THE EVOLUTION OF EARTH'S SURFACE FROM THE PERSPECTIVE OF MODERN RIVERS (Invited Presentation)

The chemical composition of the continental crust reflects an integration of low and high temperature geochemical processes and bears on a wide range of geologic problems. To estimate crustal compositions, it is common to sample natural geologic archives that effectively average rock compositions in space and time such as shales, loess, and diamictites. In principle, the dissolved load of rivers can also be used as a constraint on crustal compositions given that rivers naturally average water/rock interactions within their watersheds. In contrast to rock samples, rivers average over shorter time scales, but potentially larger spatial scales. However, direct measurements of rivers are expected to be biased estimates of bulk rock chemistry on account of the preferential dissolution of accessory phases and/or effects of incongruent weathering reactions. However, it is the chemistry expressed in river systems that regulates the global carbon cycle. And, viewed another way, the biases inherent to river dissolved loads can be leveraged to constrain specific aspects of crustal composition and evolution provided an appropriate analysis framework.

Here, we will discuss ongoing efforts to infer the compositions of crustal materials undergoing weathering in different river systems through inverse modeling of solute sources and sinks. In addition to discussing a new, flexible model for river inversions (MEANDIR), we will also highlight a range of case studies (Iceland, Panama, and Puerto Rico) where we infer the compositions of primary and secondary silicates directly from river chemistry data so as to avoid strict assumptions about the nature of incongruent weathering reactions. In addition to discussing the implications of our results for crustal composition and evolution, we will also put them in the context of climate-weathering feedbacks and long-term planetary habitability.