late quaternary volcanic ash in the peatlands of...

Late Quaternary volcanic ash in the peatlands of central Alberta

S. C. ZOLTAI Canadian Forestry Service, Northern Forestry Centre, 5320-122 St., Edmonton, Alta., Canada T6H 3S5

Received February 16, 1988

Revision accepted May 17, 1988

Three distinct volcanic-ash layers were identified in peat deposits through visual, microscopic, and chemical means. These layers were related to known volcanic events through the 14C dating of 34 peat samples. The upper layer was probably Bridge River ash, deposited about 2350 years BP. The middle layer was possibly St. Helens Y tephra, deposited around 3400 years BP. The lower ash was related to Mazama tephra, deposited about 6600 years BP. Ash-enriched layers were formed in the accumulating peat as aerial deposition of tephra occurred, often for several hundred years before and after the presumed main eruption, as shown by I4C dates and peat accumulation rates. This implies periodic eruptions by the volcanoes, not all of which produced distinct ash layers in areas distant from the source. Redeposition from wind-eroded beds of Mazama ash during the dry postglacial climatic maximum is a possibility.

Trois couches de cendres volcaniques ont CtC identifkes dans les dCp8ts de tourbe par des moyens visuels, microscopiques et chimiques. Les dges au I4C de 34 Cchantillons de tourbe ont permis une mise en corrClation avec des Cvtnements volcaniques connus. La couche supCrieure correspond probablement au Bridge River; elle fut dCposCe il y a environ 2350 ans Av.P. La couche du centre est reliCe au tephra Y St. Helens, ces cendres datent d'environ 3400 ans Av.P. La couche de cendres infkrieure est corrClCe avec le tephra Mazama; elle est dgee d'environ 6600 ans Av.P. Les dates au I4C et les taux d'accumulation de la tourbe rkvklent que des couches enrichies en cendres volcaniques se sont dCveloppCes durant I'accumulation de la tourbe dues ti une ~Mimentation akrienne de tephra qui s'est manifestke frkquement pendant plusieurs centaines dlannCes avant, et aprks, ce qui est considCrC c o m e la principale truption. Ces donnkes suggkrent des eruptions volcaniques pkriodiques, cependant ce ne sont pas tous les volcans qui produisirent des couches de cendres distinctes dans les rCgions Cloigntes de la source. Une reskdimentation durant la pCriode postglaciaire de sCcheresse maximum des lits des cendres Mazama CrodCs par le vent ne peut &tre totalement excluse.

[Traduit par la revue]

Can. I. Earth Sci. 26, 207-214 (1989)

Introduction The widespread occurrence of volcanic ash in western North

America is well documented (Powers and Wilcox 1964; West- gate and Dreimanis 1967). Late Quaternary volcanic-ash beds have been associated with Glacier Peak in Washington, Mount Mazama in Oregon (Powers and Wilcox 1964), Mount St. Helens in Washington, Plinth-Meager Mountain area near Bridge River in British Columbia (Valentine et al. 1987), and Mount Bona in the St. Elias Range in Alaska (Lerbekmo and Campbell 1969).

The particle size and abundance of tephra diminish with distance from the source volcano. Alberta is hundreds of kilometres from the sources; hence only limited amounts of fine-grained tephra can be found. The distribution of the tephra is often limited to narrow plumes, determined by the prevailing winds at the time of the eruptions. Close to the source continuous volcanic ash mantles the entiie landscape, but in Alberta the ash layers are discontinuous, often concen- trated by secondary deposition in areas sheltered from the wind. Peatlands are particularly suited for the study of volcanic-ash deposition. The peatland surfaces are wet and well vegetated, trapping the atmospherically deposited tephra. Furthermore, light amounts of volcanic ashfall do not kill the vegetation and a continuous record of ash enrichment is preserved in the accumulating peat.

Peat bogs have long been used by researchers to reveal the presence of one or more volcanic-ash layers (Hansen 1949; Fryxell 1965; Westgate et al. 1969). Not only are the ash layers well preserved in this quiescent depositional environ- ment (Westgate et al. 1970), but also the peat enveloping the tephra layers provides a ready source of carbon for I4C analyses.

In central Alberta, over 500 km from the nearest of the tephra sources, the volcanic-ash layers can be very thin (1 -2 mm) and barely visible with the naked eye (Westgate et al. 1969; Lichti-Federovich 1970). Under the microscope, however, additional volcanic-ash-enriched layers that are otherwise invisible to the unaided eye may become evident. In this paper the occurrences of both visible and nonvisible volcanic-ash layers are presented and their possible source is discussed on the basis of radiocarbon dates.

Methods During 1981, 1984, and 1986 a total of 76 peatlands were

cored in Alberta and adjoining Saskatchewan, generally north of the North Saskatchewan River. A modified Macaulay peat corer with an inside diameter of 34 rnrn was used and samples were collected at the surface and at 15 cm intervals. In many peatlands more than one core was taken to determine the physical, chemical, and floristic variability of the peat deposits. A number of samples were taken with a peat profile cutter (Wardenaar 1987), which extracted 170 cm x 10 cm x 10 cm peat columns. The peatlands were both fens and bogs (Zoltai et al. 1975), the main difference being that bogs receive only atmospheric water but the fens also receive seepage water.

In the laboratory the samples were oven dried and then ashed at 480°C. The ash was treated with hot nitric and hydrochloric acid, and the residue was extracted with hydrochloric acid and filtered. The filter paper was burned, and the weight of the residual acid-insoluble ash was expressed as a percentage of the dry sample. The fdtrate was analyzed with an inductively coupled argon plasma spectrometer for Ca, Mg, K, Al, S, P, Mn, Fe, Zn, Cu, Ni, Pb, Ti, and Na. Details on

Printed in Canada I Imprime au Canada

208 CAN. J. EARTH SCI. VOL. 26, 1989

crn below surface

FIG. 1. Photograph and sketch of an undisturbed peat block showing three volcanic ash layers and 14C ages at site 7c.

the instrument and sample preparation were reported else- where (Ali et al. 1988).

The volcanic-glass content was determined under a polarizing microscope. A small amount of acid-insoluble ash was examined by counting 200 grains of soil particles, including crystalline and tephra grains. The volcanic-glass content was expressed as a percentage of all counted particles. The morphology of individual tephra grains and the abundance of vari- ous forms were noted. Selected peat samples were submitted for 14C dating to Brock University (BGS) and to Alberta Environmental Centre Vegreville (AECV).

Results Volcanic-ash layers

Volcanic ash was found in 24 of the investigated peatlands. The ash deposits can be identified visually where they occur as distinct layers, microscopically where the ash is dispersed in a peaty zone, and chemically through the analysis of some elements.

Visible volcanic-ash layers occur as distinct white- to buff- coloured layers. They can be straight and even, or they can be contorted by differential movements in the peat after deposition. In many cases the ash layers are discontinuous and have variable thickness, possibly reflecting the contour of depositional surfaces. At an intensively studied site (site 7) the peatland was cored on a 200 m grid. A visible ash layer was found between 1.5 and 2 m depth at only 6 of the 21 cored sites. Attempts to relocate an ash layer for further sampling often failed even when cores were taken only a few centimetres

from a core that showed a clearly visible ash layer. On the other hand, at many sites in the western part of the study area there are two or three parallel volcanic ash layers, separated by up to 10 cm of peat (Fig. 1).

Identification of nonvisible volcanic-ash-enriched zones can be accomplished by the examination of the acid-insoluble peat residue under a polarizing microscope. The individual volcanic- glass shards are usually < 20 pm in diameter and can be recog- nized by their noncrystalline structure. Their irregular shape distinguishes them from biogenic silicate materials. The ash- enriched zones are invariably thicker than the visible ash layers, extending both above and below the visible ash. Some of these zones are 80 cm thick. Nonvisible ash zones could be found more consistently at different locations in the same peatland than visible ash layers. At the intensively surveyed site 7 all 21 cores had an ash-enriched layer between 1.5 and 2 m.

The morphology of individual tephra grains consists of three distinct types. The most common form is thin polygonal plates with straight edges, often associated with elongated three- sided prisms. Such prisms often contain inclusions of minerals or gas. This type is informally identified as bubble-wall material. The second type is thick, irregular to somewhat elongated grains that have large amounts of gas and mineral inclusions. This material resembles comminuted pumice. The third type is elongated cylindrical shapes that have an oval cross section (needles). Enclosures of gas are often present as bubble trains elongated along the long axis.

The treatment of ashed peat residue with hydrochloric and nitric acid (aqua regia) removes the readily acid-soluble minerals leaving resistant minerals, such as quartz and feld-

ZOLTAI 209

Volconoc osh (%grams) Ac~d-lnsol orh (%weight) Tttanfum (mg kg ' ) Aluminum (mg k g ' x l ~ ' )

FIG. 2. Distribution of volcanic ash, acid-insoluble ash, titanium, and aluminum in a peat profile at site 1. The stippled bars represent the nonvisible volcanic ash zones.

spar, in the residue (Jackson et al. 1986). Volcanic glass, composed mainly of Si02 (silica), is not dissolved in the acid treatment, as shown by the close relationship between amounts of acid-insoluble ash and volcanic ash (Fig. 2).

Volcanic ash is composed of elements that are considerably different from the components of the peat. The bulk of the ash is Si02 and A1203, with significant amounts of FeO, Na20, CaO, and K20 (Westgate et al. 1970). Among the minor constituents, Ti02 is noteworthy. It probably originates from high-Ti02 magnetites present in the tephra (King et al. 1982). Most of the major elemental constituents occur commonly in peat, originating from airborne dust and from water-soluble and biologically generated sources. However, it was found that Ti and A1 are associated with high levels of volcanic ash (Si was not determined) (Fig. 2). Titanium is particularly use- ful as an indicator because background levels of Ti are low in the study area and the main source is the volcanic aerosol.

The advantage of using microscopic and chemical means to identify the nonvisible ash layers is demonstrated in Fig. 2. In the upper and middle zone the ash grain count and the Ti and Al analyses show distinct peaks, although the acid-insoluble ash is low, possibly because of the small size and low weight of the volcanic-ash grains. At the base of the lower ash zone all indicators show high values, but the underlying mineral soil is distinguished by the absence of volcanic-ash grains and low Ti values.

The examination of peat through radiocarbon dating and through visual, microscopic, and chemical methods resulted in the identification of three distinct volcanic-ash layers and ash- enriched zones. In cases where radiocarbon control was not available the tephra was assigned to an ash zone on the basis of stratigraphic position and tephra morphology.

The depth at which the ash-enriched zones and ash layers occur is not constant because of different rates of peat accumu-

lation in different kinds of wetlands. The relative position of the ash-enriched zones within the peat deposit is, however, relatively constant, except in anomalous, eroding peat deposits such as site 11. The base of the upper ash-enriched zone is about a third of the way down the peat column, the middle zone is at about the middle of the peat column, and the lower zone is near the base of the peat column (Table 1).

The upper ash-enriched zone was found at 13 locations in peat deposits in the western part of central Alberta (Fig. 3). It had an average thickness of 13 cm (Table 1). Visible ash was relatively rare in this area (sites 5, 6, and 11; Fig. 3); where present, it was thin and discontinuous (Kearney and Luckrnan 1983). The ash consists of about equal proportions of pumice and bubble-wall materials. with rare but constant vesicular needle grains.

At 12 sites in the peatlands of central Alberta the middle ash- enriched zone was found in about the same areas as the upper zone (Fig. 4). The middle zone had an average thickness of 11 cm (Table 1). Visible ash layers were common (sites 2, 7, 8, 9, and 11; Fig. 4). At site 7 three distinct layers could be discerned (Fig. I), but the ash generally occurred in a single layer. The ash consists primarily of pumice, with much lower proportions of bubble-wall material.

The lower ash-enriched zone had the widest distribution in the peatlands of central Alberta (Fig. 5). It contained one or two visible ash layers in the sites west of Edmonton, but visible volcanic ash was not detected in the peat to the north- east. The ash-enriched zone was thick, averaging 23 cm (Table 1). The ash consists almost exclusively of bubble-wall materials.

Radiocarbon dating A total of ,33 14C dates are available for samples from 10

peatlands and one lake in central Alberta where the organic

210 CAN. J. EARTH SCI. VOL. 26, 1989

TABLE 1. Mean thickness of ash-enriched zones and mean depth of bottom of ash-enriched zones in central Alberta

Mean Mean Mean % Number Number of thickness depth of total of sites observations (cm) (cm) peat depth

Upper 13 26 13k1.6 132k9.9 38k1.3 Middle 12 18 11k1.8 184k9.4 59k3.6 Lower 32 50 23k2.4 276k13.6 85k1.7

T I I I 5 1 H I

I

I

I I

I I I

ALBERTA I

FIG. 3. Distribution of the upper volcanic-ash zone in peat. Solid squares with numerals mark I4C sites; open squares are undated sites. The barbed solid line follows the distribution of Bridge River tephra (Mathewes and Westgate 1980).

deposits were directly relevant to the volcanic-ash layers and ash-enriched zones (Table 2). These dates provide an approximation of the time of visible ash deposition and the period during which the ash-enriched layer was deposited. However, most of the dates are some centimetres above or below the ash layer, necessitating a projection of the date on the basis of the rate of peat accumulation. By assuming a uniform rate of peat formation, one can calculate the rate of peat accumulation between a dated section and the surface (0 year) or between two dated sections. This rate can then be used to estimate the projected age of any point within the peat section.

The visible upper ash layer was dated at two locations, using two radiocarbon dates (Table 3). The projected dates are 2800 and 2890 years BP. The visible middle ash layer was dated at four locations, using nine 14C dates (Table 3). The projected dates fall between 3350 and 3860 years BP, with several dates around 3500 years BP. The lower ash layer was dated at seven locations, using 17 radiocarbon dates. The projected dates are between 5380 and 6920 years BP.

The ages of double ash layers related to the lower ash zone have been calculated at sites 4, 5, 8a, and 8c (Table 3). At site 4 the age difference was about 300 years; at site 5, 230 years; at site 8a, 410 years; and at site 8c, > 1000 years.

The multiple ash layers (Fig. 1) at site 7c were dated by three samples. The first (uppermost) layer was dated at 3350 years BP. The second layer, 6 cm below the first, was dated at 3860 years BP. The third layer, 6 cm below the second, was

FIG. 4. Distribution of the middle volcanic-ash zone in peat. Solid squares with numerals mark I4C sites; open squares are undated sites. The barbed solid line follows the distribution of St. Helens Y tephra (Westgate 1977).

FIG. 5. Distribution of the lower volcanic-ash zone in peat. Solid squares with numerals mark 14C sites; open squares are undated sites. The barbed solid line follows the distribution of Mazama tephra (Westgate et al. 1970).

dated at 3820 years BP. Thus the second and third layers were about the same age. The first layer was approximately 500 years younger, although it was about the same distance from the second layer as the third was from the second. Mixing by some disturbance can be ruled out, as the three discrete ash layers were exposed in an excavation for more than 20 m. This illustrates that the peat accumulation rate was far from uniform within short segments of the peat column.

The projected period during which ash was incorporated into the peat was calculated for the ash-enriched zones in the peat by calculating the age of the top and bottom of the ash enriched zone (Table 4). According to these calculations the projected ash-enrichment of the upper zone occurred between 1920 and 3010 years BP and the length of the ash-enrichment period was up to 560 years. The ash enrichment of the middle zone occurred between about 3140 and 3950 years BP with a

ZOLTAI 21 1

TABLE 2. Volcanic-ash layers dated by I4C analysis of peat and lake sediment in central Alberta

Site

Location Thickness (cm)

Lat. W Long. N

52'28' 115"0Of 52'27' 115O12' 52'27' 115O12' 52'21' 115O17' 52'51' 116'28' 52'51' 116'28' 52'51' 116'28' 53"201 117'28' 53'20' 117'28' 53"20f 117'28' 54"45' 115O52' 54'45' 115O52' 53'26' 116"04' 53'26' 116"04' 53'26' 116O04' 53'26' 116"04' 53'26' 116O04' 53'26' 116'04' 53'26' 116'04' 53'26' 116"04' 53'26' 1 16'04' 52"501 116"511

Visible Nonvisible

135 - 150 153 - 163 331 -360 115-141 115-120 228 -273 228 -273 175 - 198 307 - 352 307 - 352 230-250 402 -490

55 -64 113-121 145 - 165 95-118 95-118

190 - 225 91-112 91-112 91-112 N.D.

N.D. N.D. N.D.

N.D. 109- 134

N.D. N.D. N.D. N.D. N.D. N.D. N.D.

Dated section (cm)

135- 140 141 - 146 331 -336 156- 161 202 - 206 246 - 249 353-358 181 - 189

337 -347 241 -246 440 - 445

55-56 115-116 147 - 148

105 - 106 196 - 198 96 - 97

102 - 103 108- 109 110-119 172-180

313-322 100- 108 185 - 195 300-310 90-110

210-219 260 -270 155-157 436-444 26-27 29-30 14-38

I4C age (years BP) Lab. No. Reference

-

BGS-766 BGS-767 BGS-768 BGS-769 BGS-771 BGS-770 BGS-772 BGS-773

BGS-774 BGS-776 BGS-777 AECV 325C AECV 326C AECV 327C

AECV 328C AECV 329C AECV 451C AECV 452C AECV 453C AECV 204C AECV 205C

AECV 198C AECV 207C AECV 208C AECV 209C AECV 210C AECV l00C AECV 128C WIS-343 GSC-1234 Beta 13558 Beta 13559 GSC-2648

Zoltai and Johnson (1985) This paper This paper This paper This paper This paper This paper This paper

This paper This paper This paper This paper This paper This paper

This paper This paper This paper This paper This paper Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Kubiw (1987) Westgate (1977) Lichti-Federovich (1970) Luckman et al. (1986) Luckman et al. (1986) Luckrnan et al. (1986)

- -

NOTE: N.D., no data. *Lake sediment.

maximum duration of 740 years. The lower ash-enriched zone was deposited between 5270 and 6970 years BP with a maximum duration of 1490 years.

Discussion Volcanic-ash layers can be attributed to known eruption

events on the basis of their 14C ages. Radiocarbon dating gives an approximation of the age of the dated material within statistically defined confidence limits. Possible errors in the peat stratigraphy are introduced by differential compression and settling of the peat and by different rates of peat accumulation. Furthermore, the living peat-forming vegetation is des- tined to become the soil of the future vegetation cover. Peat-forming mosses grow at their tips and have no roots, but the roots of woody and herbaceous vegetation penetrate the upper part of the peat, contaminating it with more recent materials. At the same time, tree materials that are several centuries old may be incorporated into the surface peat. Radio- carbon dates must therefore be accepted with reservation when they are used to correlate events at different locations.

The use of peat stratigraphy in a chronological sense is subject to the same inherent inaccuracies. On the other hand, continuous peat formation adds a dimension of time not available in upland tephra deposits. Concentration of tephra by sedimentation, often found in lakes or ponds, does not occur in carefully selected peat deposits. Old carbon, either from airborne or sedimentary sources, is not a problem in peat.

The upper ash, with the visible layer dated by this study at 2800 and 2890 years BP and the ash-enriched zone dated at 1920-3010 years BP (Fig. 6), is probably related to the Bridge River ash. This ash was deposited about 2600 years BP (Westgate 1977) or about 2350 years BP (Mathewes and West- gate 1980), based on radiocarbon dates of 2440-2890 years BP (Nasmith et al. 1967; Westgate and Dreimanis 1967; Westgate 1977; Mathewes and Westgate 1980). A double tephra layer in a bog at Otter Lake was dated at around 2000 years BP (Westgate 1977) and is believed to belong to a younger Bridge River unit (Mathewes and Westgate 1980). The Bridge River tephra is often chunky and contains lensoid bubbles (Valentine et al. 1987), similar to the composition of the upper ash.

212 CAN. J . EARTH SCI. VOL. 26, 1989

TABLE 3. Projected ages of visible ash layers in peat in central Alberta based on the rate of peat accumulation (see text for explanation)

Projected date Basis of age of ash fall

14C site Ash layer projection (years BP)

Upper Upper

Middle Middle Middle Middle Middle Middle

Middle

Middle Middle

Lower

Lower

Lower

Lower

Lower

Lower Lower

Lower

Lower

Lower

Lower

Lower

Lower Lower

2800 (BGS-773) 2820 (BGS-776)

3630 (AECV 328C) 3630 (AECV 328C) 3350 (AECV 451C) 3860 (AECV 452C) 3820 (AECV 453C) 2760 (AECV 2042) 3670 (AECV 205C) 3300 (AECV 207C) 4600 (AECV 208C) 3550 (WIS-343) 3140 (Beta 13558) 3680 (Beta 13559)

6170 (BGS-770) 8600 (BGS-772) 6170 (BGS-770) 8600 (BGS-772) 2800 (BGS-773) 6140 (BGS-774) 2800 (BGS-773) 6140 (BGS-774) 2820 (BGS-776) 6170 (BGS-777) 5680 (AECV 327C) 3630 (AECV 328C) 5740 (AECV 329C) 3670 (AECV 205C) 7080 (AECV 198C) 3670 (AECV 205C) 7080 (AECV 198C) 4600 (AECV 208C) 6320 (AECV 209C) 4910 (AECV 100C) 6260 (AECV 128C) 4910 (AECV 100C) 6260 (AECV 128C) 7480 (GSC-1234) 6570 (GSC-2648)

The plume of the Bridge River tephra (Mathewes and West- gate 1980) is compatible with the distribution of the upper ash layer, but it occurs 100-200 km farther north than the sup- posed northern limit of the plume (Fig. 3).

The visible middle ash has been dated by this study at 3350-3860 years BP and the ash-enriched zone at 3140- 3950 years BP (Fig. 6). These dates are comparable with the age of St. Helens Y tephra, dated at around 3400 years BP (Mullineaux et al. 1975). The St. Helens Y tephra has been divided into three subsets but only one (Y,) is believed to extend into Alberta (Mullineaux et al. 1975). However, tephras with a composition similar to that of St. Helens Yn glass have been found at several locations in Alberta, but these tephras were considerably older (3960-4430 years BP) than the main St. Helens Y, tephra (Westgate 1977), raising the possibility of at least two St. Helens Y beds in Alberta. The St. Helens Y tephra, deposited at some distance from the

source, consists of pumice (Mullineaux et al. 1975), as does the middle ash layer.

The distribution of the middle ash layer is comparable to the presumed plume of St. Helens Y ash (Westgate 1977), except that the southern boundary of the distribution should be shifted 50 km to the southeast (Fig. 4).

The lower ash layer was dated by this study at 5380-6920 years BP, the ash-enriched zone, at 5270-6970 years BP (Fig. 6). These dates may correspond to the age of the Mazama ash, dated at 6600 years BP (Fryxell 1965). The distribution of the lower ash is similar to that of the Mazama ash (Westgate et al. 1970), although the lower ash was found about 250 km farther north than the Mazama ash (Fig. 5).

The ash-enriched zones in peat were found to be considerably thicker than the visible ash layers, occurring both above and below the visible layer. At many locations the ash- enriched zone did not contain a visible ash layer. The source of mineral-grain contaminants in peat is airborne particulate deposition (Finney and Farnham 1968). The treed, shrubby, or tall-grass surface of wetlands effectively traps the aerial deposits, especially on wet surfaces. Overland flow and sediment deposition may occur in some fen peatlands during the spring runoff, but such sedimentary deposition of tephra is minimal.

Downward translocation of the deposited particulates in the peat is a possibility. Settling of the volcanic ash may occur in wetlands that have open pools on their surface with very little submerged vegetation. In most peatlands, however, such settling of the ash is prevented by the peat. In a recent study it was shown that particulates emitted by a base-metal smelter were not translocated downward (Zoltai 1988) and that the peat effectively trapped the mineral particulates. The thickness of the peat, enriched by base-metal particulates, was due to peat accumulation through the years. The same mechanism appears to have worked in the case of the volcanic-ash- enriched zones.

Chemical analyses indicate that high titanium concentrations coincide with the volcanic-ash-enriched zones. Studies by Parkash and Brown (1976) showed that peat adsorbs titanium from aqueous solutions. This tends to effectively immobilize the dissolved titanium near its original place of deposition-in the ash-enriched zone.

Multiple eruptions are known to have produced the St. Helens Y tephra and are suspected in the case of the Bridge River ash (Westgate 1977). The Mazama ash is believed to have been ejected by a single catastrophic eruption of Mount Mazama (Powers and Wilcox 1964; Fryxell 1965). During the present study, however, two distinct ash layers, found at three different locations, are separated by peat representing accumulation during several hundreds of years. Furthermore, most of the dates are considerably younger than the generally accepted eruption date of 6600 years BP (Table 3).

An explanation for the presence of multiple visible ash layers and thick ash-enriched zones lies in the climatic and atmospheric conditions following the Mount Mazama eruption. The eruption occurred during a postglacial episode of maximum warmth and drought that peaked between 6500 and 4500 years BP (Richmond et al. 1965). Effective wind erosion caused the formation of ventifacts and the deposition of loess in the northwestern United States. It is possible that the finer particles from the freshly deposited Mazama ash beds were subjected to prolonged wind erosion in the absence of vigorous plant colonization in a dry climate. Some of this ash was

ZOLTAI

TABLE 4. I4C ages of peat and projected ages of ash-enriched zones in peat in central Alberta based on rate of peat accumulation

Thickness of Projected age of Ash-enriched ash-enriched-zone Basis of age ash-enriched zone

Site zone (cm) projection (years BP)

Middle Middle Middle Middle

Middle

Lower Lower Lower Lower Lower Lower Lower

Lower

2260 (BGS-766) 3 130 (BGS-767) 3380 (BGS-769) 4460 (BGS-771) 2800 (BGS-773) 2820 (BGS-776) 1920 (AECV 325C)

2260 (BGS-766) 3130 (BGS-767) 3630 (AECV 328C) 3350 (AECV 452C) 3860 (AECV 453C) 2800 (AECV 210C)

2260 (BGS-766) 6380 (BGS-768) 6170 (BGS-770) 8600 (BGS-772) 6140 (BGS-774) 6170 (BGS-777) 5390 (AECV 326C) 5680 (AECV 327C) 5740 (AECV 329C)

Site 1 2 3 4 5 6 70 7b 7c 80 8b 8c 9 10 11

FIG. 6. Occurrence of visible (heavily hatchured lines) and nonvisible (stippled bars) volcanic-ash zones in relation to radiocarbon age. Arrowheads point to 14C date control.

-

-

-

: I Br~dge R.

Helens Y 4

: A t

#

Mazama

*

2 14 CAN. J. EARTH 11. VOL. 26, 1989

redeposited in the incipient wetlands of central Alberta. The alternative explanation of multiple Mazama ash beds is

that the main eruption was followed by further periodic emissions of ash. This possibility was not substantiated by observations near the emission source. It was noted, however, that stratification of Mazama ash layers in bogs is common in Oregon, raising questions about the sequential nature of eruptions of volcanic ash from Mount Mazama (Harward and Youngberg 1969).

Conclusions This study demonstrates that volcanic ash continued to b e

deposited in peatlands for a prolonged period after, and perhaps before, the main volcanic eruption. The depositional history of tephra is complex, with possible recurring volcanic emissions and aerial redepositions during dry climatic periods. One cannot, therefore, assume a single date for the deposition of any given volcanic-ash layer unless an inaccuracy of several hundred years is acceptable. It is also evident that a careful study of volcanic tephra in peat deposits, involving the identification of tephra sources by precise microanalysis, would be fruitful in clarifying the Holocene tephra chronology and stratigraphy.

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