Mean Rate Of Deposition Research Articles

Organic carbon proxies in black shales: molybdenum

Isotopic and elemental proxies are useful for discerning the original compositions of ancient rocks subject to later diagenetic/thermal alteration, low-rank metamorphism, outcrop weathering, etc. Recent work in the Cariaco Basin [Chem. Geol. 195 (2003) 131] has shown a high correlation between total organic carbon (TOC) content and Mo normalized to Al in these modern euxinic sediments: microlaminated, dark olive gray, silty clay (0–11.6 kyr BP), %TOC=1486∗(Mo/Al)+2.8, n=13, r 2=0.52, mean rate of deposition=36 cm/kyr; distinctly microlaminated, dark olive gray, clayey mud (11.6–14.5 kyr BP), %TOC=1622∗(Mo/Al)+0.22, n=15, r 2=0.89, mean rate of deposition=79 cm/kyr. Here, we use these relationships to estimate the original TOC contents of ancient black shales with overall characteristics similar to those of the modern Cariaco sediments. These “Group IV” black shales as defined by Quinby-Hunt and Wilde [Econ. Geol. 91 (1996) 4] are characterized by relatively high concentrations of V, Mo and Co but low Mn contents. The Cariaco regressions and those from the Carboniferous of Iowa and the Devonian of New York were used to estimate the ‘original’ TOC contents for Lower Ordovician black shales of the Baltica and Avalonia plates, where C org values were not taken. For individual samples, the Carboniferous regression produced TOC values approximately double that derived from the regression equation of the Cariaco Basin lower anoxic zone. Such variations among the results from the four regressions suggest that there is no universal proxy for TOC using Mo/Al. These calculated TOC values, however, are consistent with the higher levels of primary production predicted from the paleogeographic settings of these intervals. In general, the Mo proxy for original TOC content, while approximate, works for oxygen-deficient sites of deposition where other proxies for C loss, such as C org/S py ratios in normal (oxic) marine shales, do not apply. Estimates of original TOC from Mo content in samples spanning the geologic record, combined with paleogeography and paleoecology, should be useful in estimating pathways of C synthesis and remineralization in ancient oceans and initial hydrocarbon potential of petroleum source rocks.

Sediment and trace element trapping in a forested wetland, Chickahominy River, Virginia

The Chickahominy River, arising near Richmond, Virginia, flows southeast toward Newport News, which impounds the river for much of its water supply. Much of the bottomland between the two cities is flooded for extended periods annually. Sediment-deposition rates estimated from tree rings were used in conjunction with multi-element analyses of sediments and of selected growth rings from oak trees to estimate amounts of trapped sediment and trace elements. Mean rates of deposition at eight study sites range from 0.7 to 5.7 mm/yr and are related to stream gradient, stream power, percent wetland, hydroperiod, and land use. Deposition rates are highest downstream from the confluence of upper basin tributaries near Richmond, where stream power is low and there is a high percentage of emergent/shrub-scrub wetlands; rates decrease, along downstream reaches toward the Chickahominy reservoir. Tree-ring data suggest that mean sedimentation rates were greater during the last 50 years than during the previous 3-year period, possibly because of urban expansion in the upper basin. Sites nearest the urban area have the highest rates of sedimentation and the highest concentrations of most trace elements in sediments. Trace elements concentrated in sediment include zinc, lead, chromium, copper, nickel, tin, and cadmium. Concentrations in tree rings of zinc, copper, nickel, and lead were generally proportional to those in sediment at a site, and some inter-site correlations were also observed. Unusually high concentrations of zinc were detected in some tree rings, including some that formed before 1950. Concentrations of zinc and lead in the most recently formed rings of those trees suggest that sediment concentrations of those elements may have declined relative to earlier periods. The trapping of substantial amounts of sediment and trace elements by these forested wetlands demonstrates their importance in the maintenance of water-quality.

Uranium-series dating of the draining of an aquifer: The example of Wind Cave, Black Hills, South Dakota

Research Article| February 01, 1993 Uranium-series dating of the draining of an aquifer: The example of Wind Cave, Black Hills, South Dakota D. C. FORD; D. C. FORD 1Department of Geography, McMaster University, Hamilton, Ontario, Canada L8S 4K1 Search for other works by this author on: GSW Google Scholar J. LUNDBERG; J. LUNDBERG 1Department of Geography, McMaster University, Hamilton, Ontario, Canada L8S 4K1 Search for other works by this author on: GSW Google Scholar A. N. PALMER; A. N. PALMER 2Department of Earth Sciences, State University College, Oneonta, New York, New York 13820-4015 Search for other works by this author on: GSW Google Scholar M. V. PALMER; M. V. PALMER 2Department of Earth Sciences, State University College, Oneonta, New York, New York 13820-4015 Search for other works by this author on: GSW Google Scholar W. DREYBRODT; W. DREYBRODT 3Institut für Experimentelle Physik, Universitädt Bremen, D-2800 Bremen 33, Germany Search for other works by this author on: GSW Google Scholar H. P. SCHWARCZ H. P. SCHWARCZ 4Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (2): 241–250. https://doi.org/10.1130/0016-7606(1993)105<0241:USDOTD>2.3.CO;2 Article history first online: 01 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 D. C. FORD, J. LUNDBERG, A. N. PALMER, M. V. PALMER, W. DREYBRODT, H. P. SCHWARCZ; Uranium-series dating of the draining of an aquifer: The example of Wind Cave, Black Hills, South Dakota. GSA Bulletin 1993;; 105 (2): 241–250. doi: https://doi.org/10.1130/0016-7606(1993)105<0241:USDOTD>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 SocietyGSA Bulletin Search Advanced Search Abstract Wind Cave, South Dakota, is a three-dimensional, rectilinear maze cave developed in 90 m of Mississippian limestone and dolomite that is overlain by thick clastic cover strata. Beneath an area of 1.8 km2, 87 km of small, drained, phreatic passages are known. Exploration terminates at a water table 160 m below the highest parts of the cave. As the cave was draining, calcite precipitated from its waters.Uranium-series measurements (85) were taken on fragments of broken calcite crusts collected in the lower 93 m of the cave. The measurements reveal that it has been draining as a backwater for at least the past 300,000 yr. 234U/238U ratios in the water were remarkably constant, allowing estimation of ages beyond 400 ky B.P.After deducting glacial-interglacial cyclic oscillations (of 10- to 20-m amplitude), the water table appears to have been lowered at a steady rate of 0.4 m per 103 yr since ≥400 ky B.P. Most of this fall is attributed to the increase of hydraulic conductivity in the ~260 m of cover strata that separates the carbonate aquifer from its spring discharge points. A subaqueous calcite deposition zone extended to ~70 m below the contemporary water table at all times, where mean rates of deposition were 0.12 mm per 103 yr or less. Modeling of the precipitation kinetics suggests a well-mixed water body, with a Ca2+ concentration close to the thermodynamic equilibrium value and variations of depositional rate being controlled by hydrostatic pressure. 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.

RECORDING DENSITOMETRY OF X‐RADIOGRAPHS FOR THE STUDY OF CRYPTIC LAMINATIONS IN THE SEDIMENT OF LAKE WASHINGTON

X‐radiographs of cores of sediment from Lake Washington reveal prominent laminations not visible to the eye. Measurement of the film density with a recording densitometer provides an exact and objective record of these laminations. The strongest band, at a depth of about 16 cm in the central part of the lake, is attributed to the lowering of the lake level in 1916. The mean rate of deposition therefore has been about 3.1 mm/yr.The most prominent bands more recent than 1916 are too few to be annual, but the fine structure of these bands shown by the densitometer record may result from annual variations in deposition.The film density is strongly correlated with the dry weight and mineral residue of the sediment.

STUDIES ON SHELL FORMATION. I. MEASUREMENT OF THE RATE OF SHELL FORMATION USING Ca45 ,

1. The rate of shell formation in the oyster Crassostrea virginica Gmelin has been determined by measurements of Ca45 deposited by oysters placed in sea water containing Ca45. The experiments were carried out at Beaufort, N. C., during May and June at 21.2 to 25.4° C. Radioactivity measurements were made directly on the inner shell surface.2. The mean amount of deposition was directly proportional to the time of exposure to Ca45 for 8, 16 and 24 hours. Growth increments occurring in four hours were measurable. Individual variation in the rate of deposition was marked.3. The mean rate of deposition in the posterior region of the shell was approximately twice that in the center and anterior regions. Within a general region local areas may exhibit a still greater difference in rate. From the average radioactivity per unit area of shell surface the total amount of CaCO3 deposited in a given period can be calculated.4. Shell from which all tissue had been removed became increasingly radioactive in sea water co...

RADIOACTIVE ISOTOPES IN THE STUDY OF PERIPHERAL VASCULAR DISEASE

EVALUATION of the peripheral circulation can be accomplished with the use of radioactive isotopes. 1 Determination of a circulatory index depends on the rate of deposition of radioactive phosphorus (P 32 as a phosphate ion, in either phosphoric acid [H 3 PO 4 ] or sodium phosphate [Na 2 HPO 4 ]) in the tissues of an extremity; the index is then derived from a semilogarithmic plotting of the minute by minute increase in radioactivity in a selected site in the extremity (usually the sole of the foot). For statistical purposes, the reciprocal of the rate of deposition is used to calculate this index, according to the equations where C 20 is the theoretic number of counts at the end of twenty minutes and C 2 is the theoretic number of counts at the end of two minutes. In a series of normal persons, the mean rate of deposition of radioactive phosphorus in the tissues was actually less than in