Dynamics of the Carbonate System Across the Peruvian Oxygen Minimum Zone

Abstract

The oxygen minimum zone (OMZ) of Peru is recognized as a source of CO 2 to the atmosphere due to upwelling that brings water with high concentrations of dissolved inorganic carbon (DIC) to the surface. However, the influence of OMZ dynamics on the carbonate system remains poorly understood given a lack of direct observations. This study examines the influence of a coastal Eastern South Pacific OMZ on carbonate system dynamics based on a multidisciplinary cruise that took place in 2014. During the cruise, onboard DIC and pH measurements were used to estimate pCO 2 and to calculate the calcium carbonate saturation state (aragonite and calcite). South of Chimbote (9 • S), water stratification decreased and both the oxycline and carbocline moved from 150 m depth to 20-50 m below the surface. The aragonite saturation depth was observed to be close to 50 m. However, values <1.2 were detected close to 20 m along with low pH (minimum of 7.5), high pCO 2 (maximum 1,250 µatm), and high DIC concentrations (maximum 2,300 µmol kg −1). These chemical characteristics are shown to be associated with Equatorial Subsurface Water (ESSW). Large spatial variability in surface values was also found. Part of this variability can be attributed to the influence of mesoscale eddies, which can modify the distribution of biogeochemical variables, such as the aragonite saturation horizon, in response to shallower (cyclonic eddies) or deeper (anticyclonic eddies) thermoclines. The analysis of a 21-year (1993-2014) data set of mean sea surface level anomalies (SSHa) derived from altimetry data indicated that a large variance associated with interannual timescales was present near the coast. However, 2014 was characterized by weak Kelvin activity, and physical forcing was more associated with eddy activity. Mesoscale activity modulates the position of the upper boundary of ESSW, which is associated with high DIC and influences the carbocline and aragonite saturation depths. Weighing the relative importance of each individual signal results in a better understanding of the biogeochemical processes present in the area.

A source of isotopically light organic carbon in a low-pH anoxic marine zone

“…Carbonate chemistry in the AMZ is also highly temporal and spatially variable, as reflected by the contrasting levels of DIC concentration and pH between austral spring 2015 and summer 2018. Our results evidence that the ETSP AMZ off the coast of northern Chile embodies one of the shallowest (~100-400 m depth), low pH, high pCO 2 AMZ systems in the world 5 , similar to what has been observed off the Peruvian coast 29 . Indeed, the aragonite saturation depth can be as shallow as 100 m depth, as observed during austral spring in 2015.…”

“…In the same study area, Paulmier et al 12 observed a similar carbon deficit associated with the OMZ, which they related to oxygen loss due to thermal mechanisms during OMZ water formation in the equatorial region, since a decrease in DIC is expected in warm regions due to a lowering of gas solubility. Indeed, when we analyze pCO 2 data collected in the ETSP between 7 and 15°S 29 shows a steady reduction in pCO 2 as the Equatorial Subsurface Waters (ESSW) move southward along the Peruvian coast. Here, we explore the hypothesis that microbial processes involved in sulfur and nitrogen cycling can exert an additional partial control for such DIC deficit.…”

“…Geochemical and stable isotope data from two expeditions to the AMZ off northern Chile with contrasting oceanographic regimes highlight the biogeochemical variability of this environment. The interaction of local processes, such as the strength and spatial variation of upwelling conditions 28 , and its interconnection with regional processes, such as the intensity and displacement of mesoscale eddies 29 , in addition to the advection of tropical waters, determine surface productivity and the variability of the zonal and vertical boundaries of the AMZ core. The latter can in turn influence carbon cycling and export in this regioni.e., the source and fate of different carbon pools through the water column.…”

The OMZ and Its Influence on POC in the Tropical Western Pacific Ocean: Based on the Survey in March 2018

“…At present, the generally accepted view was that the DO threshold was 100 μmol/L (3.20 mg/L) in most sea areas of the global ocean, and there was a significant oxycline between OMZ and the upper or lower waters (Paulmier and Ruiz-Pino, 2009). The Eastern Tropical North Pacific and the Tropical Indian Ocean are typical distribution areas of OMZ, where the intensity of OMZ is relatively high (Figures 5A,C) (Karstensen et al, 2008;Breitburg et al, 2018;Hernandez-Ayon et al, 2019;D'Asaro et al, 2020). However, the intensity of the OMZ in the Western Pacific Ocean and the Tropical Atlantic Ocean is relatively weak (Figures 3A, 5B).…”

Coincident Biogenic Nitrite and pH Maxima Arise in the Upper Anoxic Layer in the Eastern Tropical North Pacific

“…Marine ODZs also exhibit unique inorganic carbon chemistry compared to the oxygenated pelagic ocean, containing, for instance, some of the lowest pH waters in the open ocean and correspondingly shallowest aragonite and calcite saturation horizons (Hernandez‐Ayon et al., 2019; Jiang et al., 2019; Millero, 2007). Due to the long residence time accumulating remineralization products, these waters contain very high dissolved inorganic carbon (DIC) resulting from aerobic respiration and correspondingly low pH.…”