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

The Tulare lake level history over the past 10,000 years is consistent with other paleoclimate records of south-central California. The record from this particular lake, if improved with respect to resolution especially in the latest Holocene, will potentially provide valuable constraints on forecasting runoff from the Sierra Nevada over the next several generations into one of the most important agricultural regions in the world. This project focuses on the magnetic properties of Tulare lake sediments in an attempt to better date the sediments and to determine the relative lake level at the time they were deposited.

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. Nearly all samples show pseudo-single domain magnetic carriers, at least as determined from their central position on a Day plot. The hysteresis loops also indicate a strong paramagnetic component. The temperature transition results are consistent with that of magnetite and goethite. The samples did not show any susceptibilities that were frequency dependent, which supports the assumption that 1) there was no influx of soil-derived superparamagnetic particles into the lake sediments and 2) extensive soil development has not occurred on the lake sediments, themselves. Approximately half of the samples possessed well behaved paleomagnetic directions, which were compared with reference records for North America. The results indicate that some of the deepest trench sediments were deposited approximately 7200-6300 C¹⁴ years (~7,000 to 8,000 cal yr B.P.) ago at a time when the lake had been hypothesized to become shallower and more oxidized. This changing environment is supported by the ARM/IRM and S-ratio data which both decreased upsection indicating an increase in magnetic grain size and an increase in hematite concentration.