The Geological Society of Korea
[ Article ]
Journal of the Geological Society of Korea - Vol. 57, No. 5, pp.691-705
ISSN: 0435-4036 (Print) 2288-7377 (Online)
Print publication date 31 Oct 2021
Received 23 Jun 2021 Revised 20 Jul 2021 Accepted 03 Aug 2021
DOI: https://doi.org/10.14770/jgsk.2021.57.5.691

10Be and 10Be/9Be in glaciomarine sediments of Ross Sea, Antarctica: implications for mid-late Quaternary paleoenvironmental changes

Chinmay Dash1 ; Min Kyung Lee2 ; Yeong Bae Seong3, ; Hyun Hee Rhee3 ; Jae Il Lee2 ; Kyu-Cheul Yoo2
1Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee 247667, India
2Division of Glacial Environmental Research, Korea Polar Research Institute, Incheon 21990, Republic of Korea
3Department of Geography, Korea University, Seoul 02841, Republic of Korea
남극 Ross Sea 빙성해양퇴적물의 10Be과 10Be/9Be: 제4기 중·후반 고환경변화에 대한 함의
Chinmay Dash1 ; 이민경2 ; 성영배3, ; 이현희3 ; 이재일2 ; 유규철2
1Department of Earth Sciences, Indian Institute of Technology Roorkee
2극지연구소 빙하환경연구본부
3고려대학교 지리학과

Correspondence to: +82-2-3290-2367, E-mail: ybseong@korea.ac.kr

Abstract

This study documents the variation of Be isotopes (10Be and 9Be) and their ratio (10Be/9Be) from a Ross Sea sediment core (LC42) with relation to mid to late Pleistocene geomagnetic and paleoclimatic changes. Significant changes in Be isotope concentration are observed during pre and post-Mid-Pleistocene Transition (MPT) periods. The 10Be concentration and 10Be/9Be ratio show a gradual decrease during the late Matuyama Chron, suggesting a relative increase in geomagnetic intensity. The overproduction episode associated with the Matuyama-Brunhes Boundary (MBB) does not show any pre- MBB precursor event. The progressively decreasing trend observed in 10Be and 10Be/9Be ratio records from 1 to 0.8 Ma, with minimum values at around 0.8 Ma, suggests a relatively open sea condition during the early part of MPT and an extended glacial condition around MIS 22. The 10Be and 10Be/9Be ratios are approximately constant during the pre-Mid Bruhnes Event (MBE) period and show a gradual increase after the MBE, indicating a climatic transition from colder (pre-MBE) to relatively warmer (post-MBE) conditions.

초록

본 연구는 남극 로스해에서 얻어진 코어(LC42)의 Be 동위원소 변화(10Be, 9Be, 10Be/9Be)를 분석하였으며 이를 제4기 고지자기와 고기후 변화의 관점에서 해석하였다. Be 동위원소는 Mid-Pleistocene Transition (MPT)를 전후로 많은 변화를 보이고 있다. 10Be의 농도와 10Be/9Be은 Matuyama Chron동안 점진적인 감소를 보이고 있으며, 이는 지자기강도의 상대적 증가를 지시한다. 반면에 많은 지역에서 보고된 Matuyama-Brunhes Boundary (MBB)와 관련된 10Be 과생성 사건은 본 연구에서는 발견되지 않았다. 1.0-0.8 Ma 동안에는10Be과 10Be/9Be은 점차적으로 감소했으며, 0.8 Ma에 최소값에 도달했다. 이는 이지역이 초기 MPT 시기 동안 상대적 open sea 환경이었으며 MIS 22에 빙하가 확장했다는 점을 시사한다. 10Be과 10Be/9Be은 Mid Bruhnes Event (MBE) 이전 시기에는 일정하였으며 MBE 이후 시기에는 점진적인 증가를 보이고 있다. 이는 한랭한(pre-MBE) 기후에서 온난한(post-MBE) 기후로의 전환을 지시한다.

Keywords:

Be isotope, Pleistocene, Antarctica, Ross Sea, glacier

키워드:

Be 동위원소, 플라이스토세, 남극, 로스해, 빙하

1. Introduction

Interaction of primary/secondary cosmic rays with the atomic nuclei in the upper atmosphere/earth’s surface produces 10Be and other cosmogenic nuclides such as 3He, 14C, 21Ne, 22Ne, 26Al, 36Cl, and 53Mn (Lal and Peters, 1967). The production of 10Be usually occurs due to the spallation of Oxygen and Carbon by the cosmic ray particles. Nucleons, protons in the upper atmosphere, and neutrons at the earth's surface dominate cosmogenic nuclide production, whereas subsurface production is dominated by muons, which weakly interact with matter and retain energy for deeper penetration (Lal, 1988). Distinct processes produce two different varieties of 10Be: meteoric 10Be is produced in the upper atmosphere, whereas in-situ 10Be is produced on Earth’s surface when cosmic rays interact with oxygen atoms within the lattices of mineral grains (Quartz) (Lal and Peters, 1967). The meteoric 10Be concentration in sediments has wider applicability for numerous geophysical and geochemical studies due to its high sensitivity to earth surface processes and its application to quartz free lithologies (Willenbring and von Blanckenburg, 2010). Meteoric 10Be is a potential tool for estimating basin-wide erosion rate (von Blanckenburg et al., 2012), reconstructing paleo-secular variations of the earth’s magnetic field (Aldahan and Possnert, 2003; Carcaillet et al., 2004; Simon et al., 2020), and estimating the sea-ice extent (Frank et al., 1995) and glacial meltwater discharge (Simon et al., 2016a). When normalized to its stable counterpart 9Be, the ratio can circumvent the variations relating to particle size or retention behavior (von Blanckenburg et al., 2012) and can be applied to measure spatio-temporal variability of continental erosion rates (von Blanckenburg and Willenbring, 2014; von Blanckenburg et al., 2015) and record deep ocean circulation patterns (von Blanckenburg et al., 1996). Due to the long half-life of 10Be (1.39×106 yr), this ratio can be applied to study ice sheet melting dynamics for more extended periods than could be achieved using other traditional particle reactive radioisotopes (t1/2 230Th = 7.5×104 yr and t1/2 231Pa = 3.2×104 yr).

At present, there exist only a few works concerning the 10Be in Antarctic sediments. Frank et al. (1995) studied sediment redistribution and paleo-productivity changes in the Weddell Sea sediment record and documented an increase in 10Be concentration during the late Quaternary interglacials. In another study, Frank et al. (2002) studied quasi-conservative behavior of dissolved 10Be from depth profiles of Antarctic seawater and suggested that the 10Be concentrations in deeper water are less affected by the scavenging process or ice melt. Other similar studies around the Antarctic used Be isotopes to distinguish glacial/post-glacial sediment sources (Sjunneskog et al., 2007; Yokoyama et al., 2016), estimate variations in sea ice cover, and study amplitude and rhythm of geomagnetic field change (Raisbeck et al., 2006). Despite previous high-resolution studies, the present understanding of the variability of Be isotope concentrations in Antarctic sediments concerning paleoenvironmental changes has remained unclear, as the complex characteristics of glacial and glacio-marine sediments do not offer an uncomplicated interpretation of climatic or chronological signals (Sjunneskog et al., 2007). Although Be isotopes are subject to reworking but have different pathways than other cosmogenic radioisotopes (e.g., 14C) (Willenbring and von Blanckenburg, 2010), hence can compensate for uncertainties associated with chronology and can illustrate changes in earth surface processes.

In the present study, we analyze 10Be (meteoric), 9Be, and their ratio from a Ross Sea sediment core to enhance our previous understanding of paleoclimatic records from Antarctic continental shelf sediments. The prime focus of this study is to analyze the applicability of Be isotopes as a proxy to reconstruct Antarctic Ice sheet melting by pulsed warming intervals during mid to late Quaternary and to study the variation of 10Be flux during the geomagnetic reversals.

Ross Ice Shelf occupies a large part of the Ross Sea embayment, fed by eastern glaciers and western ice streams (Sjunneskog et al., 2007). Together with sea ice, the ice shelf controls the ocean water circulation in decadal to millennial time scales, e.g., the ice shelf exceeded the continental shelf region during the Last Glacial Maxima (Domack et al., 1999; Ship et al., 1999), and its retreat during the post-glacial warm period resulted in renewed exposure to open ocean conditions (Sjunneskog et al., 2007; Jeong et al., 2018). As a consequence of the advance/retreat of the Antarctic ice shelf during glacial/interglacial periods, depositional environments in the Ross Sea have altered. Unlike the meteoric 10Be, the 9Be is primarily terrestrially sourced. Hence the Be isotope ratio variability in glaciomarine sequences will depend on (1) atmospheric flux, (2) sediment pathways and settling time, and (3) syn-depositional reworking. These Antarctic shelf sediment pathways are subject to change during different climactic episodes, thus proving them as a tracer to study climate-induced glaciomarine sediment dynamics.


2. Study area

The Ross Sea is an important aspect of global thermohaline circulation as it is one of the three primary sources of Antarctic Bottom Water (AABW). Flowing into the abyssal ocean, it provides cooling and ventilation of the deep ocean (Orsi et al., 2001). Jacobs et al. (2011) suggests that the AABW in the Ross Sea has recently freshened due to melt water influx from Amundsen and Bellingshausen Seas. Recent studies have suggested that upwelling of Circumpolar Deep Water (CDW) have accelerated the ice melting in the Ross Sea (Pritchard et al., 2012; Thompson et al., 2018), as it is relatively warmer (>0℃).

The Ross Sea contains the most abundant ice volume, which drains the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS) to the Ross Ice Shelf (RIS; Fig. 1a). The RIS advanced northward during the Last Glacial Maximum (LGM), covering most of the continental shelf of the Ross Sea with glacial sediments (Anderson et al., 2014). The Joides Trough, which is on the outermost area of the continental shelf, was the major ice drainage of EAIS, which transported glacial sediments during the LGM (Fig. 1b) (Anderson and Bartek, 1992; Ship et al., 1999; Harris et al., 2001; Livingstone et al., 2012; Anderson et al., 2014; Halberstadt et al., 2016; Park et al., 2018). The glacial geomorphological features collected from the seafloors indicate that the Joides Trough was the front of the grounding line, and nearby Mawson and Pennell Bank show pre-LGM 14C datings of carbonates (Taviani et al., 1993; Ship et al., 1999; Anderson et al., 2014).

Fig. 1.

Study area. (a) Map showing the location of Ross Sea, Weddell Sea, Ross Ice Shelf (RIS), East and West Antarctic Ice Sheet (b) Detailed physiographic map of Ross Sea (c) Bathymetry (in meters) map of Ross Sea showing the location of LC42 core (d) Lithostratigraphy of LC42 (Adapted from Ohneiser et al., 2019). Ohneiser et al. (2019) classified the LC42 core into three lithologically distinct units: Unit I (0-1.55 m) is dominated by gravelly muddy sand with interbeds of fine to medium sand and silt; Unit II (1.55-3.15 m) comprising of gravelly muddy sand, with interbeds of laminated and bioturbated silt; Unit III (3.15-11.78 m) consisting of clast free interbeds of laminated, sandy silt, bioturbated silt and gravelly muddy sands.

The Central Basin is just in front of the Joides Trough, where the glaciomarine process could not affect during the glacial periods, and we expect the pure sea ice variance performance between the permanent sea ice coverage or seasonally opened. The basin is surrounded by the Hallett Ridge (W), abrupt gradient change from the Joides Trough (S), Iselin Bank (E), and the mouth to the Southern Ocean (N) (Fig. 1c).


3. Methods

We acquired sediment core from the Central Basin during the 2015 R/V ARAON cruise of the Korea Polar Research Institute (KOPRI). We collected the RS15-LC42 in 2084 m depth (length: 11.75 m; 71° 49.40′ S, 178° 34.76′ E), and the magnetic polarity of this core exhibited normal-to-reversed magnetism at 8.26 ± 0.03 m (0.781 Ma) (Ohneiser et al., 2019). Assuming constant sedimentation rate and the top sample as of modern age, the age of the core was linearly determined using the geomagnetic reversal marker (Ohneiser et al., 2019).

The lithostratigraphy of the core was described in detail by Ohneiser et al. (2019). We followed the physical and chemical treatment procedures outlined by the Korea University Geochronology Laboratory, Seoul, to extract and measure meteoric 10Be from the clay rich marine sediments (Bourles et al., 1989; Jeong et al., 2018).

A total of 55 samples were taken for beryllium isotope analyses, roughly at every 30 cm. The samples were dried and grinded for homogenization. Over 200 μm fraction was separated, and only the <200 μm fraction was selected to remove size dependency related to chemical adsorption of beryllium isotope. After oxidization for removing organic materials, 0.4 mg of 9Be carrier (~1000 ppm) was added to 0.5 g of sediment samples before beryllium extraction procedures. The 10Be was extracted through a step-wise leaching procedure using a 0.04 M hydroxylamine in 25% acetic acid solution (Bourles et al., 1989), and its 9Be concentration was measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and Mass Spectrometry (ICP-MS) at Korea Basic Science Institute (KBSI). The relative error (of 1 standard deviation) for 9Be measurement is <±2.5%. 10Be analyses were performed using 6 MV AMS at the Korea Institute of Science and Technology (KIST) (Lee et al., 2011; Rhee et al., 2017). The blank for the beryllium carrier averaged is less than 6.5×10-15.


4. Results

The meteoric 10Be and 9Be data for the core taken from the Ross Sea (LC42) is shown in Table 1. The 10Be concentration (Decay corrected) varies from 5.29±0.05×108 to 0.214×108 atoms/g, with an average value of 1.52±0.02×108 atoms/g. The 10Be concentration depicts clear contrasts between different time intervals during the past 1 Ma. Between 1 to 0.8 Ma there is a steady decrease in 10Be concentration (average = 1.14±0.02×108 atoms/g), which increases sharply from 0.8 to 0.7 Ma (average= 1.96±0.03×108 atoms/g). Thereafter, the 10Be concentration remains approximately constant up to ~0.43 Ma, with an average value of 0.8±0.01×108 atoms/g, which is nearly half of the whole core's average value. After 0.43 Ma, the 10Be concentration increases steadily, showing maximum value at the top of the core. The average value of 10Be concentration during the post 0.43 Ma period is 1.85±0.02×108 atoms/g, nearly doubling the average value of the preceding interval. The 9Be concentration does not show much fluctuation throughout the core. The relative variation of average 9Be concentration during these time intervals is within 10% of the average value of the core (2.01±0.03×1016 atoms/g), except an anomalously higher value corresponding to 451 cm depth (Age: 425 Ma; 9Be concentration: 11×1016 atoms/g). The downcore variation of the 10Be/9Be ratio (Be ratio, decay corrected) mimics the variation of 10Be concentration, with an average value of 0.9±0.01×10-8 for the whole core. The interval between 1 to 0.8 Ma shows a gradual decrease of the ratio (average = 0.7±0.01×10-8), followed by a sharp increase (0.8-.7 Ma, average = 1±0.01×10-8). The most noticeable feature of the 10Be in this interval is a peak overpassing the average value of all samples. The Be ratio remains approximately steady till 0.43 Ma (average = 0.5×10-8) and further increases towards the top of the core. The average value of Be ratio after 0.43 Ma (1.25±0.01×10-8) doubles the preceding interval's average value. The time since the mid-Pleistocene can be divided into several climatic episodes based on Be isotope data from the present study and paleoclimatic inferences drawn from previous works. The glacial (light blue)/interglacial (yellow) periods corresponding to the increase/decrease of 10Be concentration are shown in Fig. 2.

The meteoric 10Be and 9Be Data from the core taken from the Ross Sea (LC-42).

Fig. 2.

(a) Global average surface temperature (GAST) changes based on deep sea oxygen isotopes over the last 2 Ma (Snyder, 2016). Our EAIS deglaciations coincide with the early Pleistocene warm period (pre-mid-Pleistocene transition, pre-MPT; 45) and the late Pleistocene warm interglacial periods (post-mid-Bruhnes transition event, post-MBE; 51, 56). (b) Climate proxy changes during the last 1 Ma. (1) GAST changes. (2) The temperature gradient between the interior EAIS temperature history (East Antarctica Dome C, EDC3; 47) and present temperature. (3) Surface accumulation rate, in centimeters of ice equivalent per year (EDC3; 57). (4) and (5) meteoric 10Be concentrations and 10Be/9Be ratios along Long Core 42, collected during the 2015 Ross Sea cruise (this study). The sky-blue boxes represent periods of meteoric 10Be depletion, which indicates more and thick sea ice coverage over the ocean surface due to a decrease in temperature that hindered the direct deposition of meteoric 10Be to the seafloor. The yellow boxes indicate seasonally opened periods. The brown box represents a weakened Geomagnetic Dipole Moment (GDM, XXI) period (Matuyama- Brunhes Boundary; MBB) that is < 20% of the averaged GDM and produced higher meteoric 10Be than the other periods (Simon et al., 2016a). The 10Be peaks during the period highlighted by the brown box should be interpreted with caution.


5. Discussion

Although using a single geomagnetic reversal event to establish the chronology of the core may impose age uncertainties on short-term climatic events, the longer climatic divisions will remain less affected. The Be isotope data from the present study shows changes over sub-millennial time scales, representing the variability of geomagnetic intensity and climate change over broader time intervals (Fig. 2). Thus, the present study encompassing many sub-millennial episodes of climatic variability during the past 1 Ma will not be affected by the age uncertainties.

5.1 Relationship with paleomagnetic changes

The Late Matuyama Chron witnessed an increase in the geomagnetic field intensity with significant peaks at around 870 ka, 910 ka, and 960 ka (Valet and Meynadier, 1993; Meynadier et al., 1994; Kok and Tauxe, 1999; Dinarès-Turell et al., 2002). These episodes of intensity maximas (spanning about 50 ka) were separated by periods of lower intensity. The 10Be and 10Be/9Be ratios show a gradual decrease during the late Matuyama Chron. Although the coarse sample resolution in the present study does not depict high-resolution geomagnetic changes during this period, a decreasing trend towards the Matuyama-Brunhes boundary (MBB) suggests a relative increase in paleointensity. A significant increase in the 10Be and 10Be/9Be values is observed during 0.8-0.7 Ma. No significant correlation is found between the Be isotopic record and other available paleoclimatic proxies during this period, suggesting a possible paleointensity variation (Ohneiser et al., 2019). The Be overproduction episode observed during this period is synchronous with U-Pb zircon age of MBB (772.7±7.2 ka) from Byakubi-E tephra (coupled with Oxygen isotope chronology) (Suganuma et al., 2015), 10Be overproduction observed in equatorial Pacific ocean (Simon et al., 2016b), the age of MBB obtained from 40Ar/39Ar chronology of North Atlantic marine cores (mean age of 773.1 ka) (Channell et al., 2010) and the peak 10Be flux (during 777-766 ka) observed in EPICA (European Project for Ice Coring in Antarctica) Dome C ice core (Raisbeck et al., 2006). This near-doubling of 10Be production during the MIS 19 resembles overproduction episodes associated with the largest geomagnetic reversals or collapse of geomagnetic dipole during long-term reversals (Raisbeck et al., 2006). Since there is no previous 10Be overproduction episode closely associated with the MBB overproduction, our results contradict the frequently reported pre- MBB precursor event (Hartl and Tauxe, 1996; Singer et al., 2005; Raisbeck et al., 2006).

Yamazaki (1999) documented high-resolution relative paleointensity variations during the Brunhes Chron from sediment cores of the Pacific Ocean. The results showed an increase in intensity after the MBB till the Mid-Bruhnes Event (MBE) (during the pre-MBE period). In contrast, a gradual decrease in intensity is observed during the post MBE period. A significant drop in intensity was observed at about 30-50, 110-120, 180-200, 280-300, 380-410, and 520-550 ka. The Be isotopic records from our study agree with the relative paleointensity records from the previous studies. The 10Be and 10Be/9Be ratios are approximately constant during the pre-MBE period and show a gradual increase after the MBE. Although the sampling resolution for the present study was high, geomagnetic reversals/excursions during the Brunhes Chron could be identified based on relative variation in the 10Be and 10Be/9Be records (Fig. 3). Be overproduction episodes are observed during 25 ka (Mono Lake excursion, dated 24-39 ka by Kent et al. (2002)), 114 ka (Blake excursion, dated 110-120 ka by Macrì et al. (2005)), 171 and 284 ka (Biwa I and Biwa II, respectively, dated 187-192 ka and 292-298 ka by Worm, (1997)), 322 ka (Calabrian Ridge 1, dated at 315-325 ka by Langereis et al. (1997)), and 378 ka, (Levantine dated at 360-370 ka by Langereis et al. (1997)).

Fig. 3.

Relative variation of 10Be concentration and 10Be/9Be ratio during different geomagnetic reversals/excursions.

5.2 Relationship with Pleistocene climatic changes

The Mid Pleistocene Transition (MPT) started at ~1.2 Ma and ended at around 0.9 Ma, shifting gradually from warmer to colder conditions (Elderfield et al., 2012; Sutter et al., 2019). The Early to Mid-Pleistocene warmer interval is evident from the Prydz Bay sediment record, with a decreasing value of δ18O and the presence of hemipelagic clay layers containing abundant planktonic/benthic species (indicative of open sea condition) (Theissen et al., 2003). The climate index approach of Sutter et al. (2019) show colder conditions towards the end of MPT, forming marine ice sheets in Weddell and the Ross Sea and prolonged glacial conditions between marine isotope stages (MIS) 25 and 22 (0.94-0.88 Ma BP). Elderfield et al. (2012) showed an abrupt increase in the ice sheet volume at the onset of MIS 22. The anomalously low southern hemisphere summer insolation suppressed the melting during the MIS 23, which renewed the ice growth during MIS 22. The progressively decreasing trend observed in 10Be and 10Be/9Be ratio records from 1 to 0.8 Ma with minimum values at around 0.8 Ma corroborates previous findings of a relatively open sea condition during the early part of MPT and an extended glacial condition around MIS 22. The warmer early-MPT period is also evident from Cape Roberts Project drilling in McMurdo Sound, with enriched biogenic carbonate-rich deposits during 1 Ma BP, overlying the older glacial deposits. (Taviani and Claps, 1998) Although the relative paleointensity at MBB reduced significantly to facilitate the overproduction of 10Be, the seasonal opening of Ross sea during warmer MIS 19 can be considered as another dominant contributing factor for the overproduction (Jouzel et al., 2007).

The past 0.8 Ma witnessed a dominant 100 Ka glacial-interglacial climatic cyclicity (Imbrie et al., 1993; Mudelsee and Schulz, 1997; Elderfield et al., 2012; Barth et al., 2018). The Milankovitch cycle shifted from 41 ka to 100 ka, causing significant temperature variations, albeit under cold conditions (post-MPT). The increase in the amplitude of 100 Ka cycles across the MBE is related to the change in earth’s eccentricity and precession. The colder conditions persisted till 430 ka (Yin, 2013), and short interglacials punctuated the extended glacial periods during this period. The interglacial periods became relatively warmer across the MIS 11 (after 430 Ka BP, MIS 1, 5, 9, and 11) (Jouzel et al., 2007; Yin and Berger, 2010), with higher atmospheric CO2 concentration (Barker et al., 2006; Lüthi et al., 2008). This climatic transition across the 430 Ka boundary is sometimes referred to as Mid-Bruhnes Transition. Pre-MBE (0.8-0.43 Ma) witnessed reduced temperature, which not only triggered the northern ice sheet growth (Snyder, 2016) but also increased the formation of the Antarctic bottom water with colder deep ocean water (Yin, 2013). The Ross Sea was seasonally open during the post-MBE warmer period, with less ice cover in vertical and lateral extent than the pre-MBE period. Our results showing high 10Be production after 430 Ka also suggest post-MBE interglacials (MIS 1, 5, 9, and 11) were relatively warmer than that of the pre-MBE period (MIS 13, 15, and 17). As there is a clear transition of the Be isotopic data between pre and post MBE period, the age of MBE in the present study can be recognized as another marker age for the core.

Ohneiser et al. (2019) compared the magnetic mineralogy of the near shelf (cores: RS15-LC42 and RS15-LC47)) and abyssal plain (cores: RS15-LC48 and RS15-LC108) sediments of Ross Sea. The inner shelf cores exhibited lower coercivity and coarser mineral magnetic population as compared to the abyssal plain sediments. The central basin is in close proximity to sources of glacial sediment with ice rafting, and gravity flows as the dominant processes of sediment supply, which get remobilized by bottom water currents (Meyers, 1982; Ehrmann, 1998; Howe et al., 1998; Domack et al., 1999; Anderson et al., 2014; Halberstadt et al., 2016). RS15-LC42 is located near the continental margin, in the flow path of high-energy cold, dense shelf waters, making a favorable condition for the deposition of coarser magnetic grains. The finer magnetic grains are carried further northward into the abyssal plain environment. The coarser sand beds along with gravel size inter-clasts and devoid of any depositional structures were indeed deposited by ice rafting, and further winnowed by strong bottom water currents (Fig. 1d) (Gilbert et al., 1998). In contrast, fine-grained interbeds with few local laminations suggest deposition under weaker bottom currents. Thus, alteration of poorly sorted sand and unsorted mud deposits can be attributed to temporal changes in sediment supply corresponding to changes in strength of bottom water circulation. Correlation of lithostratigraphy with climatic inferences from Be isotopic record of this study shows that relatively coarser sediments were deposited during the warmer climatic intervals and vice versa. Gravelly muddy sand deposits with presence of clasts at bottom of the core corroborates with relatively warmer conditions during early part of MPT. A short interval of coarse sediment deposition is observed around 8 m depth, corresponding to MBB. Although Be overproduction episode at MBB is associated with geomagnetic reversal, the coarser sedimentation suggests a short interglacial period with strengthened bottom water circulation. Progressively coarser sedimentation towards the top of the core suggests extensive deglaciation during late Quaternary period and is consistent with the gradual increase in 10Be values after MBE. Textural studies of Weddell Sea sediments suggest strengthening/weakening of bottom currents during interglacial/glacial periods, respectively (Melles, 1992; Pudsey, 1992). Bottom-current at the coring site is primarily driven by Ross Sea Bottom Water (RSBW) production. The fine-grained deposits in the inner shelf area imply intermittent reduction in RSBW production and, hence, expansion of shelf ice. We conclude that coarse-grained units in the lithostratigraphy were deposited under strengthened bottom current conditions in a relatively warm climate (interglacial), and finer units reflect weak bottom currents in a cooler climate (glacial).


6. Conclusions

The 10Be concentration in the Antarctic open marine condition is about 1.5×109 atoms/g (Sjunneskog et al., 2007). However, the LC42 record exhibit concentration of a magnitude of less than one degree throughout the core. The concluding points from the present study are:

1) The downcore variation of the 10Be and 10Be/9Be in the Ross Sea sediment core illustrates glacial-interglacial transitions during the mid to late Pleistocene period. The relatively high/low concentration of 10Be indicates relatively open marine/sea ice-dominated conditions, respectively.

2) The gradual decrease in the 10Be concentration from 1 to 0.8 Ma suggests relatively extended glacial conditions persisted during the late Matuyama Chron. This study shows that the MIS 22 was the coldest period during the late Matuyama Chron, which corroborates previous studies showing an abrupt increase in the ice sheet volume at the onset of MIS 22. The progressively increasing trend observed in the 10Be concentration after 430 Ka suggests that the post-MBE period was relatively warmer than the pre-MBE period.

3) The downcore variation of the 10Be and 10Be/9Be ratio shows some degree of correlation with the geomagnetic reversals/excursions. No precursor event associated with the MBB reversal contradicts previous investigations showing a pre-MBB overproduction episode closely associated with the MBB overproduction episode.

4) This study shows that further investigations in marginal environments of RIS may be useful to document high-resolution glacial-interglacial transitions, as sediment characteristics at this boundary between the ice shelf and open marine environment is sensitive to small changes in the amplitudes of climate cyclicity.

Acknowledgments

This research was supported by Korea Polar Research Institute (PE21090).

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Fig. 1.

Fig. 1.
Study area. (a) Map showing the location of Ross Sea, Weddell Sea, Ross Ice Shelf (RIS), East and West Antarctic Ice Sheet (b) Detailed physiographic map of Ross Sea (c) Bathymetry (in meters) map of Ross Sea showing the location of LC42 core (d) Lithostratigraphy of LC42 (Adapted from Ohneiser et al., 2019). Ohneiser et al. (2019) classified the LC42 core into three lithologically distinct units: Unit I (0-1.55 m) is dominated by gravelly muddy sand with interbeds of fine to medium sand and silt; Unit II (1.55-3.15 m) comprising of gravelly muddy sand, with interbeds of laminated and bioturbated silt; Unit III (3.15-11.78 m) consisting of clast free interbeds of laminated, sandy silt, bioturbated silt and gravelly muddy sands.

Fig. 2.

Fig. 2.
(a) Global average surface temperature (GAST) changes based on deep sea oxygen isotopes over the last 2 Ma (Snyder, 2016). Our EAIS deglaciations coincide with the early Pleistocene warm period (pre-mid-Pleistocene transition, pre-MPT; 45) and the late Pleistocene warm interglacial periods (post-mid-Bruhnes transition event, post-MBE; 51, 56). (b) Climate proxy changes during the last 1 Ma. (1) GAST changes. (2) The temperature gradient between the interior EAIS temperature history (East Antarctica Dome C, EDC3; 47) and present temperature. (3) Surface accumulation rate, in centimeters of ice equivalent per year (EDC3; 57). (4) and (5) meteoric 10Be concentrations and 10Be/9Be ratios along Long Core 42, collected during the 2015 Ross Sea cruise (this study). The sky-blue boxes represent periods of meteoric 10Be depletion, which indicates more and thick sea ice coverage over the ocean surface due to a decrease in temperature that hindered the direct deposition of meteoric 10Be to the seafloor. The yellow boxes indicate seasonally opened periods. The brown box represents a weakened Geomagnetic Dipole Moment (GDM, XXI) period (Matuyama- Brunhes Boundary; MBB) that is < 20% of the averaged GDM and produced higher meteoric 10Be than the other periods (Simon et al., 2016a). The 10Be peaks during the period highlighted by the brown box should be interpreted with caution.

Fig. 3.

Fig. 3.
Relative variation of 10Be concentration and 10Be/9Be ratio during different geomagnetic reversals/excursions.

Table 1.

The meteoric 10Be and 9Be Data from the core taken from the Ross Sea (LC-42).