Suplemento Gayana 70: 79-82, 2006

Vertical and horizontal extension of the oxygen minimum zone in the easterns south Pactific Ocean

Extensión vertical y horizontal de la zona de mínima oxígeno en el Pacífico Sur Oriental

Wolfgang Schneider^{1, 2}, Rosalino Fuenzalida^{2, 3, 4}, José Garcés-Vargas^{2, 3}, Luis Bravo², & Carina Lange^{1, 2}

1. Department of Oceanography, University of Concepcion, P.O. Box 160-C, Concepción, Chile, wschneid@udec.cl
2. Center for Oceanographic Research in the Eastern South Pacific (FONDAP-COPAS), University of Concepcion, Concepcion, Chile
3. Graduate Program of Oceanography, Department of Oceanography, University of Concepcion, Concepcion, Chile
4. Department of Marine Sciences, Arturo Prat University, Iquique, Chile.

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ABSTRACT

Recent high-resolution hydrographic measurements (1976-2001) from the eastern South Pacific (ESP) were employed combined with high-resolution data from the World Ocean Circulation Experiment to evaluate the vertical and horizontal extension of the OMZ (oxygen minimum zone, < 20 µmol kg^-1) there. Of the six permanent hypoxic regions in the world oceans, the ESP OMZ is volumetrically the fourth largest, occupying 2.74 x 10⁶ km³ and accounting for ~11% globally. Examples of variability in the vertical position of the OMZ in the water-column and also in its intensity offshore central Chile will be addressed.

We conclude that the OMZ in the ESP is partly based on old, low-oxygen waters from intermediate depths of the North Pacific Ocean, which were further depleted of oxygen off Mexico and Peru, advected into the region along the eastern Pacific coast. Coastal upwelling zones along the eastern Pacific combined with general circulation thus might provide a mechanism that allows the renewal of upper Pacific Deep Water, the oxygen poorest and oldest water mass of the world oceans.

Keywords: Oxygen minimum zone, dissolved oxygen, coastal upwelling, eastern South Pacific, Peru, Chile

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RESUMEN

Datos hidrográficos obtenidos recientemente en el Pacífico Sur oriental (PSO, 1976-2001) se combinaron con datos de alta resolución provenientes del Experimento de Circulación de Océano Mundial (WOCE) para evaluar la extensión vertical y horizontal de la zona mínima de oxígeno (ZMO, <20 µmol kg^-1). Las estimaciones de las seis regiones de hipoxia permanente en los océanos mundiales muestran que la ZMO en el PSO, ocupa 2.74 x 10⁶ km³ y un ~11% del volumen global representando la cuarta más grande. Esta investigación muestra que la ZMO en el PSO se basa parcialmente de aguas viejas de bajo contenido de oxígeno que provienen de profundidades intermedias del Pacífico del norte, que agotan su oxígeno fuera de las costas de México y Perú y son advectadas dentro de la región a lo largo de la costa del Pacífico oriental. La combinación de las zonas de surgencia costera a lo largo del Pacífico oriental y la circulación general podrían proporcionar un mecanismo que permita la renovación de la parte superior del agua Profunda del Pacifico, las masas de agua más pobres y más vieja de oxígeno disuelto de los océanos mundiales. Se abordarán ejemplos de variabilidad en la vertical de la ZOM en la columna de agua y además su intensidad costa afuera de Chile central.

Palabras Claves: Zona mínima de oxígeno, oxígeno disuelto, surgencia costera, Pacífico Sur oriental, Perú, Chile.

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The eastern South Pacific Ocean is home to upwelling ecosystems off the coasts of Peru and Chile that are recognized as the most productive systems of the world oceans. The entire region is characterized by high primary production, which is decomposed in the water column, consuming dissolved oxygen. The high oxygen demand in this region combines with sluggish ventilation to produce the sub-surface/mid-water eastern South Pacific (ESP) oxygen minimum zone (OMZ) (Wyrtki 1962, Helly & Levin 2004).

The OMZ is thick (ca. 500 m), very intense (< 20 µmol kg^-1), and attains its shallowest upper boundary depth (25-50 m) off the coasts of Peru and northern Chile (e.g. Morales et al. 1996, Strub et al. 1998). Its intensity diminishes and its upper boundary deepens further south (e.g. ~100 m off Concepción at ~37°S) (Atkinson et al. 2002).

The OMZ off Chile is associated with Equatorial Subsurface Water (ESSW), which is transported from north to south along the continental slope by the Peru-Chile Undercurrent (PUC) as far south as 48°S (Silva & Neshyba 1979). ESSW is bound by low-salinity, high-oxygen waters: Eastern South Pacific Intermediate Water (ESPIW; Emery & Meincke 1986) or Shallow Salinity Minimum Water (SSMW; Reid 1973) at the top and Antarctic Intermediate Water (AAIW; Schneider et al. 2003) at the bottom. Both upper and lower layers are potential candidates for ventilating the OMZ.

In a recent review of the OMZs in the eastern Pacific, southeastern Atlantic, Arabian Sea, and Indian Ocean, Helly & Levin (2004) quantified the area of the continental margin's seafloor covered by permanent marine hypoxia. Their calculation revealed that the OMZ has a total seafloor coverage of 1.148.000 km² (using dissolved oxygen of <0.5 ml L^-1); roughly 10% of this is in the ESP. The authors point out the impact that changes in the OMZ may have on the distribution of marine resources. The position, strength, and thickness of the OMZ in the water column can be greatly modified by El Niño/La Niña events, leading to large shifts in community structure and trophic transfers as well as changes in productivity and biogeochemical cycling (e.g., Morales et al. 1999, Ulloa et al. 2001, Escribano et al. 2004).

Despite the importance of the eastern South Pacific OMZ, its total horizontal and vertical extensions have not been previously estimated. Here we present a complete volumetric analysis of the size of the OMZ (dissolved oxygen concentration <20 µmol kg^-1, Kamykowski & Zentara 1990) in the ESP based on high resolution CTDO profiles of the last 25 years. Also, the volume of the world oceans occupied by OMZs is calculated based on the World Ocean Atlas 2001 data base (Conkright et al. 2002) and compared to the former high resolution estimate. Furthermore, the maintenance of the OMZ in the ESP is addressed.

Recent high-resolution hydrographic measurements (1976-2001) within the eastern South Pacific were employed to evaluate the vertical and horizontal extension of the OMZ (oxygen minimum zone, <20 µmol kg^-1). These new calculations estimate the total OMZ area and volume to be 12.42 x 10⁶ km² and 2.74 x 10⁶ km³, respectively. The OMZ is thickest (>600 m) offshore Peru between 5-13ºS, where it extends about 1.000 km offshore, and its upper boundary is shallowest off Peru (<150 m), shoaling towards the coast and extending well into the euphotic zone in some places (Fig. 1). Offshore, the thickness and meridional extent of the OMZ decrease until it finally vanishes at 140°W between 2°-8°S.

Figure 1. Thickness and boundary of the eastern South Pacific OMZ. Thickness is color-coded according to the color bar on the right-hand side of the figure; units are in meters. The upper boundary is shown in black contour lines with 50 m intervals.

The southern extension of the OMZ along the coast of South America gradually diminishes from 3.000 km at 15°S, to 1.200 km at 20°S, and to 25 km at 30°S; only a thin band is detected at ~37°S off Concepción, Chile. Simultaneously, the OMZ's maximal thickness decreases from 300 m at 20°S to less than 50 m south of 27°S. Off Chile, the eastern South Pacific Intermediate Water mass introduces increased vertical stability into the upper water column, complicating ventilation of the OMZ from above. In addition, oxygen-enriched Antarctic Intermediate Water clashes with the OMZ at around 30°S, causing a pronounced sub-surface oxygen front. The new estimates of vertical and horizontal OMZ distribution in the eastern South Pacific complement the global quantification of naturally hypoxic continental margins by Helly & Levin (2004) and provide new baseline data useful for studies on the role of oxygen in the degradation of organic matter in the water column and the related implications for biogeochemical cycles. Variability of the OMZ in the ESP as a whole is hard to estimate owing to a lack of data, nevertheless, variability in the vertical position of the OMZ in the water-column and also in its intensity offshore central Chile owing to wind intensity and direction, and, most likely, coastal trapped waves could be detected.

Of the six permanent hypoxic regions in the world oceans, the eastern South Pacific OMZ is volumetrically the fourth largest (Fig. 2a), occupying 2.74 x 10⁶ km³ and accounting for ~11% globally. The OMZ in the ESP partly is fed by old, low-oxygen waters advected into the region along the east Pacific coast that originated from intermediate depths of the North Pacific Ocean, and were further depleted in the intense OMZ off Mexico (Fig. 2b). Upward-directed Ekman pumping velocities, coastal upwelling off Peru, and upwelling along the eastern equatorial divergence cause a nutrient-rich ocean surface environment leading to high primary production. In-situ oxygen consumption due to the decomposition of organic matter and respiration is balanced in part by mixing with oxygen-rich surface waters and weak horizontal ventilation. Coastal upwelling zones along the eastern Pacific combined with general circulation might provide a mechanism that allows renewal of upper Pacific Deep Water, the oxygen poorest and oldest water mass of the world oceans.

Figure 2. a) OMZ thickness and b) upper boundary as defined for dissolved oxygen concentrations ?20 ìmol kg^-1 based on WOA 2001. Units for both parameters are in meters. Thickness and upper boundary are color-coded according to the right-hand color bars.

ACKNOWLEDGEMENTS

This presentation is supported by the Department of Oceanography and the Center for Oceanographic Research in the Eastern South Pacific (FONDAP COPAS), University of Concepcion, Chile, and by the University Arturo Prat, Department of Marine Sciences, Iquique, Chile.

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