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Publications (186)
Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic metha...
After 4.5 billion years as an evolving and dynamic planet, the Earth continues to evolve but with human‐altered dynamics. Earth scientists have special opportunities and responsibilities to accelerate our understanding of Earth's changes that are transforming our most remarkable home.
Since 2006, with the development of an optimized methodology for silicon purification based on alkaline fusion followed by cation-exchange chromatography, silicon isotopes have been widely measured across diverse sample types....
The carbonate compensation depth (CCD), δ13C of marine carbonate, atmospheric pCO2 and major ion composition of seawater provide constraints on how geological carbon cycle processes evolved over the Neogene. I use simple models and the LOSCAR ocean carbon system model to assess what changes in carbon fluxes to the ocean are necessary to explain obs...
Plain Language Summary
Periods of intense infiltration in the form of rainfall (storm events) or snowmelt push large volumes of dilute fluid through catchments, resulting in evacuation of dissolved solutes derived from weathering reactions. These events occur over short (daily or weekly) timescales, but these pulses of solutes released into small s...
Streamwater transit time distributions display a variable proportion of old waters (≥1 year). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from (a) the variable contributions of different flowpaths (overland flow, seepage flo...
Globally, soils store between 1500 and 2800 Pg of organic carbon (OC). The physical and chemical stability of these terrestrial soil carbon stores under plausible climate change scenarios is unclear. Soil organic carbon (SOC), especially in volcanic soils, is stabilized through mineral matrix interactions. How susceptible are these mineral-organic...
Data sharing benefits the researcher, the scientific community, and the public by allowing the impact of data to be generalized beyond one project and by making science more transparent. However, many scientific communities have not developed protocols or standards for publishing, citing, and versioning datasets. One community that lags in data man...
We measured germanium-silicon (Ge/Si) ratios in both fluid and solid phases using a series of highly constrained amorphous silica precipitation experiments at 20°C and neutral pH for a wide range of seed crystal surface areas. Silicon isotope data (δ30/28Si) on these experiments were previously reported by Fernandez et al. (2019). A distinct lag in...
You can find the full paper in the following link:
https://rdcu.be/cdwqD
Magnesium (Mg) isotopes fractionate during rock/mineral weathering and leaching, secondary mineral neoformation, adsorption/desorption, and plant-related Mg recycling, but the mechanisms and extent of fractionation are not well understood. Here, we report the fate of Mg and its isotopes during basalt weathering and soil development in the Hawaiian...
Germanium is a useful tracer of silicate weathering and secondary mineral formation in the Critical Zone because Ge/Si ratios are fractionated during incongruent weathering of silicates. We develop an estimate of the equilibrium fractionation coefficient between germanium and silicon for the precipitation of kaolinite using a solid-solution model....
In this study we demonstrate that spatial relationships between Fe and Al with SOC at the microscale display a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Check out details in the publication: doi.org/10.1016/j.gca.2019.11.030
Climate differences can induce profound changes in organo-mineral associations in soils. However, the magnitude of these modifications, whether as a direct effect of climate conditions or an indirect effect through changes in soil mineralogy, are still not fully understood. In this study, we aimed to improve understanding of how climate and resulta...
Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in...
The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate1,2. The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geological timescales³. By removing reduced carbon from Earth’s surface,...
On Thursday (11/04/2019), I will present some results of our project at EGU 2019 in Vienna - Austria (Poster 9929 at SSS5.1 - Controls on Soil Organic Matter Dynamics across scales) . This poster presents an incubation experiment using different labeled 13C amendments in top and subsoil depths of an Andosol. In this study, we have observed that the...
We have investigated how biota contributes to rapid chemical weathering of Hawaiian basalts using a reactive transport model and chemical data from a soil chronosequence. These Hawaiian soils have developed under a tropical forest with rainfall >200 cm/yr and exhibit extensive weathering on timescales of 10⁴ years. We developed a series of multicom...
Biological pumping of mineral elements (root uptake from the soil and concentration at the surface via litterfall) may be an important mechanism influencing their loss from terrestrial ecosystems by accelerating transport in runoff, though few estimates exist to assess this. In the Susquehanna Shale Hills Critical Zone Observatory (a temperate fore...
The impact of forest conversion on soil weathering is studied in a subtropical humid setting in southern Brazil (Rio Grande do Sul). A geochemical tracer of mineral weathering processes, the Ge/Si ratio, was used at the pedon and catchment scales to compare a cropland and a forest catchment. Ge/Si measurements were performed on bedrock, bulk soil,...
Reconstructions of the carbonate compensation depth (CCD) in the past have been used to inform hypotheses about the nature of weathering, tectonics, climate change, and the major ion content of the world's oceans over the Cenozoic. These reconstructions are sensitive to uncertainties in the input data, in particular, the paleodepth estimates of sed...
Critical zone science seeks to develop mechanistic theories that describe critical zone structure, function, and long-term evolution. One postulate is that hydrogeochemical controls on critical zone evolution can be inferred from solute discharges measured down-gradient of reactive flow paths. These flow paths have variable lengths, interfacial com...
This study aimed to compare soil properties across a topo sequence with precipitation and iron gradients to better understand the influence of soil mineralogy and weathering on soil organic matter (SOM) protection through organo-mineral associations at the micro-scale. Different SOM fractionation methods were tested to explore such interactions. Ov...
This study investigates the influence of water, carbon and energy fluxes on solute production and transport through the Jemez Critical Zone (CZ) and impacts on C-Q relationships over variable spatial and temporal scales. Chemical depletion-enrichment profiles of soils, combined with regolith thickness and groundwater data indicate the importance to...
Volcanic Andosols are recognized by their strong capacity to accumulate soil organic carbon (SOC), and for presenting a singular aggregation pattern. However, the factors which govern their SOC storage and aggregation hierarchy are still poorly understood. In addition, many methods of fractionation are proposed for these soils and there is no conse...
The near constant Si concentration under varying discharge observed in Gordon Gulch of the Boulder Critical Zone Observatory (CZO) indicates that the silica fluxes are strongly controlled by discharge. To identify the mechanisms supplying increased Si at high discharge (Q) we examine Si-Al-Fe-Ge in soils, streams and ground waters. We identify bedr...
Colloids can be important vectors for the transport of contaminants in the environment, but little is known about colloid mobilization at the watershed scale. We present colloid concentration, composition, and flux data over a large range of hydrologic conditions from a small watershed (Gordon Gulch) in the foothills of the Colorado Front Range. Co...
The homeostatic balance of Earth's long-term carbon cycle and the equable state of Earth's climate are maintained by negative feedbacks between the levels of atmospheric CO2 and the chemical weathering rate of silicate rocks. Though clearly demonstrated by well-controlled laboratory dissolution experiments, the temperature dependence of silicate we...
The application of multiple isotope proxies on the same location within a Critical Zone (CZ), which we term “CZ-tope”, elucidates the interactions of geochemical, geomorphological, hydrological and biological processes together with anthropogenic influences in the CZ across widely disparate timescales. We exemplify the CZ-tope approach by summarizi...
Plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems. Hydrologic and other losses are offset by inputs from atmospheric deposition and weathering. This multi-tracer study investigates these effects using 87Sr/86Sr, Ca/Sr, and Ge/Si ratios from solid (soil profiles and bedrock), biologica...
The Critical Zone (CZ) extends from the bottom of the water table to the tops of vegetation. Soil is at the heart of the CZ. Nine NSF-funded Critical Zone Observatories (CZOs) engage interdisciplinary teams of scientists in the integrated study of the CZ. Virtual Fieldwork Experiences (VFEs) that are multi-media representations of actual field site...
The US National Science Foundation (NSF) has pioneered an integrated approach to the study of Earth's Critical Zone by supporting a network of Critical Zone Observatories (CZOs). The CZOs are intensively studied and monitored sites with a focus on a range of Critical Zone processes that are well represented at the various sites. The initial network...
Sr isotopic composition and soluble chemistry of gypsum/anhydrite surface salt samples in the Atacama Desert.
An elevation-dependent relationship of the 87Sr/86Sr ratio of Holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting. In the Atacama Desert of northern Chile, gypsum and anhydrite of multiple origins exist widely on superficial materials that originated during the last 10,000 years. An important s...
Evidence for low pO2 and a ferruginous ocean characterize the mid-Proterozoic (1.8 - 0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export...
Concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals, formation and weathering of secondary clay minerals, hydrothermal input and biological cycling (formation and dissolution of opal phytoliths and growth of diatoms). We applied the Ge/Si ratio to assess the differen...
We combines Mg isotopic analyses with soil characterization methods to determine Mg isotopic compositions of bulk soils, basalts, and carbonate fractions at an arid (∼30 cm MAP) soil chronosequence on the Island of Hawaii. This chronosequence is developed on Pololu (350 ka) and Hawi (170 ka) lava flows. Both profiles contain pedogenic carbonates an...
We investigated the products and rates of chemical weathering on the Hawaiian Islands, sampling streams on Kaua’i and both streams and groundwater wells on the island of Hawai’i. Dissolved silica was used to investigate the flowpaths of water drained into streams. We found that flowpaths exert a major control on the observed chemical weathering rat...
The critical zone is the environment near the Earth's surface in which
biological, chemical, and physical processes interact and contribute to
the evolution and structure of life on Earth. Mineral dissolution is an
important process in the critical zone that supplies essential
nutrients, to the biotic foundation of ecosystems. Stream chemical
compo...
Post-eruptive volcanic landscapes evolve rapidly in response to erosion,
re-vegetation, and pedogenesis. The Hawaiian islands offer a time series
to study the evolution of surface processes on a uniform lithology and
under spatially varying but well-characterized climates. Young surfaces
retain constructional topography largely controlled by the mo...
We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic ho...
The Cornell Earth and Environmental Systems (EES) Field Program is a semester-length undergraduate field program located on the island of Hawai`i. The Hawaiian Islands are the world's most dynamic natural laboratory and the premier location for Earth systems research and education. While there are compelling reasons for students and faculty to trav...
A graphical analysis of the correlations between δc and εTOC was introduced by Rothman et al. (2003) to obtain estimates of the carbon isotopic composition of inputs to the oceans and the organic carbon burial fraction. Applied to Cenozoic data, the method agrees with independent estimates, but with Neoproterozoic data the method yields results tha...
We present a new high-resolution 1-D intermediate-complexity box model
(ICBM) of ocean biogeochemical processes for paleoceanographic
applications. The model contains 79 reservoirs in three regions that
should be generally applicable throughout much of Earth history: (1) a
stratified gyre region, (2) a high-latitude convective region, and (3)
an up...
We test the ability of a new 1-D intermediate-complexity box model (ICBM) that includes process-based C, N, P, O, and S biogeochemistry to simulate profiles and fluxes of biogeochemically reactive species across a wide range of ocean redox states. The ICBM was developed to simulate whole ocean processes for paleoceanographic applications and has be...
Silicon transport and incorporation into plant tissue is important to both plant physiological function and to the influence
plants have on ecosystem silica cycling. However, the mechanisms controlling this transport have only begun to be explored.
In this study, we used secondary ion mass spectrometry (SIMS) to image concentrations of Si in root a...
Marked negative δ13C excursions in Ediacaran-age carbonate sediments have been identified in several sections globally, but are not recognized in all sections of similar age. The presence of δ13Ccarb values as low as −12‰ has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today. The...
Germanium–silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the...
Strontium isotope ratios assist ecosystem scientists in constraining the sources of alkaline earth elements, but their interpretation can be difficult because of complexities in mineral weathering and in the geographical and environmental controls on elemental additions and losses. Hawaii is a “natural laboratory” where a number of important biogeo...
We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bed...
In central Nepal hot springs are common in a broad zone where deeply incised river gorges cross the MCT along steep stream reaches. The chemistry of the hydrothermal fluids is distinct from that of the rivers, enabling the use of chemical mass balance to estimate the hydrothermal flux. Spring exit temperatures are 25–70 °C. We combine mass balance...
The marine geochemical budget of some solutes does not add up. A test case shows that at least part of the reason may lie in the timescale over which continental weathering recovers from glaciations.
The India-Asia collision induces major topographic and drainage evolutions at continental scale. As a consequence, dynamics of processes which control the carbon cycle are strongly modified. The development of the Himalayan basin generates major erosion fluxes that, through silicate weathering and organic carbon burial, tend to consume atmospheric...