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  1. null (Ed.)
  2. Abstract

    Arising from the non‐uniform dispersal of sediment and water that build deltaic landscapes, morphological change is a fundamental characteristic of river delta behavior. Thus, sustainable deltas require mobility of their channel networks and attendant shifts in landforms. Both behaviors can be misrepresented as degradation, particularly in context of the “stability” that is generally necessitated by human infrastructure and economies. Taking the Ganges‐Brahmaputra‐Meghna Delta as an example, contrary to public perception, this delta system appears to be sustainable at a system scale with high sediment delivery and long‐term net gain in land area. However, many areas of the delta exhibit local dynamics and instability at the scale at which households and communities experience environmental change. Such local landscape “instability” is often cited as evidence that the delta is in decline, whereas much of this change simply reflects the morphodynamics typical of an energetic fluvial‐delta system and do not provide an accurate reflection of overall system health. Here we argue that this disparity between unit‐scale sustainability and local morphodynamic change may be typical of deltaic systems with well‐developed distributary networks and strong spatial gradients in sediment supply and transport energy. Such non‐uniformity and the important connections between network sub‐units (i.e., fluvial, tidal, shelf) suggest that delta risk assessments must integrate local dynamics and sub‐unit connections with unit‐scale behaviors. Structure and dynamics of an integrated deltaic network control the dispersal of water, solids, and solutes to the delta sub‐environment and thus the local to unit‐scale sustainability of the system over time.

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  3. null (Ed.)
  4. null (Ed.)
    International Ocean Discovery Program (IODP) Expedition 359 is designed to address sea level, currents, and monsoon evolution in the Indian Ocean. Seven proposed drill sites are located in the Maldives and one site is located in the Kerala-Konkan Basin on the western Indian continental margin. The Maldives carbonate edifice bears a unique and mostly unread Indian Ocean archive of the evolving Cenozoic icehouse world. It has great potential to serve as a key area for better understanding the effects of this global evolution in the Indo-Pacific realm. Based mainly on seismic stratigraphic data, a model for the evolution of this carbonate bank has been developed, showing how changing sea level and ocean current patterns shaped the bank geometries. A dramatic shift in development of the carbonate edifice from a sea level–controlled to a predominantly current-controlled system is thought to be directly linked to the evolving Indian monsoon. Fluctuations in relative sea level control the stacking pattern of depositional sequences during the lower to middle Miocene. This phase was followed by a two-fold configuration of bank development: bank growth continued in some parts of the edifice, whereas in other places, banks drowned. Drowning steps seem to coincide with onset and intensification of the monsoon-related current system and the deposition of giant sediment drifts. The shapes of drowned banks attest to the occurrence of these strong currents. The drift sediments, characterized by off-lapping geometries, formed large-scale prograding complexes, filling the Maldives Inner Sea basin. Because the strong current swept most of the sediment around the atolls away, relict banks did not prograde, and steady subsidence was balanced by aggradation of the atolls, which are still active today. One important outcome of Expedition 359 is ground-truthing the hypothesis that the dramatic, pronounced change in the style of the sedimentary carbonate sequence stacking was caused by a combination of relative sea level fluctuations and ocean current system changes. Answering this question will directly improve our knowledge on processes shaping carbonate platforms and their stratigraphic records. Our findings would be clearly applicable to other Tertiary carbonate platforms in the Indo-Pacific region and to numerous others throughout the geological record. In addition, the targeted successions will allow calibration of the Neogene oceanic δ13C record with data from a carbonate platform to platform-margin series. This is becoming important, as such records are the only type that exist in deep time. Drilling will provide the cores required for reconstructing changing current systems through time that are directly related to the evolution of the Indian monsoon. As such, the drift deposits will provide a continuous record of Indian monsoon development in the region of the Maldives. These data will be valuable for a comparison with proposed Site KK-03B in the Kerala-Konkan Basin (see Geological setting of the Kerala-Konkan Basin, below) and other monsoon-dedicated IODP expeditions. The proposed site in the Kerala-Konkan Basin provides the opportunity to recover colocated oceanic and terrestrial records for monsoon and premonsoon Cenozoic climate in the eastern Arabian Sea and India, respectively. The site is located on a bathymetric high immediately north of the Chagos-Laccadive Ridge and is therefore not affected by strong tectonic, glacial, and nonmonsoon climatic processes that affect fan sites fed by Himalayan rivers. The cores are expected to consist of a continuous sequence of foraminifer-rich pelagic sediments with subordinate cyclical siliciclastic inputs of fluvial origin from the Indian Peninsula for the Neogene and a continuous paleoclimate record at orbital timescales into the Eocene and possibly the Paleocene. 
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