PALEOMAGNETIC SECULAR VARIATION AND ENVIRONMENTAL MAGNETISM OF LATE HOLOCENE-AGED SEDIMENTS OF TULARE LAKE, CA

The lake-level record from Tulare Lake has been shown to potentially provide valuable constraints on forecasting runoff from the Sierra Nevada over the next several generations into the San Joaquin Valley of California, one of the world’s most prolific agricultural centers. This project focuses on the magnetic properties of Tulare lake sediments in an attempt to date a new sedimentary record, with which to test earlier work done on this lake. Toward this end, four trenches were dug at two localities in the southern end of the Tulare lake bed, totaling approximately six meters in depth. Each trench was sampled at two-centimeter spacing. The samples were analyzed at the UC Davis Paleomagnetics Laboratory and the Institute for Rock Magnetism in Minneapolis for both remanence directions and magnetic properties. For both sites, the location of a major unconformity is identifiable by changes in the temperature dependent susceptibility above and below the unconformity. The magnetic properties measurement system (MPMS) data indicates the presence of magnetite and goethite at both sites, but that there is less magnetite at the Atwell Island site. The ARM/IRM data for both sites decreased upsection toward the unconformity indicating an increase in magnetic grain size that may be reflective of bulk grain size as the lake shallows. The Day plots also indicate larger grains with more multi domain characteristics at the Atwell Island site than the samples from the Poso Canal site. The reduced amount of magnetite and the larger grain size, perhaps indicating a shift toward multi-domain behavior, may help to explain why the paleomagnetic secular variation dating was unsuccessful at the Atwell Island site. The paleomagnetic results at the Poso Canal site indicate that some of the deepest trench sediments were deposited in the age range of 7200-6300 C¹⁴ years (~7,000 to 8,000 cal yr B.P.) ago, and the youngest sediments were deposited approximately 1900-500 C¹⁴ years ago. Sediments of this age are consistent with relatively high lake levels during these times as suggested by earlier studies.