High resolution records of upwelling and tropical incursions of the of Gulf of California during the past 1,600 years: diatom and silicoflagellate proxy data

High-resolution upwelling records of the past 1,600 years are compared in a north-south transect (28 deg. N to 24 deg. N) of three cores from the eastern slopes of the Guaymas, Carmen, and Pescadero Basins of the Gulf of California. Evenly-spaced samples were collected from varved sediments in each core. Resolution ranges from ca.16 yr to ca. 37 yr. Diatoms and silicoflagellates capture the seasonal variation between a late fall to early spring upwelling period of high biosiliceous productivity, that is driven by northwest winds, and a summer period of warmer, more stratified waters during which these winds slacken and/or reverse direction. Northward incursions of tropical waters, which today are strongest during intense El Niño years, are recorded by increased numbers of Azpeitia nodulifera, a tropical diatom. Enhanced northward incursions of tropical waters, alternating with periods of increased late winter to early spring upwelling (indicated by abundant Octactis pulchra, a silicoflagellate), are recorded in regularly-spaced cycles of ca. 100 yr duration (possible solar cycles) between 400 A.D. and ca. 1700 A.D. in the Carmen Basin core, NH01-21 (26.3 deg. N). Disappearance of these cycles during the past ca. 200 yr may be indicative of changing surface water conditions in the modern Gulf of California. Azpeitia nodulifera records in northern core BAM80 E17 (27.9 deg. N), are evidence that the most intense of these tropical incursions appears to have occurred between ca. 940 A.D. and 1000 A.D., during the early part of the Medieval Warm Period and corresponding with the onset of expanded building in Chaco Canyon (New Mexico) by the Anasazi people. In the modern Gulf, a strong relationship exists between warmer SST in the northern Gulf and intensified monsoonal conditions in Arizona and New Mexico. We propose that a period of increased regularity of summer rains between ca. 940 A.D. and 1000 A.D would have benefited the Anasazi. Core NH01-26 (24.3 deg. N) near the mouth of the Gulf is dominated by tropical silicoflagellates and by Cyclotella spp., a diatom that indicates warm, stratified conditions. Between ca. 1280 A.D. and ca. 1820 A.D., however, Roperia tesselata, a diatom associated with enhanced late fall to early winter production in the central Gulf, becomes an important component of the diatom assemblages. Abundance cycles of R. tesselata approximate 60 yr. and may be indicative of Pacific Decadal Oscillation (PDO).