Seismically-triggered organic-rich layers in recent sediments from Gollukoy Lake (North Anatolian Fault, Turkey)

Multi-proxy analyses on the sedimentary sequence of Gollukoy Lake, which is located on the eastern North Anatolian Fault (NAF), reveals a complete and high-resolution paleoseismic record for the last 650 years. Six sedimentary events are detected in a 3.1 m-long piston core. They form distinct organic-rich intercalations within the background sedimentation, which are characterized by strong anomalies on the loss-on-ignition (LOI550) and total organic carbon (TOC) profiles, as well as by lighter colours on the X-ray radiographic images. Itrax micro-XRF core scanner data are also used to contribute to the detection and characterization of the event deposits. After the detection of the sedimentary events, their temporal correlation with the earthquakes in the historical seismicity catalogue of the NAF is tested. The youngest event is dated to 1940s by using Pb-210 and Cs-137 profiles in sediment, which coincides with the 1939 earthquake (M-s = 7.7) on the NAF. The ages of the older five events are determined based on radiocarbon dating and regional time-stratigraphic correlation. Radiocarbon dating on the bulk sediment samples does not provide reliable results due to hard-water effect. On the other hand, dating on charcoals, Ephippia of Daphnia and phragmite remains significantly improves the results and implies a mean sedimentation rate of 0.28 cm/yr. Based on this preliminary sedimentation rate, we show that organic matter content variations through our record correlate with the varve-based 6180 record of Nar Lake, which is located 350 km southwest of Gollukoy Lake. Accordingly, high-precipitation/low-evaporation climatic episodes detected in Nar Lake are represented by higher organic matter content in Gollukoy sediments. Fine-tuning the Gollukoy LOI550 record to the Nar delta O-18 record reveals that the ages of the sedimentary events in Gollukoy match with well-known historical earthquakes that occurred around the lake. Finally, the origin of the organic-rich intercalations is discussed based on macroscopic observations and organic geochemistry. The events are attributed to reworked organic matter eroded from the lake's littoral zone by seismically-induced water oscillations (seiche), and their subsequent deposition at the middle parts of the lake. These seismically-induced events are atypical compared to the classic earthquake-triggered sedimentary processes in lacustrine environments, which are shortly reviewed in order to emphasize the unusual Gollukoy Lake record