VARVE FORMATION IN THE GULF-OF-CALIFORNIA - INSIGHTS FROM TIME-SERIES SEDIMENT TRAP SAMPLING AND REMOTE-SENSING

The varved sediments that accumulate in the central Gulf of California (Guaymas and Carmen Basins) provide a record that allows for the resolution of annual to decadal-scale climate variability in this region. Time-series sediment trapping, combined with remotely sensed observations of sea surface temperature (AVHRR) and color (CZCS), have been used to examine the mechanism of varve formation in the central Gulf. SST and surface pigment concentration records for both the mainland and Baja sides of the Gulf display similar seasonal trends. High temperatures and low pigment concentrations occur synchronously on both sides of the central Gulf from June through to October. In association with the 1991/92 El Nino, warm temperatures persist in the central Gulf until December. Low SSTs typically occur from December through May, with high pigment concentrations marking the period from November through April. The summer-early fall (June-October) is a time of high terrigenous and biogenic sediment fluxes. During this time of year, the total flux is dominated by eolian-transported lithogenic material and dark laminae are deposited during this period. During El Nino years, high terrigenous fluxes may extend into winter. Light laminae represent deposition from November through May, during which time biogenic material dominates the total sediment flux. High opal fluxes beginning in November are attributed to seasonal cooling of surface water and upward mixing of nutrients. True upwelling conditions do not develop in the Gulf until late winter-spring. During this time, the total flux is still dominated by opal but the magnitude of the flux is reduced. This may be an artifact of phytoplankton grazing and not lower production rates.