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

Abstract

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°-22°S, 70°-86°W). Since the WOCE P21 line along ~17°S in 1993, the CFC-12 penetration depth increased from ~550 m to ~800 m. In 2013, CFC-12 had penetrated through the bottom of the oxygen deficient zone (ODZ, where oxygen (O2) < 4.5 μmol kg−1) at all stations, indicating that a portion of waters in this ODZ are ventilated on timescales < 60 years. Isopycnal trends in pSF6 and pCFC-12 ages versus AOU indicated oxygen utilization rates of 11.2 ± 4.7 μmol kg−1 yr−1 just above the ODZ (90–130 m) and 1.0 ± 0.5 μmol kg−1 yr−1 beneath the ODZ (400–700 m). Isopycnal trends in pSF6 ages and nutrients implied fixed N-loss rates of 0.6 ± 0.4 μmol kg−1 yr−1 at the top of the ODZ (~120 m). The pSF6 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 μmol kg−1 yr−1) within the ODZ itself (~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.