Geophysical investigation in sediment cores and its relationship with the governing sedimentary processes at Bransﬁ eld Basin, Antarctica

: This research aims to investigate sediment cores from a glaciomarine environment based on their petrophysical parameters to elucidate lithological and sedimentary issues, as well as to identify a geophysical signature based on their parameter’s response. To achieve this objective, six marine sediment cores were collected in the Central Bransfi eld Basin - Antarctica, the lengths ranging from 1.5 to 5.2 meters in water depths ranging from 304 to 1463 meters. The cores were submitted to different analyses and values from density, magnetic susceptibility, electrical resistivity, p-wave velocity, total gamma radiation, silt, clay and sand contents, and mean grain size for each core location were considered. Four large lithologies were identifi ed according to their geological and geophysical characteristics. The fi rst group is subglacial deformation till (GC16); the second is massive diamicton (GC13 and GC12), the third group is composed of diamictons from the basin (AM10) and lower slope (GC09); the fourth lithological group is composed of siliceous mud from an upper slope location (GC06A). The characteristics recorded across Central Bransfi eld Basin (from South Shetlands Islands to Antarctic Peninsula) highlighted the relationship between the lithological content and associated depositional processes with the geophysical proprieties as density and magnetic susceptibility.


INTRODUCTION
The Antarctica Peninsula (AP) is a sensitive area to global climatic and eustatic oscillations (e.g. Garcia et al. 2011, Vaughan et al. 2003. Those climate oscillations were directly refl ected in the sedimentary records in different time scales and physiographic provinces, arousing the interest of researchers to understand depositional processes on different scales and areas (Domack & Ishman 1993, Heroy et al. 2008, Khim et al. 2002, Leventer et al. 1996, Yoon et al. 2000.
During the last two decades, the Central Bransfi eld Basin (CBB) has been the target of geological and geophysical studies, mainly through seismic reflection, to understand the structural evolution of the basin, its formation processes, geometry, and subsurface stratigraphy (e.g. Nobre et al. 2020, Fabrés et al. 2000. In special, the petrophysical, geoacoustic, and geochemical properties of marine sediments are good indicators of the composition, environmental condition, and even the occurrence of past events (e.g. Anderson 1983, Heroy et al. 2008, Rǎdan et al. 2013, Yoon et al. 1994. The correlation between these data presents potential information for the recognition of distinct sedimentary deposition processes, sedimentary types and facies, and a key point for the paleoenvironmental reconstructions. Although knowledge about polar regions has advanced in the past decade, major gaps still exist especially about processes and patterns in glaciomarine environments, and therefore, many valuable aspects remain unresolved. This study aims to investigate and characterize the sediments from Central Bransfield Basin (CBB), Antarctica, considering their geophysical properties and their physiographic province.

Study area
The Bransfield Basin (BB) is a narrow NE-SW oriented submarine basin, located southeast of the South Shetlands Islands (SSI) and the NW of the AP. It is up to 80 km wide and extends over 500 km from Smith Island to Clarence Island. BB is divided into three sub-basins: Western, Central, and Eastern. Due to the sampling effort on site, this work will focus only on the CBB, which is bounded by the elevations of Deception Island and Bridgeman Island (Jeffers et al. 1991).
The CBB is 130 km wide and approximately 230 km long, with a maximum depth of 1950 m and considered highly asymmetric, with a narrow and steep SSI margin to the northwest and a wider, southeastern, AP margin , García et al. 2008, Gràcia et al. 1996, Lawver et al. 1996. CBB is physiographically distinct and can be segmented into continental shelf, slope (upper, middle, and lower), mid-slope platform, basin, and glacial troughs. The continental shelf is up to 50 kilometers wide, with depths between 200 and 300 meters and an inclination of about 1.5°, becoming shallower and narrower in the northeastern direction (García et al. 2008). The SSI margin is narrow, from 6 to 15 km, straight and up to 24° steep. In contrast, the AP margin is wide, from 70 to 85 km, with the break of the smooth platform and slopes generally less than 10° (Prieto et al. 1998). The mid-slope Platform presents an atypical physiographic configuration for a glaciated margin, with a continuous flat surface from the floor of glacial troughs down to the platform edges (García et al. 2009), and consists of the Western platform and the eastern platform. Both have a continuously flat surface (2°) starting from the floor of the glacial troughs and extending to the northeastern basin (García et al. 2011). The ocean basin is relatively narrow and flat, less than 30 km wide. It has four depth levels, which are progressively deeper from 1000 m to 1950 m, from southwest to northeast direction. Glacial troughs in the BB region have irregular longitudinal profiles, becoming shallower towards its central portion and deepening towards the basin in its most distal portions (García et al. 2008). In addition, one of the main characteristics of CBB is the presence of an underwater volcanic chain that was believed to be a result of the volcanism attributed to the rift of the basin (García et al. 2008). Volcanic edifices have different morphologies and can rise above the seabed. Together, they form a discontinuous line that extends from Deception Island to Bridgeman Island.
From an oceanographic perspective, Bransfield Strait can be considered as a transition zone between the Weddell Sea, to the southeast, and the Bellingshausen Sea, to the west (Barker et al. 1998). Its circulation pattern is characterized by the presence of two water masses: one western influx of relatively warm, shallow, less saline, and faster current, Transitional Zonal Water with Bellingshausen influence (TBW) as a result of Gerlache Strait and the Circumpolar Current; and an eastern influx of a colder, deeper, saltier and slower Transitional Zonal Water with Weddell Sea influence (TWW) coming from the Weddell Sea (Sangrà et al. 2011).

MATERIALS AND METHODS
We investigated six sedimentary cores, retrieved across CBB with gravity-corer onboard the Polar Research Vessel Almirante Maximiano (H-41; Brazilian Navy) during the Antarctica Expeditions OPERANTAR XXXIV and XXXV (summers of 2015 and 2016, respectively). The coring sites were located over a 100 km NW-SE transect between the SSI and AP, with depths ranging from 304 m to 1463 m ( Fig. 1, Table I). The retrieved cores were sectioned onboard and continuously refrigerated while shipped to Universidade Federal Fluminense (UFF -Niterói/Brazil) for analysis and storage.
Subsequently, to obtain sediment's geophysical parameters as gamma density (g/ cm³), p-wave velocity (m/s), electrical resistivity (ohm.m), and magnetic susceptibility (SI.10 -5 ), the cores were logged using the Muti-Sensor Core Logger (MSCL; Geotek Ltd.) on a one-centimeter resolution. After MSCL logging, the cores sections were visually described, considering the lithology, color (Munsell 1992), as well as the presence of biological content, sedimentary structures, and rafted debris.
Besides, samples were collected every 5 centimeters along the core. These samples were frozen and lyophilized before applying sodium hexametaphosphate as a deflocculant and finally shaken to break up the particles. Afterward, was applied the granulometric analysis using the particle analyzer Mastersize 2000 (Malvern Instruments). The results were processed based on the Gradistat routine of (Blott & Pye 2001) and sediments were classified according to their sand (%), silt (%), clay (%) contents, and D 50 values (φ). The Natural gamma spectrometry analyses were performed using the Portable Spectrometer RS-125/230 (GeoRESULTS Pty Ltd), measuring the U (ppm), Th (ppm), K (%), and total gamma radiation (nGy/h). Both analyses, granulometric and natural gamma spectrometry, were performed on a five-centimeter resolution.
Once this dataset was established, data distribution was analyzed with whisker boxplots, scatter plots, and summary statistic data as mean, median, maximum, and minimum values at Microsoft Statistica 8.0 and Grapher TM 8.0 softwares. Thereby, different lithologies could be classified by their different geophysical properties signature.

Core GC16 SSI (Continental Shelf)
The core GC16 (1.52 m core length; 304 m water depth; Supplementary Material - Table SI) is located at the continental shelf of the SSI margin, between the Robert and Greenwich Islands. The sediments are mainly composed of dominantly Black (GLEY 1 2.5/N) very poorly selected muddy sand. Sediment grain size indicating fine sand, ranging from 2.3 φ at 20 cm below seafloor (BSF) to 4.1 φ at 45 cm BSF (averaging 3.03φ). The sand, silt, and clay content are correlated with the maximum and minimum at 20 cm and 45 cm BSF, showing averages of 64.2%, 31.6%, and 4.2%, respectively (Fig. 2a). The occurrence of shell fragments was recorded at 12 cm BSF, while icerafted debris (IRD) were recorded at 72 cm, 80 cm, 91 cm, and 115 cm BSF. The measured density, porosity and p-wave velocity showed minor variation downcore, averaging 1.90 g/cm 3 , 49%, and 1653 m/s, respectively. A localized decrease in density up to 1.67 g/cm 3 at 70 cm followed by a magnetic susceptibility step increase going from 228 SI.10 -5 (at 46 cm) to 707 SI.10 -5 at 88 cm BSF. Electrical resistivity and natural gamma data display a relatively uniform profile, averaging 0.60 ohm.m and 209 nGy/h (Fig. 2a, Table SI).  Cores GC13 and GC12 (SSI Platform) The cores GC13 (4.70 m core length; 615 m water depth) and GC12 (4.14 m core length; 760 m water depth) (  Table II).
Core AM10 (Basin) and Core GC09 AP (Lower Slope) The core AM10 (5.20 m core length; 1463 m water depth, Table I) is located at the basin floor of the CBB, close to the volcanic edifices of Three Sisters, and essentially composed of mud, with five layers (up to 3 cm thick) of silty mud/sandy silt with poorly selected sediments. Seven laminations of organic matter (241 cm, 246 cm, 253 cm, 259 cm, 281 cm, 319 cm, 495 cm BSF) and dark greenish gray (GLEY1 4/10Y), greenishblack (GLEY1 3/10Y, GLEY 2.5/10Y), and Olive (5Y 4/2) layers were also recorded (Fig. 3a). The sediments grain size is characterized as fine silt, ranging from 5.92 φ at 510 cm BSF to 7.73 φ at 420 cm BSF (average 6.87 φ). The content of sand, silt, and clay is generally uniform through the  Table I) is located at the Antarctic Peninsula lower slope. The sediments are dominated by mud, with intervals of poorly selected silty mud/sandy silt. IRD occurred at 18 cm, 115 cm, 261 cm, and 300 cm BSF. Sediment color are predominant dark greenish gray (GLEY1 4/10Y), with very dark greenish gray (GLEY1 3/5Y and GLEY 2.5/10Y) layers up to 15 cm thick. The sediment grain size is classified as fine silt (averaging 6.06 φ). The amount of silt and clay showed an average of 76.5 % and 10.6 %, respectively, while an increase of sand content was recorded at 237 cm BSF (60.1 %).
Downcore similarities in the acquired MSCL physical properties at cores AM10 and GC09 are also observed. Density, porosity, and p-wave velocity data are relatively uniform with depth, averaging 1.35 g/cm 3 , 81%, 1528 m/s, respectively. At GC09, a higher peak in the density of 1.62 g/cm 3 was registered at 220 cm. Magnetic susceptibility, electrical resistivity and natural gamma response are averaging 103 SI.10 -5 , 0.45 ohm.m and 185 nGy/h (Fig. 3a, Table III).

Superficial sediments and depositional process at the Central Bransfield Basin
The SSI inner shelf area (core CG16) is characterized by the homogeneous distribution of unselected sands, with chaotically, irregular and size variable fragments dispersed along the core (Fig. 1, Fig. 2a). These sedimentary characteristics support their classification as residual sediments (Anderson 1983), suggesting the predominance of proximal processes associate with glaciers, possibly due to the proximity with the Greenwich and Robert Islands. During cold intervals (winters and/or glacial periods) the expansion and advance of the ice sheets over the SSI shelf produce severe erosion of the existing sea-floor strata. The eroded material is mainly transported subglacially at the base of ice sheet and deposited at the ice sheet grounding line. However, within the end of cold intervals and beginning of warm intervals (summer and/or interglacial periods), the depositional processes acting on the SSI shelf were mainly characterized by sediment starvation, depositional by-pass and erosion (Prieto et al. 1999). The action of these strong cyclicities characterized by the alternation of the advance and retreat of the glaciers over the SSI shelf associated with the predominance of proximal processes also were supported by the characteristics of the sediment on the studied site area. Residual sediments are recorded at shallower areas, where the drift of icebergs occurs and are strongly influenced by the sea currents that can transport fine sediments (as silt and clay), as well as the poorly selected sand and gravel (Trusel et al. 2010). The proximal environments granulometry is coarser, due to the proximity of source areas with an abundance of basal debris transported by ice (glaciers) and/or on the substrate. Those sediments are deposited as deformation till (Trusel et al. 2010) and, can be formed under the influence of landed glacial margin. In this way, they can end up forming push morainal banks, created by the glacier's advance and retreat process (Rocha-Campos & Santos 2000). In the SSI margin, García et al. (2011) recorded the deposition of the glacially eroded sediments from the archipelago that are transported by the melting water to embankments and basins. The authors describe and classify these sediments as terrestrial mud, and also suggest the possible occurrence of sand horizons associated with those deposits.
The black color recorded over the core should be a result of the sediment composition and geological formation of the SSI, dominated by rocks and sediments of Jurassic-Paleogene volcanic origin (Burton-Johnson & Riley 2015), that had not been verified in previous works.
In the SSI platform (cores GC13 and GC12; Fig. 1; Fig. 2b), the record of poorly selected sediments with low biogenic content supports their classification as massive diamicton, which are characteristic of the transitional glaciomarine sediments (García et al. 2011). Those sediments are also related to the SSI glacial erosion process and delivered towards the slope and basin by the melting water transport of terrestrial mud (with/without sand layers) through the bays, canyons, and associated lobes (García et al. 2011). While, in the lower slope (cores AM10 and GC09 ; Fig 1. Fig. 3a), our records demonstrated a lithological pattern similar to the massive diamicton, but the higher influence of the oceanic processes appear to act at this area more than in the SSI platform (cores CG13 and CG12). In the ocean basin, the occurrence of sediments characterized by clayey silt with olive-gray (7.5 Y); olive green (10 Y), and brownish-green (2.5 Y) colors was also recorded close to the AM10 site (Fabrés et al. 2000). The black color layers pattern was also recorded on the SSI platform (core CG13) and AP lower slope (core CG09), suggesting the deposition of volcanic material on those areas (Heroy et al. 2008, Yoon et al. 1994). Occurrences of volcanic material were previously recorded by Yoon et al. (1994) in the BB sediments. The authors describe layers characterized by dark color and composed mostly by well-rounded volcanoclastic material, with fragments of greenish glass and pumice with a smaller amount of terrigenous mineral (less than 40%), suggesting the proximity of the volcanically active regions, as the Three Sisters Island and the Deception Island, as possible source areas.
In the AP upper slope area (core GC06A; Fig.  1; Fig 3b) the sediments are characterized by strongly laminated greenish (2.5Y-4/3) material. Far from proximal environments (SSI and AP margin), the sediment distribution in this area is mainly governed by marine and non-glacial processes (Anderson 1983, Prieto et al. 1998). According to Anderson (1983), these sediments behave like glacial marine sediments, which are better selected and the most abundant in the Antarctic seabed. In addition, the dominance of biological material (mainly diatoms silicate frustules), and the absence of coarser materials (e.g. terrigenous material), also support the suggestion of a restricted input of the terrigenous material on the AP continental shelf and highlight the dominance of biogenic sedimentation process. This sedimentation is characterized by more prominent stratification, better selection, and intense bioturbation (Anderson et al. 1980, Domack et al. 1999. Those characteristics were previously recorded on the CBB sediments by Yoon et al. (1994), which report the occurrence of well stratified siliceous mud, mainly composed by the diatoms genera Nitzschia sp., Rhizosolenia sp. and Chaetoceros sp. Besides, colored layers (orange to brownorange) compose mainly by the diatom genera Chaetoceros spp. were associated with the annual spring bloom deposition (Leventer et al. 2006). These layers are composed of a set of mixed diatoms with a higher concentration of terrestrial material, which include angular quartz sand, silt, and clay deposited during the summer and autumn seasons. Therefore, wellstratified laminations with a high content of organic matter can be related to the increase of the superficial waters' productivity, resulting in large amounts of biogenic material exported to the seafloor. This increase in the superficial waters productivity occurs in the middle of the CBB, close to the edges of the retracting ice, as a result of melting sea ice, promoting an increase in the stability of the water column (Jordan & Pudsey 1992, Smith & Nelson 1985, and responsible for the large amounts of biogenic material in a short period, allowing the mass sedimentation of diatom frustules (Honjo et al. 1982, Smith & Nelson 1985. The laminations of organic material (diatomaceous mud) were also recorded on isolated depths of the AP lower slope, at core AM10 (Fig. 3a), indicating the predominance of biogenic sedimentation in open-marine conditions (pelagic and hemipelagic sediments) during short periods, as the spring and/or summer blooms (Fabrés et al. 2000, Heroy et al. 2008, Milliken et al. 2009).
The presence of IRD on marine sedimentary cores indicates that icebergs, sea ice, and massflow processes (e.g. turbidity and debris flow processes), and/or all those processes acting in the region during a known time interval (Anderson 1999, Yoon et al. 1997, Cofaigh et al. 2001. The type of rock in the debris can be used to identify the source area of the iceberg. Even in the modern age, under exceptional conditions, icebergs can spread far beyond their normal limits, so this interpretation must be cautious. The IRD recorded in the SSI continental margin (core GC16) and platform (cores GC13 and GC12), and in the AP lower slope (core GC09) and upper slope (core GC06A) suggest the contribution of the debris transported by icebergs and massflow processes to the central area of the CBB during the warmer periods, as recorded by (Yoon et al. 1997) at Admiralty Bay. These glaciogenic particles were not recorded at the lower slope, represented by core AM10, our deeper location, indicating the absence of iceberg discharge and mass-flow processes on the core site, and the dominance of oceanographic processes acting as the main depositional controller.
So far, there is no general scheme by which bioturbations can be categorized in glacial marine sediments. The bioturbation recorded in cores GC16 and GC06A results from the disturbance of sediments caused by benthic animals crawling on the seabed or, when in the subsurface, for reasons of habitation or feeding. Infauna activity is limited by the oxidation status of the bottom waters, the amount of organic matter in the sediment, and the redox potential of the porous fluid (Domack & Powell 2018). Bioturbation mixes the primary depositional signal (Anderson 2001) and, therefore, documentation is important to assess whether proxy data can be useful in the case of non-laminated sediments. During the process of anaerobic degradation of organic compounds, carried out by bacteria and other microorganisms methane and carbon dioxide are produced. Boyd et al. (2010) have already shown methanogenesis in subglacial sediments through an active population of organisms associated with subglacial sediments from Robertson Glacier, in the Canadian Rockies. Thus, considering the GC06A core as composed of diatoms (siliceous mud and oozes), the presence of gas at 285 cm may be the result of the methanogenesis process, due to the high productivity in the spring and summer months (Hagen & Vogt 1999, Heroy et al. 2008, Winfrey & Zeikus 1977.

Petrophysical properties of glaciomarine sediments
The petrophysical characteristics of the marine sediments recovered from the CBB presented distinct profiles, reflecting the complexity of sedimentary sources, transport mechanisms, and rework actions that occur at each site location. Looking at each parameter individually, it is possible to see that sediment density decreases towards the AP. The lithologies identified as tills presented higher densities, followed by diamictons, with the lowest values being associated with siliceous mud. Except for values associated with IRD, the density display values between 1.9 and 2.0 g/cm 3 for tills, from 1.3 to 1.8 g/cm 3 for diamicton, and 1,1 to 1.3 g/ cm 3 for siliceous mud. These values are in agreement with the data compilation presented by Wohlenberg (1982). The author identified the density for unconsolidated sediments ranging from 2.0 to 2.7 g/cm 3 for clays and 1.4 to 1.9 g/ cm 3 for organic clays. These patterns were also recorded by García et al. (2011) that found similar results for CBB and with the sediment density presented for Marshall (1975) that ranges around 1.4 g/cm 3 in clay and 1.2 g/cm 3 in siliceous mud.
The oscillation on the p-wave velocity of the CBB sediments was, in general, distinct between the SSI continental shelf (core GC16) and the other locations, showing a good agreement with density. The large fluctuations recorded can reflect the changes in the granulometry. This suggestion is supported by the p-wave velocity analysis carried out at the continental shelf from the northern Pacific Ocean by Hamilton (1980), which revealed an increase in the velocity of the compressional wave as a function of the increase in grain size. In this study, the coarse sands showed higher speeds, reaching 1836 m/s, when compared to finer sediments (1520 m/s in silty clays to 1646 m/s in silty sand), while the geoacoustic parameters are mainly influenced by the type of fluid present in the porous space. Gases are more compressible than liquids, so p-wave values are lower in gas-filled sediments. Thus, the presence of gas very possibly came to contribute to anomalies and consequently the absence of p-wave data (between 250 and 300 cm) recorded on core GC06A. The p-wave velocity in unconsolidated sediments is relatively uniform, between 1500 to 1600 m/s in siliceous oozes and even in clays (Marshall 1975). Sediments from Deception Island already . 4a: Box-whisker plots for geophysical properties per core location considering minimum, maximum, median, lower quartile, and upper quartile for groups of data and ternary diagram comparing the relative (%) sand/silt/ clay grain sizes fractions for the six studied cores. 4b: Bathymetric profile with sediment core location and description at Central Bransfield Basin. studied by (Leitão et al. 2016) presented values between 1532 m/s and 1564 m/s for fine silts.
In marine sediments, magnetic susceptibility can vary from the absolute minimum value of -1.5 SI.10 -5 (diamagnetic minerals, such as pure calcite or quartz) to a maximum of 1000 SI.10 -5 for basaltic debris rich in (titanium) magnetite (Bleil et al. 1996). That means the fluctuations in magnetic susceptibility can be interpreted by a higher iron content or, more generally, by a large amount of magnetic particles within a given volume. In this way, the highest values recorded near the SSI, linked to the tills and diamicton can be associated with the SSI geological matrix, due to the proximity with the Greenwich and Robert Islands, Three Sisters volcanic pinnacles formation, or even by debris transported from Deception Island, which is an active volcano (Burton-Johnson & Riley 2015). Distant locations from SSI, as diamicton and siliceous mud, possibly have a greater contribution of the AP rock matrix, composed mainly by sedimentary rocks (Burton-Johnson & Riley 2015), besides biogenic material, indicating sediments rich in silica, carbonates, and/or organic matter (Heroy et al. 2008). Previous studies show magnetic susceptibility values below to our records for different regions of the world, as at cores from lakes near the Black Sea, where values of magnetic susceptibility between 0 and 45 SI.10 -5 were used as proxies for a lithological description of detritic material, organic matter and carbonates (Radan et al. 2013). Magnetic susceptibility measurements on the Amazon Platform also show constant values around 10 SI.10 -5 , with high intensities reaching 42.7 SI.10 -5 (Bleil et al. 1996). According to the authors, the average value for clayey soils was 70 SI.10 -5 , and for mixed texture it was 45 SI.10 -5 , both measured between 0.2 and 0.4 m BSF. In Frobisher Bay in Canada, Andrews & Stravers (1993) studied the glacial environment where the medians values (per core analyzed) ranged from 6 to 709 SI.10 -5 agree with our records in the CBB.
The electrical resistivity does not show significant variations. However, lower values are found towards AP (core GC06A; 0.37 ohm.m; Fig.  3b; Table SII) as well as density and magnetic susceptibility parameters, probably caused by the saturation and salinity inside the pores. The electrical resistivity of water-saturated sediments depends on the sum of the resistivity of their solid and fluid portions. Since silicate grains are poor conductors, an electric current spread mainly in the fluid within the pore. Therefore, according to Schulz & Zabel (2013), the dominant transport mechanism of the electric current is electrolytic conduction of ions and molecules with excess or deficiency of electrons. According to this, the predominant water mass in the CBB and their different salinity patterns (TBW and TWW) may be influencing the electrical resistivity measured within the sediments.
The total natural gamma radiation in CBB sediments showed considerable variations. The high gamma-ray response indicates the presence of clay-rich deposits, such as shale, claystone, while relatively low gamma radiation indicates the presence of coarse-grained sandstone or carbonate rocks (Klaja & Dudek 2016). However, our data suggest that the natural radiation in CBB is mainly controlled by the Uranium content, with Thorium participation, and with a poor Potassium contribution, as widely discussed by Nobre et al. (2020) with the same dataset. As the lithologies classification, uranium levels present a slight decrease towards the AP. The highest mean value of U was found in SSI continental shelf (core GC16; 14.4ppm), while the lowest occurs at AP upper slope (core GC06A; 7.4ppm). The intermediate locations presented average values between 8.9ppm and 13.9ppm. Nobre et al. (2020) also suggested that sediment core distance from AP reflects the distribution of gamma-ray content at CBB sediments when analyzed by its proportion in terms of U/K and Th/U, resulting in higher contribution of thorium in sediments near to ISS and higher contribution of uranium in the deeper and distal cores locations, due to preservation of organic matter.
This relationship corroborates with the water content, which, as demonstrated by Klaja & Dudek (2016), tends to present an inverse correlation with total natural gamma radiation. The total amount of uranium measured by the spectrometer includes both detrital and authigenic contributions of uranium. Due to the solubility of the uranium in water when under oxidizing conditions, uranium can be partially transported in the form of Uranyl carbonate complexes (Bodin et al. 2011). Therefore, an increase in the amount of authigenic Uranium may indicate a depositional environment with oxygen depletion (Wignall & Myers 1988). Hassan et al. (1976) showed a strong correlation between the U content and the organic carbon and organic matter content. Uranium species can be adsorbed on clay minerals or associated with accessory minerals and on colloidal Fe oxide/hydroxide coatings, of the mineral grains. Thorium is commonly concentrated in residual terrigenous deposits, such as bauxite and clay due to be practically insolubility (Hassan et al. 1976). Important amounts of thorium are recorded in heavy minerals such as monazite, rutile, and zircon. Hassan et al. (1976) reported a correlation between the Th and clay mineral content. However, the authors have not proven that the clays were responsible for the Th enrichment. Considering the spectral proportions, the CBB sediments can be composed of illite, since their typical potassium levels are around 3% (Nielsen et al. 1987). According to Kühnel (1996), illite is a clay mineral characteristic of a cold climate.

Density and Magnetic Susceptibility signature
The geological and petrophysical characteristics recorded across the CBB (from SSI to AP) highlighted the relationship between the lithological content and associated depositional processes with the geophysical proprieties as density and magnetic susceptibility (Fig. 5a). In general terms, both geophysical proprieties (density and magnetic susceptibility) presented higher values as well near the SSI that decreases towards the AP (Fig. 4, Table VI).
Our records support that those distinct petrophysical ranges recognized through the site locations present a close relationship with the local lithologies, depositional process, and distance of sediments source areas. The highest petrophysical values are associated with lithologies classified as deformation tills from the SSI continental shelf (core GC16; average density 1.90 g/cm 3 ; average MS 497.41 SI.10 -5 ; Fig.  4, Table IV), followed by the diamictons from the SSI platform (cores GC13 and GC12; average density 1.56 g/cm 3 ; average MS 440.8 SI.10 -5 ; Fig. 4, Table IV), while the diamictons from the AP lower slope (cores AM10 and GC09; average density 1.35 g/cm 3 ; average MS 102.7 SI.10 -5 ; Table IV) and de siliceous mud from the AP upper slope (core GC06A (average density 1.15 g/cm 3 ; average MS 4.3 SI.10 -5 ; Fig. 4, Table IV) presents lower values. In this way, the highest values recorded in the SSI site locations are directly associated with the geological matrix of the area. The SSI continental shelf and platform present a geological matrix related to the faster glaciers decay during the warmer conditions (due to their relative small drainage area), that associated with the strong cyclicity of the advance and retreat of the glaciers, easily eroded volcaniclastic source and the high rates of meltwater production, that promote the basinward transport of a large volume of sediments, mainly by meltwater plumes and mass-gravity flows, as slumps, turbidity currents and gravitational instabilities (Prieto et al. 1999). In addition, other volcanic source areas as the Three Sisters volcanic pinnacles formation, or even by debris transported from Deception Island, can also contribute to the increase observed in the petrophysical properties (Burton-Johnson & Riley 2015).
However, in the site locations distant from SSI, as the AP diamicton and siliceous mud (cores AM10, GC09, and GC06A), the higher contribution of the AP rock matrix, composed mainly by sedimentary rocks (Burton-Johnson & Riley 2015), besides biogenic material, indicating sediments rich in silica, carbonates, and/or organic matter, with the low influence of volcanic material.

CONCLUSIONS
Four large lithological groups can be identified according to their geological and petrophysical characteristics, especially by their magnetic susceptibility and density data. The first group consists of subglacial deformation till (GC16), which presented the color black (GLEY 1 2.5 / N), higher values of density (1.90 g/cm 3 ) magnetic susceptibility (497 SI.10 -5 ), p-wave (1653 m/s) and grain size (3.02φ); the second and third group corresponds to massive diamicton from South Figure 5. 5a: Class scatter plot of density (g/cm3) and magnetic susceptibility (SI.10-5) according with lithology of deformation till, diamiction (near SSI or AP) and siliceous mud. 5b: Class scatter plot of density (g/cm3) and electrical resistivity (ohm.m) according with lithology of deformation till, diamiction (near SSI or AP) and siliceous mud.
Shetlands Islands (GC13 and GC12), basin (AM10) and lower slope (GC09), variable gray color and intermediate values of density, magnetic susceptibility, and p-wave velocity; the fourth lithological group is composed of siliceous mud from an upper slope location (GC06A) presenting an olive/brown color and gas detection, with lower values for density (1.15 g/cm 3 ), magnetic susceptibility (4 SI.10 -5 ), electrical resistivity (0.37 ohm.m) and natural gamma response (158 nGy/h).These values can be used on further research.