THE ROLE OF SILICATE WEATHERING IN SHIFTING DEVONIAN PALEOCLIMATE

Chemical weathering of Ca- and Mg-bearing silicate minerals and the subsequent trapping of carbon in marine carbonates act as a sink for atmospheric CO₂, driving shifts in global climate on geologic time scales. However, the role of silicate weathering in the fluctuating paleoclimate of the Devonian Period (419 to 359 Ma) is only in the early stages of investigation. Previous paleotemperature studies suggest that sub-tropical sea surface temperatures (SST) cooled considerably during the late Silurian to Middle Devonian before warming again in the late Middle Devonian. In addition to changes in silicate weathering, potential drivers for these climatic shifts that have been proposed in the literature (e.g., Chen et al., 2021) include the radiation of early rooted plants (cooling), and Acadian Orogeny degassing (warming).

This study aims to constrain the role of silicate weathering in the cooling and warming trends of the late Silurian to late Middle Devonian. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) and neodymium isotopes (ε_Nd) act as geochemical proxies for the lithology and source of weathered silicates. Marine conodont apatite samples from Požáry, Czech Republic (late Silurian to Early Devonian) and Pic de Bissous, France (late Middle Devonian) had previously been analyzed for sea surface temperatures (SST) via the paleotemperature proxy δ¹⁸O (Joachimski et al., 2009). We are analyzing these same samples for ⁸⁷Sr/⁸⁶Sr and ε_Nd, creating three complementary geochemical curves with no temporal uncertainty among them. We have found that ⁸⁷Sr/⁸⁶Sr inflects toward more radiogenic values within 1 My of when δ¹⁸O shifts to lower (warmer) values in the mid-Givetian (ca. 383 Ma), suggesting a link between silicate weathering and climate warming. We hypothesize that cool ocean bottom water of the Early to Middle Devonian reduced the rate of alteration to ocean crust, reducing carbon sequestration in hydrothermal systems along with weathering of low-⁸⁷Sr/⁸⁶Sr oceanic crust (cf. Coogan and Dosso, 2015). A reduced rate of subaerial basalt weathering due to cooling climate would produce similar results. Subsequent weathering of uplifted high-⁸⁷Sr/⁸⁶Sr and low-ε_Nd continental crust associated with the Acadian Orogeny could be occurring in conjunction with hypothesized Acadian degassing to drive higher pCO₂.