Debra Willard
Kotra, R. K.; Holmes, C. W.; Orem, W. H.; Hageman, P. L.; Briggs, P. H.; Meier, A. L.; Brown, Z. A.
Long cores (>1 m in length), were taken in >30 cm of water through the "moon pool" of a motorized 25 ft pontoon barge either grounded on the bank or anchored with 4 anchors. The location was established by GPS. A portable 12 ft high tripod was placed over the moon pool to hold the coring piston and for extraction of the core. Cores were taken with 10.8 cm-diameter, clear, FDA food grade polycarbonate tubing. A PVC piston with two O-ring seals was used. The piston was pushed into the bottom of the core tube to a position several cm above the bottom of the tube. A 10 m length of .25 cm diameter braided polypropylene line, attached to a ring threaded into the top of the piston, was pulled through the core tube. The core tube was then carefully lowered through the moon pool and any air trapped in the space between piston and the bottom of the core tube was removed and filled with water. When the tube was several cm above the bottom the free end of line attached to the piston was affixed to the head of the tripod. Thus when the core tube penetrates the sediment the piston remains in a fixed position a few cm above the sediment surface producing a vacuum that retards compaction. When the apparatus was set in positions the core tube was quickly thrust about 30 cm into the sediment by hand. Next, an aluminum clamp with handles was fixed to the core tube and two people then forced the core barrel in until it reached the underlying Pleistocene limestone. Next the clamp with handles was removed and a clamp with lifting rings was attached near the air/water interface. A cable through a pulley at the top of the tripod and attached to a hand winch mounted on one leg of the tripod was attached to the lifting ring on the clamp. The cable winch was necessary for extraction due to the weight of the core and strong suction created by the mud. Before extraction, the polypropylene line was removed from the tripod and fixed to the extraction clamp. This was to ensure that the piston remained in a fixed position during extraction and to retard loss of sediment from the bottom of the barrel. A benefit of the clear core tubing was that any leakage around the piston's O-ring seal could be readily observed during the extraction process. A person in the water wearing a face mask observed the core tube to be certain no sediment was lost at the sediment/water interface, to record any elevation difference between the sediment surface inside and outside the core at full penetration, and to quickly place a plastic pipe cap on the bottom of the core as it emerged from the sediment. The core was winched above the surface of the moon pool and lowered to rest vertically on deck. The pipe cap was taped tightly to the tube to prevent leakage. Excess tubing was cut off just below the piston using a large pipe cutter so the piston could be carefully removed without disturbing the sediment surface. In some cases the water above the sediment was carefully siphoned off with a tube to prevent sloshing of water during transport to the laboratory which would disturb the sediment surface. In other cases the piston was left in the core tube during transport. All cores were transported vertically. Each evening cores were transported vertically to a local hospital where X-ray photographs were prepared.
The cores were selected for analysis based on the x-rays. Those cores selected for further analysis were selected on the basis of laminations or other features which indicated the lack of disturbance. The core was placed in an extruding device vertically. The core was then extruded up into a template and sliced. This slice (hockey puck) was place on a prewieghed titanium plate and the wet weight determined. The ring was then removed and the slice was trimmed to remove the outer portion of the core. This was done to prevent any contamination that may have occurred at the side of the core barrel during the coring operation. This sample was then bagged and weighed. This weight was found to be important in the determination of water content and thus the dry weigh as water was lost during the period of initial sampling and the laboratory analysis. These sample were then stored in a refrigerator and then transhipped to the home based laboratory. For those core selected for trace metal analysis, the slices were sampled from the center of the "hockey puck" with titanium tools and place in an acid washed plastic bottle and frozen.
Major Sediment Characteristics
In the laboratory, the bagged samples were first weighed to determine the water loss. A wet aliquot was placed in pre-cleaned pre-weighed porcelain evaporating dish, dried at 40° C, cooled and re-weighed to determine water loss. This sample was dissolved in 6NHCl. The filtrate was washed three times with distilled water and dried. The residue was reweighed and reported as percent insoluble residue.
A separate aliquot (ca. 20 g wet weight) was weighed then sieved (62 micron mesh) using distilled water to obtain the amount of fine sediment in each interval. These samples were freeze-dried. Portions of dried samples were ground to a fine powder (75-100 mesh) in a grinding mill to obtain a homogeneous sample for analysis. Sample splits were made and 5 g placed into pre-cleaned, pre-weighed crucibles and heated in a muffle furnace at 450° for 6 hours so as to obtain a stable weight. The samples were then cooled and reweighed to determine loss on ignition. The remain fraction was submitted for 210Pb analysis.
Total extracted lead-210
The analysis of sediments at the USGS-Denver lab is based on counting of Po-210, in secular equilibrium with its parent Pb-210, and exploits the ability of polonium to self-plate onto silver and certain other metals. Five grams of the >62micron dried, ground sample was transferred to a 100 ml Teflon beaker and mixed with 5-10 ml of reagent grade 16N nitric acid. An amount of NIST-calibrated Po-209 spike was added and the sample swirled to mix the spike. The beaker was covered with a watch glass and allowed to stand overnight. The solution with solids was allowed to evaporate under heat lamps at 90°C. The sample was washed from the sides of the beaker using 8N Hydrochloric acid and swirled again to insure proper mixing. The solution is evaporated again and allowed to cool. One ml of 30% hydrogen peroxide was then added to the sample and the resulting mixture evaporated to dryness. These latter steps, adding peroxide and drying the mixture were repeated an additional two times. The 8 HCl was added to the sample and allowed to dry out. This step was repeated so as to ensure that nitric acid, which hinders efficient plating is virtually eliminated from the mixture. Finally 5ml of 8 HCl were added to the dried sample and the mixture transferred to a 100ml beaker using additional amounts of de-ionized water to insure essentially complete transfer. To minimize interference of several ions with plating, 5ml of hydroxylamine hydrochloride and 2ml of 25% sodium citrate were added to each sample. Additionally, 1ml of a hold back carrier, bismuth nitrate, was added to prevent deposition of Bi-212. A plastic coated magnetic bar was added to each beaker for stirring during autoplating. The pH of the solution was adjusted to between 1.85 and 1.95 using ammonium hydroxide. The beaker was placed on a hot plate and heated between 85 and 90°C for 5 minutes to reduce iron, chromium and oxidants present. Then a Teflon holder which exposes one side of a silver foil disc was placed in the solution for a minimum of 90 minutes. Rinsed and air-dried silver discs were then counted for polonium isotopes by alpha spectroscopy. Combined analytical and counting errors in determining lead-210 values are about 3%.
For purposes of intercalibration, fifteen samples of sieved sediment (from Bob Allen Core 6C) were analyzed at the GLERL laboratory for lead-210 by counting its decay product, polonium-210. Two gram samples were placed in beakers, 1 ml of calibrated polonium-209 standard solution added to each followed by 30 ml of concentrated HCl transferred slowly to avoid excessive foaming. After subsidence of foaming (in 24 hrs) the mixtures were placed in centrifuge tubes and allowed to stand for two weeks at room temperature. The mixture was then centrifuged and the supernatant decanted. The residue was rinsed, re-centrifuged and the second aliquot of supernatant added to the first. The above procedure was then repeated on the rinsed residue with the additional step of first slowly adding 10 ml of 30% hydrogen peroxide to oxidize organic matter and presumably release any bound lead-210 into the extractant. To both sets of supernatant solutions, one gram of NH2OH·HCl was added as in the Denver method to reduce the interference of dissolved iron with polonium plating and the solutions brought to a volume of 100 ml with distilled water and adjusted to a pH of 1.5 via small additions of NH4OH. Polonium isotopes were self-plated onto polished, Mylar-backed copper disks placed in the solutions (at 85°C) and gently agitated for eight hours. Exposed disks were alcohol-rinsed and counted by alpha spectrometers consisting of surface barrier detectors coupled to a multichannel analyzer. The Polonium-209 solution was calibrated by comparison with standard solutions of polonium-208 (NIST SRM 4327) and with polonium-210 in secular equilibrium with lead-210 in an NIST-traceable standard solution obtained from the U. S. Environmental Protection Agency (Environmental Monitoring systems, Las Vegas, Nevada). The activity of the polonium-208 solution has an overall uncertainty of 1.4 percent. The activity of the polonium-209 solution is consistently determined by the two independent methods and presently known with an overall uncertainty of about 2.5%. Uncertainty due to random errors decreases in time because the activity of the solution is re-checked every few years by intercomparison with new NIST-traceable aliquots of polonium-208. Overall uncertainty in counting were about 2%.
Total lead-210 and radium-226
A larger aliquot of wet sediment was washed through a 62 micron stainless steel sieve using distilled water. The fine fraction was allowed to settle, overlying water decanted and the final slurry was freeze-dried and disaggregated. At USGS-Woods Hole, 10-50 grams of the freeze-dried fraction were transferred to a plastic counting jar with a tight sealing plastic screw cap. Sealed counting jars were stored for at least 20 days to allow for the in-growth of radium-222 and lead-214 to approximate equilibrium values.
Samples were counted on a gamma detector system consisting of a germanium detector for low energy gamma rays (2000 mm2 area) and a 4096 channel multichannel analyzer. Samples were typically counted for 48 hours (depending on sample size) or until counting errors were <5%. The system has been calibrated using EPA standard pitchblende ore in the same geometry as the samples. A self-absorption correction has been applied to each sample. Results of the lead-214 analysis (for radium-226) of the Canadian ore standard DH-1a were within 3.5% of the published value. Total lead-210 results on the same standard were 1952 ± 14 dpm/g (n=2) compared with the published value of 1848 ± 54 dpm/g, a 5.6 % difference.
At the GLERL laboratory radium-226 was determined on 4.00 ± 0.02 g of dry, desegregated sediment less than 62 microns. Samples were packed into counting vials to a constant height 4.0 ± 0.05 cm to assure a fixed counting geometry and packing density. Vials were sealed with epoxy cement and stored for at least three weeks to allow for in-growth of radon-222 and its gamma-emitting progeny. Samples were placed in an intrinsic (HpGe) well detector and counted for up to two days to achieve acceptable counting statistics. Specific activity of radium-226 was determined from counts associated with the gamma photopeaks of Pb-214 (295 and 351 keV) and Bi-214 (609 KeV). Activities calculated from the three peaks were combined to yield a weighted mean reported value and standard deviation. The system was calibrated and frequently recalibrated by counting standards prepared by doping portions of Bay sediment with precisely known amounts of a radium standard solution (NIST SRM-4959). This solution has a 0.4% uncertainty in activity at the 99% confidence level due to random errors and an additional 0.8% uncertainty due to assessable systematic errors. Reported standard deviations in radium activity, including random errors associated with detector calibration, were typically 5-7%. The limit of detection was about 0.1 dpm/g for this matrix.
Total cesium-137
The GLERL well and planar detectors were also calibrated for cesium-137 using sediment doped with a standard gamma-emitting, mixed radionuclide standard traceable to the British Calibration Service. The system efficiency has also been calibrated using several NIST standard reference materials, (SRM-4354, Standard Lake Sediment; SRM-4353 Rocky Flats soil Number 1 ; SRM-4353, River Sediment). In order to achieve acceptable counting statistics, all samples having significant radiocesium activity on the 1 to 2 day count for radium, were recounted for up to five days each. The resulting overall detection limit was about 0.02 dpm/g corresponding to a minimum uncertainty in reported activities of about 10%. Cesium-137 was also determined at USGS-Woods Hole on selected samples by counting the 661.7 KeV gamma ray. Results for IAEA Standard 4018 were within 3% of published values. The detection limit was also about 0.02 dpm/g.
Trace metal analysis (lead and others)
To determine concentrations of selected trace metals (lead, uranium, barium and mercury) 0.2 grams of freeze-dried whole sediment from Bob Allen 6A, 6C and Russell Bank 19C were placed in microwave digestion vessels to which was added 20 ml of 10% Ultrapure nitric acid. The mixtures were processed by microwave digestion at 160°C for 1 hour. Ten ml of the resulting solutions were diluted 2.5x and analyzed for metals using an ICP-M system (Perkin Elmer Elate 5000A) focused to detect Pb-208 and U-238. The system, situated in a class 100 laboratory, was calibrated linearly using standard concentrations of 1, 10 and 100 ppb of lead and uranium and checked using synthetic water reference (NIST-1643C) as the external standard. Long term replication of that standard indicated an probable uncertainty in the determination of both metals as less than or equal to 5%. Concentrations of uranium reported as ppb were re-calculated in terms of specific activities (dpm/g) for comparison with lead-210 and radium-226 results.
Isolation of the Non-Carbonate Fraction
Starting from the top of core Bob Allen 6C, roughly equal weights of dried, sieved sediment from 10 contiguous 2 cm sections of core were combined to form a composite sample weighing 65 g spanning a 20 cm segment. Eight such composite samples were prepared for each 10 successive 2-cm intervals downcore which together spanned the entire 160 cm length of the core. Four grams of each ground, homogenized composite was packed into standard vials and sealed for later counting while a 60 g sample was placed in a 8 L container to which a stochiometric excess of dilute (10%) acetic acid was added to dissolve carbonates. Following the primary phase of gas evolution, the entire mixture was quantitatively transferred to large beakers, the solids allowed to settle and the supernatant decanted. Fresh 10% acetic acid was added along with a magnetic stirring bar. The mixtures were stirred continuously at ambient temperature (ca 20°C) for an additional three days to ensure complete dissolution of carbonates. Subsequently, the mixtures were allowed to settle, the supernatant discarded and residues rinsed with distilled water three times to remove soluble products. Following drying, 4 grams of each residue were packed into standard vials and sealed for subsequent gamma counting.
This 210Pb has a residence time in the atmosphere of 10 days. It is removed by rain or snow and is rapidly adsorbed to or incorporated within sediment forming at the earth surface. In south Florida, 210Pb is incorporated in the organic peat deposits. The activity of the unsupported 210Pb decreases as a function of time determined by its half-life. The "age" of a horizon is calculated by the following formula: Tage = 1/[Image] ln(A 210Pb0/ A 210Pbh ) substituting the constants, Tage = (ln(A 210Pb0/ A 210Pbh))/0.03114 where A210Pb0 is the unsupported lead activity in disintegrations per minute at time zero (the present) and A 210Pbh is the activity in disintegrations per minute at depth h. In an ideal situation the plot of 210 Pb activity will show a logarithmic decrease with depth. In order to provide a regional picture for south Florida, 17 cores were taken. The sites were co-located with other investigators so that the results could be correlated to changes in chemistry and ecology. The cores were taken with a modified piston core 10.16 cm ( 4 inches) in diameter which was capable of taking a 1 meter core.
Radioelement measurements: Along with the measurement of 210 Pb activity of each sample, the activity of 7Be and 226Ra was also determined. 7Be and 226Ra were determined by measuring the activity of the 0.578Mev energy. 226 Ra is determined by measuring the activity of 214Bi. These analyses were made with a GeLi detector coupled to a multichannel analyzer. The activity of 210Pb was determined by measuring its granddaughter, 210Po. 210Po decays solely by alpha-radiation which is extremely easy to measure.
Data Reduction and Analysis: In order to determine the distribution of 210Pb within the sedimentary column, the raw values for each segment were converted to the natural log. A standard linear best fit calculation was made on these data. If the original curve was logarithmic, the best fit would yield a straight line. To be acceptable the Best Fit must have a correlation coefficient (R2) value of greater than 0.9. The rate of sedimentation was then calculated using the calculated values of 210Pb versus depth and the formula shown above.
U.S. Department of the Interior, U.S. Geological Survey
Comments and suggestions? Contact: Heather
Henkel - Webmaster
Generated by mp version 2.8.18 on Wed Mar 12 17:57:53 2008