Climatic Rhythms in Holocene Sediments, Alfonso and Pescadero basins, Gulf of California.

Box cores, multicores and gravity cores collected during cruises in the Gulf of California from 1994 to 2001 have revealed a variety of primary and secondary sedimentary structures that provide information regarding changes in climate and oceanography of the region. Alfonso Basin and La Paz Basin on the western side and Pescadero Basin on the center slope of the east side of the Gulf are margin basins which have sills or shoreward slopes in the Oxygen Minimum Zone and preserve primary varves whose physical and associated geochemical characteristics yield information on Holocene climate and oceanographic changes in the Gulf. Primary productivity and sedimentation in the Gulf are to the dominant wind fields and the sediment record can be considered as an imperfect proxy of climate. Here we examine the sediment components and show that the sediments provide an important history of the changing environment. Laminated, hemipelagic mud, accumulating at rates of 25-50 cm/kyr, were sampled at 1 cm intervals to produce a high resolution record of organic carbon, calcium carbonate (foraminifera and coccoliths), opal silica (diatoms, radiolaria) and terrigenous content, that was examined for variations in accumulation and preservation. The grain size distribution of the fine fraction (silt and clay) was compared to dust samples obtained with 5 traps located around the La Paz Bay and also to the fine fraction of sediment collected at the Rancho Las Animas dunes. Growing evidence from deep-sea sediments and ice cores reveal that Holocene climates were unstable and punctuated by changes that are part of a 1-2 kyr climate rhythm first detected in the North Atlantic and recently elsewhere. Such rhythms have been recently attributed to variations in solar forcing associated with ocean-atmosphere feedbacks. Alfonso Basin sediments are organic carbon-rich (5-7%) with varying amounts of calcium carbonate (1-25%) and little opal silica (<4%) below the top 10 cm because of dissolution. Pescadero basin sediments are also organic rich but contain less carbonate (0-6%) and more silica. Major shifts in biogenic sedimentation occur at 3100 and 7200 YBP, typified by carbonate maxima, with smaller changes at 950, 1550, 4200 and 5200 yrs. These shifts also are reflected in sediment lamination and benthic isotopic records. The carbonate record is marked by strong productivity related dissolution cycles with two modes: ca. 350 yrs (300-370 yrs) which occurs throughout the core but is strongest prior to 3000 YBP and ca. 950 yrs (850-999 yrs) which occurs after 3100 YBP. Spectral analysis of the biogenic record confirms a 1-2 kyr climate rhythm in the Gulf and the data suggest that it is mediated by variations in monsoon intensity. The strong 350 yr variability appears related to solar cycles.