Environmental Magnetism of Cores from Peat Deposits of the Sacramento-San Joaquin Delta, California

We have conducted an environmental magnetic study of cores from 12 sites in the Sacramento-San Joaquin Delta of California. Eight of the sites are on tracts and islands that were created in the late 19th and 20th centuries when marshes were leveed and drained to create agricultural land. The other four sites are march islands that have never been drained. Most of the cores penetrate underlying clay deposits that predate the formation of the peat. Over 50 meters of core was studied from the 12 sites. Measurements were made of the acquisition and demagnetization of anhysteretic remanence and isothermal remanence, two laboratory- induced magnetizations that provide information about the concentration and mineralogy of the magnetic fraction. In all of the cores, the peat-rich intervals have fairly low magnetic intensities, indicating significant dilution of the clastic material by the organic material of the peat. Some variations in intensity can be discerned, which appear to be due to changes in climate and/or landscape. Material from the top of the cores from the farmed islands has much higher magnetic intensities, indicative of the concentration of clastic material that occurs as organic material is lost by oxidation and/or wind erosion. The most interesting results come from the bases of the cores where the transition from non-organic clays and silts to organic-rich peat is marked by a 100-fold decrease in the concentration of magnetic material. Detailed analysis of this transition indicates that it was usually very abrupt although in several cases the sudden decrease in concentration is followed by a short interval of moderate concentration before the final, lowest concentrations are attained. This intermediate state provides insights into the nature of the transition from an estuarine to a paludal environment.