Timescales of ventilation and consumption of oxygen and fixed nitrogen in the eastern tropical South Pacific oxygen deficient zone from transient tracers

摘要

The anthropogenic trace gases chlorofluorocarbon (CFC)-12 and sulfur hexafluoride (SF6) were measured during 2013 in the eastern tropical South Pacific Ocean (ETSP) offshore Chile and Peru (12 degrees-22 degrees S, 70 degrees-86 degrees W). Since the WOCE P21 line along similar to 17 degrees S in 1993, the CFC-12 penetration depth increased from similar to 550 m to similar to 800 m. In 2013, CFC-12 had penetrated through the bottom of the oxygen deficient zone (ODZ, where oxygen (O-2) < 4.5 mu mol kg(-1)) at all stations, indicating that a portion of waters in this ODZ are ventilated on timescales < 60 years. Isopycnal trends in pSF(6) and pCFC-12 ages versus AOU indicated oxygen utilization rates of 11.2 +/- 4.7 mu mol kg(-1) yr(-1) just above the ODZ (90-130 m) and 1.0 +/- 0.5 mu mol kg(-1) yr(-1) beneath the ODZ (400-700 m). Isopycnal trends in pSF(6) ages and nutrients implied fixed N-loss rates of 0.6 +/- 0.4 mu mol kg(-1) yr(-1) at the top of the ODZ (similar to 120 m). The pSF(6) and pCFC-12 ages were significantly younger than mean ages estimated from one-dimensional transit time distributions, which were difficult to constrain using the SF6 and CFC-12 tracer combination. Despite the fact that tracer concentrations tend to underestimate mean ages, and thus overestimate nutrient regeneration/consumption rates, N-loss rates were undetectable (< 0.5 mu mol kg(-1) yr(-1)) within the ODZ itself (similar to 175-400 m). When integrated over depth, the oxygen and nitrogen consumption rates determined above and below the ODZ implied total organic carbon (C) remineralization rates on the order of 0.6 +/- 0.1 mol C m(-2) yr(-1). These low C-export rates, and the decadal ventilation timescale of this ODZ, support a body of work suggesting that the ODZ may be sustained by inputs of high-tracer, low-oxygen waters from the adjacent Peru-Chile coastal upwelling system rather than by organic matter oxidation occurring locally.