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Schmidt Hammer studies in the maritime Antarctic: Application to dating Holocene deglaciation and estimating the effects of macrolichens on rock weathering
Geomorphology 155–156 (2012) 34–44
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Schmidt Hammer studies in the maritime Antarctic: Application to dating Holocene deglaciation and estimating the effects of macrolichens on rock weathering M. Guglielmin a,⁎, M.R. Worland b, P. Convey b, N. Cannone a a b
BICOM, Insubria University, Via Dunant, 3, 21100 Varese, Italy British Antarctic Survey, Madingly Road, Cambridge, United Kingdom
a r t i c l e
i n f o
Article history: Received 19 February 2011 Received in revised form 24 October 2011 Accepted 5 December 2011 Available online 14 December 2011 Keywords: Schmidt Hammer Rock weathering Lichens Holocene deglaciation Antarctica
a b s t r a c t In order to contribute to the reconstruction of the deglaciation history of the Marguerite Bay area (~68°S, Maritime Antarctic) and to estimate the rock weathering rate in this Antarctic sector, 28 sites (7 on Rothera Point and 21 on Anchorage Island) were characterised using Schmidt Hammer values. The weathering effect of two of the most widespread species of macrolichens in this area (Usnea sphacelata and Umbilicaria decussata) was tested at 5 different sites on Rothera Point. Schmidt Hammer data, in conjunction with recent 14C age, suggest a deglaciation age for the Marguerite Bay area of around 12 ka, and an average uplift rate of 5.4 mm year− 1 on Anchorage Island for the period between 3.3 and 5.2 ka. The weathering rates are extremely slow (e.g. three times slower than reported in Norway). Our data confirm that lichens exert a strong impact on weathering, decreasing the Schmidt Hammer R-values on lichenised surfaces by a factor of 3–4 compared to bare rock surfaces. The effect of lichens on weathering is mainly due to edaphic conditions and the type of the lichen involved rather the period of exposure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Background
The use of the Schmidt Hammer R-value as a proxy measure for the rate of rock weathering has experienced considerable success in geomorphological studies (e.g. Day and Goudie, 1977; Sjöberg and Broadbent, 1991; Goudie, 2006; Shakesby et al., 2006; Owen et al., 2007). It has been used in different geomorphic environments and on a large variety of landforms, including rock glaciers and pronival (or protalus) ramparts (Shakesby et al., 1987; Matthews et al., 2011), moraines (Owen et al., 2007; Shakesby et al., 2008), avalanche-related deposits (Nesje et al., 1994; Clark and Wilson, 2004), patterned ground (Cook-Talbot, 1991), landslides (Dawson et al., 1986), alluvial fans (White et al., 1998) and fossil shorelines (Matthews et al., 1986; Sjöberg, 1990; Sjöberg and Broadbent, 1991; Haslett and Curr, 1998; Trenhaile et al., 1999). The R-value obtained is dependent on a combination of several primary characteristics of the site including; altitude, snow persistence, humidity and the physical characteristics of the rock (lithology). Schmidt Hammer rebound values (R-) have also been widely used to estimate the period of exposure and therefore the relative age of bedrock surfaces, as well as of boulders embedded or located on glacial deposits (Matthews and Shakesby, 1984; McCarroll, 1989; Shakesby et al., 2006, 2008; Matthews and Owen, 2010; Matthews and Winkler, 2011).
The rate of rock surface weathering is generally thought to decline with time (Colman, 1981) but over very long timescales, often exceeding 100,000 years (Shakesby et al., 2006). Our understanding of the role which physical processes, particularly thermal cycling, play in the weathering of rock surfaces in polar environments such as Antarctica has been greatly enhanced by several studies (Hall, 1997; French and Guglielmin, 1999; Guglielmin et al., 2005; Hall et al., 2008). Equally important are the effects of biological weathering by algae, fungi, bacteria and lichens, even under the harsh environmental conditions found in Antarctica (Guglielmin et al., 2005, 2011). Certain species of lichens may increase rock surface weathering (Hall and Otte, 1990; McCarroll and Viles, 1995; Chen et al., 2000; André, 2002; Etienne, 2002; Favero-Longo et al., 2005; Guglielmin et al., 2005), while others can reduce it (Galvan et al., 1981; Schwartzman and Volk, 1989; Benedict, 1993; Viles, 1995; Chen et al., 2000; Guglielmin et al., 2011). In polar environments the occurrence of different lichens is related to snow distribution patterns. Therefore snow conditions can have a significant effect on the weathering processes and the rock weathering rate. Even though lichens may not be visible on the rock surface they may be present as endoliths (chasmo- and crypto-endolithic forms), living within the crystalline structure of the rock close to the surface in the zone penetrated by light (Friedmann, 1982; Friedmann et al., 1993; Hughes and Lawley, 2003). These have also been recognised as potentially important biological weathering agents (Friedmann, 1982; Chen and Blume, 2002; Guglielmin et al., 2005; Hall et al., 2008).
⁎ Corresponding author. E-mail address: [email protected] (M. Guglielmin). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.12.015
M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
Aspects of the deglaciation history of Marguerite Bay and Rothera Point have previously been studied by Emslie (2001), McDaniel and Emslie (2002) and Bentley et al. (2005). Bentley et al. (2005) presented data estimating past relative sea level (RSL) based on dating of sediments relative to former marine basins or inlets isostatically raised above contemporary sea level and which have become freshwater basins. The onset of deglaciation of the inner Marguerite Bay islands has been estimated to be between 9000 and 10,000 14C yr BP (Bentley et al., 2005). Sugden and Clapperton (1980) suggested that George VI Sound, at the southern end of Marguerite Bay, was free of ice by ca. 6000 14C yr BP. Emslie (2001), in a study based on dating the presence of fossil penguin colonies, estimated that deglaciation at Rothera Point and the islands of Marguerite Bay occurred at about 6000 yr BP. However, despite the monitoring of glaciers continuously over the last 30 years (Morris and Mulvaney, 1995; Vaughan and Doake, 1996; Smith et al., 1999; Cook et al., 2005), data relating to glacial evolution on the Antarctica Peninsula are rare, especially for the last 1000 years. To our knowledge the Schmidt Hammer, this cheap, portable, easily applied instrument, which has virtually no environmental impact (no sampling is required) has been used only a few times in Antarctica (Hall, 1987, 1997; White et al., 2009). Therefore, the aims of this study were: 1) To improve knowledge of the deglaciation history of Marguerite Bay area. 2) To identify the role of different types of epilithic lichens in rock weathering. 3. Study area The study sites are located at Rothera Point (67°34′S; 68°07′W) and on Anchorage Island (67°36′S; 68°11′W) in Marguerite Bay, southern Maritime Antarctic. The area experiences a cold dry maritime climate (Ochyra et al., 2008), with a mean annual air temperature of −6.4 °C (Morris and Vaughan, 1992). Rothera Point is a rocky promontory with an ice-free area of c. 1000 × 250 m. Anchorage Island is 2.5 km long and 500 m wide and is partly covered by semi-permanent snow and ice fields. At both sites the rocks are quite homogeneous diorite and granodiorite of mid-Cretaceous to early Tertiary age (Dewar, 1970). Permafrost is probably continuous in nature, although detailed spatial and thermal data are lacking. An active layer of 1.22 m (2009) has been recorded (M. Guglielmin et al., unpubl. data) at one locality on Memorial Hill, close to Rothera research station. Vegetation on Rothera Point is scattered and mainly composed of epilithic lichens (dominated by Usnea sphacelata and Umbilicaria decussata), and sporadic mosses (Convey and Smith, 1997). Anchorage Island has a mosaic of different vegetation communities (Convey and Smith, 1997; Bokhorst et al., 2007), including barren ground, epilithic macrolichen vegetation dominated by Usnea antarctica, moss-dominated communities (with Sanionia uncinata on loose fine sediments with relatively high ground moisture and Andreaea spp. in xeric sites) and vegetation patches dominated by Deschampsia antarctica. 4. Methods A Schmidt-Hammer type N was used with an impact pressure of 2.207 Nm. This is particularly suited for studies on hard rock types such as granodiorite. The Schmidt Hammer measures the percentage rebound distance (R-value) of a controlled blow by a spring-loaded hammer mass impacting on the end of a steel rod held against the test surface. R-values range from 10 to 100. A relatively new N-type hammer, such as the one used in this study, gives an average reading of ca. 78 on a test anvil. Several precautions were taken to avoid known potential sources of error associated with this method. First, to avoid the possible effect of elevation readings were limited to sites below 50 m a.s.l. Second,
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we limited our analyses to graniodiorite and granite–granodiorite which were not foliated or laminated and of a type particularly suited to this method (Goudie, 2006). Third, we selected flat and subhorizontal or horizontal surfaces of large rock outcrops, generally with a low roughness and in many cases glacially polished, primarily to avoid the effect of the roughness or selected weathering of crystals (Williams and Robinson, 1983; Owen et al., 2007). An exception to this procedure was at Rothera Point, where we selected two sites, both at 10 m a.s.l. with beach granitic–granodioritic boulders with a diameter larger than 30 cm. Fourth, measurements were undertaken in dry summer conditions to avoid possible effects of wet rock surfaces (Sumner and Nel, 2002). Given the homogeneity of the chosen surfaces we made 25 readings at each site, which Matthews and Owen (2010) consider to be sufficient to represent this type of area. Although some authors have used the maximum R-value obtained for each site (e.g. Nesje et al., 1994; Evans et al., 1999; Betts and Latta, 2000), we used the mean of the 5 highest readings following Evans et al. (1999). Further, for comparative purposes, we also calculated the upper 50% of the readings according to Katz et al. (2000). To determine the effect of different types of lichens on rock weathering, we selected two of the most widespread macrolichens for both areas (Usnea sphacelata and Umbilicaria decussata) and carried out 25 readings for rock colonised by each lichen species. The same number of readings was taken on bare rock at each site for comparison. In addition, we took further readings (between 10 and 30) directly on each examined lichen thallus until an almost constant R-value was obtained, to determine the R-value of the rock surface directly underneath both species of lichen and on other lichen species, including a small fruticose lichen (Pseudephebe pubescens) and crustose epilithic microlichens (Buellia spp., Lecanora spp., Lecidea spp.). 5. Results 5.1. Bare surfaces 5.1.1. Rothera Point area Seven sites ranging between 5 and 38 m a.s.l. (the highest point of the area) were analysed at Rothera Point (Figs. 1 and 2a). Two sites (A and B) were chosen in relation to two raised beaches located at elevations of 5 and 10 m a.s.l. At the first site (A), we analysed bare rock platforms on the beach while at the second site (B), we analysed rounded granitic– granodioritic boulders with a diameter greater than 30 cm. Site C consisted of polished and glacial scoured bare rocks very close to the front of an ice-ramp while the other 4 sites were located on the main summits along the shallow ridge that crosses Rothera Point from southeast to north-west. The range of the R-values was quite narrow (12–13 units), independent of the method used (Fig. 3a). The mean R-values achieved by the mean of the 5 highest readings are generally much more similar to the values obtained by the upper 50% of the readings than to which obtained by overall 25 readings. In terms of standard deviation the R-values achieved by the mean of the 5 highest readings (except for site C) a lower standard deviation and 95% than the other two methods, which are more similar each other. The difference between the values obtained at the same elevation (10 m a.s.l.) on a glacially scoured bare rock and on rounded beach boulders was negligible (1–2 units; Fig. 3a). There was a significant relationship between elevation and the Rvalues obtained independent of the method used (p b 0.03), with similar regression slopes (b) of between 0.22 and 0.25 (Fig. 4a). 5.1.2. Anchorage Island At Anchorage Island 21 sites were selected between 2 and 50 m a.s.l. (Figs. 1 and 2b), randomly distributed over the island at
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Fig. 1. a) Location of the study area and b) aerial view of Anchorage Island and (in the background) of Rothera Point and its air strip (Photo M.R. Worland).
sites suitable for readings. Sites 2, 3 and 15 were clearly raised beaches where the bare rock used for Schmidt Hammer readings was partially covered by rounded beach boulders. The R-values obtained at Anchorage were slightly lower for all three methods of analysis than those measured on Rothera Point, and ranged between: I) 33 and 52 calculated using the average of the 25 readings; II) between 39 and 54 using the upper 50% and III) between 40 and 55 using the mean of 5 highest readings. Moreover, the R-values obtained with these three methods (Fig. 3b) show the same patterns found at Rothera both for their mean values and standard deviation and 95% confidence.
To facilitate dating the surfaces tested with the Schmidt Hammer at site 7, a trench was dug to sample organic material. The trench revealed four layers (Fig. 5): 1) a surface layer (0–4 cm) of guano (produced by south polar skuas, Catharacta maccormicki); 2) a layer (4–22 cm) of angular pebbles within a silt–clay matrix; 3) a layer (22–26 cm) of well-rounded and polished pebbles; 4) a layer (26–56 cm) of angular, rounded and semi-rounded pebbles within a sandy matrix deeply weathered and rich in organic material overlying the bedrock. The organic material of the deeper layer was sampled and dated using 14C Beta analysis. The sample (Beta-260746) gave a conventional
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radiocarbon age of 5280 ± 50 years BP corresponding to a calibrated age (95% probability) of between 4250 and 3980 BC (i.e. between 6200 and 5920 BP). The relationship between elevation and the R-values was strong (Fig. 4b) and statistically significant independent of the method used (p b 0.01), with similar regression slopes (0.27) and slightly higher than that determined for Rothera Point. 5.1.2.1. Lichen effect. The effect of the presence of lichen on the rock surface (Fig. 6) was similar for all the lichens examined (Table 1; Fig. 7). The first reading always gave the lowest R-value with subsequent readings increasing, followed by a slight decrease and, finally, a further slight increase towards an asymptotic value. One exception to this pattern was obtained from P. pubescens, where the R-value initially increased, but then simply decreased towards an asymptotic value, without the later increase which characterised the other lichens.
Fig. 2. Location of the sites where the Schmidt Hammer readings (black dot) and the new (from: BAS travel sheet 1: Ryder Bay: Travel limits).
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The magnitude of the initial increase and subsequent decrease was very different for each lichen and was related to the size of the thallus. In general the changes were smaller for lichens with larger thalli (i.e. Usnea and Umbilicaria). Simplifying the patterns of these curves we can approximate them to semi-logarithmic curves, as illustrated in Fig. 7, where the asymptotic value represents the Rvalue of the underlying rock. The curves obtained from lichen species with larger thalli differed slightly from each other, although both give similar initial and asymptotic values, and both were much lower than those obtained from lichens with smaller thalli. From the data obtained it can be seen that the last 5 readings (Rw) were very similar to the asymptotic value and therefore provide a good estimation of the R-value of the rock underlying the lichen. Additionally we consider that the difference between the mean of the 5 highest values obtained from the bare rock and the Rw values
14
C ages (black star) were carried out: a) Rothera Point; b) Anchorage Island.
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Fig. 2 (continued).
provides an estimate of the weathering effect produced by the lichens on the rock (Table 2). Comparing the R-values for the two most widespread lichens (Table 1, Fig. 7), the curves obtained do not follow the same pattern, although both lichen values were much lower than those from bare rock. The different impact of the two lichens is also expressed by the difference between the mean R-value obtained for the lichens (R) and on the Rw values. Generally U. decussata appears to have a stronger weathering effect on the underlying rock as indicated by (a) the differences between the R-values of the bare rock (Rbr) and the first reading for the lichen (R) and (b) the differences between Rbr and Rw being higher (Table 2). 6. Discussion 6.1. Deglaciation history of Marguerite Bay and recent evolution of local glaciers The new 14C age obtained at Anchorage Island at 15 m a.s.l. (Beta260746) gives a conventional radiocarbon age of 5280±50 years BP corresponding to a calibrated age (95% probability) between 4250 and 3980 BC (6200 and 5920 BP). Considering the stratigraphic position of
the dated organic sediment, it is reasonable to interpret this age as the minimum age of the beach because this organic layer (layer 4) is the first beach deposit lying on the bedrock. Our data are slightly older with those calculated previously for neighbouring Lagoon Island (McDaniel and Emslie, 2002), who reported the minimum age of the beaches to be 4420±140 at 15.9 m a.s.l. (Beta-144170) and 4790±140 BP at 17 m a.s.l. (Beta-141915). Assuming a linear decay of uplift (following Bentley et al., 2005) and combining our data (5280 ± 50 years BP) with those of Bentley et al. (2005) for Anchorage Island of 2740 ± 140 at 8.25 m a.s.l., we estimate an uplift of 4.1 mm/year for this period. This value arises to 5.4 mm/year if we consider the data reported by Bentley et al. (2005) for the lower beach of Anchorage Island at 4.6 m a.s.l. as a minimum age. This rate corresponds quite well with the rate computed by Ivins et al. (2000). However, it is double the figure as calculated by the alternative oscillatory model also proposed in the same study for the same period, which estimated a value of 2.5–3.0 mm/yr since 5.5 ka. The stratigraphy of our trench at 15 m a.s.l. (Fig. 5) suggests three phases of marine sedimentation with different roundness of the clasts. From the older (bottom) to the younger (top) we have: 1) intermediate roundness (in the deposit are present both well rounded and angular pebbles); 2) well rounded and selected pebbles;
M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
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Fig. 3. Mean R-value, Standard deviation and 95% confidence interval obtained using the three different methods of 25 readings (A), the upper 50% of the readings (B); the 5 highest readings (C) measured at a) Rothera Point and b) Anchorage Island (site elevation on X axis).
3) very poorly rounded (angular) pebbles. These three phases could be related to the variations of the roundness found by Bentley et al. (2005) at Anchorage Island and Rothera Point on the surface clasts at different elevations with the youngest (below 4.5 m a.s.l.) and oldest (above 8 m a.s.l.) beaches exhibiting lower roundness values respect the intermediate beaches (ca. 4.5–8 m a.s.l.) which are more rounded. The phase of highest roundness should be related to a period of greater wave activity due to a reduced summer sea-ice cover and could be referred to a warm period at approximately 4000–2000 14C yr BP (Björck et al., 1996) Using the two minimum available ages for Anchorage Island and the R-values obtained by the three used methods at the elevations of these two sites (respectively, at 8.25 and 15 m a.s.l.) we calculated the relationship between the R-value and the age through linear regression (Table 3, case I). We also calculated the relationships between the R-value and the age considering the age of 3340 ± 140 at 4.6 m a.s.l. as minimum age of the lower examined beach (Table 3, case II). The predicted ages for the highest beach examined at Anchorage Island by the Schmidt Hammer (30 m a.s.l.) range between 8100 and 10,900 BP, while the age of the regional marine limit at 41 m a.s.l. ranges between 10,100 and 15,000 BP, which is at least 1000 years older than that estimated by Bentley et al. (2005). This difference may be explained by a non-linear relationship between weathering and time, as demonstrated by White et al.
(1998) studying the alluvial fan of a desert environment in Tunisia. However, without further age data for different elevations we cannot exclude the possibility that the R-value-age may decrease nonlinearly with time with a potential over estimation of the deglaciation age. However several authors (e.g. Shakesby et al., 2006) have suggested a constant weathering rate to be more appropriate for the Holocene time span. Therefore, assuming the weathering rate to be constant with time, the decreases of the R-value ranges between 0.7 ka − 1 and 1.4 ka − 1 at Anchorage Island and ranges between 0.6 ka − 1 and 1.3 ka − 1 at Rothera Point. In both cases these values are much lower than the values reported from other parts of the world, such as Norway where Shakesby et al. (2006) calculated a value of 3.1 ka − 1 during the Holocene. This low weathering rate is perhaps not surprising as, although Marguerite Bay is one of the moister areas of Antarctica, it is still relatively dry and cold and a region where the classical weathering processes (through freeze– thaw or wetting–drying cycles) are more limited (Hall, 1997). However, it is also possible that deglaciation at Rothera Point was delayed with respect to Anchorage Island due to its less exposed position and its proximity to grounded glaciers (therefore giving higher R-values, especially on the more recent surfaces). Therefore, the weathering rate applied to Rothera may in reality be lower than the figure we have calculated and, indeed, Rothera Point may have experienced a more recent deglaciation than Anchorage Island.
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A
65
60
55
50
45
R-value (unit)
40
35
30
Mean R-25
Elevation (m a.s.l.)
Mean R-up50%
Mean R-5h
25
0
B
5
10
15
20
25
30
35
40
45
50
65
60
55
50
45
R-value (unit)
40
35
30
Mean R-25
Elevation (m a.s.l.)
Mean R-up50%
Mean R-5h
25
0
5
10
15
20
25
30
35
40
45
50
Fig. 4. Relationships between R-value (Unit) and elevation (m a.s.l.) obtained using the mean of: all 25 measured sites (black rhombus), the upper 50% of the 25 readings according to Katz et al. (2000, empty triangles) and the 5 highest readings following Evans et al. (1999, black dots) measured at Rothera Point a) and Anchorage Island b). Their different linear regressions are also shown with a grey thick solid line, a black dashed line and a black thick solid line respectively.
6.2. Lichen effect Lichens are characterised by three different growth forms — crustose, foliose and fruticose. The thallus of crustose lichens forms a crust on the rock beneath it and is tightly attached to the substrate across its whole lower surface (de los Rios et al., 2002). Foliose and fruticose lichens have specific attachment points, and are also able to exert a significant weathering impact, although mainly in the proximity of the attachment structures, with differences detected between different species depending on their life form (de los Rios et al., 2002). Our data underline the importance of lichen growth form in determining the weathering effect of different species. In particular we considered the effect of Usnea sphacelata (a fruticose lichen) and Umbilicaria decussata (a foliose lichen with a central umbilicus), which are both large epilithic lichens and showed lower
first R-values compared to P. pubescens (a much smaller and minutely fruticose lichen). According to De los Rios et al. (2002), the umbilicate species (Umbilicaria decussata) may be able to penetrate more deeply into the rock, while fruticose species (Usnea sphacelata) may be more efficient at weathering the rock surface. Minutely fruticose species (P. pubescens) seem to be less efficient weathering agents, probably due to their smaller size, compared to the two macrolichens examined in this study. It is also notable that lichens with larger thalli showed a complex weathering effect which can be detected by the R-value curves (Fig. 7). This four-stage R-value curve was not apparent with the smaller P. pubescens or other microlichens and endolithic lichens examined (data not shown) where, after the initial increase of the R-values, there was simply a slight decrease towards the asymptotic value.
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Table 1 R-Values for Usnea sphacelata and Umbilicaria decussata at different sites on Rothera Point. For the labels of the sites see Fig. 2a. Sites
A
D
E
F
G
Usnea Usneaa Standard deviation. Confidence 95%. Usneab Standard deviation. Confidence 95%.
29 5 6 41 4 5
12 0 34 16 22
13 2 2 32 5 4
18 3 4 37 6 8
14 1 2 32 15 21
12 0
17 5 7 29 3 5
14 6 4 36 13 8
12 1 1 32 11 16
14 1 2 36 22 30
Umbilicaria Umbila Standard deviation. Confidence 95%. Umbilb Standard deviation. Confidence 95%. a b
29 7 12
Mean values of the first readings. Mean values of the last 5 readings.
We interpret the different stages of these curves as representing:
Fig. 5. Sketch of the trench excavated at Anchorage Island at 15 m a.s.l. showing three different phases of beach deposition. Legend: A) guano; B) angular pebbles in silt–clay matrix; C) well-rounded pebbles; D) mixed pebbles in sandy matrix.
Fig. 6. Photograph of the two macrolichens studied: a) Usnea sphacelata and b) Umbilicaria decussata.
I) Stage a: An initial increase of the R-value: Generally the first reading is the lowest because it depends on the resistance of the thallus, which is broken at the first reading. Subsequent readings of the underlying rock are higher. II) Stage b and d: The decrease in R-values reflects the progressive breaking of the rock flakes which normally contain the hyphae of epilithic and in some cases endolithic lichens together with an underlying layer of algae (e.g. Chen et al., 2000; de los Rios et al., 2002). III) Stage c (present only in Umbilicaria decussata and Usnea sphacelata): The slight increase in R-value may reflect the occurrence of a harder layer due to metabolites and or biochemicals precipitated by the lichens which have a bioprotection role, such as calcium oxalate or hematite (Carter and Viles, 2005), Independent of types of curves, the difference between the first reading and the asymptotic value of the curve (R − Rw) reflects the thickness of the rock weathered, depending on the different types of lichens involved. Among the sites the differences between the asymptotic value (Rw) and the first reading (R) was slightly higher for Umbilicaria decussata (ranging between 12 and 22) than for Usnea sphacelata (9–22) but the patterns were also different. For example at site F) Umbilicaria showed the highest value of Rw − R (22), whereas Usnea showed one of the lower values (16). This result indicates that this index is clearly independent of the age of the surface but may reflect the different edaphic conditions and probably the order of colonisation, which differs between sites depending on the edaphic conditions present. The importance of the edaphic conditions (moisture and temperature) in promoting a bioprotection or a biodeterioration effect by lichens has been underlined by Carter and Viles (2005), who reported that the same species can exert either effect depending on the micro-environmental conditions. Similarly the differences between the bare rock (Rbr) and asymptotic values obtained under the lichens (Rw) were independent of the age of the rock surface, but were generally higher for Umbilicaria (ranging between 16 and 32) than for Usnea (between 12 and 23). Also in this case the patterns were unclear as, in some sites (A and F), the value of the difference (Rbr − Rw) was higher for Usnea than for Umbilicaria. This is probably explained by differences in the times of colonisation of the two species at the different sites, but also the more aggressive and efficient mechanism of weathering of Umbilicaria.
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Fig. 7. The effect of the different type of lichens at the same site. R-values of Pseudephebe pubescens (grey dots), Usnea sphacelata (black dots) and Umbilicaria decussata (empty dots) are presented with their semi-log curves (dashed lines) and their moving mean (solid lines; n = 4). The horizontal solid black line represents the R-value obtained on the bare rock close to those colonised by lichens.
Matthews and Owen (2008) reported a high variability and large differences between R-values for bare rock and rock with endolithic lichens (Lecidea auriculata) in Norway, ranging between 12.5 and 30.2 on surfaces dated between 12 and 88 years. McCarroll and Viles (1995), working at the same site but on older surfaces (up to ca. 300 years), reported differences of between 30 and 40 units. In both cases the R-values are lower than our data. The differences between bare rocks and lichenised rocks on our sites ranged between 30 and 43 units for Usnea and between 37 and 49 units for Umbilicaria are not so different respect the much younger surfaces studied by McCarroll and Viles (1995). Matthews and Owen (2008) noted also that the difference increased on very recent surfaces (about 20–30 units in the first 40 years) and then remained almost constant on older surfaces (10 units over the next 200 years). In the current study, with surfaces older than 2000 years, the difference between bare and lichenised rock were not related to the age of the surface, but more likely to the edaphic and morphological conditions of the sites, confirming the observation of Matthews and Owen (2008). It is also important to recognise that measuring the surface age of rock colonised by lichens with a Schmidt Hammer may result in large errors unless the weathering effect of the lichen is taken into account, as demonstrated by Matthews and Owen (2008). Moreover, the role of lichens must also be considered when using cosmogenic-nuclide
Table 2 Comparison of the effect of the two most widespread lichens of the examined area: Usnea sphacelata and Umbilicaria decussata: Rw = mean R-values of the last 5 readings; R = Mean of the R-values of the first reading; Rbr = Mean R-values of the highest 5 readings on the bare rock. For the labels of the sites see Fig. 2a. Sites
A
D
E
F
G
Usnea Rw − R RBr − R Rbr − Rw
19 38 19
22 43 21
19 30 12
16 38 22
9 32 23
Umbilicaria Rw − R RBr − R Rbr − Rw
22 37 15
12 38 26
21 37 16
22 38 16
17 49 32
dating for the same reason, as failure to do so will result in a large underestimation of the calculated ages. This is an important issue because it is often recommended that sampling for cosmogenicnuclide analyses should be from habitats preferred by endolithic and epilithic lichens.
7. Conclusions This study demonstrates that the Schmidt Hammer is a valid technique both for contributing to the reconstruction of deglaciation in this part of the Antarctica, which is also currently experiencing rapid regional climate warming (Turner et al., 2002), and for estimating the weathering rate in such harsh and extreme environmental conditions. We have shown, using Schmidt Hammer readings and 14C ages, that local deglaciation of the Marguerite Bay area could have taken place ~ 12,000 year BP, which is rather earlier than previously reported (e.g. Bentley et al., 2005), and that the average uplift for the shorter examined period on Anchorage Island (between 3.3 and 5.2 ka) was 5.4 mm y − 1. Weathering rates calculated both at Anchorage Island and Rothera Point are extremely slow (three times slower than reported in Norway by Shakesby et al. (2006)), consistent with this sector of Antarctica being very dry and cold and, therefore, classical weathering processes, such as freeze–thaw and wetting–drying cycles, being extremely limited.
Table 3 Relationships between age and R-values: 1) regression equations; 2) predicted ages for the highest beach at 30 m a.s.l.; 3) predicted ages for the regional marine limit (41 m a.s.l.). 1
2
3
17 36 19
Case II Overall 25 readings Higher 50% of the readings Highest 5 readings
Age = − 669.8R + 35,719 Age = − 674.9R + 38,322 Age = − 687.8R + 39,349
8056 8086 8121
10,132 10,111 10,116
19 40 21
Case I Overall 25 readings Higher 50% of the readings Highest 5 readings
Age = − 1351.2R + 66,683 Age = − 1361.4R + 71,935 Age = − 1387.5R + 74,005
10,878 10,944 10,874
15,067 15,028 15,036
Mean
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In addition, our data confirm that lichens exert a strong impact on the weathering rate of rocks, decreasing the R-values of lichenised surfaces by 3–4 times compared to bare surfaces, and that the process of weathering of rocks by lichens is more a function of the edaphic conditions and the type of the lichen involved than the period of time. Acknowledgements We acknowledge British Antarctic Survey (BAS) for the logistic support on the field. We thank again BAS and PNRA (Progetto nazionale di Ricerca in Antartide) for funding this joint programme of research. We want also to thank Fabio Baio for his help in the field work. We want also to thank the two reviewers Prof. H. M. French and J. A. Matthews and the Editor Prof. A.M. Harvey for their useful comments and corrections that improved the manuscript significantly. References André, M.F., 2002. Rates of postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N). Geografiska Annaler 84A, 139–150. 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Schmidt Hammer studies in the maritime Antarctic: Application to dating Holocene deglaciation and estimating the effects of macrolichens on rock weathering M. Guglielmin a,⁎, M.R. Worland b, P. Convey b, N. Cannone a a b
BICOM, Insubria University, Via Dunant, 3, 21100 Varese, Italy British Antarctic Survey, Madingly Road, Cambridge, United Kingdom
a r t i c l e
i n f o
Article history: Received 19 February 2011 Received in revised form 24 October 2011 Accepted 5 December 2011 Available online 14 December 2011 Keywords: Schmidt Hammer Rock weathering Lichens Holocene deglaciation Antarctica
a b s t r a c t In order to contribute to the reconstruction of the deglaciation history of the Marguerite Bay area (~68°S, Maritime Antarctic) and to estimate the rock weathering rate in this Antarctic sector, 28 sites (7 on Rothera Point and 21 on Anchorage Island) were characterised using Schmidt Hammer values. The weathering effect of two of the most widespread species of macrolichens in this area (Usnea sphacelata and Umbilicaria decussata) was tested at 5 different sites on Rothera Point. Schmidt Hammer data, in conjunction with recent 14C age, suggest a deglaciation age for the Marguerite Bay area of around 12 ka, and an average uplift rate of 5.4 mm year− 1 on Anchorage Island for the period between 3.3 and 5.2 ka. The weathering rates are extremely slow (e.g. three times slower than reported in Norway). Our data confirm that lichens exert a strong impact on weathering, decreasing the Schmidt Hammer R-values on lichenised surfaces by a factor of 3–4 compared to bare rock surfaces. The effect of lichens on weathering is mainly due to edaphic conditions and the type of the lichen involved rather the period of exposure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Background
The use of the Schmidt Hammer R-value as a proxy measure for the rate of rock weathering has experienced considerable success in geomorphological studies (e.g. Day and Goudie, 1977; Sjöberg and Broadbent, 1991; Goudie, 2006; Shakesby et al., 2006; Owen et al., 2007). It has been used in different geomorphic environments and on a large variety of landforms, including rock glaciers and pronival (or protalus) ramparts (Shakesby et al., 1987; Matthews et al., 2011), moraines (Owen et al., 2007; Shakesby et al., 2008), avalanche-related deposits (Nesje et al., 1994; Clark and Wilson, 2004), patterned ground (Cook-Talbot, 1991), landslides (Dawson et al., 1986), alluvial fans (White et al., 1998) and fossil shorelines (Matthews et al., 1986; Sjöberg, 1990; Sjöberg and Broadbent, 1991; Haslett and Curr, 1998; Trenhaile et al., 1999). The R-value obtained is dependent on a combination of several primary characteristics of the site including; altitude, snow persistence, humidity and the physical characteristics of the rock (lithology). Schmidt Hammer rebound values (R-) have also been widely used to estimate the period of exposure and therefore the relative age of bedrock surfaces, as well as of boulders embedded or located on glacial deposits (Matthews and Shakesby, 1984; McCarroll, 1989; Shakesby et al., 2006, 2008; Matthews and Owen, 2010; Matthews and Winkler, 2011).
The rate of rock surface weathering is generally thought to decline with time (Colman, 1981) but over very long timescales, often exceeding 100,000 years (Shakesby et al., 2006). Our understanding of the role which physical processes, particularly thermal cycling, play in the weathering of rock surfaces in polar environments such as Antarctica has been greatly enhanced by several studies (Hall, 1997; French and Guglielmin, 1999; Guglielmin et al., 2005; Hall et al., 2008). Equally important are the effects of biological weathering by algae, fungi, bacteria and lichens, even under the harsh environmental conditions found in Antarctica (Guglielmin et al., 2005, 2011). Certain species of lichens may increase rock surface weathering (Hall and Otte, 1990; McCarroll and Viles, 1995; Chen et al., 2000; André, 2002; Etienne, 2002; Favero-Longo et al., 2005; Guglielmin et al., 2005), while others can reduce it (Galvan et al., 1981; Schwartzman and Volk, 1989; Benedict, 1993; Viles, 1995; Chen et al., 2000; Guglielmin et al., 2011). In polar environments the occurrence of different lichens is related to snow distribution patterns. Therefore snow conditions can have a significant effect on the weathering processes and the rock weathering rate. Even though lichens may not be visible on the rock surface they may be present as endoliths (chasmo- and crypto-endolithic forms), living within the crystalline structure of the rock close to the surface in the zone penetrated by light (Friedmann, 1982; Friedmann et al., 1993; Hughes and Lawley, 2003). These have also been recognised as potentially important biological weathering agents (Friedmann, 1982; Chen and Blume, 2002; Guglielmin et al., 2005; Hall et al., 2008).
⁎ Corresponding author. E-mail address: [email protected] (M. Guglielmin). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.12.015
M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
Aspects of the deglaciation history of Marguerite Bay and Rothera Point have previously been studied by Emslie (2001), McDaniel and Emslie (2002) and Bentley et al. (2005). Bentley et al. (2005) presented data estimating past relative sea level (RSL) based on dating of sediments relative to former marine basins or inlets isostatically raised above contemporary sea level and which have become freshwater basins. The onset of deglaciation of the inner Marguerite Bay islands has been estimated to be between 9000 and 10,000 14C yr BP (Bentley et al., 2005). Sugden and Clapperton (1980) suggested that George VI Sound, at the southern end of Marguerite Bay, was free of ice by ca. 6000 14C yr BP. Emslie (2001), in a study based on dating the presence of fossil penguin colonies, estimated that deglaciation at Rothera Point and the islands of Marguerite Bay occurred at about 6000 yr BP. However, despite the monitoring of glaciers continuously over the last 30 years (Morris and Mulvaney, 1995; Vaughan and Doake, 1996; Smith et al., 1999; Cook et al., 2005), data relating to glacial evolution on the Antarctica Peninsula are rare, especially for the last 1000 years. To our knowledge the Schmidt Hammer, this cheap, portable, easily applied instrument, which has virtually no environmental impact (no sampling is required) has been used only a few times in Antarctica (Hall, 1987, 1997; White et al., 2009). Therefore, the aims of this study were: 1) To improve knowledge of the deglaciation history of Marguerite Bay area. 2) To identify the role of different types of epilithic lichens in rock weathering. 3. Study area The study sites are located at Rothera Point (67°34′S; 68°07′W) and on Anchorage Island (67°36′S; 68°11′W) in Marguerite Bay, southern Maritime Antarctic. The area experiences a cold dry maritime climate (Ochyra et al., 2008), with a mean annual air temperature of −6.4 °C (Morris and Vaughan, 1992). Rothera Point is a rocky promontory with an ice-free area of c. 1000 × 250 m. Anchorage Island is 2.5 km long and 500 m wide and is partly covered by semi-permanent snow and ice fields. At both sites the rocks are quite homogeneous diorite and granodiorite of mid-Cretaceous to early Tertiary age (Dewar, 1970). Permafrost is probably continuous in nature, although detailed spatial and thermal data are lacking. An active layer of 1.22 m (2009) has been recorded (M. Guglielmin et al., unpubl. data) at one locality on Memorial Hill, close to Rothera research station. Vegetation on Rothera Point is scattered and mainly composed of epilithic lichens (dominated by Usnea sphacelata and Umbilicaria decussata), and sporadic mosses (Convey and Smith, 1997). Anchorage Island has a mosaic of different vegetation communities (Convey and Smith, 1997; Bokhorst et al., 2007), including barren ground, epilithic macrolichen vegetation dominated by Usnea antarctica, moss-dominated communities (with Sanionia uncinata on loose fine sediments with relatively high ground moisture and Andreaea spp. in xeric sites) and vegetation patches dominated by Deschampsia antarctica. 4. Methods A Schmidt-Hammer type N was used with an impact pressure of 2.207 Nm. This is particularly suited for studies on hard rock types such as granodiorite. The Schmidt Hammer measures the percentage rebound distance (R-value) of a controlled blow by a spring-loaded hammer mass impacting on the end of a steel rod held against the test surface. R-values range from 10 to 100. A relatively new N-type hammer, such as the one used in this study, gives an average reading of ca. 78 on a test anvil. Several precautions were taken to avoid known potential sources of error associated with this method. First, to avoid the possible effect of elevation readings were limited to sites below 50 m a.s.l. Second,
35
we limited our analyses to graniodiorite and granite–granodiorite which were not foliated or laminated and of a type particularly suited to this method (Goudie, 2006). Third, we selected flat and subhorizontal or horizontal surfaces of large rock outcrops, generally with a low roughness and in many cases glacially polished, primarily to avoid the effect of the roughness or selected weathering of crystals (Williams and Robinson, 1983; Owen et al., 2007). An exception to this procedure was at Rothera Point, where we selected two sites, both at 10 m a.s.l. with beach granitic–granodioritic boulders with a diameter larger than 30 cm. Fourth, measurements were undertaken in dry summer conditions to avoid possible effects of wet rock surfaces (Sumner and Nel, 2002). Given the homogeneity of the chosen surfaces we made 25 readings at each site, which Matthews and Owen (2010) consider to be sufficient to represent this type of area. Although some authors have used the maximum R-value obtained for each site (e.g. Nesje et al., 1994; Evans et al., 1999; Betts and Latta, 2000), we used the mean of the 5 highest readings following Evans et al. (1999). Further, for comparative purposes, we also calculated the upper 50% of the readings according to Katz et al. (2000). To determine the effect of different types of lichens on rock weathering, we selected two of the most widespread macrolichens for both areas (Usnea sphacelata and Umbilicaria decussata) and carried out 25 readings for rock colonised by each lichen species. The same number of readings was taken on bare rock at each site for comparison. In addition, we took further readings (between 10 and 30) directly on each examined lichen thallus until an almost constant R-value was obtained, to determine the R-value of the rock surface directly underneath both species of lichen and on other lichen species, including a small fruticose lichen (Pseudephebe pubescens) and crustose epilithic microlichens (Buellia spp., Lecanora spp., Lecidea spp.). 5. Results 5.1. Bare surfaces 5.1.1. Rothera Point area Seven sites ranging between 5 and 38 m a.s.l. (the highest point of the area) were analysed at Rothera Point (Figs. 1 and 2a). Two sites (A and B) were chosen in relation to two raised beaches located at elevations of 5 and 10 m a.s.l. At the first site (A), we analysed bare rock platforms on the beach while at the second site (B), we analysed rounded granitic– granodioritic boulders with a diameter greater than 30 cm. Site C consisted of polished and glacial scoured bare rocks very close to the front of an ice-ramp while the other 4 sites were located on the main summits along the shallow ridge that crosses Rothera Point from southeast to north-west. The range of the R-values was quite narrow (12–13 units), independent of the method used (Fig. 3a). The mean R-values achieved by the mean of the 5 highest readings are generally much more similar to the values obtained by the upper 50% of the readings than to which obtained by overall 25 readings. In terms of standard deviation the R-values achieved by the mean of the 5 highest readings (except for site C) a lower standard deviation and 95% than the other two methods, which are more similar each other. The difference between the values obtained at the same elevation (10 m a.s.l.) on a glacially scoured bare rock and on rounded beach boulders was negligible (1–2 units; Fig. 3a). There was a significant relationship between elevation and the Rvalues obtained independent of the method used (p b 0.03), with similar regression slopes (b) of between 0.22 and 0.25 (Fig. 4a). 5.1.2. Anchorage Island At Anchorage Island 21 sites were selected between 2 and 50 m a.s.l. (Figs. 1 and 2b), randomly distributed over the island at
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Fig. 1. a) Location of the study area and b) aerial view of Anchorage Island and (in the background) of Rothera Point and its air strip (Photo M.R. Worland).
sites suitable for readings. Sites 2, 3 and 15 were clearly raised beaches where the bare rock used for Schmidt Hammer readings was partially covered by rounded beach boulders. The R-values obtained at Anchorage were slightly lower for all three methods of analysis than those measured on Rothera Point, and ranged between: I) 33 and 52 calculated using the average of the 25 readings; II) between 39 and 54 using the upper 50% and III) between 40 and 55 using the mean of 5 highest readings. Moreover, the R-values obtained with these three methods (Fig. 3b) show the same patterns found at Rothera both for their mean values and standard deviation and 95% confidence.
To facilitate dating the surfaces tested with the Schmidt Hammer at site 7, a trench was dug to sample organic material. The trench revealed four layers (Fig. 5): 1) a surface layer (0–4 cm) of guano (produced by south polar skuas, Catharacta maccormicki); 2) a layer (4–22 cm) of angular pebbles within a silt–clay matrix; 3) a layer (22–26 cm) of well-rounded and polished pebbles; 4) a layer (26–56 cm) of angular, rounded and semi-rounded pebbles within a sandy matrix deeply weathered and rich in organic material overlying the bedrock. The organic material of the deeper layer was sampled and dated using 14C Beta analysis. The sample (Beta-260746) gave a conventional
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radiocarbon age of 5280 ± 50 years BP corresponding to a calibrated age (95% probability) of between 4250 and 3980 BC (i.e. between 6200 and 5920 BP). The relationship between elevation and the R-values was strong (Fig. 4b) and statistically significant independent of the method used (p b 0.01), with similar regression slopes (0.27) and slightly higher than that determined for Rothera Point. 5.1.2.1. Lichen effect. The effect of the presence of lichen on the rock surface (Fig. 6) was similar for all the lichens examined (Table 1; Fig. 7). The first reading always gave the lowest R-value with subsequent readings increasing, followed by a slight decrease and, finally, a further slight increase towards an asymptotic value. One exception to this pattern was obtained from P. pubescens, where the R-value initially increased, but then simply decreased towards an asymptotic value, without the later increase which characterised the other lichens.
Fig. 2. Location of the sites where the Schmidt Hammer readings (black dot) and the new (from: BAS travel sheet 1: Ryder Bay: Travel limits).
37
The magnitude of the initial increase and subsequent decrease was very different for each lichen and was related to the size of the thallus. In general the changes were smaller for lichens with larger thalli (i.e. Usnea and Umbilicaria). Simplifying the patterns of these curves we can approximate them to semi-logarithmic curves, as illustrated in Fig. 7, where the asymptotic value represents the Rvalue of the underlying rock. The curves obtained from lichen species with larger thalli differed slightly from each other, although both give similar initial and asymptotic values, and both were much lower than those obtained from lichens with smaller thalli. From the data obtained it can be seen that the last 5 readings (Rw) were very similar to the asymptotic value and therefore provide a good estimation of the R-value of the rock underlying the lichen. Additionally we consider that the difference between the mean of the 5 highest values obtained from the bare rock and the Rw values
14
C ages (black star) were carried out: a) Rothera Point; b) Anchorage Island.
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Fig. 2 (continued).
provides an estimate of the weathering effect produced by the lichens on the rock (Table 2). Comparing the R-values for the two most widespread lichens (Table 1, Fig. 7), the curves obtained do not follow the same pattern, although both lichen values were much lower than those from bare rock. The different impact of the two lichens is also expressed by the difference between the mean R-value obtained for the lichens (R) and on the Rw values. Generally U. decussata appears to have a stronger weathering effect on the underlying rock as indicated by (a) the differences between the R-values of the bare rock (Rbr) and the first reading for the lichen (R) and (b) the differences between Rbr and Rw being higher (Table 2). 6. Discussion 6.1. Deglaciation history of Marguerite Bay and recent evolution of local glaciers The new 14C age obtained at Anchorage Island at 15 m a.s.l. (Beta260746) gives a conventional radiocarbon age of 5280±50 years BP corresponding to a calibrated age (95% probability) between 4250 and 3980 BC (6200 and 5920 BP). Considering the stratigraphic position of
the dated organic sediment, it is reasonable to interpret this age as the minimum age of the beach because this organic layer (layer 4) is the first beach deposit lying on the bedrock. Our data are slightly older with those calculated previously for neighbouring Lagoon Island (McDaniel and Emslie, 2002), who reported the minimum age of the beaches to be 4420±140 at 15.9 m a.s.l. (Beta-144170) and 4790±140 BP at 17 m a.s.l. (Beta-141915). Assuming a linear decay of uplift (following Bentley et al., 2005) and combining our data (5280 ± 50 years BP) with those of Bentley et al. (2005) for Anchorage Island of 2740 ± 140 at 8.25 m a.s.l., we estimate an uplift of 4.1 mm/year for this period. This value arises to 5.4 mm/year if we consider the data reported by Bentley et al. (2005) for the lower beach of Anchorage Island at 4.6 m a.s.l. as a minimum age. This rate corresponds quite well with the rate computed by Ivins et al. (2000). However, it is double the figure as calculated by the alternative oscillatory model also proposed in the same study for the same period, which estimated a value of 2.5–3.0 mm/yr since 5.5 ka. The stratigraphy of our trench at 15 m a.s.l. (Fig. 5) suggests three phases of marine sedimentation with different roundness of the clasts. From the older (bottom) to the younger (top) we have: 1) intermediate roundness (in the deposit are present both well rounded and angular pebbles); 2) well rounded and selected pebbles;
M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
39
Fig. 3. Mean R-value, Standard deviation and 95% confidence interval obtained using the three different methods of 25 readings (A), the upper 50% of the readings (B); the 5 highest readings (C) measured at a) Rothera Point and b) Anchorage Island (site elevation on X axis).
3) very poorly rounded (angular) pebbles. These three phases could be related to the variations of the roundness found by Bentley et al. (2005) at Anchorage Island and Rothera Point on the surface clasts at different elevations with the youngest (below 4.5 m a.s.l.) and oldest (above 8 m a.s.l.) beaches exhibiting lower roundness values respect the intermediate beaches (ca. 4.5–8 m a.s.l.) which are more rounded. The phase of highest roundness should be related to a period of greater wave activity due to a reduced summer sea-ice cover and could be referred to a warm period at approximately 4000–2000 14C yr BP (Björck et al., 1996) Using the two minimum available ages for Anchorage Island and the R-values obtained by the three used methods at the elevations of these two sites (respectively, at 8.25 and 15 m a.s.l.) we calculated the relationship between the R-value and the age through linear regression (Table 3, case I). We also calculated the relationships between the R-value and the age considering the age of 3340 ± 140 at 4.6 m a.s.l. as minimum age of the lower examined beach (Table 3, case II). The predicted ages for the highest beach examined at Anchorage Island by the Schmidt Hammer (30 m a.s.l.) range between 8100 and 10,900 BP, while the age of the regional marine limit at 41 m a.s.l. ranges between 10,100 and 15,000 BP, which is at least 1000 years older than that estimated by Bentley et al. (2005). This difference may be explained by a non-linear relationship between weathering and time, as demonstrated by White et al.
(1998) studying the alluvial fan of a desert environment in Tunisia. However, without further age data for different elevations we cannot exclude the possibility that the R-value-age may decrease nonlinearly with time with a potential over estimation of the deglaciation age. However several authors (e.g. Shakesby et al., 2006) have suggested a constant weathering rate to be more appropriate for the Holocene time span. Therefore, assuming the weathering rate to be constant with time, the decreases of the R-value ranges between 0.7 ka − 1 and 1.4 ka − 1 at Anchorage Island and ranges between 0.6 ka − 1 and 1.3 ka − 1 at Rothera Point. In both cases these values are much lower than the values reported from other parts of the world, such as Norway where Shakesby et al. (2006) calculated a value of 3.1 ka − 1 during the Holocene. This low weathering rate is perhaps not surprising as, although Marguerite Bay is one of the moister areas of Antarctica, it is still relatively dry and cold and a region where the classical weathering processes (through freeze– thaw or wetting–drying cycles) are more limited (Hall, 1997). However, it is also possible that deglaciation at Rothera Point was delayed with respect to Anchorage Island due to its less exposed position and its proximity to grounded glaciers (therefore giving higher R-values, especially on the more recent surfaces). Therefore, the weathering rate applied to Rothera may in reality be lower than the figure we have calculated and, indeed, Rothera Point may have experienced a more recent deglaciation than Anchorage Island.
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M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
A
65
60
55
50
45
R-value (unit)
40
35
30
Mean R-25
Elevation (m a.s.l.)
Mean R-up50%
Mean R-5h
25
0
B
5
10
15
20
25
30
35
40
45
50
65
60
55
50
45
R-value (unit)
40
35
30
Mean R-25
Elevation (m a.s.l.)
Mean R-up50%
Mean R-5h
25
0
5
10
15
20
25
30
35
40
45
50
Fig. 4. Relationships between R-value (Unit) and elevation (m a.s.l.) obtained using the mean of: all 25 measured sites (black rhombus), the upper 50% of the 25 readings according to Katz et al. (2000, empty triangles) and the 5 highest readings following Evans et al. (1999, black dots) measured at Rothera Point a) and Anchorage Island b). Their different linear regressions are also shown with a grey thick solid line, a black dashed line and a black thick solid line respectively.
6.2. Lichen effect Lichens are characterised by three different growth forms — crustose, foliose and fruticose. The thallus of crustose lichens forms a crust on the rock beneath it and is tightly attached to the substrate across its whole lower surface (de los Rios et al., 2002). Foliose and fruticose lichens have specific attachment points, and are also able to exert a significant weathering impact, although mainly in the proximity of the attachment structures, with differences detected between different species depending on their life form (de los Rios et al., 2002). Our data underline the importance of lichen growth form in determining the weathering effect of different species. In particular we considered the effect of Usnea sphacelata (a fruticose lichen) and Umbilicaria decussata (a foliose lichen with a central umbilicus), which are both large epilithic lichens and showed lower
first R-values compared to P. pubescens (a much smaller and minutely fruticose lichen). According to De los Rios et al. (2002), the umbilicate species (Umbilicaria decussata) may be able to penetrate more deeply into the rock, while fruticose species (Usnea sphacelata) may be more efficient at weathering the rock surface. Minutely fruticose species (P. pubescens) seem to be less efficient weathering agents, probably due to their smaller size, compared to the two macrolichens examined in this study. It is also notable that lichens with larger thalli showed a complex weathering effect which can be detected by the R-value curves (Fig. 7). This four-stage R-value curve was not apparent with the smaller P. pubescens or other microlichens and endolithic lichens examined (data not shown) where, after the initial increase of the R-values, there was simply a slight decrease towards the asymptotic value.
M. Guglielmin et al. / Geomorphology 155–156 (2012) 34–44
41
Table 1 R-Values for Usnea sphacelata and Umbilicaria decussata at different sites on Rothera Point. For the labels of the sites see Fig. 2a. Sites
A
D
E
F
G
Usnea Usneaa Standard deviation. Confidence 95%. Usneab Standard deviation. Confidence 95%.
29 5 6 41 4 5
12 0 34 16 22
13 2 2 32 5 4
18 3 4 37 6 8
14 1 2 32 15 21
12 0
17 5 7 29 3 5
14 6 4 36 13 8
12 1 1 32 11 16
14 1 2 36 22 30
Umbilicaria Umbila Standard deviation. Confidence 95%. Umbilb Standard deviation. Confidence 95%. a b
29 7 12
Mean values of the first readings. Mean values of the last 5 readings.
We interpret the different stages of these curves as representing:
Fig. 5. Sketch of the trench excavated at Anchorage Island at 15 m a.s.l. showing three different phases of beach deposition. Legend: A) guano; B) angular pebbles in silt–clay matrix; C) well-rounded pebbles; D) mixed pebbles in sandy matrix.
Fig. 6. Photograph of the two macrolichens studied: a) Usnea sphacelata and b) Umbilicaria decussata.
I) Stage a: An initial increase of the R-value: Generally the first reading is the lowest because it depends on the resistance of the thallus, which is broken at the first reading. Subsequent readings of the underlying rock are higher. II) Stage b and d: The decrease in R-values reflects the progressive breaking of the rock flakes which normally contain the hyphae of epilithic and in some cases endolithic lichens together with an underlying layer of algae (e.g. Chen et al., 2000; de los Rios et al., 2002). III) Stage c (present only in Umbilicaria decussata and Usnea sphacelata): The slight increase in R-value may reflect the occurrence of a harder layer due to metabolites and or biochemicals precipitated by the lichens which have a bioprotection role, such as calcium oxalate or hematite (Carter and Viles, 2005), Independent of types of curves, the difference between the first reading and the asymptotic value of the curve (R − Rw) reflects the thickness of the rock weathered, depending on the different types of lichens involved. Among the sites the differences between the asymptotic value (Rw) and the first reading (R) was slightly higher for Umbilicaria decussata (ranging between 12 and 22) than for Usnea sphacelata (9–22) but the patterns were also different. For example at site F) Umbilicaria showed the highest value of Rw − R (22), whereas Usnea showed one of the lower values (16). This result indicates that this index is clearly independent of the age of the surface but may reflect the different edaphic conditions and probably the order of colonisation, which differs between sites depending on the edaphic conditions present. The importance of the edaphic conditions (moisture and temperature) in promoting a bioprotection or a biodeterioration effect by lichens has been underlined by Carter and Viles (2005), who reported that the same species can exert either effect depending on the micro-environmental conditions. Similarly the differences between the bare rock (Rbr) and asymptotic values obtained under the lichens (Rw) were independent of the age of the rock surface, but were generally higher for Umbilicaria (ranging between 16 and 32) than for Usnea (between 12 and 23). Also in this case the patterns were unclear as, in some sites (A and F), the value of the difference (Rbr − Rw) was higher for Usnea than for Umbilicaria. This is probably explained by differences in the times of colonisation of the two species at the different sites, but also the more aggressive and efficient mechanism of weathering of Umbilicaria.
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Fig. 7. The effect of the different type of lichens at the same site. R-values of Pseudephebe pubescens (grey dots), Usnea sphacelata (black dots) and Umbilicaria decussata (empty dots) are presented with their semi-log curves (dashed lines) and their moving mean (solid lines; n = 4). The horizontal solid black line represents the R-value obtained on the bare rock close to those colonised by lichens.
Matthews and Owen (2008) reported a high variability and large differences between R-values for bare rock and rock with endolithic lichens (Lecidea auriculata) in Norway, ranging between 12.5 and 30.2 on surfaces dated between 12 and 88 years. McCarroll and Viles (1995), working at the same site but on older surfaces (up to ca. 300 years), reported differences of between 30 and 40 units. In both cases the R-values are lower than our data. The differences between bare rocks and lichenised rocks on our sites ranged between 30 and 43 units for Usnea and between 37 and 49 units for Umbilicaria are not so different respect the much younger surfaces studied by McCarroll and Viles (1995). Matthews and Owen (2008) noted also that the difference increased on very recent surfaces (about 20–30 units in the first 40 years) and then remained almost constant on older surfaces (10 units over the next 200 years). In the current study, with surfaces older than 2000 years, the difference between bare and lichenised rock were not related to the age of the surface, but more likely to the edaphic and morphological conditions of the sites, confirming the observation of Matthews and Owen (2008). It is also important to recognise that measuring the surface age of rock colonised by lichens with a Schmidt Hammer may result in large errors unless the weathering effect of the lichen is taken into account, as demonstrated by Matthews and Owen (2008). Moreover, the role of lichens must also be considered when using cosmogenic-nuclide
Table 2 Comparison of the effect of the two most widespread lichens of the examined area: Usnea sphacelata and Umbilicaria decussata: Rw = mean R-values of the last 5 readings; R = Mean of the R-values of the first reading; Rbr = Mean R-values of the highest 5 readings on the bare rock. For the labels of the sites see Fig. 2a. Sites
A
D
E
F
G
Usnea Rw − R RBr − R Rbr − Rw
19 38 19
22 43 21
19 30 12
16 38 22
9 32 23
Umbilicaria Rw − R RBr − R Rbr − Rw
22 37 15
12 38 26
21 37 16
22 38 16
17 49 32
dating for the same reason, as failure to do so will result in a large underestimation of the calculated ages. This is an important issue because it is often recommended that sampling for cosmogenicnuclide analyses should be from habitats preferred by endolithic and epilithic lichens.
7. Conclusions This study demonstrates that the Schmidt Hammer is a valid technique both for contributing to the reconstruction of deglaciation in this part of the Antarctica, which is also currently experiencing rapid regional climate warming (Turner et al., 2002), and for estimating the weathering rate in such harsh and extreme environmental conditions. We have shown, using Schmidt Hammer readings and 14C ages, that local deglaciation of the Marguerite Bay area could have taken place ~ 12,000 year BP, which is rather earlier than previously reported (e.g. Bentley et al., 2005), and that the average uplift for the shorter examined period on Anchorage Island (between 3.3 and 5.2 ka) was 5.4 mm y − 1. Weathering rates calculated both at Anchorage Island and Rothera Point are extremely slow (three times slower than reported in Norway by Shakesby et al. (2006)), consistent with this sector of Antarctica being very dry and cold and, therefore, classical weathering processes, such as freeze–thaw and wetting–drying cycles, being extremely limited.
Table 3 Relationships between age and R-values: 1) regression equations; 2) predicted ages for the highest beach at 30 m a.s.l.; 3) predicted ages for the regional marine limit (41 m a.s.l.). 1
2
3
17 36 19
Case II Overall 25 readings Higher 50% of the readings Highest 5 readings
Age = − 669.8R + 35,719 Age = − 674.9R + 38,322 Age = − 687.8R + 39,349
8056 8086 8121
10,132 10,111 10,116
19 40 21
Case I Overall 25 readings Higher 50% of the readings Highest 5 readings
Age = − 1351.2R + 66,683 Age = − 1361.4R + 71,935 Age = − 1387.5R + 74,005
10,878 10,944 10,874
15,067 15,028 15,036
Mean
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