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Bedrock Exposure Ages and Their Implication in the Larsemann Hills

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Bedrock Exposure Ages and Their Implication in the Larsemann Hills 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.
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