HR: 09:20h
AN: B21E-06    [Abstracts]
TI: Photochemical Production of Dissolved Inorganic Carbon from Oceanic Colored Dissolved Organic Matter: a Gentle Approach to Measuring a new "Wild Card" Carbon Cycle Term
AU: * Zafiriou, O C
EM: ozafiriou@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
AU: Wang, W
EM: wwang@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
AU: Johnson, C G
EM: cjohnson@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
AB: BACKGROUND: Massive oceanic photochemical remineralization (termed "photo-CO2") has been reported[1-3]: CDOM + hv -----$>$ CO2 (DIC) CDOM = Colored Dissolved Organic Matter. DIC = Dissolved Inorganic Carbon. The oceanic carbon cycle cannot be understood without quantifying photo-CO2 fluxes and their sensitivity to environmental variables. The optical model of Johannessen implies a global marine photo-CO2 of $\sim$1015 mol C or 12 Gt C a-1[4]; Kieber and Mopper find photo-CO2 formation rates in the NW Sargasso Sea of $\sim$20 nmol kg-1 hr-1, extrapolating to $\sim$1.3 Gt C a-1[5-7; D. Kieber pers. comm, 2003]. CURRENT METHOD: To achieve essential sensitivity, $<$1 micromole CO2 per day, prior workers remove 99.9+%\ of the DIC (Pool Depletion method - PD). PD users acidify, strip CO2 out by bubbling, readjust pH, irradiate, and analyze. PD's chemically rough sample-handling might give rise to impossible-to-evaluate artifacts. NEW APPROACH: We designed and are implementing a gentle Pool Isotope Exchange (PIE) method, that retains the seawater carbonate system and avoids bubbling. At pH$\sim$8, we exchange[8] the natural DI12C pool (98.9% 12C) with $\sim$400 ppm 13CO2 ($<$1.5% 13\2C) to minimize the DI12C pool that dilutes new-formed photo-12CO2 (from DOM carbon, $\sim$98.9%12C). Rates of DI12C formation in incubations are then measured by isotope ratio mass spectrometry (IRMS). The PIE procedure's steps are: Sample and sterile filter seawater; Exchange DIC to near-completion; Seal incubation aliquots in quartz tubes; Irradiate aliquots with dark controls; Convert aliquots DIC to CO2; Trap and purify; Measure 13/12C ratios. Calculate fluxes from isotope ratios, their rates of change, and [DIC]. PIE STATUS: all but the first and last Steps are novel and have required extensive development. Present progress, sensitivity, and prospects for improvement will be summarized. PIE currently gives detectable, moderately reproducible signals in non-estuarine coastal (East-Coast US) seawater. Many coastal water samples will likely be quantitatively assessable by PIE. BLUE-WATER PROSPECTS: assessing the global photo-CO2 flux requires open ocean data, since CDOM there likely differs chemically from coastal CDOM[9]. However, currently no method is sensitive enough to measure it. An estimate of the requirements for future blue water photo-CO2 work will be presented, based on Nelson et al.'s blue water model of microbial CDOM production and CDOM loss by photobleaching, constrained with seasonally-cycle data at BATS[9]. REFERENCES: 1. Johannessen, S.C., W.L. Miller and J.J. Cullen (1999). Eos, Trans. Am. Geophys. Un., 80:0S63, Abstract OS 12M-02 (2000 AGU-ASLO Ocean Sciences Meeting. January 24-28, 2000, San Antonio, TX). 2. Johannessen, S., L. Ziolkowski and W. Miller (2000). (Abstracts of Pacifichem 2000, Vol. 1, ENVR-332). 3. Miller, W.L. and S.C. Johannessen (1999). Eos, Trans. Am. Geophys. Un., 80:0S93, Abstract 0521K-09 (2000 AGU-ASLO Ocean Sciences Meeting, January 24-28, 2000. 4. Johannessen, S. C.H. A photochemical sink for dissolved organic carbon in the ocean. Ph.D Thesis, Dalhousie University, CA. 179pp May, 2000. 5. Kieber, D. J., K. Mopper and J.G. Qian (1999). Trans. Am. Geophys. Un. 80: OS 128, Abstract #0522J-01 6. Kieber, D., K. Mopper and J. Qian (2000). Abstracts of Pacifichem 2000, Vol. 1, ENVR-28). 7. Kieber, D.J., K. Mopper, J. Qian and O.C. Zafiriou 8. Zeebe, R.E., D.B. Wolf-Gladrow and H. Jansen (1999). Mar.Chem. 65, 135-153. 9. Nelson, N.B, D. A. Siegel, and A.F. Michaels (1998) Deep-Sea Res. 45, 931-957.
DE: 4805 Biogeochemical cycles (1615)
DE: 4294 Instruments and techniques
DE: 1050 Marine geochemistry (4835, 4850)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting