Document: EUG-3-41-22

Holocene paleoecology of Southern California: Methods, results, and testing of climate model simulations.

WAHL, E.R.*

University of Minnesota, St. Paul, MN 55108 USA ¹

Abstract:
A pollen-based paleoenvironmental record is developed from wet meadows in the Southern California mountains. The record focuses on time windows of 9-10ka, 6ka, and 3ka (14C years) , corresponding to early- and mid-Holocene climate modelling experiments and to late-Holocene climate changes recorded in coastal estuaries in the region. Pollen spectra from a regional network of surface samples are used to determine possible modern analogs for the fossil data; the surface samples are associated with specific temperature and precipitation values determined from regional instrumental data. Analysis of the surface samples indicates that a cutoff value in the Square Chord Distance technique of ~0.20 jointly minimizes the chance of either falsely accepting or falsely rejecting a particular surface sample as an analog. Preliminary results from analysis of the modern analogs "selected" by the fossil pollen indicate: 1) the early Holocene (near 9-10ka) was characterized by temperature and precipitation ranges similar to those of today; 2) the mid Holocene (near 6ka) was characterized by conditions slightly warmer and drier than today; and 3) the late Holocene (near 3ka) was characterized by conditions almost identical to today's. In the early Holocene, the fossil pollen assemblages do not contain significant percentages from regional dry-indicating plants, making them unlike pollen spectra in the western Sierra Nevada during this period. Coupled with the analog analysis, these results strongly suggest that conditions in the Southern California mountains near 9-10ka were significantly more mesic (relative to today's climate) than conditions in the Sierra Nevada at this time. A N-S gradient of increased relative moisture from the Northern to Southern California mountains in the early Holocene is consistent with model simulations of an enhanced summer monsoon and slightly enhanced coastal winter precipitation in the American Southwest at this time. The slightly drier-than-present conditions inferred for the mid Holocene are consistent with a possible eastward shift of the monsoon in one set of model simulations, but are not well-explained by a second set of simulations in which the monsoon does not shift and summer and annual precipitation and surface wetness are all increased over their early Holocene levels.

Keywords: Southern California, Holocene, Paleoecology, Paleoclimate Simulations

This abstract is being presented at: 2:00 PM in session:
Oral Session #32: Paleoecology.