MACNBR   00242
MUSEO ARGENTINO DE CIENCIAS NATURALES "BERNARDINO RIVADAVIA"
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Did Patagonian eruptions drive Cenozoic icehouse?
Autor/es:
BELLOSI, E.S., CORBELLA
Lugar:
Santa Rosa - La Pampa
Reunión:
Congreso; VII Congreso Latinoamericano de Sedimentología y XV Reunión Argentina de Sedimentología; 2016
Institución organizadora:
Asociacion Argentina de Sedimentologia
Resumen:
Climate of Earth experienced several first-order alterations. The last of them transformed the ice-free Late Cretaceous-Early Paleogene Greenhouse system into the present Icehouse world. This change begun by the middle Eocene (50-49 Ma) and conducted to the first Cenozoic glaciation when Antarctic ice sheets extended to the sea, close to the Eocene-Oligocene boundary (33.9 Ma, Oi-1 Glaciation). Several factors have been proposed to explain this long-term global cooling-drying trend, coincident with lowering of atmosphere CO2 concentration and deepening of calcite-compensation depth in oceans. Some of these factors consider enhanced silicate weathering by mountain uplift or by ant/termite activity (C sequestration), changes in Earth?s orbital configuration (low eccentricity, high obliquity), diminished volcanic outgassing, carbon storage in the North polar area, spreading of grassland ecosystems, photosynthetic fixation, and changes of ocean-atmospheric circulation systems by tectonicpaleogeographic events. Such factors are controversial for different reasons, or are not chronologically coincident with the climate alteration. We hypothesize that abiding volcanic eruptions in NW Patagonia caused Late Paleogene drop in global temperature. Volcanic edifices were located in two belts between 39o30?- 44o S. The external Pilcaniyeu volcanic belt was active during 60-27 Ma, with maximum activity in 50-43 Ma and 34-29 Ma. Proximal records include rhyolitic ignimbrites, tuffs and breccias, obsidian, dacites, and andesite-dacite lava flows related to calderas. The internal El Maitén belt was active during 34-10 Ma, with a peak at the beginning of this interval, and generated ignimbritic and plinian dacites, rhyolitic breccias; and andesitic, rhyolitic and basaltic lavas formed in stratovolcanoes and fissure eruptions. Other volcanic belts developed by the Middle and Late Eocene in southern Chile (Regions X-XI), but these rocks are scarcely preserved. Geochemical analysis indicate that distal tephras resulted from high-energy explosive Plinian to sub-Plinian eruptions, although phreatoplinian volcanism cannot be excluded because of dominant very fine grain-size even in proximal deposits (Bellosi, 2010). A sizable part of distal fine tephras accumulated in a vast emerged extra-Andean region of central and northern Patagonia. They are mainly preserved in two terrestrial units: the Middle Eocene Koluel-Kaike and Middle Eocene-Early Miocene Sarmiento formations (and equivalent units). They are entirely formed by fine vitric ashes and accumulated without significant hiatuses and with scarce reworking. Maximum thickness (375 m) is recorded in south-central Chubut. Timing of pyroclastic accumulation is based on litho- and biostratigraphic surveys (~100 localities), numerous radiosotopic analyses and magnetostratigraphy, indicating that mostly corresponds to the Middle Eocene-Early Oligocene. Another fraction of ashes would have travelled long distances and fell down in southern oceans, as occurs nowadays. A conservative calculation of the surface covered by distal ashes (excluding near-volcano deposits) in central and northern Patagonia and coastal areas is ~6 x 105 km2. The estimated decompacted volume is about 2 x 105 km3. The amount of ash supplied to the oceans is unknown, but probably exceeds several times this number. These values are extremely high and are between the greatest known (e.g. Ignimbrite Flare-Up, John Day and Balder Fms., Lava Creek ash beds). Some of them, contemporary of Patagonian eruptions, could have added their effects. These huge volcanic eruptions supplied large amount of sulphur gases to the atmosphere, along with Fe and Si-rich minerals (plus Cu, Zn, P) to land and oceans. Both products provoke two coupled cooling effects: 1) highly reflective sulphuric acid droplets as stratospheric aerosols amplify sunlight reflection; and 2) iron and silicon fertilization of seawater increase phytoplankton biomass and drawdown concentration of atmospheric CO2. A little part of S volatiles was captured in Patagonian ash deposits (mean content: 850 ppm), and also crystallized as sulfates (gypsum, anhydrite, baritine), which concentrate in the lower part of the Sarmiento Formation (Feruglio, 1949). Average content of SiO2 and Fe2O3 in ash is 66% and 5%, respectively. The earlier open-vegetated landscapes recognized in Middle Eocene (~44 Ma) deposits of central Patagonia could be one of the first evidences of the Paleogene cooling-drying trend (Bellosi and Krause, 2014). A second step (40 Ma) was the advent of grass-dominated habitats documented by phytoliths; and later (39-38 Ma) the accelerating hypsodonty of mammalian herbivores and the expansion of dung beetles.