Modeling the Radiocarbon Reservoir Effect in Lacustrine Systems

Abstract

The modern water (both pre- and post-atmospheric nuclear testing) of most lakes has an anomalously old apparent radiocarbon age due to what is commonly referred to as the reservoir effect. In contrast to marine settings, this 14C offset phenomenon is primarily caused by pre-aged carbon discharged to lakes by rivers and/or groundwater. In this paper, a 2-component box model based on the principle of 14C mass balance in lake water and in the early diagenesis zone was formulated to address the relative importance of terrestrial inputs, autochthonous production, and biogeochemical processes in the 14C reservoir of a lacustrine system. The model was tested using observed data from Lake Qinghai, the largest inland water body in China. Our inverse modeling using Markov chain Monte Carlo (MCMC) techniques yields best estimates of the delta-14C of DIC in river (~118% modern) and groundwater (~76% modern), as well as the delta-14C of DOC in river water (~70% modern) during the post-bomb era. Assuming that these parameters remain constant over time, our modeling indicates that both the DIC and DOC pool of this lake have reservoir ages of about 1500 yr for the pre-bomb era, generally consistent with estimates obtained by extrapolation of the age-depth models of 2 sediment cores to the sediment surface.