Magazine article Oceanus

Continental Margin Particle Flux: Seasonal Cycles and Archives of Global Change

Magazine article Oceanus

Continental Margin Particle Flux: Seasonal Cycles and Archives of Global Change

Article excerpt

The boundaries between the oceans and the continents are dynamic regions for the production, recycling, and deposition of sedimentary particles. In general, rates of biological productivity along continental margins are significantly higher than in the open ocean. This is due to a variety of factors including coastal upwelling of nutrient-rich waters and nutrient input from continental runoff. While continental margins account for only about 10 percent of the global ocean area, 50 percent of the total marine organic carbon production is estimated to occur in this limited region, with much of it exported to the deep sea.

Despite the recognition that continental margins exert a strong influence on global biogeochemical cycles, there have been relatively few attempts to quantify either the magnitudes or nature of temporal variability in these regions' particle fluxes. In addition, the high sediment accumulation rates that characterize many continental margins make them ideal depositional settings for preserving high resolution records of past climate change.

In the summer of 1990 my University of South Carolina colleagues (Eric Tappa, Carol Pride, Eileen Kincaid, and Kathy Tedesco) and I initiated the first of three time series sediment trapping programs designed to study particle fluxes in continental margin basins. The basins we selected for study include Santa Barbara Basin (offshore California), Guaymas Basin (Gulf of California), and Cariaco Basin (Venezuelan margin). All three field programs are currently ongoing, with the Guaymas Basin project now in its eighth year. The work in Cariaco Basin is part of a large international effort entitled CARIACO (CArbon Retention In A Colored Ocean) involving researchers from other US and Venezuelan institutions. At all three locations, we are fortunate to have local marine labs that provide much needed logistical and ship support. They are the Southern California Marine Institute (Los Angeles), Estacion de Investigaciones Marinas de Margarita (Isla de Margarita, Venezuela), and CIB (Guaymas, Mexico).

These basins share a number of common features. All three are sites of seasonal, wind-driven coastal upwelling and high primary productivity. Additionally, all three basins are marked by oxygen-depleted conditions [ILLUSTRATION OMITTED]. The Cariaco and Santa Barbara basins are separated from regions farther offshore by sills that isolate the deep waters in the basins and cause them to become anoxic. The situation in the Guaymas Basin is somewhat different, in that the oxygen-depleted waters occur in mid water column and are associated with Pacific Intermediate Water that flows into the Gulf of California at depths between 500 and 1,000 meters. In all three basins, sediments accumulate in clearly defined layers within the anoxic zones due to the absence of benthic organisms and the consequent lack of bioturbation. A pair of these "laminae," referred to as a varve and consisting of one dark layer and one light layer, represents a year of deposition in each basin. Such sediments serve as natural archives for studying annual- to decadal-scale changes in past climatic conditions.

The basic objectives of the sediment trapping programs in all three basins are similar. First, we document seasonal to interannual changes in sediment fluxes and then relate this variability to changing hydrographic and climatic conditions. Second, we use the observed seasonal variability in the fluxes of different sediment types to develop models of varve formation for each basin. Finally, we identify and calibrate the best proxies for studying climate change in each basin and then apply these to sediment cores in order to reconstruct records of past climate change. In addition, for the Cariaco Basin study we are using an array of sediment traps placed throughout the water column to measure changes in carbon flux with depth. This allows us to evaluate whether anoxia results in enhanced preservation of organic matter. …

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