Redating the moraines in the Kromer Valley (Silvretta Mountains) – New evidence for an early Holocene glacier advance

Abstract

Among the many well-preserved moraine systems that are found throughout the Austrian Silvretta Mountains, a set of prominent moraines in the upper Kromer Valley located in front of the ‘Little Ice Age’ positions are of particular interest as they have been allocated to contrasting time periods in the past. Initial assumptions associated them with the transition period from the Younger Dryas to the early Holocene, but first exposure dating results published in 2006 suggested a relation to the ‘8200-year event’. However, since then, the lack of comparable evidence elsewhere in the Alps prompted a re-evaluation of the moraines at this site based on the recalculation of the original ages with the recently available lower ¹⁰Be production rate (‘Northeast North America’) and by dating additional boulders in the Kromer Valley and in the neighbouring Kloster Valley. The newly sampled boulders (n = 9) show, depending on interpretation, moraine stabilization around 10,000 years. These ages implicate that glacier termination occurred considerably earlier than during the previously suggested 8200-year cooling event, but also clearly after the climatic cold period of the Younger Dryas (>11,700 years) and the Preboreal Oscillation (~11,400 years). Consequently, our results suggest that the Kromer Valley moraines represent a marked glacier advance during the Boreal period before the final recession to mid- and late Holocene glacier size.

Keywords

Alps early Holocene exposure dating cosmogenic nuclides moraines

Introduction

After the Last Glacial Maximum (LGM), multiple glacier advances of decreasing extent during the late glacial period (Heiri et al., 2014; Ivy-Ochs et al., 2008) have created a sequence of successive moraine systems in many Alpine valleys (Maisch, 1982). Generally, the moraines of the multi-phased ‘Egesen’ stadial (Younger Dryas (YD) cold period) are easily recognizable and in many valleys represent final evidence of former glacier margins prior to the late Holocene (Ivy-Ochs et al., 2008). However, in some places, well-developed moraines of another advance can be found, usually within a distance of several hundred metres beyond the respective ‘Little Ice Age’ (LIA) positions. The equilibrium line altitudes (ELAs) associated with the palaeoglaciers are roughly 100 m below the LIA ELA. A typical example was identified in the Western Austrian Ferwall Mountains and named ‘Kartell stadial’ after a distinct moraine set in the Kartell cirque of the upper Moos Valley (Figure 1; Fraedrich, 1979; Sailer, 2001). The Kartell locality was ¹⁰Be dated by Ivy-Ochs et al. (2006), and based on the original ages (10,800 ± 1000 years), the glacier advance was assigned to the closest multi-centennial climatic cold event, the Preboreal Oscillation (PBO) centred around 11,400 years (Fisher et al., 2002; Rasmussen et al., 2007; Van der Plicht et al., 2004).

Figure 1.

Map of the reconstructed palaeoglaciers in the study area on the basis of Hertl (2001). Abbreviations in the overview map represent other investigated sites – T.N.: Tsidjiore Nouve; BE.: Belalp; ST.: Steingletscher; KA.: Kartell.

To affirm the age of the Kartell advance elsewhere, additional moraines in a comparable location in the Alps needed to be dated. The Kromer Valley in the neighbouring Silvretta Mountains, ~20 km SW of the Kartell site, seemed to provide an appropriate sequence of well-preserved moraines. They were first identified by Kinzl (1929) and were subject to further systematic studies by Gross (1974), Gross et al. (1977) and Hertl (2001). Situated well up-valley from the sequence of the multi-phased Egesen stadial moraines, the moraine system in the Kromer Valley represents a glacier margin delimited by end and lateral moraines. All other moraines in comparable stratigraphical positions <~1 km in front of the LIA moraines in the Silvretta Mountains were named after these and allocated to the ‘Kromer stadial’ (Gross, 1974; Gross et al., 1977; Hertl, 2001). It was assumed that the Kromer stadial represented an advance contemporaneous to the Kartell stadial (Hertl, 2001).

However, when actually exposure dated, the ¹⁰Be data yielded surprising results (8010 ± 360 to 8690 ± 410 years) with an average age of 8410 ± 690 years (Kerschner et al., 2006). Resorting to these data scattered closely around the prominent ‘8200-year event’ (Alley and Agustsdottir, 2005; Alley et al., 1997; Prasad et al., 2009), Kerschner et al. (2006) cautiously assumed a connection between this Holocene cold event and the Kromer stadial glacier advance. Thus, for the first time, the existence of a larger glacier during the 8200-year event than during the LIA was indicated. Since then, however, no other moraine systems in the Alps could be correlated to the 8200-year event. On the contrary, recent investigations from Glacier de Mont Miné in the southwestern Swiss Alps show that large glaciers advanced during the 8200-year event, but remained clearly smaller than during the LIA (Nicolussi and Schlüchter, 2012).

Moreover, since the calculation of the first ¹⁰Be ages in the Kromer Valley, new evidence now generally indicates that lower ¹⁰Be production rates should be applied in the North Atlantic and European regions (e.g. Balco et al., 2009; Goehring et al., 2012; Heyman, 2014; Small and Fabel, 2015; Young et al., 2013). Consequently, the ages shift towards the earlier Boreal period. In light of these considerations, we have deemed it necessary (1) to recalculate the results published in Kerschner et al. (2006) and (2) to amend them with new ¹⁰Be exposure ages from the Kromer Valley locality and the neighbouring Kloster Valley.

The Kromer stadial

Kromer Valley site

The Kromer Valley, in the northern Silvretta Mountains of western Austria, is a short valley about 4 km long that drains northwards into the Ill River Valley, a tributary of the Rhine River. Lying within the Austroalpine Silvretta Nappe, the research area is mainly dominated by orthogneiss, amphibolites and paragneiss (Bertle et al., 1980). Along the southern catchment boundary, coinciding with the main Silvretta Mountain divide, the highest peak (Grosses Seehorn) reaches an altitude of 3121 m a.s.l. Four small-sized (<0.2 km²) north-facing glaciers are currently found under the steep cirque walls. In regard to climate, the research area is situated in a transitional position between the humid north-western Alpine fringe and the dryer central Alps. The upper Kromer Valley exhibits average annual precipitation sums between 1600 and 2000 mm for the period 1961–1990 (Geographie Innsbruck, 2013).

The prominent, well-preserved and boulder-rich moraines in the Kromer Valley form two adjoining lobes stemming from glaciers that advanced from the Schweizer and Kromer cirques and from another glacier that originated in the Litzner and Verhupf cirques (Figure 1). A detailed description is found in Kerschner et al. (2006). Both glaciers extended downward to an altitude of ~2150 m a.s.l. where the Schweizer–Kromer glacier formed several successive terminal moraine ridges in a flatter section of the valley floor, while the Verhupf–Litzner glacier terminated in a relatively steeper section of the valley. Multiple right lateral moraines of the Verhupf–Litzner glacier can be traced for a distance of ~1 km up-valley, where they are intersected discordantly by a relict rock glacier from a small cirque NW of the Lobturm peak (2867 m a.s.l.).

Both Kromer stadial end moraines are ~0.6–0.8 km down-valley from the LIA moraines. Beyond the Kromer terminus, a further prominent and well-preserved but boulder-free right lateral moraine bears witness to an earlier advance with a glacier tongue reaching to ~1980 m a.s.l. as previously identified by Hertl (2001). Based on the local sequence of moraines, Hertl (2001) assigned this undated moraine to a fourth Egesen substadial (‘Egesen IV’) which he identified at a total of four different sites in the Silvretta Mountains. The ‘Egesen III’ substadial was identified nearly 3 km down-valley from the Kromer stadial moraines (Hertl, 2001). The maximum Egesen stadial glacier was a complex dendritic glacier with a glacier end near the village of Partenen in the Montafon valley. It can be reconstructed from numerous lateral moraines. A more detailed description of the moraines and the research area can be found in Hertl (2001) and Kerschner et al. (2006).

Based on the recent availability of high-resolution airborne laser scanning data (spatial resolution: 1 m × 1 m), we reconstructed the glaciers in the study area again and calculated their ELAs with an accumulation area ratio (AAR) of 0.67 (Gross et al., 1977; Kerschner, 1990), since in many studies it has led to reliable results in the Alpine region (e.g. Maisch, 1982). The reference LIA ELA is taken from Kerschner et al. (2006). The calculated ELAs are slightly higher than those given by Hertl (2001), and consequently, the lowering of the ELA is somewhat smaller (−60 and −70 m; Table 1).

Table 1.

Glacier stadials in the Kromer, Kloster and Upper Ill Valleys. ELAs are calculated with an accumulation area ratio of 0.67. Stadial names according to Hertl (2001).

Stadial	Glacier	ELA	Reference LIA ELA	ΔELA	Source
‘Kromer’	Schweizer–Kromer Glacier	2475	2545	−70	This paper
	Verhupf–Litzner Glacier	2490	2550	−60	This paper
	Kloster Valley Glacier	2565	2635	−70	This paper
‘Egesen IV’	Kromer Valley Glacier	2440	2555	−115	This paper
	Kloster Valley Glacier	2535	2645	−110	Hertl (2001)
‘Egesen III’	Upper Ill Valley Glacier	2365	2565	−180	Hertl (2001)
‘Egesen II’	Upper Ill Valley Glacier	2245	2565	−320	Hertl (2001)
‘Egesen I’	Upper Ill Valley Glacier	~2195	2575	~−380	Hertl (2001)

LIA: ‘Little Ice Age’; ELA: equilibrium line altitude.

Kloster Valley site

The Kloster Valley extends ~4 km in a north-northeasterly direction and is the neighbouring valley to the Southeast of the Kromer Valley (Figure 1). At present, the small Kloster Valley Glacier (~0.8 km²) still exists in the upper cirque west of the Schneeglocke (3223 m a.s.l.), the highest peak in the catchment. The LIA moraine is well preserved and reaches down to an altitude of ~2300 m a.s.l. Some 2 km down-valley from the LIA position there are two end moraines each forming an arc traversing the valley floor, both separated by a distance of ~100 m in a flat part of the valley. These were assigned by Hertl (2001) to the Kromer stadial based on the moraine sequence, their ΔELAs and their morphological attributes, that is, freshness and the abundance of large clasts are comparable to the Kromer-type locality.

Methods

Exposure dating with the cosmogenic nuclide ¹⁰Be

Sample sites

The Kromer moraines were exposure dated with the cosmogenic nuclide ¹⁰Be (Dunai, 2010; Lal, 1991; Nishiizumi et al., 1993). All sample positions (Figure 2) were carefully chosen in order to prevent dating boulders that may have been potentially deposited by non-glacial processes or that may have undergone excessive post-depositional modification. In all cases, quartz veins on clast-supported boulders located distinctly above the surrounding setting were sampled by means of a hammer, chisel and battery-powered saw. Details on the sample sites and the properties of the sampled boulders are listed in Table 2.

Figure 2.

(a, b, c) Sample sites and their corresponding exposure ages in 1000 years.

Table 2.

Sample and boulder properties. Latitude, longitude and altitude in WGS84 coordinates from Vorarlberg-Atlas (vogis.cnv.at). Shielding correction includes the effects caused by mountain topography, dip and strike of the various boulder surfaces. Density is 2.7 g/cm³ throughout.

Sample no.	Latitude (DD)	Longitude (DD)	Altitude (m a.s.l.)	Sample thickness (cm)	Shielding correction	Boulder size (~m³)	Height above ground (m)	Moraine system	Location	Description
KR-1*	46.90848	10.03922	2219	1	0.983	2.5	1.7	Verhupf–Litzner	In boulder field ~40 m within the western lateral moraine, ~260 m from the terminus	Clast supported
KR-2*	46.90884	10.03927	2211	1	0.966	1	1	Verhupf–Litzner	In boulder field ~20 m on proximal side of the western lateral moraine, ~240 m from the terminus	Clast supported
KR-3*	46.90972	10.03810	2181	1	0.961	4.5	1.5	Schweizer–Kromer	Right moraine arc representing second terminal moraine crest	Clast supported
KR-4*	46.91019	10.03733	2169	2	0.982	4.5	1.5	Schweizer–Kromer	Proximal side of second terminal moraine	Clast supported
KR-5*	46.91119	10.03725	2138	1	0.958	3	1.2	Schweizer–Kromer	Right-central position, ~80 m within the outermost terminal moraine	Cirque floor on glacial till
KR-201	46.91062	10.03534	2173	1	0.943	2	1	Schweizer–Kromer	Crest of second terminal moraine	Clast supported
KR-203	46.90597	10.04295	2257	0.8	0.979	4.5	3	Verhupf–Litzner	Boulder field on proximal side of right lateral moraine, ~450 m from the terminus	Clast supported
KR-205	46.90970	10.04167	2166	3	0.966	4	1	Verhupf–Litzner	Left-central position, ~50 m on proximal side of glacier terminus, ~15 m west of the Litzner stream	Embedded in glacial matrix
KR-303	46.90190	10.04681	2383	1	0.936	1	1.5	Verhupf–Litzner	Moraine crest of outermost right lateral moraine, ~1000 m from the terminus	Clast supported
KR-304	46.90065	10.04784	2394	2	0.965	0.5	1	Lobturm rock glacier	Location at head of discordant relict rock glacier, ~80 m within the Kromer lateral moraine	Clast supported
KR-305	46.90202	10.04458	2309	0.5	0.967	1.5	1.5	Verhupf–Litzner	Boulder field on proximal side of morainic ridge ~80 m beyond the LIA extent and ~150 m inside the Verhupf–Litzner lateral moraine	Clast supported
KL-1	46.89581	10.08011	2146	0.8	0.960	1	0.5	Klostertal	Location on crest of outermost end moraine, ~30 m SE of stream	Clast supported
KL-2A	46.89564	10.07842	2149	1	0.960	18	2	Klostertal	Huge boulder (~18 m³) on crest of the inner end moraine, 20 m SE of stream	Clast supported
KL-2B	46.89564	10.07842	2149	1	0.960	18	2	Klostertal	Same boulder as KL-2A	Clast supported

Published in Kerschner et al. (2006).

Exposure age calculations

Sample preparation was carried out at the Laboratory of Ion Beam Physics at ETH Zurich and AMS measurements were performed at ETH on the 6-MV tandem accelerator and 0.5-MV tandem accelerator Tandy. The ETH in-house ¹⁰Be AMS standard S555 (isotope ratio: 95.5 × 10⁻¹²; Kubik and Christl, 2010) was applied for samples KR-1 to KR-5, whereas the ⁹Be/¹⁰Be ratios for the remaining samples were referenced to the 07KNSTD standard (2.85 × 10⁻¹²; Nishiizumi et al., 2007). Following Schimmelpfennig et al. (2012), we chose the regional ¹⁰Be production rate for northeastern North America (NENA; Balco et al., 2009) of 3.87 ± 0.19 atoms/g (SiO₂)/year at sea level from the currently available regional production rates in the circum-North-Atlantic area as it has proven useful in the Alps for the time period investigated in this paper (Claude et al., 2014). We use the time-dependent production model (Lm) of Lal (1991) and Stone (2000). The exposure ages are presented in Table 3 and were calculated according to Balco et al. (2009) with the Cronus Earth Calculator (http://hess.ess.washington.edu/), version 2.2. Based on considerations by André (2002), a rock surface erosion rate of 1 mm/1000 years was used throughout. For comparison, we also added the ages calculated with a zero-erosion rate in Table 3, but we will only comment on the erosion-corrected ages. No additional snow correction was applied since most boulders are situated in wind-swept positions, where winter snow is usually thin or missing altogether (see Kerschner et al., 2006).

Table 3.

AMS measurement properties and calculated ages.

Sample no.	AMS standard^a	¹⁰Be atoms/g	AMS measured error (%)^b	¹⁰Be age, no erosion (a)	External uncertainty (a)	¹⁰Be age, erosion corrected (a)^c	External uncertainty (a)
KR-1*	S555	2.46 × 10⁵	4.8	9650	660	9730	670
KR-2*	S555	2.51 × 10⁵	4.7	10,070	680	10,160	690
KR-3*	S555	2.32 × 10⁵	5.0	9550	670	9630	680
KR-4*	S555	2.28 × 10⁵	4.5	9340	620	9420	630
KR-5*	S555	2.30 × 10⁵	4.0	9790	620	9870	630
KR-201	07KNSTD	2.14 × 10⁵	9.6	9890	1060	9970	1080
KR-203	07KNSTD	2.52 × 10⁵	12.5	10,570	1420	10,670	1440
KR-205	07KNSTD	1.99 × 10⁵	5.2	9170	660	9240	670
KR-303	07KNSTD	2.41 × 10⁵	4.3	9710	630	9790	640
KR-304	07KNSTD	2.59 × 10⁵	5.0	10,140	710	10,220	720
KR-305	07KNSTD	1.98 × 10⁵	4.5	8070	530	8120	540
KL-1	07KNSTD	2.25 × 10⁵	5.1	10,400	730	10,490	740
KL-2A	07KNSTD	1.93 × 10⁵	7.5	8900	790	8970	810
KL-2B	07KNSTD	2.07 × 10⁵	5.3	9560	690	9640	700

The ETH in-house standard S555 assumes an isotope ratio of 95.5 × 10⁻¹² (Kubik and Christl, 2010). 07KNSTD is based on an isotope ratio of 2.85 × 10⁻¹² (Nishiizumi et al., 2007).

Uncertainties represent 1σ deviation which comprises AMS counting errors and errors based on the normalization to blanks and standards.

Erosion corrected with 1 mm/1000 years.

Published in Kerschner et al. (2006) and recalculated in this paper with ¹⁰Be production rates according to Balco et al. (2009).

Results

The spatial distribution of the sample ages is depicted in Figure 2. All ages are also listed in Table 3 with external uncertainties representing ±1σ. The original five samples (KR-1 to KR-5), first presented in Kerschner et al. (2006), were recalculated and, because of the lower production rate, exhibit ages ~1000–1500 years older than originally published in Kerschner et al. (2006). Of the additional samples taken in the context of this study, the calculated ages scatter from ~8120 ± 540 (KR-305) to 10,670 ± 1440 years (KR-203; see Figure 3).

Figure 3.

All available Kromer stadial exposure ages. The grey bars represent the cold periods of the Younger Dryas (YD), Preboreal Oscillation (PBO), 9200-year and 8200-year events. Other abbreviations – SK: Schweizer–Kromer moraine system; VL: Verhupf–Litzner moraine system; IVL: Inner Verhupf–Litzner moraine (see Figure 2); RG: Rock glacier; KL: Kloster Valley moraine system.

Four ages ranging from 9420 ± 630 (KR-4) to 9970 ± 1080 years (KR-201) stem from the Schweizer–Kromer end moraines, while five ages (9240 ± 670 years; KR-205 to 10.670 ± 1440 years; KR-203) are associated with the Verhupf–Litzner moraine system. Of these, KR-1, KR-2 and KR-205 belong to the lower lobus area, whereas KR-203 and KR-303 were taken along the series of right lateral moraine ridges up to ~1000 m up-valley from the end moraines. KR-304 (10,220 ± 720) represents a boulder located on top of the terminus of a relict rock glacier that dissects the Kromer moraine discordantly. KR-305 was extracted from a boulder on the proximal side of a rather inconspicuous morainic ridge about 80 m outside the LIA terminal position of Litzner Glacier. It exhibits the outstanding young age of 8120 ± 540 years.

In the nearby Kloster Valley, three samples were extracted from the dual moraine system. KL-1 on the outer moraine offers an age of ~10,490 ± 740 years, while on the inner moraine KL-2A exhibits an age of 8970 ± 810 years and KL-2B of 9640 ± 700 years.

The data set was checked for outliers with the reduced chi square test (see Balco, 2011). It shows that KR-305 is a clear outlier. Furthermore, based on geomorphological considerations, KR-305 may represent an independent younger glacier advance. Matrix-supported boulder KR-205 is considered questionable since in retro perspective we cannot exclude the possibility of tilting, a temporary sediment cover with a successive exhumation due to its location in proximity to the Litzner stream or, because of its location, an above average snow cover. Hence, neither is included in further analyses.

To determine the age of moraine stabilization, we chose two different approaches: (1) the calculation of the mean and its error, and (2) the frequency distribution from a Kernel Density Estimation (KDE; see Vermeesch, 2012). The errors given for the mean ages were determined from the mean values and the external errors with the help of cumulated probability densities (CPDs). Gaussian KDEs (Vermeesch, 2012) were computed with the density plotter program by Vermeesch (2012) and are depicted in Figure 4. There they are placed in the greater context of other recent studies of early Holocene moraines in the Alps. The most probable ages (highest f(x) of KDE) and the 68% quantile (similar to the 1σ range) are placed in relation to the arithmetic means.

Figure 4.

Comparison of Kernel Density Estimations (KDEs) derived from exposure ages of different early Holocene moraine systems in the Alps.

In compliance with these approaches, the average ages and KDEs of the following four selections of samples were calculated (see Table 4):

Selection 1: the entire Kromer Valley site without the outliers KR-205 and KR-305;

Selection 2: the Schweizer–Kromer moraine system;

Selection 3: the Verhupf–Litzner moraine system without the outliers KR-205 and KR-305;

Selection 4. all samples of the Kloster Valley moraine system.

Table 4.

Comparison of age calculations for different sample selections.

Sample selection	Sample names	Description	Arithmetic mean and 1σ error (years)	Max. f(x) of KDE (years) and 68% confidence range
(1)	KR-1, KR-2, KR-3, KR-4, KR-5, KR-201, KR-203, KR-303, KR-304	Most representative samples at the Kromer-type locality (Kromer Valley)	9940 ± 910	9900/9240–10,600
(2)	KR-3, KR-4, KR-5, KR-201	Most representative samples of the Schweizer–Kromer moraine system	9720 ± 810	9740/9320–10,080
(3)	KR-1, KR-2, KR-203, KR-303	Most representative samples of the Verhupf–Litzner moraine system	10,090 ± 1000	10,020/9420–10,720
(4)	KL-1, KL-2A, KL-2B	All samples of the Kloster Valley moraine system	9700 ± 980	9660/8580–10,800

KDE: Kernel Density Estimation.

Table 4 shows that both approaches yield practically similar results.

Expanding the number of absolutely dated boulders did not change the results conceivably. Averaging samples KR-1 to KR-5 from Kerschner et al. (2006) gives a mean of 9760 ± 710 years. Thus, the new ages are not significantly different from the previously published and recalculated sample ages.

Discussion

Exposure ages and their spatial distribution

Based on the mean ages of the Schweizer–Kromer and Verhupf–Litzner moraine systems (Table 4), a distinct age difference between both adjacent glacier lobes cannot be recognized. This is in agreement with the morphological situation and the similar ΔELAs of −60 and −70 m, respectively, which likewise suggest a coeval glacier advance of both Kromer Valley glaciers. Furthermore, as the mean age and the ELA of the neighbouring Kloster Valley moraines are not significantly different than in the Kromer Valley, a simultaneous occurrence of glaciers in these valleys is assumed.

Interpreting the age of moraine stabilization

All four selections of samples yield mean ages within a 400-year period between 9700 ± 980 (selection 4; Kloster Valley) and 10,090 ± 1000 years (selection 3; Verhupf–Litzner system). The calculation of the unweighted arithmetic mean age and its error is based on the supposition that the average age and its statistical error from a specific moraine system are the best estimate of the actual age of moraine stabilization. In such a case, there would be a trade-off between blocks with some pre-exposure (occurrence usually quite rare) and those subject to post-depositional shielding or excessive post-depositional surface erosion (Putkonen and Swanson, 2003). Since the latter processes are generally more common, the average age tends to likely represent a minimum age for the stabilization of the dated feature (Briner et al., 2005; Hallet and Putkonen, 1994; Heyman et al., 2011; Putkonen and Swanson, 2003). As most boulders are clast-supported, post-depositional exhumation should not be an issue here, while higher-than-average surface erosion due to frost shattering and frost-induced excessive spallation might be a process worth considering in some cases.

In addition, KR-304 represents prolonged discontinuous permafrost activity subsequent to the recession of the Kromer glacier. Under the assumption of a rock glacier surface velocity of ~1 m/year (Barsch, 1988), its morphostratigraphic position indicates continued rock glacier activity for about a century following the melt-back of the glacier. Consequently, the exposure age of 10,220 ± 720 years renders not only an age for rock glacier stabilization but also a minimum age for moraine deposition. Based on all these considerations, we suggest that the mean ages of the various moraines should be considered lowest age constraints and that a somewhat older age for moraine stabilization should be assumed.

Exposure ages and their association with early Holocene climate

Based on the recalculation of the original ages and the additional sampling of further boulders, and with the presently available sea level – high latitude production rates of nearly 4 atoms/g (SiO₂)/year – a stabilization of the Kromer moraines during the Boreal period around 10,000 years seems to be reasonable. Even the application of slightly higher ¹⁰Be production rates, inducing a shift in ages by up to a few hundred years, does not lead to a significantly different conclusion. Hence, our investigation points to the age of a glacier position at least 1800 years prior to the 8200-year event. Therefore, our data suggest that glacier activity at the Kromer site should be placed in connection with an earlier phase of cooler and/or wetter climate conditions clearly preceding the 8200-year event. This disagrees with the ages given by Kerschner et al. (2006), which were calculated with the higher sea-level production rate of 5.1 ± 0.3 atoms/g (SiO₂)/year. Considering that this dating method tends to yield minimum ages for moraine stabilization, a correlation to the 9200-year event (Fleitmann et al., 2008) can also be excluded. However, evidence for a correlation with the older PBO centring around 11,400 years is also weak. Even when considering all ¹⁰Be ages to be minimum ages (Putkonen and Swanson, 2003), and the rock glacier with its single age of 10,220 ± 720 years (KR-304) to be a younger feature than the moraine, does only sample KR-203 show an age (10,670 ± 1440 years) that comes close to the nearest older distinct climatic cold phase of the PBO, but it has a rather large error. Beyond that, no plausible linkage to the YD can be made. It is conceivable that the ages shown here may indicate the last period of relatively large glacier extents in the early Holocene before the onset of warmer early to mid-Holocene climate conditions.

Furthermore, in the Kromer Valley, the undated boulder-free ‘Egesen IV’ substadial in the sense of Hertl (2001), located beyond the Kromer positions (Figure 1), may correspond to the Kartell stadial in the Ferwall Mountains (Kerschner et al., 2006). A comparison of both palaeoglaciers in the Kromer Valley reveals a ΔELA difference of 40–50 m, which is similar to the ΔELA difference between the Kromer- and Kartell-type localities (45–50 m). In two comprehensive studies of glacier history in the Ferwall Mountains (Sailer, 2001) and the adjacent Silvretta Mountains (Hertl, 2001), the ELAs and ΔELAs of all Kartell stadial localities in the Ferwall Mountains and ‘Egesen IV’ and Kromer stadial localities (latter both Silvretta) were calculated. The broad array of ΔELAs range from −150 to −60 m and suggest that these moraines may not have all formed coevally, but possibly successively in at least two different phases. This assumption is substantiated by (1) field evidence of some valleys exhibiting two moraine sequences in the respective stratigraphic position (e.g. Kromer Valley, Kloster Valley, Ochsen Valley, Hertl, 2001) and (2) the earlier PBO or late YD age at the Kartell-type locality with the larger ΔELA (–120 m) and the younger Boreal age of two Kromer stadial localities with the smaller ΔELAs (−60 and −70 m). Hence, the older moraines mentioned above may possibly correlate with the Kartell stadial (late YD/earliest Holocene), while the younger system corresponds to the Kromer stadial (Boreal period).

In a recent investigation by Schimmelpfennig et al. (2012), moraines at Tsidjiore Nouve in western Switzerland were exposure dated and yielded a time period of 11,450 ± 380 (moraine I, Figure 4) and 11,210 ± 200 years (moraine II, Figure 4) for their stabilization, which is clearly older than the ages reported here. A further study by Schimmelpfennig et al. (2014) of two moraines at Steingletscher in central Switzerland used the ‘Arctic’ production rate (Young et al., 2013). Recalculated with the NENA production rate (Balco et al., 2009), their outermost Holocene moraine (moraine I, Figure 4) exhibits an age of 11,100 ± 200 years and a younger inner moraine (moraine II, Figure 4) dates to 10,400 ± 400 years. Another comparable glacier position at Belalp (Valais, Switzerland) was dated by Schindelwig et al. (2011) to 10,620 ± 220 years (recalculated with the NENA production rate, moraine IV, Figure 4). These moraine systems support the proposition of two different successive periods of moraine stabilization in the early Holocene, the first one before 11,000 years and the second one nearly 1000 years later. In the Alpine region, some non-glacial evidence has been found to support cooler climatic conditions at ~10,200 years (Joannin et al., 2013). Likewise, a multiproxy investigation of lake sediments in the Austrian Lower Tauern Range by Schmidt et al. (2009) indicates a cool and wet period climaxing at ~10,200 years. Further studies show temperature declines between ~10,600 and 10,200 years (Tinner and Kaltenrieder, 2005) and from ~10,700 to 10,500 years (Heiri et al., 2004). In any case, the period of advanced early Holocene glaciers clearly larger than LIA in the Alps came to an end by the latest 10,000 years, which is confirmed among others by Nicolussi and Patzelt (2000), who showed that between ~10,100 and 8900 years the Austrian Pasterze glacier was smaller than in AD 2000.

An age of 10,000–10,500 years for the Kromer moraines also agrees with the NGRIP δ¹⁸O record (Rasmussen et al., 2007) that implicates a slow increase in temperature for about one and a half millennia following the PBO. Climatic conditions still generally favourable to glacier formation during this time period have also been recorded from various other sites in the North Atlantic region (e.g. Berner et al., 2010; Björck et al., 1997, 2001; Bond et al., 1997; Lind and Wastegård, 2011; Rasmussen et al., 2007, 2011). Compared with southwestern Norway, an age of ~10,000 years for the Kromer advance would correspond to the widespread Erdalen 1 re-advance (Bakke et al., 2005; Dahl et al., 2002; Matthews et al., 2008; Nesje, 2009), while an older age of ~10,500 years could be an equivalent to the Jondal 2 re-advance of Bakke et al. (2005), both of which were considerably larger than the LIA extent of the glaciers.

Finally, the outlier KR-305 (8120 ± 540 years), lying on an indistinct morainic ridge only 80 m beyond the LIA moraine of Litzner Glacier, may represent a glacier position in connection to the 8200-year event. While large glaciers, for example, Glacier de Mont Miné (Nicolussi and Schlüchter, 2012), remained well behind their LIA maximum because of a longer reaction time, the possibility of smaller glaciers, such as in the Kromer Valley, reaching LIA extents during the ~130-year cold period of the 8200-year event cannot be ruled out. However, more samples from this position would be needed to verify this eventuality.

Conclusion

The re-evaluation of the Kromer-type locality in the Austrian Silvretta Mountains has demonstrated that the previously suggested glacier response to the ‘8200-year event’ (Kerschner et al., 2006) cannot be confirmed. The recalculation of the previously published exposure ages with the NENA ¹⁰Be production rate (Balco et al., 2009) while amending additional samples also from associated moraines in the neighbouring Kloster Valley points to a considerably older age for moraine stabilization. Based thereupon, an early Holocene glacier advance preceding the 8200-year and 9200-year climatic events can be ascertained. However, the mean age ranging from 9700 ± 980 years at the Kloster Valley site to 10,090 ± 1000 years for the Verhupf–Litzner Glacier also suggests glacier termination clearly later than the PBO (~11,400 years) and in clear subsequence to the even earlier YD cold period (>11,700 years). Consequently, the stabilization of the Kromer moraines can be considered to represent the final cessation of a period of early Holocene glacier activity in the Alps following the recession of glaciers from their late YD/PBO positions. Thus, in accordance with other recent studies carried out in the Alps, our results point to a final phase of glacier activity beyond their LIA positions during the Boreal time period.

Footnotes

Acknowledgements

We are grateful to the INTIMATE EU COST Action in the form of two short-term scientific missions to ETH Zurich for sample preparation. We acknowledge the ETH AMS group for their laboratory support and the conduction of AMS measurements.

Funding

This study was funded in the scope of project P23601 by the Austrian Science Foundation FWF.

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Redating the moraines in the Kromer Valley (Silvretta Mountains) – New evidence for an early Holocene glacier advance

Abstract

Keywords

Introduction

The Kromer stadial

Kromer Valley site

Kloster Valley site

Methods

Exposure dating with the cosmogenic nuclide 10Be

Sample sites

Exposure age calculations

Results

Discussion

Exposure ages and their spatial distribution

Interpreting the age of moraine stabilization

Exposure ages and their association with early Holocene climate

Conclusion

Footnotes

Acknowledgements

Funding

References

Exposure dating with the cosmogenic nuclide ¹⁰Be