Science Highlight Importance of bottom nepheloid layers on the transport and delivery of sediment to the eastern Cariaco Basin, Venezuela
Laura Lorenzoni1, Frank E. Muller-Karger1,4, Robert C. Thunell3, Eric Tappa3, Claudia Benitez-Nelson3, David Hollander1, Ramón Varela2, Yrene Astor2 and Chuanmin Hu1
1University of South Florida, College of Marine Science, St. Petersburg, FL, USA ; 2Fundación La Salle de Ciencias Naturales, EDIMAR, Venezuela ; 3University of South Carolina, Department of Geological Sciences, Columbia, USA ; 4University of Massachusetts Dartmouth, School for Marine Science and Technology, MA, USA
Due to their high topographic relief and susceptibility to erosion, mountainous coastal rivers deliver high sediment loads to continental margins. Yet, the coastal biogeochemistry associated with these small rivers remains poorly characterized. Understanding the transport of lithogenic material from the continent to the coastal ocean is also critical to reconstruct past environments and predict the effect of anthropogenic activities on sediment delivery to the ocean (Milliman and Syvistski, 1992). The Cariaco Basin is an anoxic basin located off the coast of Venezuela (Figure 1) and stores one of the most detailed marine paleoceanographic sediment records (Yarincik et al., 2000). The CARIACO (CArbon Retention In A Colored Ocean) oceanographic time-series project (http://www.imars.usf.edu/CAR/), located in the Cariaco Basin, seeks to understand the linkages between surface processes and sediment deposition at the bottom of the basin.
The eastern Cariaco Basin receives the discharge from three small mountainous rivers (Manzanares, Neverí and Unare). The Neverí and Unare empty onto the wide, gentle sloping and shallow (100 m depth) Unare Platform. The Manzanares empties near a submarine canyon near the city of Cumana (Figure 1). These rivers were examined during the rainy season in September 2003 and 2006. Optical transmissometer measurements collected in the water column and near the bottom were coupled with particulate organic matter (POM) observations to understand sediment distribution and composition.
Suspended sediments entering the basin sank to the bottom within 10 km from the mouth of the rivers. Bottom nepheloid layers (BNLs) were identified in all sampled locations, and seem to be an important dispersal mechanism of terrigenous sediments. BNLs extended up to 50 km from the river mouths, reaching the 100m isobath and effectively transporting sediment into the deep Basin. BNLs were observed near the mouths of the three rivers, although they were more extensively developed over the Unare Platform. Their thickness varied from 2 to 20 m, thickest close to the shelf break. BNLs in the Cariaco Basin may be maintained by wind- or tidally driven resuspension events and local currents. Intermediate nepheloid layers (INLs) were also observed near the shelf break and, in particular, near the Manzanares River.
Though higher in particle concentration, BNLs were, in general, lower in particulate organic carbon (POC), nitrogen (PON) and phosphate (POP), compared to measurements in the overlying water column. This suggested that BNLs may not be a primary mechanism for delivering terrigenous POM to the deeper anoxic waters of the Basin during the rainy season. Terrigenous sediments originating in the Coastal Mountain Range of Venezuela are found in the deep sediments of Cariaco Basin. As the sediments travel from the coast to the interior of the basin they may serve as mineral ballast for marine particulate organic carbon (POC).
Data from the sediment trap array maintained at the time-series station (Thunell et al., 2007) suggest that terrigenous sediment from the BNLs may be delivered to the deep basin in pulses, in response to atmospheric events. Similar observations have been made off the Namibian and Oregon coasts (Inthorn et al., 2006; Hales et al., 2006). This is particularly important because the marine-derived POC that was produced and settled on the shelf during the upwelling season could be effectively sequestered to the deep. BNLs also represent a potential, transitory source of iron to the suboxic zone of the Cariaco Basin (Percy et al., 2007).
We conclude that BNLs and INLs play an important role in the seaward transport of particulate material in the Cariaco Basin. However, we still need to fully characterize composition and mechanisms of transport to understand their role in biogeochemical cycles and continental shelf dynamics. Understanding this is critical not only to interpret the paleoceanographic record of the Cariaco Basin correctly, but also to predict future changes linked to anthropogenic activities.
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