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CSIRO Marine and Atmospheric Research
Past Seminars

Seminar Abstract

Net and gross production in the equatorial Pacific from continuous
O2/Ar ratio measurements and the triple isotope composition of O2

Simultaneous measurements of O2/Ar ratios in the mixed layer and the triple isotope composition of dissolved O2 allow constraining the basic metabolic rate processes (photosynthesis, respiration and net production). Dissolved O2/Ar ratios are indicative of net production because O2 and Ar share similar solubility properties with respect to physical mechanisms, but only O2 is biologically influenced (Craig, Jenkins, Emerson, Quay and colleagues). Photosynthesis attenuates the oxygen isotope anomaly of atmospheric O2, which originates in the stratosphere, providing an index of the fraction of dissolved O2 derived from photosynthesis in situ.

From measurements of these O2 properties and a suitable wind speed-gas exchange parameterization, one can calculate net and gross production rates of oceanic ecosystems. One can make extensive measurements of the O2 properties in the oceanic mixed layer, yielding experimental values of net and gross production at larger scales and higher resolution than is possible with traditional approaches.

We have achieved a significant advance for the O2/Ar method by building a membrane inlet mass spectrometer that allows continuous shipboard analysis of O2, Ar, N2, and eventually CO2 and other gases. It was successfully deployed for the first time on transects at 110°W and 95°W, 8°S to 8°N, in the Equatorial Pacific in October/November 2003, followed by additional cruises at 125°W, 140°W, 155°W, and 170°W between June and September 2004. The mass spectrometric measurements were accompanied by discrete O2/Ar measurements and isotopic measurements, along with continuous O2 concentration measurements.

The results give gross production values and allow us to compute absolute Ar supersaturations, which may provide insight into the origin of physical supersaturation. On the 2003 cruise, a short-term reproducibility of 0.05 percent was achieved for the O2/Ar ratio, with a sampling frequency greater than twice/minute. Meridiona l gradients across the equatorial upwelling and small-scale local phenomena were resolved. Net production estimated from wind speed-based gas exchange parameterizations was near zero north of the equatorial upwelling, and about 12 mmol/m2/d south of it. Gross production estimated from 170 measurements ranged up to 150 mmol/m2/d. The corresponding equatorial net to gross O2 production ratios were approximately 0 to the north and 0.07 to the south.

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CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
Peter Oke, (Oceanographic seminars) CSIRO Marine Research (03) 6232 5387
Piers Dunstan, (Biological seminars) CSIRO Marine Research (03) 6232 5382
Katrina Nitschke, Antarctic Climate and Ecosystems CRC (03) 6226 2265 & IASOS, University of Tasmania (03) 6226 2509