THE AMAZON SHELF SETTING - TROPICAL, ENERGETIC, AND INFLUENCED BY A LARGE RIVER

A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) investigate d the oceanic processes near the mouth of the Amazon River in order to understand the fate of its enormous discharge of water, solutes and p articulates. In addition to receiving a large fluvial discharge, the c ontinental shelf near the Amazon mouth is situated on the equator and experiences an extremely energetic physical regime. As such, it repres ents an end member in the spectrum of coastal marine settings, with re gard to latitude, energy and discharge. The oceanic processes occurrin g on the Amazon shelf reflect these environmental characteristics. A r ange of interdisciplinary interactions was observed on the Amazon shel f in response to its low latitude, among them: plume dynamics have lit tle influence from Coriolis acceleration, riverine particles experienc e intense weathering conditions, high primary productivity occurs duri ng all seasons, and shoreline sedimentation involves mangrove vegetati on. The high energy conditions of the Amazon shelf result in: water mo tions dependent on suspended-sediment distributions, trace-metal adsor ption controlled by seabed dynamics, severe restriction of macrobentho s, and deep physical reworking of sedimentary strata (to 1 m or more). The great discharge of fluvial materials (water, solutes, particulate s) directly or indirectly causes three-dimensional estuarine-like proc esses and very high rates of primary productivity, sediment accumulati on and carbon burial to occur on the shelf. Although AmasSeds research can link a wide range of interdisciplinary oceanic processes to latit ude, energetics or discharge, in many cases the observed process is st rongly influenced by a coupling of these characteristics. For example, the dominance of Fe and Mn oxides in controlling redox reactions is a result of tropical weathering that concentrates the Fe and Mn and of intense seabed reworking that regularly reoxidizes these phases. There fore, the importance of Fe and Mn oxides is a result of both latitude and energy considerations. A fourth characteristic of the Amazon syste m is its tectonic setting, which determines physiographic features suc h as drainage-basin size and shelf width. Low latitude and great disch arge characterize all areas of the wet tropics; energy expenditure and tectonic setting vary with specific location. All four characteristic s must be considered when extrapolating AmasSeds observations to other areas and attempting to predict or interpret oceanic processes.