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Arcs and global biogeochemical cycles Louis Derry Cornell University PowerPoint Presentation
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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”?.

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slide1

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”?

slide2

Weathering of Ca, Mg silicates consumes CO2

CaAl2Si2O8 + 2CO2 + 3 H2O <-> Ca++ + Al2Si2O5(OH)4 +2 HCO3-

Ca++ + 2 HCO3- <-> CaCO3 + H2O + CO2

_________________________________________________________________

CaAl2Si2O8 + CO2 + 2 H2O <-> CaCO3+ Al2Si2O5(OH)4

net

Na, K silicates are much less efficient sinks for CO2

(because we don’t make Na2CO3 in the oceans)

slide3

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

slide4

Temperature dependence of weathering rate

from laboratory studies follows an Arrhenius law:

k = A exp(-Ea/RT)

Typical values of Ea for feldspars are 60 – 65 kJ mol-1 K-1.

Implies a temperature sensitivity equivalent to ≈ 2.3x increase in k between 20 and 30˚C.

Large river studies don’t really show this kind of T sensitivity. Why?

arcs a critical sink in the global c cycle
Arcs – a critical sink in the global C cycle?
  • Underrepresented in our data and our thinking
  • No big rivers
  • Large groundwater fluxes (unmeasured!)
  • Interesting features
  • Ca, Mg rich compositions
  • Fast kinetics in volcanic rocks
  • Tectonically active
  • Wet
  • High erosion rates
  • Frequent resurfacing

Climate sensitivity?

Milliman: sed yields are ≈10x global average

slide6

Weathering in basalt provinces

Basalt weathering appears to show strong climate sensitivity.

(data mostly from Dessert et al., 2003)

slide7

Weathering in basalt provinces

T dependence:

exponent ≈ 0.076 is equivalent to laboratory Arrhenius data, e.g. for plagioclases. Equivalent to Ea = 62 kJ mol-1 K-1

slide8

Luzon, Philippines

  • active arcs
  • ophiolites
  • typhoons
slide9

Rivers draining W side of Pinatubo

Big, full of fresh pumice, and completely uncharacterized

Large groundwater discharge directly to ocean

Pumice fills river channels

Pinatubo

S. China Sea

slide11

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