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Workshop Research Themes
(1) Riverine Inputs Currently, some very basic questions remain regarding the influences of drainage basin processes in determining the flux of carbon and other bioactive elements from terrestrial environments to the ocean via rivers. For example, what are the temporal scales (annual discharge cycle, El Nino - La Nina, 100-year flood events) and spatial scales (sub-basin, flood plain, river channel) that may be relevant to the global carbon cycle? The decadal-scale storage of terrestrial sediments within some river basins may be much more important than previously recognized (Stallard, 1998). Where is carbon stored/sequestered within the river system and for how long? The magnitude and duration of storage is likely to be a function of both basin size and tectonic framework. Presently, the size of this predominantly terrestrial carbon pool and the timing for its mobilization is poorly understood. This storage phenomena undoubtedly also influences the composition of particulate materials discharged. As a result, what fraction (dissolved, colloidal, particulate) carries the signal of interest to the ocean? What are the ages of POC/DOC delivered by different rivers and RiOMar environments to the ocean? Recent findings suggest that the ages of DOC and POC being discharged from rivers are generally older and more variable (i.e., across different river systems) than previously believed (Raymond and Bauer, 2001). This variability is likely to be some function, or combination of, many of the above-mentioned factors, including geological setting, short- and long-term climatic change, and human activities. In the face of such variability in source, composition and age, more information is clearly required to better constrain the contributions from different river types, and under temporally and spatially varying regimes. In
addition, practically all river basins are directly affected by human
influences, including agricultural and land-use changes, urbanization,
dredging and dam construction, which will impact the size and mobility
of basin storage. Terrestrial carbon sinks have been altered (created
and destroyed) in the past by man. How has this influenced the dynamics
of carbon storage within river systems, and, as a consequence, the amounts
and phases (POC/DOC) that enters the ocean? A better understanding of
carbon cycling in less-impacted and anthropogenically influenced RiOMar
environments is therefore needed. RiOMars receive large inputs of allochthonous, terrestrial organic carbon via rivers and autochthonous, marine organic carbon resulting from high coastal productivity rates. Previous studies have quantified these input rates for several RiOMar environments, however, the interactions between the sedimentological, geochemical and biological controls on carbon cycling within the water column remain unclear due to a lack of well-integrated studies on thee systems. Major rivers impact the adjacent marine environment in the form of a turbid river plume, affecting a wide range of chemical, geological, biological and environmental parameters. For example, turbid plumes typically create light-limiting conditions nearshore which inhibit primary production. Further offshore however, turbidity decreases as a result of particle settling and primary production can be very high locally because of river-derived nutrients. One aspect that warrants further study is the sequential timing of processes in margin waters-particularly the influence of physical forces such as resuspension events. With the development of reliable satellite ocean color analysis in case 2 waters (nearshore waters with high productivity and high turbidity) comes the possibility that new production can be estimated remotely via surface chlorophyll, but this requires that the relationship between primary production and export out of surface waters are known on at least a regional scale. In the open ocean, the relative rates of primary production and export varies greatly as a function of food web dynamics (Buesseler, 1998), with export from the photic zone generally less that 10% of primary production, but at times as great as 50%. Quantification of this relationship is lacking in river-dominated margin environments. Estimates based on organic carbon remineralization rates and particle residence times, predict that well over half of the organic carbon produced in surface waters of shallow coastal environments may reach the seabed in the form of unremineralized particulate carbon (Wollast, 1991). Benitez-Nelson et al. (2000) report a wide range of production: export ratios in the Gulf of Maine (2%-56%) using 234Th:238U disequilibrium. It is therefore likely that export in river-dominated environments is a large (but variable) fraction of production. A better understanding of the controlling processes is needed. (3) Sediment - Water Interface Processes in the Margin: Transformation, Transport and Burial RiOMar environments undoubtedly represent the largest modern repository of particulate organic carbon. During interglacial times, approximately 80-85% of global carbon burial occurs in continental margins, primarily in RiOMar environments (Berner, 1982; Hedges and Keil, 1995). Terrestrial materials that are supplied to passive margins via rivers appear to be efficiently retained and processed within the margin environment. In contrast, major river systems on collision margins may be sites where a substantial portion of terrestrial inputs bypass the shelf and are discharged onto the continental slope and beyond. It is unclear how the retention of terrestrial materials within the margin affects the fate of autochtonous organic carbon in these environments. Annually, the total organic carbon burial in marine sediments is equivalent to less than one-third of the riverine organic carbon discharge---indicating that riverine organic matter is rapidly mineralized or preferentially transported off the margin (Hedges and Keil, 1995). Recent studies indicate a much lower organic carbon preservation rate in RiOMar environments than in other aquatic environments with similar sedimentation rates and particle surface areas (Aller, 1998; Keil et al, 1997). It is likely that the low organic carbon preservation potential in river-dominated margins is directly related to the salient physical, sedimentological and geochemical features of these environments (e.g., organic carbon association with mineral matter, abundance of Fe and Mn oxides, intense/frequent biological and/or physical mixing of the seabed). A much more detailed understanding of processes at the sediment-water interface is required. (4) High Resolution Record in Margin Sediments Rivers
provide a continuous integrated recording of terrestrial vegetation and
climate that is uniquely recorded in RiOMar sediments. Although the stored
sedimentary information is multidimensional, the entrained organic compounds
provide an unparalleled wealth of structural and isotopic information.
These telltale patterns are first imprinted within soil profiles, where
climatic conditions and local bedrock determine the types and amounts
of vegetation and minerals that are formed. Erosion eventually translates
this cumulative record to the river, through the floodplain, and into
adjacent continental margin sediments. Since long-term preservation of
any materials on eroding landscapes is rare, we must ironically turn to
coastal marine sediments to understand the extremes of climatic history,
which are typically most strongly expressed on land. Click here for a printable version. |