Gabriele Uenzelmann-Neben

Lomonosov Ridge—A double-sided continental margin

The first two traverses of marine multichannel seismic data across the Lomonosov Ridge (central Arctic), by the German research icebreaker Polarstern and the Swedish icebreaker Oden , demonstrate a prograded margin toward the Amerasian side and fault-bounded half grabens toward the Eurasian side of the ridge. Nearly 450 m of undisturbed flat-lying strata have been deposited on top of the peneplaned ridge since it rifted from the Barents-Kara Sea margin and subsided below sea level in early Tertiary time (64-56 Ma).

Neogene sedimentation history of the Congo Fan

Abstract High-resolution seismic reflection data were interpreted to develop a model for the Neogene sedimentation in the Congo Fan area. Identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the Early and the Late Quaternary. A single sediment source system during the Late Paleogene⧹Early Neogene gradually changes into a two sediment source system in the Late Neogene. Since the Late Quaternary, the Congo River has been the prevailing sediment source. The evolving Benguela Current further modified the system and led to a high biogenic productivity. Evidence for gas in the sediments can be found in the whole area of investigation. Additionally, indications for a tectonic control of the Congo Canyon have been found in the distribution of sedimentary units and unit thicknesses.

The southern Weddell Sea: combined contourite-turbidite sedimentation at the southeastern margin of the Weddell Gyre

Abstract Sedimentary processes in the southeastern Weddell Sea are influenced by glacial-interglacial ice-shelf dynamics and the cyclonic circulation of the Weddell Gyre, which affects all water masses down to the sea floor. Significantly increased sedimentation rates occur during glacial stages, when ice sheets advance to the shelf edge and trigger gravitational sediment transport to the deep sea. Downslope transport on the Crary Fan and off Dronning Maud and Coats Land is channelized into three huge channel systems, which originate on the eastern, the central and the western Crary Fan. They gradually turn from a northerly direction eastward until they follow a course parallel to the continental slope. All channels show strongly asymmetric cross sections with well-developed levees on their northwestern sides, forming wedge-shaped sediment bodies. They level off very gently. Levees on the southeastern sides are small, if present at all. This characteristic morphology likely results from the process of combined turbidite-contourite deposition. Strong thermohaline currents of the Weddell Gyre entrain particles from turbidity-current suspensions, which flow down the channels, and carry them westward out of the channel where they settle on a surface gently dipping away from the channel. These sediments are intercalated with overbank deposits of high-energy and high-volume turbidity currents, which preferentially flood the left of the channels (looking downchannel) as a result of Coriolis force. In the distal setting of the easternmost channel-levee complex, where thermohaline currents are directed northeastward as a result of a recirculation of water masses from the Enderby Basin, the setting and the internal structures of a wedge-shaped sediment body indicate a contourite drift rather than a channel levee. Dating of the sediments reveals that the levees in their present form started to develop with a late Miocene cooling event, which caused an expansion of the East Antarctic Ice Sheet and an invigoration of thermohaline current activity.

Seismic characteristics of sediment drifts: An example from the Agulhas Plateau, southwest Indian Ocean

Sediment drifts provide information on the palaeoceanographic development of a region. Additionally, they may represent hydrocarbon reservoirs. Because of this, sediment drift investigation has increased over the last few years. Nevertheless, a number of problems remain regarding the processes controlling their shape, the characteristic lithological and seismic patterns and the diagnostic criteria. As an example, sediment drifts from the Agulhas Plateau, southwest Indian Ocean, are presented here. They show a variety of seismic features and facies including an asymmetric mounded geometry, changes in internal reflection pattern, truncation of internal reflectors at the seafloor and discontinuities. This collection of observations in combination with the local oceanography appears to comprise a diagnostic tool for sediment drifts.

West Antarctic ice sheet change since the Last Glacial Period

The potential for rapid deglaciation, or collapse, of the 2–million–square–kilometer West Antarctic Ice Sheet (WAIS) in response to climate change is one of the most serious environmental threats facing mankind. The WAIS is a marine ice sheet with large parts of its ice grounded below sea level. Complete collapse would result in a global sea level rise of approximately 5 meters, with immense social, economic, and ecological consequences.

A seismic reconnaissance survey of the northern Congo Fan

Abstract To study the time-varying influence of the Congo River and the Benguela Current on the deposition at the Angola Continental Margin, a high-resolution reflection seismic survey was carried out on the northern Congo Fan. Four seismostratigraphic units have been defined for the upper 800 m (1000 ms TWT) of the data. The units record different depositional environments, ranging from pre-establishment of the Congo River drainage system to the influence of the Benguela Current. An indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material. The ascent of salt is recorded up to the Pliocene. Gas that has migrated out of Lower Cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes. In a few places, this gas even ascends to the ocean floor along structural pathways through the topmost unit.

Agulhas Plateau, SW Indian Ocean: New evidence for excessive volcanism

A new set of seismic reflection and refraction lines has been interpreted regarding the basement and crustal structure of the southern Agulhas Plateau. A large number of extrusion centres were identified. Lava flows dip away from those extrusion centres and form subparallel-stratified sequences. We interpret those extrusion centres as the result of excessive volcanism in course of the separation of the southern Agulhas Plateau from the Maud Rise. Since the sedimentary layers appear to be little affected by the volcanism, that episode obviously ceased before onset of sedimentation in Late Cretaceous times. We have not found evidence for continental fragments within overthickened, predominantly oceanic crust. We therefore propose that the Agulhas Plateau belongs to the world-wide suite of Large Igneous Provinces (LIP) of predominantly oceanic origin.

Sediment deposits in the Cape Basin: Indications for shifting ocean currents?

The Benguela Current system, running off southwest Africa, is one of the worlds largest upwelling regions. The current has strongly influenced sedimentary features on the continental margin. To unravel its development, seismic stratigraphy, tied to drilling results from Ocean Drilling Program Leg 175 sites 10851087, was established. Four units, Southern Cape Basin (SCB)-1 to SCB-4, were defined for the Cenozoic sediments. The upper unit, SCB-1 (1.5 Ma), characterized by continuous high-amplitude reflectors, represents global cooling and glacial-interglacial cycles. Unit SCB-2 (14 Ma), distinguished by low-amplitude reflections, is associated with the onset of the upwelling system and establishment of the modern circulation pattern in the Cape Basin. Slump scarps are concentrated along the middle and upper shelf slope, suggesting they are caused by a combination of mass movements triggered by bottom currents and slope instabilities because of increased deposition associated with the upwelling. A westward extension and/or movement of upwelling filaments is interpreted from the observed seaward shift of scarp locations with time. Erosion associated with stronger currents probably thinned unit SCB-2 in the south. The two lower units, SCB-3 and SCB-4 (56 Ma), probably represent material eroded from the shelf break and deposited during a major Oligoceneearly Miocene regression that is consistent with a significant uplift of southern Africa. The basal reflector SCB-D of unit SCB-4 is associated with the prominent reflector D or L described in previous publications.

Seismic evidence for long-term history of glaciation on central East Greenland shelf south of Scoresby Sund

A seismic reflection transect crossing the central East Greenland continental margin south of the Scoresby Sund fjord system provides information regarding the long-term history of expansion and retreat of the Inland Ice. The shelf and slope sediments can be divided into three first-order units; the upper unit is interpreted to have a glacial origin. Within the up to 1000-m-thick glacial unit, six sequences were identified, representing at least as many phases of extensive ice sheet grounding on the shelf. Varying amounts of progradation and aggradation probably reflect successive phases in the glacial evolution of the region.

Playing jigsaw with Large Igneous Provinces—A plate tectonic reconstruction of Ontong Java Nui, West Pacific

Ontong Java Nui is a Cretaceous large igneous province (LIP), which was rifted apart into various smaller plateaus shortly after its emplacement around 125 Ma in the central Pacific. It incorporated the Ontong Java Plateau, the Hikurangi Plateau and the Manihiki Plateau as well as multiple smaller fragments, which have been subducted. Its size has been estimated to be approximately 0.8% of the Earth’s surface. A volcanic edifice of this size has potentially had a great impact on the environment such as its CO2 release. The break-up of the “Super”-LIP is poorly constrained, because the break-up and subsequent seafloor spreading occurred within the Cretaceous Quiet Period. The Manihiki Plateau is presumably the centerpiece of this “Super”-LIP and shows by its margins and internal fragmentation that its tectonic and volcanic activity is related to the break-up of Ontong Java Nui. By incorporating two new seismic refraction/wide-angle reflection lines across two of the main sub-plateaus of the Manihiki Plateau, we can classify the break-up modes of the individual margins of the Manihiki Plateau. The Western Plateaus experienced crustal stretching due to the westward motion of the Ontong Java Plateau. The High Plateau shows sharp strike-slip movements at its eastern boundary towards an earlier part of Ontong Java Nui, which is has been subducted, and a rifted margin with a strong volcanic overprint at its southern edges towards the Hikurangi Plateau. These observations allow us a re-examination of the conjugate margins of the Hikurangi Plateau and the Ontong Java Plateau. The repositioning of the different plateaus leads to the conclusion that Ontong Java Nui was larger (~1.2% of the Earth’s surface at emplacement) than previously anticipated. We use these finding to improve the plate tectonic reconstruction of the Cretaceous Pacific and to illuminate the role of the LIPs within the plate tectonic circuit in the western and central Pacific.