Carbon Isotope Evidence for Changes in Cenozoic Mid-Latitude Primary Productivity and Water Use Efficiency
Abstract
The carbon isotope composition of pedogenic carbonates (δ13Ccarb) from paleosols has been extensively used as a proxy to estimate atmospheric pCO2 over the Phanerozoic. Given that atmospheric CO2 has a higher δ13C than soil-respired CO2, higher δ13Ccarb should reflect higher atmospheric pCO2. 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 δ13Ccarb. For example, paleosol δ13Ccarb records from the mid-latitudes in central Asia and western North America increase despite decreasing pCO2 during the late Cenozoic, which suggests that other factors play an important role in determining the isotopic composition of pedogenic carbonates. We suggest that these records are primarily recording changes in primary productivity due to changing water use efficiency rather than changes in atmospheric pCO2 and therefore propose a novel use of paleosol carbonate records to understand paleo-ecosystem dynamics. Here, we use a large database of Cenozoic paleosol δ13Ccarb to estimate broad changes in soil respiration and primary productivity in both Asia and western North America. Overall, δ13Ccarb in most localities either increases or remains constant, despite large declines in atmospheric pCO2. However, throughout the entire Cenozoic, there is substantial spatial variability in δ13Ccarb. The long-term increase and/or constancy of δ13Ccarb indicates a decrease in plant productivity as atmospheric pCO2 declined, likely reflecting the influence of a reduction in plant CO2 fertilization. The sustained spatial variability reflects the effect of tectonics in altering regional climate and, hence, plant productivity. We estimate changes in water use efficiency by combining independent estimates of paleo-precipitation to estimate how this critical ecosystem parameter changes with variable atmospheric pCO2. These results point to a new method to extract critical paleo-ecosystem characteristics and understand the coupling between climate, hydrology, and terrestrial primary productivity.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2018
- Bibcode:
- 2018EGUGA..2019426C