Coupled effects of climate, lithology, and morphometrics on river water Li isotope records

Abstract:

Refining the controls of the continental lithium (Li) cycle can have profound implications in our understanding of landscape denudation, the geologic carbon cycle, climate-tectonic interactions, and changes in ocean chemistry through geologic time. The Li isotope composition of river water is a measure of the congruency of silicate weathering throughout a drainage catchment. The properties of drainage catchments that modulate the transfer of Li across landscapes – including climate, lithology, and morphometrics – have been individually discussed in previous studies but not considered wholesale. In this study, we compile a global dataset of published river water Li isotope compositions (n = 792) and compare them to their catchment properties. Through statistical approaches, including regression analysis and analysis of variance, we assess the strength of variable correlations with river water chemistry and the coupling of catchment properties with one another. We find that that river water Li isotope compositions cannot be adequately explained by individual catchment properties, but that the coupling of either climatic (mean annual temperature and precipitation) or morphometric (mean local relief, catchment area, and mean hillslope angles) properties with lithologic (rock type proportions) properties can. To further investigate the apparent coupling of catchment properties, we narrow our focus to 3 well-documented river systems on Earth. For each transcontinental drainage, we employ hierarchical clustering analysis to divide samples into subpopulations based off the similarity in their catchment properties, determine which (if any) catchment properties drive the sample clustering, and perform Li isotope fractionation modeling to see if the river chemistry of these sample subpopulations exhibit a preference for certain fractionation modes. We find that sample clustering and their preferred fractionation mode are often predicated by morphometric properties, with climatic or lithologic properties playing important but albeit subordinate roles. Overall, with these findings, we provide a testable framework that relates catchment properties with possible modes of landscape denudation that manifest in river Li isotope records.