Current Research in EEPS: Evan J. Ramos, The University of Texas at Austin
You Can’t Rule Anything Out: Probing Controls of Silicate Weathering in Floodplains
Silicate weathering underpins life-sustaining element cycles and mitigates a permanent Greenhouse state on Earth over geologic time. Despite the known importance of silicate weathering, debate regarding its primary drivers today and over geologic time remains active. Much attention has been dedicated to understanding silicate weathering in high-relief, mountainous environments for they are thought to yield the greatest silicate weathering fluxes of all continental settings. Less is known about silicate weathering in low-relief floodplains, where the magnitude of silicate weathering fluxes varies widely—from 10% of the cumulative drainage flux in the Amazon to upwards of 70% in the Ganges—and the transit of sediment and water is more challenging to predict. In this talk, I will present findings from two studies that elucidate key processes responsible for silicate weathering in floodplains and highlight areas where our mechanistic description of floodplain weathering needs refinement. Both studies are motivated by and rely upon the usage of Li isotopes: an isotopic system which is sensitive to the formation of secondary phyllosilicate (clay) minerals from the breakdown of primary silicates.
In the first study, we compile a global dataset of published river water Li isotope compositions (N = 757) and determine their catchment-average climatic, lithologic, and morphometric properties to test the hypothesis that silicate weathering intensity (i.e., the balance between physical erosion and silicate weathering), and not individual drainage catchment properties, controls river δ7Li values. Through various statistical tests, we verify that multiple catchments properties are required to significantly explain the range of observed δ7Li values. To then test the control of silicate weathering intensity on river δ7Li values, we use catchment properties as input to canonical erosion and weathering equations to compute catchment-average silicate weathering intensities. We find that the greatest mismatch between predicted and reported silicate weathering intensities are found in catchments with high weathering intensity and low relief, and especially in those with high mean annual temperature and precipitation. Altogether, these findings suggest that the paradigm of erosion-driven or steady-state weathering may not apply to floodplain environments and, by extension, the environmental drivers of river δ7Li values need further refinement.
In the second study, we measure the major/trace element and Li isotope composition of paleosol clays (< 2 μm size fraction) and basin-bounding source rocks to quantify silicate weathering intensities of a well-preserved ancient floodplain in the Bighorn Basin, Wyoming, USA. Over an interval where climate and sediment deposition are relatively steady, we discern distinct weathering trends as a function of the paleo-landscape position. Soils that formed proximally to the active channel contain more weatherable igneous sediments whereas soils that formed more distally contain more shale sediments. As a result, sediments closer to the active channel undergo consistently more intense weathering than their distal counterparts. These findings newly illustrate that hydrodynamic sorting of sediments during floodplain deposition impose a primary influence on silicate weathering intensity, providing critical information about the loci of silicate weathering in modern floodplains.