13. Site 1242¹

Shipboard Scientific Party²

INTRODUCTION

Site 1242 (proposed Site COC-4A) is located at 7°51.352´N, 83°36.418´W in a shallow basin (1364 m water depth) within the structurally complex intersection between Cocos Ridge and the Mesoamerican Trench (Fig. F1). The site is in a graben on the crest of the Cocos Ridge (Fig. F2), which is in turn crosscut by younger normal faults striking roughly east-northeast (von Huene et al., 2000). The crust underlying the site was probably formed at the Galapagos hotspot, roughly coeval with the formation of seafloor crust ~15-16 m.y. ago at the Cocos-Nazca Rise (Hey et al., 1977) (Fig. F3).

The seismic profile at Site 1242 (Fig. F4) documents the hemipelagic fill of a small subsiding basin ~460 m thick. Flat-lying reflectors mark the upper part of the sediment sequence. The geometry of seismic reflections is complex and is clearly affected by faulting in the lowermost part of the profile. At least one hiatus is likely in the deeper section. Because of this complexity, the drilling target here is limited to a penetration of 250 meters below seafloor (mbsf). Dominant sediments in the region are hemipelagic clay, with occasional ash layers that may record the history of volcanism in Central America (Ledbetter, 1985).

At present, Site 1242 is located under the warm, relatively low-salinity waters of the intertropical convergence in the Panama Basin (Ocean Climate Laboratory, 1999) (Fig. F5). Nutrients at the sea surface are low, and biological productivity is relatively low for a continental margin setting (Fig. F6).

A tectonic backtrack path on the Cocos plate (Pisias et al., 1995) moves Site 1242 southward and to the west relative to South America. The site was probably located close to the equator and at shallower depths early in its history, at a position near the Galapagos hotspot. Site 1242 is likely to record changes in the pool of warm and relatively low-salinity surface waters north of Panama Basin that are associated with the heavy rainfall under the intertropical convergence. The Pleistocene to Pliocene sediments will also provide evidence for variations in upwelling and biological production on the Costa Rica margin.

Plate tectonic backtrack locations (see Fig. F6 in the "Leg 202 Summary" chapter) can be used to predict general features of oceanographic change at Site 1242, 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. F7). 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 suggests that from 13 m.y. ago, sea-surface temperatures at Site 1242 would have been significantly (~5°C) cooler and saltier (by 2 units) than today, because at that time, the site was within the equatorial upwelling system and the South Equatorial Current. The pycnocline is relatively stable along the backtrack path. Silicate concentrations are substantially lower than present values in the interval representing 4-6 Ma but return to modern values as the backtrack path enters the equatorial upwelling system (>10 Ma). The nutrients nitrate and phosphate are relatively stable along the backtrack path for the past ~6-8 m.y., suggesting that younger sediments may favor biogenic silica relative to older sediments, perhaps because of the recent proximity to the Mesoamerican coast. 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 bottom water chemistry at Site 1242 (Fig. F8) is influenced by the lower reaches of an anomalously thick oxygen minimum zone between North Pacific Deep Water and remnants of North Pacific Intermediate Water (Tsuchiya and Talley, 1998). These waters of North Pacific origin are relatively depleted in oxygen and ¹³C and enriched in nutrients (Kroopnick, 1974). Moreover, the high organic carbon flux on the Central American margin and relatively little mixing with low-salinity surface waters, due to the formation of a strong pycnocline, lead to one of the deepest and strongest oxygen minimum zones in the world. Thus, this site offers an excellent opportunity to assess the relevance of nitrification vs. denitrification processes in a region that may have potentially contributed to global nutrient budgets (Ganeshram et al., 1995).

Given the relatively shallow depth of Site 1242 well above the regional lysocline, variations in carbonate dissolution are mainly controlled by rates of carbonate rain from surface waters and organic carbon degradation within the sediments.

The primary objective at Site 1242 is to provide a continuous Pliocene to Holocene sediment record to assess the variability of upper-ocean processes at high resolution, including biogeochemical cycles and biota as well as variations in the Atlantic to Pacific salinity contrast and dynamics of the intertropical convergence associated with 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-113