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The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end-members and provide methodology for systems with multicomponent mixing. and Hawaiian soil data sets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the endmember compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems.

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Glaciers are geologically important yet transient ecosystems that support diverse, biogeochemically significant microbial communities. During the melt season glaciers undergo dramatic physical, geochemical, and biological changes that exert great influence on downstream biogeochemical cycles. Thus, we sought to understand the temporal melt-season dynamics of microbial communities and associated geochemistry at the terminus of Lemon Creek Glacier (LCG) in coastal southern Alaska. Due to late season snowfall, sampling of LCG occurred in three interconnected areas: proglacial Lake Thomas, the lower glacial outflow stream, and the glacier’s terminus. LCG associated microbial communities were phylogenetically diverse and varied by sampling location. However, Betaproteobacteria, Alphaproteobacteria, and Bacteroidetes dominated communities at all sampling locations. Strict anaerobic groups such as methanogens, SR1, and OP11 were also recovered from glacier outflows, indicating anoxic conditions in at least some portions of the LCG subglacial environment. Microbial community structure was significantly correlated with sampling location and sodium concentrations. Microbial communities sampled from terminus outflow waters exhibited day-to-day fluctuation in taxonomy and phylogenetic similarity. However, these communities were not significantly different from randomly constructed communities from all three sites. These results indicate that glacial outflows share a large proportion of phylogenetic overlap with downstream environments and that the observed significant shifts in community structure are driven by changes in relative abundance of different taxa, and not complete restructuring of communities. We conclude that LCG glacial discharge hosts a diverse and relatively stable microbiome that shifts at fine taxonomic scales in response to geochemistry and likely water residence time.

Flooding from extreme weather events (EWE), such as hurricanes, exports large amounts of dissolved organic matter (DOM) to both estuaries and coastal waters globally. Hydrologic connectivity of wetlands to adjacent river channels during flood events can potentially have a major control on the DOM exported to coastal waters after EWEs. In this study, a geographic information system based flood model was used to: (1) determine the volume of flooded wetlands in a river corridor following Hurricane Matthew in 2016; (2) compute the resulting volume fluxes of DOM to the Neuse River Estuary-Pamlico Sound (NRE-PS), in eastern North Carolina and (3) use the flood model to quantify the wetland contribution to DOM export. The flood model-derived contributions were validated with a Bayesian Monte Carlo mixing model combining measurements of DOM quality: specific UV Absorbance at 254 nm (SUVA 254 ), spectral slope ratio (S R ), and stable isotope ratios of dissolved organic carbon (δ 13 C-DOC). Results indicated that (1) hydrologic connectivity of the freshwater riparian wetlands caused the wetlands to become the primary source of organic matter (OM) that was exported into the NRE-PS after Matthew and (2) this source lingered in these coastal waters in the months after the storm. Thus, in consideration of the pulse-shunt concept, EWE such as Hurricane Matthew cause pulses of DOM from wetlands, which were the primary source of the OM shunted from the terrestrial environment to the estuary and sound. Wetlands constituted ca. 48% of the annual loading of DOC into the NRE and 16% of DOC loading into the PS over a period of 30 days after Hurricane Matthew. Results were consistent with prior studies in this system, and other coastal ecosystems, that attributed a high reactivity of DOM as the underlying reason for large CO 2 releases following EWE. Adapting the pulse-shunt concept to estuaries requires the addition of a "processing" step to account for the DOM to CO 2 dynamics, thus a Frontiers in Marine Science | www.frontiersin.org 1 February 2020 | Volume 7 | Article 18 Rudolph et al. Wetland DOM Mobilized by Extreme Eventsnew pulse-shunt process is proposed to incorporate coastal waters. Our results suggest that with increasing frequency and intensity of EWE, strengthening of the lateral transfer of DOM from land to ocean will occur and has the potential to greatly impact coastal carbon cycling.

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A new ion exchange chromatography method is presented for the isolation of high field-strength elements (HFSE) from freshwater and seawater samples that have undergone iron co-precipitation. Large volumes of water can be condensed through the application of iron co-precipitation, but clean separation of elements from the precipitate proves difficult. Our technique is a five-column process designed to separate the HFSE, including rare earth elements such as neodymium and hafnium, before removing the iron and isolating uranium. Subsequent isolation of Nd and Hf was achieved using established ion exchange chromatography methods. The efficacy of our chemistry was verified through measurements of analytical reference materials -both standard solutions and seawater samples -subjected to the chemical separation methods described. Elution results indicate high yields (> 90%) determined by concentration measurements of a known reference material added to each column. Comparison of Accepted ArticleThis article is protected by copyright. All rights reserved.isotopic compositions for seawater (U, Nd), and standard solutions (NIST SRM 960, U) with previously published values are identical within uncertainty. Compositions were identical between solutions (Spex CLMS3, Spex PLND2) that underwent different iron co-precipitation procedures. Reference materials (JNdi-1, NIST SRM 960) run on the mass spectrometers for isotopic determinations were in agreement with universally accepted values for these materials and indicate high precision.