Minimum bedrock exposure ages and their implications: Larsemann
Hills and neighboring Bolingen Islands, East Antarctica
HUANG Feixin1,∗, LI Guangwei1, 6, LIU Xiaohan1, KONG Ping2, JU Yitai3, FINK David4, FANG Aimin5, YU Liangjun2 1. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China 2. Institute of Geology and geophysics, Chinese Academy of Sciences, Beijing 100029, China 3. China Metallurgical Geology Bureau, Beijing 100025, China 4. Australian Nuclear Science and Technology Organization, Menai, NSW 2234, Australia 5. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China 6. Graduate University of the Chinese Academy of Sciences，Beijing 100049, China
Abstract: Considerable controversy exists over whether or not extensive glaciation occurred during the Last Glacial Maximum (LGM) in the Larsemann Hills. In this study we use the in situ produced cosmogenic nuclide 10Be (half life 1.51 Ma) to provide minimum exposure ages for five bedrock samples and one erratic boulder in order to determine the last period of deglaciation in the
Larsemann Hills. Three bedrock samples taken from Friendship Mountain (the highest peak on the
Mirror Peninsula, Larsemann Hills; ~2 km from the ice sheet) have minimum exposure ages ranging from 40.0 to 44.7 ka. The erratic boulder from Peak 106 (just at the edge of the ice sheet) has a younger minimum exposure age of only 8.8 ka. The minimum exposure ages for two bedrock samples from Blundell Peak (the highest peak on Stornes Peninsula, Larsemann Hills; ~2 km from the ice sheet) are about 17 and 18 ka. On Bolingen Islands (southwest to the Larsemann
Hills; ~10 km from the ice sheet), the minimum bedrock exposure age is similar to that at
Friendship Mountain (i.e., 44 ka). Our results indicate that the bedrock exposure in the Larsemann
Hills and neighboring Bolingen Islands commenced obviously before the global LGM (i.e.,
20-22ka), and the bedrock erosion rate at the Antarctic coast areas may be obviously higher than in the interior land.
Key Words: East Antarctica, Larsemann Hills, Bolingen Islands, 10Be exposure age, Erosion rate
∗ corresponding author. E-mail: [email protected]
1 The East Antarctica Ice Sheet (EAIS) contains more than 26 million km3 of ice, about 83% of the total volume of ice in Antarctica. If the EAIS melted, it would cause a further global sea level rise of more than 60 m (Denton et al., 1991; Anderson, 1999). Study of the Cenozoic and Pliocene evolution of the Antarctic Ice Sheet behavior is important for reconstructing global paleo-climate evolution (Denton et al., 2002). The Larsemann Hills (Fig. 1) is the second largest of four major ice-free areas along East Antarctica’s coastline (Hodgson et al., 2001), and is adjacent to the
Lambert Glacier/Amery Ice Shelf system, an important feature of Antarctica’s cryosphere.
Chronicling the last deglaciation in the Larsemann Hills will provide important constraints on the history of the EAIS since late Pleistocene (Domack et al., 1998; Harris et al., 1998; Ingólfsson et al., 1998; Taylor et al., 2004). Application of the technique of exposure dating bedrock and erratic boulders by in situ produced cosmogenic nuclides (such as 10Be, 26Al, 3He and 21Ne, etc.) has been widely accepted as a most useful tool to quantify the timing and mode of glacial evolution in
Antarctica (Ackert et al., 1999; Brook et al., 1993, 1995; Bruno et al., 1997; Fogwill et al., 2004;
Fink et al., 2006; Mackintosh et al., 2007; Huang et al., 2008). Few studies of exposure ages in the
Larsemann Hills have yet been reported. Our preliminary results of minimum 10Be bedrock exposure ages at Friendship Mountain, Mirror Peninsula, indicate that Friendship Mountain was exposed before ~45 ka (Huang et al., 2005). This study, which presents new minimum bedrock exposure ages, shows that the bedrock exposure in the Larsemann Hills took place over a considerable period of time, from 45 ka (at least) to 9 ka assuming no erosion. The bedrock exposure age in the Bolingen Islands (southwest to the Larsemann Hills) is similar to that at
Friendship Mountain, which suggests that these two regions may have similar deglaciation histories.
1. REGIONAL GEOLOGICAL BACKGROUNDS
The Larsemann Hills (69º12′-69º28′ S, 76º00′-76º30′ E) lie on the east coast of Prydz Bay, East
Antarctica, and covers an area of about 50 km2. The Larsemann Hills contain four peninsulas
(Mirror Peninsula, Broknes Peninsula, Grovness Peninsula, and Stornes Peninsula), together with many offshore islands (Fig. 2). The Dalk Glacier lies to the south of the Larsemann Hills and currently flows west to east. The Bolingen Islands, ~20 km farther southwest of the Larsemann
Hills, consist of several ice-free islands, which lie ~10 km offshore from the Amery Ice Shelf (Fig.
2 1). Due to difficult access, the Bolingen Islands have been studied much less than the Larsemann
Hills.
The Larsemann Hills expose part of the east Antarctic high metamorphic terrane of the late
Proterozoic to early Paleozoic, and have experienced the Grenville and the Pan-Africa orogenic events. Bedrock of the Larsemann Hills and Bolingen Islands are composed mainly of paragneiss, orthogneiss, migmatized gneiss, quartzite and mafic granulite (Tong et al., 1997). Previous research on the Larsemann Hills shows that the glacial landforms in this region include nivation hollows, glaciofluvial deposits, V-form valleys, and local glacial striations. Extensive bedrock erosion features and tafoni pits are common. The coastline of the Larsemann Hills is relatively flat, and lacks evidence of isostatic rebound following deglaciation (Li et al., 1993; Burgess et al.,
1994; Hodgson et al., 2001). Importantly, glacial tills of the LGM are absent in the Larsemann
Hills, so considerable controversy remains regarding whether or not extensive glaciation occurred here during the LGM (Burgess et al., 1994). Recent 14C dating of lake sediments in Progress and
Reid Lakes (Hodgson et al., 2001, 2005) gave basal radiocarbon ages of >40ka suggesting that the ice sheet had not advanced across various parts of the Larsemann Hills since at least the global
LGM and more likely not after ~ 40 ka. However, determining ages much older than 40 ka (even though they may exist in those lakes) extends beyond the ability of 14C dating method (Chou et al.,
1990).
Fig. 1 Site of the Larsemann Hills and Bolingen Islands, East Antarctica
3 We investigated the geologic and geomorphologic characteristics of the Larsemann Hills in the austral summers of 2001 and 2005, with our preliminary emphasis on the bedrock exposure ages at Friendship Mountain, Mirror Peninsula. The in situ cosmogenic nuclide 10Be bedrock minimum exposure ages at Friendship Mountain ranged from 40.0-44.7 ka, while an erratic boulder (with a diameter of ~1 m) at the top of Peak 106 (southeast of Friendship Mountain) has a minimum exposure age of 8.8 ka. With the assumption of zero erosion and inheritance this indicates that the last deglaciation of Friendship Mountain occurred considerably before the LGM
(Huang et al., 2005). This paper reports on our continuing study of minimum bedrock exposure ages at Larsmann Hills, and reports new minimum bedrock exposure ages from Stornes Peninsula, and the neighboring Bolingen Islands.
2. SAMPLING AND LABORATORY TREATMENT
Previously studied bedrock samples E001, E003, and E007 were collected from top to bottom of Friendship Mountain (peak altitude is 150m), and an erratic boulder sample 0124-4 comes from the crest of Peak 106, to the east of Friendship Mountain and just on the west edge of the ice sheet.
In this paper, we further analyzed two bedrock samples (I02 and I03) from the crest of Blundell
Peak, on the south of Stones Peninsula. Sample I02 comes from ~10 m below the top of Blundell
Peak, and I03 comes from ~10 m below I02. We also collected and analyzed a bedrock sample
VK002 from near the top of one of the offshore islands in the Bolingen Islands. Table 1 shows the details of all samples.
Geographically, the Mirror Peninsula lies in the east of the Larsemann Hills. Friendship
Mountain, located in the south of the Mirror Peninsula, lies ~3 km from the Chinese Zhongshan
Station. The Stornes Peninsula is located in the west of the Larsemann Hills. Blundell Peak, located in the south of the Stornes Peninsula, is adjacent to a small ice cap of ~4 km2, which probably separated from the ice sheet during the last deglaciation. Approximately 20 km farther southwest, the Bolingen Islands lie to the north of the Amery Ice Shelf. Antarctic terrane is ~10 km to the southeast, and is currently covered by the ice sheet (Figs. 1 and 2). Because of sea water and floating ice, we required a helicopter to reach an outlying island in the Bolingen Islands group, and only took a few bedrock samples. We determined locations and elevations of all sample sites with hand-held GPS, and corrected these by topographic maps if necessary. Elevation
4 uncertainties should be less than 20 m. The bedrock samples analyzed were taken from flat slopes, and sampling depths were <5 cm. We avoided sites sheltered by high landforms or erratic boulders, as well as areas with tafoni (probably associated with extraordinarily high bedrock erosion rates) during sampling. Thus, shielding and depth corrections were not considered in this work.
Chemical preparations were carried out in the cosmogenic nuclide laboratory at the Institute of
Geology and Geophysics, Chinese Academy of Sciences. Samples were first crushed to 0.1-1.0 mm size, then separated magnetically. Quartz samples were purified by leaching 4 or 5 times in a hot ultrasonic bath with a mixed solution of HF and HNO3 (Kohl et al., 1992), and were completely dissolved together with about 0.5-0.8 mg 9Be carrier. Be was separated by ion chromatography, and its hydroxides were precipitated, then baked to oxides at 850ºC. The 10Be concentrations were measured by the accelerator mass spectrometry (AMS) at the Australian
Nuclear Science and Technology Organisation (ANSTO), Australia (Fink et al., 2004). Details of the 10Be measurement procedures, calibration and background corrections can be found in Fink et al. (2007) and Kong et al. (2007).
Fig. 2 Sample positions in the Larsemann Hills 1-Land; 2-Sea; 3-Glacier; 4-Lake; 5-Peak; 6-Exploration station; 7-Sample position
3. RESULTS AND DISCUSSION
Table 1 shows both the measured 10Be nuclide concentrations (atoms/g) and calculated
5 minimum exposure ages. The minimum exposure ages shown in Table 1 are calculated using the scaling method of Lal (1991), modified by Stone (2000) for Antarctica air pressures. This study uses production rates of 5.1 atoms/g·yr for 10Be at sea level and high latitude for the calculations.
In calculating the exposure ages with erosion, we have used the values of 2.7 g/cm3 for rock density and 150 g/cm2 for mean path length in rocks.
The bedrock minimum exposure ages at Friendship Mountain are 40.0-44.7 ka, which are much earlier than the LGM. The bedrock minimum exposure age of the Bolingen Islands is similar, about 43.8 ka. However, the bedrock minimum exposure ages at the crest of Blundell Peak
(Stornes Peninsula ) are only 17.1-18.3 ka, roughly coinciding with the LGM. The erratic boulder on top of Peak 106 has a much younger minimum exposure age, only 8.8 ka. Because the bedrock samples at Friendship Mountain and on the Bolingen Islands are relatively distant from the present edge of the ice sheet (2-10 km), their ages represent the early period of the bedrock exposure in the Larsemann Hills and Bolingen Islands. Blundell Peak is ~2 km away from the current ice sheet, yet is near the separated remnant ice cap, and the relative younger exposure ages (~18 ka) should represent the middle period of the bedrock exposure. Peak 106 is just on the edge of the ice sheet, and the erratic boulder we sampled sits on its crest. Consequently, sample 0124-4 should not have been affected by sheltering or overturning and its exposure age should represent the final period of the deglaciation, when the edge of the ice sheet arrived at its present position (Fig. 3).
Fig. 3 Bedrock minimum exposure ages and positions in the Larsemann Hills
Several previous studies have investigated the timing of deglaciation in the Larsemann Hills.
6 Gellieson (1991) determined 14C ages of lake sediments in the Larsemann Hills and offshore
islands. He concluded that deglaciation in the Larsemann Hills started around 12.0 ka, with
islands near the coast being ice free since 9.5 ka and the current coast exposed since 4.5 ka.
However, Burgess et al. (1994) found 24,950 yr old lichens in the sediment of Nella Lake
(Broknes Peninsula), indicating that at least some areas in the Larsemann Hills were ice free
during the LGM. Hodgson et al. (2001) systematically dated core sediments in numerous lakes
from the coastline to the edge of the ice sheet. The 14C ages show that the Mirror Peninsula and
Broknes Peninsula were ice free during the LGM, while the deglaciation of the Stornes Peninsula
occurred after the LGM. The 14C ages of the substrate sediment (fibroid blue algae and lichen) in
Progress Lake (near Friendship Mountain; Fig. 2) exceeded 44 ka, indicating that Progress Lake
was ice free at least since 44 ka. However, as the 44 ka age approaches the limit of the 14C dating
method, it is not possible to determine at what time before 44 ka ago the deglaciation of
Brokness and Mirror Peninsulas commenced. Hodgson also tried to date the older (>44 ka)
sediments in the Larsemann Hills using the 238U method, but could not get an accurate result due
to low uranium concentrations (Hodgson et al. 2001). Later, Hodgson et al. (2005) carried out
comprehensive research on a sediment core taken from the Reid Lake, north of Nella Lake. The
14C ages show that sediments at 68-102 cm of the core yield 41.8-43.8 ka dates, yet the lower
section of the core (below 102 cm) was beyond the limit of the 14C dating method and could not
be dated accurately. Our 10Be minimum bedrock exposure ages at Friendship Mountain (and the
Bolingen Islands) are consistent with the 14C ages (44 ka and 43.8 ka) from the Progress Lake
and Reid Lake core sediments (Hodgson et al. 2001, 2005). Thus, the deglaciation in the
Larsemann Hills and Bolingen Islands may have commenced at ~45 ka, or depending on the
magnitude of erosion corrections to these minimum exposure ages, even much earlier than 45ka.
Hodgson et al. (2005) proposed a polynomial age depth model to calculate the age for the section of the Reid Lake core below 102 cm, and concluded that the age of the lowest section was between 137 ka and 148 ka (i.e., deglaciation at Reid Lake started before the Last Glacial Cycle).
The minimum bedrock exposure ages at Friendship Mountain (near Reid Lake) are much later than 137 ka and 148 ka. If deglaciation in the Reid Lake region started before Last Glacial
Cycle, then we need to explain the factor of three reduction in 10Be concentration that would cause such a large difference between the calculated minimum exposure ages (i.e ., ~45 ka) and the
7 actual exposure ages for the bedrock samples based on the estimated age of basal Reid Lake sediments by Hodgson (i.e., ~140 ka).
A reduction in 10Be concentration can occur due to environmental shielding processes ( i.e., seasonal snow cover, local topographical highs, or local material rootless such as glacial tills).
However, these processes would make it highly unlikely that a uniform age of 40-45 ka would result along the vertical transect from different elevations at Friendship Mountain (and the
Bolingen Islands). Neither can the difference result from differential bedrock sheeting or spalling, or break-off from the sampling site, because those situations would occur randomly, and lead to a scatter of minimum bedrock exposure ages for sites with similar actual exposure ages. Thirdly, bedrock erosion can also result in lower apparent model calculated exposure ages. Again, the consistency of the minimum bedrock ages at Friendship Mountain and Bolingen Island indicate similar erosion rates. Many literatures show that the average erosion rate in the Antarctica is far less than on other continents, usually only between about 0.1-1.0 mm/ka (Nishiizumi et al., 1991;
Brook et al., 1995; Ivy-Ochs et al., 1995; Fogwill et al., 2004), but these location are totally different glacial environments from the Larsmann Hills. They are not coastal, most are in the interior areas, where it is so dry and cold that no erosion takes place. Actually, four years rock surface lowering rates of 0.015 and 0.022 mm/yr had been observed for the Larsemann and
Vestfold Hills, respectively (Spate et al., 1995). To reduce a true exposure age from ~140 ka to that of 40-45 ka would require erosion rates of ~11.7-12.0 mm/ka, which is very close to the erosional equilibrium value for the measured 10Be concentration of E002 in the Laresemann Hills and VK002 in the Bolingen Islands (~12.3 and 12.6 mm/ka, respectively). Thus, the erosion rates in the Antarctica coast areas are about tens of times higher than in the interior land.
4. CONCLUSION
The results of in situ cosmogenic nuclide 10Be dating yield minimum, zero-erosion bedrock exposure ages at Friendship Mountain (Larsemann Hills) and the neighboring Bolingen Islands of
40.0-44.7 ka. These dates are consistent with 14C ages of lake sediments from the Mirror Peninsula, and indicate that the last bedrock exposure in the Larsemann Hills and Bolingen Islands commenced before 45 ka. This considerably predates the LGM. Based on the exposure age of an erratic boulder on Peak 106, the last deglaciation in the Larsemann Hills and neighboring
8 Bolingen Islands lasts to after the LGM. The results also suggest that the erosion rates in the
Antarctic coast areas may be obviously higher than in the interior land.
ACKOWLEDGEMENT We thank the Chinese Polar Research Administration for field logistic supports during the 18th and 22th Chinese Antarctic Research Expedition (CHINARE). Bill
Isherwood reviewed the manuscript and made useful and constructive suggestions. This work was supported by the National Science Fund of China (Grant Number 40506003 and 40631004) and the Polar Science Strategy Research Fund (Grant Number 20070219).
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Table 1. Sample sites and Be isotope measurements, 10Be minimum exposure ages in the Larsemann Hills and Bolingen Islands
Sample Location Lat. S./Long. E. Elevation 10Be rate quartz（g） 10Be/9Be error 10Be measured 10Be Minimum
(m) (atoms/g·yr) （10-12） （%） （×106 atoms/g） exposure age（ka）
E001 Friendship Mountain, Mirror Peninsula 69°23.64′/76°24.11′ 150 7.414 94.0756 812.3 4.8 293,798 40.0±3.3 E003 Friendship Mountain, Mirror Peninsula 69°23.59′/76°24.32′ 125 7.227 17.2190 163.4 8.4 319664 44.7±4.8 E007 Friendship Mountain, Mirror Peninsula 69°23.46′/76°24.55′ 56 6.717 20.4710 160.2 15.2 267037 40.1±6.7 0124-4 Peak 106, Mirror Peninsula 69°24.77′/76°24.95′ 108 7.110 28.8447 52.7 10.3 62,262 8.8±1.1 I02 Stornes Peninsula 69°25.57′/76°06.25′ 150 7.414 52.2736 121.2 4.3 126,656 17.2±1.3 I03 Stornes Peninsula 69°25.51′/76°06.31′ 140 7.337 48.8840 119.0 5.1 133,517 18.3±1.5 VK002 Bolingen Island 69°26.07′/75°44.27′ 80 6.888 83.9334 457.4 2.5 298,676 43.8±3.1 The errors in the 10Be minimum exposure ages include not only the error of 10Be/9Be, but also the AMS system error (2%), 10Be production rate error (6%) and the concentration error of the 10Be carrier (1%).
13