Estella Weigelt

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.

Seismostratigraphy of the Siberian Sector of the Arctic Ocean and adjacent Laptev Sea Shelf

A new seismostratigraphic model has been established within the Arctic Ocean adjacent to the East Siberian Shelf on the basis of multichannel seismic reflection data acquired along a transect at 81°N. Ages for the sedimentary units were estimated via links to seismic lines and drill site data of the US Chukchi Shelf, the Lomonosov Ridge, and the adjacent Laptev Shelf. Two distinct seismic units were mapped throughout the area and are the constraints for dating the remaining strata. The lower marker unit, a pronounced high-amplitude reflector sequence (HARS), is the most striking stratigraphic feature over large parts of the Arctic Ocean. It indicates a strong and widespread change in deposition conditions. Probably, it developed during Oligocene times when a reorientation of Arctic Plates took place, accompanied by the gradual opening of the Fram Strait, and a widespread regression of sea level. The top of the HARS likely marks the end of Oligocene/early Miocene (23Ma). An age estimate for the base of the sequence is less clear but likely corresponds to base of Eocene (˜56Ma). The second marked unit detected on the seismic lines parallels the seafloor with a thickness of about 200ms two-way travel time (160 m). Its base is marked by a change from a partly transparent sequence with weak amplitude reflections below to a set of continuous high-amplitude reflectors above. This interface likely marks the transition to large-scale glaciation of the northern hemisphere and therefore is ascribed to the top Miocene (5.3 Ma).

Early Pliocene change of deposition style in the Cape Basin, southeastern Atlantic

Many effects of climate change, currents and tectonics on sedimentary deposition can be detected in seismic reflection patterns. To unravel the influence of these different forces on sediments of the southwest African margin, we established a seismostratigraphy, tied to the drilling results from Ocean Drilling Program Leg 175 Sites 1081–1087. Three depositional regions are distinguished for the Cape Basin: In the Northern and Middle Cape Basin, continuous high-amplitude reflectors characterize the upper acoustic sequences, indicating a strong, and probably climate-related, alternation in deposited matter, as well as an undisturbed deposition accompanied by high sedimentation rates due to enhanced upwelling. In the Middle Cape Basin, large reflection-free zones indicate the presence of gas. In contrast, in the Southern Cape Basin, less upwelling-related deposition restricted the thickness of sedimentary layers. Low-amplitude reflectors and slump scarps indicate a permanent reallocation of accumulated matter by mass movements on the steeper slope of the southern margin. A prominent feature in the sequences of the Northern and Middle Cape Basin is a marked increase in seismic reflection amplitudes since the Early Pliocene (ca. 4.4 Ma) in correspondence to a rise of cyclicity in reflector strength. We suggest a reorganization in the depositional system associated with enhanced upwelling as a consequence of the closure of the Central American Sea-way. The upwelling system probably became more susceptible to short pulse variations (∼10 k.y.) in climate and currents. With it the composition of deposited matter severely changed, which in turn is documented in the high-amplitude reflectors.

Exploring the long-term Cenozoic Arctic Ocean climate history: a challenge within the International Ocean Discovery Program (IODP)

The global climate evolution during Cenozoic times is characterized by the transformation from warm Paleogene oceans with low latitudinal and bathymetric thermal gradients into the more recent modes of circulation characterized by strong thermal gradients, oceanic fronts, cold deep oceans, and cold high-latitude surface waters. Our understanding of this long-term Cenozoic climate history is mainly based on the continuous and high-resolution records from the low and mid-latitudes, whereas records from the high latitudes, especially the high northern latitudes, are strongly limited. From the central Arctic Ocean, information is restricted to sedimentary sections recovered on Lomonosov Ridge during the single scientific drilling campaign of the Integrated Ocean Drilling Program (IODP) in 2004—the “Arctic Coring Expedition (ACEX).” By studying the unique ACEX sequence, a large number of scientific discoveries that describe previously unknown Arctic paleoenvironments have been obtained during the last decade. However, major key questions dealing with the Cenozoic climate history of the Arctic Ocean on its course from Greenhouse to Icehouse conditions remain unanswered. In this review paper, we present (1) the main highlights of the ACEX expedition and (2) why there is a need for further scientific Arctic drilling together with the plan, objectives and strategy for a drilling campaign on Lomonosov Ridge (“ACEX2”). ACEX2 is scheduled for 2018 as a mission-specific platform approach within the new International Ocean Discovery Program (IODP).