GLODAP Radiocarbon

Introduction

Radiocarbon was one of the core WOCE measurements and therefor collected on most of the one-time survey lines. These new dataincreased the total number of high quality oceanic radiocarbon measurements by approximately an order of magnitude over whatexisted previously. The WOCE definition of high quality for radiocarbon data was a measurement precision of 4 o/oo based on countingerrors. Radiocarbon was included as a WOCE core measurement because it was the only tracer measured which provided time constraintson deep ocean processes. Additionally, the component to radiocarbon which resulted from atmospheric weapons testing is a usefultracer for investigating thermocline mixing and ventilation processes. Because radiocarbon enters the ocean from the atmosphereas 14CO2 bomb-produced radiocarbon is a useful proxy for studies of the anthropogenic CO2 flux into the ocean.

Methods

When WOCE fieldwork began in the early 1990s the accelerator mass spectrometry (AMS) method for analyzing small volume samples (~250 mL) was not fully operational. Therefore, in the Pacific Ocean deep waters (depth > 1500m) were generally sampled using large volume samples (~250 L). Large volume water samples were collected using the same Gerard barrels and counted using the same laboratories, equipment and methods as had been used on previous expeditions such as GEOSECS, TTO and SAVE. Details of the large volume procedure have been described in detail previously and are not repeated here (Stuiver et al. 1974; Stuiver and Polach 1977; Key et al. 1994; Key 1996; Stuiver et al. 1996). The AMS procedures are also well described in the literature (McNichol et al. 1994, 2000; Osborne et al. 1994; Schneider et al. 1994; Key et al. 1996; von Reden et al. 1999). A careful error analysis of replicate AMS results from the Pacific WOCE program was done by Elder et al. (1998). They concluded that the accuracy of the WOCE AMS measurements was approximately 4 o/oo. A similar comparison for large volume samples was precluded by the extreme expense of replicate sampling. Based on numerous tests such as comparisons from deep water samples at the same location and different times, AMS and large volume samples from the same depth at the same location and time and larger scope consistency checks leads us to believe that the accuracy of the large volume results is approximately 4 o/oo. It is important to note that the uncertainty reported for radiocarbon in the WOCE and GLODAP compilations is the laboratory based error only (primarily counting error). With a few exceptions (results from German cruises on Meteor and Polar Stern) AMS and large volume results can be differentiated by the bottle number: bottle numbers greater than 80 indicate large volume Gerard barrel samples and traditional β counting methods while smaller bottle numbers indicate AMS sampling and measurement methods.

Significant detail about the radiocarbon data can be found in the final cruise reports. In the Pacific individual reports were prepared for each expedition and these are available at the WOCE Web site. Since the Indian Ocean was carried out as a single continuous expedition only one report was prepared. The final report for the Indian Ocean radiocarbon results is available from the GLODAP Publications.

All of the radiocarbon data were assigned quality control flags based on WOCE guidelines. Because historical data seldom existed, that sort of comparison was limited. For most data the only sort of comparison possible was a consistency check within each station and within neighboring stations from the same cruise. Very limited larger scale comparisons with other methods have been tested (see for example Keller et al. 2002 and Gruber et al. 2000). To date no significant systematic differences have been found between the WOCE data and historical data collected since the 1970s. Therefore, none of the radiocarbon results included in the GLODAP collection have been adjusted in any manner similar to that necessary for some of the GLODAP inorganic carbon measurements.

Separation Technique

In addition to the measured radiocarbon values, the GLODAP compilation also tabulates estimates of the bomb-produced and natural (pre-bomb) components. Wherever potential alkalinity values exist the separation into components was done using the method of Rubin and Key (2002). When potential alkalinity data was not available the components were estimated using the silicate method described by Broecker et al. (1995) with the additional calibration step and latitude constraint used by Rubin and Key (2002).

Maps and Inventories

The GLODAP radiocarbon maps and inventories were prepared in the same manner as other parameters with the following exceptions: For measured radiocarbon and bomb radiocarbon the entire globe was mapped at once rather than by basin; In the Atlantic Ocean data from the 1980s was used rather than WOCE era due to the fact that WOCE Atlantic radiocarbon sampling was extremely limited. Significantly more detail on the mapping procedure can be found in Key et al. (2004).

References

  • Broecker, W.S., S. Sutherland, W. Smethie, T.-H. Peng and G. Ostlund. 1995. Oceanic radiocarbon: Separation of the natural add bomb components. Global Biogeochemical Cycles 9(2):263-288.
  • Elder, K.L., A.P. McNichol and A.R. Gagnon. 1998. Reproducibility of seawater, inorganic and organic carbon 14C results at NOSAMS. Radiocarbon 40(1):223-230.
  • Gruber, N., K. Keller, and R.M. Key. 2000. What story is told by oceanic tracer concentrations? Science 290(5491), 455-456, 2000.
  • Keller, K., R.D. Slater, M. Bender and R.M. Key. 2002. Decadal scale trends in North Pacific nutrient and oxygen concentrations: Biological or physical explanation, Deep-Sea Research II 49(1-3):345-362.
  • Key, R.M., D. Muus and J. Wells. 1994. Zen and the art of Gerard barrel maintenance, in Joyce, T., and C. Corry (Eds). 1994 WOCE Operations Manual, Volume 3: The Observational Programme, Section 3.1: WOCE Hydrographic Programme, Part 3.1.3: WHP Operations and Methods. WHP Office Report WHPO 91-1, WOCE Report No. 68/91, Revision 1, Woods Hole, Mass., USA.
  • Key, R.M. 1996. WOCE Pacific Ocean radiocarbon program. Radiocarbon 38(3):415-423.
  • Key, R.M., P.D. Quay, G.A. Jones, A.P. McNichol, K.F. von Reden and R.J. Schneider. 1996. WOCE AMS Radiocarbon I: Pacific Ocean results; P6, P16 & P17. Radiocarbon. 38(3):425-518.
  • Key, R.M., A. Kozyr, C.L. Sabine, K. Lee, R. Wanninkhof, J. Bullister, R.A. Feely, F. Millero, C. Mordy, T.-H. Peng. 2004. A global ocean carbon climatology: Results from GLODAP. Global Biogeochemical Cycles, Vol. 18, GB4031.
  • McNichol, A.P., G.A. Jones, D.L. Hutton, A.R. Gagnon, and R.M. Key. 1994. Rapid analysis of seawater samples at the National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole, MA. Radiocarbon, 36(2):237-246.
  • McNichol A.P., R.J. Schneider, K.F. von Reden, A.R. Gagnon, K.L. Elder, NOSAMS, R.M. Key and P.D. Quay. 2000. Ten Years After-The WOCE AMS Radiocarbon Program. Nucl. Instr. And Meth. In Phys. B. 172:479-484.
  • Osborne, E.A. A.P. McNichol, A.R. Gagnon, D.L. Hutton and G.A. Jones, Internal and external checks in the NOSAMS sample preparation laboratory for target quality and homogeneity. Nucl. Instr. and Methods in Phys. Res., B92, 158-161, 1994.
  • Rubin, S., and Key, R.M. 2002. Separating natural and bomb-produced radiocarbon in the ocean: The potential alkalinity method. Global Biogeochem. Cycles 16(4):10 1029.
  • Schneider, R.J. A.P. McNichol, M.J. Nadeau and K.F. von Reden. 1995. Measurements of the oxalic acid I/oxalic acid II ratio as a quality control parameter at NOSAMS, In Proceedings of the 15th International 14C Conference. Radiocarbon 37(2):693-696.
  • Stuiver, M., S.W. Robinson, H.G. Ostlund and H.G. Dorsen. 1974. Carbon-14 calibration between the University of Washington and the University of Miami GEOSECS laboratories. Earth Planet. Sci. Lett. 23:65-68.
  • Stuiver, M. and H.A. Polach. 1977. Discussion: Reporting of 14C data. Radiocarbon 19(3):355-363.
  • Stuiver, M., G. Ostlund, R.M. Key and P.J. Reimer. 1996. Large volume WOCE radiocarbon sampling in the Pacific Ocean. Radiocarbon 38(3):519-561.
  • Von Reden, K.F., J.C. Peden, R.J. Schneider, M. Bellino, J. Donoghue, K.L. Elder, A.R. Gagnon, P. Long, A.P. McNichol, T. Morin, D. Stuart J. Hayes and R. Key. 1999. High-precision measurements of 14C as a circulation tracer in the Pacific, Indian, and Southern Oceans with accelerator mass spectrometry, American Institute of Physics, Conference Proceedings Series, 8th International Conference on Heavy Ion Accelerator Technology, Ed.: Kenneth Shepard, Woodbury, NY 11797, 410-421.
Last modified: 2021-03-17T18:30:24Z