BACKGROUNDThe paleoceanography of the southeast Pacific Ocean off the coast of South America has received little attention in past drilling experiments and remains essentially unknown to this day. The Deep Sea Drilling Project (DSDP) did not visit the western margin of South America and provided only two sites in the Peru Basin (Sites 320 and 321). The Ocean Drilling Program (ODP) examined 10 sites on the Peru Margin during Leg 112, which provided key evidence for sedimentation and geochemical processes in this productive and nearly anoxic sediment system. ODP Leg 141 examined the tectonics of the Chile Triple Junction and got a first look at sediments at five sites on the margin of southern Chile, but these sites provided relatively little material suitable for paleoceanographic studies. Leg 202 seeks to remedy this situation by examining a broad range of sites off South America, from central Chile to the equator (Fig. 1; Table 1, Table 2). Depth and latitudinal transects of sites will facilitate study of changes in vertical ocean circulation and surface ocean processes as they change through time.
Modern subsurface circulation of the southeast Pacific is illustrated in water mass properties in profiles of dissolved oxygen, phosphate, and salinity in a meridional transect of the eastern Pacific (Fig. 2). Bottom water presently enters eastern basins of the South Pacific from the south, below 3 km water depth (Lonsdale, 1976). After transiting north, accumulating nutrients and losing oxygen in the North Pacific, much of the Pacific Deep Water exits as a middepth southward flow between 1 and 3 km. This middepth outflow and its importance to Pacific (and global) distributions of nutrients has been known for nearly 30 yr (Reid, 1973). But how it changes through time remains a mystery. Recent debate centers on the role of Southern Hemisphere winds in maintaining the global thermohaline circulation (Toggweiler and Samuels, 1993). Much of the advective export of phosphate and nitrate from the Pacific occurs in the southward return flow between 1 and 3 km in the eastern Pacific, where concentrations of these nutrients are highest (Wunsch et al., 1983). Thus, changes in this flow have the potential to change nutrient budgets in the Pacific and global ocean (Berger et al., 1997).
At intermediate water depths, water mass properties of the Pacific Ocean are highly asymmetric. Antarctic Intermediate Water (AAIW) is relatively depleted in phosphate and contains abundant oxygen because it forms in substantial contact with the atmosphere. This combination of processes results in relatively high 13C in AAIW (Kroopnick, 1985). At present, AAIW is for the most part restricted to the Southern Hemisphere. North Pacific Intermediate Water (NPIW), which forms in the northwest Pacific with relatively little interaction with the atmosphere (Talley, 1993) contains abundant nutrients, but the NPIW is relatively low in oxygen and has low 13C. An exceptionally steep property gradient between the southern source and northern source water masses occurs, at present, south of the equator. These intermediate water masses are found in the eastern Pacific, typically at depths of ~500-1000 m.
At the shallow end of the intermediate water (typically a few hundred meters water depth) is the classical oxygen minimum zone (OMZ), driven by degradation of organic matter sinking out of the euphotic zone and modified by ocean circulation (Wyrtki, 1962). North of the equator, the OMZ is extremely intense (<0.2 mL/L O2) between 200 and 900 m. This broad depth range of this OMZ reflects the presence of NPIW, which is depleted in oxygen because it exchanges relatively little with the atmosphere in its northern source areas, as well as the exceptionally high export of organic matter from productive upwelling systems along the eastern boundary of the Pacific Ocean (Tsuchiya and Talley, 1998). A relatively shallow and abrupt pycnocline below low-salinity surface waters helps to maintain the shallow oxygen minimum north of the equator.
The large pools of oxygen-poor water at intermediate depths in the modern eastern Pacific Ocean (Tsuchiya and Talley, 1998) both north and south of the equator are major sites of denitrification and represent collectively the largest sink for nitrogen in the world's oceans. By acting as governors of the average oceanic nitrate concentration, these regions (along with the Arabian Sea) have the potential to act as climate rheostats by altering the global fertility and, thus, the rate of CO2 fixation within the sea. Indeed, temporal decreases and increases in export production off northwestern Mexico, off Peru, and in the Arabian Sea do appear to have modulated the oxygen content in upper intermediate depth waters and the consequent intensity of denitrification during the late Quaternary (Codispoti and Christensen, 1985; Ganeshram et al., 1995). Whether or not such variations occurred on a broad scale over long time periods remains an open question. For example, it is unclear whether biological production of the eastern tropical Pacific was higher (Lyle et al., 1988) or lower (Loubere, 2000) than at present during the last ice age.
Near the equator, the OMZ is shallower (300-400 m depth) and oxygen values return to typical deep Pacific values of 1 mL/L by ~700 m depth (Fig. 2). Farther south, off central Chile, a "double" OMZ reflects southward advection of oxygen-depleted waters from the Peru margin at ~200-500 m depth in the poleward-flowing Gunther Undercurrent (GU), above the relatively oxygen-rich AAIW near ~500-1000 m. Pacific Central Water (PCW; of Northern Hemisphere origin) comprises the deeper oxygen minimum from ~1500 to 2000 m depth. Oxygen is slightly higher (and nutrient contents lower) in the deep basin because of the incursion of Circumpolar Deep Water (CPDW) below ~3000 m.
Near-surface waters of the eastern Pacific Ocean also exhibit exceptional spatial variability, reflecting the influence of the Peru (or Humboldt) Current, the largest and most continuous Eastern Boundary Current in the global oceans (Fig. 3). Cool waters of the Peru Current (Fig. 4) are advected northward from Chile to offshore reaches of Peru (Strub et al., 1998). Coastal upwelling off central Peru maintains this cool flow (Toggweiler et al., 1991), and these waters merge to feed the westward-flowing South Equatorial Current, which is in turn maintained by equatorial upwelling as the Equatorial Cold Tongue. On an interannual scale, Liu and Huang (2000) argue based on modern heat budgets that warming in the region over the past 50 yr is related to reduction of the wind-driven advection of cool water off the eastern boundary and that such effects extend even to the western Pacific Ocean. A primary question for ocean drilling is whether long-term variability of the equatorial cold tongue is related to changing character of eastern boundary waters.
Near the equator, east of the Galapagos Islands, the Equatorial Front separates the cold, salty waters
of the Peru Current from warmer and fresher tropical waters of the Northern Hemisphere. North of
the equator, the Panama Basin region is noted for its extreme warmth (often >30°C), exceptionally
low salinity (near 32 practical salinity units [PSU]), and strong shallow pycnocline (typically
centered near 20-40 m water depths) (Fig. 4, Fig. 5). These features north of the equator reflect high
rainfall relative to evaporation (Magaña et al., 1999), which stabilizes the water column and
diminishes vertical mixing of heat and other properties. A significant fraction of the net fresh water
flux to the Panama Basin originates in the Atlantic Ocean or Caribbean Sea (Jousaumme et al.,
1986), so low salinities here reflect the transport of fresh water from the Atlantic to Pacific Basins
via the atmosphere. The dynamics of this transport are important because this relatively small
transport of fresh water helps to maintain the relatively high salinity of the Atlantic Oceana key
parameter in maintaining the global thermohaline "conveyor belt" circulation dominated by North
Atlantic Deep Water (Zaucker et al., 1994; Rahmstorf, 1995).
The region is highly productive but is significant in the world's oceans for not consuming all nutrients at the sea surface (Fig. 6). High phosphate concentrations in tropical surface waters here (Levitus et al., 1993) are now thought to reflect biological limitations associated with iron or other so-called micronutrients. This is significant because any change in the net nutrient utilization would also cause a change in the net flux of CO2 and other biologically mediated gases from the sea surface.
Many of the proposed sites are located on bathymetric rises with pelagic or hemipelagic sedimentation. Most are relatively isolated from continental margin turbidites and tectonics by the Peru-Chile Trench or other bathymetric features. Age constraints are based on magnetic lineations. For these estimates, we use the magnetic anomaly age model of Cande and Kent (1995), which is in reasonable agreement with orbitally tuned sedimentary age models of the last 5 m.y. and with radiometric dates at older intervals. Maps of the crustal magnetic lineations, with our proposed sites superimposed, are illustrated as Figure 7, Figure 8, Figure 9, Figure 10.
In Figure 11, we show the backtrack paths of the proposed drill sites relative to a fixed South America. Movement of South America in the absolute framework is very small compared to the oceanic plate motions, so these backtrack paths approximate real geography. Absolute poles of rotation for different time intervals are given in Table 3. In Figure 11, the absolute poles for South America were added to the absolute poles for each respective crustal plate to calculate the position of each drill site relative to South America.
Sites on the Nazca and Cocos plates backtrack paths for 0 to 19 Ma are based on the analysis of Pisias et al. (1995). Absolute poles of rotation for the Cocos plate (used for alternate Sites COC 2A, COC-3A, and COC-4A) were calculated using the pole of relative rotation between the Cocos and Pacific plates (36.823°N, 108.629°W, W = 2.09°/m.y. [DeMets et al., 1990]) and absolute poles for the Pacific plate (0- to 5-Ma pole = 61.6°N, 82.5°W, W = 0.97°/m.y. and for 5- to 20-Ma pole = 70.3°N, 74.4°W; W = 0.73°/m.y. [Cox and Engebretson, 1985]).
Absolute poles for the Nazca plate (used for Sites CAR-1C, CAR-2C, PAN-2A, NAZCA-10A,
NAZCA-14A, NAZCA-16A, NAZCA-17A, SEPAC-9A, and SEPAC-10A) were calculated using
relative motions between the Pacific and Nazca plates (pole = 55.58°N, 90.10°W; W = 1.42°/m.y.),
and the absolute Pacific poles noted above. For the interval from 0 to 5 Ma, the resulting absolute
pole for the Nazca plate is essentially identical to the pole given by Gripp and Gordon (1990). For
the interval 19 to 42 Ma, Nazca plate sites were rotated using the absolute poles of Duncan and
Absolute poles of rotation for the Antarctic plate (used for alternate Site SEPAC-5A) are from Cox and Engebretson (1985) and for South America are from Duncan and Richards (1991).
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