Composition Maps of Surface Sediments of the Eastern Tropical Pacific Ocean

Abstract

The eastern tropical Pacific Ocean is a region having high primary productivity gradients associated with upwelling at coastal margins, equatorial divergence, and advection of nutrients by currents (see Introduction chapter, this volume). These gradients produce compositional changes in the surface sediments of the region. Understanding these modern sedimentation patterns is a vital step toward interpreting past environments and establishing how these differed from those in the modern world. The eastern equatorial Pacific region is topographically subdivided into the Guatemala Basin on the Cocos Plate, the Panama Basin (which straddles the Cocos and Nazca plates), and the Peru and B Vauer basins on the Nazca Plate (Fig. 1). The East Pacific Rise separates these eastern Pacific Ocean basins from the Pacific Basin proper. Sill depths generally are deep, about 3 km in most cases, thus they are only partly separated from the main mass of Pacific Ocean deep water. The primary latitudinal patterns of productivity and sedimentation are caused by zonal banding of the currents and by equatorial divergence. Nutrient-poor waters from the western Pacific Ocean are advected eastward into the eastern tropical Pacific region at about 5°N, while the South Equatorial Current transports nutrient-rich waters from the margin of South America. Along the equator, productivity associated with the equatorial divergence provides another zonal pattern (Koblentz-Mishke et al., 1970). Superimposed upon this latitudinal pattern are (1) high productivity associated with coastal upwelling and (2) nutrient cycling near the continental margins. This pattern is readily apparent in the depth to the brown-green color change in the uppermost seafloor sediments in the region (Fig. 2). Brown surface sediments are indicative of oxidizing sediment conditions, where Fe(III) is the stable oxidation state of Fe. As the flux of organic carbon to the sediments increases, the sediments become green at depth, where the Fe is reduced to Fe(II). The depth to this boundary is a good semiquantitative measure of both organic carbon flux and sedimentary redox intensity. In the eastern tropical Pacific Ocean, a shallow Fe(III)/Fe(II) boundary can be found along coastal margins and in the equatorial region. Around the Galapagos Islands, there is a complex structure to this pattern where island-induced upwelling occurs and where the equatorial undercurrent splits and occasionally surfaces (Pak and Zaneveld, 1973; Leetma, 1982). Sediments under the nutrient-poor North Equatorial Countercurrent form a tongue of brown sediment that extends eastward at 5°N. This brown-green color change is immediately discernible by macroscopic core description. Other patterns of sediment composition also exist, but can be studied only by chemical analysis of the sediments.