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

Friday, 18 December 2015: 12:05
2012 (Moscone West)
Jeremy K Caves1, Samuel H Kramer1, Daniel E Ibarra2 and C Page Chamberlain3, (1)Stanford University, Stanford, CA, United States, (2)Stanford University, Earth System Science, Stanford, CA, United States, (3)Stanford University, Environmental Earth System Science, Stanford, CA, United States
The carbon isotope composition of pedogenic carbonates (δ13Ccarb) 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, δ13Ccarb records from the mid-latitudes in central Asia and western North America show increasing trends in δ13Ccarb 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 pCOand 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 δ13Ccarb after the early Eocene, and spatially heterogeneous δ13Ccarb values across western US basins. We combine this δ13Ccarb data with compilations of atmospheric pCO2 to estimate soil respiration and plant productivity. The long-term increase in δ13Ccarb 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 δ13Ccarb 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.