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For any suite of optical dating samples two issues that must be considered are: do zero-age samples yield an optical age of zero, and are the optical ages consistent with independent stratigraphic and chronologic information?
A test of the zero-age of dune sands was performed by dating samples from the crest, lee slope and stoss slope of an active dune in southwestern Manitoba. Three surface samples showed that, using 1. These ages are consistent with expectations for the crest, lee slope and stoss slope, respectively. Optical ages of late Holocene dune sand units at the Brookdale Road section, southwestern Manitoba, were consistent with radiocarbon ages from organic matter within intervening buried soils.
The suite of optical and radiocarbon ages from the Brandon Sand Hills provides a record of dune activity and stability for the region, and tentatively identifies periods of eolian activity at about 2 over 50 dating sites sandy hills, 3. Durante el presente estudio una prueba de edad-cero de las dunas de arena fue realizada a partir de muestras provenientes de la cima, y de las laderas a sotavento y a barlovento de una duna activa situada al sudoeste de Manitoba.
Estas edades concuerdan con lo que se esperaba para la cima y las laderas a sotavento y a barlovento, respectivamente. Increasingly, optical dating is used to develop records of dune chronologies on the Great Plains of North America, and elsewhere Stokes and Gaylord, ; Stokes and Swinehart, ; Wolfe et al. Whereas radiocarbon dating is still a commonly used technique for determining Holocene chronologies in these and other paleoenvironments, the suitability of eolian sediments for optical dating and the absence of organic material suitable for reliable radiocarbon dating in dune sands, has led to the common use of optical dating of these sediments.
Despite the acceptance of optical dating, there is still a need for rigorous testing of the technique as applied to these and other environments. The primary objective of this study was to over 50 dating sites sandy hills the present chronology for the timing of eolian activity in the Brandon Sand Hills, southwestern Manitoba, using optical dating of dune sands.
In achieving this objective, we also tested the ability of optical dating to provide zero and modern ages for an active sand dune, and compared optical ages of dune sands with radiocarbon ages of organic matter within intervening paleosols. This is the easternmost dune occurrence on the southern Canadian prairies, residing within the aspen parkland ecoregion of the prairie ecozone. Most of the sand dunes in the area are presently stabilized by parkland and forest vegetation. Exposed sections within the stabilized dunes typically reveal several metres of sandy eolian deposits, separated by buried soil horizons, representing periods of past eolian activity and stability, respectively.
Accelerator mass spectrometry AMS and radiometric age determinations have been obtained from organic matter contained within these buried soils David, ; Wolfe et al. Numerous uncertainties are associated with age estimates derived from radiocarbon analyses of organic matter in buried soils, in part because organic matter within a soil is derived from a variety of sources accumulated over the soil residence time.
Thus, a conservative approach to interpreting radiocarbon ages of buried soils is to consider them as maximum limiting ages for any overlying deposits David, and as minimum limiting ages for the parent eolian sand and any underlying deposits Muhs et al. Radiocarbon dating of soil organic matter in the Brandon Sand Hills indicates recurrent intervals of eolian stability and activity in the past years. Although precise regional correlations are precluded by dating uncertainties, periods of most notable paleosol development are interpreted at 0.
Eolian activity is inferred to have occurred during the intervening periods, possibly corresponding to periods of drought.
As eolian activity was inferred by the absence of paleosols in these studies, the main objective of this study was to apply optical dating techniques to intervening dune sands to improve the chronology of dune activity in this area. On the Canadian prairies, optical dating has also been used as a means of determining the timing of the most recent eolian activity, typically by collecting samples 50 to cm below the surface of stabilized dunes David et al.
In these circumstances, it is assumed that samples collected close to the surface will give an adequate estimate of the time of last activity, because of the relatively rapid rate of deposition of dune sands and subsequent stabilization by surface vegetation. Using modern, partially active sand dunes on the Canadian prairies as analogues for older stabilized dunes, it has been noted that the last portion of the dune to stabilize is typically the head, between the crest and the brink of the dune Fig.
Therefore, samples for optical dating have been collected over 50 dating sites sandy hills the head of stabilized dunes to determine the timing of most recent activity Wolfe et al. One finding from these studies is that many stabilized sand dunes on the Canadian prairies have been active within the last few hundred years, with many optical ages reported being between 70 and years old Wolfe et al. For such relatively young ages, the accuracy of the optical age can be affected by a zero-age error.
This error represents the difference between the actual equivalent dose of the sample at the time of deposition, and the expected value of zero for a sample that has been fully bleached by sunlight.
This error may arise from variation in the duration and spectrum of sunlight exposure of the over 50 dating sites sandy hills grains prior to burial. In Wolfe et al. These samples revealed equivalent doses of 0. Other researchers have reported similar non-zero equivalent doses and ages for modern samples e. As part of the present study, we compared the ages of samples collected at the surface, and at 50 cm depth, from the lee slope, crest and stoss slope of an active dune in order to evaluate the zero-age error and the resolution in ages derived from different parts of the dune.
Eleven samples were collected in the summer of from the surface of the active dune, from shallow pits dug into the active dune, and from two sections within stabilized sand dunes. Field and laboratory reference s, sample depths and locations are recorded in Table I. The active dunes appear transitional between transverse and parabolic dunes.
In historical times the area of active dunes has diminished from about ha in to about ha in Wolfe et al. A sand dune at the easternmost edge of this complex Fig. Comparison of successive aerial photographs of the dune indicates that it has migrated approximately 6 m between andfor an average rate of about 0. Shay et al. Additional measurements of erosion and deposition from May to October in each year, indicated net erosion on the stoss slope of the dune, deposition in the crestal area, and deposition on the lee slope Shay et al.
These findings are similar to a month study by Wolfe and Lemmen in the Great Sand Hills, which found erosion on the upper and lower stoss slopes of a parabolic dune, deposition between the crest and the brink of the dune, and deposition on the lee slope. Six samples were collected from the active dune in the Spirit Sands Fig.
Three surface samples were collected from the lee slope, crest and stoss slope of the dune to act as zero-age test samples. These samples were collected over 50 dating sites sandy hills removing approximately the upper 3 mm of sand grains from the surface of the active dune and placing them in light-tight one-litre containers. Samples were collected on a sunny day.
Three sub-surface samples were also collected in order to compare the relative ages of the buried samples to the zero-age test samples on the lee slope, crest and stoss slope of the active dune. These samples were collected by digging pits and inserting a light-tight sampling tube into the section at a depth of 50 cm.
Additional samples were collected 30 cm above and below the sub-surface samples for dosimetry analyses. Samples were collected from the Brookdale Road section site MB that had been ly radiocarbon dated. The site is a roadcut through the north wing arm of a stabilized easterly-trending parabolic dune in the northwestern portion of the Brandon Sand Hills Fig. The section contains a series of buried soils separated by beds of humus-free dune sands, that overly deltaic deposits.
The section was first dated by David using radiometric dating of over 50 dating sites sandy hills matter extracts from four organic-rich paleosol A-horizons in eolian sands. Samples were collected from the upper few centimetres of the paleosols, except for the lowest sample GSC which was collected from the central portion of the paleosol richest in organic matter. These buried soils were subsequently re-sampled for dating by Wolfe et al. In dating these, Wolfe et al.
Only two of the four sampled paleosols were dateable, due to insufficient humic acids in the other soil horizons. It was found that the radiometric ages were comparable to those obtained from AMS dating, but that the former may be too young by a few hundred years.
This age difference could be attributed to differences in sampling locations, or to the incorporation of younger mobile carbon leached from paleosols higher in the profile into the lower paleosol used in the radiometric dating. As noted by Wolfe et al. Indeed, detailed studies by Abbott and Stafford indicate that humic acid extractions minimize both contamination by younger organic acids and recycling of older organic matter, and show that humic acid extractions give radiocarbon ages closest to radiocarbon ages of plant macrofossils.
With these uncertainties in mind, three sediment samples were collected from dune sands in this section in order to determine the timing of dune sand deposition, and to compare the optical ages of dune sands to the radiocarbon ages of the organic horizons. Samples were collected beneath a tarpaulin to exclude light exposure, and by inserting a light-tight sampling tube into the pits dug horizontally in the section at the desired depth. Additional samples were collected 30 cm above and below the optical dating samples for dosimetry analyses.
A second exposure in a over 50 dating sites sandy hills parabolic dune from the Henry Fast section site MB was sampled for optical dating of dune sands. Three buried soils within this section were ly sampled for AMS dating, but contained insufficient organic carbon for reliable ages.
Two samples were collected to determine the timing of dune sand deposition in lieu of any radiocarbon chronology. These five optical ages are compared to the suite of radiocarbon ages from the Brandon Sand Hills Wolfe et al. Optical dating measures the time elapsed since mineral grains were last exposed to sunlight.
A detailed description of the apparatus and methods used can be found in Ollerhead et al. Environmental beta and gamma dose rates were calculated from measured or assumed K, U, Th, Rb and moisture contents Table IIusing the conversion factors of Nambi and Aitkenas updated by Adamiec and Aitkenand the beta attenuation factors of Mejdahl Cosmic-ray dose rates were estimated according to the prescription of Prescott and Hutton Internal dose rates were calculated as per footnotes to Table II, and the total dose rates for the active dune and section samples were calculated.
Over 50 dating sites sandy hills the section samples, the apparent ages were corrected for anomalous fading using the approach described by Huntley and Lamothe The corrected optical ages for these samples are shown in Table IV. For two of these SFU-O andthe apparent optical ages were within the range of those calculated for the surface samples, and cannot be differentiated from them.
The 50 cm-depth sample SFU-O collected from the stoss slope of the dune is the only sample to have an equivalent dose that is ificantly greater than the other samples. Water contents used for dose rate calculations were 0. Notes: U, M and L are upper, middle and lower dosimetry samples taken from 30 cm above the sample location, from the sample location itself, and from 30 cm below it.
The K content within the grains was assumed to be The effective dose rate from alpha particles emitted by U an Th within the grains was assumed to be 0. A condition of secular equilibrium is assumed for both the uranium and thorium decay chains.
Dose rates, equivalent doses D e and apparent ages for surface and near-surface samples. For a surface sample, where only the upper 3 mm of surface grains was collected, the dose rate used was determined for the corresponding sample collected 50 cm below the surface, and is shown in brackets. Dose rates, equivalent doses D e and ages uncorrected, and corrected for anomalous fading for section samples.
The delay is the time between laboratory irradiation and equivalent dose measurement. Anomalous fading rate is expressed in percent per decade where a decade is a factor of 10 in time since irradiation, for a starting point 2 days after irradiation. Figures in brackets are assumed, based on figures for other samples from the same site with mineralogy expected to be similar. Details of the measurements are in Huntley and Lamothe The non-zero equivalent doses for the surface samples are attributed to the sunlight exposure at deposition having put electrons into the traps being sampled for optical dating as well as evicting them, there thus being a dynamic equilibrium.
The resulting equivalent doses and corresponding ages Table III, columns 5 and 7 are all smaller, as expected.
With the assumption that each 50 cm-depth sample at burial was in the same state as its corresponding surface sample was at collection, it is possible over 50 dating sites sandy hills calculate ages for them that should be correct. This has been done first by taking the equivalent dose for the 50 cm-depth sample, and subtracting the equivalent dose for the corresponding surface sample; the are shown in Table V. Since the resulting values obtained using the two different bleaches were consistent, they were averaged and divided by the dose rates to obtain the ages.
Figure 3 compares optical ages at Brookdale Road section site MB to calibrated radiocarbon ages derived from ages reported by David and Wolfe et al. The lower three eolian deposits in the section are optically dated to about 2. The first notable observation is that the optical ages are in the correct chronologic sequence, both in relation to their stratigraphic positions and to the radiocarbon ages.