CONTROLS ON VOLCANIC ARC WEATHERING RATES INFERRED USING COSMOGENIC NUCLIDES

Chemical weathering of highly reactive mafic and ultramafic igneous rocks may be a key sink in the global carbon cycle. Understanding how uplift of these rocks during arc-arc and arc-continent collisions through earth history has affected the evolution of global climate, including the onset of icehouse periods, requires improved constraints on the relative sensitivity of their weathering rates to physical erosion vs. climate. If weathering rates depend chiefly on erosion, then tectonic uplift of mafic and ultramafic rocks may have a strongly destabilizing effect on global climate. Conversely, if weathering rates are limited primarily by temperature or runoff, then a negative feedback mechanism between weathering and climate may attenuate the effects of rock uplift. This work characterizes the relationship between chemical weathering rates, physical erosion rates, and climate in tropical, montane watersheds in Puerto Rico that are underlain by volcanic arc rocks and associated ophiolitic serpentinite. Key to this analysis are new constraints on long-term erosion rates on these rocks from cosmogenic Cl-36 produced in situ in magnetite. These cosmogenic erosion rates are paired with classical measurements of stream solute fluxes and sediment geochemistry across runoff gradients to quantify the limits to volcanic arc rock and serpentinite weathering rates. 

This work is divided into three chapters. Chapter 2 constrains the altitude scaling behavior of Cl-36 production in magnetite. This allows erosion rates to be determined more accurately in watersheds near sea level in Puerto Rico. Chapter 3 demonstrates that volcanic arc rock weathering rates in the humid tropics are more strongly limited by physical erosion than by climatic factors. However, a positive correlation between erosion and runoff observed in this landscape may enhance the coupling between climate and weathering rates. Chapter 4 finds that, in contrast to volcanic arc rocks, serpentinite weathering is strongly limited by runoff and weakly limited by erosion. These results are presented as empirical power-law relationships that can be readily applied in global carbon cycle modeling.