Abrupt environmental changes within a late Holocene lacustrine sequence south of the Marmara Sea (Lake Manyas, N-W Turkey): possible links with seismic events

A coring campaign in Lake Manyas (Kuş Gölü) on the southern coast of the Sea of Marmara (Turkey) has provided insights into basin infilling processes during the upper 11 metres of the sedimentary record. Combined sedimentary features between 5 and 4 m depth have been explained by a seismite. A brittle mixed layer (uniquely rich in seeds and ostracod valves) was widely detected at 4 m downcore. This has been interpreted as being the result of a seiche either through a salt inundation linked to a tsunami in the Sea of Marmara (the large-scale scenario) or a local hydrothermal fluid discharge (the small-scale scenario). Ostracod valves, which are usually not preserved in Lake Manyas sediment, are either incorporated here as tsunami debris (providing a spatially averaged snapshot of the regional assemblage) or are locally preserved following a temporary geochemical/hydrochemical change of the water of unknown duration (but probably several decades). The brittle mixed layer overlies cracks resulting from the direct effects of the seismic shock wave on slightly compacted sediment. At around 964 cm depth palynological and particle-size analyses indicate a change from backswamp to open lacustrine conditions. The end of the backswamp has been correlated with a decrease in the level of the Sea of Marmara from 4000 to 3000–2800 yr ago (uncal.), caused by regional tectonic movements. An age–depth model is presented based on two AMS radiocarbon dates, on the palynological detection of the end of the Beyşehir Occupation Phase (130 cm above the event) and on maximal sedimentation accumulation rates obtained by radionuclide analyses. The entire study sequence covers the last 4300 yr. The paper examines the possibility of an Early Byzantine age for the seismic event. This may have been the historical event at AD 460 that destroyed Cyzicus (the ancient Erdek, 20 km north of the lake) and various villages in the interior and was felt widely in the then-known world. Other possibilities include various local and regional events in that same period: the Early Byzantine Tectonic Paroxysm and the large earthquake in AD 447 that affected the entire Sea of Marmara region.