11. Site 12401

Shipboard Scientific Party2

INTRODUCTION

Site 1240 (proposed Site PAN-2A) is located at 0°1.311´N, 86°27.758´W, about midway between mainland Ecuador and the Galapagos Islands (Fig. F1). North of Carnegie Ridge at 2921 m water depth in the Panama Basin, the site is in a small east-west-trending trough in classical abyssal hill topography typical of seafloor spreading centers. Relief is 200-300 m between hills and valleys (Fig. F2). The basaltic oceanic crust underlying the site was formed at the Cocos spreading center about 3 m.y. ago (Hey et al., 1977). A tectonic backtrack path of the Nazca plate (Pisias et al., 1995) moved the site westward and slightly to the south at greater ages, but given the relatively young age of the site, tectonic movement here was slight.

Site 1240 was chosen in an abyssal valley close to the equator to monitor variations in equatorial upwelling at high resolution. A seismic profile (Fig. F3) documents pelagic drape over the abyssal hills, with a slight thickening of the section within the valleys. Sediment cover of 250-280 m is relatively thick for the region, given the young age of the crust, suggesting that this abyssal valley acts as a sediment trap to collect the heavy biogenic rain below the productive equatorial upwelling system. Dominant sediments in the region are foraminifer-bearing nannofossil-diatom ooze, with some intervals of diatom ooze and occasional ash layers.

Equatorial upwelling is strong over Site 1240 (Fig. F4), driven by southeasterly trade winds, especially in Southern Hemisphere winter. The strongest upwelling occurs just south of the equatorial front that separates cool, relatively high salinity surface waters south of the equator from the warm, low-salinity waters of the Panama Basin (Strub et al., 1998). Nutrient-rich waters of the Equatorial Undercurrent (EUC) serve as source waters for the upwelling (Levitus et al., 1993), but nitrate and phosphate are not fully utilized by the phytoplankton in spite of high biomass in near-surface waters (Figs. F5, F6). A limited supply of micronutrients such as iron, for which the EUC is a major source, may play an important role as a regulator of production (Murray et al., 1994).

Site 1240 is likely to record changes in upwelling and biological production, as well as long-term changes in upper-ocean temperature and pycnocline depth. The surface-ocean properties of the eastern equatorial Pacific are sensitive to interannual to decadal oscillations such as those of the well-known El Niņo Southern Oscillations (ENSO) (Cane, 1986) as well as longer-term changes associated with the Pleistocene ice ages (Pisias and Mix, 1997; Andreasen and Ravelo, 1997; Mix et al., 1999; Lea et al., 2000; Beaufort et al., 2001).

Although Site 1240 is relatively young, which limits the extent of plate movement associated with tectonic drift, even small changes in position may be important near the equator. Plate tectonic backtrack locations can be used to predict general features of oceanographic change at Site 1240, assuming that overall conditions in the region remain constant and that the only change in the system is drift of the site location relative to this fixed oceanographic background (Fig. F7). In this analysis, we ignore changes in the position of the continental margin through time. Sampling of modern oceanographic atlas values at the paleosite locations suggests that from 3 m.y. ago, sea-surface temperatures at Site 1240 may have been slightly cooler (~1°C), salinities were higher, and pycnocline depths were shallower than today. These changes are predicted because Site 1240 was south of the equator just a few million years ago. Here, upwelling into the South Equatorial Current (SEC) is greater than just north of the equator, so we might expect a slight increase in biological production at greater ages due to this geographic effect. In the absence of other regional changes in ocean circulation or tectonics, we would expect that biogenic sediment accumulation rates at Site 1240 would have been relatively high in the past. Eolian sediment components may also have been more prevalent at greater ages when Site 1240 backtracks across the northern edge of the Atacama dust plume near the equator (Molina-Cruz, 1977). Significant deviations from these general trends, if detected in the sediment cores, would imply changes in regional oceanographic or climatic conditions, or errors in the tectonic backtrack or age models.

The deep waters at Site 1240 (Fig. F6) derive from middepth waters that enter the Panama Basin at its sill depth of ~2000-2500 m, both in the north from the Guatemala Basin and in the south from the Peru Basin, as well as from deeper Peru Basin Water that enters the Panama Basin through the Ecuador Trench (Lonsdale, 1976). Anomalously high geothermal heating in the basin helps to maintain a short residence time (40-50 yr) of waters within the basin. In spite of this active circulation, Panama Basin deep waters are relatively depleted in oxygen and 13C and enriched in nutrients relative to waters at equivalent depths outside the basin because of in situ oxidation of 13C-depleted organic matter (Kroopnick, 1974). As a result of these processes, the long-term history of benthic foraminiferal 13C here combines global, regional, and local effects (Mix et al., 1995).

The primary objective at Site 1240 is to provide a continuous sedimentary sequence of late Pliocene to Quaternary sediment to assess variability of upper-ocean processes, including equatorial upwelling, at millennial to orbital timescales.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

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