Pleistocene – Holocene Rainfall and Productivity Records in Alfonso Basin Baja California Sur

Abstract:

Northwesterly winds dominate the southern Gulf of California during most of the year and are associated with high primary productivity due to upwelling. The North American Monsoon causes the wind direction to reverse during the summer months and this brings humid and warms conditions to the region. This climatic temporal-variation generates alternate sedimentation and as a consequence laminated deposits of biogenic and terrigenous composition, preserved by the presence of an oxygen minimum zone. A 5.4 m long piston core (36N) was recovered from Cuenca Alfonso during the DIPAL IV cruise on May 2011. The core was sampled at 1 cm intervals to produce high-resolution proxy records of primary productivity (biogenic opal, total organic carbon and carbonate content) and rainfall (terrigenous content).  The core spans from late Pleistocene to the present and it’s the longest analyzed record for the basin for the above-mentioned proxies. In general, the proxies show a negative trend towards the present, suggesting a more productive basin and wetter climate in the past. During climatic events like the Bølling-Allerød and the Holocene climatic optimum (HCO) CaCO₃ content decreased and opal content increased; suggesting the presence of stronger NW winds and nutrient-richer waters. In contrast, during the Younger Dryas and 8.2 Kyr events the opal content decrease and the CaCO₃ increase.  At 5.5 KyBP the negative trends in all components intensify, especially during the Medieval Warm Period (MWP). An exception to this occurs during the little ice Age (LIA) where the opal and CaCO₃ increase. The organic carbon content increases at the end of the HCO and during the MWP, and decreases during the BA, YD, 8.2 Kyr and LIA. The terrigenous content follows the 24°N summer insolation trend, but shows an increase during warm events (BA, HCO and MWP), and can be attributed to migration of the average position of the Intertropical Convergence Zone driven by variations in solar insolation.