H21C-0834
Rare Earth Element Behavior During Incongruent Weathering and Varying Discharge Conditions in Silicate Dominated River Systems: The Australian Victorian Alps
The distribution of rare earth elements (REE) and trace elements was measured by ICP-MS on fresh, slightly weathered and weathered granite and surface water samples from a network of 11 pristine rivers draining the Australian Victorian Alps during (i) high and (ii) low discharge conditions. River water REE concentrations are largely derived from atmospheric precipitation (rain, snow), as indicated by similar Chondrite normalized REE patterns (higher LREE over HREE; negative Ce anomalies, positive Eu anomalies) and similar total REE concentrations during both dry and wet seasons. Calculations based on the covariance between REE and Cl concentrations and oxygen and hydrogen isotopes indicate precipitation input coupled with subsequent evaporation may account for 30% o 100% of dissolved REE in stream waters. The dissolved contribution to the granitic substratum to stream water comes mainly from the transformation of plagioclase to smectite, kaolinite and gibbsite and minor apatite dissolution. However, since most REE of the regional granite are present in accessory minerals (titanite, zircon, etc.) they do not significantly contribute to the river REE pool. REE concentrations drop sharply downstream as a result of dilution and chemical attenuation. A trend of downstream enrichment of the heavier REE is due to selective partitioning of the lighter REE (as both free REE or REECO3 complexes) to hydrous oxides of suspended Al which, in turn, is controlled by a downstream increase of pH to values > 6.1 (for free REE) and > 7.3 (for REECO3 complexes). Although most circumneutral waters were supersaturated with REE phosphate compounds, precipitation of LnPO4 is not believed to have been a dominant process because the predicted phosphate fractionation pattern is inconsistent with the observed trends. Negative saturation indices of hydrous ferric oxides also militate against surface complexation onto goethite. Instead, REE attenuation most likely resulted from adsorption onto hydrous aluminium oxide. Seasonally, higher total REE concentrations during the dry season are due to longer residence time of water within rock fractures as well as high rainfall REE concentrations which, for the summer of 2007, might be related to organic carbon rich dust released after bushfires in the region. Lower pH values and low oxidation potentials at that time also contribute to raise REE concentrations through desorption.
H21C-0835
Seasonality of Rare Earth Element concentrations and fluxes in the Amazon river and its main tributaries
Many studies carried out on the Amazon River illustrate the complex functioning of this river in terms of
geochemistry. Concerning the REE, (Sholkovitz and Szymczak 2000) and (Hannigan and Sholkovitz 2001,
Gerard et al, 2003) summarized the actual knowledge we have on the Amazon river. In this study we present
a 2-year time series on dissolved REE geochemistry in the Amazon River at Óbidos station (S01°56'50",
W55°30'40"), which is the ultimate gauging station on the Amazon River upstream from the marine influence
and from the three main Amazon River tributaries, The Negro River at Serrinha (S00°28'55", W064°49'48)
station, the Solimões River at Manacapuru stations (S03°20'43", W60°33'12") and the Madeira River at Porto
Velho (08°44'12", W63°55'13"), and the Curuaí floodplain, one of the largest várzea located in between
Manaus and Óbidos. REE concentrations were measured by ICP-MS in LMTG Laboratory (France).
The main results are: -a substantial seasonal variation in REE concentrations that is correlated with
discharge. This variation repeats itself from yaer to year, and is also reflected in a compilation of literature
data that reflects different years and dates of sampling; - an absence of seasonal variation in REE patterns
and Ce* anomalies; -
a monthly weighted annual Nd flux to the surface Atlantic Ocean of 607 ± 43 T.yr-1, which is at least 1.6 times
larger than the currently used estimate based on one single measurement during the low water stage. A
mass balance of the major tributaries shows quasi-conservative behavior of the LREE and an excess of
observed HREE during the high water stage. Additional observations are necessary to see if this feature is
recurrent or whether it reflects inherent organizational and analytical difficulties involved in the monthly
sampling of all Amazonian rivers. Persistence of such a HREE excess requires a source such as suspended
matter sorbed REE that transfer to the dissolved phase at tributary confluences or during passage through
the adjacent floodplain lakes of the Amazon. This hypothesis is qualitatively supported by the observation
that during the high water and falling water stage the floodplain exit waters display higher REE concentrations
than Amazon mainstem.
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H21C-0836
Riparian Dendrochemistry: Detecting Rare-Earth Elements in Trees along an Effluent- Dominated Desert River
This research documents spatial and temporal patterns of effluent uptake by riparian trees through development of a new and innovative application for dendrochronology, specifically dendrochemistry. The rare-earth element (REE) gadolinium (Gd), is a known micro-pollutant that enters streams from wastewater treatment plants. Gd was first used in select medical procedures in 1988 and subsequently discharged via treatment plants into waterways. Trees uptake Gd but do not utilize it, thereby providing a specific presence/absence date stamp in tree rings and making it an ideal marker of effluent water use by trees. Results from this study along an effluent-dominated portion of the Santa Cruz River in southeastern Arizona, show elevated levels of Gd in surface flows and the presence of Gd in cottonwood (Populus fremontii) growth rings. The first indication of Gd in tree rings occurred around 1988, and concentrations increased through 2000 followed by a sharp decline from 2001-2005. These dendrochronological results suggest that a clogging layer prevented effluent from infiltrating and recharging groundwater tables during the 2001-2005 drought period, thus reducing concentrations of Gd and other REEs in the groundwater tables. Since riparian trees depend on groundwater for some or all of their water needs, a reduction of Gd in tree rings indicates reduced effluent concentrations in groundwater and therefore a limited connection between the river and the groundwater due to a clogging layer. The impact of effluent quality on the chemical composition of tree rings is a useful monitoring tool to evaluate temporal patterns of surface water quality, the extent of surface and groundwater interactions, and the influence of effluent on riparian ecosystems.
H21C-0837
Rare Earth Elemental Signatures in Fungal Fruiting Bodies as Probes into Mineral Breakdown Reactions in Post-glacial Landscapes
The application of rare earth element (REE) abundances in low temperature geochemistry and biogeochemistry has improved our understanding of the cycling of various micro- and macronutrients from the bedrock into terrestrial ecosystems. In many continental rocks, REEs are concentrated in accessory phases such as apatite and monazite. These phosphate mineral phases break down readily and may be especially important nutrient sources, particularly for P and Ca, in recently glaciated terrains. Several studies (e.g., 1-3) have suggested that the presence of ectomycorrhizal (ECM) fungi, due to the organic acids they secrete, may play an especially important role in this weathering process. A field-based experiment implementing mesh bags doped with specific mineral compositions confirmed that ECM fungal tissues do record the REE signatures of the minerals they break down (4). In an effort to understand the relative role different ECM fungi may play in mineral breakdown reactions, we have measured REE abundances in tissues of several ECM fruiting bodies. Our preliminary data include Russula, Suillus Americana, Leccinum and Lactarius ECM fungi from three postglacial landscapes. At a given site, the relative abundance of REEs varies between the different ECM fungi. Interestingly, we found distinctions in tissue La/Ce values at two of the sites. Leccinum, a deep rooter, shows much lower La/Ce than the companion Russula and Lactarius samples from the same site. Similarly Suillus tissues demonstrated lower La/Ce when compared to Russula growing nearby. Lower La/Ce is consistent with enhanced dissolution of the mineral apatite, a common accessory phase. While the influence of symbiotic host (beech vs. oak vs. pine) may play some role in the distinctive REE signatures recorded by the fruiting bodies, we attribute the observed differences to organic acid production and tendency to colonize in different horizons of the soil profile. (1) Wallander, Plant and Soil, 2000; (2) Blum et al., Nature, 2002; (3) Hoffland et al., Front Ecol Environ., 2003; (4) Hagerburg et al., Plant and Soil, 2003.