Taylor Glacier Research Articles

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

AbstractBlood Falls is a hypersaline, iron‐rich discharge at the terminus of the Taylor Glacier in the McMurdo Dry Valleys, Antarctica. In November 2014, brine in a conduit within the glacier was penetrated and sampled using clean‐entry techniques and a thermoelectric melting probe called the IceMole. We analyzed the englacial brine sample for filterable iron (fFe), total Fe, major cations and anions, nutrients, organic carbon, and perchlorate. In addition, aliquots were analyzed for minor and trace elements and isotopes including δD and δ18O of water, δ34S and δ18O of sulfate, 234U, 238U, δ11B, 87Sr/86Sr, and δ81Br. These measurements were made in order to (1) determine the source and geochemical evolution of the brine and (2) compare the chemistry of the brine to that of nearby hypersaline lake waters and previous supraglacially sampled collections of Blood Falls outflow that were interpreted as end‐member brines. The englacial brine had higher Cl− concentrations than the Blood Falls end‐member outflow; however, other constituents were similar. The isotope data indicate that the water in the brine is derived from glacier melt. The H4SiO4 concentrations and U and Sr isotope suggest a high degree of chemical weathering products. The brine has a low N:P ratio of ~7.2 with most of the dissolved inorganic nitrogen in the form of NH4+. Dissolved organic carbon concentrations are similar to end‐member outflow values. Our results provide strong evidence that the original source of solutes in the brine was ancient seawater, which has been modified with the addition of chemical weathering products.

Dust Transport to the Taylor Glacier, Antarctica, During the Last Interglacial

AbstractChanges in the composition of dust trapped in ice provide evidence of past atmospheric circulation and earth surface conditions. Investigations of dust provenance in Antarctic ice during glacial and interglacial periods indicate that South America is the primary dust source during both climate regimes. Here, we present results from a new ice core dust archive extracted from the Taylor Glacier in coastal East Antarctica during the deglacial transition from Marine Isotope Stage 6 to 5e. Radiogenic strontium and neodymium isotopes indicate that last interglacial dust is young and volcanic, in contrast to the observed preindustrial and Holocene (Marine Isotope Stage 1) dust composition. The dust composition differences from the last interglacial and current interglacial period at the site require a profound difference in atmospheric transport and environmental conditions. We consider several potential causes for enhanced transport of volcanic material to the site, including increased availability of volcanic material and large‐scale atmospheric circulation changes.

The Neoproterozoic “Blood Falls” in Tarim Craton and Their Possible Connection With Snowball Earth

AbstractBlood Falls refers to the outflow of iron‐rich brine plumes under the Taylor Glacier in Antarctica, which have distinctive blood‐like red color. It was proposed that during the Cryogenian, Fe oxides of terrestrial origin accumulated under glaciers via a mechanism similar to the modern Blood Falls, although reports of iron deposits of such novel origin are lacking in literature. The Quruqsay iron formation is hosted in Neoproterozoic glacial sequences in the northern margin of the Tarim Craton in NW China and consist of 15 lenticular iron orebodies. The iron ores are mainly composed of fine Fe‐oxide aggregate and clastics, with the latter sometimes occurring as dropstones indicating glacial fluvial deposition. The hematite minerals show positive Ce and Y anomalies but lack positive Eu anomalies, with flat shale‐normalized rare Earth elements patterns. The hematite minerals also display remarkably negative εNd values and close‐to‐0 δ56Fe values, which collectively indicate that iron was sourced from continents rather than mid‐ocean ridge hydrothermal vents. Additionally, δ18O values of the hematite are consistent with the large ice‐volume during the Snowball Earth event. Evidence from the Quruqsay iron deposit suggest oxidation of terrestrial sub‐glacier iron‐bearing discharge in oceans of extensive glaciation, correlating with the predicted Blood Falls‐type iron deposits in the Neoproterozoic. The Quruqsay iron formation is a unique hematite‐rich iron deposit implying that subglacial lakes can be a significant repository for Fe2+ and that the volume of the Neoproterozoic glaciers may be much higher than the current glacier.

Aeolian Material Transport and Its Role in Landscape Connectivity in the McMurdo Dry Valleys, Antarctica

AbstractArid regions, particularly polar and alpine desert environments, have diminished landscape connectivity compared to temperate regions due to limited and/or seasonal hydrological processes. For these environments, aeolian processes play a particularly important role in landscape evolution and biotic community vitality through nutrient and solute additions. The McMurdo Dry Valleys (MDV) are the largest ice‐free area in Antarctica and are potentially a major source of aeolian material for the continent. From this region, samples were collected at five heights (~5, 10, 20, 50, and 100 cm) above the surface seasonally for 2013 through 2015 from Alatna Valley, Victoria Valley, Miers Valley, and Taylor Valley (Taylor Glacier, East Lake Bonney, F6 (Lake Fryxell), and Explorer's Cove). Despite significant geological separation and varying glacial histories, low‐elevation and coastal sites had similar major ion chemistries, as did high‐elevation and inland locations. This locational clustering of compositions was also evident in scanning electron microscopy images and principal component analyses, particularly for samples collected at ~100 cm above the surface. Compared to published soil literature, aeolian material in Taylor Valley demonstrates a primarily down‐valley transport of material toward the coast. Soluble N:P ratios in the aeolian material reflect relative nutrient enrichments seen in MDV soils and lakes, where younger, coastal soils are relatively N depleted, while older, up‐valley soils are relatively P depleted. The aeolian transport of materials, including water‐soluble nutrients, is an important vector of connectivity within the MDV and provides a mechanism to help “homogenize” the geochemistry of both soil and aquatic ecosystems.

A Horizontal Ice Core From Taylor Glacier, Its Implications for Antarctic Climate History, and an Improved Taylor Dome Ice Core Time Scale

AbstractIce core records from Antarctica show mostly synchronous temperature variations during the last deglacial transition, an indication that the climate of the entire continent reacted as one unit to the global changes. However, a record from the Taylor Dome ice core in the Ross Sea sector of East Antarctica has been suggested to show a rapid warming, similar in style and synchronous with the Oldest Dryas—Bølling warming in Greenland. Since publication of the Taylor Dome record, a number of lines of evidence have suggested that this interpretation is incorrect and reflects errors in the underlying time scale. The issues raised regarding the dating of Taylor Dome currently linger unresolved, and the original time scale remains the de facto chronology. We present new water isotope and chemistry data from nearby Taylor Glacier to resolve the confusion surrounding the Taylor Dome time scale. We find that the Taylor Glacier record is incompatible with the original interpretation of the Taylor Dome ice core, showing that the warming in the area was gradual and started at ∼18 ka BP (before 1950) as seen in other East Antarctic ice cores. We build a consistent, up‐to‐date Taylor Dome chronology from 0 to 60 ka BP by combining new and old age markers based on synchronization to other ice core records. The most notable feature of the new TD2015 time scale is a gas age—ice age difference of up to 12,000 years during the Last Glacial Maximum, by far the largest ever observed.

Trans-dimensional Bayesian inversion of airborne transient EM data from Taylor Glacier, Antarctica

Trans-dimensional Bayesian inversion of airborne transient EM data from Taylor Glacier, Antarctica

Did the Black-Mat Impact/Airburst Reach the Antarctic? Evidence from New Mountain Near the Taylor Glacier in the Dry Valley Mountains

AbstractDetailed microscopic investigations of horizons in a surface paleosol, part of a pedostratigraphic stack of tills at New Mountain, Antarctica, dated to the middle Miocene climatic optimum e...

Genomic and physiological characterization and description of Marinobacter gelidimuriae sp. nov., a psychrophilic, moderate halophile from Blood Falls, an antarctic subglacial brine

Antarctic subice environments are diverse, underexplored microbial habitats. Here, we describe the ecophysiology and annotated genome of a Marinobacter strain isolated from a cold, saline, iron-rich subglacial outflow of the Taylor Glacier, Antarctica. This strain (BF04_CF4) grows fastest at neutral pH (range 6-10), is psychrophilic (range: 0°C-20°C), moderately halophilic (range: 0.8%-15% NaCl) and hosts genes encoding potential low temperature and high salt adaptations. The predicted proteome suggests it utilizes fewer charged amino acids than a mesophilic Marinobacter strain. BF04_CF4 has increased concentrations of membrane unsaturated fatty acids including palmitoleic (33%) and oleic (27.5%) acids that may help maintain cell membrane fluidity at low temperatures. The genome encodes proteins for compatible solute biosynthesis and transport, which are known to be important for growth in saline environments. Physiological verification of predicted metabolic functions demonstrate BF04_CF4 is capable of denitrification and may facilitate iron oxidation. Our data indicate that strain BF04_CF4 represents a new Marinobacter species, Marinobacter gelidimuriae sp. nov., that appears well suited for the subglacial environment it was isolated from. Marinobacter species have been isolated from other cold, saline environments in the McMurdo Dry Valleys and permanently cold environments globally suggesting that this lineage is cosmopolitan and ecologically relevant in icy brines.

The physical limnology of a permanently ice‐covered and chemically stratified Antarctic lake using high resolution spatial data from an autonomous underwater vehicle

AbstractWe used an Environmentally Non‐Disturbing Under‐ice Robotic ANtarctic Explorer to make measurements of conductivity and temperature in Lake Bonney, a chemically stratified, permanently ice‐covered Antarctic lake that abuts Taylor Glacier, an outlet glacier from the Polar Plateau. The lake is divided into two lobes – East Lobe Bonney (ELB) and West Lobe Bonney (WLB), each with unique temperature and salinity profiles. Most of our data were collected in November 2009 from WLB to examine the influence of the Taylor Glacier on the structure of the water column. Temperatures adjacent to the glacier face between 20 m and 22 m were 3°C colder than in the rest of WLB, due to latent heat transfer associated with melting of the submerged glacier face and inflow of cold brines that originate beneath the glacier. Melting of the glacier face into the salinity gradient below the chemocline generates a series of nearly horizontal intrusions into WLB that were previously documented in profiles measured with 3 cm vertical resolution in 1990–1991. WLB and ELB are connected by a narrow channel through which water can be exchanged over a shallow sill that controls the position of the chemocline in WLB. A complex exchange flow appears to exist through the narrows, driven by horizontal density gradients and melting at the glacier face. Superimposed on the exchange is a net west‐to‐east flow generated by the higher volume of meltwater inflows to WLB. Both of these processes can be expected to be enhanced in the future as more meltwater is produced.

Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle

Abstract. Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components requiring large sample volumes. For margin sites to be useful as ancient ice archives, the ice stratigraphy needs to be understood and age models need to be established. We present measurements of trapped gases in ice from Taylor Glacier, Antarctica, to date the ice and assess the completeness of the stratigraphic section. Using δ18O of O2 and methane concentrations, we unambiguously identify ice from the last glacial cycle, covering every climate interval from the early Holocene to the penultimate interglacial. A high-resolution transect reveals the last deglaciation and the Last Glacial Maximum (LGM) in detail. We observe large-scale deformation in the form of folding, but individual stratigraphic layers do not appear to have undergone irregular thinning. Rather, it appears that the entire LGM–deglaciation sequence has been transported from the interior of the ice sheet to the surface of Taylor Glacier relatively undisturbed. We present an age model that builds the foundation for gas studies on Taylor Glacier. A comparison with the Taylor Dome ice core confirms that the section we studied on Taylor Glacier is better suited for paleo-climate reconstructions of the LGM due to higher accumulation rates.

Constraining the recent history of the perennially ice-covered Lake Bonney, East Antarctica using He, Kr and Xe concentrations

Lake Bonney is a perennially ice-covered lake in the McMurdo Dry Valleys (MDVs) that has long been studied in order to provide constraints on the paleoclimate of West Antarctica. The lake is divided into two lobes, West Lake Bonney (WLB) and East Lake Bonney (ELB) that are separated by a narrow ridge. The two lobes currently receive surface melt water during austral summers from glacier-fed ephemeral streams and this meltwater enters the lake via a narrow ring, or moat, of liquid water that forms around the lake during summer. The West Lobe also receives water from direct input of melt water from Taylor glacier and saline water from irregular subglacial discharge.Here, we combine previously published He data from Lake Bonney with new Kr and Xe concentration data to examine the signatures of water recharge via the seasonal moat and these data are used to constrain a model for He, Kr and Xe transport within both WLB and ELB over about the last 5000–6000yrs. A detailed numerical simulation is presented that combines diffusive transport of noble gases within the stratified water column of Lake Bonney, along with ice ablation at the top of the ice cover, partitioning of noble gases between water and ice, plus exchange of noble gases between WLB and ELB. Results strongly suggest that open moats have only operated for about 2–3 centuries within the last millennium. These results are corroborated by the high concentration of He, especially within WLB, which points to a history of ice cover with no open moats operating for both lobes for at least about 5 millennia. In addition, the distribution of He, Kr and Xe suggest that a significant rise of the water level of Lake Bonney associated with a warmer period may have been interrupted by a roughly 4–5 century long cold period during which the moats were not large enough to allow air saturated water into the lake, with this cold period ending about one century ago. In addition, during this cold period, there is evidence for a thickening of the ice cover. In the last century Lake Bonney levels have been rising, pointing either to more extended austral summers or elevated summer temperatures, a rise that is compatible with the heavy noble gas record in the upper portion of the lake.A preliminary analysis that includes all stable noble gases heavier than He, suggests that glacial melt water (GMW) directly from Taylor Glacier below the ice cover may have been a significant input into the deepest parts of WLB, but not ELB.

An englacial hydrologic system of brine within a cold glacier: Blood Falls, McMurdo Dry Valleys, Antarctica

ABSTRACTTaylor Glacier hosts an active englacial hydrologic system that feeds Blood Falls, a supraglacial outflow of iron-rich subglacial brine at the terminus, despite mean annual air temperatures of −17°C and limited surface melt. Taylor Glacier is an outlet glacier of the East Antarctic ice sheet that terminates in Lake Bonney, McMurdo Dry Valleys. To image and map the brine feeding Blood Falls, we used radio echo sounding to delineate a subhorizontal zone of englacial brine upstream from Blood Falls and elongated in the ice flow direction. We estimate volumetric brine content in excess of 13% within 2 m of the central axis of this zone, and likely much higher at its center. Brine content decreases, but remains detectable, up to 45 m away along some transects. Hence, we infer a network of subparallel basal crevasses allowing injection of pressurized subglacial brine into the ice. Subglacial brine is routed towards Blood Falls by hydraulic potential gradients associated with deeply incised supraglacial valleys. The brine remains liquid within the subglacial and englacial environments through latent heat of freezing coupled with elevated salt content. Our findings suggest that cold glaciers could support freshwater hydrologic systems through localized warming by latent heat alone.

Dust composition changes from Taylor Glacier (East Antarctica) during the last glacial-interglacial transition: A multi-proxy approach

Mineral dust is transported in the atmosphere and deposited in oceans, ice sheets and the terrestrial biosphere. Temporal changes in locations of dust source areas and transport pathways have implications for global climate and biogeochemical cycles. The chemical and physical characterization of the dust record preserved in ice cores is useful for identifying of dust source regions, dust transport, dominant wind direction and storm trajectories. Here, we present a 50,000-year geochemical characterization of mineral dust entrapped in a horizontal ice core from the Taylor Glacier in East Antarctica. Strontium (Sr) and neodymium (Nd) isotopes, grain size distribution, trace and rare earth element (REE) concentrations, and inorganic ion (Cl− and Na+) concentrations were measured in 38 samples, corresponding to a time interval from 46 kyr before present (BP) to present. The Sr and Nd isotope compositions of insoluble dust in the Taylor Glacier ice shows distinct changes between the Last Glacial Period (LGP in this study ranging from ∼46.7–15.3 kyr BP) the early Holocene (in this study ranging from ∼14.5–8.7 kyr BP), and zero-age samples. The 87Sr/86Sr isotopic composition of dust in the Taylor Glacier ice ranged from 0.708 to 0.711 during the LGP, while the variability during the early Holocene is higher ranging from 0.707 to 0.714. The εNd composition ranges from 0.1 to −3.9 during the LGP, and is more variable from 1.9 to −8.2 during the early Holocene. The increased isotopic variability during the early Holocene suggests a shift in dust provenance coinciding with the major climate transition from the LGP to the Holocene. The isotopic composition and multiple physical and chemical constraints support previous work attributing Southern South America (SSA) as the main dust source to East Antarctica during the LGP, and a combination of both local Ross Sea Sector dust sources and SSA after the transition into the Holocene. This study provides the first high time resolution data showing variations in dust provenance to East Antarctic ice during a major climate regime shift, and we provide evidence of changes in the atmospheric transport pathways of dust following the last deglaciation.

Hydraulic conductivity of active layer soils in the McMurdo Dry Valleys, Antarctica: Geological legacy controls modern hillslope connectivity

Spatial variability in the hydraulic and physical properties of active layer soils influences shallow groundwater flow through cold-desert hydrological systems. This study measures the saturated hydraulic conductivity and grain-size distribution of 90 soil samples from the McMurdo Dry Valleys (MDV), Antarctica—primarily from Taylor Valley—to determine what processes affect the spatial distribution of saturated hydraulic conductivity in a simple, mineral-soil-dominated natural hillslope laboratory. We find that the saturated hydraulic conductivity and the grain-size distribution of soils are organized longitudinally within Taylor Valley. Soils sampled down-valley near the coast have a higher percentage of fine-sized sediments (fine sand, silt, clay) and lower saturated hydraulic conductivities than soils collected up-valley near Taylor Glacier (1.3×10−2 vs. 1.2×10−1cm/s). Soils collected mid-valley have intermediate amounts of fines and saturated hydraulic conductivity values consistent with a hydrogeologic gradient spanning the valley from high inland to low near the coast. These results suggest the organization of modern soil properties within Taylor Valley is a relict signature from past glaciations that have deposited soils of decreasing age toward the mouth of the valley, modified by fluvial activity acting along temporal and microclimate gradients.

Buried ice in Kennar Valley: a late Pleistocene remnant of Taylor Glacier

AbstractBuried glacier ice is common in the McMurdo Dry Valleys and under ideal climatic and geomorphological conditions may be preserved for multimillion-year timescales. This study focuses on the analysis of ~300 m2 of buried glacier ice in lower Kennar Valley, Quartermain Range. The mapped ice is clean,<10 m thick and covered by a~25 cm sandy drift. The mouth of Kennar Valley is occupied by a lobe of Taylor Glacier, an outlet glacier from Taylor Dome. Based on ice–sediment characteristics, air bubble concentrations and stable isotopic analyses from three ice cores, the lower Kennar Valley ice is glacial in origin. These data coupled with a previously reported exposure age chronology indicate that the buried ice was deposited by a late Pleistocene advance of Taylor Glacier, probably during an interglacial interval. The surface of the buried glacier ice exhibits a patterned ground morphology characterized by small, dome-shaped polygons with deep troughs. This shape possibly reflects the final stages of ice loss, as stagnant, isolated ice pinnacles sublimate in place. This study highlights how polygon morphology can be used to infer the thickness of clean buried ice and its geomorphological stability throughout Antarctica, as well as other in cold, arid landscapes.

EnEx-RANGE - Robust autonomous Acoustic Navigation in Glacial icE

Within the Enceladus Explorer Initiative of the DLR Space Administration navigation technologies for a future space mission are in development. Those technologies are the basis for the search for extraterrestrial life on the Saturn moon Enceladus. An autonomous melting probe, the EnEx probe, aims to extract a liquid sample from a water reservoir below the icy crust.A first EnEx probe was developed and demonstrated in a terrestrial scenario at the Bloodfalls, Taylor Glacier, Antarctica in November 2014. To enable navigation in glacier ice two acoustic systems were integrated into the probe in addition to conventional navigation technologies. The first acoustic system determines the position of the probe during the run based on propagation times of acoustic signals from emitters at reference positions at the glacier surface to receivers in the probe. The second system provides information about the forefield of the probe. It is based on sonographic principles with phased array technology integrated in the probe’s melting head. Information about obstacles or sampling regions in the probe’s forefield can be acquired. The development of both systems is now continued in the project EnEx-RANGE. The emitters of the localization system are replaced by a network of intelligent acoustic enabled melting probes. These localize each other by means of acoustic signals and create the reference system for the EnEx probe.This presentation includes the discussion of the intelligent acoustic network, the acoustic navigation systems of the EnEx probe and results of terrestrial tests.

Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

An understanding of the mechanisms that control CO2 change during glacial-interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2(δ(13)C-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ(13)C-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in δ(13)C-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ(13)C-CO2 that suggest rapid oxidation of organic land carbon or enhanced air-sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bølling (14.6-14.3 ka) and Holocene (11.6-11.4 ka) intervals are associated with small changes in δ(13)C-CO2, suggesting a combination of sources that included rising surface ocean temperature.

Measurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production rates

Carbon-14 (14C) is incorporated into glacial ice by trapping of atmospheric gases as well as direct near-surface in situ cosmogenic production. 14C of trapped methane (14CH4) is a powerful tracer for past CH4 emissions from “old” carbon sources such as permafrost and marine CH4 clathrates. 14C in trapped carbon dioxide (14CO2) can be used for absolute dating of ice cores. In situ produced cosmogenic 14C in carbon monoxide (14CO) can potentially be used to reconstruct the past cosmic ray flux and past solar activity. Unfortunately, the trapped atmospheric and in situ cosmogenic components of 14C in glacial ice are difficult to disentangle and a thorough understanding of the in situ cosmogenic component is needed in order to extract useful information from ice core 14C. We analyzed very large (≈1000kg) ice samples in the 2.26–19.53m depth range from the ablation zone of Taylor Glacier, Antarctica, to study in situ cosmogenic production of 14CH4 and 14CO. All sampled ice is >50ka in age, allowing for the assumption that most of the measured 14C originates from recent in situ cosmogenic production as ancient ice is brought to the surface via ablation. Our results place the first constraints on cosmogenic 14CH4 production rates and improve on prior estimates of 14CO production rates in ice. We find a constant 14CH4/14CO production ratio (0.0076±0.0003) for samples deeper than 3m, which allows the use of 14CO for correcting the 14CH4 signals for the in situ cosmogenic component. Our results also provide the first unambiguous confirmation of 14C production by fast muons in a natural setting (ice or rock) and suggest that the 14C production rates in ice commonly used in the literature may be too high.

Antarctic aquifers may teem with life

Antarctica's Blood Falls are well named. There, the white tongue of the Taylor glacier is stained crimson, as if the ice itself has been wounded. The iron in the water comes from the weathering of the bedrock beneath the ice, a process enhanced by microbial action. This unique feature is much more than a curiosity--it is a portal into the Antarctic subsurface, a hint at what lies beneath, says Jill Mikucki at the University of Tennessee in Knoxville.

Deep groundwater and potential subsurface habitats beneath an Antarctic dry valley.

The occurrence of groundwater in Antarctica, particularly in the ice-free regions and along the coastal margins is poorly understood. Here we use an airborne transient electromagnetic (AEM) sensor to produce extensive imagery of resistivity beneath Taylor Valley. Regional-scale zones of low subsurface resistivity were detected that are inconsistent with the high resistivity of glacier ice or dry permafrost in this region. We interpret these results as an indication that liquid, with sufficiently high solute content, exists at temperatures well below freezing and considered within the range suitable for microbial life. These inferred brines are widespread within permafrost and extend below glaciers and lakes. One system emanates from below Taylor Glacier into Lake Bonney and a second system connects the ocean with the eastern 18 km of the valley. A connection between these two basins was not detected to the depth limitation of the AEM survey (∼350 m).