Cryogenic origin for brine in the subsurface of southern McMurdo Sound, Antarctica

Sampling of interstitial fluids during deep coring in southern McMurdo Sound, Antarctica, revealed the presence of seawater-sourced, hypersaline brine at depths >200 m below the sea-floor. Na-Cl-Br and SO4-Cl-Br relationships are consistent with a concentration mechanism that involves the removal of pure H2O as ice and precipitation of mirabilite (Na2SO4·10H2O) during progressive freezing of seawater. The brine is in Neogene subglacial, glacimarine, and marine facies that record advance and retreat of glaciers through the Ross Sea embayment. In this environment, sea ice formation in semi-isolated marine basins that occupied flexural troughs along ice sheet margins produced dense brines that sank and infiltrated the permeable subglacial sediment. Repeated cycles of glacial advance and retreat provided multiple opportunities for batches of seawater to be transformed into brine that now is in the subsurface of southern McMurdo Sound. Results demonstrate the feasibility of brine formation via seawater freezing and attest to the potential of a cryogenic origin for subsurface brines in high-latitude regions of the Northern Hemisphere, as proposed by some workers.Published587-5902.2. Laboratorio di paleomagnetismo3.8. Geofisica per l'ambienteJCR Journalrestricte

Cryogenic origin for brine in the subsurface of southern McMurdo Sound, Antarctica

Sampling of interstitial fluids during deep coring in southern McMurdo Sound, Antarctica, revealed the presence of seawater-sourced, hypersaline brine at depths >200 m below the sea-floor. Na-Cl-Br and SO4-Cl-Br relationships are consistent with a concentration mechanism that involves the removal of pure H2O as ice and precipitation of mirabilite (Na2SO4·10H2O) during progressive freezing of seawater. The brine is in Neogene subglacial, glacimarine, and marine facies that record advance and retreat of glaciers through the Ross Sea embayment. In this environment, sea ice formation in semi-isolated marine basins that occupied flexural troughs along ice sheet margins produced dense brines that sank and infiltrated the permeable subglacial sediment. Repeated cycles of glacial advance and retreat provided multiple opportunities for batches of seawater to be transformed into brine that now is in the subsurface of southern McMurdo Sound. Results demonstrate the feasibility of brine formation via seawater freezing and attest to the potential of a cryogenic origin for subsurface brines in high-latitude regions of the Northern Hemisphere, as proposed by some workers

Physical Properties of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica

Whole-core measurements of Wet Bulk Density (WBD), compressional (P)-wave velocity (Vp), and Magnetic Susceptibility were measured at a sampling interval of 1 or 2 centimetres (cm) throughout the AND-2A drill core for initial core characterisation and on-site correlation with seismic modeling to predict target-reflector depth. Measurements were made using a GEOTEK (Multi-Sensor-Core-Logger MSCL). Density and velocity standards were measured together with core runs of 3-6 metres (m) (and occasionally up to 18 m) throughout the entire depth range to monitor data quality. Drift of the magnetic susceptibility sensor was also monitored and corrected where necessary. These physical properties show a large range of values, reflecting the different nature of the various lithologies including extremely high velocity and density values in individual clasts, and the effects of cementation on porosity. A downcore increase in WBD and Vp occurs in the upper 200 m, however, no systematic trend exists at greater depths although large fluctuations on a m-decimetre- (dm) scale occur. Magnetic susceptibility is generally low (\u3c100 x\u3e10-5 SI), however, four intervals of high (\u3e600 x 10-5 SI) susceptibility occur at 560, 800, 980 and 1 080 mbsf, indicating a relatively greater contribution of volcanic-derived material to the core site in the lower half of the AND-2A core

Fracture Logging of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica

Fractures in AND-2A drillcore were documented in this study. Over 4100 fractures of all types were logged. A population of 510 steeply-dipping, petal, petal-centreline and core-edge induced fractures is present, reaching a maximum density of c. 10 fractures/metre. Subhorizontal induced extension fractures are also abundant. There are 1008 natural fractures in the core, including faults, brecciated zones, veins and sedimentary intrusions. Kinematic indicators document dominant normal faulting, although reverse faults are also present. The natural fractures occur in strata ranging in age from the Miocene to the Plio-Pleistocene

Operations Overview for the ANDRILL Southern McMurdo Sound Project, Antarctica

During the austral spring of 2007-08, a 1138 metre (m)-long rock and sediment core (ANDRILL [AND]-2A) was recovered from beneath the land-fast sea-ice in southern McMurdo Sound (SMS) in 384 m of water. A custom-built drilling system comprising an UDR-1200 rig, jack-up platform, hot water drill, sea riser, and diamond-bit wireline coring string was set up on the sea-ice approximately 32 kilometres (km) from Scott Base (NZ) and McMurdo Station (USA). The drilling system employed technology developed to handle challenging environmental conditions, including drilling from an 8 metre-thick sea-ice ‘platform’ that moved both laterally and vertically, tidal currents, and high winds. Drill site set up commenced in early September 2007, and the first AND-2A core was recovered on 10 October 2007. Drilling operations continued until 5 December 2007. Science operations were conducted at the drill site, in both the borehole and a purpose-built laboratory complex, and at the Crary Science and Engineering Center (CSEC), McMurdo Station (USA). Drill site science operations involved downhole logging, which was carried out in the borehole casing and in parts of the open hole, fracture studies, and physical properties measurements. Core was transported by helicopter from the drill site to McMurdo Station, where it was split, scanned, described, and sampled for initial characterisation. Once initial studies were completed, the core was packed into crates for shipment to the Antarctic Research Facility (ARF; core repository) at Florida State University in the United States

Cryogenic origin for brine in the subsurface of southern McMurdo Sound, Antarctica

Sampling of interstitial fl uids during deep coring in southern McMurdo Sound, Antarctica, revealed the presence of seawater-sourced, hypersaline brine at depths >200 m below the seafl oor. Na-Cl-Br and SO4-Cl-Br relationships are consistent with a concentration mechanism that involves the removal of pure H2O as ice and precipitation of mirabilite (Na2SO4·10H2O) during progressive freezing of seawater. The brine is in Neogene subglacial, glacimarine, and marine facies that record advance and retreat of glaciers through the Ross Sea embayment. In this environment, sea ice formation in semi-isolated marine basins that occupied fl exural troughs along ice sheet margins produced dense brines that sank and infi ltrated the permeable subglacial sediment. Repeated cycles of glacial advance and retreat provided multiple opportunities for batches of seawater to be transformed into brine that now is in the subsurface of southern McMurdo Sound. Results demonstrate the feasibility of brine formation via seawater freezing and attest to the potential of a cryogenic origin for subsurface brines in high-latitude regions of the Northern Hemisphere, as proposed by some workers

Palaeomagnetism of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica

We conducted initial palaeomagnetic studies on cores from site AND-2A (77°45.488’S, 165°16.605’E, ~383.57 metres water depth). A total of 813 samples were collected that span from the top of the section down to the base at 1138.54 metres below sea floor (mbsf). Samples were collected every one or two metres down the core, with paired (pilot) samples being collected about every ten to twenty metres to allow us to assess the demagnetisation behaviour of the samples using either alternating field (AF) or thermal demagnetisation. With the exception of only a few intervals, AF demagnetisation was observed to resolve a characteristic remanent magnetisation (ChRM) as well or better than thermal demagnetisation. Thermal demagnetisation was particularly ineffective in many intervals owing to thermal alteration that was common above 500°C and was evident in some samples even at low temperatures. Above Lithostratigraphic Unit (LSU) 8 (436.18 mbsf), where lithologies are generally more coarse grained than lower in the section, resolving a ChRM is difficult and recent overprints or a drilling overprint are a concern. Within LSU 8 and below, most samples have a ChRM that can be resolved. The ChRM is most likely an original depositional magnetisation throughout most of this lower section, although orthogonaldemagnetisation diagrams contain evidence that normal polarity overprinting affects some intervals. Based on 40Ar/39Ar dates and diatom datums, the magnetozones identified from the base of the hole up to ~266mbsf are consistent with spanning from either Chron C6n (18.748-19.772 Ma) or C6An.1n (20.040-20.213 Ma) up through Chron C5Br (15.160-15.974 Ma). Above this, intervals of constant polarity are isolated within longer stratigraphic intervals of uncertain polarity, making their correlation with the geomagnetic polarity timescale (GPTS) speculative and highly dependent on ages obtained from other dating methods. One exception is a reversed-to-normal polarity transition that occurs at ~31 mbsf and is interpreted to most likely be the Brunhes/Matuyama boundary. The spacing of polarity reversals below 266 mbsf and their correlation with the GPTS indicates that this part of the stratigraphic section was deposited between 15 to 20 Ma at a mean sedimentation rate of about 18 centimetres (cm)/ thousand year (k.y.)

ANDRILL Southern McMurdo Sound Project Scientific Prospectus

During the austral summer of 2007 the ANtarctic DRILLing program (ANDRILL) will drill from a sea-ice platform in southern McMurdo Sound to obtain new information about the Neogene Antarctic cryosphere and evolution of Antarctic rift basins

Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica

In 2007, the Antarctic Geological Drilling Program (ANDRILL) drilled 1138.54 m of strata ~10 km off the East Antarctic coast, includ ing an expanded early to middle Miocene succession not previously recovered from the Antarctic continental shelf. Here, we pre sent a facies model, distribution, and paleoclimatic interpretation for the AND-2A drill hole, which enable us, for the fi rst time, to reconstruct periods of early and middle Miocene glacial advance and retreat and paleo environmental changes at an ice-proximal site. Three types of facies associations can be recognized that imply signifi cantly different paleoclimatic interpretations. (1) A diamictite-dominated facies association represents glacially dominated depositional environments, including subglacial environments, with only brief intervals where ice-free coasts existed, and periods when the ice sheet was periodically larger than the modern ice sheet. (2) A stratified diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-infl uenced depositional environments and is more consistent with a very dynamic ice sheet, with a grounding line south of the modern position. (3) A mudstone-dominated facies association generally lacks diamictites and was produced in a glacially infl uenced hemipelagic depositional environment. Based on the distribution of these facies associations, we can conclude that the Antarctic ice sheets were dynamic, with grounding lines south of the modern location at ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma and during the Miocene climatic optimum, ca. 17.6–15.4 Ma, with ice-sheet and sea-ice minima at ca. 16.5–16.3 Ma and ca. 15.7–15.6 Ma. While glacial minima at ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma were characterized by temperate margins, an increased abundance of gravelly facies and diatomaceous siltstone and a lack of meltwater plume deposits suggest a cooler and drier climate with polythermal conditions for the Miocene climatic optimum (ca. 17.6–15.4 Ma). Several periods of major ice growth with a grounding line traversing the drill site are recognized between ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma, with evidence of cold polar glaciers with ice shelves. The AND-2A core provides proximal evidence that during the middle Miocene climate transition, an ice sheet larger than the modern ice sheet was already present by ca. 14.7 Ma, ~1 m.y. earlier than generally inferred from deep-sea oxygen isotope records. These fi ndings highlight the importance of high-latitude ice-proximal records for the interpretation of far-fi eld proxies across major climate transitions

Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica

In 2007, the Antarctic Geological Drilling Program (ANDRILL) drilled 1138.54 m of strata ~10 km off the East Antarctic coast, includ ing an expanded early to middle Miocene succession not previously recovered from the Antarctic continental shelf. Here, we pre sent a facies model, distribution, and paleoclimatic interpretation for the AND-2A drill hole, which enable us, for the fi rst time, to reconstruct periods of early and middle Miocene glacial advance and retreat and paleo environmental changes at an ice-proximal site. Three types of facies associations can be recognized that imply signifi cantly different paleoclimatic interpretations. (1) A diamictite-dominated facies association represents glacially dominated depositional environments, including subglacial environments, with only brief intervals where ice-free coasts existed, and periods when the ice sheet was periodically larger than the modern ice sheet. (2) A stratified diamictite and mudstone facies association includes facies characteristic of open-marine to iceberg-infl uenced depositional environments and is more consistent with a very dynamic ice sheet, with a grounding line south of the modern position. (3) A mudstone-dominated facies association generally lacks diamictites and was produced in a glacially infl uenced hemipelagic depositional environment. Based on the distribution of these facies associations, we can conclude that the Antarctic ice sheets were dynamic, with grounding lines south of the modern location at ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma and during the Miocene climatic optimum, ca. 17.6–15.4 Ma, with ice-sheet and sea-ice minima at ca. 16.5–16.3 Ma and ca. 15.7–15.6 Ma. While glacial minima at ca. 20.1–19.6 Ma and ca. 19.3–18.7 Ma were characterized by temperate margins, an increased abundance of gravelly facies and diatomaceous siltstone and a lack of meltwater plume deposits suggest a cooler and drier climate with polythermal conditions for the Miocene climatic optimum (ca. 17.6–15.4 Ma). Several periods of major ice growth with a grounding line traversing the drill site are recognized between ca. 20.2 and 17.6 Ma, and after ca. 15.4 Ma, with evidence of cold polar glaciers with ice shelves. The AND-2A core provides proximal evidence that during the middle Miocene climate transition, an ice sheet larger than the modern ice sheet was already present by ca. 14.7 Ma, ~1 m.y. earlier than generally inferred from deep-sea oxygen isotope records. These fi ndings highlight the importance of high-latitude ice-proximal records for the interpretation of far-fi eld proxies across major climate transitions.Published2352-23651.8. Osservazioni di geofisica ambientale2.2. Laboratorio di paleomagnetismo3.8. Geofisica per l'ambienteJCR Journalreserve