Dissertation Defense

PRESENTATION/TALK

Coupled Transport, Fractionation and Stabilization of Dissolved Organic Matter and Rare Earth Elements in the Critical Zone

Marshall 531

Angelica Vazquez-Ortega, Soil Water and Environmental Sciences (SWES), will present her doctoral thesis research.

Abstract:

It is important to understand the processes that influence critical zone (CZ) evolution to ensure its sustainability. This thesis reports on laboratory and field experiments designed to measure the behavior of biogenic and lithogenic chemical species and their interaction in the CZ from column to pedon to catchment scales. We postulated that interactions between biogenic organic matter and rock-derived metals drive coupled processes of carbon stabilization and chemical weathering and denudation.  We tested this hypothesis in three different laboratory and field experiments conducted in the context of ongoing integrated surface earth process studies in the Jemez River Basin Critical Zone Observatory (JRB-CZO). First, in an investigation of the role of particle surface chemistry on dissolved organic matter (DOM) reactive transport, we observed that secondary mineral coatings (Al and Fe (oxy)hydroxides) on primary silicate (quartz) surfaces play a major role in sequestering aromatic and “humified” constituents into sorbate form, thereby significantly retarding their subsurface transport and bioavailability. Further, reinfusion to OM-reacted porous media of a different DOM source resulted in exchange reactions consistent with a new zonal model of organic matter adsorption at mineral surfaces. The elucidation of biogeochemical mechanisms that stabilize carbon (C) in soils is critical to understanding C flux feedbacks to climate change, including the impacts of land-use management.

     This dissertation also aimed to examine the influence of water and DOM through fluxes on CZ weathering processes. Rare earth elements and yttrium (REY) were selected as a focus for this purpose because of their known coherent trends in reactivity toward organic ligands common to soils and sediments. Specifically, trends in REY fractionation were explored for their utility to inform on biogeochemical weathering processes in forested terrain derived from rhyolitic bedrock in the JRB-CZO.  Since various mineral weathering mechanisms are expected to differentially influence REY release, fractionation, and transport, the relative importance of such processes should be reflected in REY signatures of bulk soil, pore solutions, and surface waters deriving from hillslope and catchment discharges and remaining in weathered regolith.  Our studies showed:  (1) REY depletion trends with depth in bulk soils are correlated with topographically-induced variation in water and dissolved organic carbon flux (reflected in negative correlations between total water and carbon fluxes, including WYs 2011, 2012, and 2013) and solid phase REY concentrations measured at the same depths; (2) REY and dissolved organic carbon (DOC) concentrations in stream waters are strongly correlated during snowmelt periods of high discharge, consistent with REY complexation and mobilization in association with organic ligands during the period of shallow subsurface flow; (3) preferential sequestration of Eu occurs during formation of dominantly biogenic (microbially-produced) secondary Mn(IV)-oxides, apparently explaining patterns of enrichment of Eu in the soil matrix; and (4) the incremental increase in positive Ce-anomalies with depth in the soil matrix are apparently controlled by adsorption or co-precipitation with secondary Fe-(oxy)hydroxide minerals.

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