On the role of chemical weathering of continental arcs in long-term climate regulation: A case study of the Peninsular Ranges batholith, California (USA)

Abstract

The long-term stability of atmospheric pCO2 is dominantly determined by the balance between the rate of CO2 input from magmatic/metamorphic degassing and the efficiency of CO2 uptake by silicate weathering via eventual precipitation as marine carbonate and by organic carbon burial. Silicate weathering is thought to represent a negative feedback to changes in atmospheric pCO2. The Late Cretaceous-Early Cenozoic was characterized by elevated atmospheric pCO2 and greenhouse climate, widely thought to be due to increased magmatic flux from mid-ocean ridges, flood basalts and continental arcs. Of interest here is the role of continental arcs in modulating long term atmospheric CO2 contents and climate. Continental arc magmatism is accompanied by rapid uplift and erosion due to magmatic/tectonic thickening of the crust, thus, the development of continental arcs might also enhance the global efficiency of silicate weathering, in turn increasing the efficiency of carbon sequestration. To assess the contribution of continental arcs to global carbon sinks, we conducted a case study in the Cretaceous Peninsular Ranges batholith (PRB) and associated forearc basin in southern California, USA, representing one segment of a Mesozoic circum-Pacific continental arc system. Arc magmatism occurred between 170-85 Ma, peaking at 100 Ma. Erosion occurred during magmatism and continued well into the early Eocene, with forearc sediments representing the products of this protracted unroofing of the arc. By calculating the depletion of Ca in forearc sediments relative to their arc protoliths, we estimate chemical weathering fluxes (in equivalent CO2) to be ∼106 mol km−2yr−1 near the end of magmatism, decreasing to ∼105 mol km−2yr−1 by the Early Eocene. Integrated over the entire magmatic and post-magmatic history of the batholith, the total amount of CO2 consumed through chemical weathering is comparable to that degassed. However, during magmatism, the regional degassing flux exceeds the regional weathering flux even though the efficiency of weathering is increased due to magmatic and tectonic uplift. After magmatism ends, continued erosion from remnant topography causes the arc to transition into a net regional sink, increasing the global efficiency of chemical weathering and amplifying cooling after magmatism ends. Understanding how atmospheric CO2 and climate vary over long timescales requires an understanding of how magmatic orogenies enhance degassing but also increase the strength of the global silicate weathering feedback.