Project Report

My research aims to reconstruct a record of radiocarbon (14C) that directly samples atmospheric CO2 through the last glaciation, tied to a robust independent timescale. Atmospheric concentrations of 14C vary over time due to changes in production rate of 14C in the atmosphere as well as the storage and movement of carbon through the ocean, atmosphere, and biosphere. When 14C records are corrected for changes known be caused by changes in the production rate of 14C, atmospheric records of 14C can be used to trace changes in the movement of carbon by comparison with existing 14C dated marine records. Such comparison would lead to an improved understanding of the history of the carbon cycle turnover rates and a more precise knowledge of the relationship between the carbon cycle and climate change. Improving our understanding of the history of the carbon cycle is critical to the development of climate models used to make climate change predictions based on the current anthropogenically driven global climate change. Without a better understanding of these critical paleoclimate changes, and the ability to simulate them in our prediction models, the reliability of these models is highly uncertain. Speleothems have long been used in paleoclimate reconstructions, because they can be dated using 234U-230Th series (U-Th) measurements, can display annual banding, and can grow continuously for tens of thousands of years, which also makes them potential sources of records of atmospheric 14C. . Speleothems form from cave drip water, which has a 14C concentration influenced by the age of the soil carbon and host bedrock. Soil gas is 14C is very close to equilibrium with atmospheric 14C, but bedrock is geologically old carbon, and is 14C free or ‘dead’. The effect of this mixture is the dead carbon fraction (DCF), which is determined by comparing the 14C measurements during a period of overlap with the tree ring record of atmospheric 14C. The offset between the datasets is then subtracted throughout the remainder of the record to result in a DCF corrected atmospheric 14C record. This procedure involves the implicit assumption that the correction has remained constant through time. Comparison with the tree ring record indicates that the DCF correction in my own work on speleothems appears to remain constant across the Younger Dryas/Holocene transition, an abrupt climate change event 12,900 yrs BP. My work in Wuhan, China during the East Asian Pacific Summer Institiute (EAPSI) aimed to determine the environmental factors that control DCF in speleothems. Scientists at China University of Geosciences (CUG) have carried out environmental monitoring at Heshang Cave since 2003, to study the relationship between modern climatic changes and geochemical proxies in speleothems. To be able to properly interpret 14C in speleothems in terms of the paleoclimate, long-term modern monitoring studies such as this study at Heshang Cave is essential. During EAPSI I made two field trips to Heshang Cave to collect soil gas, cave air, drip water, and calcite samples to measure for 14C to study the changes in DCF during the East Asian Summer Monsoon. Additionally, I drilled high-resolution samples from a large stalagmite that had been previously removed from Heshang Cave. The samples of stalagmite will also be measured for 14C to study the variations in DCF over the last glacial cycle, in comparison with the modern environmental conditions. A modern tree sample was also collected for reconstruction of a local atmospheric 14C record, to more closely constrain the causes of changes in DCF by accounting for local variations in atmospheric 14C. Analysis of all of these samples is currently in progress, both by myself at University of California, Irvine and also by Chinese scientists at CUG.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Magana Alexandra L
United States
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