HR: 0800h
AN: H11C-0654    [Abstracts]
TI: The Effect of Land-Surface Subsidence Processes on the Remaining Peat in the Sacramento-San Joaquin Delta, California, USA
AU: * de Fontaine, C S
EM: fontaine@usgs.gov
AF: U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, United States
AU: Drexler, J Z
EM: jdrexler@usgs.gov
AF: U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, United States
AB: The Sacramento-San Joaquin Delta (henceforth, the Delta) was once a 1,400 km² tidal wetland that was drained and reclaimed for agricultural production beginning in the mid-nineteenth century. Drainage of the organic, peat soils has resulted in land-surface subsidence in the region to as much as 8 m below sea level. Although the factors causing land-surface subsidence are well known (principally oxidation of organic matter), little is known about the physical and chemical condition of the remaining peat. As part of a project called Rates and Evolution of Peat Accretion through Time in the Delta, the physical and chemical condition of the remaining peat layer was studied on four farmed islands and four relatively undisturbed marsh islands. Peat cores were collected from all sites using a modified Livingstone corer. Core samples were analyzed for bulk density, percent organic matter and carbon, and for radiocarbon. On the marsh islands, the thickness of the relatively undisturbed peat layer ranged from 4.2 to 8.2 m, and on the farmed islands, it ranged from 1.2 to 1.5 m. On the farmed islands, the unoxidized and intact peat layer lies underneath a 0.4- to 0.9-m-thick layer of compacted and oxidized peat. Radiocarbon ages obtained from Scirpus seeds within the basal peat of the farmed islands indicate that peat formation began 6,035-6,785 cal yr BP. Although peat is still forming on marsh islands today, the youngest intact peat at the farmed island sites formed 4,415-4,985 cal yr BP. The mean bulk density of marsh island peat (0.17 g/cm³) is less than the intact and unoxidized peat layer on the farmed islands (0.21 g/cm³; Student's t-test, p < 0.0001), and much less than the compacted and oxidized zone at the surface of the farmed islands (0.65 g/cm³; Student's t-test, p < 0.0001). In the peat samples, there is an inverse relationship between bulk density and organic matter. The higher bulk densities and lower carbon content of the upper compacted and oxidized peat layer on farmed islands are a result of surface drainage for agricultural practices. Although the remaining intact peat below this zone does not appear to be oxidized, it has been altered through physical compaction.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 1865 Soils (0486)
DE: 1890 Wetlands (0497)
SC: Hydrology [H]
MN: 2007 Fall Meeting