The sensitivity of the carbon cycle to mountain building
The global carbon cycle is sensitive to mountain building, because (i) erosion exposes rocks to weathering reactions that can sequester or emit CO2, (ii) erosion facilitates the removal and long-term storage of organic matter, and (iii) decarbonation of subducted carbonate rocks emits “metamorphic” CO2. Quantifying the role of mountain uplift and erosion for the balance of these carbon sources and sinks remains complex. In this talk, I analyze the chemistry of rivers that span well-defined erosion-rate gradients to quantify the erosion sensitivity of rock-weathering and organic carbon export. These analyses suggest that landscapes eroding at moderate erosion rates may be efficient carbon sinks, whereas carbon sources dominate in rapidly eroding mountains. I will then discuss how degassing of metamorphic CO2 and sediment storage on floodplains may modulate this finding.
Rain, rivers, and reefs
Volcanic ocean islands are well suited for studying climatic controls on erosion because they typically have relatively homogeneous bedrock, steep rainfall gradients, and remnant surfaces that constrain their age, initial topography, and vertical motions relative to sea level. They also often host coral reefs, which protect coastal communities and infrastructure from storms, floods, and rising sea levels. In this talk, I’ll examine the influence of rainfall and discharge variability on bedrock river incision on the Hawaiian Island of Kaua‘i. I’ll also present a new compilation of river incision efficiencies inferred from eroded canyon volumes and lava flow ages across the Azores, Hawaiian, Cape Verde, and Canary Islands. These erosional efficiencies increase with mean annual rainfall over nearly the full range of rainfall rates experienced on Earth. Yet, the scaling between rainfall and erosion rates vary between island chains. I’ll discuss some possible explanations for these discrepancies in climatic control and consider the implications on couplings between orogeny and orography. In the second part of the talk, I’ll examine controls on coral reef widening under relative sea level rise and present a non-dimensional framework for mapping out environmental influences on reef development.