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Oxygen Minimum Zones in the Eastern Tropical Atlantic and Pacific Oceans

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Oxygen Minimum Zones in the Eastern Tropical Atlantic and Pacific Oceans Progress in Oceanography 77 (2008) 331–350 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans Johannes Karstensen *, Lothar Stramma, Martin Visbeck Leibniz-Institut für Meereswissenschaften an der Universität Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany article info abstract Article history: Within the eastern tropical oceans of the Atlantic and Pacific basin vast oxygen minimum zones (OMZ) Received 16 November 2005 exist in the depth range between 100 and 900 m. Minimum oxygen values are reached at 300–500 m Accepted 5 May 2007 1 depth which in the eastern Pacific become suboxic (dissolved oxygen content <4.5 lmol kgÀ ) with dis- Available online 4 April 2008 1 solved oxygen concentration of less than 1 lmol kgÀ . The OMZ of the eastern Atlantic is not suboxic and 1 has relatively high oxygen minimum values of about 17 lmol kgÀ in the South Atlantic and more than Keywords: 1 40 lmol kgÀ in the North Atlantic. About 20 (40%) of the North Pacific volume is occupied by an OMZ Oxygen minimum zones 1 1 when using 45 lmol kgÀ (or 90 lmol kgÀ , respectively) as an upper bound for OMZ oxygen concentra- Water mass spreading 3 tion for ocean densities lighter than < 27.2 kg mÀ . The relative volumes reduce to less than half for the Eastern tropical Atlantic rh Eastern tropical Pacific South Pacific (7% and 13%, respectively). The abundance of OMZs are considerably smaller (1% and 7%) for the South Atlantic and only 0% and 5% for the North Atlantic. Thermal domes characterized by upward Suboxic Water age displacements of isotherms located in the northeastern Pacific and Atlantic and in the southeastern Atlantic are co-located with the centres of the OMZs. They seem not to be directly involved in the gen- eration of the OMZs. OMZs are a consequence of a combination of weak ocean ventilation, which supplies oxygen, and respi- ration, which consumes oxygen. Oxygen consumption can be approximated by the apparent oxygen utili- zation (AOU). However, AOU scaled with an appropriate consumption rate (aOUR) gives a time, the oxygen age. Here we derive oxygen ages using climatological AOU data and an empirical estimate of aOUR. Averaging oxygen ages for main thermocline isopycnals of the Atlantic and Pacific Ocean exhibit an exponential increase with density without an obvious signature of the OMZs. Oxygen supply originates from a surface outcrop area and can also be approximated by the turn-over time, the ratio of ocean vol- ume to ventilating flux. The turn-over time corresponds well to the average oxygen ages for the well ven- tilated waters. However, in the density ranges of the suboxic OMZs the turn-over time substantially increases. This indicates that reduced ventilation in the outcrop is directly related to the existence of sub- oxic OMZs, but they are not obviously related to enhanced consumption indicated by the oxygen ages. The turn-over time suggests that the lower thermocline of the North Atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated South Atlantic. The turn-over time approach itself is independent of details of ocean transport pathways. Instead the geographical loca- tion of the OMZ is to first order determined by: (i) the patterns of upwelling, either through Ekman or equatorial divergence, (ii) the regions of general sluggish horizontal transport at the eastern boundaries, and (iii) to a lesser extent to regions with high productivity as indicated through ocean colour data. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction sources and sinks in the ocean interior which further modify their horizontal and vertical gradients compared to a ‘transport only’ Ventilation of the ocean’s interior originates from the ocean sur- distribution. For some properties, like oxygen, it is relevant to face where air–sea exchanges set water mass conditions and sup- delineate the partitioning between contributions due to ocean ven- ply oxygen to the surface mixed layer. Large-scale gradients of tilation/transport and those due to biogeochemical cycling. surface properties combined with ocean transport and mixing pro- Oxygen is a key parameter for biogeochemical cycles and as cesses generate complex horizontal and vertical structures of the such a major player in the carbon system of the ocean (Bopp interior ocean properties. Some properties, as dissolved oxygen et al., 2002) as well as the marine nitrogen cycle (Bange et al., content (referred to in the following as oxygen), have additional 2005). Changes in marine oxygen levels have also been linked to the global climate system (Keeling and Garcia, 2002; Mataer and * Corresponding author. Tel.: +49 431 600 4156; fax: +49 431 600 4102. Hirst, 2003). Volumes of the interior ocean that are relatively poor E-mail address: [email protected] (J. Karstensen). in oxygen are often called oxygen minimum zones (OMZs). OMZs 0079-6611/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2007.05.009 332 J. Karstensen et al. / Progress in Oceanography 77 (2008) 331–350 are presumed to have varied significantly in extend over geological the Pacific Ocean there are two large OMZ regions, one in the North times and possibly altered the ocean carrying capacity of carbon Pacific off Central America and one in the South Pacific off Peru and and nitrogen significantly (e.g. Meissner et al., 2005). Chile both reaching far into the central Pacific. Here the minimum OMZs are located in regions with specific characteristics in oxygen values may even reach the suboxic level with values below 1 1 both, biogeochemical cycling and physical ocean ventilation. From 4.5 lmol kgÀ (0.1 ml lÀ )(Fig. 1b). Although more oxygenated 1 the biogeochemical point of view the OMZs are located in eastern with minimum values on the order of 20–40 lmol kgÀ the spatial boundary upwelling areas with high productivity and rather com- distribution in the Atlantic is similar to that in the Pacific with the plex cycling of nutrients (e.g. Helly and Levin, 2004). In respect to OMZ on the eastern side to the north and south of the equator. As the physics of ocean ventilation OMZs are seen as a consequence of the direct ventilation of North Pacific waters is confined to densi- 3 minimal lateral replenishment of surface waters (Reid, 1965) since ties <26.6 kg mÀ (Suga et al., 2008) the North Pacific has a much they are located in the so called ‘shadow zones’, unventilated by more intense OMZ than all other oceans. In contrast, the subtrop- the basin scale wind-driven circulation (e.g. Luyten et al., 1983). ical southern hemisphere oceans south of 20°S are typically well 1 The possible interplay of the two mechanisms, supply by ocean cir- ventilated with oxygen concentrations above 90 lmol kgÀ , as a culation and demand by ocean biogeochemistry makes under- consequence of intense water mass formation over a wide outcrop standing, modelling and prediction of OMZs location, strength area (Karstensen and Quadfasel, 2002). The minimum oxygen con- and in particular their temporal variability a challenging task. centration in the Indian Ocean in this climatology is located in the Model based scenarios of the ‘‘future ocean” predict an overall dis- northeast part of the Arabian Sea. solved oxygen decline and consequent expansion of OMZs under Strong interannual to decadal time variations of oxygen exist in global warming conditions (e.g. Luyten et al., 1983). Also on sea- the upper 100 m of the world ocean. There, relatively small linear sonal to interannual time scales variability can be high: Helly trends are superimposed on large decadal-scale fluctuations (Gar- and Levin (2004) report 60% reduction in the sea floor area influ- cia et al., 2005). The lowest oxygen content in the late-1950s was 1 enced by the eastern South Pacific OMZ (<20 lmol kgÀ ) off Peru followed by high content in the mid-1980s and by low content in and northern Chile during El Niño years. This can not be translated the late-1990s. In the upper thermocline of the northeast subtrop- into a reduction of the oceans OMZ volume as most of the seafloor ical Pacific between 1980 and 1997 an increase of 20–25% apparent area is on the continental shelf break and an offshore retreat would oxygen utilization (AOU) was described (Emerson et al., 2001). The detach the OMZ from the sea floor thus reducing its sea floor area solubility of oxygen is negatively correlated with temperature extent substantially. (Weiss, 1970) but Garcia et al. (2005) could not find a consistent There is no agreed upon threshold in oxygen that defines an oxygen-to-heat relation that satisfactorily explains the oxygen OMZ. Three terms have been used to describe the overall oxygen content changes for all time periods. conditions: anoxic, suboxic, and oxic. Anoxic waters have virtu- This work, will focus on the time mean situation based on data ally no oxygen and high levels of sulfide are released into the observed mainly during the last 20 years. In the eastern tropical water column. A prominent example is the Black Sea (Murray Atlantic and Pacific thermocline domes (marked by white crosses et al., 2005). When oxygen is detectable but below about in Fig. 1a) are present and cover mainly the Central Water layer. 1 1 4.5 lmol kgÀ (0.1 ml lÀ ) the water is called suboxic (Warren, These domes are permanent, quasi-stationary features on the east- 1995; Morrison et al., 1999). This threshold is rather well defined ern side of thermal ridges extending zonally across the ocean ba- as oxygen derived from nitrate is used for the remineralization of sins.
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