Arcs and global biogeochemical cyclesLouis DerryCornell 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”?

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

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

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

Weathering of Ca, Mg silicates consumes CO2

Na, K silicates are much less efficient sinks for CO2

(because we don’t make Na2CO3 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 ratefrom 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?

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?

0 400 800 1200 1600 20000.0

0.5

1.0

1.5

2.0

R² = 0.786770601814333

basaltLinear (basalt)Iceland

Runoff, mm

CO2

flux,

106

mol

km

-2

R2 = 0.43

Basalt weathering appears to show strong climate sensitivity.(data mostly from Dessert et al., 2003)

Weathering in basalt provinces

-15 -10 -5 0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

mod-wetExponential (mod-wet)arid

Mean annual T, ˚C

CO2

flux,

106

mol

km

-2

FCO2 = 0.13 exp(0.076•T)

T dependence:exponent ≈ 0.076 is equivalent to laboratory Arrhenius data, e.g. for plagioclases. Equivalent to Ea = 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

PinatuboS. China Sea

Pumice fills river channels

0 200 400 600 800 1000 1200 1400 1600 1800 20000

200

400

600

800

1000

1200

1400

1600

SiO2, µmol/L

(Ca

+ M

g)sil

, µm

ol/L

Himalaya

Arc rivers are different ….

Philippines

A few important themes

How do arcs interact with global biogeochemical and water cycles?

C fluxesother elemental fluxes (Si, Fe, Sr, …..)geothermal fluxesground water fluxes

coupling with erosion and mass wasting

generation of continental crust