Reconstructing the Pleistocene connectivity history of the Black Sea and the Caspian Sea using strontium isotopes

Abstract

Quantifying the timing and nature of the connection between two hydrographic systems is critical to understanding the impact of different drivers on basin hydrology. The evolution of the Black Sea and the Caspian Sea is driven by a complex combination of tectonics and climate, resulting in extreme water level fluctuations in these two basins and multiple connection and isolation events between each other and the open ocean throughout their geological history.

The Quaternary connectivity history between the Black Sea and the Caspian Sea has previously been reconstructed using palaeontological, phylogenetic and geochemical studies. However, these records are often contradictory, lack precision, are rarely continuous and are commonly qualitative. Strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr), by contrast, can provide a direct and quantitative means of reconstructing the connectivity history between the two basins and the open ocean. The ⁸⁷Sr/⁸⁶Sr of
non-marine systems is sensitive to changing water sources and is incorporated and preserved in the calcitic shells of aquatic organisms. The ⁸⁷Sr/⁸⁶Sr measured on well-preserved fossil carbonates can therefore be used to obtain past ⁸⁷Sr/⁸⁶Sr of the basin and, consequently, to identify possible water sources as the connectivity of the basin changes.

This study adds substantial new data to the very sparse Sr isotopic dataset that exists in the modern Black Sea, Caspian Sea and the rivers that drain into them. A numerical box model parameterized with these fluvial data is used to constrain the past hydrological budget of each basin and to reconstruct the inter-basin connectivity required to produce palaeo salinity estimates and the ⁸⁷Sr/⁸⁶Sr measured on fossil ostracods collected from sedimentological sections and deep sea cores across the Black and Caspian Sea region.

The new ⁸⁷Sr/⁸⁶Sr records show that the Caspian Sea, in addition to present-day fluvial sources, also received significant amount of water from the Amu Darya river throughout most of the Pleistocene. The Caspian Sea also sustained a connection with the Arctic ocean in the earliest part of the Pleistocene between 2.7 to ∼2 Ma. The overlapping ⁸⁷Sr/⁸⁶Sr values of the coeval Black and Caspian seas during the early Pleistocene indicate that the two basins were connected during the time, via both one way flow from the Caspian Sea into the Black Sea, as well as a two way exchange between them. This Black- Caspian Sea connection was persistent until ∼380 ka, after
which Mediterranean input into the Black Sea began to dominate the ⁸⁷Sr/⁸⁶Sr signal of Black Sea water. Although, the Caspian Sea may have continued to flow into the Black Sea until ∼150 ka, divergence of the Caspian Sea and the Black Sea ⁸⁷Sr/⁸⁶Sr record suggests progressive isolation of these basins from each other, while the Black Sea experienced increasing frequency of connection with the Mediterranean Sea.

The timing of connection between the Black and the Caspian seas throughout the Pleistocene and between Black and the Mediterranean seas prior to 500 ka appears to be unrelated to the glacial - interglacial cyclicity, suggesting that tectonics was the primary control on the basin connectivity. However, over the last 500 ka, the Black Sea connection to the Mediterranean Sea was mainly driven by eustatic sea level changes at the Bosporus and the Black Sea connection to the Caspian Sea was impacted by additional meltwater fuelled discharge, reflecting the glacial interglacial cycle. This new connectivity record of the Black Sea and the Caspian Sea to each other and the open ocean therefore provides new insight into the relative impact of climate and tectonics on the evolution of these basins.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Rachel M Flecker (Supervisor) & David A Richards (Supervisor)