Biological turnovers in response to marine incursion into the Caspian Sea at the Plio-Pleistocene transition

https://doi.org/10.1016/j.gloplacha.2021.103623Get rights and content

Highlights

  • Marine water entered the Caspian Sea at the end of the Pliocene (2.75 Ma).

  • Geochemical shifts occurred, with complete biotic turnover in algal communities.

  • Biotic turnovers were most likely associated with changes in salinity.

  • Current sea-level change may drive similar events today and in the near future.

  • Human-made shipping canals may drive hydrological imbalance in seas worldwide.

Abstract

Marine influence on low-salinity environments can trigger aquatic ecosystem shifts, including biodiversity turnovers. High-resolution palaeoenvironmental records of marine connection events are particularly valuable, as they provide natural laboratories to understand analogous oceanographic and biodiversity turnover events in present-day climate- and anthropogenically-induced incursions. One such incursion event occurred across the Plio-Pleistocene transition when water from the open ocean spilled into the Eurasian continental interior, inundating the Caspian area. Here we record the so-called Akchagylian marine incursion using well-dated palynological and geochemical records of the Lokbatan section (Azerbaijan). Immediately prior to the intensification of northern hemisphere glaciations (~2.75 Ma), fresh-brackish peri-Paratethyan dinocyst assemblages were replaced by monospecific assemblages of the marine dinocyst, Operculodinium centrocarpum sensu Wall and Dale (1966). This indicates that the Caspian Sea experienced a marine incursion during a period of global high sea level. The marine incursion also registered in the geochemical record as a peak in excess‑strontium and carbonate content. Marine influence on the Caspian ceased after ~2.46 Ma and a second biological turnover took place, with low-salinity tolerant peri-Paratethyan dinoflagellate communities replacing the marine assemblages. The large-scale Akchagylian marine incursion episode shows the extreme degree of biodiversity change that marine influence on fresh-brackish water basins could trigger. Similar processes are increasingly relevant to present-day marginal and landlocked basins, which face ever-greater incursions from marine species and water due to both climate-mediated sea-level rise and human-made infrastructure projects.

Introduction

Connections between the global ocean and its marginal basins have occurred in the past whenever sea level has risen above the spill-point separating the water masses and such events have been recorded in sedimentary archives (e.g. Krijgsman et al., 1999; Svitoch et al., 2000; Chepalyga, 2007; Yanina, 2014; Virtasalo et al., 2016; Andreetto et al., 2021). The initiation of water flow across a spill-point can have vital implications for sea level and surface area, and can affect water chemistry and circulation patterns in one, or both basins (e.g. Schönfeld, 1997; Hernández-Molina et al., 2006; Nicholls and Toumi, 2013; Marzocchi et al., 2016; Stoica et al., 2016; Arpe et al., 2018; Grothe et al., 2020). In turn this can trigger limnological changes and potentially drive fast, catastrophic turnovers in biodiversity (e.g. Orhon, 2014; Castellanos-Galindo et al., 2020).

Examples driven by recent sea-level change, as well as by human-made infrastructure projects (e.g. Suez, Panama, White Sea-Baltic, Volga-Baltic and Volga-Don canals) provide evidence of both the incipient and ongoing effects of cross-contamination of water between different marine and freshwater systems (Slynko et al., 2002; Galil et al., 2008; Castellanos-Galindo et al., 2020; Son et al., 2020). The documented impacts can include significant alterations to salinity and productivity as well as the introduction of non-native invasive species, both of which have a knock-on effect on biodiversity, species interactions and ecosystem services (Castellanos-Galindo et al., 2020; Weiskopf et al., 2020). Further proposed large-scale infrastructure projects include the Istanbul Canal, that would make an additional connection from the Marmara Sea to the Black Sea (Orhon, 2014), the Dead Sea Peace Conduit, which would pipe water from the Red Sea into the Dead Sea (Gavrieli et al., 2005), and the Eurasia Canal, which would connect the Black Sea and Azov Sea system to the Caspian Sea via the Kuma-Manych depression (Bekturganov and Bolaev, 2017). These projects are likely to invite similar effects even if they are meticulously planned and executed.

Throughout most of the Pliocene, the Caspian was a lake, isolated from the global ocean (Reynolds et al., 1998). However, around the time of the Plio-Pleistocene transition, the Caspian Sea became connected with the global ocean, causing the prevalence of marine-like conditions (Jones and Simmons, 1996). This event is referred to as the Akchagylian marine incursion episode (Lazarev et al., 2021). A large area surrounding the Caspian Sea was submerged, causing up to a five-fold increase of its surface area (Van Baak et al., 2019) and introducing exotic biota into the basin (Richards et al., 2018; Lazarev et al., 2019). It had been hypothesised as early as the 1930s that the Arctic Ocean was the source of marine water for the Akchagylian marine incursion episode (Kovalevsky, 1933; Agalarova et al., 1940; Muratov, 1951) and this hypothesis has been restudied in recent years (Krijgsman et al., 2019). Geochemical evidence provided by strontium isotopes suggests the ingress of marine water at this time (Van Baak et al., 2019). Palaeontological evidence for a connection with the Arctic Ocean comes from the presence of boreal-arctic foraminifera in sediments of the marine incursion interval (Agalarova et al., 1940; Richards et al., 2018). These lines of enquiry are further supported by the close genetic relationship between Arctic and Caspian species and their molecular clock divergence dates that match with the Plio-Pleistocene transition interval (Palo and Väinölä, 2006; Vandendorpe et al., 2019).

The modern-day Caspian Sea is a large brackish endorheic waterbody mostly fed by the Volga River (Leroy et al., 2020). Its salinity ranges from slightly over 13 psu in the south and central basins, to less than 5 psu in the northern basin. Its fauna and flora have a low biodiversity (due to the many changes in salinity over the past millions of years) but contain many endemic taxa (owing to long periods of isolation). The biota of the Caspian Sea (and the Pontocaspian biota in general) displays a great plasticity in reaction to salinity (Hoyle et al., 2019; Leroy et al., 2020). Dinoflagellate cyst spectra in surface samples are dominated by Impagidinium caspienense, a species typical of the Caspian Sea and the Aral Sea (Marret et al., 2004; Mudie et al., 2017). Lingulodinium machaerophorum, with short processes in response to reduced salinity, is also often abundant (Mertens et al., 2009; Leroy et al., 2013).

In this study, we present millennial-scale resolution records (Milankovitch type) of dinoflagellate cysts and other aquatic palynomorphs, as well as sediment geochemistry. These records are derived from a Caspian Sea sedimentary sequence taken from the South Caspian Basin in eastern Azerbaijan (Fig. 1) that covers the time period before, during and after the Akchagylian marine incursion. The same sedimentary sequence has been studied palynologically (terrestrial pollen) and radiometrically dated by Hoyle et al. (2020). The continental pollen records allowed the detection of glacial-interglacial cycles, facilitating unprecedented precision in age control in order to discuss the connection event at the resolution of marine isotope stages (MIS). Therefore, we aim to further constrain the chronology and timespan of the marine incursion episode, both discussing the event within its palaeoclimatic context and elucidating the mechanisms and environmental consequences of marine incursion episodes. The example presented here explicitly demonstrates the effects of marine water ingress on a low-salinity system.

Section snippets

Sedimentary archive and chronology of the studied interval

The Lokbatan section is ~10 km west of Baku and exposes the top of a broadly north-south trending anticline. The section lies north of the town of Lokbatan, on the road between Lokbatan and Qobu (N40.343067°, E 49.743161°, Fig. 1). The studied interval belongs to the latest Pliocene and early Pleistocene, comprising the uppermost Productive Series, the Akchagylian and the lower part of the Apsheronian. The primary emphasis is placed on events within the Akchagylian. The lithology of this

Dinoflagellate cyst and other aquatic palynomorphs records

Based on the CONISS analysis of the dinocyst records, seven dinocyst assemblage zones (DAZ-1 to DAZ-7; Fig. 2a and b) may be identified. Zones DAZ-4 and DAZ-6 were each split further into sub-zones (DAZ-4a to c and DAZ-6a and b). Preservation of palynomorphs was poor below DAZ-1, and this interval (not included in the CONISS analysis) has been named zone P. Micrographs and SEM images of some of the most important dinocysts encountered during this study are included in the supplementary material.

Discussion

The biotic (aquatic palynology) and abiotic (sedimentological and geochemical) records allow reconstruction of the changing biota and lacustrine environment within the Caspian Sea before, during and after the Akchagylian marine incursion. Furthermore, an additional constraint to the chronological framework of the marine incursion episode is provided and discussed within its palaeoclimatic context.

Conclusions

We have presented a detailed record of biological turnovers caused by a marine incursion into the Caspian Sea during a period of global high sea levels immediately prior to the intensification of Northern Hemisphere glaciations. The Akchagylian marine incursion initiated at the end of the Pliocene (~2.75 Ma) with the moment of maximum marine influence, as indicated by dinocyst and geochemical records, at the peak of interglacial MIS G7 (2.74 Ma). Dinocyst assemblages were severely affected by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by BP Exploration Operating Company Limited as a grant to SL at Brunel University London. LLM is supported by the Madrid Talent Attraction Program (Programa de Atracción de Talento de la Comunidad de Madrid, modalidad 1, 2019-T1/AMB-12782). Thanks to Manuel Sala-Pérez, Francesca Sangiorgi, Arjen Grothe and Ali Soliman for discussion of the dinocyst taxonomy and ecology during preparation of this manuscript. We are also very grateful to Sergei Lazarev, Wout Krijgsman, Dan

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