Using Carbon Isotopes in Cenozoic Soil Carbonates to Quantify Primary Productivity from Mid-Latitude Regions

Abstract:

The carbon isotope composition of pedogenic carbonates (δ¹³C_carb) from paleosols has been extensively used as a proxy to estimate atmospheric pCO₂ over the Phanerozoic. However, a number of other factors – including the concentration of plant-respired CO₂ and the isotopic composition of both atmospheric and plant-respired carbon – influence the δ¹³C of pedogenic carbonates. For example, δ¹³C_carb records from the mid-latitudes in central Asia and western North America show increasing trends in δ¹³C_carb despite decreasing pCO₂ during the late Cenozoic, which suggests that other factors play an important role in determining the isotopic composition of pedogenic carbonates. Instead, we suggest that these records are primarily recording changes in primary productivity rather than changes in atmospheric pCO₂ and therefore propose a novel use of paleosol carbonate records to understand paleo-ecosystem dynamics.

Here, we compile existing paleosol carbonate records, and present three new records from Wyoming, to estimate soil respiration and primary productivity in western North America during the Paleogene and early Neogene. We observe both an overall increase in δ¹³C_carb after the early Eocene, and spatially heterogeneous δ¹³C_carb values across western US basins. We combine this δ¹³C_carb data with compilations of atmospheric pCO₂ to estimate soil respiration and plant productivity. The long-term increase in δ¹³C_carb indicates a decrease in plant productivity as conditions became more arid across much of the western US, congruent with both records of regional uplift and of global cooling. Furthermore, significant spatial heterogeneity in δ¹³C_carb indicates that regional factors, such as the presence of paleolakes and/or local paleotopography may have provided a second-order control on local and regional productivity. Thus, our results provide a first-order estimate linking changes in primary productivity with regional tectonics and global climatic change.