Debate over the ocean bomb radiocarbon sink: Closing the gap

摘要

There has over recent years emerged a controversy as to how much of the radiocarbon released into the atmosphere by nuclear weapons testing has been taken up by the ocean. Hesshaimer et al. [1994] made a case based on stratospheric and tropospheric measurements coupled with estimates of total bomb radiocarbon yield that it was not possible to explain both atmospheric observations and existing ocean-based bomb radiocarbon uptake estimates [ Broecker et al., 1985, 1995]. They therefore proposed that estimates of the oceanic sink should be revised downward by about 25%. One reason for concern over this discrepancy is that the widely used wind speed dependent air-sea gas exchange parameterization of Wanninkhof [1992] is scaled to give an average exchange rate matching that given by the ocean bomb-radiocarbon budget. An example of an application of the Wanninkhof [1992] parameterization is in estimating ocean CO2 uptake based on direct measurements of the air-sea pCO2 difference [Takahashi et al., 1997]. Such estimates scale linearly with the air-sea gas exchange coefficient. Further, as has been highlighted by the Hesshaimer et al. [1994] study, an understanding of the global budget of radiocarbon is an important issue in and of itself. In this paper, a number of new approaches to assessing the size of the ocean bomb radiocarbon sink are explored, and estimates are given for the total ocean bomb radiocarbon inventory during both the mid-1970s and the mid-1990s. The revised estimates of bomb-radiocarbon ocean uptake yield a mid-1970s inventory in closer agreement with that proposed by Hesshaimer et al. [1994] than the inventory obtained by the Broecker et al. [1995] study.