Opal in Gulf of California sediments as a tool to estimate the vertical component of water exchange between the Gulf and the Pacific Ocean

Water exchange between the Gulf of California and the Pacific has a significant vertical component. Surface (0-200 m) gulf water flows out into the Pacific, and deep (200-600 m) water flows into the Gulf. We propose a biogeochemical method to estimate the vertical component of water exchange assuming a steady state for the concentration of nutrients in the Gulf and using the annual input of dissolved Si needed to support the production of opal accumulated in its sediments. An estimated average opal accumulation of (273.3 +/- 6.8, mean +/- SE)x10(9) mol SiO2 year(-1) for the whole Gulf was extracted from literature sources. Annual averages for H4SiO4, at the mouth of the Gulf and for the depth intervals 0-200 m and 200-600 m, were used to calculate the annual average vertical component of water exchange between the Gulf and the Pacific needed to balance this opal accumulation, i.e., 0.23 +/- 0.02 Sv. This relatively low value is only similar to 3% of the whole average water exchange. This vertical component value was used to calculate the net annual nitrate input from the Pacific into the Gulf, and with the Redfield ratio it was transformed to new phytoplankton production for the whole Gulf [(10.67 +/- 3.66)x10(9) kg C year(-1)].