Gohl, Karsten
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Item type:Publication, Past interaction of ocean circulation and West Antarctic Ice Sheet dynamics in the Amundsen Sea SectorThe West Antarctic Ice Sheet is highly sensitive to ocean forcing and is currently experiencing grounding line retreat and ice shelf thinning. Since the response of the ice sheet to a warming world is unknown, it is important to analyze past warmer-than-present periods, e.g. the Pliocene, for comparison with possible future changes. Two drill sites, Site U1532 and Site U1533 of IODP Expedition 379, were linked to a large network of 2D high-resolution seismic reflection data to analyze changes of the West Antarctic Ice Sheet from the latest Miocene to the Pleistocene. During the Pliocene, between 4.2 to 3.2 Ma, a warm period was recognized with a highly dynamic West Antarctic Ice Sheet, correlating with reduced ocean bottom current activity. This period can be described as an overarching warm period with several advance and retreat phases of the West Antarctic Ice Sheet in the Amundsen Sea Sector.doctoral thesis209 177 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Geodynamic and Palaeobathymetric Reconstruction between Canada and Greenland(2013-04-24); ; ; The Baffin Bay and the Labrador Sea are sedimentary basins of Cretaceous to Eocene age, located between Canada and Greenland. They are separated by the bathymetric high of Davis Strait which - as a polar gateway limits the water transport between both. While oceanic crust has long been identified from magnetic spreading anomalies in the Labrador Sea, the crust of Baffin Bay and Davis Strait remained a subject of debate. To analyse the tectonic evolution of the area, research expeditions in 2008 and 2010 collected geophysical data. I here present P-wave velocity and density models along a 710-km-long line in southern Baffin Bay and a 315-km-long line across the Davis Strait. Seismic reflection and magnetic anomaly data support and compliment the models. Previous studies are confirmed: Baffin Bay is floored by oceanic crust, the Davis Strait by stretched continental crust. By plate kinematic modelling I developed the tectonic evolution of southern Baffin Bay and the Davis Strait area. I further present a palaeobathymetric reconstruction of the Davis Strait as polar gateway.doctoral thesis311 150 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Paleobathymetric Reconstruction, Modeled Ocean Circulation, and Sedimentation History in the Weddell Sea, Antarctica(2015-11-17); ; ; The Weddell Sea basin is of particular significance for understanding climate processes, including the generation of ocean water masses and their influences on ocean circulation as well as the Antarctic ice sheets dynamics. The sedimentary record, preserved in the basin serves as an archive of the pre-glacial to glacial development, ocean circulation and tectonic evolution. This thesis focuses on understanding the sedimentation history and reconstructing paleo-water depths, using all available multichannel seismic lines and existing drilling sites, with the aim to apply the paleo-water depths to General Circulation Models (GCM) of the Weddell Sea basin. A series of sedimentary thicknesses grids (pre-glacial, transitional, full-glacial) and paleobathymetric grids produced in this work are essential contributions for numerical climate simulations and ocean circulations. These sedimentary thickness grids allow the comparison of sedimentary regimes of the pre-glacially dominated and glacially dominated stages of Weddell Sea history. The pre-glacial deposition with thicknesses of up to 5 km was controlled by the tectonic evolution and sea-floor spreading history interacting with terrigenous sediment supply. The transitional unit shows a relatively high sedimentation rate and has thicknesses of up to 3 km, which may be attributed to an early formation of the East Antarctic Ice Sheet having partly advanced to the coast or even inner shelf. The main deposition centre of the full-glacial unit lies in front of the Filchner-Ronne Ice Shelf and has sedimentation rates of up to 140-200 m/Myr, which infers that ice sheets grounded on the middle to outer shelf and that bottom-water currents strongly impacted the deep-sea sedimentation in the middle Miocene. The paleobathymetric grids at 15, 34 and 120 Ma are reconstructed by using a backstripping technique and applied to constrain paleoclimate models. Coupled GCM runs are forced by global warm climatic boundary conditions of the Mid-Miocene and the new Weddell Sea paleobathymetry data. The GCM model results suggest that deep water formation and ocean circulation are especially sensitive to the paleobathymetric configuration of the Weddell Sea which is mainly characterized by a more southerly shelf break than at present or in previous paleobathymetric reconstructions for the Miocene. The southwards shifted shelf break of the Weddell Sea results in dramatic changes in simulated mixed layer depth and bottom water formation. Intensification of this bottom water plays a significant role in sediment distribution and the geomorphology of the Weddell Sea margin, e.g. through the build-up of a number of large sediment drifts. In addition to the paleobathymetric study of the Weddell Sea, I carried out two seismic interpretation studies in the southeast Weddell Sea and along the Dronning Maud Land margin. Large deposition centers, the Crary Trough Mouth Fan and prominent sediment ridges, are interpreted as glacial deposits of the southeastern Weddell Sea. Two giant, sinuous, NE-SW-oriented sediment ridges are interpreted as turbidity-contourites, due to the complicated down-slope/along-slope processes occurring across their margins. The large catchment area, abundant sediment supply, fluctuating sea level and ice sheet dynamics are the major contributions for the sedimentation. The remarkable increase in mass-transport deposits during the Late Miocene and Middle to Late Pliocene is related to the build-up of pore overpressure during rapid sediment accumulation as well as changing sea level and may be triggered by glacio-isostatic paleoearthquakes. Based on seismic reflection data and well data acquired on the continental margin offshore Dronning Maud Land, the sedimentation processes are investigated. My investigations reveal that the Jutul-Penck Graben system on the Dronning Maud Land plays a significant role in erosion, transport, and deposition of sedimentary material. I further found seismic chimney structures in this region for the first time and attribute their formation to volcanic processes.doctoral thesis386 157 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Geophysical investigation of the West Antarctic continental margin between Wrigley Gulf and the Amundsen Sea Embayment(2015-11-20); ; ; The aim of this thesis is the investigation of the Westantarcitc continental margin between Wrigely Gulf and the Amundsen Sea Embayment based on a geophysical dataset collected during two expedition of the research vessel Polarstern in 2006 and 2010. This study presents a compilation of 3D potential field forward modelling results inculding a set of seismic reflection and refraction profiles to investigate the lithospheric architecture and tectonic development of this poorly known part of Antarctica. The results presented in this study decipher the crustal structure and tectonic development in this area for the first time.doctoral thesis360 157 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, The crustal evolution of the Chatham Rise: Mid-Cretaceous Hikurangi Plateau collision and breakup between Zealandia and Antarctica(2020-03-02); ; ; The breakup of supercontinents is often associated with the changing polarity of tectonic forces from lithospheric convergence to lithospheric divergence. The initiation of the last supercontinent disintegration occurred simultaneously with the breakup of Gondwana. During the mid-Cretaceous, the East Gondwana margin underwent a remarkably fast transformation from a long-lived active subduction margin to a passive continental rifted margin, which led to the separation of southern Zealandia from West Antarctica. Recent studies suggest that the cessation of subduction and onset of extension in southern Zealandia was initiated by the collision and subduction of the thick oceanic Hikurangi Plateau with the East Gondwana subduction zone. However, little is known about the crustal structure of the Chatham Rise, east off New Zealand, although the Chatham Rise played a central role in change in tectonic forces. In particular, the nature of the southern Chatham Rise margin and the SE Chatham Terrace, an area of anomalously shallow seafloor hosting abundant seamounts and guyots, is poorly constrained. To investigate the role of the Hikurangi Plateau collision and subduction on the onset of extension and rifting in southern Zealandia, geophysical data including wide-angle reflection and refraction seismic, multi-channel seismic reflection, and potential field data were acquired during RV Sonne cruise SO246 in 2016. Geophysical data were collected along four profiles across two sub-provinces of the Chatham Rise, the SE Chatham Terrace and adjacent oceanic crust. P-wave velocity and gravity modelling of the new geophysical data yield insights into the crustal structure and therefore the breakup mechanism of the southern Chatham Rise margin, constrain the extent of the Hikurangi Plateau underthrusted beneath the Chatham Rise, and enhance our understanding of the driving forces behind the abrupt change from subduction to rifting along the East Gondwana margin. Along the Chatham Rise, the P-wave velocity models highlight distinct differences in the crustal thickness between the eastern and western sub-provinces, but also reveal common characteristics in crustal composition. The crust of the western Chatham Rise is up to 25 km thick, whereas the eastern Chatham Rise is substantially thinner (14-18 km). Modelled P-wave velocities and densities suggest a similar geology for both parts. The Chatham Rise mainly consists of greywackes, meta-greywackes, and schist in the upper crust, and their high-temperature equivalents in the lower crust. This is consistent with its past position at an active continental margin. Seismic imaging and gravity data show that the 10-16 km thick Hikurangi Plateau is restricted to the lower crust of the western Chatham Rise. The geophysical data suggest that the Hikurangi Plateau does not reach as far south as previously proposed. Furthermore, southward thinning of the lower crustal layer along the westernmost profile, together with previously published data, indicates that a piece of the subducted oceanic Phoenix Plate is still present below the Chatham Rise and southern Zealandia. The crustal thickness of the SE Chatham Terrace varies between 5 and 8 km, which can be correlated to slightly thinner or thicker than Pacific oceanic crust (~6 km thickness). The velocity structure can be interpreted as similar to Pacific oceanic crust, but at the same time also shows characteristics of hyper-extended continental crust. Since graben structures are present, I interpret the SE Chatham Terrace as a broad continent-ocean transition zone, which consists of very thin continental crust modified by magmatic activity. Typical Pacific oceanic crust has been only found close to the easternmost Chatham Rise and is presumably not older than 88 Ma. The Pacific oceanic crust is separated from the Chatham Rise by a highly faulted area, which I interpret as exhumed lower continental crust. High-velocity lower crust (VP > 7 km/s) has been identified along the eastern Chatham Rise and at the easternmost Chatham Rise. These two areas of high-velocity lower crust are interpreted as magmatic underplating and intrusions. Seaward-dipping reflector sequences typical of volcanic-rifted margins are completely absent along the southern Chatham Rise margin. Moreover, the southern Chatham Rise margin is largely fault-controlled, but the geophysical data do not support the presence of exhumed and serpentinised upper mantle, which is typical for magma-poor margins. On this basis, I interpret the southern Chatham Rise margin as a unique hybrid rifted margin, which shows features typical of both, volcanic-rifted and magma-poor margins. Based on these observations, I developed a tectonic model that explains the multi-stage tectonic evolution of the southern Chatham Rise margin. Accordingly, the Hikurangi Plateau entered the subduction zone at ~110 Ma. Subsequently, convergence velocities slowed down until subduction ceased at ~100 Ma. The thicker crust of the western Chatham Rise is a result of the subduction and underthrusting of the Hikurangi Plateau, which most likely attenuated subsequent crustal extension along the western Chatham Rise. Slowing subduction in the sector of the Hikurangi Plateau led to development of subduction-transform edge propagator (STEP) faults on both sides of the plateau after 110 Ma. I suggest that the Hikurangi Plateau collision, together with fragmentation of the Phoenix Plate by these STEP faults, triggered and / or contributed to the previously hypothesized global-scale plate reorganisation event between 105 and 100 Ma. At the same time, rifting and crustal extension in southern Zealandia started. The rifting in southern Zealandia and the evolution of the southern Chatham Rise were likely the result of complex slab dynamics triggered by the Hikurangi Plateau subduction. First, rifting was initiated by shallowing of the subducted slab due to the higher buoyancy of the young and thick Hikurangi Plateau. Initial extension was oblique to the margin and arc, and led to the reactivation of former arc-parallel E-W thrust faults as normal faults. With prolonged extension, new generations of NE-SW normal faults started to form, and lower crust was exhumed along the easternmost tip of the Chatham Rise was initiated. Secondly, progressive eclogitisation of the land-ward Phoenix Plate slab is likely to have caused the slab to rollback after convergence ceased. This led to a prolonged episode of rifting during which extension focussed on the southern Chatham Rise margin (i.e. the SE Chatham Terrace and Bounty Trough). Finally, the style of extension changed after most of the Phoenix Plate slab became detached at around 90 Ma. The slab detachment opened a pathway for deep-seated and hot upwelling mantle, which resulted in (I) intrusions and magmatic underplating, (II) formation of the first oceanic crust along the easternmost tip of the Chatham Rise, (III) alkaline magmatism on the Chatham Island between 85 and 82 Ma and (IV) magmatic overprint of the SE Chatham Terrace leading to seamount formation. After 85 Ma, spreading segments became connected and the formation of the young Pacific-Antarctic Ridge led to the final separation of Zealandia from Antarctica.doctoral thesis703 276