Conference Name: Geological Society of America Annual Meeting & Exposition 2010: Reaching New Peaks in the Geosciences
By burning fossil fuels and disturbing soils through land use changes such as forestry and agriculture, humans affect the carbon cycle by altering the amount that goes into Earth’s atmosphere and the amount stored in sediments. We looked at carbon storage in pre- and post-settlement alluvium in two small tributaries of the Grand River near Allendale, Michigan. Approximately 300 samples from 18 alluvial sections, and 10 vibracores were described and analyzed for texture, moisture content, loss on ignition, and carbonate content. In both sections and cores, a coarse layer interpreted to be stream gravel indicates an unconformity between upper and lower sediment sequences. Two radiocarbon analyses suggest that the lower sediment sequence is as old as mid Holocene in age. Burial of in situ tree stumps and trash indicates that the upper sequence is younger than settlement (~1820 A.D.). Our preliminary analysis indicates that pre-settlement alluvium has a mean organic content of 5.0 ± 4% (one standard deviation), while the mean organic content of post-settlement alluvium has a percentage of 4.1 ± 2%. The difference in variation between the upper and lower sediments could suggest a different set of processes in organic storage. Our current interpretation of these sediments suggests that the lower sequence is lacustrine or slack water sediments and the upper sequence is alluvial overbank sediment. The radiocarbon ages suggest that the lower sediment sequence formed during the Nipissing high stand when the Grand River and many of its tributaries were flooded. This interpretation could explain the difference in variation between organic carbon content. Work in progress includes dating the upper sequence using Cesium 137 to determine rates of carbon storage.