Atlantic Coastal Plain Sediments Research Articles

Characterizing Ground-Motion Amplification by Extensive Flat-Lying Sediments: The Seismic Response of the Eastern U.S. Atlantic Coastal Plain Strata

ABSTRACTWe examine the effects that Atlantic Coastal Plain (ACP) strata have on ground motions in the eastern and southeastern United States. The ACP strata consist of widespread, nearly flat-lying sediments, the upper portions of which are unconsolidated or semiconsolidated. The ACP sediments are deposited primarily on crystalline basement rocks, creating large velocity and density contrasts with the underlying rocks. At 211 sites on ACP strata to thicknesses of 4000 m, we compute spectral ratios relative to the average of four bedrock sites west or northwest of the strata. Sites consist of stations of Earthscope’s USArray Transportable Array (TA), and temporary deployments in the Southeast Suture of the Atlantic Margin Experiment (SESAME), Eastern North American Margin (ENAM) experiment, and the DCShake deployment in Washington, D.C. For the TA and SESAME stations, we use signals from 13 teleseisms and three regional earthquakes as input, combining the north and east components of motion after taking the Fourier transforms. We also include similarly processed site responses from the ENAM and DCShake arrays that were computed in earlier studies. Results show prominent, fundamental resonance peaks at frequencies determined by reverberations in the entire sediment column, and that often define the largest amplifications for each frequency. As frequencies increase, these resonance peaks migrate to thinner ACP strata and increase in amplitude. The peaks are well defined at frequencies below about 1 Hz, but become narrower and less defined regionally at higher frequencies. We develop simple equations to characterize amplification versus ACP thickness, which we approximate by cosine and Gaussian curves with amplifications of 1 on bedrock and rising to the resonance peak, and then decreasing to an average amplification at thicknesses greater than twice the resonance peak. Comparisons with other site corrections for the central and eastern United States based on sediment thickness show similarities on thin ACP strata but divergence on thicker sediments. The results also demonstrate the effectiveness of using teleseismic arrivals to characterize the site responses of sedimentary sequences.

Resonant Frequency Derived from the Rayleigh-Wave Dispersion Image: The High-Impedance Boundary Problem

ABSTRACTWe present a simple and automated approach to estimate primary site-response resonance, layer thickness, and shear-wave velocity directly from a dispersion image for a layer over half-space problem. We demonstrate this for high-impedance boundary conditions that lie in the upper tens of meters. Our approach eliminates the need for time-consuming dispersion curve picking and 1D shear-wave velocity inversion for large data volumes that can capture velocity structure in profile. We highlight important relationships between dispersion characteristics and resonance parameters through synthetic modeling and field data acquired over Atlantic Coastal Plain sediments. In this environment, shallow soil conditions are critical to accurately estimate earthquake site response. We suggest that this image processing approach can be applied to a range of high-impedance conditions, at a range of scales, or can provide model constraints for more complex velocity structures.

Constraining Properties of Sedimentary Strata Using Receiver Functions: An Example from the Atlantic Coastal Plain of the Southeastern United States

ABSTRACTThickness and seismic velocities of sedimentary sequences strongly affect their response during earthquakes, which can prolong and amplify ground motions. We characterize shallow structure of Atlantic Coastal Plain (ACP) sediments using a passive-seismic approach based on high-frequency P-to-S receiver functions. We map the site-specific fundamental frequency for 64 USArray Transportable Array stations and confirm that the method yields results similar to those from traditional spectral ratio techniques, with fundamental frequencies between 0.1 and 1 Hz. In addition, using sediment S-wave reverberations and P-to-S phase arrival times measured directly from the receiver functions, we invert for average S- and P-wave velocity profiles of the ACP sedimentary strata. We find that VS increases with depth following a power-law relationship (VS∝z) whereas the increase of VP with depth is more difficult to constrain using converted wave methods; therefore, we choose to use the Brocher (2005) relationship to obtain VP through a VP/VS relationship. Finally, we use the variation of measured S-reverberation amplitudes with depth to validate these velocity profiles. These results have implications for seismic shaking across the ACP, which covers large portions of the eastern United States.

Amplification of Earthquake Ground Motions in Washington, DC, and Implications for Hazard Assessments in Central and Eastern North America

AbstractThe extent of damage in Washington, DC, from the 2011 Mw 5.8 Mineral, VA, earthquake was surprising for an epicenter 130 km away; U.S. Geological Survey “Did‐You‐Feel‐It” reports suggest that Atlantic Coastal Plain and other unconsolidated sediments amplified ground motions in the city. We measure this amplification relative to bedrock sites using earthquake signals recorded on a temporary seismometer array. The spectral ratios show strong amplification in the 0.7 to 4 Hz frequency range for sites on sediments. This range overlaps with resonant frequencies of buildings in the city as inferred from their heights, suggesting amplification at frequencies to which many buildings are vulnerable to damage. Our results emphasize that local amplification can raise moderate ground motions to damaging levels in stable continental regions, where low attenuation extends shaking levels over wide areas and unconsolidated deposits on crystalline metamorphic or igneous bedrock can result in strong contrasts in near‐surface material properties.

Erosional and depositional history of the Atlantic passive margin as recorded in detrital zircon fission-track ages and lithic detritus in Atlantic Coastal plain sediments

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages >800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

On the Geologic Structure at the Epicenter of the 1886 Charleston, South Carolina, Earthquake

The study focuses on evidence of Cenozoic faulting in the epicentral area of the 1886 Charleston, South Carolina, earthquake and its connection with Mesozoic structure. The seismic data consist of several reflection profiles collected near Summerville, South Carolina, in the period 1975–1983. Reprocessing of the data reveals an extensive early Mesozoic extensional basin, approximately 20 km in width, between Summerville and Charleston. The basin is delineated by the geometry of reflections that image early Mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies. Cenozoic compressional reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary of the basin is marked by a sharp gradient in the magnetic field. Folded Cretaceous and Tertiary Atlantic Coastal Plain sediments in association with diffractions and truncated reflections from the early Mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted. Instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin. Modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km. We propose that the Charleston earthquake occurred due to compressional reactivation of a Mesozoic fault in a localized zone of intense early Mesozoic continental rifting.

Q for P Waves in the Sediments of the Virginia Coastal Plain

The seismic quality factor Q for P waves in Atlantic Coastal Plain sediments is estimated using data from the 2004 U.S. Geological Survey seismic survey in eastern Virginia. The estimates are based on spectral ratios derived from reflections and sediment-guided P waves in Late Cretaceous and Tertiary sediments within the annular trough of the Late Eocene Chesapeake Bay impact structure. The estimates of Q for the frequency range of 10–150 Hz are from 75 to 100, with the best estimate of 80 based on multichannel stacking of spectral ratios from receivers in the offset range of 200–2000 m. This result is approximately a factor of 2 larger than the results previously reported for the Charleston, South Carolina, area, and it is approximately one-half of that recently reported for the Mississippi Embayment.

Spatial and Temporal Variability in Colloid Dispersion as a Function of Groundwater Injection Rate within Atlantic Coastal Plain Sediments

A subsurface injection experiment was conducted on the USDOE's Savannah River Site (SRS) to determine the influence of pump‐and‐treat remediation activities on the generation and transport of groundwater colloids. The impact of colloid generation on formation permeability at injection rates ranging from 19 to 132 L min−1 was monitored using a set of six sampling wells radially spaced at approximate distances of 2.0, 3.0, and 4.5 m from a central injection well. Each sampling well was further divided into three discrete sampling depths that were pumped continuously at a rate of ∼0.1 L min−1 throughout the course of the injection experiment. Discrete samples were collected for turbidity and chemical analysis. Turbidity varied greatly between sampling wells and zones within a given well, ranging from <1 to 740 NTU. The two sampling wells closest to the injection well displayed the greatest response in terms of turbidity to increases in injection rate. Transient spikes in turbidity generally corresponded to incremental increases in the injection rate that were followed by a decrease in turbidity to a stable injection rate–dependent level. Mineralogical analysis of the resulting suspensions confirmed the presence of kaolinite, goethite, and to a much lesser degree, quartz and illite, with many of the particles too large (>1 μm) to be readily mobile within the formation. Turbidity measurements taken during this study indicate that colloid mobilization induced by water injection was both spatially and temporally heterogeneous. Furthermore, colloid release did not follow simple predictions based on shear force, presumably due to the complexities encountered in real heterogeneous systems. These findings have important implications to our understanding of how colloids and the co‐contaminants are mobilized in the subsurface environment, as well as for the development of monitoring practices that minimize the creation of colloidal artifacts. Technical and logistical obstacles encountered in conducting such an extensive field experiment are also discussed.

Chemical Reduction of U(VI) by Fe(II) at the Solid−Water Interface Using Natural and Synthetic Fe(III) Oxides

Abiotic reduction of 0.1 mM U(VI) by Fe(II) in the presence of synthetic iron oxides (biogenic magnetite, goethite, and hematite) and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids. U(VI) was rapidly and extensively (> or = 80%) reduced in the presence of synthetic Fe(III) oxides and highly Fe(II) oxide-enriched (18-35 wt % Fe) Atlantic coastal plain sediments. In contrast, long-term (20-60 d) U(VI) reduction was less than 30% in suspensions of six other natural solids with relatively low Fe(III) oxide content (1-5 wt % Fe). Fe(II) sorption site density was severalfold lower on these natural solids (0.2-1.1 Fe(II) nm(-2)) compared tothe synthetic Fe(lII) oxides (1.6-3.2 Fe(II) nm(-2)), which may explain the poor U(VI) reduction in the natural solid-containing systems. Addition of the reduced form of the electron shuttling compound anthrahydroquinone-2,6-disulfonate (AH2DS; final concentration 2.5 mM) to the natural solid suspensions enhanced the rate and extent of U(VI) reduction, suggesting that AH2DS reduced U(VI) at surface sites where reaction of U(VI) with sorbed Fe(II) was limited. This study demonstrates that abiotic, Fe(II)-driven U(VI) reduction is likely to be less efficient in natural soils and sediments than would be inferred from studies with synthetic Fe(III) oxides.

Ecology, physiology, and phylogeny of deep subsurface Sphingomonas sp.

Several new species of the genus Sphingomonas including S. aromaticivorans, S. stygia, and S. subterranea that have the capacity for degrading a broad range of aromatic compounds including toluene, naphthalene, xylenes, p-cresol, fluorene, biphenyl, and dibenzothiophene, were isolated from deeply-buried (>200 m) sediments of the US Atlantic coastal plain (ACP). In S. aromaticivorans F199, many of the genes involved in the catabolism of these aromatic compounds are encoded on a 184-kb conjugative plasmid; some of the genes involved in aromatic catabolism are plasmid-encoded in the other strains as well. Members of the genus Sphingomonas were common among aerobic heterotrophic bacteria cultured from ACP sediments and have been detected in deep subsurface environments elsewhere. The major source of organic carbon for heterotrophic metabolism in ACP deep aquifers is lignite that originated from plant material buried with the sediments. We speculate that the ability of the subsurface Sphingomonas strains to degrade a wide array of aromatic compounds represents an adaptation for utilization of sedimentary lignite. These and related subsurface Sphingomonas spp may play an important role in the transformation of sedimentary organic carbon in the aerobic and microaerobic regions of the deep aquifers of the ACP.

Induction of aromatic catabolic activity in Sphingomonas aromaticivorans strain F199.

Enzyme induction studies with Sphingomonas aromaticivorans F199 demonstrated that both toluene and naphthalene induced expression of both naphthalene and toluene catabolic enzymes. However, neither aromatic compound induced expression of all the enzymes required for complete mineralization of either naphthalene or toluene. Activity measurements in combination with gene sequence analyses indicate that growth on either aromatic substrate in the absence of the other is, therefore, sub-optimal and is predicted to lead to the build-up of metabolites due to imbalance in toluene or naphthalene catabolic enzyme activities. Growth on toluene may be further inhibited by the co-expression of two toluene catabolic pathways, as predicted from gene sequence analyses. One of these pathways may potentially result in the formation of a dead-end intermediate, possibly benzaldehyde. In contrast, either p-cresol or benzoate can support high levels of growth. Analyses of promoter region sequences on the F199 aromatic catabolic plasmid, pNL1, suggest that additional regulatory events are modulated through the interaction of BphR with Sigma54 type promoters and through the binding of a regulator upstream of p-cresol catabolic genes and xylM. We hypothesize that the unusual gene clustering in strain F199 is optimized for simultaneous degradation of multiple aromatic compound classes, possibly in response to the heterogeneous composition of aromatic structures in the fossil organic matter present in the deep Atlantic Coastal Plain sediments from which this bacterium was isolated.

Bacterial reduction of crystalline Fe (super 3+) oxides in single phase suspensions and subsurface materials

Microbiologic reduction of synthetic and geologic Fe{sup 3+} oxides associated with four Pleistocene-age, Atlantic coastal plain sediments was investigated using a dissimilatory Fe reducing bacterium (Shewanella putrefaciens, strain CN32) in bicarbonate buffer. Experiments investigated whether phosphate and anthraquinone-2,6-disulfonate, (AQDS, a humic acid analogue) influenced the extent of crystalline Fe{sup 3+} oxide bioreduction and whether crystalline Fe{sup 3+} oxides in geologic materials are more or less reducible than comparable synthetic phases. Anaerobic incubations (10{sup 8} organisms/mL) were performed both with and without PO{sub 4} and AQDS that functions as an electron repository and shuttle. The production of Fe{sup 2+} (solid and aqueous) was followed with time, as was mineralogy by X-ray diffraction. The synthetic oxides were reduced in a qualitative trend consistent with their surface area and free energy: hydrous ferric oxide (HFO) > goethite > hematite. Bacterial reduction of the crystalline oxides was incomplete in spite of excess electron donor. Biogenic formation of vivianite [Fe{sub 3}(PO{sub 4}){sub 2}{center_dot}8H{sub 2}O] and siderite (FeCO{sub 3}) was observed; the conditions of their formation was consistent with their solubility. The geologic Fe{sup 3+} oxides showed a large range in reducibility, approaching 100% in some materials. The natural oxides were equally or more reducible thanmore » their synthetic counterparts, in spite of association with non-reducible mineral phases (e.g., kaolinite). The reducibility of the synthetic and geologic oxides was weakly effected by PO{sub 4}, but was accelerated by AQDS. CN32 produced the hydroquinone form of AQDS (AHDS), that, in turn, had thermodynamic power to reduce the Fe{sup 3+} oxides. As a chemical reductant, it could reach physical regions of the oxide not accessible by the organism. Electron microscopy showed that crystallite size was not the primary factor that caused differences in reducibility between natural and synthetic crystalline Fe{sup 3+} oxide phases. Crystalline disorder and microheterogeneities may be more important.« less

Microbial Abundance and Activity in a Low-Conductivity Aquifer System in East-Central Texas

aquifer sands (28‐31 m), even though the total organic carbon content was very low. Rates of anaerobic H2, lactate, and formate consumption were also high in aquifer sands, relative to the other strata. The higher microorganism numbers and activities in the aquifer sediments likely reflect the importance of increased electron donor and acceptor transport in higher hydraulic conductivity sands, relative to other strata. The presence of sulfate and near absence of other electron acceptors (O2 ,N O 3 , and Fe 3+ ), absence of methanogens, high numbers of SRB, and relatively high sulfate reduction activity suggest that sulfate reduction is the dominant terminal electron-accepting process in the deep aquifer. In contrast to aerobic Atlantic Coastal Plain sediments, aquifer sediments at 30 m in this east-central Texas aquifer contain greater numbers of anaerobes than aerobes. Low pH and high sulfate contents argue for oxidation of pyrite in or near the shallow aquifer. This pyrite oxidation supplies sulfate to deeper aquifers. Pyrite from the deep aquifer sediments, with the greatest abundance and activity of SRB, is too enriched in 34 S (up to 58‰) to have formed from modern or typical groundwater sulfate. Overall, our results suggest that the resident microbiota are capable of cycling carbon and sulfur in the aquifer, but probably without substantial remineralization of pyrite in the formation.

Delineating paleochannels using shallow seismic reflection

Discontinuous confining units beneath industrialized areas provide effective pathways for contaminants to move from one aquifer to another within Atlantic Coastal Plain sediments. Shallow seismic reflection can provide coherent high resolution images of thin, alternating sand and clay sequences in many shallow water table environments — useful, cost‐effective information which can significantly enhance environmental investigations.

Taxonomic study of aromatic-degrading bacteria from deep-terrestrial-subsurface sediments and description of Sphingomonas aromaticivorans sp. nov., Sphingomonas subterranea sp. nov., and Sphingomonas stygia sp. nov.

Phylogenetic analyses of 16S rRNA gene sequences by distance matrix and parsimony methods indicated that six strains of bacteria isolated from deep saturated Atlantic coastal plain sediments were closely related to the genus Sphingomonas. Five of the strains clustered with, but were distinct from, Sphingomonas capsulata, whereas the sixth strain was most closely related to Blastobacter natatorius. The five strains that clustered with S. capsulata, all of which could degrade aromatic compounds, were gram-negative, non-spore-forming, non-motile, rod-shaped organisms that produced small, yellow colonies on complex media. Their G + C contents ranged from 60.0 to 65.4 mol%, and the predominant isoprenoid quinone was ubiquinone Q-10. All of the strains were aerobic and catalase positive. Indole, urease, and arginine dihydrolase were not produced. Gelatin was not liquified, and glucose was not fermented. Sphingolipids were present in all strains; 2OH14:0 was the major hydroxy fatty acid, and 18:1 was a major constituent of cellular lipids. Acid was produced oxidatively from pentoses, hexoses, and disaccharides, but not from polyalcohols and indole. All of these characteristics indicate that the five aromatic-degrading strains should be placed in the genus Sphingomonas as currently defined. Phylogenetic analysis of 16S rRNA gene sequences, DNA-DNA reassociation values, BOX-PCR genomic fingerprinting, differences in cellular lipid composition, and differences in physiological traits all indicated that the five strains represent three previously undescribed Sphingomonas species. Therefore, we propose the following new species: Sphingomonas aromaticivorans (type strain, SMCC F199), Sphingomonas subterranea (type strain, SMCC B0478), and Sphingomonas stygia (type strain, SMCC B0712).

Microbial acetogenesis as a source of organic acids in ancient Atlantic Coastal Plain sediments

Research Article| October 01, 1996 Microbial acetogenesis as a source of organic acids in ancient Atlantic Coastal Plain sediments Francis H. Chapelle; Francis H. Chapelle 1U.S. Geological Survey, 720 Gracern Road, Suite 129, Columbia, South Carolina 29210 Search for other works by this author on: GSW Google Scholar Paul M. Bradley Paul M. Bradley 1U.S. Geological Survey, 720 Gracern Road, Suite 129, Columbia, South Carolina 29210 Search for other works by this author on: GSW Google Scholar Author and Article Information Francis H. Chapelle 1U.S. Geological Survey, 720 Gracern Road, Suite 129, Columbia, South Carolina 29210 Paul M. Bradley 1U.S. Geological Survey, 720 Gracern Road, Suite 129, Columbia, South Carolina 29210 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (10): 925–928. https://doi.org/10.1130/0091-7613(1996)024<0925:MAAASO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Francis H. Chapelle, Paul M. Bradley; Microbial acetogenesis as a source of organic acids in ancient Atlantic Coastal Plain sediments. Geology 1996;; 24 (10): 925–928. doi: https://doi.org/10.1130/0091-7613(1996)024<0925:MAAASO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Field and laboratory evidence shows that deeply buried (90–888 m) fine-grained sediments of the Atlantic Coastal Plain contain viable acetogenic microorganisms, and that these microorganisms actively produce organic acids. Concentrations of formate, acetate, and propionate in pore waters extracted from fine-grained sediments ranged from 50 μM to 5 mM and were much higher than in adjacent pore waters associated with sandy sediments (<2 μM). Laboratory studies showed that asceptically cored fine-grained sediments incubated under a H2 atmosphere produced formate and acetate, and that H14CO3− was converted to 14C-acetate and 14C-formate over time. An enrichment culture of these acetogenic microorganisms was recovered from one long-term incubation that showed the presence of several morphologically distinct gram-positive, rod-shaped bacteria. These microorganisms were capable of growth under autotrophic (H2 + CO2), heterotrophic (syringate), and mixotrophic (H2 + CO2 + syringate) conditions. These results suggest that microbial acetogenesis, rather than abiotic processes, is the most important organic acid-producing mechanism during low-temperature (∼ 30 °C) diagenesis of Atlantic Coastal Plain sediments. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dating of shallow groundwater: Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr

This paper describes a direct comparison of apparent ages derived from 3H/3He, chlorofluorocarbons (CCl3F and CCl2F2), and 85Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved‐gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption‐desorption processes at this location.

Factors influencing the abundance and metabolic capacities of microorganisms in Eastern Coastal Plain sediments

The abundance and metabolic capacities of microorganisms residing in 49 sediment samples from 4 boreholes in Atlantic Coastal Plain sediments were examined. Radiolabeled time-course experiments assessing in situ mirobial capacities were initiated within 30 min of core recovery. Acetate (1-(14)C- and(3)H-) incorporation into lipids, microbial colony forming units, and nutrient limitations were examined in aliquots of subsurface sediments. Water-saturated sands exhibited activity and numbers of viable microorganisms that were orders of magnitude greater than those of the low permeability dense clays. Increased radioisotope utilization rates were observed after 6-24-h incubation times when sediments were amended with additional water and/or nutrients. Supplements of water, phosphate, nitrate, sulfate, glucose, or minerals resulted in the stimulation of microbial activities, as evidenced by the rate of acetate incorporation into microbial lipids. Additions of water or phosphate resulted in the greatest stimulation of microbial activities. Regardless of depth, sediments that contained >20% clay particles exhibited lower activities and biomass densities, and greater stimulation with abundant water supplementation than did sediments containing >66% sands and hydraulic conductivities > 200 μm sec.(-1).

The influence of microbial activity and sedimentary organic carbon on the isotope geochemistry of the Middendorf Aquifer

Microorganisms present in deep Atlantic coastal plain sediments affect the geochemical evolution of groundwater and its chemical and isotopic composition, yet the factors controlling their origin, distribution, and diversity are poorly understood. The evolution of the groundwater chemistry, the fractionation of stable carbon isotopes, and the groundwater age are all indicators of the inorganic and microbial reactions occurring along a given flow path from groundwater recharge to groundwater discharge. In this study, tritium, 14C, and groundwater chemistry along three flow paths of the Middendorf aquifer in South Carolina were analyzed. The 14C ranged from 89 percent modern carbon (pmC) in the recharge zone to 9.9 pmC in the distal borehole; the δ13C remained relatively constant at ∼−22‰, suggesting microbial oxidation of organic carbon. Carbon isotope analyses of particulate organic carbon from core sediments and groundwater chemistry were used to model the carbon chemistry; the groundwater ages obtained from 14C ranged from modern to 11,500 years B.P. The highest frequencies of occurrence, numbers, and diversity of aerobic and anaerobic bacteria were found in boreholes near the recharge zone where the calculated ages were &lt;1000 years B.P. The transport of microorganisms from the recharge zone may be responsible for this distribution as well as the electron acceptors necessary to support this diverse community of bacteria. The presence of both aerobic heterotrophs and anaerobic sulfate‐ and iron‐reducing bacteria in the core sediments suggested the occurrence of anaerobic microsites throughout this otherwise aerobic aquifer. The highest in situ microbial respiration rate, as determined by modeling, was found along a flow path near the recharge area. It is likely that the electron acceptors necessary for supporting a diverse microbial community are depleted by the time the groundwater residence time in the Middendorf aquifer exceeds several hundred years.

The Influence of the Coastal Plain Sedimentary Wedge on Strong Ground Motions from the 1886 Charleston, South Carolina, Earthquake

The effect of Atlantic Coastal Plain sediments on ground motion from the 1886 Charleston, South Carolina, earthquake was studied using linear regression analysis and ground motion modeling. Statistical tests applied to 264 Modified Mercalli intensity reports at epicentral distances less than 400 km showed that intensities within the Lower Coastal Plain (near the coast) averaged from 0.5 to 1.0 units less than intensities within other areas over comparable distances. Simulated ground acceleration time histories were generated along two profiles: one trending northeast from the epicenter along the coast and another trending northwest toward the Fall Line. The modeling results (which assume linear behavior) indicate that the thick coastal sediments attenuate high frequency motions (&gt;3-10 Hz), while amplifying lower frequency motions. In contrast, locations on thinner sediments to the northwest experience amplification of high frequency motions. The overall effect is that peak acceleration decays more slowly with distance in the thinning sediments to the northwest of the epicenter than in the thick sediments along the coast.