Date of Award


Document Type


Degree Name

Master of Science (MS)


College of Science and Mathematics


Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Ying Cui

Committee Member

Clement A. Alo

Committee Member

Danlin Yu


Modern carbon dioxide (CO2) levels are well known from instrumental observation (just exceeded 415 ppmv on May 13th, 2019). CO2 levels (pCO2) in the past, however, are difficult to obtain, especially for geologic time older than 800 thousand years (kyr). Many proxies have been used to infer past CO2 levels in the geologic records, but the results are often incomplete and inconsistent. Here, I assess the uncertainty of a new pCO2 proxy that has great potential to reconstruct continuous pCO2 records across the entire Phanerozoic. This new proxy is based on stable carbon isotope fractionation (Δ13C) of C3 land plants because growth chamber experiments and field observations suggest pCO2 goes up as Δ13C increases for pCO2 ranges from 198 to 4200 ppm. Although this proxy has been applied widely in the Cenozoic, recent studies raise concerns that the Δ13C of C3 land plants can be affected by mean annual precipitation (MAP), plant species and mean annual temperature (MAT) in addition to pCO2. Modern C3 land plants reveal a positive correlation between Δ13C and MAP, as well as MAT. The effect of both MAP and CO2 on Δ13C, however, is unknown, making it difficult to interpret the carbon isotope signals in the sedimentary records. The main objective of this work is to assess the extent to which the uncertainty of pCO2 reconstruction in the geological records can be reduced given independent knowledge of MAP and MAT. I hypothesize that if MAP is known at any given time in the geologic past, then pCO2 can be estimated with reduced uncertainty. This hypothesis is tested by accounting for changes in MAP in the Quaternary using multi-regression relationship obtained from large modern dataset. Least square multi-regression suggests +0.2‰ changes in Δ13C per 100 mm yr-1 changes in MAP while holding pCO2 constant, and -1.8‰ changes in Δ13C per 100 ppm changes in pCO2 while holding MAP constant [Δ13C = 25.1 + (0.002) * MAP - (0.02) * pCO2 (r2 = 0.40, p < 0.0001)]. This study provides potential for accounting for changes in MAP usage of this regression equation to offer more precise pCO2 estimates in the geological records. The reduction in pCO2 uncertainty using this unique proxy can help better understand how climate will change in the future due to anthropogenic CO2 emissions.

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