A new record of provenance and hydroclimate in Santa Barbara Basin, California spanning the last 250 years

Abstract:

Precipitation events in southern California generate enhanced sediment flux to Santa Barbara Basin (SBB), and the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) influence the occurrence and intensity of these events. In order to improve understanding of how enhanced sediment flux from precipitation events affects the well known laminated sediment sequences in SBB, bulk sediment from a new SBB box core (SPR0901-04BC, 62 cm length, 588 m water depth) was analyzed at high-resolution for elemental and mineralogical content. Detrital elemental concentrations increase in samples associated with floods and strong El Niño events, however elemental enrichments vary among these event types, possibly reflecting differences in source area or fluctuations in source contribution. Calcium and sodium concentrations and enrichments oppose those of other detrital elements, suggesting a difference in source area for these two elements, likely the upper reaches of the Santa Clara River. Geochemical weathering index results indicate the sediment deposited during major flood events has undergone increased chemical weathering relative to background sediment deposition in the basin. Weathering index values and clay mineral concentrations, derived from diffractogram peak areas, follow the PDO Index and reflect the frequency of strong El Niño events. During the cool, dry PDO phase when La Nina events are prolonged, weathering index values and clay concentrations decrease, while during the warm, wet PDO phase when strong El Niño events are frequent, weathering index values and clay mineral concentrations rapidly increase with subsequent rainfall events. This pattern can be attributed to a trend of decreasing erosion during dry periods such that only the less chemically weathered soil profile surface is transported to the basin. During wet intervals increasing erosion into the soil profile releases the more intensely weathered sediment and higher clay content of the B horizon. Thus the detrital sediment in this box core records contribution changes from different source areas as well as the intensity and timing of precipitation events, especially those generated during strong El Niño events, and the influence of the PDO phase.