Refinement of Miocene sea level and monsoon events from the sedimentary archive of the Maldives (Indian Ocean)

International Ocean Discovery Program (IODP) Expedition 359 cored sediments from eight borehole locations in the carbonate platform of the Maldives in the Indian Ocean. The expedition set out to unravel the timing of Neogene climate changes, in particular the evolution of the South Asian monsoon and fluctuations of the sea level. The timing of these changes are assessed by dating resultant sedimentary alterations that mark stratigraphic turning points in the Neogene Maldives platform system. The first four turning points during the early and middle Miocene are related to sea-level changes. These are reliably recorded in the stratigraphy of the carbonate sequences in which sequence boundaries provide the ages of the sea-level lowstand. Phases of aggradational platform growth give precise age brackets of long-term sea-level high stands during the early Miocene and the early to middle Miocene Climate Optimum that is dated here between 17 to 15.1 Ma. The subsequent middle Miocene cooling coincident with the eastern Antarctic ice sheet expansion resulted in a long-term lowering of sea level that is reflected by a progradational platform growth. The change in platform architecture from aggradation to progradation marks this turning point at 15.1 Ma. An abrupt change in sedimentation pattern is recognized across the entire archipelago at a sequence boundary dated as 12.9–13 Ma. At this turning point, the platform sedimentation switched to a current-controlled mode when the monsoon-wind-driven circulation started in the Indian Ocean. The similar age of the onset of drift deposition from monsoon-wind-driven circulation across the entire archipelago indicates an abrupt onset of monsoon winds in the Indian Ocean. Ten unconformities dissect the drift sequences, attesting changes in current strength or direction that are likely caused by the combined product of changes in the monsoon-wind intensity and sea level fluctuations in the last 13 Ma. A major shift in the drift packages is dated with 3.8 Ma that coincides with the end of stepwise platform drowning and a reduction of the oxygen minimum zone in the Inner Sea. The strata of the Maldives platform provides a detailed record of the extrinsic controlling factors on carbonate platform growth through time. This potential of carbonate platforms for dating the Neogene climate and current changes has been exploited in other platforms drilled by the Ocean Drilling Program. For example, Great Bahama Bank, the Queensland Plateau, and the platforms on the Marion Plateau show similar histories with sediment architectures driven by sea level in their early history (early to middle Miocene) replaced by current-driven drowning or partial drowning during their later history (Late Miocene). In all three platform systems, the influence of currents on sedimentations is reported between 11 and 13 Ma

Compositional variations in calciturbidites and calcidebrites in response to sea-level fluctuations (Exuma Sound, Bahamas)

compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, was determined. Two types of gravity deposit were present in the cores of ODP Leg 101, Site 632A, i.e., calciturbidites and calcidebrites. In analogy with earlier studies, the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e., platform interior, platform edge, and platform slope. Calciturbidites deposited during interglacial, sea-level highstands show a dominance of non-skeletal grains, largely derived from the platform interior, while calciturbidites of glacial, sea-level lowstands, show a dominance of skeletal platform-edge to platform-slope-derived grains. Thus, the calciturbidite composition can be used to reconstruct the position of absolute sea level. In addition, the mud content of the calciturbidites increased after Marine Isotope Stage 11. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform-edge-derived sediments during varying sea-level positions. The Bahamian carbonate platform is located in a tectonically stable passive-margin setting and the gravity-flow deposits were laid down in an environment exclusively controlled by eustatic sea-level fluctuations. This study shows that all types of gravity-induced carbonate deposits, calciturbidites, and calcidebrites, were deposited in response to global eustatic sea-level variations. The sediment composition could be linked directly to sediment input from specific facies realms along the carbonate platform margin. Hence, sediment composition analysis is a strong tool that may be used to discriminate between gravity-induced deposition triggered by eustatic sea-level changes and that related to tectonic events, when analyzing resedimentation processes in sedimentary basins.Geoscience & EngineeringCivil Engineering and Geoscience

Tsunami Deposits and Their Morphological Effects: A Regional Scale Approach

Tsunami are high energy events which have heavy morphological impacts on the shore lines. A review of the literature shows that these impacts are very difficult to differentiate from large storms impacts in most of the cases. This paper begins with a description of tsunami impacts, at a local scale, in terms of erosion and of accumulation. It studies cases where tsunami accelerate “normal” behaviour of the coast or, on the other hand go against expected normal behaviour. In order to better understand the relative roles of tsunamis and storms, the local forms are replaced within a regional scale/context and some important regions (Mediterranean Sea, New Zealand, the Antilles) are taken as examples of the interrelation between storm-forced evolution and tsunami – controlled behaviour