Determining Mechanisms and Rates of Geologic Respiration at Watershed Scales
Mineralization of organic carbon in rocks, a process known as geologic respiration, is a major CO2 source to the atmosphere and thus a major control on Earth’s climate over geologic timescales. The goal of this research project is to resolve a current paradox surrounding the relationship between erosion and geologic respiration, and to test a novel geochemical tracer of this process (rhenium). The relationship between geologic respiration and physical erosion rates is being characterized by studying watersheds with contrasting tectonic uplift, erosion, and sediment yields.
with Miguel Goni and Brian Haley. Funding from NSF EAR-1655506 |
Role of Nitric Acid in Chemical Weathering and Pedogenic Thresholds
Ecosystem nitrogen supply strongly influences the biogeochemical cycling and availability of other essential nutrients in temperate forests, especially calcium. Short-term additions of nitrogen often increase dissolved nitrate fluxes and decrease soil pH, which can stimulate soil calcium loss. However, the long-term effect of high nitrogen availability on ecosystem calcium supply is more difficult to determine, and may depend on calcium supply and weathering from different types of bedrock. Strontium isotope ratios (87Sr/86Sr) provide a reliable method to distinguish whether long-term sources of calcium to ecosystems originate from bedrock versus atmospheric sources. We are examining nutrient concentrations and 87Sr/86Sr ratios in tree needles and soil and rock pools from forests in the Oregon Coast Range across a wide natural range in soil nitrogen content. Our goal is to evaluate the interactions between long-term nitrogen accumulation and bedrock type on calcium availability.
with Steve Perakis. Funding from NSF EAR-2045135.
Molybdenum isotope dynamics in the terrestrial environmentIn recent years molybdenum (Mo) isotopes in ocean sediments have become widely recognized as a potential tracer of ocean oxygenation through geologic time. However, the isotopic composition of the global riverine Mo flux, which is an important variable in the Mo isotope budget, is still poorly constrained. We are examining Mo isotope ratios from a variety of weathering profiles, focusing on the effect of organic matter interactions, redox conditions, and atmospheric inputs. This work takes place across age and climate gradients in Hawaii, volcaniclastic and granitic sites in Puerto Rico, the Oregon Coast Range, and the Amazon Basin. We have also analyzed a suite a streamwaters from across the US Critical Zone Observatory Network to assess the role of lithology and climate in controlling the dissolved Mo flux to the oceans.
with Elizabeth King and Annette Trierweiler. Funding from NSF EAR-1053470 |
Mineral aerosol dust deposition and contribution to elemental and nutrient cycling
The goal of this research is to quantify dust inputs to soils and ecosystems in the Caribbean and in Central America, downwind of the Sarahan dust plume. Dust deposition fluxes are notoriously difficult to quantify accurately, but this is a necessary first step in evaluating the role of dust as a geochemical and biogeochemical pathway. Our work demonstrates the power of using radiogenic isotopes of Sr and Nd to quantify dust deposition inputs on a watershed scale, averaged over weathering timescales. In the Rio Icacos watershed in the Luquillo Mountains, we found that Saharan dust contributes a mass flux of 210 +/- 70 kg/ha/yr. This dust flux has a significant impact on soil chemistry, particularly trace element budgets. Ongoing work is examining the spatial controls on dust deposition, as well as the role of dust in providing ecosystem nutrients such as phosphorus and base cations.
with Matt McClintock, Gilles Brocard, Jane Willenbring, and Stephen Porder |
Controls on chemical weathering and stream solute fluxes in Oregon's Coast Range and Cascade Mountains
Chemical weathering reflects a complex interaction between mineral reactivity, hydrology, and geomorphic and tectonic forces. Stream solute concentration-discharge relationships offer a powerful approach to understanding chemical weathering and the relative importance of individual drivers. The concentrations of solutes and their relative proportions vary with stream discharge in complex ways. In the Oregon Cascades, analyses of the long-term stream discharge and stream chemistry records from the H.J. Andrews Long Term Ecological Research site reveal intriguing variability in concentration-discharge patterns, ranging from major dilution to near-chemostatic behavior. By analyzing this variability in combination with measurements of solute tracers including Ge/Si and 87Sr/86Sr, we have identified a major role of subsurface rock fracturing archtecture in controlling chemical weathering fluxes.
Role of molybdenum in terrestrial nitrogen fixation
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Molybdenum is an essential component of biogeochemical cycling, most notably as a component of the nitrogenase enzyme used in biological nitrogen (N) fixation. While the important role of phosphorus (P) in limiting N fixation in ecosystems has been well documented, occurrence and prevalence of molybdenum (Mo) limitation is largely unknown. We are investigating nutrient limitation of N fixation in the Oregon Coast Range, of the Pacific Northwest, USA. Our study examines how Mo, P and N interact across landscape-wide background varaiation in soil N content and heterotrophic N-fixation. We are studying N-fixation in soil, forest floor litter, and in the cyanolichen Lobaria pulmonaria.
with Jade Marks, Steve Perakis, Katy Dynarski, and Chris Catricala
with Jade Marks, Steve Perakis, Katy Dynarski, and Chris Catricala
Reduction-oxidation processes and element mobilization in the Critical Zone
Soils are critically important natural resources at the interface of the biosphere, lithosphere, hydrosphere, and atmosphere. Soil formation has a profound impact on these spheres, yet the rates and processes involved in soil formation are not well understood. This project examines U-series disequilibria, a promising new geochemical tracer that may be used to understand soil development and the associated geochemical fluxes. We are studying soil chronosequences from Merced, CA, and Hawaii as well as soil climosequences in Hawaii. Hawaii is an ideal natural laboratory to do this research because it is uniquely possible to study gradients where only one of the major soil forming factors varies while the others are held constant.
with Elizabeth King, Aaron Thompson, Maria Chapela Lara, and Heather Buss |