Understanding Molybdenum Isotope Dynamics in Terrestrial Environments Public Deposited



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  • This dissertation works towards determining the mechanisms driving the Mo isotopic composition of soils, and how these signals may be used to refine the use of Mo as a proxy of biogeochemical processes. The first step towards quantifying Mo fractionation in soils is to determine the mechanisms controlling Mo accumulation, loss, and mobility. To do this, I first measured the abundance and isotopic composition of Mo across soil climatic gradients on the Hawaiian Islands. The Maui Climate Gradient (MCG) is a 410 kyr precipitation gradient where redox state (Eh) is well-constrained. I observed higher bulk soil Mo concentrations and greater mobilization of Mo in anoxic soils, and determined that Mo adsorption was two orders of magnitude greater onto organic matter relative to short range ordered (SRO) Fe- (oxyhyrdr)oxides. Since high rainfall coincides with an accumulation of organic matter, these results suggest a shift from Fe- (oxyhydr)oxides to organic matter control of Mo at high rainfall sites. I then focused on the Kona Climate Gradient (KCG), located on a 10 kyr lava flow where soils are undergoing initial stages of chemical weathering. Across the KCG, I observed net accumulation of Mo with increasing precipitation in surface soil horizons, suggesting that atmospheric inputs were a significant source of Mo in soils. The isotopic composition of soil Mo is also offset from bedrock values, confirming that Mo isotope shifts are occurring. The isotopically lightest Mo signature is found in the driest sites, and the isotopically heaviest Mo signature is found in surface horizons of the wettest sites. To reconcile these observations, I measured the Mo isotopic composition of precipitation, groundwater, and vegetation and concluded that while the Mo cycle is significantly affected by isotopic fractionation mechanisms within soils, it is also modulated by atmospheric inputs and subsequent Mo adsorption onto organic matter. My next research project followed up on these findings from Hawaiian soils by calculating the extent of Mo isotope fractionation during adsorption onto organic matter. At pH 4, fractionation between the solution and adsorbed phase (98Mo) was 1.4‰ and fit an equilibrium fractionation model. As pH increased from 2 to 7, Mo adsorption onto IHA decreased, but the degree of fractionation increased to a maximum of 1.8‰. I compared laboratory results to Mo isotope patterns in precipitation, foliage, organic horizon, surface mineral soils, and bedrock for 12 forested sites across the Oregon Coast Range. Fractionation of precipitation-derived Mo onto the organic horizon aligned well with laboratory results, suggesting that organic matter influences the Mo isotope composition of soils by preferentially adsorbing light Mo. The magnitude and direction of Mo fractionation during adsorption onto organic matter is similar to fractionation of Mo onto Fe- and Mn- (oxyhydr)oxides, which has implications for the interpretation of the sedimentary Mo record and its use as a paleoredox tracer. In addition to organic matter adsorption and desorption processes, the dissolution of Fe- (oxyhydr)oxides, colloid dispersion, and shifts in pH have the potential to mobilize Mo and other trace metals in soil. To determine trace metal mobilization as a function of redox, soil mineralogy, and colloid dynamics, I measured trace metal mobilization via colloids and the aqueous phase during two consecutive, 8-day redox cycles. In soils with high clay content and low permeability, reducing conditions drove trace metal mobility. Comparatively, in soils with low clay content and high permeability, trace metal mobilization was independent of redox state. My results provide evidence that lithology remains an overarching factor governing trace metal mobility in soils. The Mo isotopic composition of the soil solid, colloids, and the aqueous phase did not reflect the redox history of soils, suggesting that the additional fractionation mechanisms such as organic matter and atmospheric inputs complicate the utility of Mo as a tracer of redox in soils. My final research endeavor investigated the impact of Mo cycling in the terrestrial environment on the magnitude and isotope signature of the Mo flux to the oceans. I collected Mo from a series of rivers and groundwater sources along the Hawaiian Islands where the stage of chemical weathering varies as a function of lithological age. Groundwater dominates the Mo flux during initial stages of chemical weathering, and the dissolved Mo isotopic composition of groundwater is only slightly fractionated from bedrock. With increasing age, and stage of chemical weathering, rivers become the main vector for water transport to the oceans. Rivers draining predominately shallow flowpaths have Mo isotopic signatures that reflect fractionation processes as Mo cycles through the terrestrial environment. However, the input of groundwater to riverine base flow overprints small-scale fractionation mechanisms and narrows the range of the isotopic signature of the global Mo flux. The chapters of this dissertation seek to address the mechanisms driving isotope fractionation patterns during terrestrial biogeochemical cycling. My data shows that Mo is fractionated within soils during adsorption onto organic matter, and that the input of isotopically heavy Mo from atmospheric inputs is a previously unrecognized source of Mo to soils that may alter the trajectory of fractionation patterns. These conclusions suggest that the utility of Mo as a redox tracer in soils is suppressed by additional fractionation mechanisms. Nevertheless, these insights into Mo biogeochemical cycling contribute to better understanding and prediction of how riverine isotope signatures have likely varied as a function of chemical weathering throughout Earth’s history.
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