Paleoclimatic study of Kumlubent Lagoon based on Oxygen-18 and Carbon-13 isotope data from sequenced mollusc shells on the western coast of the Sea of Marmara, NW Türkiye

Oxygen isotope (δ¹⁸O) and carbon isotope (δ¹³C) data obtained from marine carbonate shells allow the understanding of paleoclimatic conditions. The mass difference between the isotopes of an element leads to a change in the ratio of heavy and light isotopes within the structure of an organism due to certain events. In this study, we present the paleoclimatic findings from the analysis of fossils obtained from sediment cores of a lagoon on the Marmara coast, located in the coastal region of Kumlubent on the Gelibolu peninsula, Çanakkale.

In the Kumlubent lagoon, two sediment cores were taken from depths of 3.40 m and 2.78 m respectively. The cores were mainly composed of gray sand material intercalating with distinct intervals of silt-size materials, sapropels, and layers of pebbles suggesting event-type sedimentations.   Morphological identification of the fossil shells in the sediment cores was examined by using light microscopy as well as a Scanning Electron Microscope (SEM). Oxygen and carbon isotope measurements in the carbonate shells were measured by Inductively Coupled Mass Spectrometry (ICP-MS) analyses.  The values obtained were converted to δ¹⁸O-δ¹³C values based on the ratio "¹⁸O:¹⁶O seawater-¹³C:¹²C" to track depletion and enrichment. Our results show that the marine fossil community in Kumlubent lagoon includes 35 different species and 25 genera. The abundance of species in the sediment cores varied significantly indicating possible intervals of high primary production. The most commonly found genus in the cores were Bittium species — in particular Bittium latreillii, Bittium reticulatum, and Bittium sp.— - which were selected for the oxygen and carbon isotope analyses. Using the isotopic behavior diagram constructed from the collected data, we investigated the relationship between the behavior of the δ¹⁸O isotope ratio and the dry periods of the environment in which the Bittium species, as observed in the recovered sediment cores. δ¹⁸O data in the ranged from -2.63‰ to -0.94‰, and δ¹³C values ranged from 0.25‰ to 1.98‰. Paleotemperature calculations for Kumlubent carbonate shells indicated seawater temperatures between 24.8°C and 32.0°C during the formation of the shell material. δ¹⁸O and δ¹³C values showed opposite increase-decrease patterns reflecting varying precipitation-evaporation and varying freshwater input in the Kumlubent region in the Sea of Marmara.