Geomorphologic controls on Geomorphologic controls on the age of particulate organic the age of particulate organic

carbon from small carbon from small mountainous riversmountainous rivers

Lonnie Leithold and Neal Blair, Lonnie Leithold and Neal Blair, North Carolina State North Carolina State

UniversityUniversity

Funded by NSF Integrated Carbon Cycle Research

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

Why POC?Why POC?

POC moves with and is largely bound to mineral POC moves with and is largely bound to mineral grainsgrains

POC bound to mineral grains is gained and lost to POC bound to mineral grains is gained and lost to various degrees as the grains move through surficial various degrees as the grains move through surficial environments, dependent on their residence times environments, dependent on their residence times in those environmentsin those environments

POC, therefore, can tell us a great deal about POC, therefore, can tell us a great deal about sedimentary processes, both present and pastsedimentary processes, both present and past

Why POC?Why POC?

POC in small mountainous POC in small mountainous watershedswatersheds

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

POC in small mountainous POC in small mountainous watershedswatersheds

Particles in these Particles in these systems generally have systems generally have relatively short relatively short residence times in near-residence times in near-surface reservoirs (e.g., surface reservoirs (e.g., the regolith, channel the regolith, channel floor, floodplain)floor, floodplain)

One consequence is that One consequence is that ancient sedimentary ancient sedimentary carbon (kerogen) carbon (kerogen) survives to be recycled survives to be recycled into modern systemsinto modern systems

POC in small mountainous POC in small mountainous watershedswatersheds

We can expect that We can expect that such residence such residence times will vary times will vary from watershed to from watershed to watershed, watershed, dependent on dependent on geomorphic geomorphic processesprocesses

Small mountainous and Small mountainous and upland rivers on active upland rivers on active marginsmargins

Temperate climate with Temperate climate with abundant, episodically abundant, episodically intense rainfallintense rainfall

Underlain almost Underlain almost exclusively by sedimentary exclusively by sedimentary rocksrocks

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

Based on 13C and 14C 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 Concentration and 1414C age of C age of OC in a soil profileOC in a soil profile

Data from Gaudinski et al., 2000

~ 1550 yrs

Post modern

14C = 40

Fm = 1.03

• Higher yields in Higher yields in these systems are these systems are accommodated by accommodated by bedrock erosionbedrock erosion

• Deep gully Deep gully incision, for incision, for example, delivers example, delivers virtually virtually unweathered unweathered bedrock and bedrock and kerogen to the kerogen to the river networkriver network

Based on published rock to regolith conversion rates, in order for high sediment yields (>600-800 tons kmBased on published rock to regolith conversion rates, in order for high sediment yields (>600-800 tons km -2-2 yr yr-1-1) to be ) to be sustained, direct bedrock erosion sustained, direct bedrock erosion mustmust be involved be involved

By this criterion, bedrock erosion is estimated to contribute to about ¾ of small mountainous and ½ of upland rivers 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)catalogued by Milliman and Syvitski (1992)

From Source to SinkFrom Source to Sink Can POC in the Can POC in the

offshore offshore stratigraphic stratigraphic record tell us record tell us about watershed about watershed processes in the processes in the past?past?