The preserved carbonate record in Holocene sediments of the Alfonso and Pescadero basins, Gulf of California, Mexico

Multi-core and gravity cores were collected from Alfonso Basin, located on the western side of the Gulf of California with a sill in the oxygen minimum zone (OMZ), and from Pescadero Basin located on the landward slope within the OMZ depth range. These locations have anoxic to dysoxic bottom waters which preserve primary depositional laminae whose physical and geochemical characteristics yield information on climatic and oceanographic changes over the last 7000 to 9000 years. y productivity proxies preserved in the basin sediments have been used to define palaeoclimates. The sediment cores were sampled at 1 cm intervals, which composite 25–35 cm ka−1. Organic carbon, carbonate, opal and terrigenous content analyses were made of the laminated, hemipelagic mud. Sediment cores from Alfonso Basin are organic carbon-rich (5–7%), with varying carbonate content (1–25%) and little opal (<4%); they span 7700 years of record. Sediment cores from eastern Pescadero are also organic carbon-rich (2.5–4.3%), but contain less carbonate (0–6%) and more silica (8–21%); they span 9800 years of record. ogenic record was used as the basis for defining three climatic periods during the Holocene. (a) The early period, c. 10 000–7200 calendar (cal) years BP, is represented almost exclusively in the record for eastern Pescadero Basin (Alfonso Basin cores sample only the last 5800 years). It is characterized by high productivity and variability resulting from strong northwesterly winds and upwelling. (b) The middle period, 7200–4200 cal years BP, characterized by a general and steady decrease in productivity. Variability also reduces and the period appears to be one of climatic stability. (c) The late period, 4200 cal years BP to the present, during which there is an increase in productivity in the east but it declines in the west and with an increase in the carbonate-opal east-west asymmetry between the two basins. The records are marked by strong climate-ocean variability cycles with two modes −200 years (throughout the record) and c. 830 years (after 3000 cal years BP) — that appear related to latitudinal shifts of the Inter-tropical Convergence Zone (ITCZ), produced by solar cycles.