Before sediment coring, an access borehole should be drilled using the hot water drilling method. The diameter of the borehole may range from 10 to ~60 cm [24] and the depth from a few to thousands of meters [4,22]. When the hot-water drill penetrates the ice base, the borehole water level will adjust to the local hydrological level, achieving pressure equilibrium [3]. The sediment coring tools are then deployed, first through the air filled and then water filled parts of the borehole, through the ocean or lake water column if present, to reach the sediment, which is sampled and retrieved to the surface as sediment core samples [8].

The access borehole closes at rates of between 3 to ~6 mm/h [25,26], i.e., it refreezes at an L113 change to a rate that depends on the ice temperature surrounding the borehole, the water pressure, the initial borehole size, and the water composition [24]. Sub-ice instruments can only be deployed and retrieved during the window of opportunity between the time the borehole is drilled/reamed and the time it refreezes to a certain size [24]. Otherwise, instruments can become stuck in the borehole. Therefore, sediment sampling devices are always cylindrical shaped and primarily designed and selected based on the hot water borehole diameter and water pressure [27]. The stabilizing frame [28] and overhanging trigger system [29] used in lake and ocean sediment coring are not applicable due to their size (Figure 2).

 

In certain subglacial lake settings, any contamination from the surface is strictly forbidden, thus sediment coring tools should be sterilized and cleaned prior to being deployed into the borehole [30,31].

The ANDRILL (Antarctic Geological Drilling) project on Ross Ice Shelf [32] used a basic winch and tower, a high-strength cable (with or without power transmission capacity), a control unit, and a platform for sub-ice sediment coring. The cable, winch, and tower were designed to manage loads from between 1500 and 4500 kg [33]. The operational height between the tower top pulley to the borehole surface is generally between 3–6 m, while heights of more than 10 m can be achieved using a hydraulic powered crane [34]. Assembly and disassembly operations of the sampling devices are usually executed by manual rather than by automatic auxiliary equipment.

Subglacial sediments are more heterogeneous compared to normal aquatic sediments, due to the multiple sources of their deposits throughout their histories [12]. The basic composition of such sediments is generally the same as marine sediments (silt, clay, sand fractions, gravels, for example.) but the structure and mechanical properties are strongly influenced by the overlying ice layer dynamics [10]. In some drilling sites, the shear strength of cores obtained can be more than 70 kPa at a depth of 2 m [35], and the surface biological debris (2–4 mm) and deeper interbedded gravel (4–64 mm) become the main factors preventing corers from going deeper [6]. Therefore, coring tools equipped with additional kinetic energy units have been developed to penetrate deeper [11,23].

Although many attempts have been made to collect subglacial sediment samples, only a few studies have successfully recovered sediment samples from beneath Antarctic subglacial settings. Coring results have generally shown low success rates and penetration performance. In this paper, we review the design and characteristic structure of representative sediment coring tools developed for Antarctic subglacial aquatic environments. We provide some constructive suggestions and our expectations regarding the technologies are also included in order to express new ideas on how to improve current coring techniques and corer designs.

With the possible exception of the sediment corer used on the Novolazarevskiy ice shelf [39] (which was not identified), the first subglacial sediment gravity corer is the ‘Benthos model 2171′ used at Site J-9 (82° 22′ S, 168° 38′ W) as a part of the Ross Ice Shelf Project [40]. During the austral summer of 1977, 58 short cores with lengths of up to 122 cm were collected at Site J-9 [40,41]. Samples were 400 km from the open water of the Ross Sea, where the ice thickness was about 420 m, the marine water column was 237 m, and the distance between the water surface to the seafloor was 597 m [40]. The access borehole was drilled through the ice shelf using a flame-jet drill developed by the Browning Engineering Corporation and produced a hole 30–80 cm in diameter [42]. This corer was also used to obtain 30–60 cm cores from beneath the Fimbulsen Ice Shelf in 1991–1992 [8,43]. The ‘Benthos model 2171′ gravity corer can be equipped with a 2.4 m long liner (ID 67 mm), has a total weight of 110 kg, and can be equipped with a 3 m core barrel [8,40].

The ‘Wintle’ gravity corer was purposely built for sub-ice-shelf sediment sampling. It is 150 cm in length and its maximum diameter is 12 cm [6,44]. This corer was first used at site AM02 (69°42.8′ S, 72°38.4′ E) on the Amery Ice Shelf, where it collected a 144 cm core ~80 km south of the floating ice shelf front in the 2000–2001 drilling season, as part of the Australian National Antarctic Research Expeditions (ANARE) Amery Ice Shelf Oceanographic Research (AMISOR) program [6]. This 144-cm core was X-ray radiographed to examine for any sedimentary structures and ice-rafted debris (IRD) granules (2–4 mm) and pebbles (4–64 mm) were counted for every 5 cm interval downcore [44]. In the 2003–2004 season, a second 47 cm core was collected from site AM01b (69°25.86′ S, 72°26.77′ E), which is 50 km west of the AM02 site. Four other cores between 60 and 124 cm long were obtained from sites AM03 to AM06 in the 2005–2006 and 2009–2010 seasons [10].

On 16 October 2006, the ANDRILL team used a gravity corer at the AND-1B site (77°53′22″ S, 167°5′22″ E), after drilling the hot water borehole (~40 cm in diameter) and prior to the deployment of the sea riser [35]. This project aimed to recover the water–sediment interface and a few decimeters of sediment below the surface. Eight coring attempts were made and 7 cores with lengths of up to 53 cm were recovered [35]. The small (~80 kg, maximum diameter 22 cm) gravity corer (Figure 5a) was designed and built by AWI (Alfred Wegener Institute). The corer was fitted with a plastic core barrel that was a UWITEC standard plastic liner that was 1.0 or 1.5 m long and 59.5/63 mm in ID/OD and was without a cutter and core catcher [37]. The weight of the corer can be adjusted by adding or reducing lead weight loops. A self-contained tension triggering valve mechanism (lock unit) was installed at the top of the corer. The valve is kept open during the lowering and penetrating steps; it then closes once enough instantaneous tension is exerted on the lifting bar (Figure 5b,c). After passing the corer through the access borehole, it is lowered to 5–25 m above the sea-floor and stabilized for about 1–2 min. Then the surface winch is switched to a ‘free-wheel’ mode and the corer penetrates into the sediment by its gravitational potential energy. After penetration, the surface operator exerts a sudden force to the cable to close the sealing lid. The core with the intact water–sediment interface is kept under vacuum in a space formed by the sealing lid, the core tube, and sediment [35].

The AWI gravity corer was again deployed by the ANDRILL team at Coulman High beneath the Ross Ice Shelf between November 2010 and January 2011, as part of preparations for a planned drilling project targeting an Early Cenozoic sequence [37]. In total, 28 cores with lengths of up to 129 cm were obtained from four hot water drilling sites [37].

 

The AWI gravity corer was used in several hot water boreholes (~40 cm) at the Ekström Ice Shelf between November 2018 and January 2019, as part of the Sub-EIS-Obs project [45]. Seven gravity coring attempts were made but only one succeeded in obtaining a water–sediment interface sample with a 46 cm core (Figure 6b) from the drill site, where the ice thickness was 332 m and the seafloor was located ~700 m from the ice surface.

The UWITEC (an Austrian engineering company) gravity corer is designed to sample soft and watery surface sediments with the aim of preserving the water–sediment interface [46]. This gravity corer has a streamlined shape and is equipped with an automatic core catcher (Figure 7). It weighs 5–8 kg and has a maximum OD of no more than 150 mm (without the ball core catcher). The core tube dimensions are standard, with a UWITEC liner with ID/OD measurements of 59.5/63 mm and a length of 80–120 cm. The corer can be released to ‘free-fall’ several meters above the sediment surface. The line-ball core catcher and valve flap automatically seal the tube’s lower and upper ends, respectively, when coring is completed. Additional dead weights can be added if needed, with each tube circular ring weighing 4 kg [46]. The UWITEC gravity corer has successfully retrieved a surface sediment core from a coring site (72°00.259′ S, 68°29.022′ W) at Lake Hodgson [47], where the ice thickness is 3.7 m and the distance between the ice surface and the sediment is 93.4 m [22].

After being modified by Northern Illinois University (NIU), the corer was renamed the NIU/UWITEC gravity multi-corer (Figure 8) [1]. This multi-corer is a combination of three standard UWITEC gravity corers, installed on a trisection frame (Figure 8a). Its working principle is the same as the standard UWITEC gravity corer, but it can take three replicate cores simultaneously after being self-triggered upon striking the bottom sediment. This corer has been used in at least two sites since it was designed, including at the Subglacial Lake Whillans (~60 cm diameter access borehole) site by the WISSARD (Whillans Ice Stream Subglacial Access Research Drilling) team in January 2015 (Figure 8b), where it collected eight good cores with lengths between 20 and 40 cm [48,49]. However, seven attempts were unsuccessful (e.g., corer was stuck in the borehole) because the corer assembly is relatively large, is not streamlined, and is quite light [1,50]. Recently, the corer was deployed at Subglacial Lake Mercer Whillans (~60 cm diameter access borehole) by the SALSA (Subglacial Antarctic Lakes Scientific Access) team in the 2018–2019 season, and at least six water–sediment interface cores were obtained [51].

 

This short corer (Figure 9) was developed cooperatively by BAS (British Antarctic Survey) and UWITEC, with the aim of collecting surface sediment cores in sub-ice-shelf settings. It consists of a valve head, which also functions as a dead weight, a single liner, and an orange-peel shaped core catcher. This corer was used during 2014–2015 and 2015–2016 field seasons by BAS to sample sub-ice-shelf sediments. Approximately 30 cm diameter access boreholes were drilled by a hot water drilling system through the Filchner-Ronne Ice Shelf, where ice thickness ranges between 750–891 m. In total, seven cores were obtained with a combined total length of 265 cm in the 2014–2015 season and at least three cores of up to 55 cm were obtained in the 2015–2016 field season [52]. Three cores with lengths up to 75.5 cm were obtained from BAS drill sites on the Filchner-Ronne Ice Shelf during the 2016–2017 field season, where ice thickness ranges between 597–615 m and water column thickness ranges between 440–643 m.

To obtain long sediment core samples through a tether without an electric power supply, the SLE (Subglacial Lake Ellsworth) team proposed a heavy gravity corer (Figure 10) with a 270 kg head weight, enabled for preparative sterilization and cleaning [31]. This corer is equipped with a single tube core barrel that is 3.7 m long and has a 60 mm inner diameter to achieve a reasonable balance between corer diameter and corer weight. The combination of a long and heavy tube with a heavy load was designed for deeper penetration. The entire corer is driven into the sediment by only gravitational potential energy [8].

This corer was planned to be used in the subglacial Lake Ellsworth in the 2012–2013 season by passing through an access borehole approximately 36 cm wide and ~3000 m deep [36] to obtain old sediments. The seismic reflection data analysis showed that the lake’s sediment is highly porous with a low density, very similar to material found in the deep ocean floor. The sedimentary sequence thickness was estimated to be at least 2 m [53]. Unfortunately, the hot-water drilling failed to access the lake [54] and the SLE gravity corer has not been applied.

The WHOI (Woods Hole Research Institute) gravity corer (Figure 11) is the largest tool in SALSA’s lineup, with a total length of 9.1 m. Its purpose is to take up to ~6.0 m core samples by its heavy weight (~1130 kg). At subglacial Lake Mercer in 2018–2019 season, the WHOI corer was deployed through a ~60 cm diameter access borehole and it made contact with the sediment bed at a depth of 1084 m. Two cores with lengths of 70 cm and 170 cm were obtained with this corer, setting a new length record in subglacial lake sediment coring [55,56].

The BAS/UWITEC percussion corer was developed by the BAS in collaboration with UWITEC [8]. It consists of a hammer unit, the valve head, and a core barrel unit (Figure 13). The maximum diameter of this corer is 14 cm and the 3 m long version with three hammer weights makes a total of ~130 kg. The corer was designed to pass through a ~30 cm wide access borehole. The double-tube core barrel unit consists of an outer barrel (ID/OD 64/70 mm) made of stainless steel, a 3 m long UWITEC PVC liner (ID/OD 59.5/63 mm), a double layered orange peel core catcher, and a cutter nose (ID/OD 59/74 mm). This corer can be deployed to core 6 m long sediment samples if it is equipped with a 6 m double-section core barrel. Lowering, lifting, and downhole coring procedures can be manually accomplished by a single winch cable through surface operations.

This BAS/UWITEC manually operated percussion corer was used during the 2011–2012 austral summer in the hot-water drilling project at Larsen C and George VI ice shelves. A total of 1160 cm of sediment cores were recovered with a maximum penetration of 290 cm [14,57,58]. BAS recovered several cores using the BAS/UWITEC corer, including (i) cores with lengths up to 115 cm from beneath Pine Island Glacier during 2012–2013 [59,60]; (ii) a 135.5 cm long core from FNE2 drilling site during the 2016–2017 season; (iii) a 152 cm long core from FNE3 drilling site during the 2016–2017 season; (iv) a ~10 cm long coarse gravel core from beneath Rutford Ice Stream drill site, where ice thickness was 2152 m during the 2018–2019 season [61], respectively. This corer was deployed at several hot-water boreholes (~40 cm in diameter) at the Ekström ice shelf between November 2018 and January 2019 as part of the Sub-EIS-Obs project [45]. Eight coring attempts resulted in 7 cores with a total length of ~826 cm and single core lengths of up to 200 cm.

Another UWITEC percussion corer deployed in the Antarctic is the UWITEC KOL Kolbenlot percussion piston corer, a commercial product used for lake sediment coring, working at depths of no more than 140 m [46]. This 2 m long corer is manually operated from the ice surface [62]. Three overlapping 2 m long cores with a total penetration depth of 3.76 m were obtained at the Subglacial Hodgson Lake core site (72° 00.256′ S, 68° 29.022′ W), where the ice was 3.7 m thick and the lake bed was located 93.4 m below the ice surface [22].

The JLU (Jilin University, China) manually operated hammer corer was used at the Neumayer Station III in Antarctica to obtain a long core sample (>100 cm) with an undisturbed water–sediment interface. The corer consists of a hammer unit, the main frame, and a core barrel unit (Figure 14). The maximum diameter of this corer is 23 cm and it weighs ~130 kg, including its 45 kg hammer. The single core tube (UWITEC liner) measures 59.5/63 mm (ID/OD) excluding the outer barrel, and the cutter (ID/OD 59/66 mm) with the core catcher and the water–sediment interface protector are attached at the lower end of the core tube. The coring procedure is generally the same as that for the BAS/UWITEC, except for an additional step included to preserve the water–sediment interface. The water–sediment interface can be kept at an undisturbed state in the liner, even after the corer is lifted to ice surface. The JLU manually operated hammer corer was deployed at the Ekström ice shelf between November 2018 and January 2019 as part of Sub-EIS-Obs project [45] and it successfully obtained a 128 cm long core with a well-preserved water–sediment interface (Figure 15).

The SLW (Subglacial Lake Whillans) hydraulic percussion corer (Figure 16) or NIU (Northern Illinois University) percussion corer was designed by S. Vogel and DOER-Marine to obtain deeper, hard, and over-consolidated sediment cores, such as subglacial till cores [1,33]. The corer is designed to be sterilized and it measures a total length of ~14 m and has a ~220 mm maximum OD, including a 5 m long core barrel with ~100/141 mm ID/OD. It is powered by a hydraulic system (motor, piston, for example) that includes a ~900 kg percussion drop mass; a power, control, and telemetry unit; and a lifting assembly. A smart Kevlar-reinforced fiber-optic cable (4500 kg load ability) connected to the multipurpose winch is used to deploy the corer.

To ensure that the pullout strains between core barrel and sediments do not exceed the cable’s maximum tension capacity, the corer can enable a ‘hydraulic helping’ mode. First, the hydraulics can be commanded to force pressurized water down to the gap between liner and core barrel, then, the water exiting via jet holes in the core cutter head is forced up the outside of the core barrel to decrease the friction between the corer and sediments [1,8,33].

This corer was deployed to the hot water drilling site at the Subglacial Lake Whillans (84.240° S, 153.694° W) in late January 2013 within the WISSARD project (Figure 17). Unfortunately, the corer’s working modes failed to work as designed due to a malfunction in the surface smart winch. Eventually, used as a gravity corer, it was finally able to collect a 0.4 m long gravity sediment core [1].

The SLE percussion piston corer (Figure 18a) was designed and manufactured by UWITEC and BAS [7]. The corer is designed to be sterilized and has a length of 5.8 m and a maximum diameter of 20 cm [7,8]. This corer was the first ‘real-time visual corer’ to study the Antarctic subglacial sediment environment. It consists of five main parts, including a control interface, the hammer, the core barrel, the piston, and the piston-clutch [8]. The control interface contains power converters, control electronics, and communication modems. The hammer has a linear motor drive hammer mechanism sealed in the pressure chamber. The core barrel (3-m long version), liner, cutter (hardened steel) and double-layered orange peel core catcher were borrowed from the UWITEC corer design [7]. The corer’s clutch can prevent the piston from moving further to deform the sediment core when the core barrel is full or when sediment penetration has ceased [8]. Cameras are integrated with the piston, pressure chamber, and near the clutch; thus, operators working on ice surface can monitor and control the coring procedure through a conducting tether (Figure 18b,c). The coiled umbilical cable can compensate for the distance changes occurring between the control interface unit and the hammer unit during coring [8].

This corer has been specifically and progressively designed to implement technical innovations in subglacial aquatic sediment sampling, such as controlled coring through real-time monitoring, the ability to obtain a long core with a well-preserved water–sediment interface, and the separation between the control cabin and the power cabin. Unfortunately, this corer has not yet been deployed due to the same reason [54] cited for the SLE gravity corer.

The Caltech piston corer was designed and built by a California Institute of Technology (Caltech)-led team that conducted sediment coring beneath Ice Stream B (at camp UpB), West Antarctica [16]. This corer design was derived from the marine piston corer, which is 6 m long with a 50 mm liner ID. It is equipped with a cutter, a metal core catcher, and a plastic liner fitted into the steel core barrel. Several cores of up to 400 cm long [19] were recovered through about 50 hot-water-drilled access holes with an average depth of ~1030 m at Upstream B, where sediments are deposited about 600 m below sea level [8]. More detailed information about this corer’s design and its working principle were not published.

The WISSARD piston corer (Figure 20) was based on the design of the previous Caltech piston corer. It was designed and built by the University of California Santa Cruz to collect non-deformed sediment cores at the bottom of subglacial Lake Whillans [8,33]. This borehole corer is equipped with a 3 m long core barrel with a 58 mm ID [1]. The corer is designed to be sterilized and is mainly made of stainless steel. On its upper stage, there is a tube containing a coiled Kevlar release cord that is used to dead drop the corer [8]. Several external guiding plates at the upper end of the corer increase the possibility of vertical insertion. This corer needs to be released precisely at a few meters above the lake floor, triggered by a wireline messenger [8]. The piston unit stops at the sediment surface during penetrating and lifting. This corer can obtain cores without significant compression or disturbance. The WISSARD piston corer was used to recover an 80 cm long core during the second phase of scientific operations at the subglacial Lake Whillans drilling site in late January 2013 [1].

The water–sediment interface is the prime area for detecting microbial life, especially in the ultra-oligotrophic lakes such as Lake Ellsworth [23]. To be able to collect such samples, a mini-push piston corer was designed and built by the SLE team to obtain the crucial water–sediment interface in this subglacial lake. This corer is mounted on the front (lower end) of the Lake Ellsworth Probe, which is sterilized before deployment, and samples are capped upon retrieval (Figure 21) [7]. The corer is 39 cm long and is designed to obtain a composite core sample up to 22 cm long and contains the water–sediment interface [23]. All metallic parts of the corer are constructed from titanium (grade V), which, along with the nonmetallic components, ensure the validity of trace Fe analysis of all water samples [23]. The SLE probe includes a video camera and lighting system to enable continuous monitoring of the area below the probe. After the sediment surface is sighted and assessed visually by the surface operator, the corer is then deployed to start coring [8]. The coring deployment by itself takes approximately 3.5 min with a penetration speed of 1 mm/s [23]. To simplify the description, the whole coring process is divided into two phases, the penetrating (Figure 22b) phase and the ball valve sealing phase (Figure 22c). During the penetrating phase, the lead screw drives the whole corer assembly to penetrate the sediment. After the corer reaches the target penetration depth, the corer assembly automatically initiates the sealing phase, i.e., the lead screw continues to rotate, moving the ball drive plate down, which rotates the ball valve by 90° and seals the core tube end [23].

The piston keeps a static position relative to the probe body (mounting head) during the whole coring process, thus preventing fluid intrusion from the outside [23]. Since the core water freezes during the probe’s ascent back through a cold (approximately −18 °C) air-filled section, the piston was designed with a volume compensation mechanism (Figure 23) that can compensate for the water volume change when water inside the core tube freezes.

This corer passed testing on artificial laboratory sediments as well as deep-marine sediments (1020 m water depth). The results show that this corer can obtain samples in all the sediment types assessed, and the piston unit and other mechanisms worked well [23]. Unfortunately, the BAS hot-water drill failed to access Lake Ellsworth in 2013 and the corer has not been tested in the Antarctic.

A push corer inner tube assembly was deployed at the AND-1B site (77°53′22″ S, 167°5′22″ E) by the ANDRILL drilling team in October 2006, using a sea riser and a drill string which were lowered to within a few meters of the seafloor. Different from the AWI gravity corer previously deployed at same site, the push corer is not cable-operated but is installed on the drill string and is directly ‘pushed’ into the sediment without rotation or the use of drilling mud. This corer is 1.6 m [35] long and it is attached to the end of an 83 mm diameter drill bit [15]. Using this method, four cores, varying in length from 33 to 156 cm, were collected by the drill string-mounted ODP (Ocean Drilling Program) liner. Most cores were lost during the lifting operation, resulting in only 216 cm of sediment cores obtained from sediments at depths of between 0.70 m and 10.18 m [35]. The reason we list this non-cable-suspended corer here is to provide a results contrast with the AWI gravity corer that was deployed at the same access borehole.