Soil Carbon Isotope Values and Paleoprecipitation Reconstruction

Anthropogenic climate change has significant impacts at the ecosystem scale including widespread drought, flooding, and other natural disasters related to precipitation extremes. To contextualize modern climate change, scientists often look to ancient climate changes, such as shifts in ancient precipitation ranges. Previous studies have used fossil leaf organic geochemistry and paleosol inorganic chemistry as paleoprecipitation proxies, but have largely ignored the organic soil layer, which acts as a bridge between aboveground biomass and belowground inorganic carbon accumulation, as a potential recorder of precipitation. We investigate the relationship between stable carbon isotope values in soil organic matter (δ13CSOM) and a variety of seasonal and annual climate parameters in modern ecosystems and find a statistically significant relationship between δ13CSOM values and mean annual precipitation (MAP). After testing the relationship between actual and reconstructed precipitation values in modern systems, we test this potential paleoprecipitation proxy in the geologic record by comparing precipitation values reconstructed using δ13CSOM to other reconstructed paleoprecipitation estimates from the same paleosols. This study provides a promising new proxy that can be applied to ecosystems post‐Devonian (∼420 Ma) to the Miocene (∼23 Ma), and in mixed C3/C4 ecosystems in the geologic record with additional paleobotanical and palynological information. It also extends paleoprecipitation reconstruction to more weakly developed paleosol types, such as those lacking B‐ horizons, than previous inorganic proxies and is calibrated for wetter environments. Plain Language Summary Rainfall is very important to plant health and function. When plant material is deposited onto the ground, it becomes soil. This soil retains records of plant chemistry. We tested whether this plant chemistry recorded amount of rainfall over a wide range of environments, and found that soil chemistry does record rainfall. When tested in fossil soils, the soil chemistry as it remained of prior plant deposition could be used to calculate ancient rainfall, millions of years ago. Key Points Soil carbon isotope values can be used to reconstruct precipitation Reconstructed paleoprecipitation using carbon isotope values of organic matter are comparable to reconstructions with other proxies These soil isotope geochemistry findings validate prior work linking aboveground plant biomass isotope ecology