B13G-01 INVITED
Rejuvenation and aging of carbon in rivers: Sources, exports and interactions among fractions in the Amazon and other systems
Carbon in all its forms plays a key role in riverine ecosystems. Organic carbon (OC) sustains heterotrophic activity that produces CO2 and interacts with minerals through sorptive processes. Dissolved inorganic carbon (DIC) serves as a source for carbon fixation by autotrophs and as a key control on geochemical reactions. Organic and inorganic carbon are subject to interconversion through biologically mediated processes. In turn, they are part of an open fluvial system that receives exports from terrestrial environments in the form of OC, respired soil CO2, and weathering products; processes allocthonous carbon within its channels; sequesters OC for tens to thousands of years through floodplain sediment deposition; interacts with the atmosphere via gas exchange; and ultimately transports carbon to the oceans or inland lakes. Natural 13C and 14C isotopes provide a unique capability to identify the sources of carbon to rivers, processes leading to its transformation in transit, and residence times on land and within rivers. They can also provide an indicator of the degree of biologically mediated coupling between DIC and OC fractions, We illustrate the downstream evolution of DIC and OC fractions in the Amazon river system, discussing the extent and controls on coupling among fractions. Previous results demonstrate that: respiratory generation of CO2 in the lowlands can come to dominate the isotopic composition of DIC, essentially flushing out and replacing DIC via evasion; the OC fueling this respiration is often a very small, labile component effectively decoupled from bulk OC fractions; and in turbid systems with abundant sediment supplies, sorptive protection on mineral surfaces may decouple DOC and POC and couple POC to the upstream sediment erosion and depositional cycles. As a result, we observe longitudinal trends towards DIC equilibration with the labile fraction of OC, reaching or approaching 14C values close to the atmosphere's. POC responds strongly to sediment dynamics, which may result in strong seasonal and longitudinal variability in age overprinted over a broader long-term trend towards younger POC downstream. In contrast, bulk DOC often appears distinct from DIC and POC and displays a remarkable uniformity in age (decades) across all systems, unless autotrophic production is pervasive. Finally, we assess global patterns in riverine carbon fraction age and coupling among fractions, comparing them to those in the Amazon and framing them in the context of global patterns of carbon fraction distribution and flux to the oceans.
B13G-02 INVITED
Biogeochemical and Remote Sensing Approaches to Evaluating the Sources and Fates of DOM and Particles in the U.S. Middle Atlantic Bight
Estuaries and the coastal ocean experience a high degree of variability in the composition and concentration of particulate and dissolved organic matter (DOM) as a consequence of riverine/estuarine fluxes of terrigenous DOM, sediments, detritus and nutrients into coastal waters and associated phytoplankton blooms. Our approach integrates biogeochemical measurements (elemental content, stable isotopes and molecular analyses), optical properties (absorbance) and remote sensing to examine the sources and fates of organic matter in the U.S. Middle Atlantic Bight (MAB). Remote sensing of chromophoric DOM (CDOM) is applied to track terrigenous DOM and quantify dissolved organic carbon (DOC) within the coastal ocean. Satellite validation analyses demonstrate successful retrieval of particulate organic carbon (POC), DOC, and CDOM absorption coefficient (aCDOM) for coastal ocean waters using the MODIS-Aqua and SeaWiFS satellite sensors with absolute percent differences between satellite and field measurements of 8±6.4% for DOC, 18±15% for aCDOM(355 nm) and 25±17% for POC. The seasonal processes that influence CDOM distributions in the MAB include freshwater discharge, photooxidation, and wind-induced vertical mixing in autumn and winter. DOC and POC distributions are driven primarily by river/estuary discharge, net ecosystem production, and the ocean circulation pattern along the shelf and continental slope. Satellite data analysis of aCDOM can be used to quantify photooxidation rates, track the inputs of terrigenous DOM from rivers or estuaries into the coastal ocean and beyond, and trace water masses with different CDOM signatures. Satellite-derived DOC and POC can be applied to quantify the fluxes of DOC and POC entering the coastal ocean and exported from the continental margin to the open ocean, estimate net C production, and assess the standing stocks of DOC and POC.
B13G-03
A geochemical approach to assessing the relative contributions of rivers, wetlands and island drains to dissolved organic matter in the Sacramento-San Joaquin River Delta
The Sacramento-San Joaquin River Delta (Delta) is a hydrologically complex system which, in addition to being used for crop production, development, and recreation, provides drinking water to over 23 million Californians. Understanding how existing Delta habitats currently affect dissolved organic matter (DOM) dynamics seasonally will help us predict how future changes in land use might influence water quality. This study assessed whether different sources (rivers, wetlands, open water, island drains) contribute DOM to the Delta, examined the composition of the added DOM, and then assessed the relative importance of DOM derived from these sources to the quality of water arriving at Clifton Court for export into the California State Water Project. We used a geochemical fingerprinting approach to characterize the DOM originating from different Delta habitats that employed a number of analyses including absorbance, fluorescence, lignin content and composition, δ13C and δ15N isotopic composition, structural groupings determined by 13C-NMR, and specific disinfection byproduct (DBP) formation potential. These qualitative parameters adequately distinguished DOM derived from four distinct sources and, thus, enabled us to use a linear mixing model to estimate the contribution of these sources to DOM exiting the Delta. Comparison of water entering the Delta via the Sacramento and San Joaquin Rivers versus water exiting the Delta at Clifton Court indicated that the Delta was a source of DOM throughout the year, particularly in February, March, and May when concentrations already were high in incoming river water. Water passage through shallow wetlands and subsided islands significantly increased DOM concentrations, while deep open water habitats supporting submerged aquatic vegetation had little discernable effect on DOM concentrations and composition. Wetlands contributed greatest amounts of DOM in the spring and summer, in contrast to the island drains, which appeared to be important sources of DOM during the winter. Results indicate that significant changes in land use, particularly the introduction of shallow wetlands, could alter existing patterns of DOM concentrations in the Delta.
B13G-04
Temporal Dynamics of Terrestrial Organic Matter Transfer to the Ocean
Little is known about the amount of time that elapses between the synthesis of terrestrial organic matter by vascular plants and its ultimate incorporation into marine sediments, yet this information is critical to investigations of terrigenous carbon export to the world's oceans. Although bulk radiocarbon analyses of dissolved and particulate phases yield important clues about the scale of such ‘terrestrial residence times', their interpretation is often complicated by the influence of potentially large and variable inputs from petrogenic or other non-vascular plant sources. In order to more fully resolve these components, we performed compound-specific radiocarbon and stable carbon analysis of several higher plant and petroleum biomarkers extracted from margin sediments adjacent to an array of river systems that vary in drainage basin size and relief, bedrock lithology, and climatic setting. A first- order relationship appears to exist between their radiocarbon age and latitude, implying a strong influence of temperature on the rate of organic matter cycling within, and export from, watersheds. Coupled molecular isotope mass balances further suggest that up to half of the total organic carbon preserved in some river-dominated margin sediments is ultimately weathered from ancient sedimentary rocks. In order to further apportion the temporal characteristics of the more active carbon pools, detailed down core Δ14C profiles of individual, vascular plant-derived fatty acids were constructed for anoxic marine basins located at both tropical and temperate latitudes and compared to the Δ14C evolution of atmospheric CO2 in a simple, steady state modeling framework. Results indicate that the majority of these compounds were sequestered in various temporal reservoirs by several thousand 14C years before delivery to marine sediments, with the remaining inventory delayed for a far shorter time of only one or two decades. Moreover, the fraction of each fatty acid homologue passing through these ‘millennial' and ‘decadal' reservoirs systematically decreases at shorter chain length, possibly reflecting the influence of different molecular-level properties such as degradation rate and/or physiochemical sorption. The implications of these findings for climate and carbon cycle reconstructions will also be explored.
B13G-05
Organic Acid Concentrations in Rivers Within the Amazon River Drainage Basin
The composition of the dissolved organic matter pool in both fresh and marine waters is largely unknown. Concentrations of low-molecular-weight organic acids (oxalate, citrate, glycolate, formate, acetate, succinate) have been determined in Brasilian (18 rivers sampled) and Peruvian (19 rivers sampled) rivers within the Amazon River drainage basin. Succinate concentrations were below the detection limit in all rivers. The dominant acid varied among the sampled rivers, indicating that organic acid concentrations depend on river basin characteristics. Organic-acid carbon comprised a highly significant, but variable, fraction of total dissolved carbon, with a range of 3-90%, indicating that organic-acid-derived carbon may be an important source of biologically labile carbon within the Amazon River drainage basin.
B13G-06
CDOM Characterization of Surface Waters and Groundwater in Tampa Bay, FL: A Spectroscopic and Radioisotopic Approach
Carbonate platforms surrounding subtropical estuaries allow water and chemical constituents to be easily transferred between subsurface aquifers and overlying surficial waters. During times of low rainfall, a significant portion of the water found in the rivers that supply these estuaries originates from springs within the streams themselves. This fluctuation in headwater source often results in variability in the chemistry of the organic material and therefore CDOM optical properties. In addition to the impact of groundwater on stream chemistry, an estuary can be influenced by waters rising directly from beneath the estuary. In the Tampa Bay region, CDOM concentration and optical properties within groundwater wells suggest unique sources and biogeochemical pathways of organic material. Shallow aquifers resemble surface runoff in concentration of DOC and CDOM, as well as in CDOM spectral properties. In contrast, the deep aquifers show higher fluorescence efficiencies and blueshifting of fluorescence. Overall, the depth dependency indicates that CDOM in shallow groundwater has terrestrial sources whereas deep groundwater (where water is estimated to be >10,000 years old) has properties more similar to marine humics. Discussed here will be the results from a March-April 2006 study coupling spectroscopic and radioisotopic techniques to better characterize sources of CDOM in a subtropical estuary. Naturally occurring isotopes of radium (used as a proxy for groundwater contribution) were measured along with CDOM fluorescence, absorption and DOC. Discerning the contribution of CDOM via groundwater, river water and marine water will prove useful in understanding carbon cycling within the Tampa Bay region.
B13G-07 INVITED
Effects of Urbanization on Organic Carbon Loads in the Sacramento River, California
To gain better understanding of the effects of urbanization on organic matter transport in rivers, we quantified total organic carbon loading from point and non-point urban sources within the metropolitan area of Sacramento and compared these loads to the amount of organic carbon carried in the downstream Sacramento River. Median total organic carbon (TOC) concentrations in the Sacramento River, non-point urban runoff and wastewater treatment plant effluent were 2.1, 8.9 and 23 mg L-1, respectively. Dissolved organic carbon (DOC) in non- point runoff and the river had similar specific UVA absorbance and disinfection byproduct formation potential, but based on radiocarbon measurements, non-point DOC was substantially older (age greater than 2000 years) than DOC in the Sacramento River. This finding suggests that DOC in non-point runoff is derived primarily from leaching of older soil organic matter. The 10th, 50th, 90th and 99th percentile contributions of urban sources to daily TOC load in the Sacramento River were 10%, 20%, 38% and 80%, respectively. Total urban TOC yield was 150 kg ha-1yr-1 and urban sources contributed 17% of the annual load of TOC in the Sacramento River below Sacramento.
B13G-08 INVITED
Non-riverine pathways of terrigenous carbon to the ocean
The extent and nature of non-riverine fluxes of carbon from land to ocean are poorly understood. Tidal pumping from highly productive coastal environments, atmospheric deposition and submarine groundwater discharge can be significant transport mechanisms for carbon to the ocean. Evidence is mounting that tidally-induced porewater fluxes ("outwelling") of dissolved organic matter (DOM) from mangroves and salt marshes alone may be similar in magnitude as the global riverine flux of DOM. Tidal pumping of dissolved inorganic carbon (DIC) might exceed organic carbon fluxes by far, but the existing knowledge on DIC outwelling is too scarce for a first global estimate. Results from two case studies on the biogeochemistry of DOM outwelling are presented, from the mangroves in Northern Brazil and the salt marshes in the Northern Gulf of Mexico. Ongoing research in the Northern Gulf of Mexico indicates that outwelling and groundwater inputs probably exceed riverine DOM fluxes in this region. Similar observations were made in Northern Brazil. There, the fate of mangrove-derived DOM could be traced from its source in the mangrove sediments to the outer North Brazil shelf by using a combination of isotopic and molecular approaches. Reversed-phase liquid chromatography / mass spectrometry (LC/MS) provided a multifaceted array of information that mirrors the molecular complexity of DOM. Statistical analyses on these data revealed significant differences between mangrove and open-ocean DOM which successively disappeared by irradiating the samples with natural sunlight. Nuclear magnetic resonance analyses yielded concurrent results. Ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) is the only technique capable of resolving and identifying individual elemental compositions in these complex mixtures. We applied this technique for characterizing mangrove-derived DOM and to assess the molecular changes that occur in the initial stages of outwelling. The different approaches concordantly show the presence of photodegraded mangrove DOM on the North Brazil shelf. During transport offshore, sunlight efficiently destroyed aromatic molecules, removing about one third of mangrove-derived DOM. The remainder was refractory and may thus be distributed over the oceans.