater fidelity on active margins. (Blair and Aller, 2012).
ACTIVE MARGINS AND RIVERINE SYSTEMS
Tectonic processes on active margins are intrinsically coupled to the transport of sediment and associated organic matter. Over geologic time scales (>1 Ma), uplift and mass wasting of sedimentary rock from uplifted accretionary wedges inject recycled organic C (e.g. kerogen), along with modern material into the marine environment (Fig. 1). The magnitude and nature of the organic carbon (OC) delivered to the marine realm can also be affected on short time scales due to event based disturbances (e.g. earthquakes, landslides). Hence, tectonic processes in active margins are intrinsically coupled with the transport of sediment and the associated organic matter. River systems located adjacent to active margins are responsible for some of the largest sediment yields on the globe (Milliman and Syvitski 1992). Importantly, those located on active margins discharge a larger percent of sediment directly to deep ocean basins (Milliman and Syvitski 1992).
The active margin C-Cycle. Accretion, uplift, and erosion of sedimentary rock on the continent bring previously buried OC to the surface. If mass wasting is sufficiently rapid, as is the norm on these margins, the exposed fossil C is recycled into the sedimentary system thereby avoiding oxidation in subaerial outcrops. The recycled fossil C is blended with younger material as sediments move across the surface.
CURRENT KNOWLEDGE ON ORGANIC CARBON ON ACTIVE MARGINS
In an active margin system, the recycled pool represents a significant portion of the OC buried in the marine environment (Blair et al., 2004) (Fig. 2). Small mountainous rivers along active margins export particulate organic carbon that is 7-75% fossil C (kerogen) in content, with the remainder derived from modern vegetation and millennial aged soil sources (Leithold et al., 2006; Drenzek et al., 2009; Blair et al., 2010). Recycled C is more inert than younger forms derived from plants and soils, and this inherent recalcitrance should lead to persistence, transit to the deep marine environment, and incorporation into the subduction zone.
Subduction zones are the ultimate sink for sediment and associated OC. To determine global C budgets and volatile production in subduction zones it is necessary to understand the recalcitrance of OC entering these regions. In active margins a significant fraction of OC reaching subduction may be the result of rapid terrestrial erosion by small mountainous rivers. OC from this source which reaches the offshore subduction environment is likely to be recycled C. The relative fates of these organics depend on reactivity and environment.
The active margin and accretionary wedge carbon cycle (a). Kerogen formation begins with the diagenesis of organic matter. Tectonically uplifted kerogen will combine with modern terrestrial sources and the mixed pool will be transported by rivers to the marine realm, where the marine pool of organic carbon will be added prior to burial, while terrestrial carbon is concurrently lost (b). During burial and diagenesis marine and modern terrestrial carbon will be lost, while kerogen will be preferentially preserved (c).
Blair, N., Leithold, E., Brackley, H., Trustrum, N., Page, M., Childress, L., 2010. Terrestrial sources and export of particulate organic carbon in the Waipaoa sedimentary system: Problems, progress and processes. Marine Geology, 270, 108 – 118.
Blair, N., Leithold, E., Aller, R., 2004. From bedrock to burial: the evolution of particulate organic carbon across couple watershed-continental margin systems. Marine Chemistry, 92, 141 – 156.
Drenzek, N., Hughen, K., Montlucon, D., Southon, J., dos Santos, G., Druffel,R., Giosan, L., Eglinton, T., 2009. A new look at old carbon in active margin sediments. Geology, 37(3), 239 – 242.
Leithold, E., Blair, N., Perkey, D., 2006. Geomorphic controls on the age of particulate organic carbon from small mountainous and upland rivers. Global Biogeochemical Cycles, 20, GB3022.
Milliman J., Syvitski, J., 1992. Geomorphic/Tectonic Control of Sediment Discharge to the Ocean: The Importance of Small Mountainous Rivers. The Journal of Geology, 100 (5), 525 – 544.