WEATHERING CONGRUENCY OF VOLCANIC ROCK EXHIBITS PROPORTIONAL BEHAVIOR TO TEMPERATURE, WITH NET INCONGRUENCY FAVORING COLD CLIMATES

The chemical weathering of volcanic rocks is thought to be a primary control on Earth’s climate spanning 10⁵ – 10⁷-year timescales. The degree to which primary volcanic minerals are converted to secondary minerals, termed weathering congruency, directly influences silicate weathering fluxes to the ocean. By studying the chemistry of river water, we seek to better understand catchment-wide environmental and geological factors that influence silicate weathering congruency in mafic and intermediate volcanic rock-dominated catchments.

In this study, we compile published riverine dissolved load Li isotope and Li/Na data and compare them to catchment properties in volcanic rock-dominated catchments. River water δ⁷Li values and Li/Na are sensitive to secondary mineral formation: when secondary minerals precipitate, they preferentially incorporate ⁶Li over ⁷Li and exclude Na, imparting river with high ⁷Li/⁶Li ratios (i.e. high δ⁷Li values) and low Li/Na ratios. Therefore, river water δ⁷Li values and Li/Na convey the relative amount of secondary mineral formation. Using digital elevation data, we determined drainage catchment geometries for each river water sample and determine catchment-averaged climate (mean annual temperature and precipitation), morphometrics (mean local relief and catchment area), and lithology from other global datasets. To control for the effect of lithology on river water chemistry, we select sample sites that have >60% intermediate or mafic volcanic rocks, which include rivers in Iceland, Azores, Lesser Antilles, Java, Reunion, and within the Columbia River drainage in the US.

Comparisons between river water chemistry and catchment properties point to the primary influence of climate on weathering congruency. Although samples varied in their tectonic settings, morphometric properties were highly consistent across study sites, allowing us to focus on the effect of climate. Among climatic factors, mean annual temperature was determined to correlate significantly with river water chemistry whereas precipitation showed no significant correlations. We found climate was the primary driver of chemical weathering congruency of mafic and intermediate volcanic igneous rocks. Lastly, we observe that colder climates yield higher relative amounts of clay formation than their equatorial counterparts. The cause of increasing weathering congruency with increasing mean annual temperature could be related to dissolution and precipition kinetics, soil pH, or the mode of precipitation delivery.