Tectonomorphic evolution of Marie Byrd Land - Implications for Cenozoic rifting activity and onset of West Antarctic glaciation

The West Antarctic Rift System is one of the largest continental rifts on Earth. Because it is obscured by the West Antarctic Ice Sheet, its evolution is still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land, an area that stretches ~ 1000 km along the rift system, in order to shed light on its development. Furthermore, we petrographically analysed glacially transported detritus deposited in the marine realm, offshore Marie Byrd Land, to augment the data available from the limited terrestrial exposures. Our data provide information about the subglacial geology, and the tectonic and morphologic history of the rift system. Dominant lithologies of coastal Marie Byrd Land are igneous rocks that intruded (presumably early Paleozoic) low-grade meta-sedimentary rocks. No evidence was found for un-metamorphosed sedimentary rocks exposed beneath the ice. According to the thermochronology data, rifting occurred in two episodes. The earlier occurred between ~ 100 and 60 Ma and led to widespread tectonic denudation and block faulting over large areas of Marie Byrd Land. The later episode started during the Early Oligocene and was confined to western Pine Island Bay area. This Oligocene tectonic activity may be linked kinematically to previously described rift structures reaching into Bellingshausen Sea and beneath Pine Island Glacier, all assumed to be of Cenozoic age. However, our data provide the first direct evidence for Cenozoic tectonic activity along the rift system outside the Ross Sea area. Furthermore, we tentatively suggest that uplift of the Marie Byrd Land dome only started at ~ 20 Ma; that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of continental West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data, although only based on few samples, imply that extensive glaciation of this part of West Antarctica may have only started since the early Miocene

A community-based geological reconstruction of Antarctic Ice Sheet deglaciation since the Last Glacial Maximum

A robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20ka, 15ka, 10ka and 5ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorit. © 2014 The Authors

Exhumation history along the eastern Amundsen Sea coast, West Antarctica, revealed by low-temperature thermochronology

West Antarctica experienced a complex tectonic history, which is still poorly documented, in part due to extensive ice cover. Here we reconstruct the Cretaceous to present thermotectonic history of Pine Island Bay area and its adjacent coasts, based on a combination of apatite and zircon fission track and apatite (U-Th-Sm)/He thermochronology. In addition, we report petrographic information for the catchments of Pine Island, Thurston Island, and Thwaites glaciers. Our data suggest that the underlying bedrock of the Pine Island and Thwaites Glacier catchments are very different and vary from granitoids to (Cenozoic?) volcanogenic sequences and low-grade metamorphics. Our thermochronology data show that the upper crustal rocks of Pine Island Bay experienced very rapid cooling during the late Cretaceous. We attribute this rapid cooling of basement rocks and associated reduction in mean elevation to tectonic denudation driven by gravitational collapse of the Cretaceous orogen along the proto-Pacific Gondwana margin. Rapid Cretaceous crustal cooling was followed by very slow cooling during the Cenozoic, with no erosional response—within the limits of thermochronological methods—to the onset of glaciation and subsequent climatic changes. Cenozoic rifting within the West Antarctic Rift appears to have had little effect on erosion processes around Pine Island Bay; instead, our data suggest Cenozoic crustal tilting toward Pine Island Trough, a major geomorphic feature previously suggested to be a branch of the rift system

Die tektonische und glaziale Geschichte des Amundsen Sektors, Westantarktis

West Antarctica has gone through major tectonic changes beginning in the Cretaceous (145 - 66 Ma). With the West Antarctic Rift System, the area also holds one of the largest continental rifts known on Earth. However, the West Antarctic Rift System is almost completely buried beneath West Antarctic Ice Sheet, as a result details of its geodynamic history are sparse and its tectonic evolution is still poorly understood. Furthermore, with the 'Amundsen Sector', West Antarctica hosts one of the most rapidly changing parts of the West Antarctic Ice Sheet. With the fastest flowing ice streams in Antarctica, especially the eastern areas are characterized by rapid ice sheet thinning and grounding-line retreat. Like data on West Antarctica's geodynamics, constraints on the deglacial history are limited to either marine sedimentary data or a few isolated nunataks. Thus it is difficult to assess the long-term deglacial history of the area, too. In this study we reconstructed the shallow crustal (~15 - 1 km) evolution of the ~1000 km long Amundsen Sector of West Antarctica and alongside the rift system. Thereby we applied three lowtemperature thermochronology analysis: apatite (U-Th-Sm)/He, apatite fission track and zircon fission track, and combined their results in inverse thermal history models. We further utilized detrital information on the dominantly ice-covered hinterland through the analysis of ice rafted debris extracted from marine sediments. Finally, to address the glacial retreat after the Last Glacial Maximum (ca. 23 - 19 ka) we applied 10Be-surface exposure dating in two key areas onshore of the eastern and mid Amundsen Sector, namely Pine Island Bay and the Kohler Range. Together, all three thermochronology systems yield early Cretaceous to early Miocene ages (121 - 18 Ma), with a dominant cluster in the mid- to late Cretaceous. Zircon fission track data ranges between 108 - 80 Ma, apatite fission track between 121 - 28 Ma, with narrow track lengths distributions and mean track lengths of 14.9 - 13.1 um, apatite (U-Th-Sm)/He mean ages between 94 and 18 Ma. Dominant (subglacial) lithologies of the coastal Amundsen Sector are igneous rocks that presumable intruded into low-grade meta-sediments of early Paleozoic times, and from our data there is no indication for un-metamorphosed sedimentary bedrock beneath the ice. Thermal history modeling identifies two major cooling stages: beginning with a period of rapid cooling during the Cretaceous, which was replaced by a stage of relative tectonic stability and rather slow cooling in the Cenozoic. This phase of strong cooling is also present in the detrital data, indicating that the majority of the Amundsen Sector was strongly affected by the extension phase along the modern northern margin of the West Antarctic. The extension was part of the Gondwana break-up and ultimately lead to the separation of West Antarctica - New Zealand. Post-Cretaceous activity appears to be limited to block tilting along the Pine Island glacial trough and an isolated Miocene cooling signal within the mid Amundsen Sector, which is interpreted to reflect the exhumation of a fault-bound horst structure separated through fault zones that are resultant of or directly linked to activity in the West Antarctic Rift System. Apart from that our data implies a rather stable tectonic setting and shows no strong indication of significant glacial erosion for most of the Cenozoic. The 10Be surface-exposure ages indicate that the sampled nunataks in the Kohler Range are ice-free since about 13 and 9 ka, respectively. Exposure ages from Pine Island Bay range from 8 ka close to the Pine Island glacial trough to 9 ka from a more coastal island. This implies a general Holocene retreat and minimum long-term average thinning rates of ~3.3 cm/yr are one order of magnitude lower than recent rates based on satellite data. Our results thereby provide insights into the significance of local ice-sheet variations and suggest that the post-Last Glacial Maximum history in the Amundsen Sector was characterized by glacial thinning as well as lateral retreat in pre- to early Holocene times

Prognostische Wertigkeit konnektivitätsbasierter Parameter für das motorische Outcome der Handfunktion bei subakuten Schlaganfallpatienten

Der Schlaganfall ist eine der weltweit führenden Ursachen für bleibende Behinderungen. Insbesondere Lähmungen der oberen Extremität führen zu weitreichenden Einschränkungen im täglichen Leben, sodass der Erholung der motorischen Funktion eine große Bedeutung für das weitere soziale und/oder berufliche Leben zukommt. Daher ist es notwendig geeignete prognostische Parameter zu finden, die eine Vorhersage der Erholung der motorischen Funktion ermöglichen. Das Ziel dieser Arbeit bestand darin die prognostische Bedeutung von klinischen Parametern, strukturellen und funktionellen Konnektivitätsparameter für das motorische Outcome der Handfunktion bei subakuten Schlaganfallpatienten zu untersuchen. Hierzu wurde bei 17 subakuten Schlaganfallpatienten die funktionelle Konnektivität mittels Resting-State-fMRT für verschiedene Verbindungen (M1il-M1kl, M1il–dPMCkl, M1il–dPMCil, M1il–SMAkl, M1il- Cbkl) sowie die strukturelle Konnektivität mittels probabilistischer Traktographie für verschiedene Verbindungen (M1il-M1kl, M1il-Ponsil, M1il-Cbkl) im subakutem Stadium ermittelt. Zudem wurden ebenfalls motorische (Nine-Hole-Peg-Test und Griffkraft) sowie klinische Parameter (NIHSS) erfasst. Anschließend wurde mittels einer schrittweisen, multiplen Regressionsanalyse überprüft inwieweit sich die Parameter eignen, um das motorische Outcome der Patienten, gemessen mit dem Motricity Index und dem Box-and- Block-Test, nach 3 und 6 Monaten vorherzusagen. Die Untersuchung ergab eine hohe prognostische Relevanz der initialen Griffkraft für den kraftassoziierten Parameter Motricity Index nach 3 und nach 6 Monaten. Zudem zeigte sich eine prognostische Relevanz der strukturellen Konnektivitätsparameter zwischen M1il-Cbkl für den Motricity Index nach 6 Monaten sowie zwischen M1il-M1kl für den Box-and-Block Test nach 3 und 6 Monaten. Für die funktionellen Konnektivitätsparameter fand sich kein prognostischer Nutzen. Zusammenfassend hat sich für das Patientenkollektiv der Studie mit vorrangig leicht bis moderat betroffenen Patienten ergeben, dass sowohl die Griffkraft als klinischer Test sowie die strukturellen Konnektivitätsparameter einen prognostischen Wert für das motorische Outcome der Handfunktion haben, jedoch lohnt sich der Aufwand der Konnektivitätsanalyse nicht, da sich daraus keine exaktere Prognose ergibt.Background: Connectivity-based predictions of hand motor outcome have been proposed to be useful in stroke patients. We intended to assess the prognostic value of different imaging methods on short-term (3 months) and long-term (6 months) motor outcome after stroke. Methods: We measured resting state functional connectivity (rsFC), diffusion weighted imaging (DWI) and grip strength in 19 stroke patients within the first days (5–9 days) after stroke. Outcome measurements for short-term (3 months) and long-term (6 months) motor function was assessed by the Motricity Index (MI) of the upper limb and the box and block test (BB). Patients were predominantly mildly affected since signed consent was necessary at inclusion. We performed a multiple stepwise regression analysis to compare the predictive value of rsFC, DWI and clinical measurements. Results: Patients showed relevant improvement in both motor outcome tests. As expected grip strength at inclusion was a predictor for short- and long-term motor outcome as assessed by MI. Diffusion-based tract volume (DTV) of the tracts between ipsilesional primary motor cortex and contralesional anterior cerebellar hemisphere showed a strong trend (p = 0.05) for a predictive power for long-term motor outcome as measured by MI. DTV of the interhemispheric tracts between both primary motor cortices was predictive for both short- and long-term motor outcome in BB. rsFC was not associated with motor outcome. Conclusions: Grip strength is a good predictor of hand motor outcome concerning strength-related measurements (MI) for mildly affected subacute patients. Therefore additional connectivity measurements seem to be redundant in this group. Using more complex movement recruiting bilateral motor areas as an outcome parameter, DTV and in particular interhemispheric pathways might enhance predictive value of hand motor outcome

Fire and Ice - Tectonic and glacial history of the Amundsen Sector, West Antarctica

West Antarctica has gone through major tectonic changes beginning in the Cretaceous (145 - 66 Ma). With the West Antarctic Rift System, the area also holds one of the largest continental rifts known on Earth. However, the West Antarctic Rift System is almost completely buried beneath West Antarctic Ice Sheet, as a result details of its geodynamic history are sparse and its tectonic evolution is still poorly understood. Furthermore, with the 'Amundsen Sector', West Antarctica hosts one of the most rapidly changing parts of the West Antarctic Ice Sheet. With the fastest flowing ice streams in Antarctica, especially the eastern areas are characterized by rapid ice sheet thinning and grounding-line retreat. Like data on West Antarctica's geodynamics, constraints on the deglacial history are limited to either marine sedimentary data or a few isolated nunataks. Thus it is difficult to assess the long-term deglacial history of the area, too. In this study we reconstructed the shallow crustal (~15 - 1 km) evolution of the ~1000 km long Amundsen Sector of West Antarctica and alongside the rift system. Thereby we applied three lowtemperature thermochronology analysis: apatite (U-Th-Sm)/He, apatite fission track and zircon fission track, and combined their results in inverse thermal history models. We further utilized detrital information on the dominantly ice-covered hinterland through the analysis of ice rafted debris extracted from marine sediments. Finally, to address the glacial retreat after the Last Glacial Maximum (ca. 23 - 19 ka) we applied 10Be-surface exposure dating in two key areas onshore of the eastern and mid Amundsen Sector, namely Pine Island Bay and the Kohler Range. Together, all three thermochronology systems yield early Cretaceous to early Miocene ages (121 - 18 Ma), with a dominant cluster in the mid- to late Cretaceous. Zircon fission track data ranges between 108 - 80 Ma, apatite fission track between 121 - 28 Ma, with narrow track lengths distributions and mean track lengths of 14.9 - 13.1 um, apatite (U-Th-Sm)/He mean ages between 94 and 18 Ma. Dominant (subglacial) lithologies of the coastal Amundsen Sector are igneous rocks that presumable intruded into low-grade meta-sediments of early Paleozoic times, and from our data there is no indication for un-metamorphosed sedimentary bedrock beneath the ice. Thermal history modeling identifies two major cooling stages: beginning with a period of rapid cooling during the Cretaceous, which was replaced by a stage of relative tectonic stability and rather slow cooling in the Cenozoic. This phase of strong cooling is also present in the detrital data, indicating that the majority of the Amundsen Sector was strongly affected by the extension phase along the modern northern margin of the West Antarctic. The extension was part of the Gondwana break-up and ultimately lead to the separation of West Antarctica - New Zealand. Post-Cretaceous activity appears to be limited to block tilting along the Pine Island glacial trough and an isolated Miocene cooling signal within the mid Amundsen Sector, which is interpreted to reflect the exhumation of a fault-bound horst structure separated through fault zones that are resultant of or directly linked to activity in the West Antarctic Rift System. Apart from that our data implies a rather stable tectonic setting and shows no strong indication of significant glacial erosion for most of the Cenozoic. The 10Be surface-exposure ages indicate that the sampled nunataks in the Kohler Range are ice-free since about 13 and 9 ka, respectively. Exposure ages from Pine Island Bay range from 8 ka close to the Pine Island glacial trough to 9 ka from a more coastal island. This implies a general Holocene retreat and minimum long-term average thinning rates of ~3.3 cm/yr are one order of magnitude lower than recent rates based on satellite data. Our results thereby provide insights into the significance of local ice-sheet variations and suggest that the post-Last Glacial Maximum history in the Amundsen Sector was characterized by glacial thinning as well as lateral retreat in pre- to early Holocene times

Where is the West Antarctic Rift System in the Amundsen Sea and Bellingshausen Sea sectors?

The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm. The current model, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula. We present the range of existing hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for the eastern West Antarctica with implications for glacial onset and developments

Thermotectonic And Geomorphic Evolution Of Marie Byrd Land And The Pine Island Bay Area

Due to extensive glacial cover, the evolution or the West Antarctic Rift System (WARS) and the proto- Pacific margin of Gondwana are still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land and the Pine Island Bay area. The goal of our study is to decipher the long-term thermotectonic evolution of this area, infer its denudation history, and to provide estimates for paleotopography and potential links to the glaciation history. Our data show that during early and mid-Cretaceous subduction along the proto-Pacific margin, all of Marie Byrd Land and the Pine Island Bay area experienced rapid exhumation. This rapid exhumation continued for about 25 Myr after subduction stopped, presumably driven by tectonic denudation related to continental breakup between Zealandia and West Antarctica and rifting activity of the WARS. This late Cretaceous extension period was related to rapid topography reduction, as expressed by the formation of the West Antarctic erosion surface close to sea level, and was probably related to free-boundary gravitational collapse of the Gondwanide orogen. By ~60 Ma, rapid exhumation stopped, which we interpret as cessation of the first WARS rifting period. Cretaceous rapid exhumation was followed by very low exhumation rates throughout the Cenozoic, which we explain by tectonic quiescence and subdued topography. After ~30 Ma, and restricted to the western Pine Island Bay area, rapid exhumation resumed, presumably coeval with large-scale crustal tilting of the eastern Pine Island Bay area towards the Pine Island trough. We interpret this as indicating renewed activity of the WARS, (i) suggesting that Cenozoic rifting activity was much more localized than Cretaceous rifting, and (ii) corroborating previous assumptions that the WARS branches from the continental interior into the Amundsen Sea. Our structural model, based on the thermochronology data, kinematically links the rift branches reaching into the Bellingshausen and Amundsen Seas with the Byrd Subglacial Basin and the Bentley Subglacial Trench, requiring dextral transtension for the Bellingshausen Sea area and sinistral transtension for the Amundsen Sea area. Furthermore, our data suggest that enhanced denudation along the flanks of the Marie Byrd Land dome and thus presumably its uplift only started at ~20 Ma, that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of continental West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data imply that initiation of extensive glaciation of this part of West Antarctica may only have started since the early Miocene

The extent of the West Antarctic Rift System in the Amundsen Sea and Bellingshausen Sea sectors

The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm and in the eastern Marie Byrd Land, Ellsworth Land, Thurston Island and Antarctic Peninsula crustal blocks. One of the current models, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula, basically following the eastward migrating collision of the Phoenix Plate with the Antarctic Plate. We present the range of existing and novel hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for West Antarctica with implications for glacial onset and developments