44-19 Modeling the effects of Paleozoic land plant evolution on marine Si isotope systematics

Abstract:

Silicate weathering helps to regulate Earth’s climate through sequestration of atmospheric CO₂ into marine sediments during secondary mineral formation. The rise of vascular land plants in the latter half of the Paleozoic Era is hypothesized to have increased silicate weathering rates significantly through plant-rock interactions including carbonic acid production in soils, exudation of organic compounds by plant roots, mechanical breakdown of rocks by plant root systems, and alteration of water flow in soils. This increase in weathering rates could have altered the long-term carbon cycle by accelerating the removal of CO₂ from the atmosphere, but the impact of this increased weathering rate depends on its congruency. Incongruent weathering means that a large proportion of primary dissolved products released by weathering are incorporated into secondary mineral phases. The degree of congruency reflects how much of the dissolved products of weathering are delivered to the ocean and available for marine carbonate formation. River water dissolved Si (dSi) concentrations and dSi δ³⁰Si values are controlled by weathering congruency. During incongruent weathering reactions, river dSi becomes isotopically enriched in ³⁰Si due to the preferential partitioning of ²⁸Si into secondary clays. The formation of clays also traps Si on the continents, reducing the flux of dSi to the oceans. Since rivers and groundwater discharge are the primary inputs of dSi from the continents to the surface ocean, changes in weathering congruency should be reflected in the δ³⁰Si compositions of seawater and the skeletons of siliceous sponges and radiolarians that lived in that seawater. To determine if increased weathering driven by the evolution of land plants in the Devonian was more congruent (less formation of secondary silicate mineral phases) or incongruent (more formation of secondary phases), we employ a mathematical mass-balance model of the marine silica cycle. We then compare our model predictions with measurements of δ³⁰Si values from Paleozoic sponge spicules and radiolarians. The results of this model-data comparison will provide new insights into changes in fluxes and the δ³⁰Si values of weathering inputs and seawater [dSi] across this interval. 

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.