Research articleFlooding of the Caspian Sea at the intensification of Northern Hemisphere Glaciations
Introduction
Semi-isolated basins like the Mediterranean and Paratethys (former Black Sea-Caspian Sea domain) are exceptionally suited for the study of (paleo-) environmental changes as a function of relative sea level fluctuations. These land-locked areas are at present only connected to the open ocean via shallow and narrow gateways, and decreased hydrological exchange had profound influences on environmental conditions (e.g. temperature, salinity, humidity; de la Vara et al., 2016). For example, increased Mediterranean-Paratethys connectivity played an important role during the latest Messinian (Van Baak et al., 2016, Van Baak et al., 2017), drastically changing salinities in both basins (Krijgsman et al., 2010; van Baak et al., 2015), allowing bidirectional faunal exchange (Stoica et al., 2013; Grothe et al., 2018). Sea level dropped in the basins and both Paratethys and Mediterranean temporarily transformed into enormous endorheic lakes.
Paratethys water level changes can indirectly influence the hydrological budget of the Mediterranean Sea and at the same time control temperatures on the Eurasian continent. A large Paratethys sea acts as a thermal regulator, allowing an oceanic climate to develop over the continental interior of Eurasia, thereby decreasing Eurasian summer temperatures and as a result decreasing the strength of the South Asian summer monsoon (Ramstein et al., 1997). Therefore, a transition from a series of small endorheic lakes to a large interconnected Paratethyan sea will significantly influence the atmospheric continent-ocean contrast. A large Paratethys sea may further affect the North African monsoon circulations and decreases sensitivity to orbital forcing in this region, prohibiting the oscillation between arid (desert) and humid (green Sahara) periods paced by precession (Zhang et al., 2014).
One of the most extreme changes in Paratethys surface area occurred during the Pliocene-Pleistocene transition interval, with the so-called Akchagylian flooding of the Caspian Basin. During most of the Pliocene, the Black Sea and Caspian basins were endorheic lakes of much smaller size compared to the present-day (Fig. 1a). Coarse-grained deposits of the paleo-Volga delta prograded ~700 km southward into the South Caspian Basin (SCB) to form the main reservoirs of the South Caspian oil province (Abdullayev et al., 2012). This long-term fluvio-deltaic lowstand was rapidly replaced by highstand brackish-marine conditions for much of the Early Pleistocene (e.g. Van Baak et al., 2013). Related to this sea-level rise, the surface area of the Caspian Sea increased fivefold, predominantly over the relatively flat northern shelf region (Fig. 1b), and the Caspian Sea reconnected to the Black Sea, which in turn potentially drained into the Mediterranean (Popov et al., 2006).
Causal mechanisms of the Akchagylian flooding of the Paratethys are still poorly understood and a process-based understanding of the consequences is conspicuously lacking due to a general absence of high-resolution age constraints (Krijgsman et al., 2019). Widely varying age estimates between 4.2 and 1.2 Ma have been proposed and it is currently unknown how this flooding correlates to the global marine record and the terrestrial records of Central Asia (see Van Baak et al., 2013 for details). Here, we combine high-resolution magnetostratigraphy and 40Ar/39Ar geochronology to place the Plio-Pleistocene flooding of the Caspian Basin and its reconnection to the Black Sea into a global framework. We create a basic magnetostratigraphic age model for the Jeirankechmez section in Azerbaijan to place the large scale paleoenvironmental changes, previously documented using micropaleontology and palynology (Richards et al., 2018), in a chronological context. Water fluxes to the Caspian Basin are quantitatively constrained using 87Sr/86Sr ratios. Finally, we discuss the causes and consequences of the major hydrological shift in Central Eurasia occurring during the Plio-Pleistocene transition interval.
Section snippets
The Jeirankechmez section
The Plio-Pleistocene flooding of the Caspian Basin can be excellently studied in land-based sections in Azerbaijan. In the Gobustan region, southwest of the capital Baku, long and complete records of Plio-Pleistocene sediments are exposed in outcrops continuous for several kilometers along strike (Fig. 2). The Jeirankechmez section crops out in a series of interconnected hills, covering a total stratigraphic thickness of ~1600 m on the southeastern flank of an anticline along the Jeirankechmez
Magnetostratigraphy
Paleomagnetic sampling and processing followed standard techniques as described by e.g. Van Baak et al. (2013). 268 levels were sampled with double cores at each level. Paleomagnetic measurements were performed at the paleomagnetic laboratory Fort Hoofddijk of Utrecht University (The Netherlands). Bulk magnetic susceptibility was measured for each specimen on an AGICO MFK1-FA. A total of 217 specimens were thermally demagnetized up to a maximum temperature of 620 °C (typical for the
87Sr/86Sr analyses
To quantitatively assess the potential connectivity changes related to the arrival of marine faunas in the Akchagylian, we measured 87Sr/86Sr isotopic composition on ostracods. In semi-isolated basins such as the Paratethyan seas, 87Sr/86Sr are very indicative of changing basin connectivity and changes in riverine input (for details see Vasiliev et al., 2010). All 87Sr/86Sr analyses were performed on handpicked ostracod specimens of Ilyocypris bradyi and Caspiocypris. Strontium isotopic
Eurasian continental hydrology across the iNHG
The Caspian Sea drainage basin plays an important role in the paleoenvironmental evolution of Central Eurasia. At present, the Volga river supplies >80% of the total water flux into the Caspian Sea and its discharge is regulated by precipitation variations in the high northern latitudes. Long rivers like the Volga, but also the Amu Darya and Syr Darya transport water for 1000's of kilometers, and allow for climatic processes of the high northern latitudes and even the Pamir and Himalaya
Conclusion
The water level of the Caspian Sea, the biggest lake on Earth, acts distinctly out of phase compared to the open ocean sea level. We show that the intensification of Northern Hemisphere Glaciations around 2.7 Ma coincided with a major water level increase in the Caspian Sea. A marine connection to the Caspian Sea may have existed between 2.7 Ma and 2.4 Ma, after which a lowering of the salinity in the Caspian Sea occurred. During the early Pleistocene, the Caspian Sea remained a significantly
Acknowledgements
This work was financially supported by the Netherlands Geosciences Foundation (ALW) with support from the Netherlands Organization for Scientific Research (NWO) through the VICI grant nr. 865.10.011 of WK and by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES). All Dutch and Azerbaijani students who contributed to the Jeirankechmez fieldworks are thanked for their efforts. We thank the two anonymous reviewers for their insightful comments.
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2021, Quaternary InternationalCitation Excerpt :The chronology of the Akchagylian regional stage of the Caspian region of Eastern Paratethys is traditionally bracketed between ca. 3.5 and 2 Ma (Trubikhin, 1977; Semenenko and Pevzner, 1979; Nevesskaya and Trubikhin, 1984; Nevesskaya et al., 2005). Although the recent radiometric data suggested a younger age estimates for early Akchagylian (van Baak et al., 2019), the position of the highest water stand is reliably dated at the Pliocene-Pleistocene transition (Krijgsman et al., 2019). During the maximum of the Akchagylian transgression its level could exceed 50–150 m above the present world sea level, and decreased down to the present sea level by the beginning of Calabrian, as it is deduced from (Popov et al., 2010; Krijgsman et al., 2019).
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