Browsing by Subject "radiocarbon"

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    Carbon-14 Age Dating Calculations for Minnesota Groundwaters
    (2018) Alexander, Scott C; Alexander, E Calvin Jr.
    Groundwater dating techniques can be applied to flow systems with time scales from hours to tens of millennia. For the purposes of this report age and residence time are used interchangeably. For waters with ages ranging up to about 30,000 to 40,000 years carbon-14 (14C), or radiocarbon dating, can be a useful technique (Han et al., 2012). Han and Plummer (2013, 2016) reviewed 14C groundwater dating models. In particular, converting a measured 14C activity to an “age” is complicated by exchange of carbon in surficial, soil, and groundwater environments. Groundwater age is, however, not defined by simple piston flow past an arbitrary point like a well. Mixing occurs at several scales from advection and dispersion along a single flow path, to mixing of multiple flow paths, to mixing within a borehole intersecting multiple aquifers. In practice all groundwaters are a mixture of waters with varying subsurface residence times (Bethke and Johnson, 2008; Cartwright et al., 2017). Efforts to reconcile complex geochemistry and flow paths with geochemical models and calculations have been made by many; classic efforts include Deines et al. (1974), Wigley et al. (1978), Plummer et al. (1990), ranging to work by Coetsiers and Walraevens (2009). This document outlines field and analytical techniques we have used to acquire the carbon isotopic data from nearly 700 wells in Minnesota. Determinations of the ages or residence times of Minnesota groundwaters are widely used in scientific and management studies all around Minnesota (Alexander and Alexander, 1989). In a typical county atlas about 100 wells are measured for groundwater chemistry, stable isotopes of hydrogen (H) and oxygen (O), and tritium content. A selection of about ten water wells with no measurable tritium are then resampled for the radioactive isotope carbon-14 (14C) and the stable isotopes carbon-13 (13C) and carbon-12 (12C). Three major groups of studies have been conducted in Minnesota. Many original analyses were done as part of research on groundwater age in the Mt. Simon aquifer and were extended with a radium study (Lively et al., 1992) with funding in large part by Legislative Committee on Minnesota Resources (LCMR). At this same time a variety of small-scale studies were conducted in a variety of geologic settings across Minnesota. The United States Geological Survey (USGS) oversaw several projects to define “flow tubes” in selected aquifers across Minnesota (Delin, 1990; Smith and Nemetz, 1996) also with LCMR funding. The Minnesota Department of Natural Resources (DNR) as part of the County Atlas Program and Mt. Simon recharge studies (Berg and Pearson, 2012) has conducted 14C age dating with significant support from the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR) with additional funding from the Clean Water Fund.
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    Constraining above- and belowground uncertainties in tropical montane biogeochemistry
    (2021-08) Looker, Nathaniel
    Sustaining water resources and soil organic carbon (SOC) storage in the face of global change requires understanding how vegetation and soils function across landscapes. Field-based characterization of vegetation and soils is increasingly complemented or substituted by the use of satellite imagery or geospatial products derived from statistical models. This dissertation comprises three studies presenting strategies for drawing inferences on vegetation and soils from field-, satellite-, and model-based sources of information while quantifying associated uncertainties and biases. All studies focused on a mountainous region in central Veracruz, Mexico. The first study evaluated parameter uncertainty in satellite-based analysis of the seasonality, or phenology, of tropical montane vegetation. Phenological parameters and uncertainties were estimated using imagery with high spatial resolution (5 m) but low temporal resolution. The double-logistic phenology model performed well for cloud forest vegetation but poorly characterized the dynamics of other land-cover types, as reflected in large parameter uncertainties. Significant trends were detected in cloud forest phenology across gradients of topoclimate and forest composition. Accounting for parameter uncertainty was critical to the unbiased quantification of these trends. The second study assessed potential improvements in landscape-specific SOC predictions through the integration of regional-to-global statistical models and local soil data. Off-the-shelf models underestimated SOC stocks by a factor of three, on average. Calibration using local soil data included within global databases corrected this linear bias, while calibration using a more representative dataset corrected disproportionate underestimation in SOC storage hotspots. The calibration approach permitted joint prediction of top- and subsoil SOC storage and can accommodate auxiliary field data to reduce prediction uncertainties. The third study quantified bias in SOC stocks and radiocarbon activity due to soil volume change across land-use gradients, using novel and existing approaches to estimate volume change. Ignoring volume change associated with deforestation and grazing inflated SOC stocks and introduced a previously unrecognized negative bias in radiocarbon activity, causing SOC appear to older. Post hoc adjustments for volume change, using the same data required to calculate SOC stocks, may improve confidence in estimates of land-use impacts on SOC dynamics. Collectively, these results underscore the importance of accounting for uncertainty when integrating multiple information sources to characterize the spatial and temporal heterogeneity of vegetation and soils in complex landscapes.