CONSTRAINING THE SEDIMENT SOURCE IN THE GUAYMAS BASIN, GULF OF CALIFORNIA: RESULTS FROM IODP EXPEDITION 385

The Guaymas Basin is a major trap for sediment in the Gulf of California; however, the primary source of sediment into the basin is still uncertain. Both the Colorado River and Baja Peninsula are possible sources of sediment. The marine ecosystem inside the Gulf of California was established when the Baja Peninsula began rifting apart from mainland Mexico around 6 Ma. Primary production in the Gulf is not limited by temperature or solar irradiance due to it being located in the subtropics, meaning nutrient supply must be limiting phytoplankton growth. The two major phytoplankton groups competing for nutrients in the Gulf are siliceous diatoms and calcareous coccolithophores. Understanding the phytoplankton response to nutrient delivery can help us better understand the control on carbon export in the Gulf. Increased runoff during periods of heightened North American Monsoons can potentially deliver significant amounts of sediment to the Gulf and facilitate phytoplankton growth. Understanding the connection between primary production and sediment source helps us better understand the nutrient delivery systems available in restricted basins like the Gulf. This study uses roughly 350 meters of sediment collected during International Ocean Discovery Program (IODP) Expedition 385 from Hole A at Site U1545 to conduct a provenance study of the detrital zircon grains delivered to the basin. Zircon grains were separated from sediment cores of Hole U1545A using density (water wheel) techniques and picked accordingly. Using EDS on an SEM, zircon grains were confirmed within the detrital sediment. I determined that the grain size of the separated fraction of sediment ranges from clay to larger sand sized grains. Elemental zirconium has been found all over the core from the bottom to the top. If a sufficient number of zircon crystals of a large-enough size are found, Lu-Hf isotope and trace element analysis will be used to constrain their source formations. An εHf signal coming from the Baja Peninsula is expected to be much more constrained and positive as the baja peninsula has a more mantle derived source. The signal from the North American Continent, however, will be more varied and negative as there is a wider geologic setting with a more crustal derived component over the continent.