Sebastian Lindhorst

Monsoon-induced partial carbonate platform drowning (Maldives, Indian Ocean)

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fields. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensification of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

Periplatform drift: The combined result of contour current and off-bank transport along carbonate platforms

Hydroacoustic and sedimentological data from the western leeward flank of the Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion, which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward-thinning wedge of the periplatform drift at the foot of the bank escarpment displays along-slope and downslope variations in sedimentary architecture. Sediments are muddy carbonate sands that coarsen basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth-related facies belts develop, whereas strike-discontinuous sediment lobes, scarps, and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration.

Distribution and characteristics of marine habitats in a subpolar bay based on hydroacoustics and bed shear stress estimates - Potter Cove, King George Island, Antarctica

Marine habitats worldwide are increasingly pressurized by climate change, especially along the Antarctic Peninsula. Well-studied areas in front of rapidly retreating tidewater glaciers like Potter Cove are representative for similar coastal environments and, therefore, shed light on habitat formation and development on not only a local but also regional scale. The objective of this study was to provide insights into habitat distribution in Potter Cove, King George Island, Antarctica, and to evaluate the associated environmental processes. Furthermore, an assessment concerning the future development of the habitats is provided. To describe the seafloor habitats in Potter Cove, an acoustic seabed discrimination system (RoxAnn) was used in combination with underwater video images and sediment samples. Due to the absence of wave and current measurements in the study area, bed shear stress estimates served to delineate zones prone to sediment erosion. On the basis of the investigations, two habitat classes were identified in Potter Cove, namely soft-sediment and stone habitats that, besides influences from sediment supply and coastal morphology, are controlled by sediment erosion. A future expansion of the stone habitat is predicted if recent environmental change trends continue. Possible implications for the Potter Cove environment, and other coastal ecosystems under similar pressure, include changes in biomass and species composition.

The climate-archive dune: Sedimentary record of annual wind intensity

Active dunes contain an unread high-resolution archive of past wind strength. Variations in the grain-size spectrum of an active dune through time are compared with historical time series of wind speed. Annually averaged, sediment-derived wind intensity and instrumental records of wind speed show a correlation as high as 0.75. The potential of eolian dunes to gain long-term data series of wind intensity in areas and for time periods lacking an instrumental record is demonstrated by reconstructing 20th century wind-intensity variations in the southern North Sea area. The approach can be used in both recent and fossil dune systems. Potential applications include the validation of climate models, the reconstruction of supraregional wind systems, and the monitoring of future shifts in the climate system.

Reef slope geometries and facies distribution: controlling factors (Messinian, SE Spain)

Abstract Sea-level fluctuations and changes in sediment grain size are widely thought to be the main factors controlling carbonate platform slope geometries. Two successive clinoform bodies from the Upper Miocene Cariatiz carbonate platform (SE Spain) were selected to analyze geometry and facies distribution in relation to sea-level oscillations. Facies occurring in these clinoform bodies are from top to bottom reef-framework, reef-framework debris, Halimeda breccia, Halimeda rudstone, and bioclastic packstone, as well as siltstone and marl. Slope geometry and facies, composition, and distribution, are significantly different in each clinoform body. These differences are the result of the interaction of several factors such as coral growth, in situ slope carbonate production, rockfalls and sediment gravity flows, hemipelagic rain, reworking of reef-slope facies and siliciclastic input. Changes in accommodation were related to sea-level fluctuations and controlled the relative impact of these factors. A sea-level fall took place in the time between deposition of the selected clinoform bodies and changed the hydrographical conditions of the basin. These changes influenced the presence of Halimeda and the grain-size distribution, and consequently the slope geometries. Reef-slope geometry is not exclusively controlled by changes in grain size. The stabilization by organic binding is proposed to be a significant factor controlling the slope deposition.

Lowstand wedges in carbonate platform slopes (Quaternary, Maldives, Indian Ocean)

Seismic, hydroacoustic and sedimentological data were used to analyse the response of atoll‐slope sedimentation in the Maldives to the late Quaternary sea‐level change. The slope deposits, as imaged in multichannel seismic profiles, are arranged into stacked aggrading to backstepping basinward thinning wedges. In a piston core recovered at the lower slope of one of the atolls, the sediment texture ranges from packstone to rudstone. Major components are blackened bioclasts, the large benthic foraminifers Operculina and Amphistegina, together with Halimeda debris and red algae. Radiocarbon dating at a core depth of 66 cm indicates that the wedge sedimentation stopped or was largely reduced after 16 ka BP. Therefore, the atoll‐slope deposits largely consist of sediment formed in situ and deposited during the last glacial lowstand in sea‐level. This is in apparent contradiction to the concept of highstand shedding of tropical carbonate platforms, which requires slope sedimentation during sea‐level highstands, when the platform is flooded. Rather than intrinsic factors, such as sediment bypass along the steep slope, the extrinsic process of current winnowing of the slope appears to be a major controlling factor in the production of this feature. This process may be relevant for other case studies of carbonate platforms, as currents may be accelerated around such edifices, leading to slope winnowing and sediment deposition in more current‐protected zones. The study results also have consequences for the interpretation of outcrop and seismic subsurface data of carbonate platform slope series, because such slope sediment wedges are not necessarily formed during sea‐level highstands, but can consist of lowstand wedges only.

Facies variability in mixed carbonate???siliciclastic platform slopes (Miocene)

Miocene tropical carbonate platform slopes in southern Spain contain classical reef-slope facies distribution but also an unexpected abundance of serpulid-rich facies, locally forming build-ups. Two sections from the Miocene Sorbas and Níjar Basins were mapped and analyzed petrographically in order to identify the factors determining this facies variability. Reef-slope facies is intercalated with serpulid-rich facies and siliciclastic bodies. Serpulids are the pioneers colonizing the substrate in zones of quiet hydrodynamic conditions after hydrographical changes such as eventual river discharge. The interplay of sea-level changes and hydrographical conditions, together with episodic terrestrial influx, control lateral and along-slope facies variability as well as the facies distribution across the carbonate platform. Neither a deterministic distribution of facies belts nor a stochastic partitioning of facies in mosaics can accurately explain the facies distribution. A new model is proposed to explain facies variability in the context of intrinsic and extrinsic factors.

Upper Pleistocene cold-water corals from the Inner Sea of the Maldives: taphonomy and environment

Cold-water corals of the Late Pleistocene (21,400–22,500 BP) are recorded from the sea-bottom of two inter-atoll channels (Kardiva Channel at 457-m depth and Malé Vaadhoo Channel at 443-m depth) of the eastern row of the Maldives archipelago. Coral assemblages are composed mainly by Lophelia pertusa and secondarily by Madrepora oculata and Enallopsammia rostrata. These cold-water coral patches are places where the benthic life, mainly sessile, is concentrated, which is clearly absent off-rubble patches. The main epibionts are tube-dwelling polychaetes (mainly Spirorbis and Serpula), bryozoans, siliceous sponges, barnacles, gorgonids, solitary corals, encrusting foraminifera, and microbial mats. The analysis of epibionts assemblages shows different biocoenoses between both studied sites as well as a dependency of the epibiont coverage with regard to the coral genus. Some living benthic organisms such as brachiopods, bivalves, gastropods, barnacles, and ophiuroids find refuge among coral branches. The common record of juvenile specimens of vagile organisms such as small ophiuroids, is probably related to the nursery function of the cold-water corals in spite they are fossils. Environmental requirements of Recent cold-water corals (Lophelia, Madrepora and Enallopsammia) differ of conditions at both sampling sites with sensibly lower oxygenation degree and density of waters than needed for cold-water corals. Therefore, it is proposed that the present-day oxygen and density conditions are the factors which inhibit modern cold-water coral growth in the inter-atoll channels.