Degree Granting Department
Frank E. Muller-Karger
Amazon Shelf, Bottom Nepheloid Layers, Cariaco Basin, Organic Carbon, Sediment transport, Small Mountainous Rivers
The aim of this dissertation was to understand lithogenic suspended sediment transport mechanisms and distribution in two river-influenced margins: The Cariaco Basin, Venezuela, and the Amazon Shelf, Brazil. Lithogenic sediment input in the Cariaco Basin is controlled by small mountainous rivers (SMR), while in the Amazon Shelf it is dominated by the Amazon River, the largest river in the world in terms of freshwater discharge (~20% of global riverine discharge). Optical transmissometer measurements were coupled with particulate organic matter (POM) observations to understand changes in the geochemical composition of suspended sediment and spatial/temporal distributions over the two regions of interest. In the Cariaco Basin sampling was conducted during the rainy seasons of September 2003, 2006 and 2008, and during the upwelling period (dry season) of 2009. Our results suggest that bottom nepheloid layers (BNL) originating at the mouth of the SMR discharging into the Cariaco Basin are a major delivery mechanism of terrigenous sediments to the basin's interior year-round. Intermediate nepheloid layers (INL) were also observed near the shelf break (~100m) and appear to effectively carry terrigenous material laterally from the shelf to deep waters, thereby providing a plausible supply mechanism of the terrestrial material observed in sediment traps, deployed >70 km offshore as part of the CARIACO Ocean Time-Series. These findings highlight the importance of small, local rivers in the Cariaco Basin as sources of terrestrial material. Indeed, the low isotopic composition of particulate organic carbon (δ13Corg, ~-30 - -24 ‰) carried by the BNL suggests that this material was continentally derived. BNL δ13Corg also changed with season, indicating that the geochemical composition of BNL reflects particle source. These nepheloid layers contained relatively low POM concentrations (average of 10%), agreeing well with published data, yet the fine sediment of the BNL may serve as mineral ballast, enhancing the sinking velocities of POC and thus increasing the efficiency of the biological pump in Cariaco. We suggest that during the transition between the upwelling and rainy season BNL deliver sediment to the deep Cariaco Basin in pulses. During upwelling, BNL are retained on the inner shelf by onshore Ekman transport associated with upwelling. The nepheloid layers are later released as the upwelling subsides; this, coupled with high river discharge rates, may explain the seasonal pulse of sediment observed at the end of the upwelling period (May) in the sediment trap array.
The SMR in Cariaco also have the capacity to deliver large amounts of sediment to the Cariaco Basin during episodic events, such as earthquakes and floods. During September 2008 a sediment density flow was observed in the eastern Cariaco Basin, likely triggered by a magnitude 5.2 earthquake that occurred on August 11, 2008 off the city of Cumaná. Elevated suspended sediments near the bottom were observed at the mouth of the Manzanares Canyon (> 90 g m-2, over a depth of 165 m) and decreased to ~11 g m-2 (over a depth of 40 m) 42 Km away from the canyon's mouth at the CARIACO Ocean Time-Series site (10.5° N, 64.67° W). The sediment flux associated with this single event was ~ 10% of the total annual sediment flux that typically reaches the Cariaco Basin deep seafloor. Average carbon to nitrogen atomic ratios (C/N) as well as C and N isotopic composition confirm that most of the organic matter transferred by the sediment flow was of continental origin (C/N ratios of ~19.3, δ13C of -27.04 ‰, and δ15N of 6.83 ‰). The Manzanares River mouth is located at the head of the canyon, and likely supplies most of the fine grained sediments and fresh organic carbon that accumulate in the upper part of the canyon. This suggests that the canyon is an active depositional center, and its proximity to the Manzanares River and Cariaco Basin is critical for sediment supply offshore, which in turn can have a significant impact on the long-term sequestration of carbon into the deep basin.
The nutrient and sediment biogeochemistry of the outer Amazon Shelf was studied in February-March 2010 to replicate observations made by the AmasSeds study in 1989-1991. These transects roughly corresponded to the AmasSeds Open Shelf (OS) and River Mouth (RM) transects. Onshore winds (~6 m s-1) contained the Amazon plume within ~120 Km of the coast; the plume was visible only in the mid-shelf stations located closest to the coast in the OS transect. Within the river plume, surface dissolved inorganic nutrient concentrations were near zero, except for silicates (4-6 μM). Coupled with oxygen supersaturation (AOU < 1), this suggested complete biological uptake of the major dissolved inorganic nutrients (N, P). Dissolved organic carbon (DOC) was also highest within the plume (average of 116 μM), decreasing to ~73 μM in oceanic waters. Total suspended solids (TSS) in surface waters within the plume were ~1-1.5 mg l-1, decreasing to ~0.2-0.3 mg l-1 in all other sampled stations both over the shelf and in deeper waters. TSS were highest within BNL (22-33 mg l-1) observed over the inner shelf; BNL were not observed outside the area of the Amazon plume. Suspended particulate organic carbon (POCsusp) showed a depleted δ13C isotopic signal (~-25 ‰ to -28 ‰) in surface and bottom waters, suggesting terrestrial provenance. Within the BNL, %POC was low (0.6-0.9%, as compared to 7-18% in surface waters), showing extensive and rapid decomposition of organic matter over the shelf. Atomic C/N ratios in particulate organic matter both in surface waters and within BNL were relatively close to Redfield's (8-14) and relatively stable over the area sampled. Particulate atomic organic carbon vs. particulate organic phosphorous (POC/POP) ratios were also low within the BNL (~110) and increased offshore (>500), suggesting a direct input of particulate P from the Amazon River or from reworked surface sediments. The fraction of POC in surface sediments was also low (0.73 ±; 0.56%; N = 5) and relatively uniform across the region sampled. We estimated instantaneous fluxes of 38.7 metric tons TSS s-1, 0.24 metric tons POC s-1 and 6.42 x 10-3 metric tons POP s-1 northwestward over an area extending between ~50 Km and 120 Km offshore. Our TSS estimates are 30% lower than those calculated by Nittrouer et al. (1986) during peak discharge of the Amazon. We also calculated that some 1.50 Tg yr-1 of DOC were being flushed northwestward along the outer shelf annually, which represent ~6% of the total DOC transported by the Amazon.
By analyzing these two geographical settings it was possible to compare and contrast transport mechanisms of continentally-derived material and establish the relative importance of each mechanism in their different environment. There is still much to be understood regarding BNL in the Cariaco Basin, such as their role within the Manzanares Submarine Canyon with regards to sediment contribution and deposition. Additionally, during the last 30 years, anthropogenic influences on the small rivers around the Basin have significantly altered the drainage and sediment loads, yet reliable data to quantify the level of influence and change over time are not available. We need a better understanding of the natural variability of these small, tropical fluvial systems, trends and impact of episodic events, to better interpret the climate record stored at the bottom of the basin and predict future ecosystem changes in the region. In the Amazon Shelf, more accurate estimates of DOC, POC and POP fluxes northwestward are warranted. The magnitude of the Amazon River discharge dampens changes that have occurred in the last 20 years within the Amazon Basin, suggesting that historic Amazon Shelf sediment and carbon estimates are still valid. The data presented here adds to the growing body of literature that highlights the significance of river-influenced continental margins as sites of organic carbon deposition, remineralization export and sequestration.
Scholar Commons Citation
Lorenzoni, Laura, "Sediment transport and distribution over continental shelves: a glimpse at two different river-influenced systems, the Cariaco Basin and the Amazon Shelf." (2012). Graduate Theses and Dissertations.