EARLY OLIGOCENE GLOBAL COOLING, VOLCANIC IRON FERTILIZATION, AND THE IGNIMBRITE FLARE-UP OF SOUTHWESTERN NORTH AMERICA

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.