Study of marine sedimentary deposits in the hypoxic/anoxic environment of Amvrakikos Gulf
Loading...
Date
2025-07-14
Authors
Πρανδέκου, Αμαλία
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The Amvrakikos Gulf is a semi-enclosed marine basin in western Greece. Its rich biodiversity and unique ecological features led to its designation as a wetland of international importance under the Ramsar Convention. Despite its significance, the gulf faces numerous challenges that have gradually degraded its water quality to critical levels, resulting in ongoing environmental degradation. Its unique geomorphology and oceanographic conditions make it a valuable case for understanding the onset, progression, and impacts of hypoxic and anoxic conditions in coastal semi-enclosed marine environments. Its restricted hydrodynamic regime, combined with significant freshwater inflow and anthropogenic interventions, has rendered it highly susceptible to oxygen depletion. This thesis investigates the environmental evolution of the Amvrakikos Gulf from the Mid-to Late Holocene up to the present day, with a particular focus on the development and expansion of bottom water hypoxia and anoxia, using a multi-proxy approach.Thirteen sediment cores were collected across various sub-basins of the gulf, including both short cores (<50 cm) for recent decades and long cores (>160 cm) for Holocene reconstructions. These cores were analyzed for benthic and planktonic foraminiferal assemblages, sediment grain size, total organic carbon (TOC), elemental geochemistry (ICP-MS), stable isotopes (δ¹³C, δ¹⁸O), and maceral analysis. Two environmental quality indices, Foram-AMBI and Foram Stress Index (FSI), were employed to assess ecological status over time. Radiometric dating techniques, including ¹⁴C and ²¹⁰Pb, were used to establish age-depth models and calculate sedimentation rates.The results highlight a strong temporal and spatial variability in the bottom water oxygenation of the Gulf. During the Mid- to Late Holocene, periods of intensified organic matter deposition and bottom water oxygen depletion were identified, particularly after 2.5 ka. These events were synchronous with known climatic humid periods and regional anthropogenic influences. The distribution of benthic foraminiferal taxa, notably the dominance of opportunistic species such as Ammonia tepida and Bulimina elongata, and intermittent azoic layers, provided robust evidence for recurring anoxic phases. Elemental ratios such as Sr/Ba, Fe/S and Ti/Ca, along with TOC values, further supported these interpretations.In recent decades (post-1960), hypoxia and anoxia have become increasingly severe and widespread. The short-core record reveals a dramatic deterioration in ecological conditions, particularly from the 1990s onwards. This deterioration coincides with anthropogenic activities, including dam constructions on the Arachthos and Louros rivers, intensive agricultural runoff, and rapid aquaculture development. Foraminiferal data from these periods indicate reduced diversity, elevated abundance of tolerant taxa, and more frequent azoic conditions. Notably, the most adverse environmental conditions for the total gulf occurred from 1990 to 2000 and from 2005 to 2010 coinciding with the recorded fish mortality events.Climatic data, including temperature and precipitation records, were also analyzed in relation to ecological proxies. In areas that are affected more by the Ionian water, the bottom environmental conditions tend to improve with decreases in temperature. In the rest of the gulf, the oxygen depletion of the bottom water layer is dependent on multiple causes such as organic matter input and surface salinity.The findings of this thesis underscore the vulnerability of semi-enclosed basins to oxygen stress and highlight the importance of multiproxy sediment analyses in reconstructing past marine conditions. The integration of multiproxy sediment analyses allows for a comprehensive perspective understanding of both natural and anthropogenic influences on marine hypoxia. Moreover, the case of Amvrakikos Gulf provides broader insights into the response of coastal systems to climate change and human pressures, with direct implications for environmental management, fisheries, and conservation policy in the Mediterranean and beyond.
Description
Keywords
Amvrakikos Gulf, Enclosed basin, Anoxia, Hypoxia, Benthic foraminifera, Geochemical proxies