Timescales of ventilation and consumption of oxygen and fixed nitrogen in the eastern tropical South Pacific oxygen deficient zone from transient tracers. (September 2019)

• Record Type: Journal Article
• Authors: Sonnerup, Rolf E.; Chang, Bonnie X.; Warner, Mark J.; Mordy, Calvin W.
• Is Part Of: Deep sea research. Volume 151(2019)
• Publication Date: 2019-09
• Subjects: Oceanography -- Periodicals; Océanographie -- Périodiques; 551.4605
• DOI: 10.1016/j.dsr.2019.103080
• Languages: English
• ISSNs: 0967-0637

Abstract

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 p SF6 and p CFC-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 p SF6 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 p SF6 and p CFC-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. Highlights: CFC-12 penetration in the ETSP has increased from ~550 m in 1993 to ~800 m in 2013. Waters in the ETSP O2 deficient zone are ventilated on timescales <60 years. In-situ N-loss was not detectable within the O2 deficient zone.