The Pontocaspian basins in a grain of sand : Coastal sedimentary architecture, forcing mechanisms, and faunal turnover events in restricted basins

This thesis explores the effects of limited interbasinal connectivity on clastic coastal sedimentary environments around the Pontocaspian basins, and how external forcing mechanisms controlled environmental changes and lake-level variations since the Pliocene. The Pontocaspian region constitutes a vast restricted sedimentary domain including the Black Sea, Caspian Sea, Aral Sea, Marmara Sea, and several Anatolian lakes. Since the Pliocene, these basins recorded limited, intermittent or non-existent marine connection with the open ocean. The basins repeatedly changed from exorheic systems connected with the Mediterranean Sea to endorheic systems disconnected from the marine realm. The dynamic evolution of these restricted basins produced frequent and drastic environmental changes. This resulted in a unique combination of sedimentary architecture, forcing mechanisms and subsequent faunal turnover events greatly differing from typical marine settings. By developing well-documented facies models, the interplay between the water and sediment fluxes into the basins, and basin physiography, is identified as the major internal controls on deposition in such restricted basins. The amount of accommodation space, the basin slope gradient and the depositional water depth are of prime importance to sedimentation processes. Based on these factors, revised water depths of the boundaries delimiting the different depositional environments are proposed. This thesis additionally suggests to carefully select the appropriate sedimentary environmental classification when working in restricted basins. Base on a combination of several proxies, such as cyclostratigraphy, stable isotopic ratios, clay mineralogy, biostratigraphy and climate modelling, the impacts and mechanistic pathways of orbital and glacial cyclicity are evaluated for the restricted Pontocaspian basins. This research suggests that orbital cycles affected regional climate conditions over time, driving major changes in the hydrological budget of the restricted basins. However, global climatic variations driven by glacial cycles constituted the main external drivers as they generated episodic reconnection events between the Caspian Sea, the Black Sea and the rising open ocean. This thesis finally investigates the effects of current environmental changes on the endangered endemic Pontocaspian fauna. A comparison of spatial and temporal variability within coastal environments, with mollusc fauna distributions and abundances, allows evaluating the resilience and recovery potential of this endangered biota. This research identified salinity variations, both natural and manmade, as the major forcing mechanisms generating faunal turnover events. This integrated sedimentological-biological approach may provide effective conservation policy strategies to tackle the biodiversity crisis currently recorded in the Pontocaspian basins