Jennifer M. Cotton

New constraints on using paleosols to reconstruct atmospheric pCO2

Ecosystem and climatic changes due to anthro po genic emissions of CO 2 are of increasing importance to society. One way to predict these changes in the future is to study warming events in the geologic past. The main control of Earth’s temperature over geologic time is the concentration of atmospheric CO 2 ; however, the precise relationship between CO 2 and temperature is not fully understood. Therefore, it is essential to be able to quantify atmospheric CO 2 concentrations in the geologic past, especially at times of rapid climate change analogous to current atmospheric changes. One widely applied proxy relates the carbon isotopic composition of pedogenic carbonates to atmospheric pCO 2 , but one of the key variables (soil-respired CO 2 ; S[z]) is not well constrained. This study presents a new proxy where soil-respired CO 2 is related to mean annual precipitation (MAP) by the following equation: S(z) = 5.67(MAP) – 269.9, R 2 = 0.59, SE = 681 ppm. This proxy, validated for modern atmospheric pCO 2 levels, constrains the primary source of uncertainty in the soil carbonate paleobarometer. We apply this proxy to make atmospheric pCO 2 reconstructions for examples from the Cenozoic, Mesozoic, and Paleozoic and calculate CO 2 estimates that are in better agreement with estimates from other independent proxies and model results than previous pedogenic carbonate reconstructions. The implications of the uncertainties attributed to pedogenic carbonate paleobarometry as well as guidelines for using this method are discussed. By making individual measurements rather than assumptions for each isotopic value and combining those analyses with an S(z) estimate for each paleosol considered using our new proxy described herein, we can obtain increased precision for atmospheric pCO 2 reconstructions.

Constraining the early Eocene climatic optimum: A terrestrial interhemispheric comparison

The early Eocene climatic optimum was a period of major climatic and environmental change that was caused by perturbations to the global carbon cycle. Recent work from terrestrial sections in the Northern Hemisphere demonstrates that the period was characterized by different responses in the terrestrial and marine realms, suggesting that traditional causal mechanisms may not adequately explain the dynamics of the early Eocene climatic optimum. Here, we present a new high-resolution multiproxy record of terrestrial climatic and environmental conditions during the early Eocene climatic optimum from the Southern Hemisphere and compare this reconstruction to other marine and terrestrial records. Similar to Northern Hemisphere terrestrial records, there is a transient peak period of atmospheric carbon isotope enrichment as well as increased temperatures and precipitation, which indicate that terrestrial environmental responses to the early Eocene climatic optimum were broadly consistent in temperate settings worldwide. This global consistency in terrestrial records demonstrates differences in peak warming time scales and carbon isotope responses between marine and terrestrial systems, which further constrain potential causes for the early Eocene climatic optimum to multiple-system or nontraditional mechanisms and highlight the importance of paired records for understanding past climate.

Climate, CO2, and the history of North American grasses since the Last Glacial Maximum

Bisoscapes: Bison isotopes show that temperature and precipitation outweigh CO2 changes for North American grass landscapes. The spread of C4 grasses in the late Neogene is one of the most important ecological transitions of the Cenozoic, but the primary driver of this global expansion is widely debated. We use the stable carbon isotopic composition (δ13C) of bison and mammoth tissues as a proxy for the relative abundance of C3 and C4 vegetation in their grazing habitat to determine climatic and atmospheric CO2 controls on C4 grass distributions from the Last Glacial Maximum (LGM) to the present. We predict the spatial variability of grass δ13C in North America using a mean of three different methods of classification and regression tree (CART) machine learning techniques and nine climatic variables. We show that growing season precipitation and temperature are the strongest predictors of all single climate variables. We apply this CART analysis to high-resolution gridded climate data and Coupled Model Intercomparison Project (CMIP5) mean paleoclimate model outputs to produce predictive isotope landscape models (“isoscapes”) for the current, mid-Holocene, and LGM average δ13C of grass-dominated areas across North America. From the LGM to the present, C4 grass abundances substantially increased in the Great Plains despite concurrent increases in atmospheric CO2. These results suggest that changes in growing season precipitation rather than atmospheric CO2 were critically important in the Neogene expansion of C4 grasses.

Positive feedback drives carbon release from soils to atmosphere during Paleocene/Eocene warming

The Paleocene-Eocene Thermal Maximum (PETM) is the most rapid climatic warming event in the Cenozoic and informs us how the Earth system responds to large-scale changes to the carbon cycle. Warming was triggered by a massive release of 13C depleted carbon to the atmosphere, evidenced by negative carbon isotope excursions (CIE) in nearly every carbon pool on Earth. Differences in these CIEs can give insight into the response of different ecosystems to perturbations in the carbon cycle. Here we present records of δ13Ccc of pedogenic carbonates and δ13Corg from preserved soil organic matter in corresponding paleosols to understand changes to soil carbon during the PETM. CIEs during the event are larger in pedogenic carbonates than preserved organic matter for corresponding paleosols at three sites across two continents. The difference in the CIEs within soil carbon pools can be explained by increased respiration and carbon turnover rates of near-surface labile soil carbon. Increased rates of labile carbon cycling combined with decreases in the amount of preserved organic carbon in soils during the PETM suggests a decrease in the size of the soil carbon pool, resulting in a potential increase in atmospheric pCO2 and a positive feedback with warming. The PETM is a model for how the earth system responds to warming, and this mechanism would suggest that soils might serve as a large source for atmospheric CO2 during warming events.

Using Stable Carbon and Nitrogen Isotopes of Hair to Teach About Sustainable Agriculture Through Active Learning

ABSTRACT The call for reform of science education is nearly three decades old (National Commission on Excellence in Education, 1983), but the implementation of such education improvements in the form of active learning techniques in large enrollment classes remains difficult. Here we present a class project designed to increase student involvement and quantitative analysis skills in a large enrollment lecture geared towards nonscience majors. We use the stable carbon and nitrogen isotopic analyses of hair to teach students about the impacts of industrial agriculture through quantitative assessment of diet in an environmental science course. Assessment of student learning, which was determined through exam questions and also through a feedback questionnaire, was overwhelmingly positive, demonstrating the usefulness of this technique in bringing active learning to the large enrollment classroom.