Validity limits of carbon tetrachloride as an ocean tracer

Abstract

We investigate non-conservative behavior of carbon tetrachloride (CCl 4) in the ocean by evaluating concurrent data of this tracer and of the chlorofluorocarbon CFC-12 from three zonal sections in the South Atlantic (∼30°S, 19°S, 11°S; METEOR cruises M22/5, M15/3, M28/1) and from two sections in the western Weddell Sea ( POLARSTERN cruises ANT XIII/4, ANT XV/4). The issue is of interest biogeochemically and for uses of CCl 4 as a transient ocean tracer. For the South Atlantic a simple model is employed that simulates the meridional tracer transfer into the Central Water and Antarctic Intermediate Water from their southerly outcrops. From a joint fit for the three sections we deduce a CCl 4 depletion rate of approximately 22% per year for temperatures exceeding 13°C, which confirms a previous estimate and exceeds rates due to hydrolysis by up to about 50-fold. A tracer utility of CCl 4 in warm ocean waters thus hardly exists. However, below ∼13°C the decomposition rates decrease sharply, and they become negligibly small below about 3°C (rate <0.1% per year, compatible with rates due to hydrolysis). In the Weddell Sea we do not find positive evidence of a CCl 4 destruction at depth (upper limit 1% per year), in keeping with the South Atlantic result. In the western Weddell Sea deep waters we deduce a apparent CCl 4/CFC-12 ratio age of about 30 years. We confirm a previous claim of a CCl 4 deficiency in newly formed Weddell Sea deep and bottom waters, which we deduce to amount to approximately 32% relative to CFC-12. We ascribe this deficiency to CCl 4 loss within the ventilated source waters (possibly due to interaction with sea or shelf ice), combined with a slower gas transfer from the atmosphere into the upper waters (contribution ∼12%). It is argued that CCl 4 deficiencies relative to more stable tracers should be common in newly formed deep and bottom waters, and that assessing such initial deficiencies is a prerequisite for using CCl 4 as a tracer. An open question is a CCl 4 instability at reduced oxygen concentrations, although the critical oxygen level appears to be lower than reported previously (Tanhua et al., Mar. Chem. 54 (1996) 159). Moreover, temperature might only be a proxy for the real agent that governs CCl 4 destruction. The actual mechanism of decomposition remains unknown, but judging from an Arrhenius plot a first-order chemical reaction can be excluded. It is estimated that the ocean contributes roughly 8% to the total environmental destruction of CCl 4.