HR: 0800h
AN: PP11B-0514    [Abstracts]
TI: Gulf of California Sediment and Proxy SST Records Suggest a Post 6 ka Development of the Arizona Monsoon and Solar Forcing of Cycles
AU: * Barron, J A
EM: jbarron@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Bukry, D
EM: dbukry@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AB: Summer monsoonal rains in Arizona and adjacent areas are mainly due to pulses of moisture traveling northward up the Gulf of California (GOC). Modern studies reveal that northern GOC SSTs must exceed 26 deg. C before monsoonal rainfall develops in Arizona and western New Mexico, and over 80 percent of the rainfall in this region occurs after northern GOC SSTs exceed 28.5 deg. C. Warming of GOC occurs progressively from south to north in the late spring, as northwest winds, which dominate in the late fall to early spring, decrease in strength, and tropical waters penetrate northward along the western coast of the GOC. Sediment (CaCO₃ and opal) and microfossil (diatom and silicoflagellate) proxies spanning the past 15,000 years from cores in the central GOC suggest that waters of the northern GOC were too cold between ca. 11 and 6 ka to allow development of monsoonal rains in Arizona. Evidence for a post 6 ka intensification of monsoonal rains in Arizona and adjacent areas includes: 1) increased frequency of arroyo cutting in Arizona after ca. 5 ka, 2) increased evidence of paleofloods in Arizona and SW Utah after ca. 6 ka, and 3) the renewal of aggradation of alluvial fans in the Mojave Dessert at ca. 6 ka after a lull in their formation between ca. 11 and 6 ka. Supportive pollen evidence includes : 1) the late Holocene appearance of summer flowering annuals and C-{4} grasses in SE Arizona, and 2) the post 6 ka appearance of a warm, mixed biome in the highlands of northwest Mexico. Other pollen evidence and the scarcity of early and middle Holocene packrat middens in the American southwest, however, have been cited as evidence of increased monsoonal rains during the early and middle parts of the Holocene It is likely that the Gulf of Mexico was the main source of monsoonal moisture in the American southwest prior to ca. 6 ka, especially in the regions east of Arizona. A northward displacement of the Intertropical Convergence Zone in the Caribbean prior to ca. 5.4 ka possibly led to increased advection of monsoonal moisture from the Gulf of Mexico. Marked increases in tropical diatoms (Azpeitia nodulifera) and silicoflagellates (Dictyocha aculeata) in the central GOC between ca.A.D. 940 and 1020 and again between ca. A.D. 1100 and 1140 argue for substantial warming of SSTs, implying that the Arizona Monsoon was enhanced during this part of the Medieval Climate Anomaly. Detailed study of GOC sediment records of the past 2,000 years suggests that solar variability drives long-term climatic cycles there. Increased opal and an upwelling flora, normally dominant between November and April when northwest winds blow down the GOC, correspond to intervals of solar minima; whereas increased CaCO₃ and tropical microfossils, which are typical of the summer to early fall, coincide with solar maxima.
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4910 Astronomical forcing
DE: 4944 Micropaleontology (0459, 3030)
DE: 4954 Sea surface temperature
DE: 4964 Upwelling (4279)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting