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Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Quaternary time scales for the Pontocaspian domain: Interbasinal
connectivity and faunal evolution
W. Krijgsman
a,⁎
, A. Tesakov
b
, T. Yanina
c
, S. Lazarev
a
, G. Danukalova
d
, C.G.C. Van Baak
e
,
J. Agustí
f
, M.C. Alçiçek
g
, E. Aliyeva
h
, D. Bista
i
, A. Bruch
j
, Y. Büyükmeriç
k
, M. Bukhsianidze
l
,
R. Flecker
i
, P. Frolov
b
, T.M. Hoyle
a
, E.L. Jorissen
a
, U. Kirscher
m
, S.A. Koriche
n
,
S.B. Kroonenberg
o
, D. Lordkipanidze
l
, O. Oms
p
, L. Rausch
q
, J. Singarayer
n
, M. Stoica
q
,
S. van de Velde
r
, V.V. Titov
s
, F.P. Wesselingh
r
a
Department of Earth Sciences, Utrecht University, Budapestlaan 17, Utrecht 3584, The Netherlands
b
Geological Institute of the Russian Academy of Sciences, Pyzhevsky 7, Moscow 119017, Russia
c
Lomonosov Moscow State University, Moscow 119991, Russia
d
Russian Academy of Sciences, Institute of Geology of the Ufimian Scientific Centre, K. Marx St. 16/2, Ufa 450077, Russia
e
CASP, West Building, Madingley Rise, Madingley Road, Cambridge CB3 0UD, UK
f
ICREA. Institut Català de Paleoecologia Humana i Evolució Social (IPHES), Universitat Rovira i Virgili, Tarragona, Spain
g
Department of Geology, Pamukkale University, Denizli 20070, Turkey
h
Geological Institute of Azerbaijan (GIA), H. Javid Av. 29A, AZ1143, Baku, Azerbaijan
i
BRIDGE, School of Geographical Sciences and Cabot Institute, University of Bristol, University Road, Bristol BS8 1SS, UK
j
Senckenberg Forschungsinstitut Senckenberganlage, Frankfurt 25 60325, Germany
k
Department of Geological Engineering, Bülent Ecevit University, Incivez/Zonguldak 67100, Turkey
l
The National Museum of Georgia, 3 Purtseladze St., 0107, Tbilisi, Georgia
m
Earth Dynamics Research Group, Department of Applied Geology, WASM, Curtin University, Perth, Australia
n
Department of Meteorology and Centre for Past Climate Change, University of Reading, UK
o
Department of Applied Earth Sciences, Delft University of Technology, Delft 2600, The Netherlands
p
Fac. de Ciències Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain
q
Department of Paleontology, Bucharest University, Bălcescu Bd. 1, Bucharest 010041, Romania
r
Naturalis Biodiversity Center, P.O. Box 9517, Leiden 2300, The Netherlands
s
Russian Academy of Sciences, Institute of Arid Zones, Chekhova 41, Rostov-on-Don 344006, Russia
ABSTRACT
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 con-
tradicting 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.
1. Introduction
The Black Sea and Caspian Sea basins are the present-day remnants
of the ancient Paratethys Sea (Laskarev, 1924), an epicontinental
water-mass that developed since the earliest Oligocene in central Eur-
asia as the northern branch of the Tethys Ocean. It was separated from
the southern, Mediterranean branch by the Alpine-Caucasus-Himalayan
orogenic belt that progressively formed by ongoing tectonic collision of
the Eurasian plate with the African-Arabian and Indian plates (Rögl,
1999;Popov et al., 2006). In Oligo-Miocene times, the Paratethys Sea
covered large parts of Europe and Asia, stretching from southern Ger-
many in the west to western China in the east. A complex combination
of Mio-Pliocene tectonic uplift, glacio-eustatic sea level fluctuations,
and sedimentation by major deltaic systems, progressively filled the
https://doi.org/10.1016/j.earscirev.2018.10.013
Received 13 July 2018; Received in revised form 9 October 2018; Accepted 16 October 2018
⁎
Corresponding author.
E-mail address: w.krijgsman@uu.nl (W. Krijgsman).
Earth-Science Reviews 188 (2019) 1–40
Available online 18 October 2018
0012-8252/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/BY/4.0/).
T
marginal sedimentary basins in the west and east. Consequently, the
Paratethys sea drastically retreated, which influenced the regional cli-
mate of Eurasia (Ramstein et al., 1997) and facilitated mammal (in-
cluding hominid) migration between Africa, Asia and Europe (Bar-Yosef
and Belmaker, 2011).
Throughout its entire history, Paratethys formed a series of re-
stricted basins, separated by shallow, tectonically active, gateways, that
were extremely sensitive to small climatic and tectonic variations (e.g.,
Popov et al., 2006;Palcu et al., 2017). The semi-enclosed basin con-
figuration resulted in extreme palaeoenvironmental dynamics including
anoxic, hypersaline, brackish to fresh water conditions. Some salinity
regimes in the basin, that were supposed to require connectivity to the
open ocean, have been a major scientific puzzle for many centuries
(Fig. 1:Kircher, 1678). The long-lived isolated position of the basins,
combined with the exceptional palaeoenvironmental conditions, cre-
ated faunal communities that are endemic to the Paratethys region, and
that waxed and waned through geological history (e.g., Harzhauser
et al., 2002). They obtained maximum extension during the latest
Messinian (Lago-mare) times ~5.5 Ma, when Paratethyan faunas oc-
cupied the entire Mediterranean Basin as well (e.g., Guerra-Merchán
et al., 2010;Stoica et al., 2016). Today the Paratethyan faunas are at
their minimum: the so-called Pontocaspian communities are now al-
most entirely restricted to small enclaves in the major deltaic and es-
tuarine systems of the northern Black Sea as well as the Caspian Sea
(Grigorovich et al., 2003;Yanina, 2012a). The origin, evolution and
migration of these characteristic Pontocaspian faunal elements is still
not fully understood (e.g., Wesselingh et al., 2008).
The Pliocene-Quaternary co-evolution of the Black Sea-Caspian Sea
is dominated by major changes in sea/lake levels. These may have been
driven by external components, resulting from opening and closing
gateways to the Mediterranean or Arctic ocean, as well as by internal
components where hydrological and climatic changes induced by gla-
cial-interglacial cycles may have created periods of intermittent inter-
basinal connectivity (e.g., Badertscher et al., 2011;Yanina, 2014). A
simplified scenario is that the two basins were isolated during low-
stands, when individual water levels, environmental conditions, and
faunal composition were largely determined by the local hydrological
budgets. The two basins became connected during highstands of the
Caspian Sea. During such periods, overflow of the Caspian Sea through
the Manych low-land connection north of the Greater Caucasus enabled
faunal exchange (Fig. 2). Environmental conditions became similar in
both basins by mixing of the water masses and consequently migration
and blending of the Pontocaspian fauna took place. Additionally, the
Black Sea became connected to marine waters of the Mediterranean
during interglacial highstands and the Pontocaspian biota were mar-
ginalized. These marine transgressions did not reach the Caspian Basin.
Sea/lake level changes in the Pontocaspian region have been gigantic.
Over 1000 m lake level rise has been proposed for the late Pliocene
Productive Series-Akchagylian transition in the South Caspian Basin
(see van Baak et al., 2017 and references therein). Also on short time
scales, lake level changes were very significant as shown by Caspian
Holocene variations of ~100 m affecting historic settlements on the
Caspian coast (Kroonenberg et al., 2007). The Quaternary sea/lake
level history of the Black Sea –Caspian Sea domain is still enigmatic
Fig. 1. Ancient map of the Pontocaspian region after Kircher (1678), who in his “Mundus Subterraneus”already envisaged that the Caspian Basin must have been
connected to the open ocean to explain its relatively high salinity (> 10 ‰) today. The Caspian Sea is in fact an isolated long-lived lake since at least 2.6 Ma. Kircher
considered a subterraneous channel to the Persian Gulf for the marine connection. The location of this marine connection is still enigmatic today.
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
2
and awaits consistent interpretations, although significant progress has
been made recently by geochemical proxies (e.g. strontium ratios,
oxygen isotopes) that may clarify the timing and amount of con-
nectivity (Major et al., 2006;Badertscher et al., 2011).
Understanding the fundamental mechanisms and processes that
influenced both the geological and historical changes in sea level,
connectivity, climate, and environment (e.g. salinity, anoxia, etc.) is
also crucial for a coherent understanding of the future economical and
sustainable developments in the region. The immensely rich hydro-
carbon fields of both the Black Sea and especially the South Caspian
Basin are the product of changing interbasinal connectivity, that gen-
erated the anoxic source rocks of the Oligocene Maikop Series, the
deltaic reservoir rocks of the Pliocene “Productive Series”and the
brackish water cap rock of the Plio-Pleistocene Akchagylian clays (e.g.,
Hinds et al., 2004;Vincent et al., 2010). Paleoenvironmental changes in
the region are causing the biodiversity crisis that the Pontocaspian
fauna is experiencing today (e.g., (Grigorovich et al., 2003;Popa et al.,
2009).
One of the key problems to understand the complex and intertwined
geological history of the Pontocaspian region is the absence of reliable
stratigraphic correlations between the Black Sea and Caspian Sea and
between the lake/marine and continental domains. The lack of open
marine faunal assemblages in the Paratethys generally hampers a
straightforward correlation to the standard geological time scale, and
the presence of mainly endemic faunas resulted in regional time scales
for the different Paratethyan subbasins (e.g., Hilgen et al., 2012;
Nevesskaya et al., 2003). For the Quaternary, individual time scales
have been developed for the Caspian and the Black Sea region, both
mainly based on their own characteristic faunal (usually mollusc) as-
semblages from rich, but local sites (e.g., Yanina, 2014). Cross-
correlation has mainly been done on biostratigraphic arguments, be-
cause radiometric, magnetostratigraphic and astronomical data are
scarce and/or their results controversial. Carbon dating has been
thoroughly applied to Holocene rocks (e.g., Yanina, 2014 and refer-
ences therein), but radiometric datings (K/Ar or Ar/Ar) of older Qua-
ternary rocks are very rare, because of the common lack of intercalated
volcanic ashes (e.g., Chumakov and Byzova, 1992). Magnetostrati-
graphic correlations to the geomagnetic polarity time scale have been
widely produced for the lower Quaternary successions (e.g.,
Molostovsky, 1997). They are more problematic for rocks younger than
780 kyr (last full reversal of the magnetic field), because there this
technique only works if it is possible to sample with a resolution that is
high enough (< 1 ka) to pick up the reversal excursions of the Brunhes
chron (Laj and Channell, 2007;Singer et al., 2014). Consequently,
many of the regional stage boundaries are still poorly dated and serious
age uncertainties exist between the current geological time scales for
the region.
Here, we present a comprehensive overview of the existing strati-
graphic and geochronologic data for the Caspian Sea, the Black Sea and
adjacent continental domains. The main result will be an update of the
Quaternary geological time scale for the Pontocaspian region which
will allow better understanding of the succession of geological events,
especially dealing with the major changes in interbasinal connectivity
and faunal evolution. We will furthermore discuss the state-of-the-art
on the existing techniques and mechanisms that allow a better under-
standing of Quaternary paleoenvironments, interbasinal connectivity,
fauna migration patterns and sea level change in these long-lived
anomalohaline lake systems, and the corresponding environmental
changes in the terrestrial counterparts.
Fig. 2. Present-day drainage area of the Pontocaspian domain. Yellow circles denote the locations of the stratotype sections of the main Quaternary stages of the
Caspian Basin and Black Sea Basin: 1) Akchaghylian on Krasnovodsk peninsula (Turkmenistan), 2) Apsheronian on Apsheron Peninsula (Azerbaijan), 3) Bakunian in
Baku (Azerbaijan), 4) Kuyalnikian (Ukraine), 5) Gurian (Georgia), 6) Chaudian on Cape Chauda (Crimea) and 7) Uzunlarian (Crimea). White circles denote the
locations of key sections: 1) Pyrnuar (N38.93, E56.26), 2) Malyi Balkhan (N39.27, E54.97), 3) Yuzhny Urundzhik (N39.27, E54.50), 4) Ushak (N40.45, E53.37), 5)
Lokbatan (N40.33, E49.75) and Jeirankechmez (N40.24, E47.09), 6) Duzdag (N40.70, E46.92) and Bozdag (N40.80, E46.84), 7) Pantashara (N41.23, E46.36) and 8)
Kvabebi (N41.48, E45.68) and Kushkuna (N41.25, E45.44).
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
3
2. Quaternary time scales of the Pontocaspian domain
2.1. The Caspian Sea region
The Caspian Sea is a lake: it is the world’s largest endorheic water
body (Fig. 2) extending over 1200 km in latitude (36°-47°N), and 195-
435 km in longitude (46°-56°E). The surface area and water volume of
the Caspian Basin critically depend on the regional hydrological bal-
ance. The Caspian Sea is divided into three subbasins of roughly similar
surface area, but widely differing in depth and volume. The North
Caspian Basin (< 15m deep, 1% volume) is separated by the Man-
gyshlak sill from the Middle Caspian Basin (< 800m, ~33%) which is
in turn separated by the Apsheron threshold from the South Caspian
Basin (< 1025m, ~66%) (Panin et al., 2005;Zonn et al., 2010). At
present, Caspian water level is ~27 m below global sea level, which
gives a surface area of ~371,000 km
2
and volume of ~78,200 km
3
. The
Caspian Basin is a huge reservoir of anomalohaline (often referred to as
brackish) water. It is highly sensitive to climatic changes in its catch-
ment area (3.5 million km
2
), which extends far northward to the central
part of the East European Plain (Panin et al., 2005;Zonn et al., 2010).
During the Quaternary, the catchment extended to include almost en-
tire Central Asia. Today, the Caspian catchment contains forests and
steppes in the Volga and Ural valleys and mountainous forests and arid
regions in the Caucasus and Transcaspian areas. The salinity of the
present-day Caspian Sea changes from 1‰near the Volga delta in the
north to 13.5 ‰in the south (Dumont, 1998). The Volga discharge
provides 85-90% of the total fresh water influx and forms the main
element in the hydrological budget (Agapova and Kulakova, 1973;
Mamedov, 1997;Zonn et al., 2010).
In late Miocene (Pontian) times the Caspian Basin was still con-
nected to the Black Sea, forming the final phase of the ancient
Paratethys Sea (Popov et al., 2004, 2006;Krijgsman et al., 2010). The
Caspian Basin became isolated from the Black Sea in the earliest Plio-
cene (Van Baak et al., 2016a), when a major drop in its water level
resulted in far southward retreat of lake environments and an asso-
ciated progradation of the Volga’sfluvio-deltaic deposits that reached
the South Caspian Basin (Fig. 3a). This so-called Productive Series is the
main South Caspian hydrocarbon reservoir unit (e.g., Hinds et al.,
2004). The nature and extent of lake conditions in the southern basin at
the time are unknown as these deposits are often several km below
surface. Since its isolation from the Black Sea and open ocean, the
Caspian Basin has experienced numerous transgressions and regressions
with water level fluctuations of several tens to hundreds of meters re-
sulting in enormous changes of its shoreline, especially in the flat
northern part (Varuschenko et al., 1987;Svitoch, 2010a;Yanina, 2014).
Our review of the Caspian stratigraphy starts with the late Pliocene
Akchagylian transgression that resulted in the first of several lake
phases that extend all the way to the modern Caspian Sea. Here we
review the definitions and commonly used Plio-Quaternary strati-
graphic subdivisions in the Pontocaspian domain, including their geo-
chronological constraints, which allow a detailed correlation of the
Caspian Sea with the Black Sea and the terrestrial and global ocean
records.
2.1.1. Akchagylian
2.1.1.1. Description. During the Akchagylian age (late Pliocene-earliest
Pleistocene) the largest Caspian transgression occurred, with shores
extending well into the middle Volga and southern Urals to the north as
well as the Sea of Azov in the west and the Aral Sea in the east (Fig. 3b).
The Caspian Basin was a saline lake with major endemic faunal
radiations but also events occurred that saw the introduction of
marine foraminifera. The widespread fine grained intervals of the
Akchagylian form the hydrocarbon cap rocks in many places in the
Caspian Basin.
The Akchagylian stage was defined by Andrusov (1912), who de-
scribed these deposits from the Krasnovodsk Peninsula on the eastern
side of the Caspian Sea (Andrusov, 1889, 1902, 1906) and named this
stage after the Akchagylian district (Fig. 2). Andrusov did not designate
a stratotype, and later the Ushak well was proposed as lectostratotype
(a stratotype used to describe a stratigraphic unit that does not have a
sufficient holostratotype). The stratigraphic profile of the Ushak well
was thoroughly studied by many authors; the Akchagylian deposits are
around 100 m thick but the base is not exposed (Dvali et al., 1932;Ali-
Zade, 1961;Cheltsov, 1965;Nevesskaya, 1975a;Danukalova, 1996). In
the stratotype area, Akchagylian deposits transgressively overlie Neo-
gene red-colored rocks and are in turn overlain by Apsheronian lime-
stones (Ali-Zade, 1961). In the Ushak profile, Ali-Zade (1961) and
Cheltsov (1965) determined: 1) a 21 m lower unit consisting of alter-
nations of calcareous clays, shelly limestones, calcareous sandstones
and two thin layers of volcanic ashes, 2) a 25 m middle unit comprising
grey sandstones with ferruginous concretions, and 3) a 59 m upper unit
consisting of an alternation of clays, siltstones, sandstones and shelly
limestones. Because the Akchagylian in the Ushak well section has no
clear boundaries with underlying and overlying units, Danukalova
(1996) suggested the Malyi Balkhan section in western Turkmenistan as
neostratotype (a stratotype designated after the holo/lectostratotype as
a replacement for it) and the Pyrnuar section as hypostratotype (an
additional stratotype,indifferent geographic context). The Pantashara
and the Bozdag/Duzdag sections in Transcaucasia (Azerbaijan) were
also proposed as neostratotypes (Danukalova, 1996).
The Akchagylian deposits are widespread in the Caspian area and
their distribution shows the maximum extent of the Akchagylian
transgression (Fig. 3b). The maximum thickness of the Akchagylian (up
to 750 m) is reached in the deep Caspian basins whereas it is generally
much thinner (0.5-10 m) in the periphery of the basin. In several sec-
tions of Azerbaijan (Lokbatan, Jeirankechmez) the Akchagylian overlies
the deltaic deposits of the Productive Series of the South Caspian Basin
(Van Baak et al., 2013;Van Baak, 2015). In eastern Georgia, (e.g.,
Kvabebi section), the Akchagylian is deposited above the so-called pre-
Akchagylian unconformity (Buleishvili, 1960), indicating syntectonic
deformation in the western Kura Basin (see Adamia et al., 2017). It is
angularly unconformable and paraconformable at some points. Ak-
chagylian deposits are overlain by conglomerate and sandy units there.
Palynological data of the northern regions show dominating taiga for-
ests (Kuznetsova, 1966) whereas the southwestern regions contain
pollen that indicate open steppe landscapes (Kovalenko, 1971;Naidina
and Richards, 2016).
The Akchagylian mollusc faunas are best characterised by the high
number of endemic mactrid and cardiid bivalve species (Danukalova,
1996). The Akchagylian is often subdivided into three substages, based
on its mollusc assemblages (Golubyatnikov, 1904, 1908;Kolesnikov,
1940;Ali-Zade, 1954;Yakchemovich et al., 1970;Paramonova, 1994).
The lower substage is marked by low variety of genera and species
containing Aktschagylia subcaspia, A. karabugasica, A. inostrantzevi,
Cerastoderma dombra dombra. The middle substage is characterized by
high species numbers within the genera Aktschagylia, Andrussovi-
cardium, Miricardium and Avicardium. The upper substage is character-
ized by low numbers of mollusc species including Aktschagylia sub-
caspia, A. ossoskovi, Cerastoderma dombra dombra and species of
Dreissena and Theodoxus. The gastropods Pirenella caspia s.l.,“Clessi-
niola”intermedia, “C.”utvensis, and “C.”vexatilis occur in all substages.
The usage of this threefold scheme may lead to an arbitrary allocation
of units, especially as it is only based on high or low mollusc species
richness. For example, other investigations demonstrated that middle
Akchagylian molluscs appeared at different stratigraphical levels,
which led to an alternative subdivision of the Akchagylian in two
substages (Popov, 1969;Trubikhin, 1977;Danukalova, 1996;
Nevesskaya et al., 2003).
The lower part of the Akchagylian is marked by a sudden occurrence
of euryhaline marine foraminifera like Cassidulina crassa,C. prima,
possibly C. reniforme, and C. obtusa,Ammonia beccarii, Cibicides loba-
tulus, Spirillina sp., Discorbis multicameratus, Miliolina aksaica
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
4
(Agalarova, 1976;Richards et al., 2018) as well as small sized biserial
Bolivinidae. The latter are nowadays considered to belong to the
planktonic genus Streptochilus (Smart and Thomas, 2006, 2007). During
this marine influx, the freshwater dominated Pliocene ostracod fauna
became replaced by an anomalohaline (“brackish”) water assemblage
consisting of Limnocythere alveolata,L. luculenta,L. tschaplyinae,Typh-
locypris gracilis, Loxoconcha eichwaldi, Candona candida, and C. combibo
(Van Baak et al., 2013;Fig. 4). The upper part of the Akchagylian is
characterized by the presence of Eucythere naphtatscholana, Amnicythere
andrussovi, A. nata, A. multituberculata,A. cymbula, Leptocythere gubkini,
Euxinocythere praebosqueti, Loxoconcha babazananica, and Camptocypria
acronasuta.
2.1.1.2. Correlation. The age of the Akchagylian is mainly based on
paleomagnetic investigations and correlation to the Geomagnetic
Polarity Time Scale (GPTS) in combination with radio-isotopic dating.
The interpreted ages of the boundaries are subject of debate. One of the
first paleomagnetic results was obtained from western Turkmenistan
and indicated that the onset of the Akchagylian in the South Caspian
Basin started slightly below a normal-reversed boundary (Khramov,
1960, 1963). We consider this now the Gauss-Matuyama boundary,
which corresponds to an age of 2.58 Ma. Note here that the first
geomagnetic polarity time scale ever published, appeared later (Cox
et al., 1963). Magnetostratigraphic investigations of numerous sections
in Turkmenistan and Azerbaijan confirmed this N-R polarity pattern
straddling the base of the Akchagylian (Trubikhin, 1977). The Pyrnuar
section in western Turkmenistan, however, showed two small reversed
zones in the lower Akchagylian, which were correlated with chrons
C2An.2r (Mammoth) and C2An.1r (Kaena) by Trubikhin (1977).
Consequently, the base of the Akchagylian was matched to the base
of the Gauss chron (C2An) at an age of 3.6 Ma, a correlation that has
been accepted in numerous publications (Trubikhin, 1977;Semenenko
and Pevzner, 1979;Sidnev, 1985;Molostovsky, 1997). Recently,
Gurarii (2015) re-investigated the polarity pattern of the Pyrnuar
section and concluded that the two reversed zones are too small to be
interpreted as Mammoth and Kaena and that the base of the
Akchagylian correlates better with the upper part of the Gauss chron
(between 3.0 and 2.6 Ma).
In the northern Pre-Caspian region, the base of the Akchagylian
Stage has been placed at the lower Nunivak chron (C3n.2n) at an age of
~4.5 Ma, but the main Akchagylian transgression event there was also
dated at the Gauss-Matuyama boundary (Yakhemovich et al., 1981,
2000). Hence, it was suggested to correlate the lower/upper substage
boundary with the Gauss/Matuyama reversal (Nevesskaya et al., 2005).
The Akchagylian deposits of the Azov region and the Northern Greater
Caucasus only show a single normal to reverse polarity change, inter-
preted as the Gauss-Matuyama boundary, although the base Akchagy-
lian is tentatively placed at the Gilbert-Gauss reversal at 3.6 Ma
(Naidina and Richards, 2016).
The ash layers from the lowermost Akchagylian of Azerbaijan have
been dated by Chumakov (Chumakov et al., 1988) using a fission track
method at 3.34 ± 0.5 Ma, although they later reported that these ages
were not very accurate and had to be confirmed by new data
(Chumakov et al., 1992). Magnetostratigraphic data from the Lokbatan
Fig. 3. Paleogeographic maps for the Plio-Pleistocene Pontocaspian region. A) Middle Pliocene; B) Late Pliocene; C) Early Pleistocene; D) Middle Pleistocene. Based
on Vinogradov, 1961, 1969 and Abdurakhmanov et al. (2002).
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
5
section were not straightforward but led to the conclusion that the
lower Akchagylian boundary should be around ~3.2 Ma (Van Baak
et al., 2013). This was in good agreement with results from the Kvabebi
section in eastern Georgia where the uppermost two normal subchrons
of Gauss were attributed to the Akchagylian (Agustí et al., 2009). In the
Kushkuna section of Azerbaijan, however, the lowermost Akchagylian
only contains one normal polarity zone, correlative to the top Gauss
(Trubikhin, 1977). Recently, the Akchagylian ash layers of the Lok-
batan and Jeirankechmez sections of Azerbaijan were radio-isotopically
dated with the
40
Ar/
39
Ar method, which resulted in a revised age of 2.7
Ma for the base of the Akchagylian in Azerbaijan (Van Baak, 2015).
This is in excellent agreement with the magnetic polarity pattern of the
Jeirankechmez section that shows the two reversed subchrons of the
Mammoth and Kaena in the upper part of the Productive Series
(Khramov, 1963;Van Baak, 2015). The earlier magnetostratigraphic
correlation of Lokbatan was thereby rejected (Van Baak, 2015).
In conclusion, most evidence and interpretations converge towards
two different ages for the base of the Akchagylian (Fig. 5). The “classic
option”mainly relies on the magnetostratigraphic correlation of the
Pyrnuar section and dates the base of the Akchagylian to the base of the
Gauss at an age of 3.6 Ma. This is the officially accepted age in Russian
stratigraphy (Provisions, 2003;Nevesskaya et al., 2005). The “young
option”mainly depends on sections in Azerbaijan, where the integra-
tion of
40
Ar/
39
Ar dating and magnetostratigraphy indicates an age of
2.7 Ma for the base of the Akchagylian, in the uppermost normal part of
the Gauss chron (Khramov, 1960;Van Baak, 2015).
2.1.2. Apsheronian
2.1.2.1. Description. Caspian Sea conditions in the Early Pleistocene
Apsheronian were similar to today. The Caspian Basin was occupied by
an anomalohaline lake. A major salinity decrease during the late
Akchagylian corresponds with the almost complete extinction of
characteristic Akchagylian bivalve faunas and subsequently the
typical Apsheronian fauna evolved. This endemic fauna became
increasingly dominated by extant endemic Caspian groups. During
the Apsheronian, the Caspian Basin was mostly an isolated basin that
may have had rare, short-lived connections only to the Black Sea via the
Manych Strait (Fig. 3c).
The Apsheronian stage was established based on outcrops on the
Apsheronian peninsula (Fig. 2) near Baku (Azerbaijan), and the Bailov
Cap district was proposed as lectostratotype locality (Andrusov, 1923;
Nevesskaya, 1975b). There, the Apsheronian deposits conformably
overlie the Akchagylian deposits and are overlain, with an angular
unconformity, by Bakunian strata (Nevesskaya, 1975b;Stratigraphy,
Fig. 4. Distribution of the main ostracod species, foraminifera and charophyta in the Caspian Basin during the Plio-Pleistocene, based on literature data. 1) Cyprideis
torosa;2)Ilyocypris gibba;3)I. bradyi;4)Cyprinotus salinus;5)Eucypris sp.; 6) Pseudocandona compressa (juvenile); 7) Limnocythere aralensis;8)Zonocypris membranae;
9) Darwinula stevensoni;10) Limnocythere alveolata; 11) L. luculenta; 12) L. tschaplyinae; 13) Typhlocypris gracilis; 14) Loxoconcha eichwaldi (different stages of evo-
lution); 15) Candona candida; 16) C. combibo; 17) Eucythere naphtatscholana; 18a) Amnicythere andrussovi; 18b) A. palimpsesta; 18c) A. normalis; 18d) A. saljanica; 19)
Amnicythere nata; 20) Leptocythere gubkini; 21) A.multituberculata; 22) Euxinocythere praebosqueti; 23) A. cymbula; 24) Loxoconcha babazananica; 25) L. petasa; 26)
Camptocypria acronasuta; 27) Caspiocypris filona; 28) Tyrrhenocythere bailovi; 29) Xestoleberis chanakovi; 30) T. amnicola donetziensis; 31) T. azerbaidjanica; 32) T.
papillosa; 33) Camptocypria acronasuta; 34) Bacunella dorsoarcuata; 35) Loxoconcha gibboides; 36) L. lepida; 37) Euxinocythere bosqueti; 38) Cytherissa bogatschovi; 39)
Euxinocythere bacuana; 40) A. quinquetuberculata; 41) Loxoconcha endocarpus
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
6
1982).
The size of the Caspian Sea slightly decreased compared to the
Akchagylian, so Apsheronian deposits are, almost everywhere, found
conformably overlying Akchagylian strata (Stratigraphy, 1982). Paly-
nological data mainly reveal treeless landscapes, indicating an in-
tensification of the continental climate and an increasing aridity when
compared to the Akchagylian (Naidina and Richards, 2016).
The Apsheronian Stage is subdivided into three substages, based on
changes in the composition of the mollusc fauna (Andrusov, 1923;
Kolesnikov, 1940;Zhidovinov et al., 2000;Sidnev, 1985). The lower
substage is characterized by a species-poor assemblage of low saline to
fresh water bivalves (Dreissena, Corbicula, Apsheronia) and gastropods
(Lymnaea, Streptocerella, Turricaspia, Theodoxus). The middle substage is
marked by the first occurrence of the mollusсgenera Parapsheronia and
Didacna. The upper zone is characterized by a general depletion of the
saline fauna and the disappearance of the ribbed apsheronids of the
middle interval. This upper (Tyurkyanian) interval is poorly
recognizable in many parts of the Caspian Basin and in some regions
only a twofold division has been used (Stratigraphy, 1982).
Euryhaline foraminifera of the genus Ammonia spp. have been ob-
served at the base of the Apsheronian, suggesting a brief increase in
salinity. Most of the ostracod species from the Akchagylian continued to
evolve within the Apsheronian stage but became more frequent. The
Apsheronian comprises the ostracod genera Leptocythere and Caspiolla;
the middle Apsheronian is marked by a widespread radiation within the
genus Leptocythere (Fig. 4). Tyrrhenocythere azerbaidjanica,T. papillosa,
T. bailovi,Cyprideis torosa and Cytherissa bogatschovi are also commonly
observed (Fig. 4).
The Tyurkyanian stage was defined by Khain (1950) for the con-
tinental deposits (up to 100 m) between the Apsheronian and Bakunian
successions (Stratigraphy, 1984). These deposits contain fresh water
molluscs (Viviparus diluvianus,Valvata piscinalis,V.antiqua, Bithynia sp.,
Lithoglyphus naticoides,Pisidium amnicum,P. cf. supinum,P. cf. sub-
truncatum, Sphaerium rivicola,Unio sp., Dreissena sp.) and correspond to
Fig. 5. Plio-Quaternary time scales for the Pontocaspian domain. GPTS with Systems and Stages are after Hilgen et al. (2012), oxygen isotope curve with numbered
Marine Isotope Stages (MIS) is after Lisiecki and Raymo (2005). M=Mammoth (C2An.2r), K=Kaena (C2An.1r), R= Reunion (C2r.1n), O=Olduvai (C2n), C.M.=
Cobb Mountain (C1r.2n), J=Jaramillo (C1r.1n). Ages of the Cobb Mountain Subchron are after Channell (2017). On the right side are the time scales for the Caspian
Basin, Black Sea Basin and the terrestrial domain (mammal (MN/MQ) zonation and regional (MNR/MQR) biochronological units).
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
7
a strong (~150 m; Fig. 6) regressional phase (Danukalova et al., 2016;
Svitoch, 2013a;Svitoch, 2013b;Yanina, 2012a;Yanina, 2012b;
Zastrozhnov et al., 2018). In the fluvio-lacustrine Tyurkyanian deposits,
ostracods typical of shallow fresh water environments are dominant
(e.g., Ilyocypris bradyi, I. gibba, Pseudocandona compressa, Candona ne-
glecta, Cypris subglobosa, Darwinula stevensoni, Eucypris sp.). The Tyur-
kyanian deposits are well-expressed in the eastern margin of the South
Caspian Basin and are locally present in the North Caspian Basin where
they are known from drill cores and have a maximum thickness of 38 m
(Danukalova et al., 2016). Here, we consider the Tyurkyanian as the
upper part of the Apsheronian.
2.1.2.2. Correlation. An intensive paleomagnetic campaign to study the
Apsheronian deposits in the Pontocaspian region took place in between
1960 and 1980 (Khramov, 1960, 1963;Trubikhin, 1977;Kochegura
and Zubakov, 1978;Semenenko and Pevzner, 1979;Yakhemovich
et al., 1981). The magnetostratigraphic correlations of sections in
Azerbaijan and Turkmenistan were straightforward and have placed
the Akchagylian/Apsheronian boundary at the base of a normal
polarity interval, that was assumed to correspond to the Olduvai
subchron (C2n), corresponding to an age of 1.95 Ma (Kochegura and
Zubakov, 1978;Semenenko and Pevzner, 1979;Sidnev, 1985).
Recently, multidisciplinary investigations based on integration of
paleomagnetic, Ar/Ar dating, ostracod faunal analysis and lithological
changes have been conducted again in the Kura Basin of Azerbaijan and
found the base of the Apsheronian close to, but slightly below the base
of the Olduvai (Van Baak et al., 2013). Paleomagnetic data from the
Duzdag section in Azerbaijan confirm that the base of the Apsheronian
corresponds to a normal polarity interval (Pevzner, 1986). The
magnetic polarity pattern at Duzdag, however, indicates that this
normal polarity interval correlates to the Reunion subchron rather
than to the Olduvai, giving a slightly older date of ~2.1 Ma for the basal
Apsheronian.
Further revisions of the boundary have been proposed based on the
new definition of the Neogene-Quaternary boundary. In 1998, the
Interdepartmental Stratigraphic Committee of Russia (ISC) established
the Neogene-Quaternary boundary on Russian territory at the top of the
Olduvai subchron corresponding to an age of 1.8 Ma (Provisions, 1998).
With an aim of correlating the Russian national stratigraphic chart to
the global Geological Time Scale, the Akchagylian/Apsheronian
boundary was made equal to the Neogene-Quaternary boundary, and
therefore arrived at an age of 1.8 Ma. Since this time, many time scales
in the Pontocaspian region place the base of the Apsheronian at the top
of the Olduvai subchron (Nevesskaya et al., 2005).
The Tyurkyanian deposits are suggested to correspond to the low-
ermost part of the Middle Pleistocene (Semenenko and Pevzner, 1979;
Trubikhin, 1977). According to fission track data, however, the age of
the Tyurkyanian is 1050 to 950 ka (Ganzey, 1984).
In conclusion, two different ages exist for the top of the
Akchagylian/base of the Apsheronian (Fig. 5). The official geological
Fig. 6. Schematic reconstruction of the Black Sea and Caspian Sea water-level curves in comparison to global oxygen isotopes records of Lisiecki and Raymo (2005)
during the Pleistocene to Holocene. Associated interbasinal water exchanges marked by arrows pointing to the right for Mediterranean waters flooding into the Black
Sea, arrows pointing to the left for Caspian Sea waters flooding into the Black Sea and double arrows for bidirectional water exchange between Black Sea and Caspian
Sea. N.B. Two options exist in literature regarding the position of the Singilian: *Svitoch (2013b and references therein) and **Zastrozhnov et al. (2018 and
references therein).
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
8
time scale of Russia dates the Akchagylian/Apsheronian boundary at
1.8 Ma, resulting in a “long Akchagylian”of 1.8 Myr. The alternative
correlation, based on the Duzdag section of Azerbaijan, dates this
boundary at > 2.1 Ma, suggesting a “short Akchagylian”of 0.6 Myr if
the age of 2.7 for the base Akchagylian in Azerbaijan is used, or an
“intermediate Akchagylian”of 1.5 Myr if the age of 3.6 Ma for the base
of the Akchagylian is used (Fig. 5).
2.1.3. Bakunian
2.1.3.1. Description. Conditions in the Middle Pleistocene Bakunian
stage resembled the modern Caspian Sea in size, fauna and salinity
regimes (Fig. 3d). During highstands punctuated overflows towards the
Black Sea existed. Widespread carbonate rocks formed during warmer
phases of the Bakunian in the South Caspian Basin; these are the main
source for building material there.
The Bakunian Stage was defined by Sjögren (1891) and comprises
the sedimentary strata that conformably overlie the Apsheronian de-
posits in the eastern part of the Apsheron peninsula of Azerbaijan
(Fig. 2). Sjögren (1891) did not propose any stratotype and
Golubyatnikov (1904) suggested the section Gora Bakinskogo Yarusa
on the Apsheron Peninsula, which was later thoroughly studied by
other researchers (Nalivkin, 1914;Fedorov, 1957;Mamedov and
Aleskerov, 1988;Svitoch et al., 1992;Svitoch and Yanina, 1997;
Yanina, 2005). The Neftyanaya Balka section was suggested as an ad-
ditional reference section (Yanina, 2012b, 2013). Neftyanaya Balka is
situated in the Kura Basin and has the advantage that it contains well
defined stratigraphic boundaries for the Bakunian stage. The mollusc
fauna of the Bakunian stratotype is characterised by a number of bi-
valve species including Didacna parvula, D. catillus, D. rudis, D. cardi-
toides, D. eulachia, D. mingetschaurica, D. pravoslavlevi, D. lindleyi. The
taxa D. parvula and D. catillus are index-species, while D. rudis and D.
carditoides are considered characteristic species (Bogachev, 1932a,
1932b;Fedorov, 1957, 1978a;Vekilov, 1969;Svitoch et al., 1992;
Yanina, 2005, 2013;Nevesskaya, 2007;Svitoch and Yanina, 2007).
Sediments of the Bakunian are widespread on most Caspian coasts.
In tectonic depressions, Bakunian deposits are deeply buried, and are
only exposed in local uplifted structures. Sediments represent a range of
facies and their thickness varies between a few meters on high terraces,
to hundreds of meters (maximum thickness ~500 m) in the depressions.
Spores and pollen indicate a relatively cold and humid climate in the
early Bakunian, with forest formation (birch, alder, oak, maple, elm) on
the western coast (Abramova, 1974;Filippova, 1997) and forest-steppe
assemblages (pine, birch, alder, elm) in the lower Volga region
(Moskvitin, 1962). The late Bakunian was marked by moderately warm
and humid climate, as expressed by forest-steppes (oak, caracas, hop-
hornbeam) in the Kura Basin (Svitoch et al., 1998), widespread ever-
greens in Dagestan (Abramova, 1974), and steppes in the valleys of the
Ural river (Yakhemovich et al., 1986).
The base of the Bakunian is determined by a significant transgres-
sion that reached its maximum extent in the first half of the Middle
Pleistocene (=lower Neopleistocene), but was much smaller than the
Akchagylian transgression (Fig. 3c). The presence of Bakunian Didacna
parvula, D. rudis, D. carditoides, D. catillus in the Black Sea (Chaudian
Stage) and in the Manych Depression (Fedorov, 1978a;Popov, 1983;
Svitoch et al., 1998, 2010;Yanina, 2005, 2006) represents Caspian
overflows into the Black Sea Basin through the Manych Strait.
The Bakunian Stage is commonly subdivided into two substages
based on different mollusc fauna, in particular the bivalve genus
Didacna (Fedorov, 1957, 1978a;Moskvitin, 1962;Vekilov, 1969;
Popov, 1983;Mamedov and Aleskerov, 1988;Yanina, 2005, 2012b,
2013;Nevesskaya, 2007). Three morphological groups of the Didacna
genus exist: the crassoidal, catilloidal and trigonoides groups (Svitoch,
1967;Yanina, 2005, 2013;Nevesskaya, 2007). The faunas of the lower
Bakunian are dominated by the first two groups and furthermore in-
cludes taxa like Didacna parvula, D. catillus and D. fedorovi. In addition,
Dreissena rostriformis is widely distributed. The most characteristic
species are Didacna parvula and D. catillus. The upper Bakunian pre-
dominantly contains didacnas of the transitional crassoidal-catilloidal
group (D.rudis, D. carditoides) and crassoidal group (D. eulachia, D.
mingetchaurica, D. pravoslavlevi, D. bacuana) and representatives of the
catilloidal and trigonoidal groups are rare. The most characteristic
species are Didacna rudis, D. carditoides and D. eulachia (Yanina, 2005,
2013;Nevesskaya, 2007).
The transgressive base of the Bakunian is marked by a level rich in
euryhaline foraminifera, represented mainly by Ammonia spp., Nonion
sp. and Cibicides lobatulus. The Bakunian ostracod community is domi-
nated by endemic species adapted to unusual salinity settings such as
Eucythere naphtatscholana, Loxoconcha eichwaldi, L. petasa, L. babaza-
nanica, Tyrrhenocythere azerbaidjanica, T. papillosa, T. amnicola do-
netziensis, Cytherissa bogatschovi and Bacunella dorsoarcuata (Fig. 4).
Furthermore, many leptocytherid and candonid species occur. During
the Bakunian, a morphological transformation of the carapace features
took place, especially in loxoconchids. Several early stage morphotypes
evolved into new species that characterize the present-day Caspian Sea
ostracod fauna, including Loxoconcha endocarpus,L. lepida and L. gib-
boides.
The Urundzhikian stage has been defined by Fedorov (1946) as an
independent stratigraphic unit corresponding to the final stage of the
Bakunian transgressive cycle (Fedorov, 1993, 1999). The stratotype of
the Urundzhikian is the section Yuzhny Urundzhik in Western Turk-
menistan (Fedorov, 1946). As an additional reference section Neftya-
naya Balka in Azerbaijan was suggested, which contains well defined
stratigraphic boundaries (Svitoch and Yanina, 2007). Urundzhikian
deposits are known from the southern Caspian Basin only (Kura Basin,
Apsheron peninsula and southwest Turkmenistan). They are char-
acterized by numerous Didacna species: D. eulachia, D. mingetschaurica,
D. pravoslavlevi, D. colossea, D. shirvanica, D. bergi, D. karelini, D. por-
sugelica, D. čelekenica, D. rudis, and D. carditoides. Trigonoidal and ca-
tilloidal didacnas are rare. Characteristic species are D. eulachia, D.
pravoslavlevi and D. kovalevskii. The size of the endorheic Urundzhikian
lake phase slightly exceeded the area of the modern Caspian Sea. Here,
we consider the Urundzhikian as the upper part of the Bakunian.
2.1.3.2. Correlation. The beginning of the Bakunian transgression is
generally considered to correlate with the lower part of the Middle
Pleistocene as defined on the International Quaternary Chart (Fedorov,
1978a;Rychagov, 1997;Yanina, 2012a, 2013;Svitoch, 2013a;
Zastrozhnov et al., 2018). The age of Bakunian deposits is ~600 ka
according to fission track analysis, and 378-480 ka according to
thermoluminescent dating (Lavrishchev et al., 2011a, 2011b). Based
on palynological data, the lower Bakunian is correlated to isotope
stages MIS 18-16 (750-625 Ka) and the upper Bakunian to MIS 15-13
(625-475 ka). The large size and thickness of Didacna shells and the
high carbonate content of Urundzhikian sediments suggest warm
climatic conditions. The Urundzhikian transgression is consequently
correlated to the interglacial stage MIS 11 (Svitoch and Yanina, 2007;
Yanina, 2012a).
Magnetostratigraphic data indicate that the Bakunian pre-
dominantly correlates with the normal Brunhes Chron, which provides
a maximum age of 780 ka (Asadullayev and Pevzner, 1973). A recent
study of the Xocashen section in western Azerbaijan places the Ap-
sheronian-Bakunian boundary between the Jaramillo and Bruhnes
chrons, in a reversed subchron correlative to chron C1r.1r with an age
of 0.88-0.85 Ma (Van Baak et al., 2013), but these authors did not
distinguish the Tyurkyanian.
In conclusion, the age of the lower Bakunian boundary is con-
strained to the age interval between 0.88 and 0.75 Ma (Fig. 5). The
official geological time scale of Russia places the Apsheronian/Baku-
nian boundary at the Brunhes/Matuyama reversal at an age of 0.78 Ma.
2.1.4. Khazarian
2.1.4.1. Description. During the Middle-Late Pleistocene Khazarian and
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
9
Hyrcanian phases the Caspian Sea experienced large sea level changes
and episodic overflow into the Black Sea occurred (Fig. 6). In between
these high-stands, deep regressions took place and essentially the
modern Caspian system evolved.
The Khazarian Stage has been defined by Andrusov (in
Pravoslavlev, 1913) and comprises the sedimentary strata that con-
formably overlie Bakunian deposits in the North Caspian Basin and the
Manych Strait. Fedorov (1953) suggested a twofold subdivision into a
lower and upper Khazarian, which is officially accepted in Russian lit-
erature. Lower Khazarian deposits in boreholes on the Apsheron pe-
ninsula were also called “Gyurgyanian”(Dashevskaya, 1936, 1940), a
name that can be found in some stratigraphical schemes (e.g.,
Nevesskaya, 2007). Many researchers have considered another unit in
the northern Pre-Caspian area, the Singilian, either at the base of the
lower Khazarian (Pravoslavlev, 1913, 1932;Fedorov, 1957, 1978a;
Moskvitin, 1962;Goretskiy, 1966;Rychagov, 1997), or as an in-
dependent unit between Bakunian and lower Khazarian intervals
(Sedaikin, 1988;Svitoch and Yanina, 1997;Yanina, 2012b;Svitoch,
2013b). In these scenarios the Singilian is considered time-equivalent
with the Urundzhikian (Fig. 6). Popov (1983), however, considered the
Singilian as the regressive phase of the lower Khazarian. More recently,
the regressional Singilian phase was placed between the early and late
Khazarian transgressions (Zastrozhnov et al., 2018).
The early Khazarian transgression corresponds to a sea level high
stand of +15 m (Fedorov, 1957;Vasiliev, 1961;Rychagov, 1997)or
+20-25 m (Svitoch and Yanina, 2007). The lower Khazarian deposits
are widespread along the Caspian coast, penetrating far inland along
paleodepressions. The sediment thickness ranges from a few meters to
thick sequences in the depressions. The maximum thickness is recorded
in the Kura Basin, where it reaches 600 m (Vekilov, 1969). Spore and
pollen data indicate a three-stage change in vegetation: from goosefoot-
sagebrush steppe at the beginning of early Khazarian time, to taiga type
forests in its middle and periglacial steppe at the end (Zastrozhnov
et al., 2018).
The lower Khazarian is characterized by the broad development of
the trigonoidal group of didacnas (Didacna subpyramidata, D. paleo-
trigonoides, D. gurganica, D. mishovdagica, D. trigonula, D. trigonoides
chazarica), representatives of the crassoidal group (Didacna pra-
voslavlevi, D. nalivkini, D. delenda, D. apscheronica, D. ovatocrassa, D.
subcrassa, D. pontocaspia tanaitica) and rare species of the catilloidal
group (Didacna dilatata, D. subcatillus,D. vulgaris, D. lindleyi, D. adac-
noides). Characteristic species for the lower Khazarian are Didacna
subpyramidata and D. paleotrigonoides. Numerous other anomalohaline
molluscs are present as well (Monodacna caspia, Hypanis plicatus,
Adacna vitrea, etc.). Typical ostracods are Caspiolla gracilis, Candoniella
subellipsoida, Leptocythere arevina, L. martha, L. quinquetuberculata, L.
gibboida, Bacunella dorsoarcuata, Loxoconcha endocarpa, Cyprideis littor-
alis and Cytherissa cascusa (Ushko and Shneider, 1960;Sedaikin, 1988;
Svitoch and Yanina, 1997). Among foraminifera, Ammonia caspica and
Mayerella brotzkajae occur (Yanko, 1989).
The early Khazarian spans several interglacials and glacials.
Khazarian deposits of the Manych Depression include two overflow
phases (Fedorov, 1978a;Svitoch and Yanina, 1997;Yanina, 2005,
2012a). The early Khazarian transgressions mostly developed under
relatively cold climate conditions but some of the transgressions de-
veloped during interglacial intervals. Traces of permafrost are found in
deposits of the Northern Pre-Caspian region, which represented a
periglacial interval during glacial intervals (Grichuk, 1954;Moskvitin,
1962;Zhidovinov et al., 1984). Deciduous woods developed in Dage-
stan (Abramova, 1974). Glaciers existed in the Caucasus mountains and
cold and humid conditions existed in Azerbaijan during these glacials
(Milanovsky, 1966;Dumitrashko et al., 1977;Aleskerov, 1990). Lower
Khazarian spores and pollen from boreholes in the Southern Caspian are
dominated by grassy plants and trees (pine, fir, birch, alder, willow)
(Vronsky, 1976).
The late Khazarian transgression (Fig. 7a) corresponds to a sea level
high stand of -10 m (Kaplin et al., 1977a;Popov, 1983;Svitoch and
Yanina, 1997), but lacked any connection with the Black Sea Basin
(Fig. 6). Common gigantism of shells, high carbonate content in the
sediment, and the presence of oolites indicates warm climate conditions
(Yanina, 2014). Salinities ranged between 10 to 12‰in the northern
part and up to 14-15‰in the southern part of the Caspian Basin, i.e.
higher than today (Yanina, 2014). Pollen assemblages testify to mod-
erately warm interglacial climates (Abramova, 1974).
The upper Khazarian is marked by widespread species of the cras-
soidal Didacna group (Didacna surachanica, D. subcrassa, D. hyrcana, D.
nalivkini, D. delenda, D. ovalis, D. karabugasica, D. subovalis, D. ovato-
crassa, D. schuraosenica). Trigonoidal and cattiloidal forms are rare. The
index species of the upper Khazarian is Didacna surachanica (Fedorov,
1957;Yanina, 2005;Nevesskaya, 2007). In more oligohaline and fresh-
water deposits Corbicula fluminalis is common.
Foraminifera are represented by Ammonia novoeuxinica, A. caspica,
Elphidium capsicum, E. shochinae, Frorilus trochospiralis, Majerella brotz-
kajae, Cornuspira minusculla and Milioniella risilla (Yanko, 1989). The
Khazarian ostracod fauna is a continuation of the Bakunian fauna
(Fig. 4). The most common species are Candona schweyeri,Fabae-
formiscandona sp.,Eucythere naphtatscholana,Cyprideis torosa,Lox-
oconcha eichwaldi,L. petasa, L. lepida, L. babazananica, Tyrrhenocythere
azerbaidjanica, T. papillosa, T. amnicola donetziensis, Euxinocythere ba-
cuana, Amnicythere andrussovi (and its associated morphotypes), A.
striatocostata, Bacunella dorsoarcuata and Cytherissa bogatschovi
(Sedaikin, 1988).
After the main late Khazarian transgression, but before the
Khvalynian transgression, another transgression has been described
from boreholes in the North Caspian Basin; the Hyrcanian (or
Girkanian) transgression (Fig. 6;Popov, 1955, 1967;Goretskiy, 1957;
Yanina, 2013;Sorokin et al., 2018). Hyrcanian deposits contain
“Khvalynian-like”fauna of Didacna subcatillus,D. cristata, D. pallasi, D.
subcrassa, but also the mainly freshwater Corbicula fluminalis. The
widespread occurrence of C. fluminalis is indicative of the warm water
character of the basin. The Hyrcanian mollusc fauna in the Manych
Depression shows that Caspian waters were draining to the Black Sea
(coevally with the last phase of the Karangatian transgression in that
basin) through the Manych Strait (Popov, 1983;Yanina, 2014).
The Khazarian sediments (including the Hyrcanian in their upper
part) are generally separated from the lower Khvalynian deposits by the
Atelian regression (Fig. 6), when the Caspian Sea level was significantly
lowered (Fig. 7b). Based on seismic-acoustic profiling, the maximum
lowstand during the Atelian is estimated at -120 to -140 m (Lokhin and
Maev, 1990;Maev, 1994). Vast areas of the Caspian shelf were exposed
and river incisions were deep (Fedorov, 1978a;Rychagov, 1997). Ate-
lian deposits contain mammal remains of the Upper Paleolithic faunal
complex, including mammoth, horse, reindeer, etc., indicative of
tundra-steppe and cold arid continental climate. Towards the end of the
Atelian epoch, the climate became even warmer. In the vegetation, the
share of arboreal pollen (birch, pine and spruce trees) increased while
elm, oak and linden re-appeared. The importance of xerophytes de-
creased, while grasses and herbaceous vegetation expanded. Steppe and
forest-steppe environments became dominant (Grichuk, 1954;
Chiguryaeva and Khvalina, 1961;Moskvitin, 1962;Bolikhovskaya
et al., 2017).
2.1.4.2. Correlation. The lower Khazarian is generally considered to be
equal to the upper part of the Middle Pleistocene, while the upper
Khazarian is correlated with the lower part of the Upper Pleistocene.
According to Th-U, TL and electron spin resonance data, lower
Khazarian deposits are dated at ages of > 300, > 250, 148-177, and
142-108 ka. They have a normal polarity (Brunhes) and one or two sub-
zones of reverse polarity (Shkatova, 2010). The Khazarian comprises
three transgressive stages which correspond to MIS 10, 8 and 6 (Fig. 6;
Yanina, 2014).
Upper Khazarian deposits are dated by Th-U and TL methods at 127-
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
10
87 and 122-(130)-84 ka (Shkatova, 2010), 144-90 ka (Geochronology…,
1974), 130-91 ka (Leontiev et al., 1975), and 122-106 ka (Shakhovets,
1987;Shkatova, 2010). According to Uranium-Ionium dates, the age of
the late Khazarian transgression was estimated at between 114 and 75
ka (Leontiev et al., 1975;Rychagov, 1997), and between 115 and 81 ka
(Arslanov et al., 1978). Results of electronic paramagnetic (spin) re-
sonance (ESR) revealed ages between 140 and 85 ka (Bolikhovskaya
and Molodkov, 1999, 2008;Molodkov and Bolikhovskaya, 2009) and
108-85 ka (Shkatova, 2010). These results led to the conclusion that the
late Khazarian transgression phase took place between 127 and 122 ka,
the initial regression phase at 117–114 ka BP, and the full regression
phase at 114–85 ka, coeval with MIS 5e, 5e-d, and 5d-a, respectively
(Shkatova et al., 2010). Late Khazarian and Hyrcanian deposits of the
Srednyaya Akhtuba section (lower Volga area) were OSL dated at
112 ±5 ka, 102± 5, 87 ±4 ka indicating they also correspond to MIS
5(Yanina et al., 2017a, 2017b). Paleomagnetic measurements of upper
Khazarian sediments confirmed the occurrence of a reversal excursion
in five sections. Its age (from Th–U, TL and ESR measurements) is es-
timated between 117 ± 7.5 and 89 ± 11 ka (Shkatova, 2010), and thus
is most likely to correspond to the reversed polarity Blake event (~120
ka). It is generally accepted that the late Khazarian transgression cor-
responds to the Eemian Interglacial.
Most age constraints indicate that the upper Khazarian deposits
have an age between 125-85 ka, corresponding to MIS 5 (Fig. 8). The
Hyrcanian transgression corresponds to the final part of the interglacial
interval MIS 5a (85 ka). The Atelian regression then most likely peaked
at an age of 85-75 ka. The final phase of regression is dated by OSL at
48 ± 3 (Yanina et al., 2017b) and by
14
Cat45–41 ka (Bezrodnykh
et al., 2017) suggesting it corresponds to the first half of the interstadial
warming of MIS 3.
2.1.5. Khvalynian
2.1.5.1. Description. The Khvalynian stage developed in the Late
Pleistocene glacial period. Very high transgressions with an overflow
event towards the Black Sea interchanged with very deep regressions.
Salinities were somewhat depressed compared to today and a unique
landscape (Baer knolls) developed in the North Caspian plains.
The Khvalynian Stage has been defined by Andrusov (in
Pravoslavlev, 1913) and comprises the sedimentary strata that trans-
gressively overlie the Atelian and Khazarian deposits in the North
Caspian Basin and Manych Strait. The Khvalynian transgression is by
far the most extensive sea-level rise in the Late Pleistocene history of
the Caspian Sea (e.g., Yanina, 2014)(Fig. 7c). Khvalynian deposits are
generally subdivided into lower and upper Khvalynian highstand in-
tervals that are separated by the Enotaevka regression (Fig. 6).
The well preserved paleocoastlines of the early Khvalynian Sea
Fig. 7. Paleogeographic maps for the late Pleistocene Pontocaspian region. Arrows indicate the water flow direction in the gateway regions. All maps are based on
Yanina (2014).
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
11
allow precise determination of facies distributions. Its sedimentary
thickness is usually only a few meters, but can reach over 100 m in the
Kura and Western Turkmenistan basins. The lower Khvalynian deposits
comprise widely different sediments, including the characteristic liman-
type “chocolate clays”of the lower Volga area that probably formed by
accumulation of fine brown sediments that derived from the periglacial
landscapes of the hinterland (Moskvitin, 1962;Goretskiy, 1966;
Makshaev and Svitoch, 2016;Tudryn et al., 2016). Palynological data
confirm a cold climate (Abramova, 1974;Yakhemovich et al., 1986).
The early Khvalynian water level reached +50m, and Caspian Sea
water spilled over to the Black Sea via the Manych Strait (e.g., Popov,
1983;Varuschenko et al., 1987;Yanina, 2014). During this period the
North Caspian Basin was much larger than today (Fig. 7c).
The lower Khvalynian and upper Khvalynian are separated by the
Enotaevka regression (Brotsky and Karandeeva, 1953), which reached a
maximum lowstand at -105 m (Maev, 1994). Terrestrial Enotaevka
deposits and numerous unconformities are described from the coastal
regions (Leontiev and Fedorov, 1953;Vasiliev, 1961;Rychagov, 1974;
Fedorov, 1978a;Svitoch and Yanina, 1997). The regressive Enotaevka
unit in the North Caspian Basin consists of deltaic deposits with marked
cross-bedding (Roslyakov et al., 2007). It is prominent in the deeper
parts and pinches out towards the shore where it is characterized by an
unconformity (e.g. Svitoch and Yanina, 1997). According to pollen
data, arid cool climate conditions existed (Sorokin et al., 1983). During
the late Khvalynian transgressive stage, sea level reached about 0 m
(which is 27 m above todays Caspian levels: (Fedorov, 1957;
Varuschenko et al., 1987;Rychagov, 1997). No overflow to the Black
Sea existed. Upper Khvalynian deposits are found on all Caspian coasts
in the interval of -20 to 0 m, but their thickness does not exceed a few
meters.
In the lower Khvalynian Didacna ebersini, D. parallella, D. protracta
are abundant, while D. cristata, D. subcatillus, D. praetrigonoides, D. de-
lenda and D. zhukovi are rare. Characteristic species are D.parallella and
D. protracta. On the eastern coast, these species are replaced by D.
cristata and D.zhukovi.The thin shells of these molluscs indicate rela-
tively low water temperatures compared to the present-day Caspian
Sea. The structure of mollusc fauna is indicative of relatively low sali-
nities, although regional variations existed. Salinities in the North
Caspian Basin (3-4‰) were slightly higher than today, whereas sali-
nities in the southern basin (~11 ‰) were slightly lower (Yanina,
2014). The upper Khvalynian deposits comprise clays, silts and sands
with Monodacna caspia,Dreissena polymorpha,D. grimmi, and gastro-
pods. Multiple Didacna species include D. praetrigonoides,D. parallella,
D. protracta, and more rarely D. subcatillus.Didacna praetrigonoides,a
rare species in the early Khvalynian, became dominant. The salinity of
the main water body of the late Khvalynian basin was very similar to
the early Khvalynian (Yanina, 2014). The relative abundance and thick
shells of the molluscs indicate warmer conditions in late Khvalynian
times then during early Khvalynian. Palynological data confirm a
general warming of the Caspian region (Grichuk, 1954;Abramova,
1974;Vronskiy, 1974;Vronsky, 1976;Sorokin et al., 1983;
Yakhemovich et al., 1986).
Ostracods of the Khvalynian are mainly represented by Loxoconcha
gibboides, L. endocarpa, Caspiolla gracilis, Leptocythere propinqua, L.
martha, Paracyprideis enucleate, Cyprideis torosa (Sedaikin, 1988). For-
aminifera are dominantly brackish water species (Yanko, 1989).
2.1.5.2. Correlation. The ages of the Khvalynian transgressions are
rather ambiguous and remain subject of discussion (Kaplin et al.,
1972, 1977a, 1977b;Kvasov, 1975;Leontiev et al., 1975;Arslanov
et al., 1978, 2016;Svitoch and Yanina, 1997;Rychagov, 1997;
Bezrodnykh et al., 2004;Badyukova, 2007;Yanina, 2014). Based
mostly on (now considered outdated) TL dates, the age of the lower
Khvalynian deposits was estimated at 70-40 ka, and the age of the
upper Khvalynian deposits at 20-10 ka (Leontiev et al., 1975;Rychagov,
1997). According to
14
C and
230
Th/
234
U data the age of the Khvalynian
is 19-8 ka (Kvasov, 1975;Svitoch and Yanina, 1997;Leonov et al.,
2002;Chepalyga et al., 2008;Svitoch et al., 2008;Tudryn et al., 2013;
Arslanov et al., 2016). Based on the radiocarbon dates from drilling the
North Caspian Basin, lower Khvalynian ages are 30-21 ka and upper
Khvalynian ages 19-12 ka (Bezrodnykh et al., 2004, 2015). The OSL
dating of the chocolate clay in the lower Volga area resulted in ages of
15 ± 1 and 13 ± 0.5 ka (Yanina et al., 2017b). Paleomagnetic studies
of the lower and upper Khvalynian sediments revealed the presence of
two magnetic excursions, identified at several sections (Seroglazka,
Lenino, and Yenotaevka), which are likely to correspond to the
Laschamps (41 ka) or Mono Lake (32 ka) events (Laj and Channell,
2007). The discrepancies between these results from different methods,
which may even lead to inversions in the chronology, show that the
ages for these transgressions are not yet well established (Tudryn et al.,
2013).
According to Sorokin et al. (2018), the lower Khvalynian trans-
gression correlates with the global interstadial warming of the younger
half of MIS 3 at an age of 30-21 ka, and is caused by an increase in the
surface runofffrom the catchment area. The Khvalynian sea level rise
was interrupted at the LGM (time of maximum cooling and aridization,
MIS 2) and resumed when the ice sheet was decaying. The warm phases
of Bølling and Allerød promoted ice sheet melting along with thawing
of permafrost, the latter having been widespread in the Volga drainage
basin (Sorokin et al., 2018). The Khvalynian came to its end at the first
sharp warming that resulted in the rise of the Caspian level and is
generally taken as marking the Pleistocene/Holocene boundary.
In conclusion, it seems that most age constraints for the base of the
Khvalynian converge on an age of 35 ka (young option), while the top is
3
4
5
LR04 δ18O (‰)
1
2
6
10
12
14
16
18
20
5
7
9
11
15
8
Apsh.
Chaudian Uzunlarian Karangatian
Neoeuxinian
Chernom-n
Khazarian Khvalynian
Novocaspian
BLACK SEA
BASIN
CASPIAN SEA
BASIN
Bruhnes GPTS 2012
Calabrian Ionian
Quaternary
Tarantian
Bakunian
Holocene
System
Subseries/
Stages
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Age, Ma
?
Fig. 8. Quaternary time scales for the Pontocaspian domain. GPTS with Systems
and Stages is after Hilgen et al. (2012), oxygen isotope curve with numbered
Marine Isotope Stages (MIS) is after Lisiecki and Raymo (2005). On the right
side are the time scales for the Caspian Basin and the Black Sea Basin.
W. Krijgsman et al. Earth-Science Reviews 188 (2019) 1–40
12
estimated at ~10 ka (Fig. 8). The age of the maximum Enotaevka re-
gression separating the lower and upper Khvalynian is around ~15 ka.
2.1.6. Novocaspian
2.1.6.1. Description. The Holocene Novocaspian stage represents the
modern Caspian Sea settings and faunas.
The Novocaspian (Holocene) Stage, defined by Bogachev (1903),
comprises the sedimentary strata that transgressively overlie the
Khvalynian deposits in the North Caspian Basin. Novocaspian deposits
are developed on all Caspian coasts below -19 (-20) m. In the North
Caspian Basin, they are represented by shallow water sands with nu-
merous fresh-and anomahaline molluscs species. The Novocaspian is
separated from the upper Khvalynian by the regressive Mangyshlakian
facies (defined by Zhukov, 1945), that represents the deltaic pro-
gradation of the Volga and Ural rivers during a major sea level fall up to
–80 m or even –113 m (Figs. 6, 7d) (Varuschenko et al., 1987;
Bezrodnykh et al., 2004, 2015). The Mangyshlakian deposits contain
peats and sands with plant detritus and species poor assemblages of
fresh water and oligohaline molluscs (Dreissena polymorpha, Lymnaea,
Unio) but lack Didacna species (Sorokin, 2011).
The Novocaspian mollusc fauna is marked by various Didacna spe-
cies of the crassoidal and trigonoidal groups: Didacna crassa, D. baeri, D.
trigonoides, D. pyramidata, D. longipes, D. barbotdemarnii. In addition,
numerous other anomalohaline molluscs are present like Monodacna
caspia, Hypanis plicatus, Adacna vitrea, etc. Characteristic for the
Novocaspian are the species D. baeri and D. trigonoides, plus the entry of
Cerastoderma glaucum (Fedorov, 1953, 1957).The uppermost beds
contain Mytilaster minimus and Abra segmentum, which were anthro-
pogenically introduced into the Caspian from the Azov/Black Sea
during the 20th century. Spores and pollen spectra contain up to 25% of
tree pollen, mainly pine and birch, indicative of the relatively humid
climate in the Holocene (Sorokin, 2011).
2.1.6.2. Correlation. Judging from the absolute age determinations, the
Mangyshlakian regression peaked between 10 and 8 ka, and the
Novocaspian deposits are all younger than 7 ka (Fig. 8). The AMS
14
C
data from boreholes in the Volga delta suggest a lowstand around 8000
BP. Volga delta data indicate a continuously rising sea level between
5000 and 3000 BP until a highstand was reached at -25 m around 2600
BP (Overeem et al., 2003;Kroonenberg et al., 2008). During the
historically well-known mediaeval Derbent regression (Rychagov,
1997) Caspian Sea levels dropped to -34 m, and possibly even -45 m
as recorded in the deeper parts of the offshore Kura delta in Azerbaijan.
A second highstand around 300 BP is documented in an outermost
barrier in Dagestan (Kroonenberg et al., 2007). The two highstands
appear to coincide with two well-known periods of increased
precipitation in Eurasia, the 2600 BP event (Van Geel and Renssen,
1998) and the Little Ice Age, whereas the Derbent regression seems to
be coeval with the Warm Mediaeval Period (Kroonenberg et al., 2007).
2.2. The Black Sea region
The Black Sea is today a marginal sea of the Mediterranean (Fig. 2)
and has a surface area of 436,400 km
2
(excluding the Sea of Azov), a
maximum depth of 2,212 m, and a volume of 547,000 km
3
. Its E-W
extent is about 1175 km (27°27'-41°42') and it stretches ~800 km N-S
(46°33'- 40°56'). At present, it is the world's largest meromictic water
body; deep waters do not mix with the upper water layers that receive
oxygen from the atmosphere. As a result, over 90% of the deep Black
Sea volume is anoxic. Circulation patterns are primarily controlled by
basin topography and fluvial inputs, which result in a strongly stratified
vertical structure. The Black Sea has a positive freshwater balance: it
receives more fresh water from the rivers and rainfall than it loses from
evaporation. The Black Sea consequently experiences an estuarine type
of water transfer with the Mediterranean Sea via a shallow threshold
(35-40 m) at the Bosphorus Strait, with bottom inflow of dense
Mediterranean water below a surface outflow of fresh Black Sea water
into the Marmara Sea. The salty Mediterranean inflow mixes with the
basin’s fresher waters, which results in an average salinity of 18 –22‰
for the Black Sea surface waters, i.e. much lower than the Mediterra-
nean (37 –38‰).
During Quaternary to Recent times, periods of isolation and episodic
connection with the Mediterranean Sea (through the Marmara and
Aegean seas) largely controlled the paleoenvironmental conditions in
the Black Sea (Zubakov, 1988;Badertscher et al., 2011;Van Baak et al.,
2016b). The ancient Bosphorus Strait may have been slightly deeper
than today, as the sill depth in the Paleozoic bed rock is estimated at
~85 m in the Dardanelles Strait (Algan et al., 2001). However, the
Bosphorus gateway itself evolved only in the Middle Pleistocene
(McHugh et al., 2008). When global/Mediterranean Sea levels were
above the Bosphorus sill, marine water connections existed all the way
to the Black Sea. During such connection phases the Black Sea level
tracked that of global sea levels. When the Mediterranean levels were
below the sill, the Black Sea turned into an isolated, saline lake basin.
Major rivers like the Danube, Dniester, Dnieper, and Don (via the Sea of
Azov), supply fresh water to the Black Sea; together they drain a large
part of continental Europe (Fig. 2). During intervals with a positive
water balance the Black Sea level remained at the sill height and one-
directional flow towards the Mediterranean Sea occurred. In times of
negative water budgets, lake levels dropped until the total inflow
(precipitation and river influx) equalled the evaporation in the Black
Sea Basin (e.g., de la Vara et al., 2016). Salinities during these lake
phases were typically in the oligohaline-mesohaline ranges, very si-
milar to today’s Caspian Sea.
In the northeast, the Black Sea is connected to the Sea of Azov
through the Kerch Strait (Fig. 2). The Sea of Azov (45°12′-47°17′N,
33°38′-39°18′E) has a surface area of 39,100 km
2
and is 13 m deep in
its central part. Two large rivers, the Don and Kuban, flow into the Sea
of Azov. Annually 49.2 km
3
waters flows to the Black Sea, while 33.8
km
3
returns, resulting in an average salinity of ~11 ‰. Recently, an-
thropogenic reduction of river drainage strengthened the inflow of the
Black Sea waters and increased the average salinity up to 13.8 ‰. In the
geological history, the Sea of Azov frequently desiccated during glacio-
eustatic lowstands and the Don and Kuban rivers directly drained into
the Black Sea, south of the modern Kerch Strait (