THE IMPACT OF LAND-USE CHANGES ON CARBON STORAGE IN SMALL STREAMS, OTTAWA COUNTY, MICHIGAN

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 of carbon that goes into Earth’s atmosphere and the amount stored in sediments. We looked at organic 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, organic carbon content, and carbonate. 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 indicate that the lower sediment sequence is as old as middle Holocene. Burial of in situ tree stumps and recent trash indicates that the upper sequence is younger than European settlement (~1820 A.D. in west Michigan). Our preliminary analysis indicates that pre-settlement alluvium has a mean organic content of 5.0 ± 3.9% (one standard deviation), while the mean organic content of post-settlement alluvium is 4.1 ± 2.3%. The difference in variation between the upper and lower sediments could suggest a different set of processes in organic carbon storage. Our current interpretation of these sediments suggests that the lower sequence is lacustrine or slack water sediment 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 during the middle Holocene. This interpretation could explain the difference in variation between organic carbon content. Work in progress includes dating the upper sediment sequence using cesium-137 soil dating to determine rates of organic carbon storage in alluvium since European settlement.