EARLY OLIGOCENE GLOBAL COOLING, VOLCANIC IRON FERTILIZATION, AND THE IGNIMBRITE FLARE-UP OF SOUTHWESTERN NORTH AMERICA CATHER, Steven M.1, DUNBAR, Nelia W.1, MCDOWELL, Fred W.2, MCINTOSH, William C.1, and SCHOLLE, Peter A.1, (1) New Mexico Bureau of Geology, New Mexico Tech, 801 Leroy Pl, Socorro, 87801, (2) Department of Geological Sciences, The University of Texas, Austin, TX 78712
We propose that widespread, middle Cenozoic explosive volcanism in SW North America influenced global paleoclimate. Recent shipboard experiments have shown that fertilization of the oceanic photic zone with nanomolar concentrations of Fe produced dramatic increases in phytoplankton productivity and uptake of CO2. Drawdown of atmospheric CO2 has been attributed to Fe fertilization of the ocean following the relatively small volume (3-5 km3) Pinatubo eruption. The ignimbrite flare-up of late Eocene - early Miocene age in Mexico and SW U.S.A. was perhaps the largest Phanerozoic episode of explosive volcanism on earth, and emplaced nearly 4x105 km3 of silicic ignimbrite during hundreds of individual 102-103 km3 eruptions. All eruptions undoubtedly contributed to the global dust flux, but those in the Sierra Madre Occidental (SMO) of Mexico were particularly important because they accounted for ~3/4 of the erupted volume of the ignimbrite flare-up. Much of the SMO was located within the zone of easterly trade winds, and thus would have contributed a large portion of plinian and co-ignimbrite fallout ash directly to the Pacific Ocean. Major ignimbrite volcanism in SMO occurred in two pulses that corresponded to paleoclimatic events. The first pulse (~38-28 Ma) accounted for perhaps 2/3 of SMO ignimbrite volcanism. It encompassed the ~35-30 Ma global shift to heavier d13C values in benthic foraminifera that may have resulted from enhanced marine productivity and increased burial rates of organic carbon. Assuming a typical carbon/iron molar ratio of 105 for phytoplankton in Fe-limited areas, we calculate that readily available, adsorbed Fe on ash from the first pulse of SMO ignimbrite volcanism may have increased production of organic carbon by 1018 moles. If significant dissolution of fine vitric ash occurred in the photic zone, carbon production may have been vastly greater. Early Oligocene global cooling, beginning with Oi-1 glaciation at ~33.5 Ma and continuing to ~26 Ma, may have resulted from the crossing of a threshold as atmospheric CO2 declined. No particular spike of ignimbrite activity can be demonstrated at 33.5 Ma. The second, volumetrically smaller pulse of ignimbrite activity in SMO (~24-20 Ma) culminated ~23 Ma, closely coincident with a short-lived phase of early Miocene cooling and Mi-1 glaciation.