SILICATE WEATHERING AND MIDDLE 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 poorly understood. Although the Devonian Period has been generally regarded as a greenhouse climate, previous paleotemperature studies suggest that sub-tropical sea surface temperatures (SST) cooled considerably during the late Early to Middle Devonian before warming again in the early Late Devonian.

This study aims to constrain the role of silicate weathering in the warming climate of the late Middle Devonian. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) act as a geochemical proxy for the lithology of weathered silicates. Marine conodont apatite samples from Pic de Bissous, France had previously been analyzed for SST via the paleotemperature proxy δ¹⁸O (Joachimski et al., 2009). We analyzed these same samples for ⁸⁷Sr/⁸⁶Sr, creating complementary δ¹⁸O and ⁸⁷Sr/⁸⁶Sr curves with no temporal uncertainty between them. We 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 would produce similar results. Either mechanism results in higher pCO₂ and warming, which would increase weathering of high-⁸⁷Sr/⁸⁶Sr continental crust.