Arcs and global biogeochemical cycles Louis Derry Cornell University How does weathering, erosion and geothermal activity in arc terranes impact global biogeochemical cycles? example of C cycle How does surface processing of primary material impact transition to continental crust? CaAl 2 Si 2 O CO H 2 O Ca ++ + Al 2 Si 2 O 5 (OH) 4 +2 HCO 3 - Ca HCO 3 - CaCO 3 + H 2 O + CO 2 _________________________________________________________________ CaAl 2 Si 2 O 8 + CO H 2 O CaCO 3 + Al 2 Si 2 O 5 (OH) 4 Weathering of Ca, Mg silicates consumes CO 2 Na, K silicates are much less efficient sinks for CO 2 (because we dont make Na 2 CO 3 in the oceans) net Weathering as f(Temp, runoff, erosion) in large rivers climate sensitivity not as strong as expected erosion rate plays an important role (tectonics + climate) runoff relief Temperature erosion Gailardet et al., 1999 Temperature dependence of weathering rate from laboratory studies follows an Arrhenius law: k = A exp(-E a /RT) Typical values of E a for feldspars are 60 65 kJ mol -1 K -1. Implies a temperature sensitivity equivalent to 2.3x increase in k between 20 and 30C. Large river studies dont really show this kind of T sensitivity. Why? Arcs a critical sink in the global C cycle? Interesting features Ca, Mg rich compositions Fast kinetics in volcanic rocks Tectonically active Wet High erosion rates Frequent resurfacing Underrepresented in our data and our thinking No big rivers Large groundwater fluxes (unmeasured!) Milliman: sed yields are 10x global average Climate sensitivity? Basalt weathering appears to show strong climate sensitivity. (data mostly from Dessert et al., 2003) Weathering in basalt provinces T dependence: exponent is equivalent to laboratory Arrhenius data, e.g. for plagioclases. Equivalent to E a = 62 kJ mol -1 K -1 Weathering in basalt provinces Luzon, Philippines active arcs ophiolites typhoons Rivers draining W side of Pinatubo Big, full of fresh pumice, and completely uncharacterized Large groundwater discharge directly to ocean Pinatubo S. China Sea Pumice fills river channels Arc rivers are different . Philippines A few important themes How do arcs interact with global biogeochemical and water cycles? C fluxes other elemental fluxes (Si, Fe, Sr, ..) geothermal fluxes ground water fluxes coupling with erosion and mass wasting generation of continental crust