Large Rivers I: Biogeochemistry and Fluxes
Presiding: S Leclair, Department of Earth and Environmental Sciences, Tulane University; R Aalto, University of Washington
H23C-01 13:30h
Carbon Fluxes in the Amazon and Mekong River systems: Relative Importance of Outgassing versus Fluvial Export
The "role" of large continental rivers in the global carbon cycle is most typically expressed as the fluvial export of total organic and dissolved inorganic carbon from land to the ocean. A closer examination suggests that the organic carbon fluxes may be underestimated by a factor of about 2 and that continental sedimentation may constitute an additional sink, but that outgassing to the atmosphere substantially mitigates both those sinks. The relative distributions of outgassing versus fluvial export is evaluated for two contrasting rivers systems.
H23C-02 13:45h
Modeling Spatial and Temporal Dynamics of Riverine Sediment and Carbon Delivery to the South China Sea
The export of carbon from land to sea by the process of erosion and fluvial transport represents a major link in the global cycle and one which remains poorly understood and inadequately quantified. Large continental rivers are particularly significant in this respect but river basin development has a profound impact on inputs of freshwater, sediment and carbon to the oceans. The South China Sea is one of the most significant interchanges between terrestrial and ocean stores on the planet. High fluvial suspended sediment loads are driven by the active tectonic setting, rapid weathering rates, high rainfall and human activities which promote erosion. It follows that the controls on the delivery of carbon from terrestrial sources to the SCS region and the extent to which river basin development activities enhance supply and/or inhibit delivery is of fundamental importance for modeling carbon dynamics. In addition to the direct input of terrestrial carbon from fluvial supply, the buoyancy affect of freshwater plumes drives the upwelling of nutrients. Developing a capability for modeling water, sediment and carbon fluxes within large river basins is therefore an important task. Hydrological data archives provide a source of information for evaluating the impact of recent land use change and basin development on water and sediment fluxes, though the compilation of databases is a time consuming task. Adapting a methodology developed by the authors to investigate sediment yield variability within the Yangtze River, the paper is concerned with the delivery dynamics of two large fluvial systems (Pearl/Xi Jiang and the Mekong) which drain into the South China Sea. The approach employs a GIS framework to investigate temporal and spatial dynamics of sediment delivery, its influence on carbon pathways and the statistical significance of controlling catchment variables. While the Xi Jiang system exhibits heterogeneity in sediment yield trends, a notable decline in water and sediment flux throughout the Mekong system is apparent following the closure of the Manwan Dam in the Chinese section of the basin.
H23C-03 14:00h
From the Andes to the Atlantic: the Evolution of Organic Matter in the Amazon River System
We compare the compositions of dissolved, fine and coarse particulate organic matter fractions (DOM, FPOM and CPOM respectively) from 31 river sites in Bolivia and Peru with 18 sites along the Amazon mainstem. The diversity of sites - ranging from wet and dry Andean headwater environments, to depositional foreland reaches, to major lowland rivers - allows us to assess the compositional evolution of organic matter along a 5000 km transect draining Peru and a 3000 km transect draining Bolivia. Organic matter size fractions were assessed by concentration, elemental (%OC, %N, C/N), isotopic (13C, 14C, 15N), lignin phenol, hydrolysable amino acid, and mineral surface area analyses. Similar to previous results from the lower Amazon and from Bolivian tributaries, the degree of mineral association was the most important factor in determining the composition of riverine organic matter. However, organic matter within a size class evolves considerably from the Andes to the lowlands. The Bolivian transect showed OM fractions becoming more diagenetically altered downstream, but the Peruvian transect showed a more complicated picture. Together, data suggest that underlying changes to sediment surface area and mineralogy may be the primary control of organic matter composition with fractions, as well as between fractions. These findings highlight the importance of organo-mineral associations and the need to describe mineral phases associated with riverine organic matter.
H23C-04 INVITED 14:15h
New Insights into the Biogeochemistry of Particulates in the Lower Mississippi River and Adjacent Margin: Transport and Transformation Processes
During the past few years, a number of research projects in the lower Mississippi River and on the adjacent margin have greatly increased our understanding of biogeochemical cycling in this major river system. In particular, we have gained a better appreciation for the dynamic nature of large river systems and the importance of processes that operate on time scales of minutes to days and on space scales of centimeters to decimeters. An overview of these new insights will be presented, which represents the work of many colleagues and students over the past 5 years. These include: (1) our current assessment of the geochemical characteristics and flux of suspended materials within the lower Mississippi; (2) an evaluation of the seasonal storage, diagenetic transformation and remobilization of particulates within the lower Mississippi; (3) recent work quantifying the deposition of particulate materials on the adjacent margin; and (4) initial assessment of the fate (burial or export) of particulate materials in the coastal margin. Gram-quantity suspended particulate samples have been collected (1-3 times per month) over the past two years at a station in the lower river. Analyses of geochemical tracers (7Be, 234Th, 137Cs, 210Pb, 234/238U) reveal a large seasonal variability in particulate source and residence time within the basin, including remobilization of older stored sediments. Within the lower 500 km of the Mississippi, riverine suspended sediments are deposited on the riverbed during decelerating flows and can remain there for 3-20 months, depending on the annual hydrograph. Substantial diagenetic alteration occurs during these periods of seasonal storage, thereby impacting the characteristics and partitioning of materials that enter the coastal zone. Riverine particulates are delivered to the adjacent shelf as a diffuse suspended sediment plume and as a highly concentrated near-bottom suspension (fluid mud). Extensive remineralization and reususpension/export takes place within margin sediments after deposition. As a result, less than half of the organic carbon delivered to the shelf is buried there on a decadal time scale.
http://www.tulane.edu/~riomar
H23C-05 14:30h
Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean
Global predictions are provided on the terrestrial flux of sediment, on a river-by-river basis, under Modern and pre-Anthropocene conditions. Sediment delivery is influenced basin characteristics, regional climate, and reservoirs. Pre-Anthropocene sediment flux was likely 15.5 BT/yr. Human activities have increased fluvial sediment erosion by 2.3 BT/yr. Modern delivery of sediment is presently 12.6 BT/yr, due to an estimated trapping of 100 BT of sediment behind reservoirs. Africa and Asia have greatly reduced sediment loads, Indonesia now delivers much more sediment. Seasonal patterns of sediment delivery are conditioned by precipitation patterns, snow-release periods, and human-influenced water release from reservoirs.
H23C-06 14:45h
How Much Water and Suspended Sediment Does a Large Tropical Island Shed During a Major Hurricane? Hydrologic and Geomorphologic Effects of Hurricane Georges, September 1998, Puerto Rico
On September 21-22 1998, Hurricane Georges, a category-3 hurricane on the Saffir-Simpson scale, produced heavy rainfall, flooding, and landslides in the mountains and coastal plains of Puerto Rico. In general, rainfall, runoff, and sediment yield vary across the 8,711 square kilometer island of Puerto Rico because of an orographic barrier, the Cordillera Central. Mean annual runoff for the island is estimated to be 910 mm (equal to 8 billion cubic meters), which is about 57 percent of mean annual precipitation: 1,600 mm (14 billion cubic meters). Mean annual suspended-sediment discharge from Puerto Rico into surrounding coastal waters is estimated at 5.9 (+/- 3.2) million metric tonnes. The largely mountainous watersheds of the island are small (tens to hundreds of square kilometers), channel gradients are steep, and most stream valleys tend to be well-incised and narrow. Major rainstorms are intense but brief. As a consequence, flood waters rise rapidly (minutes to tens of minutes) with peak discharges several orders of magnitude above base discharge, and flood waters recede quickly (hours). Major storms transport a substantial portion of suspended sediment from uplands to the coast, based on data from a set of nine suspended-sediment monitoring stations representative of typical conditions in Puerto Rico. During Hurricane Georges, U. S. Geological Survey and National Weather Service rain-gage networks recorded 2-day rainfalls that ranged from about 100 mm to 630 mm (average was 300 mm, equal to about 2.6 billion cubic meters of water). Many streams rose more than 5 meters, resulting in severe flooding in northern, southwestern, and western watersheds. Landslides dissected roads and isolated communities on both the northern and southern slopes of the Cordillera Central. More than twice the mean annual discharge flowed from some watersheds (approximately 1 billion cubic meters of water for the entire island) carrying with it one to six times the mean annual load of sediment from some watersheds. A total of approximately 2.4 million metric tonnes of suspended sediment were discharged from the island to the coastal areas and insular shelf, equal to an average suspended-sediment yield of 280 metric tonnes per square kilometer. Suspended-sediment concentrations for streams draining the steeper, drier, and less vegetated southern watersheds were four to five times greater than the concentrations for streams draining the lower relief, wetter, and more vegetated northern watersheds. The runoff from this single storm was about 13 percent of mean annual runoff and was responsible for about 40 percent of mean annual suspended-sediment discharge from the island.