OS22A-01
The Fate of Particle-bound Terrestrial Material from Rivers to the Sea: The Mississippi and Amazon River Systems
Particle-bound materials (carbon, nutrients, transition and trace elements) that originate within the watershed and are transported via rivers to the ocean, undergo many transformations en route. Knowledge of what transformations take place (and where these transformations occur) is crucial to our understanding of river-ocean systems and their response to global change. Uranium isotopes are sensitive indicators of the spatial and temporal extent to which terrestrial materials are altered during sedimentation and diagenesis cycles en route to the ocean. In concert with other shorter-lived naturally occurring radioisotopes (7Be, 210Pb), uranium isotopes provide valuable information about the residence time, pathways and fates of particle-bound terrestrial materials. Studies within the lower (tidal fresh) Mississippi River and adjacent ocean margin and on the Amazon Shelf provide insights as to the relative role of the lower river and the open margin as sites for intensive sediment processing. Preliminary results indicate that the net loss of particle-bound constituents such as Fe, Mn, U and organic carbon (due to sedimentation/diagenesis processes) is much greater within the lower Mississippi River than on the adjacent shelf. In contrast, more limited data suggests that the lower Amazon may be the site of net gain in particle-bound constituents and the adjacent shelf the site of net loss of terrestrial signal.
OS22A-02
Sources of Organic Carbon to a River-Dominated Coastal Shelf: the Use of Biomarkers to Track Terrestrial Organic Matter Inputs
Because of high sedimentation rates in the coastal ocean, particularly along river-dominated ocean margins, estuarine and coastal sediments can be important repositories for organic carbon derived from both allochthonous and autochthonous sources. Using molecular biomarker analyses (C:N, bulk isotopes, lignin, and compound-specific isotopes) this study examines the sources of organic carbon to the Louisiana shelf within the dispersive path of the Mississippi River. Surface sediment samples were collected from a grid of stations across the Mississippi river plume and adjacent waters during two cruises in the spring and fall of 2000. Lignin biomarker data shows a mixture of sources to the sediments. Sediments near the mouth of the Mississippi River are chemically distinct from those found in the central portion of the shelf. Based on lignin acid:aldehyde ratios and stable C isotope analyses, terrestrial material delivered in this region is comparatively less degraded and richer in C4 plant carbon. A mixing model based on parameters defining riverine, marsh, and marine organic carbon suggests the highest OC inputs to the shelf in spring were from marine sources (58% to 63% marine OC) while the river supplied the most OC (63% to 76% river OC) during the fall sampling period. This model furthermore estimated marsh inputs ranging from 20% to 30% of the OC in spring and 13% to 19% during the fall, in contrast to the prevailing notion that local terrestrial sources provide insignificant inputs to the carbon pool in this environment.
OS22A-03
Foraminifera and Thecamoebians as hydrodynamic indicators for Amazon estuarine system
The Amazon mangrove forest in Brazilian territory is one of the most extended in the world. It goes from Ponta do Tubarao (4S e 43W) to Cape Orange (5N e 51W) along 2,250 km of coast line. Because the Amazon River System influence, it can be divided into two regions; one with river influence toward north and the other without river influence. In order to characterize the mangrove environment hydrodynamic on both sides of the Amazon River System, foraminifera and thecamoebians assemblages were investigated in the sediment of two estuaries; Araguari to the North (1 15S - 50 30W) and Caete to the South (0 50S - 46 30W). For both estuaries forams and thecamoebians species distribution are atypical relative to other world regions. In both, there are only few calcareous forams and almost all are small and possible of being transported in suspension. Typical estuarine species were not identified. The typical mangrove forams which are agglutinated species were dominant in both estuaries. However, the Caete estuary has a large number of forams species (29), indicating better efficiency in mixing fresh and salt water in comparison to the Araguari. On the other hand, the Araguari has big richness of thecamoebians species (15) indicating fresh water prevalence. The fresh water predominance is due to the Amazon water plume being diverted to the Amapa coast where the Araguari estuary is located. The foraminifera species was also used to determine the salt water penetration in the estuary. In the Caete estuary, salt water penetrates to about 40 km while in the Araguari it does coincide with the limit of the bore tide wave "pororoca" penetration, 45 km. Based on the species succession (forams to thecamoebians species) the Araguary estuary can be divided into three regions controlled by turbidity: the outer, middle and inner estuary. The Caete species succession is not that clear and only could be divided based on salinity into outer and inner estuary. In both estuaries forams and thecamoebians assemblages are efficient tools to distinguish hydrodynamic patterns. The Amazon water plume flowing toward North interferes in salt water penetration in the Araguari estuary, which inhibits forams development but favoring the thecamoebians. On the Amazon south side, the macrotidal regime and smaller volume of fresh water in the estuaries contribute to fresh and salt water mixing and thus resulting in homogeneity of forams and thecamoebians species distribution.
OS22A-04
Mid-Holocene Drought in the Andes and Associated Impacts on Hydrology of the Amazon River
Pollen, charcoal, and radiocarbon analyses were performed on a 2m-long sediment core obtained from Lake Tapera (coastal Amapa) to provide the paleoenvironmental history of this part of Amazonia. Detrended Correspondence Analysis was applied to the pollen data to improve visualization of sample distribution and similarity. The chronology was based on seven AMS radiocarbon dates, which allowed the establishment of a basal age (8,060 yrs BP) and identification of a sedimentary hiatus lasting 5,500 years (c. 7,100-1600 yrs BP) in Lake Tapera. Because the timing of the hiatus overlapped with the highest Holocene sea-level (5,000 yrs BP), which would have increased the local water table preventing the lake from drying out, it is clear that sea-level was not important in maintaining the lake level. As Lake Tapera apparently depended on riverine flood waters, the sedimentary gap was probably caused by reduced Amazon River discharge, due to an extremely dry period in the Andes (8,000-5,000 years BP), when precipitation levels markedly decreased. One of the impacts of this drought in the Andes was a c. 100m drop in Lake Titicaca water depth. The contrasting presence before and after the hiatus of Andean pollen (river transported) in the record of Lake Tapera supports this interpretation. The pollen analysis also shows that when sedimentation resumed in 1,620 cal. years BP, vegetation around the lake was changed from forest into savanna. This record demonstrates the need to improve our understanding of climate changes and the extent of their associated impacts on the environment.
OS22A-05
Modern Environmental Changes on Amapa Coastal Plain under Amazon River Influence
The Amazonian coastal environment is very dynamic compared to other coasts. It is situated at the edge of the Earth's largest forest, and is segmented by fluvial systems, with the biggest being the Amazon River. The rivers are particularly influenced by the Intertropical Convergence Zone (ITCZ), which controls the water and particle discharge, and the flooding regime. Moderate and strong El Nino conditions correlate with low-precipitation periods, and La Nina events cause precipitation to increase. These variables and others related to the Amazon dispersal system create an interesting area for the study of global and regional environmental changes. The Araguari River floodplain on the Amapa coast is influenced by natural processes of global scale such as ENSO events and ITCZ, and by local processes such as Amazon River discharge, tides and tidal bore (pororoca). Anthropogenic processes such as extensive water-buffalo farming also promote environmental changes. Time- series analyses of remote sensing images and suspended sediment have shown that the maximum turbidity zone inside Araguari River is related to the pororoca phenomenon. The pororoca remobilizes sediment from the river bottom and margins, developing sediment suspension >15 g/l as it passes - creating fluid muds. The pororoca also introduces Amazon- and shelf-derived sediment into the Araguari estuary. Measurements during eight spring-tide cycles indicate erosion of 3 cm of consolidated mud and deposition of 1 cm. The pororoca also influences the remobilization and cycling of nutrients and consequently affects the distribution of benthic organisms, including benthonic foraminifera and thecamoebians. For more than a century, the coastal plain has had water-buffalo farming (>42,000 animals today), which modifies the drainage system and affects sedimentary processes. Areas with more buffalo trails have higher suspended-sediment concentration (SSC) during the dry season and lower SSC during the rainy season. This relationship is reversed in drainages without the influence of the herd. The development of small channels, one meter deep by one meter wide, induced by the buffalos can evolve into a large drainage network in a short period of time. The Santana Creek started as a buffalo trail and in three decades it evolved into a network with a main channel 200 m wide and 6 km long. Despite the Amazon River having the largest influence in the region, the Araguari River also has a considerable water discharge of 2.4x103 m3/s and particulate discharge of 7x105 tons/y, and is able to impose changes to the Amapa coastal environments and inner shelf. The natural closure of the Araguari north arm during middle of the 19th century has induced the development of a wide coastal plain in the Cape Norte region, and decreased fresh water to the Carapaporis channel. On the other hand, deforestation for farm development and buffalo farming has influenced the hydrologic regime, sediment and nutrient balance.
OS22A-06
Amazon Shelf Sediment Accumulation Rate and Subaqueous Delta Growth
The Amazon drainage basin is one of the largest basins on Earth extending over 7x106 km2, and water and sediment produced are carried by the Amazon River. Sediment brought to Amazon shelf is spread on a subaqueous delta along the coast from river mouth to the Guiana coast, 1600 km to the north. Delta width varies from 200 km at the river mouth to 40 km at Cape Orange (Brazil). The subaqueous delta has topset, foreset and bottomset beds. The topset beds are found in water depths between 10 and 30-40 m, the foreset are between 30-40 m and 70 m, and bottomset are from 70 to 75-80 m. Shallow seismic data reveals that the beds of the foreset prograde oceanward at a larger rate than the topset buildup, indicating that shelf subsidence rate is less than sediment accumulation rate. The subaqueous delta progrades over carbonate-rich sandier substrate that extends to the shelf break. The substrate is molded by tidal currents into ridges normal to subaqueous-delta progradation. Subaqueous-delta sediments are sandy near the river mouth and become muddier toward north. Muddy sediments move seaward and partially fill the inter-ridge depressions. Near the river mouth, sediment magnetic susceptibility measured along cores is higher and more variable (250-550 x 106 SI), and in the distal portion is lower and less variable (280-350 x 106 SI). Despite diverse sediment types, accumulation rates based on C-14 dating in the foreset beds near the river mouth and distal portion are very similar, 5.1 mm/y and 4.3 mm/y. Accumulation rates based on Pb-210 are two orders of magnitude greater (100-600 mm/y). Particulate organic matter derived from the terrestrial basin and from marine plankton production are incorporated and buried in the delta. Biogeochemical processes transform part of the organic matter into methane gas, which can escape to the atmosphere or be trapped in sediment. High-resolution, shallow seismic data indicate a large amount of methane gas in the subaqueous delta distributed over 31,000 km2.
OS22A-07
Processes and Patterns of Fluvial Sediment Transport and Accumulation on Continental Shelves
Wet-tropical settings produce much weathering and sediment supply to the coastal ocean. The processes that control the fate of the sediment have some distinctions from those operating in other latitudinal settings. For example, river hydrographs are sufficiently constant that sediment is delivered throughout the year, as opposed to temperate latitudes where there is a dramatic seasonal variability. In tropical locations, fluctuations in trade-wind conditions (speed, direction) dominate the physical forcing that drives sediment transport. Sedimentation active on the Amazon shelf and in the Gulf of Papua (Papua New Guinea) can be contrasted with that active in temperate settings off North American (Eel, Columbia), European (Po, Rhone) and Asian (Yangtze, Huanghe) rivers. Among the differences are the mechanisms that lead to high-concentration gravity flows (fluid muds). Because much sediment is transported across shelf by these mechanisms, they have a dominant control on the fate of particles and the patterns of sediment accumulation (e.g., clinoform deposits). The continuous discharge of sediment in wet-tropical settings allows tidal currents to intensely rework the seabed. Coupled with other processes (e.g., estuarine trapping, intensified trade-winds) this leads to very turbid boundary layers (>10 g/l) and extensive physical mixing of surficial seabed sediments. Interdisciplinary impacts are to remineralize geochemical components and to inhibit benthic habitation. In contrast, temperate settings have some of the same processes operating, but in a different fashion. In particular, episodic river discharge and/or storm activity can create different results (e.g., wave-generated fluid muds) and sedimentary products (e.g., flood and storm deposits). Integrated over time scales of centuries, evidence for operational processes in tropical and temperate coastal settings are preserved in high-resolution stratigraphic records. Among the fluctuations recorded are impacts of El Nino and longer-term climatic variability, as well as human perturbations. Together, tropical and temperate records provide a diverse and comprehensive history of terrestrial and marine processes, which will become important for demonstrating the impact global change has on Earth surface processes.
OS22A-08
Tidal Asymmetry in Amazon Estuary
Estuaries and coastal areas are the last reservoir of fine sediments yield in the continental basins. One of the
most interesting questions for each particular system is where the fine sediments are trapped in larger
quantities.
In the early 90's, a thorough measurement project was held over the Amazon Continental Shelf and coastal zone.
One of the results of this project was the register of a huge amount of sediments in suspension over the Shelf,
where the turbidity maximum would be located.
In order to promote the formation of a turbidity maximum zone, conditions for trapping and deposition are
necessary as well as a source of energy to keep sediments in suspension, in general in a feed back way.
Several mechanisms can be pointed as responsible for trapping sediments in estuaries and coastal areas.
Sediments are carried by currents and the question could be established as which are the main features in the
hydrodynamics which contributes to the formation of the turbidity maximum? Flocculation induced by the changes
in the flow structure, mainly the reduction in turbulence, can be regarded as one of the mechanisms which
contribute to the formation of the turbidity maximum In fact, in-situ measurements showed floc sizes as big as
600µm over the Amazon Shelf. Other feature which strongly affects hydrodynamics and hence trapping sediments
is the creation of recirculation zones, related to the estuarine morphology, this aspect, especially important in
open areas, is also important in the Amazon Shelf. The role of the shear stress asymmetry and the transport
capacity asymmetry both related to the salt stratification structure are also shown as important mechanisms for
sediment trapping in this environment.
Focusing the understanding of the dynamics of fine sediments in Amazon estuary, the tide asymmetry in the
Amazon estuary is analyzed. Changes in the vertical tide asymmetry are observed along the estuary. A positive
vertical tide asymmetry is observed in the estuarine region with river influence, which is not indicative of positive
velocity asymmetry, as is currently considered for tidal bays, without significant river discharge. Over the Shelf,
where salinity intrusion and fluid mud bottom layers occur, negative tidal asymmetry is observed. Causes for the
changes of the asymmetry and their consequences for the sediment transport are addressed in this work.
http:www.oceanica.ufrj.br/costeira/projetos/amazonas/index.html