Gregg R. Davidson

The stable isotopic composition and measurement of carbon in soil CO2

Abstract The theoretical minimum difference between the δ 13 C of soil CO 2 (a concentration) and soil-respired CO 2 (a flux) was previously thought to be 4.4‰. It is shown here that the difference is a function of the δ 13 C of soil-respired CO 2 , with possible minimum values less than 4.4‰. A simple equation is given which allows the δ 13 C of soil CO 2 or soil-respired CO 2 to be calculated from four measurable variables. Sampling methodology is investigated demonstrating the need for correcting measured soil-respired CO 2 samples for CO 2 contamination in base reagents, for contributions from atmospheric CO 2 , and for isotopic fractionation at the base-solution/air interface.

The evolution of an oxbow lake in the Mississippi alluvial floodplain

The history of sediment accumulation in an oxbow lake located on the Mississippi alluvial floodplain was reconstructed based on sedimentation rates determined using 14C activities from bulk sediment fractions and from 210Pb and 137Cs measurements. Higher rates of sediment accumulation consistent with frequent flooding when first abandoned 3,800 to 5,000 years before the present were followed by slower sedimentation rates consistent with migration of the Mississippi river away from the oxbow and less frequent flooding. This low sedimentation rate persisted for several thousand years until the surrounding land was cleared for agricultural use in the late 19th century. A 50-fold increase in the rate of sediment accumulation has persisted from the time of land clearing to the present, doubling the total mass of accumulated sediment in a single century.

Geochemical evidence of preferential flow of water through fractures in unsaturated tuff, Apache Leap, Arizona

Abstract The occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the Apache Leap Research Site near Superior, Arizona. Water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel, from the saturated zone in a vertical borehole (USW UZP-4), and from both the unsaturated and saturated zones in an angled borehole (DSB). The geochemistry and14C activity of water samples from the DSB suggest that most of the recharge to the saturated zone has occurred through fractures, especially beneath the ephemeral streams. Evidence of substantial recent recharge through fractures was found in saturated-zone samples from the mine haulage tunnel using 3H, δ34S and SO42−/Cl− analyses. Evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the DSB using geophysical measurements from the borehole, water-content analyses from core samples, and 14C and 3H analyses from pore water extracted from preserved core samples. Post-bomb 14C activity was measured in pore water near fractures just above the saturated zone.

Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agricultural use: A 210Pb, 137Cs, and 14C study in Mississippi, USA

Sediment cores were collected from 2 sites in the forested fringe of an oxbow lake surrounded by land that was converted from forest to agricultural use in the late 19th century. The 2 sampling areas were selected to represent areas of high (West site) and low (east site) current sediment accumulation rates, based on distance from a perennially discharging stream. Modern (post settlement and land clearing) sediment accumulation rates were calculated using (super 210) Pb and (super 137) Cs on bulk sediment samples from 2 cores from each site. Two additional cores were collected from each site for radiocarbon analysis of twig cellulose with the assumption that most twigs in the sediment within the forested fringe fell from overhead and are contemporaneous with the sediment. Only the West site, however, yielded sufficient identifiable twig material for analysis. Modern sediment accumulation rates based on (super 210) P and (super 137) Cs fall between 0.2-0.4 cm/yr at the East site, and 0.7-1.3 cm/yr at the West site (nearest the stream inlet), with approximate agreement between the (super 210) Pb and (super 137) Cs methods. Modern sediment accumulation rate based on bomb-pulse (super 14) C activity of twigs from cores from the West site is approximately 1.0 cm/yr, in agreement with the (super 210) Pb and (super 137) Cs results results. Historic sediment accumulation rates were estimated at the West site using twigs from deeper intervals with pre-bomb 14C activity. Sediment covering approximately 1000 yr of pre-settlement sediment accumulation exhibited evidence of minor bioturbation or in-washing of reworked material, but with a clearly lower accumulation rate of less than 0.1 cm/yr.

Root-Zone Hydrology: Why Bald Cypress in Flooded Wetlands Grow More When it Rains

Bald cypress (Taxodium distichum) is known to respond to increases in precipitation with increased radial growth even when rooted in continuously saturated sediments where water is not a growth-limiting factor. Measurements of δ18O, Cl−, 3H and hydraulic head in surface water and shallow groundwater in an oxbow lake-wetland in northern Mississippi show that rapid downward flow of surface water into the root zone is initiated only after precipitation-induced increases in surface water depth exceed a threshold value. Rapid flow of surface water through the root zone has the potential to introduce oxygen to sediments that would otherwise be anoxic, facilitating nutrient uptake and growth. Climatic reconstruction using tree rings from bald cypress in this environment appears possible because increases in precipitation generally correlate well with increases in water level, which in turn enhances the delivery of oxygenated water to the roots.

Hydrologic evolution of the Edwards Aquifer recharge zone (Balcones fault zone) as recorded in the DNA of eyeless Cicurina cave spiders, south-central Texas

The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modified by karst processes influenced by structural and stratigraphic controls. While the modern confined Edwards Aquifer flows through cavernous voids at the base of the escarpment, air-filled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed.

Simultaneous determination of mercury and organic carbon in sediment and soils using a direct mercury analyzer based on thermal decomposition–atomic absorption spectrophotometry

The purpose of this work was to study the feasibility of using a direct mercury analyzer (DMA) to simultaneously determine mercury (Hg) and organic matter content in sediment and soils. Organic carbon was estimated by re-weighing the sample boats post analysis to obtain loss-on-ignition (LOI) data. The DMA-LOI results were statistically similar (p<0.05) to the conventional muffle furnace approach. A regression equation was developed to convert DMA-LOI data to total organic carbon (TOC), which varied between 0.2% and 13.0%. Thus, mercury analyzers based on combustion can provide accurate estimates of organic carbon content in non-calcareous sediment and soils; however, weight gain from moisture (post-analysis), measurement uncertainty, and sample representativeness should all be taken into account. Sediment cores from seasonal wetland and open water areas from six oxbow lakes in the Mississippi River alluvial flood plain were analyzed. Wetland sediments generally had higher levels of Hg than open water areas owing to a greater fraction of fine particles and higher levels of organic matter. Annual loading of Hg in open water areas was estimated at 4.3, 13.4, 19.2, 20.7, 129, and 135 ng cm(-2) yr(-1) for Beasley, Roundaway, Hampton, Washington, Wolf and Sky Lakes, respectively. Generally, the interval with the highest Hg flux was dated to the 1960s and 1970s.

Using lake sedimentation rates to quantify the effectiveness of erosion control in watersheds

The effectiveness of erosion control methods is difficult to measure, hampering the development of management practices and preventing accurate assessment of the value of erosion control structures over time. Surface erosion can vary widely over an area, particularly if gully erosion is present, and the use of sediments transported in streams for quantifying erosion is hindered by the highly variable nature of fluvial sediment loads. When a watershed drains into a lake, accumulated sediments have the potential to yield information about historic rates of sedimentation that can be used to evaluate the effectiveness of previous erosion control measures. In the present study, sediments from five natural oxbow cutoff lakes in the Mississippi River alluvial floodplain were dated using lead-210 decay rates and bomb-pulse derived cesium-137 with the goal of relating trends in sedimentation rate to reductions in erosion due to management practices. It was found that the radioisotope dating methods were best used in concert with known dates for implementation of management practices. Changes in sedimentation rate over time frames as short as 12 years were detectable. Larger lakes showed smaller changes in sedimentation rate relative to smaller lakes.

ACCURATE LACUSTRINE AND WETLAND SEDIMENT ACCUMULATION RATES DETERMINED FROM 14C ACTIVITY OF BULK SEDIMENT FRACTIONS

In the absence of identifiable macrofossils in lacustrine sediments, radiocarbon dating must rely on pollen or bulk sediment fractions. Bulk sediment fractions are not generally preferred because they contain an unknown mixture of organic material of variable age, they may contain dead carbon such as lignite that is difficult to eliminate, and material of aquatic origin may be subject to reservoir effects. If the various processes that contribute carbon to the system are relatively constant over time, however, changes in 14C activity with depth may be used to accurately estimate sediment accumulation rates even if the absolute ages are erroneous. In this study, fine-grained fractions (250710 m organic material, humic acids extracted from <250-m fraction, and untreated <250-m fraction combusted at low temperature) were analyzed and compared with terrestrial plant stems (twigs), charcoal, and wood fragments in sediments from an oxbow lake in Mississippi, USA. The 14C activities of the bulk fractions were highly linear with depth and produced consistent calculated sediment accumulation rates similar to, and perhaps more reliable than, rates determined using twigs or charcoal.

Extraction of 14C from pore water in unsaturated rock using vacuum distillation

We discuss a vacuum-distillation method for obtaining representative (super 14) C samples from dissolved inorganic carbon in rock pore-waters. Distillation offers four advantages over current centrifugation and compression methods for obtaining pore-water carbon: 1) carbon recovery is possible from rocks that will not yield water by centrifugation or compression; 2) the mass required for (super 14) C analysis can typically be obtained in a single extraction, eliminating the need for storing and combining multiple pore-water collections; 3) water and carbon are extracted and isolated simultaneously, reducing the number of required steps and the potential for contamination; and 4) distillation requires less equipment at lower cost than centrifugation or compression. In this study, isotopic fractionation resulting from incomplete recovery of carbon during distillation was too high for stable isotope applications, but was relatively minor for (super 14) C applications. The lighter isotopes were favored in the recovered phase, resulting in samples depleted in (super 14) C by a maximum of 4%. Mass balance calculations indicate that there may be a significant reservoir of carbon absorbed to mineral surfaces that is only partially removed by this method. Incorporation of adsorbed carbon into the recovered sample did not measurably alter the (super 14) C activity.