Middle-late Holocene climate variability in La Paz Basin, southern Gulf of California

Sediments from DIPAL III-K47 core collected at 830 m depth in the western slope within the oxygen minimum zone of La Paz Basin, southern Gulf of California provide a detailed record of paleoceanography and paleoclimatology for the tropical Pacific on centennial time scales for the past 7300 years. The sedimentary sequence is compose of hemipelagic sediments and is laminated throughout its entire length (145 cm). According to the preliminary age model based on radiocarbon AMS dates, core covers the period from ca 7300 to 1000 cal yr BP. The estimated sedimentation rates are between 0.20 and 0.29 mm/yr. Radiolarian assemblages, geochemical (major and trace elements Al, Ba, Ca, K, Si Ti, Zr and Zr/Al and Ba/Al ratios) and magnetic susceptibility are used as proxies of variations of oceanic circulation patterns, paleoproductivity, aeolian activity and precipitation. Eighty-two intervals were sampled for radiolarians and the core was sampled at 1-cm intervals to produce records of major and trace elements. Factor Analysis of the radiolarian abundances counted in sediments samples identified three assemblages. The first one (Arachnocorallium calvata, Lithomelissa setosa, Lithomelissa thoracites and Peridium longispinum) suggests winter-spring like conditions (cold and dry), Gulf of California Water persistence and a relative increase in productivity that might become from the east-to-west upwelling gradient. The second radiolarian assemblage (Tetrapyle octacantha group and Phorticium pylonium group) was interpreted as stratification of the column water and the incursion of warm, oligotrophic Tropical Surface Water that remind summer-fall like conditions. The third assemblage (Clathrocircus stapedius, Phorticium pylonium group, Lithomelissa pentacantha, Phormacantha hystrix, Phormospyris stabilis scaphipes, Lithomelissa thoracites, Pseudocubus obeliscus, Druppatractus irregularis and Druppatractus variabilis), suggests a mix water column that favors the organic carbon flux to deeper layers that increase the susbsurface-intermediate radiolarians food supply. Major changes in all records suggests four climate scenarios for the past 7300 years and a multi-centennial drought from ca 2100 to 1500 cal yr BP is suggested. From ca 7300 to 5300 cal yr BP, in general, productivity is high but shows some changes, probable related to the persistence of warm and oligotrophic tropical waters. Between ca 5300 and 4000 cal yr BP seems that the primary productivity induced by upwelling decrease abruptly, however some species of radiolarians are favored probably by turbulence and mesoscale gyres, terrigenous recorded a gradual change from wetter to dryer conditions. The interval from ca 4000 to 2700 cal yr BP is in general characterized by fluctuations in productivity. Radiolarian assemblages suggest that two different physical mechanisms could promote the productivity in the region (upwelling and gyres). From ca 2700 to 1000 cal yr BP it seems that the productivity is driven mainly by the east-west gradient across the Gulf of California due to the intensification of NW winds in the region.