Geomorphologic controls on the age of particulate organic carbon from small mountainous rivers Lonnie Leithold and Neal Blair, North Carolina State University Funded by NSF Integrated Carbon Cycle Research

Particulate organic carbon (POC) is only a small fraction (~1%) of the material discharged by rivers, but it is an extremely important fraction Particulate organic carbon (POC) is only a small fraction (~1%) of the material discharged by rivers, but it is an extremely important fraction Why POC?

POC moves with and is largely bound to mineral grains POC moves with and is largely bound to mineral grains POC bound to mineral grains is gained and lost to various degrees as the grains move through surficial environments, dependent on their residence times in those environments POC bound to mineral grains is gained and lost to various degrees as the grains move through surficial environments, dependent on their residence times in those environments POC, therefore, can tell us a great deal about sedimentary processes, both present and past POC, therefore, can tell us a great deal about sedimentary processes, both present and past Why POC?

POC in small mountainous watersheds The POC discharged by small mountainous rivers, which collectively carry about half of the total annual discharge to the ocean, is distinct from that discharged by larger, passive margin rivers The POC discharged by small mountainous rivers, which collectively carry about half of the total annual discharge to the ocean, is distinct from that discharged by larger, passive margin rivers

POC in small mountainous watersheds Particles in these systems generally have relatively short residence times in near-surface reservoirs (e.g., the regolith, channel floor, floodplain) Particles in these systems generally have relatively short residence times in near-surface reservoirs (e.g., the regolith, channel floor, floodplain) One consequence is that ancient sedimentary carbon (kerogen) survives to be recycled into modern systems One consequence is that ancient sedimentary carbon (kerogen) survives to be recycled into modern systems

POC in small mountainous watersheds We can expect that such residence times will vary from watershed to watershed, dependent on geomorphic processes We can expect that such residence times will vary from watershed to watershed, dependent on geomorphic processes

Small mountainous and upland rivers on active margins Small mountainous and upland rivers on active margins Temperate climate with abundant, episodically intense rainfall Temperate climate with abundant, episodically intense rainfall Underlain almost exclusively by sedimentary rocks Underlain almost exclusively by sedimentary rocks

Leithold, Blair, and Perkey, 2006, Global Biogeochemical Cycles v.20

Based on  13 C and  14 C mass balance calculations

At the lower end of yield in SMRs, riverine POC loads are dominated by young plant material delivered by processes including sheetwash and episodic shallow landsliding

Concentration and 14 C age of OC in a soil profile Data from Gaudinski et al., 2000 ~ 1550 yrs Post modern  14 C = 40 F m = 1.03

Higher yields in these systems are accommodated by bedrock erosionHigher yields in these systems are accommodated by bedrock erosion Deep gully incision, for example, delivers virtually unweathered bedrock and kerogen to the river networkDeep gully incision, for example, delivers virtually unweathered bedrock and kerogen to the river network

Based on published rock to regolith conversion rates, in order for high sediment yields (> tons km -2 yr -1 ) to be sustained, direct bedrock erosion must be involved Based on published rock to regolith conversion rates, in order for high sediment yields (> tons km -2 yr -1 ) to be sustained, direct bedrock erosion must be involved By this criterion, bedrock erosion is estimated to contribute to about ¾ of small mountainous and ½ of upland rivers catalogued by Milliman and Syvitski (1992) By this criterion, bedrock erosion is estimated to contribute to about ¾ of small mountainous and ½ of upland rivers catalogued by Milliman and Syvitski (1992)

From Source to Sink Can POC in the offshore stratigraphic record tell us about watershed processes in the past? Can POC in the offshore stratigraphic record tell us about watershed processes in the past?