12. Site 1241¹

Shipboard Scientific Party²

INTRODUCTION

Site 1241 (proposed Site COC-2A) is located at 5°50.570´N, 86°26.676´W on a gently sloping sediment-covered ramp on the north flank of Cocos Ridge (Fig. F1) at 2027 m water depth in the Guatemala Basin. Some small bathymetric highs that rise a few hundred meters above the ramp probably represent volcanic outliers from the main edifice of Cocos Ridge (Fig. F2). The crust underlying the site was probably formed at the Galapagos hotspot, roughly coeval with the formation of seafloor crust ~11-13 m.y. ago at the Cocos-Nazca Rise (Hey et al., 1977) (Fig. F3).

A seismic profile at Site 1241 (Fig. F4) documents pelagic drape 400-500 m thick. To the south, the thinning of the sediment cover on Cocos Ridge along with bathymetric evidence for fossil channels indicates that some downslope transport of sediment has occurred. Dominant sediments in the region are foraminifer-bearing nannofossil ooze, lithifying to chalk at depth, and occasional ash layers that may record the history of volcanism in Central America (Ledbetter, 1985).

At its present location, Site 1241 (Fig. F5) is under the warm and relatively low salinity waters of the Panama Basin (Levitus and Boyer, 1994). Nutrients at the sea surface are low, and biological productivity is substantially lower than at the equator (Fig. F6) in spite of an anomalously shallow pycnocline (Ocean Climate Laboratory, 1999). In its recent history, the site is well positioned to monitor the location of the Intertropical Convergence Zone in the eastern Pacific.

A tectonic backtrack path on the Cocos plate (Pisias et al., 1995) moves Site 1241 southward and slightly to the west relative to South America (see Fig. F6 in the "Leg 202 Summary" chapter). The site was probably located close to the equator and probably at shallower depths during the late Miocene. Site 1241 is expected to provide a record of surface water salinity reduction that occurred in response to the closure of the Isthmus of Panama during late Miocene time (Haug et al., 2001), along with a record of volcanism in Central America based on the accumulation rates of ash (Sigurdsson et al., 2000).

Plate tectonic backtrack locations can be used to predict general features of oceanographic change at Site 1241 under the assumptions 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. F8). In this analysis we ignore changes in the position of the continental margin through time, which may be significant in the region of northern South America and the Central American Isthmus. Sampling of modern oceanographic atlas values at the paleosite locations suggest that from 10 to 11 m.y. ago, sea-surface temperatures at Site 1241 would have been significantly (~3°C) cooler and saltier (by one unit) than today because, at that time, the site was within the equatorial upwelling system and the South Equatorial Current. The pycnocline depth would have been much shallower in the past, as well, resulting in higher sea-surface nutrient concentrations and primary productivity prior to 6 Ma. In the absence of other regional changes in ocean circulation or tectonics, we would expect that biogenic sediment accumulation rates at Site 1241 would be relatively high prior to 6 Ma. 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.

On shorter timescales within the late Neogene, Site 1241 is likely to record oscillations of ice-age climate in changing sea-surface salinity and pycnocline depth, which are both associated with the heavy rainfall under the intertropical convergence. Here, glacial-interglacial sea-surface temperatures are thought to be relatively stable, with changes of just 1°-2°C inferred from species abundances of radiolarians (Pisias and Mix, 1997) and foraminifers (Mix et al., 1999).

Deep waters of the Guatemala Basin (Fig. F7) currently derive from middepth waters of the North Pacific that enter through fracture zones in the East Pacific Rise (Tsuchiya and Talley, 1998). Waters of North Pacific origin are relatively depleted in oxygen and ¹³C and enriched in nutrients (Kroopnick, 1974). As a result of this oxygen-depleted source water, high amounts of organic rain on the Central American margin, and relatively little mixing with low-salinity surface waters, the region has one of the deepest and strongest oxygen minimum zones in the world and is thus an important site of denitrification and potential control of global nitrate budgets (Ganeshram et al., 1995). Carbonate dissolution is significant in deeper waters of the Guatemala Basin, and sites deeper than 3400 m (e.g., Ocean Drilling Program (ODP) Sites 844 and 845) have been below the carbonate compensation depth for the past 10 m.y. (Lyle et al., 1995). The relatively shallow depth of Site 1241 will facilitate study of changes in the upper reaches of the lysocline in Guatemala Basin and will complement our understanding of deepwater variability in the region. Based on the likely tectonic backtrack of the site to the south and to shallower water depths in the past, it is likely to monitor intermediate waters during late Miocene and early Pliocene time.

The primary objective at Site 1241 is to provide a continuous late Neogene sedimentary sequence to assess variability of upper-ocean processes, including the reorganization of equatorial Pacific surface circulation and the development of the Atlantic to Pacific salinity contrast, both associated with the closure of the Isthmus of Panama and other late Neogene climate changes.

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.

Ms 202IR-112