Past and Present Coastal Upwelling Along the Western Americas

Abstract

Every year, increasing insolation during spring and summer sets up a broad pattern of equatorward winds along the western shores of North and South America. These winds are responsible for large-scale advection of nutrient-rich waters to the surface and, therefore, the elevated productivity of the California and Peru/Humboldt eastern boundary current systems. Because the Earth's radiation balance is involved, eastern boundary currents probably responded to past changes in summer insolation and could be sensitive to future changes in greenhouse gas concentrations. The hydrography of the nutrient-like trace element Cd and its incorporation into the geologic record provide a unique way to quantify the sensitivity of coastal upwelling to climate change, using the effect of orbital changes in insolation during the Holocene as a test case. In California, we have used the Cd content of carefully cleaned shells of the benthic foraminifer Elphidiella hannai to infer changes in estuarine dissolved Cd concentrations that are directly linked to coastal upwelling. Shells of E. hannai have been recovered from sediment sections that span much of the Holocene from several California estuaries. We report here that mean Cd/Ca ratios in shells of E. hannai from San Francisco Bay dated 7.5 ka are considerably higher (402 +/- 54 nmol/mol, n = 9) than in tests dated 900-1900 AD (274 +/- 15 nmol/mol, n = 19). Based on a comparison of wind forcing with an X-year time series of nearshore Cd, this suggests that mean wind stress during the upwelling season was about twice as high during the early Holocene than it is today. Further interpretation, however, will require a better understanding of the origin of an increase in variance of replicate Cd/Ca determinations in downcore intervals. The modern hydrography of Cd also shows that an inter-hemispheric comparison of upwelling records will be necessary to distinguish changes due to variations in solar forcing from changes due to the evolution of the El Nino/Southern Oscillation through the Holocene.