Links Between Global Climate and the Evolution of Cold Upwelling on the California Margin Since the Late Miocene
Ravelo, A.C.; LaRiviere, J.; and Dekens, P.
The evolution of upwelling regions is thought to be an important component of global climate changes because of the potential for upwelling regions to affect carbon sequestration and ventilation. The development of upwelling regions can also contribute the evolution of sea surface temperature spatial patterns, which can drive changes in atmospheric pressure gradients and climate. By analyzing alkenone saturation in deepsea sediments from cores along the California margin, we reconstructed sea surface temperature since the late Miocene (~12 Ma to present). We find that sea surface temperatures were 5-10°C warmer than today in the late Miocene all along the California margin and cooled gradually until the Pleistocene, reaching modern temperatures by about 2 Ma. The Southern California Bight locations cooled most rapidly during Northern Hemisphere Glaciation. We compared our sea surface temperature records to records of paleoproductivity and atmospheric pCO2 content to assess the linkages between them. Overall, this paleoclimate study has several important implications. First, that cooling along the margin was nearly uniform (with no change in the north-south gradient) indicating that changes in upwelling, rather than changes in the strength of the California Current probably explain the temperature changes. Second, that the warmth of the late Miocene and early Pliocene, when paleoproductivity was relatively high, suggests that temperature is not indicative of the strength of upwelling-driven biological productivity. Third, that the cooling of the California margin, and not the strength of biological production, was tightly coupled to global climate change. Fourth, that the warmth of the late Miocene occurred when pCO2 was relatively low, similar to preindustrial values, indicating that climate and greenhouse gas forcing were not tightly coupled to each other and that other forcings must be important during some warm climate states. And finally, that cooling of the California margin occurred as other upwelling regions around the globe cooled through the Pliocene and Pleistocene, suggesting that global changes in large-scale thermocline structure may have been responsible for the development of cold upwelling as the Ice Ages developed.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013