The primary scientific objectives of Leg 202 are to assess climate and oceanographic changes and to investigate the role of such changes in biogeochemical systems in the southeast Pacific. The drilling experiment contained three major elements that probe the Earth's system on three different but compatible scales: tectonic (millions of years), orbital (tens to hundreds of thousands of years), and centennial to millennial (hundreds to thousands of years). The sediment records obtained during Leg 202 will allow the testing of a broad set of hypotheses on these three timescales:
On each of these scales, we obtained sediment records suitable for the study of upper ocean circulation, subsurface water masses, climate on land as reflected in eolian and fluvial inputs to the ocean, the role of biogeochemical fluxes in a changing system, and processes related to chemical diagenesis.
Our operational goals were to maximize the depth and latitude range of sites to sample different parts of the water column and upper ocean currents from southern Chile to Central America (Fig. F1; Tables T1, T2). We targeted sites with a range of time spans and sedimentation rates appropriate to the scale of questions to be addressed (Fig. F2). Some sites (1236, 1237, and 1241) targeted low sedimentation rates of <30 m/m.y. to obtain long sequences of climate change in the Neogene and, in some cases, the late Paleogene that are not subject to severe burial diagenesis. Other sites (1238 and 1239) targeted moderate sedimentation rates of 3080 m/m.y. to assess orbital-scale climate oscillations at a resolution suitable for the tuning of timescales and examination of changing responses to orbital forcing during the late Neogene. Sediments that accumulated rapidly, at rates of 802000 m/m.y., were recovered from high southern latitudes (Sites 12321235) and near the equator (Sites 1240 and 1242) to assess equator-to-pole climate linkages at both millennial and orbital scales. At all Leg 202 sites we recovered and verified continuous sedimentary sections as long as possible by drilling multiple advanced hydraulic piston corer (APC) holes and assembling composite sections in real time as drilling progressed.
Here, we summarize the shipboard results of Leg 202 and highlight the progress made at sea in each of the three experimental time frames and within a broad range of processes related to changing climate, ocean circulation, and biogeochemical systems.
Near-surface waters of the eastern Pacific exhibit enormous spatial variability, reflecting the influence of the Humboldt Current, the largest and most continuous eastern boundary current in the global oceans (Fig. F3). Off southern Chile, cool waters of the Antarctic Circumpolar Current impinge on the continent and form a transition zone between the southward-flowing Cape Horn Current and the northward-flowing Humboldt Current. Here, the westerly winds bring heavy rainfall to the coastal mountains and the Andes, resulting in high sediment fluxes to the ocean (Lamy et al., 1998, 2001). Cool and relatively low-salinity waters of the Humboldt Current are advected northward from Chile to offshore reaches of Peru (Strub et al., 1998) (Fig. F4A). Coastal upwelling driven by southerly winds along the coast off of central Chile and Peru helps to maintain this cool flow and brings nutrients to the sea surface (Fig. F4B) to feed productive ecosystems (Toggweiler et al., 1991). These eastern boundary waters merge to feed the westward-flowing South Equatorial Current, which is in turn maintained by equatorial upwelling as the equatorial cold tongue (Fig. F4A) (Wyrtki, 1981; Bryden and Brady, 1985). On scales ranging from decades to tens of thousands of years, changes in the rate of wind-driven advection of cool water off of the eastern boundary contribute to major variations in the cold tongue, with recognizable effects extending to the western Pacific (Liu and Huang, 2000; Pisias and Mix, 1997; Feldberg and Mix, 2002).
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 (~32 psu) (Fig. F4C), and a strong, shallow pycnocline (typically centered near 2040 m depth). 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. Some of the net freshwater flux to the Panama Basin originates in the Atlantic or Caribbean (Jousaumme et al., 1986), so low salinities here are also associated with the transport of freshwater from the Atlantic to Pacific Basins via the atmosphere. The dynamics of this transport are important because this relatively small transport of freshwater 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).
Modern subsurface circulation of the southeast Pacific is illustrated in profiles of dissolved oxygen, phosphate, and salinity in a meridional transect of the eastern Pacific (Fig. F5). Bottom water presently enters eastern basins of the South Pacific from the south, below 3 km depth (Lonsdale, 1976; Tsuchiya and Talley, 1998). 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 have been known for nearly 30 yr (Reid, 1973), but how this outflow changes through time remains a mystery. Debate centers on the role of Southern Hemisphere winds in maintaining 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 depth 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 the budget of nutrients in the Pacific Ocean and the 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 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, near the equator. These intermediate water masses are found in the eastern Pacific, typically at depths of ~5001000 m.
At a few hundred meters water depth, the classical oxygen minimum zone (OMZ) is driven by the 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). The broad depth range of this OMZ reflects the presence of NPIW, which is depleted of 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 North Pacific (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.
Near the equator, the OMZ is shallower (300400 m depth) and oxygen values return to typical deep Pacific values of 1 ml/l by ~700 m depth (Fig. F5). Farther south, off central Chile, a "double" OMZ reflects southward advection of oxygen-depleted waters from the Peru margin at ~200500 m depth in the poleward-flowing Gunther Undercurrent (GU) above the relatively oxygen-rich AAIW near ~5001000 m. Pacific Central Water (PCW) of Northern Hemisphere origin comprises the deeper oxygen minimum from ~15002000 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.
The large pools of oxygen-poor water at intermediate depths in the modern eastern Pacific (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 fertility and thus the rate of CO2 fixation within the sea. Indeed, temporal decreases and increases in export production off of northwestern Mexico, off of 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.
The equatorial region is highly productive but is significant in the world's oceans for not consuming all nutrients at the sea surface (Fig. F4B). 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 (Mix, 1989; Jasper et al., 1994; Farrell et al., 1995a).
Most of the sites drilled during Leg 202 examine sediments overlying oceanic crust formed either at oceanic spreading centers (Sites 1232 and 1240) or on bathymetric ridges formed by hotspot volcanism (Sites 12361239, 1241, and 1242). Three sites are located on the continental margin of Chile (Sites 12331235). The sites on the margin may have been influenced by local tectonic effects of subduction at the Peru-Chile Trench, but the time spans covered here (a few hundred thousand years at most) are short enough that tectonic movements at these margin sites can be ignored.
The sites off the margin cover longer time spans, and all are subject to plate tectonic movements. These sites can be "backtracked" to estimate their paleogeographic position relative to South America (Fig. F6), using poles of rotation for the Nazca and Cocos plates (Pisias et al., 1995). The sites may also be backtracked in water depth, assuming they have subsided in response to long-term cooling of the underlying crust (Parsons and Sclater, 1977). Such estimates work reasonably well for normal oceanic crust but are more difficult to apply to oceanic ridges and plateaus that may have experienced different thermal, tectonic, and volcanic histories than the regional oceanic spreading centers (Detrick and Crough, 1978).
Age constraints on the volcanic crust are based mostly on seafloor magnetic lineations (Hey et al., 1977; Lonsdale and Klitgord, 1978; Herron et al., 1981; Cande and Leslie, 1986), along with basal sediment ages developed by shipboard biostratigraphy. For quantitative age 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.
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. Movement of South America in the absolute framework is very small compared to the oceanic plate motions, so these backtrack paths approximate real geography. 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 (Sites 12411242) 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.; 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 (Sites 1232 and 12361240) 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.
Major tectonic events undoubtedly influenced the environments of the southeast Pacific. Evaluating these effects is a major element of the Leg 202 experiment. Drake Passage likely began to open about 29 Ma, and a deepwater connection between ocean basins was present by ~2324 Ma (Barker and Burrell, 1977). Numerical models predict a decrease of the Chile-Peru surface current and a distinct decrease in Antarctic deep and bottom water export to the north as water masses are entrained into a growing circumpolar circulation (Mikolajewicz et al., 1993).
Uplift of the Andes would have caused significant changes in atmospheric circulation and wind-driven oceanic surface circulation. Because it is difficult to separate tectonic and climatic influences on sedimentation, regional elements of this uplift history are controversial. Nevertheless, sedimentation in the tropical Atlantic, as well as hiatuses and paleobotanical evidence on land, suggest major Andean uplift events in the last 10 Ma (Curry, et al., 1995; Gregory-Wodzicki, 2000; Harris and Mix, 2002).
Neogene tectonic closure of the Central American Isthmus from 13.02.7 Ma resulted from the subduction of the Pacific Cocos and Nazca plates, and hotspot volcanics of the Cocos and Carnegie Ridges beneath the North and South American plates and later the Caribbean plate (Duque-Caro, 1990; Dengo, 1985; Collins et al., 1996). The final closure has always been an attractive candidate for the ultimate cause of the Pliocene intensification of Northern Hemisphere glaciation since ~3.1 Ma, but the details of the isthmus formation and the climatic mechanisms that govern the response to this remain uncertain.
Next Section | Table of Contents