Mid-to late holocene paleoenvironmental evolution of the bottom water layer in the semi-enclosed Amvrakikos Gulf

The Amvrakikos Gulf, a semi-enclosed marine basin in western Greece, has undergone substantial environmental changes over the past 6,000 years due to geomorphological evolution, climatic variability, and human influences. This study aims to reconstruct the paleoenvironmental evolution of the bottom water layer of the gulf during the Mid-to Late Holocene. To achieve this, two sediment cores from the western and eastern basins were analyzed using an interdisciplinary approach, including foraminiferal and geochemical proxies. The cores comprise fine-grained sediments while the foraminifera fauna exhibited high abundances with low diversity, primarily dominated by species commonly associated with eutrophic seafloors throughout the examined period especially in the inner parts of the gulf. Furthermore, foraminifera and geochemical proxies indicate that the inner gulf (east basin) presents lower salinity and higher environmental stress than the west basin indicating higher influence of the rivers than the Ionian water intrusions. The results delineate three distinct phases in the environmental evolution of the Amvrakikos Gulf. From 6 to 3.8ka BP, the gulf exhibited relative oligotrophic conditions with high salinity and frequent marine intrusions from the Ionian Sea, facilitating enhanced oxygenation in bottom waters. Between 3.8 and 2.5ka BP, increased humidity and higher riverine input led to reduced salinity and higher eutrophication, particularly in the eastern basin. At that time, foraminifera proxies indicated that a low-energy environment has begun to establish, marking the onset of the hypoxic environment that is observed today. From 2.5ka BP to the present, the gulf has experienced escalating environmental degradation, with a general trend toward reduced salinity and oxygen depletion with several anoxic events especially in the eastern basin accelerating in humid periods. • Foraminifera and geochemistry predict seafloor conditions in Mid-to Late Holocene. • Inner gulf shows persistent stress from limited oxygen and high fluvial inputs. • Environmental conditions have been more adverse in the last 2,500 years. • Enrivonmental changes are affected by changes in salinity and local geomorphology.