Evaluating the Accuracy of Biogeochemical Cycling Rates from Transient Tracers

Transient tracers, such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF₆), have been used to quantify oxygen utilization (OUR) and nutrient regeneration (NRR) rates in the ocean interior. Depth integration of these rates have been used to estimate surface ocean carbon export on basin scales. However, there are draw-backs to the transient tracer methods because mixing affects the tracer and oxygen/nutrient fields differently, biasing the rates inferred, causing uncertainty in the overall productivity estimates.

We address the biases and uncertainties in tracer-based OURs and NRRs by testing the tracer-based approaches against model output, where the biogeochemical cycling rates are known. Expanding on previous work, we adopt a multi-model approach based on output from several three-dimensional models of moderate (1°) resolution run with biogeochemistry. Reasonable agreement between the known oxygen consumption term in the models and the inferred OURs exists at mid-depth in the subtropical North Pacific during certain time periods (1990s) which may reflect the alignment of tracer age (inferred by matching transit time distributions to modeled tracer concentrations) and apparent oxygen utilization contours within the along-isopycnal flow here. Over most of the ocean, however, OURs and NRRs can be significantly biased. Our findings with regard to the regions and times for which the transient tracer age methods work best will help put error bars on OURs and NNRs from recent and upcoming GO-SHIP repeat hydrography cruises and the earlier WOCE cruises. We will present our results with focus on the Pacific P2, P16, P18, and P6 WOCE/GO-SHIP sections.