, Using tree ring cellulose as a tool to estimate past tritium inputs to the ocean.
, Earth Planetary Science Letters. 237: 341:353, 2005|
Tritium (H-3) concentrations in tree rings should reflect ambient precipitation. Thus, to improve knowledge of the H-3 input to the oceans, we developed a new technique to measure H-3 concentrations in annual tree rings. Measurements of H-3 were made on cellulose, the primary constituent of wood, as the isotopic signal of its carbon bound hydrogen atoms should be unchanged since biosynthesis. Traditional cellulose extraction techniques from softwoods are slow and were found to not yield reproducibly pure cellulose. Therefore, a new microwave method was developed which reduces extraction times from 3-5 days to approximately 3 h. Potential H-3 contamination from the hydroxyl groups of the cellulose molecule was subsequently removed by exchange with H-3-free NaOH, thus avoiding the dangers of working with large amounts of cellulose nitrate. The validity of the technique was tested by presenting a H-3 time series from a cedar tree which grew in Tollymore Forest Park, Northern Ireland, for comparison with H-3 data from the Valentia weather station. We find that the H-3 in the cellulose clearly reflects the H-3 in precipitation with no significant smearing of the bomb signal. A simple box model illustrates that the maximum reservoir residence time of source water for the tree is less than 1 yr, suggesting that groundwater is not a major source of water for this tree. In general, however, the groundwater input needs to be quantified for accurate H-3 reconstructions to be made. This work demonstrates the potential of using H-3 in wood cellulose as a proxy for H-3 in precipitation and, thus, opens the door to reconstruction of past H-3 inputs to the ocean.
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