Using tree ring cellulose as a tool to estimate past tritium inputs to the ocean

Abstract

Tritium ( 3H) concentrations in tree rings should reflect ambient precipitation. Thus, to improve knowledge of the 3H input to the oceans, we developed a new technique to measure 3H concentrations in annual tree rings. Measurements of 3H 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 3H contamination from the hydroxyl groups of the cellulose molecule was subsequently removed by exchange with 3H-free NaOH, thus avoiding the dangers of working with large amounts of cellulose nitrate. The validity of the technique was tested by presenting a 3H time series from a cedar tree which grew in Tollymore Forest Park, Northern Ireland, for comparison with 3H data from the Valentia weather station. We find that the 3H in the cellulose clearly reflects the 3H 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 3H reconstructions to be made. This work demonstrates the potential of using 3H in wood cellulose as a proxy for 3H in precipitation and, thus, opens the door to reconstruction of past 3H inputs to the ocean.