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Cryostratigraphy of the Klondike “muck” deposits, west-central Yukon Territory E. Kotler and C.R. Burn
Abstract: Four late Quaternary cryostratigraphic units are recognized in the unconsolidated valley-bottom deposits of the Klondike area, Yukon Territory. Three of the units, in ice-rich, loessal sediments of pre-Wisconsinan or Wisconsinan age, collectively compose the King Solomon Formation. They are overlain by a Holocene organic unit. The units are distinguished by their cryostratigraphic characteristics and oxygen-isotope ratios of included ground ice. The basal unit is the Last Chance Creek Member, a pre-Late Wisconsinan deposit, containing preserved ice wedges (δ18 O ≈ –28 to –26‰; δD ≈ –225 to –209‰). The overlying Quartz Creek Member, a Late Wisconsinan unit, is dominated by organic-rich loess. Massive ice is noticeably absent, although the sediments are ice rich. The isotopic composition of ice in this unit is characteristic of full-glacial conditions (δ18 O ≈ –32 to –29‰; δD ≈ –234 to –257‰). An abrupt change to warmer and wetter conditions at the end of glaciation, prior to the Holocene, is recorded by the icerich, colluviated Dago Hill Member (δ18 O ≈ –28 to –21‰; δD ≈ –164 to –225‰), which began accumulating by 11.62 14C ka BP. Large ice wedges originate in this unit, and, in places, penetrate the underlying full-glacial sediments. Even higher δ18 O and δD values occur for ice in the Holocene organic unit (δ18 O ≈ –25 to –20‰; δD ≈ –164 to –189‰). The majority of the massive icy bodies in the King Solomon Formation are ice wedges, but pool ice and aggradational ice are also exposed, especially in the Dago Hill Member. Massive icy beds formed by groundwater intrusion into permafrost occur at the lower contact of the Quartz Creek Member. Résumé : On reconnaît quatre unités cryostratigraphiques du Quaternaire tardif dans les dépôts non consolidés de fond de vallées dans la région du Klondike, Territoire du Yukon. Ensemble, trois de ces unités dans des sédiments de lœss, anté-Wisconsiniens ou Wisconsiniens, riches en glace, constituent la Formation de King Solomon. Elles sont recouvertes d’une unité organique datant de l’Holocène. Les unités se distinguent par leurs caractéristiques cryostratigraphiques et par les rapports oxygène-isotope de la glace incluse dans le sol. L’unité à la base est le membre de Last Chance Creek, un dépôt anté-Wisconsinien tardif, qui contient des fentes remplies de glace préservée (δ18 O ≈ –28 à –26‰; δD ≈ –225 à –209‰). Au-dessus, un loess riche en matière organique domine le membre de Quartz Creek, une unité Wisconsinienne tardive. L’absence de glace massive est remarquable, bien que les sédiments soient riches en glace. La composition isotopique de la glace dans cette unité est caractéristique de pleines conditions glaciales (δ18 O ≈ –32 à –29‰; δD ≈ –234 à –257‰). Le membre à colluvions et riche en glace de Dago Hill (δ18 O ≈ –28 à –21‰; δD ≈ –164 à –225‰), qui a commencé à se bâtir vers 11,62 14C ka avant le présent, signale un changement abrupt à des conditions plus chaudes et plus humides à la fin de la glaciation, avant l’Holocène. De grandes fentes remplies de glace prennent leur origine dans cette unité et, en certains endroits, elles pénètrent dans les sédiments sous-jacents pleinement glaciaires. On retrouve des valeurs encore plus élevées de δ18 O et de δD pour la glace dans l’unité organique datant de l’Holocène (δ18 O ≈ –25 à –20‰; δD ≈ –164 à –189‰). La plupart des amas massifs de glace dans la Formation de King Solomon proviennent de fentes remplies de glace, mais des bassins de glace et de la glace accrétionnaire sont aussi exposés, surtout dans le Membre de Dago Hill. On retrouve des lits massifs glacés, formés par l’intrusion d’eau souterraine dans le pergélisol au contact inférieur du membre de Quartz Creek. [Traduit par la Rédaction]
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Introduction Beringian Quaternary deposits have been the focus of numerous studies in Alaska and Yukon Territory (e.g., Tyrrell Received June 16, 1999. Accepted December 23, 1999. E. Kotler.1 Department of Earth Sciences and OttawaCarleton Geoscience Centre, Carleton University, Ottawa, ON K1S 5B6, Canada. C.R. Burn. Department of Geography and Environmental Studies and Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, ON K1S 5B6, Canada. 1
Corresponding author. Present address: RR3, Cobble Hill, BC V0R 1L0, Canada. (e-mail:
[email protected]).
Can. J. Earth Sci. 37: 849–861 (2000)
1917; Péwé 1955, 1989; Harington and Clulow 1973; Péwé et al. 1997). In the Klondike area, these sediments primarily contain loess and are generally ice- and organic-rich (Fraser and Burn 1997). The loess overlies auriferous creek gravels and is both colluviated and in primary depositional position. The sediments are locally called “muck” and are often exposed by mining. Two sedimentological units have been recognized within the “muck” deposits: (1) a lower silt unit, consisting of primary and redeposited loess of Late Wisconsinan age; and (2) an overlying organic unit; a disorganized to crudely bedded Holocene deposit consisting mostly of organic debris and peat (Fraser and Burn 1997). No regional examination of the cryostratigraphy or isotopic characteristics of the ice in these deposits has been completed, although French and © 2000 NRC Canada
850 Fig. 1. Location map of the Klondike area, Yukon Territory, indicating sites where sections of “muck” were examined in summer 1997.
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al. 1995; Björk et al. 1996; Fraser and Burn 1997; Severinghaus et al. 1998). The maximum spatial extent of McConnell glaciation occurred between 18 000 and 22 000 14 C BP, or 22 000 and 27 000 calendar years ago (DukRodkin and Hughes 1991).
Regional setting
Pollard (1986) examined massive ice exposed on lower Hunker Creek (Fig. 1), and Naldrett (1982) and Fraser and Burn (1997) described ice wedges within the two units. Cryostratigraphy has been used in the western Canadian Arctic, Yukon, Alaska, and Siberia to infer permafrost conditions during and after sediment deposition (e.g., Mackay 1978; Black 1983; Burn et al. 1986; Vasil’chuk and Vasil’chuk 1995; Burn 1997). The principal foci of cryostratigraphy are identification of ground ice types and examination of their relations with adjacent sediments (e.g., Mackay and Dallimore 1992; Murton and French 1994; French 1998). The stable-isotope composition of ground ice (δ18O and δD) is an important component of cryostratigraphy, used to assist discrimination between units and interpretation of paleoclimate (e.g., Mackay 1983; Burn et al. 1986; Michel 1990). The objectives of this paper are (1) to examine the cryostratigraphy and isotope geochemistry of ice-rich Late Quaternary deposits in the Klondike area, and (2) to interpret paleoclimatic and permafrost conditions subsequent to deposition of the sediments. There are no formal names for the cryostratigraphic units recognized within the Klondike deposits, and we propose a classification to assist identification, description, and correlation of the sediments. Radiocarbon dating has been used to constrain times of sediment deposition, and ages are presented in uncalibrated radiocarbon years before present (BP). Time in radiocarbon years is not linearly related to calendric dates, but key dates are the beginning of the Holocene and end of Younger Dryas cold interval at 10 000 14C BP or 11 450–11 600 calendar years ago; the beginning of the Bølling-Allerød “warm” interval at 12 200 14C BP or 14 700 calendar years ago; and the onset of Late Wisconsinan McConnell glacial conditions in the Klondike after 27 000 14C BP or about 31 000 calendar years ago (Alley et al. 1993; Bard et al. 1993; Bartlein et
The 2000 km2 study area is bounded by Klondike River to the north, Indian River to the south, Flat Creek to the east, and Yukon River to the west and comprises five major drainage basins, those of Bonanza, Hunker, Quartz, Dominion, and Sulphur creeks (Fig. 1). The area lies within Klondike Plateau, a Tertiary-age highland bounded by Kluane Plateau to the south and Tintina Trench to the north (TempelmanKluit 1980; Mathews 1986). The Klondike Plateau consists of medium- to high-grade Proterozoic and Paleozoic metasedimentary, ultramafic, and gneissic rocks (Green 1972; Mortensen 1990). The dominant bedrock in the study area is fragmented quartz- mica schist and phyllite. During at least four Tertiary to Early Pleistocene glaciations, westerly flowing ice approached the Klondike, but the area remained unglaciated (Duk-Rodkin et al. 1995). Proglacial Klondike Gravel was deposited by 730 ka BP on Tertiary-age White Channel Gravel. Subsequent valley incision has resulted in the occurrence of these gravel units in intermediate- to high-level terraces of the area (Hughes 1987). The Middle Pleistocene Reid glaciation, which ended by 190 ka BP, was less extensive, with the ice front 150 km east of Dawson (Bostock 1966; Berger et al. 1996). A small proportion of the loess at the bottom of some valleys in the study area may have been deposited during this glacial interval (Milner 1976). The majority of the loess now present in the area was deposited during the McConnell glaciation, after about 27 ka BP. Loess deposition terminated at the end of the McConnell glaciation, following an abrupt transition to moist conditions, allowing accumulation of organic material from the start of the Holocene (Fraser and Burn 1997). The Klondike area lies in the northern boreal forest and within the zone of extensive discontinuous permafrost (Fig. 2). The mean annual air temperature at Dawson is −5.1ºC, while a mean annual temperature of –1.7ºC has been measured at the top of permafrost in undisturbed ground (Environment Canada 1982; Smith et al. 1998). A permafrost thickness of 60 m has been measured at Dawson, but perennially frozen ground is often absent from south-facing slopes and beneath deciduous woodland (French et al. 1983). In addition to the ground ice in the “muck” deposits, numerous open-system pingos have formed at the base of slopes in the area under the influence of permafrost (Hughes 1969). Ice-wedge casts in the White Channel and Klondike gravels (Milner 1976; Morison 1985; Duk-Rodkin 1996) indicate that permafrost developed in central Yukon during the late Tertiary and Early Pleistocene (Burn 1994; Froese 1997). Sand wedges of Reid age, exposed along Stewart River east of the study area, indicate that periglacial conditions recurred during the Middle Pleistocene (Hughes 1987). Massive ice and ice wedges within McConnell and postMcConnell deposits demonstrate the establishment and persistence of permafrost from the Late Wisconsinan (French and Pollard 1986; Fraser and Burn 1997). © 2000 NRC Canada
Kotler and Burn
851 Table 1. Results of radiocarbon age determinations. 14
Lab No.
Type
C age (years BP)
Site
Sample description
BGS-2015 BGS-2019
BDC BDC
49 000±9000 45 500±5800
1 1
BGS-2018
BDC
40 060±2800
1
TO-6968
AMS
23 520±210
5
TO-6967
AMS
22 300±190
5
Beta-111606
BDC
13 910±70
3
TO-6869
AMS
11 620±90
6
BGS-2017
BDC
10 178±200
7
BGS-2016
BDC
10 125±380
7
Wood above gravel Rhizomes above gravel, Last Chance Creek Member Wood from cryoturbated facies, Last Chance Creek Member Twigs above tephra bed, Quartz Creek Member Twigs below tephra bed, Quartz Creek Member Twigs and grass from squirrel nest, Quartz Creek Member Twigs from base of Dago Hill Member Wood fragments, 2nd generation ice wedges, Dago Hill Member Twigs and peat, 1st generation ice wedges, Dago Hill Member
Note: β-decay (BDC) age determinations were obtained at Brock University and Beta Analytic Inc. and accelerator mass spectrometer (AMS) age determinations at the University of Toronto. Location numbers refer to those in Fig. 1.
Methods The study sites, where sections exposed by mining were examined, are shown in Fig. 1. Sections were logged in the field and units were identified based on their colour, texture, structure, nature of included organic material and ground ice characteristics. Cryostructures in the “muck” deposits were described using the terminology of Murton and French (1994), but their cryofacies system was not used, because the classification requires volumetric ice content. Accurate estimates of volumetric ice content require that the porosity of the mineral sediments, the bubble content of the ice, and, at temperatures near 0ºC, the unfrozen water content are known. Even if the latter is ignored, the other two variables are rarely known, making cryofacies designations impractical in the field. Detailed particle-size determinations from sites throughout the study area were reported by Fraser and Burn (1997, Table 1), so laboratory grain-size determinations were not made in this study. Field inspection of the sediments indicated the mineral material was fine sand and silt, as indicated by Fraser and Burn (1997). Organic material was collected from the units for radiocarbon dating. These samples were extracted frozen from the sediment, described briefly, wrapped in aluminum foil, and sealed in plastic bags. Ice-wedge ice and pore ice were sampled from the various identified units at the exposed sections for stable-isotope analysis. To prevent contamination by surface meltwater, the area surrounding each sample site was dried, and a papertowel dam built to keep it dry. Frozen surface material was removed to a depth of 2 cm from a 15 cm square using a clean chisel. The sample was then extracted by chiselling off small pieces into a plastic bag, which was sealed, and placed within a second bag. Sealed samples were allowed to melt, and the water was transferred to plastic vials for storage and
Fig. 2. Permafrost map of Yukon Territory (after Heginbottom et al. 1995).
transport. Samples were analyzed for 18O and D concentrations at the University of Ottawa. Oxygen was extracted by equilibration with CO2 and hydrogen by reduction with zinc, following methods of the G.G. Hatch Isotope Laboratory; further details are in Kotler (1998). Results are presented using the δ-notation, where δ represents the per mil (‰) relative difference of 18O/16O or D/H in a sample with respect to Vienna Standard Mean Ocean Water (VSMOW; Craig 1961a). © 2000 NRC Canada
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One to two litre samples of frozen sediment were cut from sections to determine the total ice content. To obtain the volume of the frozen sample, each was immediately wrapped in plastic, immersed in a bucket of water, and the overflow was captured in a graduated plastic cylinder. Samples were sealed and transported to Mayo, Yukon Territory, for laboratory analysis, where the excess water was poured off and weighed, and the remainder of the sample was dried to determine the gravimetric water content. The volumetric ice content was estimated from these data.
Ground ice in the Klondike area Descriptions of ground ice in the Klondike “muck” deposits have focussed on the abundant ice wedges near the top of the Pleistocene silts and in the Holocene organic sediments (Naldrett 1982; French and Pollard 1986; Fraser and Burn 1997). The wedges form the dominant massive icy bodies in exposed sections, and have been interpreted as epi- or syngenetic (Naldrett 1982). The number and size of these ice wedges suggest formation and growth under a cooler climate than in the area today, where ice wedges are rarely active (Burn 1990; Mackay 1992). Bodies of pool ice form a second widespread type of massive ground ice. In contrast to the ice wedges, these bodies are translucent and the bubble trains radiate outward from the centres of the bodies. Dissolved solutes are concentrated toward the centre of the ice, as the central parts of these bodies are, ubiquitously, tainted. The pool ice is commonly found above ice wedges, occupying former ice-wedge troughs. Massive ice is also found at the base of sections, close to the contact with valley-bottom gravels. French and Pollard (1986) examined a body of such ice on lower Hunker Creek, and proposed that the ice was either of segregation origin or was a buried snow bank. There are several deposits similar in appearance to the materials examined by French and Pollard (1986), and, at the base of sections, these may contain angular bedrock clasts. Massive ice at the base of the Pleistocene sediments may have also formed from groundwater intrusion, particularly where the ice is sediment free and forms dikes. The intrusive form is to be expected in an area with many open-system pingos (Hughes 1969), and where icings are abundant in creek beds during winter, indicating the extent of continuous groundwater discharge. Intrusive ice bodies also occur in the Holocene unit, where they may deform organic beds. Finally, a band of ice occurs in many places above thaw contacts. This ice, which commonly joins the tops of ice wedges, is composed of aggradational ice and deformed icewedge ice that has been pushed into the sediments following repeated thermal contraction and expansion (Mackay 1993). The cryostratigraphy of the Klondike “muck” deposits is interpreted from the various forms of ground ice described, with attention to their stratigraphic position and thermal history.
Cryostratigraphy “Muck” is found on valley sides facing north and northeast and in the bottoms of narrow valleys. In general, the
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thickness of the deposits increases southward from 1 to 10 m on lower Bonanza and Hunker Creeks, to over 20 m on Quartz and Dominion Creeks (Fig. 1). We recognize four sedimentary units within these materials: the organic unit of Fraser and Burn (1997); two subdivisions of their silt unit; and an additional unit beneath it. Cryostratigraphically, the three lower units are considered distinct members of the King Solomon Formation, the ice-rich loessal sediments that are ubiquitous in valley bottoms of the Klondike area overlying auriferous creek gravel. Description of the cryostratigraphic units follows from oldest to youngest: the Last Chance Creek Member; the Quartz Creek Member; the Dago Hill Member; and the organic unit. Last Chance Creek Member Fraser and Burn (1997) only found pre-McConnell (>40 ka BP) silts in two of 23 sections they examined, and then as a veneer interbedded with underlying gravel. They did not find any massive ice in these deposits. However, the oldest cryostratigraphic unit we recognize is a continuous, 400 m long exposure, unconformably overlying creek gravel at Last Chance Creek (site 1, Fig. 1). The sediments of this unit can be divided into two distinct facies: a lower 0.75– 1 m of dark olive grey (Munsell code 5Y 3/2) massive silt with rhizomes, and an upper 0.5–0.75 m dark brown (7.5 YR 3/2) cryoturbated bed, comprising silt and organic matter, particularly twigs and small branches. Radiocarbon ages determined on rhizomes collected from the base of the unit, directly above gravel, and wood from the cryoturbated facies were 45.50 ± 5.80 ka BP (BGS-2019) and 40.06 ± 2.80 ka BP (BGS-2018), respectively (see Table 1). Both ages are at or beyond the normal limit of radiocarbon dating, and should be considered >40 ka BP. Discontinuous, planar ice lenses, up to 20 cm long, are abundant, giving the unit a planar, nonparallel lenticular cryostructure. Narrow syngenetic or epigenetic ice wedges, up to 4 m in height, extend downward beyond the base of the unit and penetrate the underlying gravel (Fig. 3). These ice wedges show at least two episodes of growth; one truncated at the base of the unit, the other at the base of the cryoturbated facies. Nine ground-ice samples were collected from this unit for stable-isotope analysis. Two of the samples were from ice wedges, while the others comprised pore ice. The range in δ18O values determined for these samples was –28.3 to −26.3‰, and the corresponding range in δD was from –225 to –209‰ (Fig. 4). Quartz Creek Member This unit is named for the Quartz Creek site (site 2, Fig. 1), where an age of 27.15 ± 0.66 ka BP (BGS-1754) was determined on turf in growth position at the base of the silt unit (Fraser and Burn 1997). The member comprises the lower, massively bedded part of the unit. A ground squirrel nest collected from Quartz Creek sediments at the base of Dago Hill (site 3, Fig. 1) was dated at 13.91 ± 0.07 ka BP (Beta-111606; W.P. Lebarge, personal communication, 1997). The nest is the youngest item recovered from the member, and indicates, approximately, the latest time that the member was exposed at the ground surface. The unit commonly overlies gravel, and was observed up to 10 m © 2000 NRC Canada
Kotler and Burn Fig. 3. Last Chance Creek Member (LCC) showing the lower silt facies and upper cryoturbated facies, both with lower thaw contacts. The ice wedge shows two episodes of growth. Photograph taken at Last Chance Creek, August 1997. The scale bar is 1.5 m. QCM, Quartz Creek Member.
thick in section. Water-jet drilling on Gold Bottom Creek (site 4, Fig. 1) indicated the thickness may be up to 17 m. The sediments are massive to bedded loess, olive brown to grey brown in colour (Munsell code 2.5 Y 4/4 to 10 YR 5/2). Several 1–10 cm thick pebbly sand beds occur in the lower portions of the unit. Mammal bones, of the types described by Harington and Clulow (1973), are found throughout the unit. Samples of twigs from above and below a continuous tephra bed observed near the base of this unit on upper Bonanza Creek (site 5, Fig. 1), were radiocarbon dated to, respectively, 23.52 ± 0.21 ka BP (TO-6968) and 22.30 ± 0.19 ka BP (TO-6967). The reversal in age of these samples indicates reworking of material, and the younger of the two presents the oldest date for deposition at the present site. Fraser and Burn (1997) also noted a 1–20 cm continuous to discontinuous tephra bed within this unit throughout the study area, and obtained a radiocarbon age of 24.02 ± 0.55 ka BP (BGS-1755) from turf buried in growth position by the tephra at Quartz Creek. The relative absence of massive icy bodies and tree and shrub material distinguishes the Quartz Creek Member from
853 Fig. 4. Co-isotope plot from 52 ground-ice samples collected in the Klondike area in summer 1997. Distinct groupings of sediments of different ages are associated with differing climatic regimes (see text for details).
the underlying Last Chance Creek Member. Despite the absence of visible ice, the Quartz Creek Member is ice rich, for all samples collected from this unit released water upon thawing. Of ten samples collected, the mean volumetric ice content was 65%. Intrusive ice was observed near the base of this unit at Quartz Creek at several places along a 1 km long section. No ice wedges occur within the Quartz Creek Member where it is overlain by Dago Hill sediments. Where younger silts do not occur above the Quartz Creek Member, there are ice wedges near the top of the unit. The lower contact of this unit is a thaw unconformity, visible where it truncates ice wedges of the underlying Last Chance Creek Member, but not where the formation directly overlies creek gravels. Nine samples of pore ice were collected from the Quartz Creek Member at Quartz Creek (site 2, Fig. 1) for stableisotope analysis. The values determined for δ18O (–31.9 to −29.3‰), and δD (–257 to –234‰), are considerably lower than values from other units (Fig. 4). Dago Hill Member The redeposited (upper) portion of the silt unit of Fraser and Burn (1997) was observed throughout the study area to comprise loess interbedded with dark organic material such as peat, rhizomes, twigs, and small branches, making the sediments very dark brown to black (Munsell code 7.5 YR 2.5/1). The sediments were seen in section up to 12 m thick near the base of Dago Hill (site 3, Fig. 1). The beds are tightly folded in places, and sandy lenses are common. The sedimentological contact with the Quartz Creek Member, where present, is planar and sharp and marked by an accumulation of organic material (Fig. 5). An 11.62 ± 0.09 ka BP (TO-6869; 13 450–13 820 calendar years ago; Stuiver et al. 1998) radiocarbon age was determined from wood collected at the bottom of this unit near the type section at the base of Dago Hill (site 6, Fig.1), and ages of 10.12 ± 0.38 ka BP (BGS-2016) and 10.18 ± 0.20 ka BP (BGS-2017) were determined from wood fragments and peat overlying gravel, near the base of this unit on Dominion Creek (site 7, Fig. 1). © 2000 NRC Canada
854 Fig. 5. Contact between Quartz Creek Member (QCM) and Dago Hill Member (DHM) at Quartz Creek (Site 2, Fig. 1). Scale bar at lower left is 1.5 m. Photograph taken in August 1997.
The sediments are ice rich, and 10–20 cm long ice lenses parallel to bedding are common. The unit has a parallel wavy layered cryostructure. Ice wedges up to 8 m in height are abundant within the member. Syngenetic and epigenetic ice wedges occur in valley bottoms (Fig. 6A), whereas very broad ice wedges, joined at their tops, are found on valley sides (Fig. 6B). These wedges, with upturned sediments at their margins, and vertical to nearly horizontal foliation, resemble anti-syngenetic wedges described by Mackay (1990). Organic and mineral material, as well as pool ice, have been incorporated into most of these anti-syngenetic wedges through erosion along their troughs. Bubbles typically occupy 30% of the ice by volume, are spherical to tubular, and commonly have orientations unrelated to foliation. The mean volumetric ice content of 43 samples collected from the Dago Hill Member was 70%, with a standard deviation of 15%. A continuous, 0.75 m thick bed of sediment-poor ice occurs at or just above the contact of this unit with the Quartz Creek Member. The ice in the bed has no foliation, but contains dispersed sediment, and 20% 1–3 mm spherical bubbles. The ice bed is penetrated by ice wedges, and is upturned at their margins. Although the lower sedimentological contact of this formation is quite evident, the lower cryostratigraphic contact is not identifiable by eye, owing to the lack of visible ice within the underlying Quartz Creek Member. The ice wedges and pool ice are truncated 1 m below the upper sedimentological contact with the organic unit. This 1 m zone contains planar to wavy aggradational ice lenses, up to 40 cm long. Although the contact with the organic unit may not be conformable, Fraser and Burn (1997) suggest relatively little denudation of the sediments occurred in valley bottoms before accumulation of organic material, because the ice wedges are not anti-syngenetic. Twenty-seven samples of pore and ice-wedge ice were collected from the Dago Hill Member. The range in δ18O values was –28.1 to –21.2‰, and in δD, –225‰ to –164‰ (Fig. 4). Thirteen samples were collected at approximately 50 cm intervals across the Quartz Creek Member – Dago Hill Member boundary at Quartz Creek, to determine the relative location of sedimentological and cryostratigraphic
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contacts (Fig. 7). The lowermost seven samples provided δ18O values between –32 and –31‰, and the uppermost three were between –27 and –25‰. These represent the samples unequivocally in the Quartz Creek Member and the Dago Hill Member, respectively. Of samples in between, one gave a δ18O value of –29.3‰, while the upper two samples collected below the organic bed that forms the stratigraphic contact between the Quartz Creek Member and the Dago Hill Member are characteristic of Dago Hill δ18O values (Fig. 7). If mixing of isotopically heavier Dago Hill Member water with the lighter Quartz Creek Member water occurred in a paleoactive layer, then the thaw unconformity representing the base of the Dago Hill Member lies about 2 m below the organic bed, as indicated in Fig. 7. Organic unit Disorganized organic material such as branches and peat make up this surficial unit, which Fraser and Burn (1997) reported up to 13.5 m thick. The lower sedimentological contact with the King Solomon Formation is unconformable, irregular and sharp. Narrow, syngenetic ice wedges occur at various levels within the unit, and can be found rejuvenating truncated ice wedges in the Dago Hill Member. At several locations, the tops of these wedges have been deformed downslope. The lower cryostratigraphic contact is a thaw unconformity, located where underlying ice wedges are truncated. The ice wedges originating at the base of the present active layer are neither as large nor as abundant as in the Dago Hill Member. Six samples from ice wedges and of pore ice were collected for isotope analysis, and provided the most enriched values of the four groups collected. δ18O values ranged from –24.6‰ to –20.3‰, and δD from –190‰ to –164‰ (Fig. 4). These values are similar to those obtained from other Holocene ground ice in central Yukon (Michel 1983; Burn et al. 1986).
Depositional and freezing history A nearly complete hillside section of the King Solomon Formation exposed at Last Chance Creek (site 1, Fig. 1) is shown in Fig. 8. This 17 m high section exposes the greatest thickness of the Last Chance Creek Member. The cryostratigraphy of the area is summarized in Fig. 9. This figure is a representative section compiled from the 21 exposures examined in 1997 and 1998. It combines the sedimentological and geocryological characteristics of the deposits and summarizes the stratigraphic positions of the various cryostratigraphic units described in the text. In addition to the physical cryostratigraphy, data have been collected on the stable isotopic composition of ground ice in the Klondike “mucks.” Stable isotope values from ground ice have been used to interpret generalized paleoclimatic conditions for sites in northwest Canada (Mackay 1983; Burn et al. 1986; Michel 1990). The interpretation is based on the isotope:temperature relation determined empirically by Dansgaard (1964) from numerous sites worldwide [1]
δ18O = 0.695T – 13.6
where T is the average annual surface air temperature, and δ18O is the weighted annual mean δ18O value in precipitation © 2000 NRC Canada
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Fig. 6. Ice wedges of the Dago Hill Member (A) characteristic syngenetic form in valley-bottom deposits (site 7, Fig. 1); (B) antisyngenetic form on hillsides (site 6, Fig. 1). Scale bars each 1.5 m. Photographs taken in July 1997.
at a site. The relation can only be used with caution, because, at any site, the isotopic composition of precipitation varies during the year, with lower δ18O values in winter than in summer (e.g., Fritz et al. 1987; Michel 1990). Maulé et al. (1994) collected soil water and shallow groundwater samples near Edmonton, Alberta, and found them to be, respectively, 0.8‰ and 2.2‰ more depleted in δ18O than weighted mean annual precipitation. Maulé et al. (1994) estimated that snowmelt contributed to 27% of the soil water and 44% of the groundwater at their study sites. These data provide an estimate of the net difference between precipitation and ground moisture under present conditions at a northern continental site. Relations similar to [1] are used routinely to reconstruct Pleistocene temperature variations from ice cores (e.g., Jouzel et al. 1993). With ground ice, some modification of the isotopic composition occurs in the soil water, making the temperature signal less precise. Last Chance Creek Member The >40 ka BP ages determined on wood pieces from this unit (Table 1) indicate deposition during, or prior to, the Reid–McConnell interglacial period, correlative with the
Boutellier Interval of unglaciated Beringia (Hopkins 1982). Syngenetic ice wedges within the unit indicate that permafrost has persisted in the Klondike from before McConnell glaciation. Ice-wedge growth must have occurred after gravel deposition, since the first stage of growth is truncated above the gravel, and there must have been further sediment to accommodate an active layer. The first paleoactive layer is likely the lower silt and ice lens-rich facies of this unit, while the upper, cryoturbated facies, which truncates the second ice-wedge growth period, may represent a second paleoactive layer. The ice lenses are likely aggradational ice, formed as the base of the active layer moved upward, either by addition of material to the ground surface, or by a decrease in surface temperature toward the end of the period, possibly with the onset of McConnell glaciation. The isotopic values are comparable to those obtained from ice within interstadial sediments in the western Canadian Arctic (Mackay 1983), further suggesting that temperatures were similar during emplacement of these sediments, most likely during an interglacial or interstadial period. In addition, the stable isotope values indicate more frigid conditions than during the Holocene, but the presence of ice wedges demon© 2000 NRC Canada
856 Fig. 7. δ18 O profile across the contact between Quartz Creek Member and Dago Hill Member at Quartz Creek (site 2, Fig. 1). The data were obtained from water which was released upon thawing of the sediment samples.
strates sufficient snow accumulation for the melt to infiltrate permafrost. Quartz Creek Member Permafrost must have existed throughout the deposition of this unit, which began by 27 ka BP (Fraser and Burn 1997), since older ice wedges have been preserved, as have mammal carcasses (Harington and Eggleston-Stott 1996). As indicated above, ice wedges that occur entirely within this unit are only found at sections without Dago Hill sediments. None of these ice wedges show evidence of syngenetic development. Both the radiocarbon ages and the isotopic composition of the ground ice suggest the unit accumulated under fullglacial conditions, since similar isotopic ratios have been determined for full-glacial sediments in the western Canadian arctic (e.g., Michel and Fritz 1982; Mackay 1983). Therefore, the absence of evidence for thermal contraction, that is, ice or sand wedges, is odd. Either snowmelt was insufficient for the development of ice wedges; snow cover was excessively thick, inhibiting thermal contraction; or the lack of ice wedges may follow from removal of the uppermost material in this unit, leaving only the lower portion of the original sediments. The absence of this member from broad valleys supports the latter hypothesis, but no anti-syngenetic ice wedges have been observed in the member, and ages obtained from organic material near the top of the member at Site 3 are close to the end of the McConnell glaciation, suggesting minimal removal of sediments. Independent evidence of a thin snow cover comes from the herbivorous glacial fauna of Beringia, which required access to forage throughout the year (e.g., Laxton et al. 1996). Some of these species, like the saiga antelope, had short legs and small
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hooves, and could only have survived in areas with thin snow cover (Harington and Cinq-Mars 1995). Hence, it is unlikely that an excessively thick snow cover inhibited thermal contraction. We conclude then, from examination of 21 exposures, that ice wedges did not form, due to insufficient snowmelt infiltration, and that existing ice wedges in this member formed in the period of accumulation of the Dago Hill Member. Dago Hill Member The oldest age (11.62 ± 0.09 ka BP) determined from sediments of the Dago Hill Member suggests accumulation began toward the end of McConnell glaciation. The abundance of organic debris at the base of this unit indicates warming three to four thousand years before the early Holocene thermal optimum (e.g., Burn et al. 1986; Mackay 1995; Burn 1997). The basal age of this member corresponds closely with the start of the Bølling–Allerød interstadial at 12.2 ka BP, recorded in Greenland ice cores (Johnson et al. 1992; Alley et al. 1993; Taylor et al. 1993). These ages also closely follow the start of the Beringian Birch Zone warm interval recorded by a rapid increase in dwarf Betula pollen, beginning 12–14 ka ago (Ager 1982; Hopkins 1982; Ritchie and Cwynar 1982). The Dago Hill Member has planar and convoluted bedding, with folding unrelated to displacement by ice wedges. Sandy lenses are abundant and the great thicknesses of this unit at some sites suggests that intense and rapid colluviation occurred, transporting organic material and silt from hillsides to valley bottoms. Evidence of this rapid accumulation in valley bottoms occurs as narrow, syngenetic ice wedges (Fig. 6A). Removal of material from hillsides has resulted in the development of the broad, anti-syngenetic wedges seen within this unit at these locations (Fig. 6B). The stable isotopic values obtained from ice within this unit, are characteristic of a range of conditions that vary from glacial to present, and represent climatic transition at the end of McConnell glaciation (Michel 1983; Burn et al. 1986). Pore and ice-wedge ice from the Last Chance Creek Member have similar isotopic ratios to the pore ice and many of the ice-wedge ice samples from the Dago Hill Member (Fig. 4), suggesting that surface temperatures at the time of deposition of these two units were similar. Organic unit The organic unit began accumulating at the start of the Holocene (Fraser and Burn 1997). The associated thaw unconformity is located at the base of aggradational ice in the top of the Dago Hill Member or Quartz Creek Member, whichever it overlies. Small epigenetic or syngenetic ice wedges penetrate older ice wedges truncated at the thaw unconformity, indicating that they did not begin growing before the start of the Holocene. The downslope deformation of these wedges is indicative of ongoing, long-term slope movement since their development. The isotopic composition of the ice in the unit encompasses values common in present materials. Stable-isotope characteristics The relation of δ18O to δD in precipitation is characterized by the Global Meteoric Water Line (GMWL; Craig 1961b) © 2000 NRC Canada
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857
Fig. 8. The King Solomon Formation exposed at Last Chance Creek (site 1, Fig. 1) in August 1997. The ice wedges in the Dago Hill Member (DHM) are abundant and extensive in comparison with those in the Last Chance Creek Member (LCC). Ice wedges are lacking in the Quartz Creek Member (QCM). The scale bar is 1.5 m long.
[2]
δD = 8 δ18O + 10
Local meteoric water lines (LMWL) have been determined for several stations in Canada, from samples of precipitation collected over periods of years (e.g., Fritz et al. 1987). A LMWL for Inuvik, N.W.T., the closest site to the Klondike for which data have been published (Vardy et al. 1998), is [3]
δD = 7 δ18O – 12.02
The slopes of LMWL from most sites are close to 8, but, at dry sites, the slope may be less (Craig 1961b), due to secondary evaporation. Results of all analyses for stable isotopes are presented in Fig. 4 as a co-isotope plot. The numbers of samples obtained from individual units were insufficient to obtain water lines for each unit, but the summary least-squares regression line for all samples is [4]
δD = 8.4 δ18O + 10.6
and the functional relation, where the precisions in determination of δ18O and δD are ± 0.2‰ and ± 2.0‰, respectively, is (Mark and Church 1977; Burn and Maxwell 1993) [5]
δD = 8.6 δ18O + 16.9
The standard errors of the slopes are 0.3 and 0.5 for eqs. [4] and [5], respectively. These relations are similar to the GMWL, suggesting that bulk and intrasedimental ice sampled from the King Solomon Formation and the organic unit may have originated directly from surface waters subject to minimal secondary evaporation. Maulé et al. (1994) found that there were no more than 3‰ differences between isotope values of soil and shallow groundwater, and that the waters were composed of both summer and winter precipitation. The variations between isotope signatures of the different units in Fig. 4, cannot be solely accounted for in this way, and, therefore, these groupings may instead represent significantly different temperature regimes associated with glacial, interstadial, and interglacial conditions. The most isotopically depleted ice, from the Quartz Creek Member, implies the coldest surface temperatures represented in the cryostratigraphy of the “muck” deposits (eq. [1]). The stable isotope values in this unit are consistent with interpretation of full-glacial conditions. In contrast, the highest stable isotope values from the Holocene organic unit are associated with interglacial conditions. The intermediate isotope values from sediments of the Last Chance and Dago Hill members represent deposition during interstadial conditions, with some values from Dago Hill Member sediments overlapping the Holocene signatures. Using (eq. [1]), the difference in mean temperature be© 2000 NRC Canada
858
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Fig. 9. Generalized cryostratigraphy of frozen, unconsolidated sediments in the Klondike area, YT. This figure is a representative section compiled from the 21 sections examined in 1997 and 1998. It combines the sedimentological and geocryological characteristics of the deposits and summarizes the stratigraphic position of the various cryostratigraphic units described in the text.
tween full-glacial and Holocene conditions is estimated at about 11°C, while the difference between full-glacial and interstadial conditions was about 6°C. In Greenland, Cuffey et al. (1995) have estimated that full-glacial temperatures were 15°C colder than in the Holocene, and similar estimates (10–14°C) have been determined in Siberia (Nikolayev and Mikhalev 1994). Ice wedges which penetrate the Quartz Creek Member are, on average, 3‰ enriched in δ18O with respect to pore ice. Mackay (1983) noted that ice wedges may be up to 5‰ depleted in δ18O with respect to host sediments, due to their infilling by snow meltwater. Therefore, the more enriched ice wedges which penetrate the Quartz Creek Member are likely from a different period, that is, the time of Dago Hill accumulation.
Regional implications The observations reported above imply that climatic amelioration from full-glacial conditions prevalent during McConnell glaciation began around 12 ka BP (~13 500 calendar years ago). The amelioration was characterized by development of a warmer and wetter climate. Since intrasedimental water can be derived from a combination of
rain and snow meltwater, then higher δ18O and δD values in the Dago Hill Member than in the Quartz Creek Member can be attributed to warmer climate, while the wetter conditions are inferred from the presence of ice wedges, the preserved woody pieces, and the sedimentary evidence of colluviation. The end of the McConnell glaciation was a period of considerable geomorphic activity, with removal of silt and organic material from valley sides and accumulation of these sediments in valley bottoms. The cryostratigraphy of the Last Chance Creek Member contains features similar to the Dago Hill Member, indicating that the climate prior to McConnell glaciation was also of interstadial character, considerably cooler than conditions in the region today. In addition to the geomorphic effects of the amelioration of climate represented by the Dago Hill Member, conditions in the region also improved for humans. While there are only a few indications of human occupation in eastern Beringia before 12 ka BP, subsequent evidence is relatively abundant (Morlan 1987; Hoffecker et al. 1993). The climate change may, therefore have been important for the peopling of eastern Beringia. A thicker snow cover may have hindered the escape of prey hunted by the first peoples, both directly by arresting their movement, and indirectly through modification of the ecosystem to which the animals were ac© 2000 NRC Canada
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customed. Hoffecker et al. (1993) suggest that the appearance of woody plants and trees used for fuel was critical for human colonization.
Conclusions From the material presented above, we conclude (1) The ice-rich loessal deposits in the study area may be considered the cryostratigraphic King Solomon Formation, comprising three members. (2) The lowermost Last Chance Creek Member, consisting of ice-rich silt, syngenetic ice wedges, and cryoturbated organic material, is a >40 ka deposit with 18O and D concentrations indicative of interstadial conditions prior to the McConnell glaciation. (3) The Quartz Creek Member consists of ice-rich Late Wisconsinan, McConnell loess. The δ18O and δD values for ice from this member are characteristic of full-glacial conditions. The member was deposited when conditions were sufficiently dry to inhibit the growth of ice wedges. (4) The Dago Hill Member represents a 3–4 ka period of colluviation at the end of McConnell glaciation. The abundance and size of ice wedges indicates more snow cover than during full-glacial conditions, while isotopic concentrations in ground ice which are higher than in the Quartz Creek Member also suggest a shift to warmer temperatures. (5) The surficial organic unit contains ground ice with 18O concentrations characteristic of Holocene sediments. Ice wedges in this unit are neither as large nor as abundant as in the Dago Hill Member. (6) Co-isotope relations suggest that most ice within the deposits was frozen directly from infiltrating surface rain or snow meltwater, although intrusive ice is present at the base of some deposits.
Acknowledgments The research was supported by the Natural Sciences and Engineering Research Council of Canada, the Northern Scientific Training Program, and the Exploration and Geological Services Division, Department of Indian Affairs and Northern Development. Logistical support provided by J.V. Clark School, Mayo, Yukon Territory and the Renewable Resources Branch, Territorial Government of Yukon is critical to continuing field research in central Yukon, as is support provided by Yukon College in the form of Northern Research Endowment Fellowships. The generosity of the Klondike placer miners and their families, as well as field assistance from Anne-Pascale Bartleman is acknowledged with gratitude. J.A. Donaldson, A.G. Lewkowicz, and F.A. Michel provided helpful discussion of the thesis from which this paper is derived. Helpful comments on radiocarbon ages from Yukon were provided by R.N. McNeely, and R.E. Morlan. The paper was improved through thoughtful reviews by L.D. Dyke, T.W.D. Edwards, C.E. Schweger, and S.A. Wolfe.
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