Palaeoecological records are required to test ecological hypotheses necessary for conservation strategies as short-term observations can insufficiently capture natural variability and identify drivers of biotic change. Here, we demonstrate the importance of an integrated conservation palaeobiology approach when making validated decisions for conservation and mitigating action. Our model system is the Razim-Sinoie lake complex (RSL) in the Danube Delta (Black Sea coast, Romania), a dynamic coastal lake system hosting unique Pontocaspian mollusc species that are now severely under threat. The Pontocaspians refer to an endemic species group that evolved in the Black Sea and Caspian Sea basins under reduced salinity settings over the past few million years. The natural, pre-industrial RSL contained a salinity gradient from fresh to mesohaline (18ppm) until human intervention reduced the inflow of mesohaline Black Sea water into the lake system. We reconstruct the evolution of the RSL over the past 2000 years from integrated sedimentary facies and faunal analyses based on 11 age-dated sediment cores and investigate the response of mollusc species and communities to those past environmental changes. Three species associations ("marine", "Pontocaspian" and "freshwater") exist and their spatio-temporal shifts through the system are documented. Variable salinity gradients developed, with marine settings (and faunas) dominating in the southern part of the system and freshwater conditions (and faunas) in the northern and western parts. Pontocaspian species have mostly occurred in the centre of the RSL within the marine-freshwater salinity gradient. Today, freshwater species dominate the entire system, and only a single Pontocaspian species (Monodacna colorata) is found alive. We show that the human-induced reduced marine influence in the system has been a major driver of the decline of the endemic Pontocaspian biota. It urges improved conservation action by re-establishing a salinity gradient in the lake system to preserve these unique species.

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The Pontocaspian (Black Sea - Caspian Sea) region has a very dynamic history of basin development and biotic evolution. The region is the remnant of a once vast Paratethys Sea. It contains some of the best Eurasian geological records of tectonic, climatic and paleoenvironmental change. The Pliocene-Quaternary co-evolution of the Black Sea-Caspian Sea is dominated by major changes in water (lake and sea) levels resulting in a pulsating system of connected and isolated basins. Understanding the history of the region, including the drivers of lake level and faunal evolution, is hampered by indistinct stratigraphic nomenclature and contradicting time constraints for regional sedimentary successions. In this paper we review and update the late Pliocene to Quaternary stratigraphic framework of the Pontocaspian domain, focusing on the Black Sea Basin, Caspian Basin, Marmara Sea and the terrestrial environments surrounding these large, mostly endorheic lake-sea systems.@en

Sedimentological facies models for (semi-)isolated basins are less well developed than those for marine environments, but are critical for our understanding of both present-day and ancient deltaic sediment records in restricted depositional environments. This study considers an 835 m thick sedimentary succession of mid-Pliocene age, which accumulated in the Dacian Basin, a former embayment of the Black Sea. Detailed sedimentological and palaeontological analyses reveal a regression from distal prodelta deposits with brackish water faunas to delta-top deposits with freshwater faunas. Sediments contain frequent hyperpycnal plumes and an enrichment in terrestrial organic material, ichnofossils and in situ brackish and freshwater faunas. Deltaic progradation created thin, sharply-based sand bodies formed by multiple terminal distributary channels, covering a wide depositional area. The system experienced frequent delta-lobe switching, resulting in numerous thin parasequences. Parasequences are overlain by erosive reddish oxidized sand beds, enriched in broken, abraded brackish and freshwater shells. These beds were formed after sediment starvation, on top of abandoned delta lobes during each flooding event. A robust magnetostratigraphic time frame allowed for comparison between the observed sedimentary cyclicity and the amplitude and frequency of astronomical forcing cycles. Our results indicate that parasequence frequencies are significantly higher than the number of time equivalent astronomical cycles. This suggests that delta-lobe switching was due to autogenic processes. We consider the observed facies architecture typical for a delta prograding on a low-gradient slope into a shallow, brackish, protected, semi-isolated basin. Furthermore, in the absence of significant wave and tidal influence, sediment progradation in such a protected depositional setting shaped a delta, strongly river-dominated.

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