Identification of extreme events in the sedimentary record relies on the correct characterisation of the deposit by means of multiple parameters and different diagnostic criteria. Multiple proxies based on well-tested geological, biological, and chemical analyses have been used to distinguish tsunami layers from other marine extreme events. However, few proxies distinguish or compare the “degree of sedimentary chaos” of an event relative to background sediments and between other extreme events. In this pilot study, the potential of Optically Stimulated Luminescence (OSL), normally used as a dating method, is demonstrated and a new potential sedimentological proxy is proposed when analysing in depth the OSL signal from single grain measurements to distinguish between underwater deposits derived from marine extreme events embedded in the texturally and compositionally homogeneous shallow shelf archive of the eastern Mediterranean. Newly and previously collected sediment samples from underwater excavations and a sediment core offshore Caesarea Maritima (Israel) were used in this experiment. Each sample was associated with either tsunami, storm, or fair weather (normal) marine conditions based on multiple proxies. In this study, single grain over-dispersion (SG-OD) values and ranges are used as a potentially novel way to quantify the degree of sedimentary chaos, thus the degree of scatter of the dose distribution as a function of the efficiency of exposure to sunlight (zeroing) of the quartz grains entrained during transport and deposition. Given that turbidity of a fluid should be associated with lower light penetration, and higher energy flow with greater sediment density (as can occur in tsunamis relative to storms), these conditions should be reflected in variations in single grain over-dispersion values, hence identifying luminescence distribution patterns related to known depositional conditions. This quantification is coupled with detailed statistical analysis of the resulting single grain equivalent dose distributions, minimally used in OSL-dating. Three types of underwater deposits were categorised based on three distribution typologies, to which, preliminary, conservative single grain over-dispersion ranges were assigned: <25% for Gaussian-like sands deposited under normal marine conditions; 25–60% for storm-laid sediments depicting bimodal distributions; and >60% for tsunami-related deposits showing asymmetric distributions with strong positive skewness. This new tool provides a means to study offshore extreme marine events, without potential dismissal of other identification criteria and proxies. Based on these results, single grain over-dispersion OSL could be considered as a potential new sedimentological proxy for identification, comparison and characterisation of marine-derived extreme events in the shallow shelf (such as tsunami deposits), as it is intrinsic to the sediment grain, and the degree of sedimentary chaos of the event itself.
Bibliographical noteFunding Information:
GIL is grateful to the Azrieli Foundation (Canada-Israel) for the award of an Azrieli Fellowship, which allowed for funding and completion of this research in Israel. This work was also supported by the Israel Ministry of Energy and Water ( 9-2010-ES ; BNG), the Israel Science Foundation ( 984/10 ; BNG), Mr. Norman Krischer, Sir Mick Davis and EcoOcean. Special thanks to M.Sc. Rami Tsadok for assistance in sample collection during our C-CAP project 2010–2011, the use of his boat and skipper expertise. This research is a contribution to IGCP Project 639 “Sea Level Change from Minutes to Millennia”. The authors would like to thank the Guest Editor of this Special Issue “Sedimentary Evidence of Geohazards: Challenges and Advances”, Dr. Catherine Chagué, and Dr. Pedro Cunha and a second anonymous reviewer, for their constructive comments which greatly improved this manuscript.
© 2018 Elsevier B.V.
- Palaeo-tsunami deposit
- Single grain
- Statistical analysis
- Tsunami toolkit
ASJC Scopus subject areas