The Holocene Deglaciation of the Byers Peninsula (Livingston Island, Antarctica) Based on the Dating of Lake Sedimentary Records

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Geomorphology 261 (2016) 89–102

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Geomorphology

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The Holocene deglaciation of the Byers Peninsula (Livingston Island, Antarctica) based on the dating of lake sedimentary records

M. Oliva a,⁎, D. Antoniades b, S. Giralt c, I. Granados d,S.Pla-Rabese,M.Torof,E.J.Liug, J. Sanjurjo h, G. Vieira a a Centre for Geographical Studies -IGOT, Universidade de Lisboa, Portugal b Department of Géographie & Centre d'Études Nordiques, Université Laval, Canada c Institute of Earth Sciences Jaume Almera, CSIC, Spain d Centro de Investigación, Seguimiento y Evaluación, Guadarrama National Park, Spain e Centre de Recerca Ecològica i Aplicacions Forestals (CREAF-CSIC), Spain f Centre for Hidrographic Studies (CEDEX), Spain g Department of Earth Sciences, University of Bristol, UK h University Institute of Geology, University of A Coruña, Spain article info abstract

Article history: The process of deglaciation in the Antarctic Peninsula region has large implications for the geomorphological and Received 30 July 2015 ecological dynamics of the ice-free environments. However, uncertainties still remain regarding the age of degla- Received in revised form 23 February 2016 ciation in many coastal environments, as is the case in the South Shetland Islands. This study focuses on the Byers Accepted 28 February 2016 Peninsula, the largest ice-free area in this archipelago and the one with greatest biodiversity in Antarctica. A com- Available online 2 March 2016 plete lacustrine sedimentary sequence was collected from ﬁve lakes distributed along a transect from the western coast to the Rotch Dome glacier front: Limnopolar, Chester, Escondido, Cerro Negro and Domo lakes. A Keywords: 14 Antarctica multiple dating approach based on C, thermoluminescence and tephrochronology was applied to the cores Byers Peninsula in order to infer the Holocene environmental history and identify the deglaciation chronology in the Byers Pen- Deglaciation insula. The onset of the deglaciation started during the Early Holocene in the western fringe of the Byers Penin- Lake sediments sula according to the basal dating of Limnopolar Lake (ca. 8.3 cal. ky BP). Glacial retreat gradually exposed the highest parts of the Cerro Negro nunatak in the SE corner of Byers, where Cerro Negro Lake is located; this lake was glacier-free since at least 7.5 ky. During the Mid-Holocene the retreat of the Rotch Dome glacier cleared the central part of the Byers plateau of ice, and Escondido and Chester lakes formed at 6 cal. ky BP and 5.9 ky, respectively. The dating of the basal sediments of Domo Lake suggests that the deglaciation of the current ice-free easternmost part of the Byers Peninsula occurred before 1.8 cal. ky BP. © 2016 Elsevier B.V. All rights reserved.

1. Introduction areas are mostly distributed along the coastal fringes and include both nunataks (areas of high relief protruding above the ice sheet) and mar- Glacier retreat in the Antarctic Peninsula (AP) region has accelerated itime environments where mean annual temperatures are slightly over the last several decades in response to increasing air temperatures below 0 °C. Mean annual temperatures in the SSI range between −1 (Cook et al., 2005; Pritchard and Vaughan, 2007; Cook and Vaughan, and −2°Catsealevel(Bockheim et al., 2013), and small changes in av- 2010; Shepherd et al., 2012). The AP is experiencing one of the fastest erage temperature and/or precipitation may thus lead to signiﬁcant rates of warming on Earth, with temperature increases of ca. 0.5 °C/decade changes in glacier mass balances (Navarro et al., 2013; Osmanoglu et al., (Turner et al., 2005; Steig et al., 2009) that have led an accelerated ice loss in 2014). The high climate sensitivity of these islands has resulted in multiple the region. According to climate projections, this reduction in glacier vol- glacial advances and retreats in response to climatic ﬂuctuations during the umes in the AP will continue in the near future (IPCC, 2013). A consequence Holocene (Ó Cofaigh et al., 2014; The RAISED Consortium, 2014). These Ho- of glacier retreat is the appearance of new ice-free land. Therefore, although locene climatic changes have also had consequences for terrestrial ecosys- these ice-free areas currently represent b0.4% of the Antarctic land surface, tems, including post-glacial rebound and the formation of marine terraces they are expected to expand in the coming decades. (John and Sugden, 1971; Fretwell et al., 2010; Watcham et al., 2011), per- This research is focused on the South Shetland Islands (SSI), one of mafrost degradation (Oliva and Ruiz-Fernández, 2015), as well as changes the most extensive ice-free regions in the western AP, where ice-free in freshwater ecosystems (Toro et al., 2013) and in the distribution of fauna (Sun et al., 2000; del Valle et al., 2002). ⁎ Corresponding author. Within the current context, the recent glacial retreat recorded dur- E-mail address: [email protected] (M. Oliva). ing the second half of the 20th century in many areas in the AP region

http://dx.doi.org/10.1016/j.geomorph.2016.02.029 0169-555X/© 2016 Elsevier B.V. All rights reserved. 90 M. Oliva et al. / Geomorphology 261 (2016) 89–102 needs to be framed within the natural pattern of glacial and climatic 2010 represents 17.3% of its 1.16 km2 surface (Oliva and Ruiz- events that have occurred during the last millennia. Consequently, Fernández, 2015). However, this pattern has not been observed in the constraining the age of the deglaciation of present-day ice-free areas Byers Peninsula, where the frontal moraine lies in contact with the in the AP has received renewed attention during recent years. This pro- Rotch Dome glacier. cess of deglaciation started after the Last Glacial Maximum and contin- The retreat of the Rotch Dome during the last few millennia has ued throughout the Holocene (Sugden and Clapperton, 1986; Ingólfsson been accompanied by the formation of marine terraces at elevations et al., 1998, 2003; Heroy and Anderson, 2005; Simms et al., 2011; Ó between 2 and 15 m a.s.l. due to post-glacial rebound (Hall and Cofaigh et al., 2014). Cosmogenic dating of glacial landforms (Seong Perry, 2004). These raised beaches surround the central plateau of et al., 2009; Balco, 2011; Balco et al., 2013) has complemented earlier the Byers Peninsula (90–140 m), above which several volcanic plugs studies focused on radiocarbon (14C) dating from marine and terrestrial stand out, such as Start Hill (265 m), Chester Cone (188 m) and records (Ingólfsson et al., 1998, 2003). Nevertheless, the timing of de- Cerro Negro (143 m) (López-Martínez et al., 2012). The only two glaciation and the resulting geomorphic implications are still poorly small ice masses on the Byers Peninsula that currently exist outside constrained in many of these ice-free environments. Byers Peninsula, the Rotch Dome are located in its NW sector, where the land surface in the westernmost part of Livingston Island, is one such example reaches its greatest elevation a.s.l. Numerous ponds and lakes have where further paleoenvironmental research on the transition between formed in depressions within the hilly landscape of the central plateau glacial and lacustrine sediments is required (Ó Cofaigh et al., 2014). Al- of the Byers Peninsula (Toro et al., 2007), five of which are the subject though a recent study of the drainage system in the Byers Peninsula pro- of this study (Fig. 1). posed that three different glacial centres existed within the Byers The Byers Peninsula is composed of marine sediments from the Peninsula during the Holocene (Mink et al., 2014), until now absolute Upper Jurassic to Lower Cretaceous and volcanic and volcaniclastic ages of the deglaciation in this peninsula have been based almost solely rocks (López-Martínez et al., 1996). The bedrock is heavily weathered on 14C ages of the basal sediments from Limnopolar Lake, which report- and fractured, with extensive evidence of intense frost shattering. As ed an estimated age of ca. 8.3 cal. ky BP (calibrated thousands of years in other ice-free environments of the SSI, periglacial processes and land- before present) (Toro et al., 2013), and of several minimum ages from forms are widespread at all elevations, with the presence of sporadic or other lake sediment records that ranged between 4 and 5 cal. ky BP discontinuous permafrost up to 20–40 m a.s.l. and continuous perma- (Björck et al., 1996). frost at higher elevations (Vieira et al., 2010; Bockheim et al., 2013). In In this study, we combine absolute and relative dating techniques of the central plateau of the Byers Peninsula the permafrost table occurs lake sediments to clarify the deglaciation process of the Byers Peninsula. at 1.35 m depth (de Pablo et al., 2014). The geomorphological distribution of glacial and periglacial landforms Climate data from 2002 to 2010 show an average annual tempera- provides information that complements the sediment-based geochro- ture of −2.8 °C at 70 m a.s.l. and precipitation ranging from 500 to nology and assists in the interpretation of paleoenvironmental stages. 800 mm (Bañón et al., 2013). Surface runoff is largely restricted to the By deriving a chronological framework for five lakes distributed along summer season and is mostly related to snow melt and active layer an east-west transect from the Rotch Dome glacier to the coast of this thawing (Toro et al., 2013). The vegetation cover is moderately abun- peninsula, we: (a) improve the limited network of points from which dant in the flat marine terraces, and is largely composed of mosses there is chronological information regarding deglaciation, (b) validate and grasses (Vera, 2011), with lichens distributed at higher locations. the use of radiocarbon (14C) and thermoluminescence dating as com- Soils in the Byers Peninsula are shallow and composed of coarse mate- plementary absolute dating techniques in Antarctic lakes to determine rial (Moura et al., 2012). the onset of lacustrine sedimentation (indicative of the age of lake formation), (c) propose a general model for glacier retreat and landscape 2.2. Study lakes evolution in the Byers Peninsula based on geochronological sequences and geomorphological evidence, and (d) compare the evolution of gla- The five lakes studied here are distributed across the central plateau cier retreat in the Byers Peninsula with regional environmental and cli- of the Byers Peninsula (Fig. 2). They have small areas (b5 ha), shallow matic proxies. depths (1.6–5.3 m at coring sites) and are located at elevations of between 45 and 100 m (Table 1). They are ice-covered except for 2–- 2. Study area 3 months during the summer. Chester Lake is the largest lake, located at the foot of Chester Cone, 2.1. Byers peninsula with a maximum depth of 5.3 m. The catchment is characterised by smooth terrain, with the lake covering over 40% of the catchment Livingston Island is the second largest island in the SSI with a surface area. Limnopolar Lake has the largest catchment, which is mostly devoid area of 818 km2. The Byers Peninsula is a ca. 60 km2 ice-free area - the of vegetation except for small scattered patches of mosses and lichens in largest in the SSI - located at its western end. It has been designated wet areas. The maximum depth of Limnopolar Lake is 5.5 m. However, an Antarctic Specially Protected Area (ASPA No. 126). Livingston Island in certain years snow dams may block its outlet during spring thaw be- constitutes 23% of the area of the SSI archipelago and represents 25% of fore bursting due to melting, leading to short-term rises in lake level of its total ice volume (Osmanoglu et al., 2014). At present, approximately up to 1 m. Escondido Lake is situated in a depression surrounded by 697 km2 (85% of the surface area) of Livingston Island is covered by gla- three peaks (110–120 m), with a bare and rounded catchment and an ciers, although the glaciated area has decreased over the last decades: in irregular shoreline. The lake is deepest (5.3 m) in its eastern part and 1956 glaciers extended over 734 km2 (89.7%) (Calvet et al., 1999). Byers it shallows to the west. Cerro Negro Lake is located in a small glacial Peninsula represents today almost half of the total ice-free area of Liv- cirque in the upper part of Cerro Negro hill, between the two summit ingston Island. peaks. The lake is surrounded by steep talus slopes, with fine-grained On Livingston Island, glacier retreat has been observed over the last sediments only in the northern slope. Cerro Negro Lake has the smallest decades in both valley glaciers and in the glacier domes that extend catchment in our study (1.5 ha), and a maximum depth of 4.5 m. Domo across the lowlands. However, the rate of glacier retreat has decelerated Lake is the closest lake to the Rotch Dome, at a distance of only 300 m between 2002 and 2011 (Navarro et al., 2013), with slightly positive from the glacial front. It is the shallowest of all the studied lakes with surface mass balances observed from 2007 to 2011 (Osmanoglu et al., a maximum depth of b2 m. Patterned ground is widespread on the sed- 2014). Glacier shrinkage has exposed new land surfaces in the small iments (sands and pebbles) distributed across the Domo Lake catch- peninsulas surrounding the Rotch Dome ice cap, including Elephant ment, indicating intense cryoturbation processes reworked by the Point where the new ice-free surface that appeared between 1956 and mass-wasting activity. M. Oliva et al. / Geomorphology 261 (2016) 89–102 91

Fig. 1. Location map of Byers Peninsula within the Antarctic Peninsula region (top, left) and the South Shetland Islands (top, right). Distribution of the study lakes in the Byers Peninsula (bottom).

3. Materials and methods sedimentary sequences. The complete sedimentary infill was recovered for all the lakes, although the basal sediments (5–10 cm) of Cerro During field-work in November 2012, we collected the complete sed- Negro Lake were lost during core recovery. A total of 22 core sections imentary sequences from four lakes distributed along an approximately (9 in Chester Lake, 4 in Escondido Lake, 3 in Cerro Negro Lake and 5 in east-west transect from the glacier to the coast (Table 1). The lake sur- Domo Lake) were obtained. The complete sedimentary infill of faces were still frozen at the time of coring, enabling the use of lake ice Limnopolar Lake was obtained in 2003 and 2008 coring campaigns (for as the drilling platform. The sediment cores were collected from the further details, see Toro et al., 2013). The sediment cores were stored deepest part of the lakes using a 60 mm UWITEC gravity corer attached in a cold room at +4 °C prior to subsampling. All cores were split longi- to a 6 m-long telescopic aluminium rod to retrieve the unperturbed tudinally and the main visible sedimentological features were described. water–sediment interfaces, while a 90 mm UWITEC piston corer was The cores were visually cross-correlated using the main lithological fea- used in the deepest lakes (Escondido and Chester) to recover their longer tures (tephras included) with the aim to both characterize the complete 92 M. Oliva et al. / Geomorphology 261 (2016) 89–102

Fig. 2. Photographs of the lakes cored for palaeoenvironmental purposes in the Byers Peninsula. sedimentary infill of the lakes as well as to select the best core to conduct luminescence, but again did not show TL or OSL emission. Both fractions environmental and climate reconstructions. were analysed by X-ray diffraction and weak evidence of quartz was ob- The age-depth model of each sedimentary sequence was established served, while anorthite, labradorite and weak albite peaks were identi- using two absolute dating techniques (luminescence dating and 14C) fied, as well as amorphous materials. Observations under a stereoscopic combined with a relative dating approach (tephrochronology). A total microscope revealed high glass content. Thus, polymineral 90–180 μm of 28 aquatic moss macrofossil samples from the sediment cores of sand grains were used for dating. Given the presence of feldspars in Chester (6), Escondido (17) and Cerro Negro (5) lakes were collected the samples (anorthite, labradorite and albite) both Infra-Red Stimulat- for Accelerator Mass Spectrometry (AMS) 14C dating at the ed Luminescence (IRSL) and TL were tested, as feldspars usually show Radiochronology Laboratory of the Centre d'Études Nordiques (Laval luminescence signals under Infra-Red stimulation and characteristic TL University, Canada). Antarctic lake sediments have a very low carbon glow curves. IR stimulation revealed very weak luminescence signals content and aquatic mosses have been found to provide the most reli- but TL showed acceptable glow curves for dating without an optical fil- able dates (Toro et al., 2013). Radiocarbon dating was not possible in ter. Thus, TL (total bleach method, Aitken, 1985) was finally used for Domo Lake sediments due to the lack of moss remnants and very low dating. 5 mg aliquots were mounted in stainless steel cups and mea- carbon content in the sediment. Radiocarbon dates were calibrated sured in an automated Risø DA-15 TL/OSL reader system in the lumines- with the software Calib version 7.0 and the SHcal13 calibration curve cence lab of the University of A Coruña (Spain). TL glow-curves were (Reimer et al., 2013), selecting the median of the 95.4% distribution measured with a coupled 9235QA photomultiplier tube (PMT). Labora- (2σ probability interval). The age-depth model for each sedimentary se- tory doses were given using a 90Sr/90Y beta source mounted on the quence was built using the Bacon R package (Blaauw and Christen, reader giving a dose rate of 0.120 ± 0.003 Gy s−1. TL measurements 2011). The construction of the chronological model of Limnopolar were performed using the total bleach method (Mejdahl, 1986) with Lake record is described in detail in Toro et al. (2013) and in the residual TL level defined by the TL remaining after 48 h bleaching Martínez-Cortizas et al. (2014). to sunlight. The TL equivalent doses were calculated using the interval Samples for luminescence dating were taken after opening the sed- 300–400 °C. The glow curves were determined applying the additive iment cores under red light. Fine sand grains were obtained by sieving dose method (Aitken, 1985) with linear extrapolation. and quartz grain extraction was attempted, but no quartz signals were The natural dose-rates were estimated from U, Th and K contents observed with thermoluminescence (TL) and Blue-OSL (Optically Stim- using inductively coupled plasma mass spectrometry (ICP-MS) on bulk ulated Luminescence) that usually show clear signals when quartz is samples. Conversion factors proposed by Guerin et al. (2011) were ap- present. Silt grains between 4 and 11 μm were also prepared for plied in this research. For assessing the external alpha contribution an a-

Table 1 Main limnological and morphological data of the lakes studied in this research.

Lake Latitude Altitude (m asl) Catchment area (km2) Lake area (km2) Coring depth (m) Coring date Selected cores for dating (cm) Longitude

Chester 62°36′41.10″ S 95 0.09 0.037 5.3 18/11/2012 CH12/05-01G (42) 61°06′02.28″ W CH12/08–01 (122.5) Escondido 62°37′06.57″ S 92 0.08 0.016 5.2 24/11/2012 ES12/03–02 (148) 61°03′36.50″ W ES12/02-01G (23) Cerro Negro 62°37′47.30″ S 100 0.015 0.0035 2.2 27/11/2012 CN12/03-01G (114.5) 61°00′19.99″ W Domo 62°37′17.49″ S 45 0.17 0.025 1.6 27/11/2012 DO12/06-01G (59.5) 60°58′32.98″ W Limnopolar 62°37′23.65″ S 65 0.58 0.0221 4.5 30/11/2008 LIM03/1, LIM08/E-D (Toro et al., 2013) 61°06′23.67″ W M. Oliva et al. / Geomorphology 261 (2016) 89–102 93

Table 2 Results of AMS 14C dating with samples from the three lakes with living and subfossil mosses.

Lake Core Lab code Sample Sediment depth Combined depth δ13C (%) Conventional radiocarbon Calibrated age code (cm) (cm) age (yr BP) (cal yr BP)

Chester Surface sample ULA-4656 Mosses Surface 0.0 −26.8 Modern – CH12/05-01G ULA-4242 CH04 2.5–2.7 2.6 −26.4 1125 ± 15 970 ± 20 CH12/05-01G ULA-4249 CH03 10.3.8–10.8 10.6 −25.4 1140 ± 20 1000 ± 30 CH12/05-01G ULA-4251 CH01 31.7–32.1 31.9 −25.8 3665 ± 20 3930 ± 50 CH12/08–01 ULA-4244 CH10 3.5 31.9 −27.6 3685 ± 20 3955 ± 5 CH12/08–01 ULA-4253 CH12 117.0–121.0 146.4 −23.6 4575 ± 20 5155 ± 125 Escondido Surface sample ULA-4657 Mosses Surface 0.0 −14.9 Modern – ES12/02-01G ULA-4650 ES-15 10.8–11.0 10.9 −24.3 1485 ± 20 1330 ± 30 ES12/02-01G ULA-4651 ES-16 15.9–16.4 16.2 −22.0 1830 ± 20 1715 ± 25 ES12/03–02 ULA-4256 ES-09 3.0–3.5 11.1 −24.0 1605 ± 15 1455 ± 10 ES12/03–02 ULA-4704 ES-10 9.0 16.8 −25.7 1810 ± 20 1655 ± 60 ES12/03–02 ULA-4248 ES-08 14.6–14.8 22.5 −23.0 2010 ± 15 1920 ± 20 ES12/03–02 ULA-4702 ES-11 19.0 26.8 −22.2 2195 ± 20 2150 ± 50 ES12/03–02 ULA-4654 ES-07 25.0–25.3 33 −24.3 2375 ± 20 2350 ± 35 ES12/03–02 ULA-4655 ES-12 30.0 37.8 −23.5 2595 ± 20 2710 ± 30 ES12/03–02 ULA-4247 ES-06 35.6–35.9 43.6 −24.8 2600 ± 20 2720 ± 45 ES12/03–02 ULA-4653 ES-13 40.5 48.3 −22.7 2925 ± 20 3010 ± 80 ES12/03–02 ULA-4652 ES-05 46.4–46.6 54.3 −23.2 3220 ± 20 3405 ± 50 ES12/03–02 ULA-4663 ES-14 50.5 58.3 −25.7 3325 ± 25 3510 ± 70 ES12/03–02 ULA-4255 ES-04 56.9–57.1 64.8 −22.4 3630 ± 15 3890 ± 65 ES12/03–02 ULA-4254 ES-02 123.5–124.0 131.6 −24.1 3765 ± 15 4060 ± 130 ES12/03–02 ULA-4703 ES-03 130.2–130.5 138.2 – 4265 ± 20 4755 ± 25 ES12/03–02 ULA-4243 ES-01 135.0–135.6 143.1 −23.6 4600 ± 20 5165 ± 140 Cerro Negro Surface sample ULA-4658 Mosses Surface 0.0 −23.4 Modern – CN12/03-01G ULA-4246 CN-01 4.4–4.6 4.5 −23.0 1855 ± 15 1740 ± 10 CN12/03-01G ULA-4701 CN-02 16.8–17 16.9 – 1955 ± 20 1860 ± 50 CN12/03-01G ULA-4250 CN-03 31.0–31.2 31.1 −22.1 2690 ± 15 2760 ± 35 CN12/03-01G ULA-4245 CN-04 41.4–41.7 41.6 −21.1 3010 ± 20 3130 ± 45 value of 0.02 was added. As water in sediments affects to ionizing radia- - Unit 3 (44–0 cm composite core depth): composed of millimeter- to tion effects (and to the dose rate estimate), water saturation of the sam- centimeter-scale alternations of undulating layers of dark brown ples was assumed during burial. The cosmic dose rates were calculated mosses and light brown clays and silty clays. Two centimeter-thick according to Prescott and Hutton (1994) from geographical data (latitude black tephra layers were present (T1, T2; Liu et al., 2015), interbed- and altitude), sample depth (including depth of the sample in the core ded with these laminations. and the water column) and density. Estimated cosmic dose rates were below 10% of the total dose rate of most samples. 4.1.1. Chronology 4. Results The age model for Chester Lake is shown in Fig. 3. The onset of lake sedimentation is based on the mid-point of the obtained TL age, which Radiocarbon dates of 28 aquatic moss macrofossil samples from the roughly agrees with the radiocarbon age located at the same depth. sediment cores of Chester, Escondido and Cerro Negro lakes are given in Unit 2 is interpreted as being completely disturbed, with similar folded Table 2, while the results of TL dating of the deepest possible samples of structures in all the cores retrieved from this lake. Unit 3 has a very low these three lakes (plus Domo Lake) are shown in Table 3. average sedimentation rate of 0.07 mm/yr, except near the two tephra layers where sedimentation rates increased to 2.67 mm/yr. 4.1. Chester Lake 4.2. Escondido Lake Three lithological units were defined from the sedimentary records, from oldest to youngest: 4.2.1. Lithology From the bottom to the top, the sedimentary infill of this lake – -Unit1(151145 cm composite core depth): composed of light contained four lithological units: brown, fine sand with small rounded pebbles and interpreted as the onset of the sedimentary infill of the lake. A single tephra layer - Unit 1 (156–152 cm composite core depth): a diamicton composed (T3; Liu et al., 2015) was also observed in this unit. of light bluish clays and rounded centimeter-scale pebbles. - Unit 2 (145–44 cm composite core depth): composed of millimeter- - Unit 2 (152–131 cm composite core depth): composed of to centimeter-scale laminated dark brown clays and silty clays with millimeter- to centimeter-scale, alternating dark brown mosses convoluted structures. layers, light brown clays and silty clays. Several millimeter-thick

Table 3 Results of TL dating with samples from the base of the four lakes.

Sediment depth Combined depth Dose-rate Number of measured Equivalent dose Apparent age Fading rate Corrected age Sample (cm) (cm) (Gy/ka) aliquots (Gy) (ka) (%) (ka)

CH-1 116.5–122 147.7 1.20 ± 0.09 24 2.98 ± 0.46 2.48 ± 0.43 8.8 ± 3.5 5.90 ± 1.71 ES-1 144.0–148.0 154 2.18 ± 0.08 4 17.25 ± 0.86 7.93 ± 0.50 –– CN-1 111.0–112.5 112 1.04 ± 010 20 3.98 ± 1.15 3.83 ± 1.17 8.7 ± 1.6 7.53 ± 2.48 DO-1 55.0–58.5 56.8 1.27 ± 0.09 22 1.48 ± 0.16 1.16 ± 0.15 8.5 ± 4.3 2.26 ± 0.72 94 M. Oliva et al. / Geomorphology 261 (2016) 89–102

Fig. 3. Age model for Chester Lake.

black tephra layers were interbedded in this sequence. between 0.1 and 0.12 mm/yr, and since the main lithological features -Unit3(131–66.5 cm composite core depth): composed of laminat- of this radiocarbon-dated section are the same as the lower non-dated ed, dark grey clays, silty clays and small pebbles with convoluted one, we estimate the ﬁnal stage of deglaciation (i.e. top of unit 1) at be- structures. The centimeter-thick tephra layer T3 was found at the tween 5870 and 6050 cal. yr BP. The convoluted nature of unit 3 pre- bottom of this unit. cludes any attempt to calculate its sedimentation rate. Unit 4 is quite - Unit 4 (66.5 cm–0 cm composite core depth): composed of the same similar to unit 2, although the mean sedimentation rate is approximate- materials as unit 2, and included two centimeter-scale black tephra ly double (0.20 mm/yr). The lowest sedimentation during this period is layers (T1 and T2; Liu et al., 2015). found in the uppermost 11 cm of the composite sequence, with 0.08 mm/yr.

4.3. Cerro negro Lake 4.2.2. Chronology The chronological model of Escondido Lake was based on 17 radio- 4.3.1. Lithology carbon ages and one luminescence date (Fig. 4). However, the TL date The retrieved sedimentary inﬁll of this lake was subdivided in four (7.9 ± 0.5 ky) could not be corrected due to the small number of ali- lithological units: quots present in the sample and was rejected due to the signiﬁcant age difference with the closest radiocarbon date (5165 ± 140 cal. yr - Unit 1 (114.5–50.0 cm core depth): composed of laminated to band- BP). The well-constrained part of Unit 2 has a sedimentation rate ed white and light and dark greyish clays and silty clays with M. Oliva et al. / Geomorphology 261 (2016) 89–102 95

Fig. 4. Age model for Escondido Lake.

undulating structure. A two centimeter-thick black tephra layer (T3) where the first moss layer has been dated at 3130 ± 45 cal. yr BP. The is found at 59.5 cm of core depth. sedimentation rates are quite variable throughout this core. As in the - Unit 2 (50.0–25.5 cm core depth): made up of faintly banded light previous two lakes, the sedimentation rate for the upper 4.5 cm shows clays and silty clays, which dipped at approximately 30°. One the minimum rate of the entire core (0.03 mm/yr), although the rate in- millimeter-thick tephra layer was located at 29.5 cm core depth. creases below this layer to 0.16–1.03 mm/yr, being highest near the T1 - Unit 3 (25.5–4.5 cm core depth): composed of dark clays and two and the T2 tephras. centimeter-thick tephra layers (T1 and T2), with ca. 5 cm of dark brown moss layers and light brown clays at the top of this unit. - Unit 4 (4.5–0 cm core depth): composed of faintly banded light clays 4.4. Domo Lake and silty clays with a microbial mat on the surface. 4.4.1. Lithology The 59.5 cm of the uppermost sedimentary infill retrieved from this lake is made up of a mixture of dark brown clays, silty-clays, coarse 4.3.2. Chronology sands and centimeter-scale, rounded volcanic pebbles. The coarse Four radiocarbon ages and one luminescence date (Fig. 5) were used sands that were more abundant in the lowermost 24 cm were to build the chronological model for Cerro Negro Lake. The reliable chro- interpreted as diamicton, whereas the clays and silts were most abun- nology is limited to the uppermost 42 cm of the sedimentary infill, dant in the uppermost 35.5 cm of the core. 96 M. Oliva et al. / Geomorphology 261 (2016) 89–102

Fig. 5. Age model for Cerro Negro Lake.

4.4.2. Chronology on the climatic reconstruction provided by geomorphological In Domo Lake there were no living or subfossil mosses, and thus no observations. suitable organic material was present on which to perform radiocarbon dating. Only one luminescence date obtained at the bottom of the core 5.1. Geochronology of lake sedimentary records in the Byers Peninsula (2.3 ± 0.7 ky) constrains the age of these sediments. The average sedimentation rate for all the sedimentary infill is 0.33 mm/yr, somewhat Lithostratigraphic analysis of the sediment cores from the five stud- higher than in the other lakes. ied lakes revealed very different patterns of Holocene sedimentation in the lakes of the Byers Peninsula (Fig. 6). While the sedimentary infills of lakes Limnopolar, Escondido and (less evidently) Cerro Negro exhibited 5. Discussion a cm-thick alternating sequence of moss layers and mineral units, the sequence from Domo Lake was composed entirely of homogeneous By combining absolute ages from luminescence and 14C dating with coarse-grained sediments. Notably, all cores from Chester Lake tephrostratigraphic correlations between the lake sequences retrieved displayed a highly disturbed post-glacial sedimentary infill with some from Limnopolar, Chester, Escondido, Cerro Negro and Domo lakes, mosses in the upper part of the sediment cores. we have developed a robust chronological framework for the Byers Pen- Furthermore, the preservation of several visible tephra layers within insula. In the following discussion, we use this chronology to evaluate all sediment sequences (Liu et al., 2015) enables direct correlations be- the history of landscape and geomorphic development of the island dur- tween cores taken from the same lake as well as between those from ing the Mid to Late Holocene. We also discuss the additional constraints Limnopolar, Escondido and Cerro Negro lakes. The Chester Lake M. Oliva et al. / Geomorphology 261 (2016) 89–102 97

Fig. 6. Luminiscence and 14C ages, together with the tephra layers for all lakes. sediment cores can only be used for inferring the age of deglaciation, higher elevation than Escondido Lake, and located in a small cirque shel- since the cores contained visible evidence of deformation and post- tered by Cerro Negro hill. This topographical setting in the shadow of glacial sediments provided inconsistent 14Cdates.Itwasthereforenot the hill is very favorable for snow accumulation, as reflectedbythelon- possible to establish a robust chronology for the sedimentary infill of gevity of snow patches present today, and is also favorable for greater this lake. The Domo Lake record showed very little change in either tex- annual persistence of the lake ice cover. It is possible that between 7.5 ture or color within the core and lacked organic matter, which, com- and 3.1 cal. ky BP Cerro Negro Lake was mostly ice and snow-covered, bined with the complete lack of organic remains for radiocarbon thus impeding organic sedimentation. In high latitude lakes extended dating, restricted the chronological control to the luminescence dating ice cover reduces the incident radiation absorbed by the lake and limits performed on the bottom sediments. the time available for moss growth (Lamoureux and Gilbert, 2005)and The basal layers in all lakes comprised coarse-grained sediments other primary producers (i.e. algal biofilms). with presence of decimeter-sized igneous rock fragments. This is In Cerro Negro Lake there is an increase in organic matter though the interpreted to represent the diamicton deposited during the deglacia- first mosses appeared later in this lake, at 3.1 cal. ky BP. Both ages fit tion of these lakes, such as was inferred for the nearby Limnopolar within the mid-Holocene warm period inferred for the northern AP be- Lake (Toro et al., 2013). The diamicton was reached in two lake se- tween 4.5 and 2.8 ky BP (Bentley et al., 2009). The presence of mosses in quences (Escondido and Domo) and likely near the base in Chester the Byers Peninsula lake sediments has been interpreted to indicate and Cerro Negro lakes. In these two lakes, the basal sediments warmer and/or drier conditions (Toro et al., 2013). (5–10 cm) were lost on the lake ice during core recovery but they The Escondido Lake record shows alternating layers of mosses and were observed and their main lithological features were similar to the clay-silty sediments until 2.3 cal. ky BP, most likely related to changing diamicton recovered from other lakes. Therefore, the basal age for Ches- environmental conditions in the catchment and therefore, in the lake ter and Cerro Negro lakes is a minimum age estimated for deglaciation, ecosystem. They are probably related to significant variations in the cli- although we suggest that it closely approximates the actual age of ice- mate regime, with varying moisture and temperature conditions in the free exposure. Byers Peninsula between 3.8 and 2.3 cal. ky BP. The presence of faintly Subsequent to the onset of sedimentation in all lakes, there was a laminated and inclined clays and silty clays in the Cerro Negro Lake sed- gradual change from coarse- to fine-grained particles with inorganic iments (Unit 3), instead of the well-defined alternation of mosses and sediments. In Escondido Lake, the first appearance of mosses occurred clays found in the Escondido Lake, may suggest the dominance of geo- at around 5.1 cal. ky BP. This pattern was not observed in Cerro Negro morphic processes in the former whereas climate fluctuations played Lake, where the base of the core was dated at 7.5 cal. ky BP and the a key role in the latter. oldest organic sediments started to be deposited at 3.1 cal. ky BP. This The upper part of the cores spans the last 1.8 cal. ky BP. However, the discrepancy in the timing of organic deposition in these two lakes may youngest mosses were dated at 1455 ± 10 and 1330 ± 30 cal. yr BP in be related to the location of Cerro Negro Lake, which is at a slightly Escondido Lake cores and at 1740 ± 10 cal. yr BP in the Cerro Negro Lake 98 M. Oliva et al. / Geomorphology 261 (2016) 89–102 core. The significance of the absence of moss layers in the uppermost in Barton Peninsula, where cosmogenic ages suggest deglaciation be- sedimentary infill of these two lakes remains unclear. Their presence tween 11.9 and 7.6 ky, and in the nearby Weaver Peninsula, where in the youngest Limnopolar Lake sediments (ca. 0.5 cal. ky BP) suggests the highest currently ice-free areas were exposed at 8.8 ky (Seong that local processes, rather than large-scale climate or environmental et al., 2009). fluctuations, may be responsible for this difference. At present, mosses It is likely that the same pattern occurred in the Byers Peninsula dur- occur in each of these three lakes. Living mosses from the studied ing this time. The loss of glacier volume of the Rotch Dome would have lakes have been radiocarbon dated and reported modern ages in all cleared the western fringe of the peninsula of ice and the glacier thin- cases, which suggests that no present-day reservoir effect exists in ning exposed a significant part of the highest lands as nunataks. Howev- these lakes. er, no reliable dates are available for the central-west area of the Byers Peninsula. Several lake records collected from this area in the late 5.2. The process of deglaciation in Byers 1980s and early 1990s reported basal ages ranging from 4 to 5 cal. ky BP (Björck et al., 1996), in all cases showing significant age differences The bottom dates of each lake sequence are indicative of the onset of with the records presented in this study (Fig. 7). However, as Björck lacustrine sedimentation, which, in turn, suggests the age of deglacia- et al. (1996) conceded, in most cases they did not reach deglacial sedi- tion at each lake. Using paleolimnological and geomorphological evi- ments, and thus their ages do not provide a chronological control for dence, several stages can be inferred for the environmental dynamics the onset of deglaciation in the Byers Peninsula. of the Byers Peninsula since the last glaciation: Marine records suggest that warmer conditions continued from 8.2 Stage 1 – Deglaciation of the western fringe and highest land (from to 5.9 cal. ky BP, with the Holocene period of minimum sea-ice cover 8.3 to 5.9 cal. ky BP) and warm water conditions (Hodgson et al., 2006; Milliken et al., The timing of the onset of the deglaciation of the Byers Peninsula is 2009). Consequently, significant glacier thinning and ice margin retreat based on the dating of the oldest moss found in the sediments of occurred in many sites in the western AP between 8 and 7 cal. ky BP, as Limnopolar Lake, in the west-central part of the peninsula, with a re- inferred from geomorphological evidence and geochronological data ported age of 7510 ± 80 cal. yr BP (Toro et al., 2013). The chronological (Bentley et al., 2006). This glacier thinning and retreat consequently in- model estimated an age for the base of lacustrine sediments of ca. creased the rates of isostatic uplift in the SSI (Watcham et al., 2011). 8.3 cal. ky BP. Therefore, the relatively flat area at the west of Limnopolar Stage 2 – Deglaciation of the central plateau (from 5.9 ky to Lake was probably the first deglaciated environment apart from the 1.8 cal. ky BP) highest nunataks and coastal areas and beaches. The NW section of The Rotch Dome continued to shrink during this phase. There are no the Byers Peninsula – the area encompassing the highest lands with el- geomorphological traces of the position of the glacier front between the evations exceeding 200 m – likely would have remained glaciated dur- deglaciation of Cerro Negro and the deglaciation of Escondido and Ches- ing this time. There is no geomorphological evidence indicating ter lakes. The luminescence age of the basal sediments in Chester Lake whether the Rotch Dome and the ice that accumulated in the NW sec- indicated an age of 5.9 ± 1.7 ky (at 148 cm depth), which is roughly tion of the Byers Peninsula were linked during this stage. In contrast, consistent with the oldest dated moss sample in the same core that the low-altitude terrain in the SW sector would likely have remained yielded and age of 5160 ± 120 cal. yr BP (at 146 cm depth). Basal lumi- ice-free. This suggests that the process of deglaciation started in the nescence dating in Escondido Lake provided an apparent age of 7.9 ± W\\SW region of the Byers Peninsula, and recession continued towards 0.5 ky, which we rejected in favor of the projected 14C-AMS age of the higher elevation areas in the eastern sector located above the equi- 5870–6050 cal. yr BP for the base of the core. Two reasons support librium line altitude (Fig. 8). such rejection: (i) the number of measured aliquots is too small to get The oldest age inferred from luminescence dating in our studied a consistent TL age; (ii) there were insufficient grains to perform fading lakes was derived in Cerro Negro Lake, at 7.5 ± 2.5 ky. The higher loca- tests to correct the obtained apparent age for this sample. Therefore, as- tion of Cerro Negro Lake with respect to the plateau suggests that the suming the uncertainties of the dates, Chester Lake was deglaciated at eastwards retreat of the Rotch Dome was accompanied by a thinning around 5.9 ky and Escondido Lake, located 2 km eastwards, at 6 cal. ky of the ice cap between 8.3 and 7.5 ky. The diminishing ice thickness dur- BP. Despite their proximity to Limnopolar Lake (i.e., 1.5–3 km), there ing the Early Holocene in response to warming climate conditions final- was a substantial difference of 2.4–2.5 ky in the timing of deglaciation. ly exposed Cerro Negro Lake and lacustrine sedimentation started at The considerable difference in time between Limnopolar, Chester 7.5 ky. Nevertheless, the lake could have remained permanently ice- and Cerro Negro lakes may be related to glacier stabilization or with covered for long periods during the following millennia, as previously minor glacier advances and retreats following the glacier shrinkage of mentioned. Glacier recession continued and glacial ice disappeared the ice cap recorded between 8.3 and 7.5 ky. Moreover, in the case of from the slopes descending from Cerro Negro to the plateau, where Chester Lake a small glacier centre may have existed during this stage blockstreams are distributed today. They are currently inactive and surrounding the Chester Cone (Martínez de Pisón et al., 1996). Such a heavily lichenized but they may have developed during this phase, sug- long period of relative glacier stabilization may have generated mo- gesting very intense periglacial processes and widespread permafrost raines. However, the lack of moraine remnants in the area may be asso- conditions in ice-free areas (Ruiz-Fernández and Oliva, in press). ciated with the high effectiveness of physical weathering processes in In the SSI, the shrinkage of the ice caps started between ca. 11 and humid Maritime Antarctica that erodes moraines shortly after their for- 9.5 cal. ky BP as a consequence of warmer conditions in the AP region, mation (Martínez de Pisón et al., 1996). The existence of a partially dis- as inferred from ice and marine cores and supported by geomorpholog- mantled moraine at the foot of Cerro Negro might be attributed to this ical records (Bentley et al., 2009). Glacier retreat was particularly rapid phase, since it shows evidence of a phase of glacier stabilization during between10.1and8.2cal.kyBP(Milliken et al., 2009). One of the impli- the long-term retreat (Ruiz-Fernández and Oliva, in press). The pattern cations of the loss of glacier volume in the SSI is the exposure of ice-free of long-term retreat of the dome-shaped glacier towards the east does terrain during the Early Holocene in low-altitude areas of flat topogra- not preclude the presence of small glaciers distributed across the penin- phy located distal from ice caps. This is the case of some coastal environ- sula, as was suggested by glacial deposits surrounding the Chester Cone ments in King George Island, such as Fildes Peninsula that remained ice- (Martínez de Pisón et al., 1996) and by drainage networks (Mink et al., free between 11 and 9 cal. ky BP (Watcham et al., 2011) and the edges of 2014). However, the location of some of the latest glacier centres sug- Barton Peninsula that became ice-free around 8 cal. ky BP when lakes gested in this study is not supported by the existing absolute dates, began to form (Oliva et al., submitted for publication). Moreover, since one of the proposed last remnants of glacier ice is precisely located some of the highest lands of the islands with steep slopes were exposed at Limnopolar Lake, known to be the first deglaciated lake in the Byers due to the shrinking ice, therefore acting as nunataks. This was observed Peninsula at ca. 8.3 ky (Toro et al., 2013). Nevertheless, an isolated M. Oliva et al. / Geomorphology 261 (2016) 89–102 99

Fig. 7. Distribution of the available dates from lake sediments in Byers Peninsula, comparing this research with former studies. glacier may have persisted over time in the area of Chester Lake, topo- indicative of different phases of glacial advance or retreat. Based on graphically protected at the foot of Chester Cone. An ice cap centred in this geomorphological evidence it is likely that glacier retreat was rela- the uplands of the NW peninsula during this stage is possible, but tively fast between Domo Lake and its present-day position. Subse- there is no evidence to assess if it was connected with the Rotch Dome. quently, the glacier appears to have experienced minor retreats and In other areas of the SSI, glaciers reached their present limits around advances during the Late Holocene that aggregated sediments to the 6.1 ky (Watcham et al., 2011; Ó Cofaigh et al., 2014). Following a period inner slope of the moraine which therefore constitutes a polygenic mo- with complex climate patterns in the AP region until 4.5 ky, terrestrial raine (Palacios et al., 2015). This pattern of moraine formation hinders and marine records indicate a phase of significant warming between the identification of periods with glacier expansion linked to colder 4.5 and 2.8 cal. ky BP (Mid-Holocene warm period; Bentley et al., and/or wetter conditions during the last two millennia. In other areas 2009). In the Byers Peninsula, the warmest and wettest conditions of the western AP such as in Rothera Point (Guglielmin et al., 2015) were recorded between 3.0 and 2.8 cal. ky BP (Björck et al., 1991, and Palmer Deep (Shevenell and Kennet, 2002) there is evidence of gla- 1996). Consequently, the ice cap in the Byers Peninsula must have cial expansion during the last millennium, namely during the Little Ice retreated significantly and by ca. 1.8 cal. ky BP the retreat of the Rotch Age (LIA). Glacial advances during the LIA have been identified in Dome reached the area surrounding Domo Lake. other areas of the SSI (Yoon et al., 2004; Hall, 2007) although in other Stage 3 – Deglaciation of the eastern ice free sector of Byers (prior to ice-free environments, such as the Barton Peninsula, glaciers have 1.8 cal. ky BP to present) been stable during the Late Holocene without major significant ad- Despite the luminescence age of the basal sediments of Domo Lake vances (Oliva et al., submitted for publication). The chronology of (2.3 ± 0.7 ky), the fact that tephra layers T1 and T2 do not appear in these neoglacial advances is not well-constrained (Bentley et al., the sequence suggests that this lake is younger than 1.8 cal. ky BP, 2009). Recently, Simms et al. (2012) suggested that the glacial expan- when T1 was deposited according to Escondido Lake sediments. There- sion between 1500 and 1700 CE was followed by retreat until fore, the area between this lake and the present-day moraine ridge of 1840 CE. Glacial retreat resulted in an uplift of 2.5 m in the SSI, which Rotch Dome (300 m distance) must have become ice-free during the was synchronous with the formation of the lowest raised beaches in Late Holocene. This date must be considered as approximate and more the Byers Peninsula (Hall and Perry, 2004). In Byers Peninsula some of absolute dates must be obtained in order to completely constrain this the ridges of the polygenic moraine may be related to the LIA. Such cli- last stage of deglaciation. mate conditions would also be favorable for more abundant long-lasting This area is hilly, with till spread across the landscape and some lakes snow-patches and might have promoted the appearance of small glacier and ponds filling intra-moraine depressions. The frontal moraine ridge patches in sheltered environments near the highest volcanic plugs, as is very steep, showing a significant height (10–25 m) with respect to well as the expansion of the two small glaciers that still exist in the the deglaciated area. The moraine is composed of unconsolidated NW corner of the Byers Peninsula. gravels and boulders in a sandy matrix, and forms a large primary longi- Finally, the recent accelerated retreat observed in many glaciers of tudinal ridge from the north to the south coast of the peninsula. In the the SSI over the last decades (Navarro et al., 2013)isnotclearlyvisible central section there are several secondary arches, which may be in the Byers Peninsula. In comparison to neighboring areas where 100 M. Oliva et al. / Geomorphology 261 (2016) 89–102

Fig. 8. Possible model for the deglaciation based on the existing dates. Question marks on the maps indicate uncertainties on the location of the glacier fronts. glaciers are now physically disconnected from the moraine system, such as Holocene. The shrinking of the glacier cleared the highest volcanic Elephant Point (Oliva and Ruiz-Fernández, 2015), today the Rotch Dome plugs of the peninsula of ice, as inferred from Cerro Negro Lake sedi- sits in contact with the moraine in the centre of the Byers Peninsula. ments, which indicated that this lake became ice-free by 7.5 ky. The highest lands in the Byers Peninsula therefore acted as nunataks pro- 6. Conclusions truding the glacial ice during most of the Mid-Holocene. The loss of glacier volume continued during the Mid-Holocene and the central plateau The analysis of ﬁve lacustrine sedimentary sequences (Domo, Cerro became gradually ice-free, as indicated by the Escondido and Chester Negro, Escondido, Chester and Limnopolar lakes) along an east-west tran- lake records, with these lake catchments becoming ice-free by sect from the Rotch Dome glacier to the coast enabled the identiﬁcation 6 cal. ky BP and 5.9 ky, respectively. The long-term eastward retreat of of the past dynamics of the westernmost ice cap in Livingston Island. It pro- the Rotch Dome during the Holocene may have occurred parallel to vides a continuous record for past environmental and climate changes dur- the existence of other small glacier spots distributed at the foot of the ing most of the Holocene in the western South Shetland Islands. The highest elevations of the peninsula. The easternmost ice-free area in Holocene retreat of this ice cap has exposed the largest ice-free environ- Byers became ice-free during the Late Holocene, and probably before ment in the SSI, with major consequences for the geomorphological and 1.8 cal. ky BP, as suggested by Domo Lake sediments. ecological dynamics in the area. The chronology of the deglaciation in the This research provides new data about the paleoenvironmental his- Byers Peninsula provides a framework for better understanding a wide tory of the Byers Peninsula, and presents new questions for future stud- range of processes connected with glacial retreat in the terrestrial ecosys- ies. In particular, the application of cosmogenic dating techniques tem, such as colonization of vegetation, distribution of permafrost and geo- would complement data about the Holocene deglaciation process in- morphological landforms and processes, formation of marine terraces. ferred from lake sediments by incorporating dates obtained from ero- The onset of lake sedimentation is indicative of the age of deglacia- sional and depositional glacial evidence. Additional effort should focus tion at each site. The retreat of Rotch Dome glacier pre-dated on the NW corner of this peninsula, where two small glaciers still 8.3 cal. ky BP in the westernmost third of the peninsula as a response exist today, but about which information on past environmental dy- to warmer climate conditions in the AP region during the Early namics is lacking. M. Oliva et al. / Geomorphology 261 (2016) 89–102 101

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