Determination of Persian Gulf Water transport and oxygen utilisation ratesusing SF6 as a novel transient tracer

Sulphur hexafluoride (SF6) has potential as a transient tracer of recently ventilated water masses, as its atmospheric burden continues to increase. N orthern Arabian Sea hydrography was examined using measurements of atmosphe ric and dissolved SF6, CFC-11, CFC-12 and CFC-113. Persian Gulf Water (PGW) was characterised by its SF6 signal, and the time elapsed since its format ion was evaluated by two approaches. Four ventilation age estimates were de rived from SF6/CFC-11, SF6/CFC-12, CFC-113/CFC-11 and CFC-113/CFC-12, and t heir agreement at the oceanic stations confirms the validity of SF6 as a tr ansient tracer. A second approach, of correcting SF6 partial pressure for P GW dilution by an optimal mixing model and referencing to the atmospheric S F6 chronology, provided a relative tracer age. This indicated a PGW flow of 0.016 (+/- 0.003) mis across the northern Arabian Sea, with an associated oxygen consumption of 10.1 mu mol/l p.a. that exceeds tracer-derived estima tes but confirms rates derived from export flux. The oxygen-deficient zone (ODZ) at 100-1000m in the Arabian Sea is the broa dest hypoxic layer in the ocean (Olson et al., 1993), influencing oceanic n itrogen cycling and atmospheric trace gases (Naqvi, 1987, Law and Owens, 19 90). Seasonal upwelling supports intensive production in this region with s ignificant export and oxygen utilisation in deeper waters, and the relative ly slow input of low-oxygen waters from the Red Sea, Persian Gulf and centr al Indian Ocean further contributes to the persistence of the ODZ (Olson et al., 1993). Persian Gulf Water (PGW) is transported through the Gulf of Om an (GOM) across the northern Arabian Sea as a westerly current, where it is evident as a saline intrusion of elevated oxygen content at 225-325m, and is also present to the east parallel to the Indian coastline (Morrison, 199 7, Plahn et al., 1999). Recent observations suggest that the impact of PGW upon the oxic and nutrient status of the northern Arabian Sea is reduced by rapid mixing and dilution within the GOM (Morrison et nl., 1997; Banse et al., 1997). Constraining estimates of PGW flow rate and oxygen demand is th erefore critical in establishing the factors that maintain the ODZ. Many previous investigations have utilised chlorofluorocarbons (CFCs) as tr ansient tracers to examine surface water penetration into the ocean interio r (Doney and Bullister, 1992; Olson et al., 1993; Haine et al., 1995). CFC- 11 and CFC-12 have been accumulating in the atmosphere since the 1930's, wh ereas CFC-113 has only been produced since the early 1970's. The reduction in CFC production in the wake of the Montreal Protocol has resulted in a lo wer accumulation rate in the atmosphere (Walker et al., 2000), and so the u tility of CFC's for determining the relative "age" of recently-ventilated w ater masses has diminished. Sulphur hexafluoride (SF6) is an inert man-made volatile produced since the early 1970's for its dielectric properties. Th e documentation of an annual atmospheric increase of 5.5-7% (Maiss et al., 1996; Geller et al., 1997) and the establishment of methodologies for repro ducible measurement of background SF6 in the ocean now suggest a viable alt ernative application for SF6 as a transient tracer of ocean dynamics (Law e t al., 1994). This potential was first examined within a study aimed at determining the c hronology and velocity of Persian Gulf Water (PGW) in the northern Arabian Sea. Transient tracers were measured at 4 stations in the Gulf of Oman (GOM 1-GOM6) and 7 stations (A1-A7) on a south-easterly transect from the Omani coastline (19 degreesN 59 degreesE) to the southern Arabian Sea (8 degreesN 67 degreesE) during the UK JGOFS ARABESQUE cruises (Fig.1). Dissolved and atmospheric measurements were obtained for SF6 during September 1994 and fo r CFCs during November-December 1994.;Dissolved SF6 was analysed in 650 ml samples with a precision of 0.012 fmol/kg SF6 and reproducibility of 1.2% f or surface waters (Law et al., 1994), with standards cross-calibrated with atmospheric standards (Max-Planck Institute). Precisions of 0.022, 0.02 and 0.006 pmol/kg and reproducibilities of 2.7%, 1.7% and 4.8% were obtained f or CFC-11, CFC-12 and CFC-113 analysis, respectively (Haine et al, 1995), w ith standards dress-calibrated with the SIO-93 scale. Atmospheric ratios we re obtained from recent CFC and SF6 chronologies for the northern hemispher e (Walker et al., 2000; Geller et al., 1997) and surface saturations using appropriate solubility derivations (Warner and Weiss, 1985; Bu and Warner, 1995; Wanninkhof, 1992).