During the last decades, rivers and their deposits in different
regions were intensively studied to better understand the late-Quaternary
landscape evolution and former human activities. One proxy for
paleoecological and paleoclimatic reconstructions is the analysis of gastropods
(snails) from carbonatic river sediments. In the scope of this study, we
investigated gastropod assemblages from a Holocene fluvial sediment–paleosol
sequence at the upper Alazani River in the southeastern Caucasus. On the one
hand, we aimed to derive reliable independent information about possible
long-lasting human activity since the late Neolithic–Chalcolithic in the
upper Alazani floodplain. This was formerly suggested by
Für ein besseres Verständnis der spätquartären
Landschaftsentwicklung und früherer menschlicher Aktivitäten wurden
während der letzten Dekaden in verschiedenen Regionen Flüsse und
deren Ablagerungen intensiv untersucht. Einen Proxy für
paläoklimatische und paläoökologische Rekonstruktionen stellen
Analysen von Gastropoden (Schnecken) aus karbonatischen Flusssedimenten dar.
Während dieser Studie wurde die Artenzusammensetzung von
Gastropodengemeinschaften einer holozänen Sediment-Paläobodensequenz
am oberen Alazani im südöstlichen Kaukasus untersucht. Zum einen
wollten wir fundierte unabhängige Informationen über mögliche
landandauernde menschliche Aktivitäten in der Aue des oberen Alazani
seit dem Spätneolithikum/Chalcolithikum erhalten. Eine solche Besiedlung
legte bereits eine frühere auf
River systems react sensitively towards different external influences such as tectonics, base level fluctuations, climate changes and anthropogenic activities. Therefore, rivers and their deposits were intensively studied in different regions during the last decades to decipher the late-Quaternary landscape evolution (Bridgland and Westaway, 2008; Schulte et al., 2015; Faust and Wolf, 2017; von Suchodoletz et al., 2015, 2018a, b; Avsin et al., 2019; Lauer et al., 2020). In addition, floodplains were preferred areas of former human activities due to easy access to water resources, fertile loamy soils and a generally flat topography. Therefore, during the last years they were also intensively studied in the context of geoarcheological research (Hassan, 1997; Morozova, 2005; Giosan et al., 2012; von Suchodoletz and Faust, 2018).
Geomorphological and geoarcheological research of floodplains and their sediments is generally carried out by (i) field-based geophysical and geomorphological analyses partly supported by digital elevation or multispectral remote sensing data (Bakker et al., 2007; Mlekuz and Budja, 2010; Syvitsky et al., 2012; von Suchodoletz et al., 2015; Babek et al., 2018) and (ii) stratigraphical and sedimentological analyses of naturally or artificially outcropped sediment sections or drilling cores. The obtained fluvial sediments are analyzed and numerically dated using sedimentological, geochemical and geochronological methods (Fuchs et al., 2011; Giosan et al., 2012; Schulte et al., 2015; von Suchodoletz et al., 2015, 2018a, b; Faust and Wolf, 2017; Avsin et al., 2019; Lauer et al., 2020). In addition, under favorable conservation conditions local and regional paleoecological and paleoclimatic information can also be obtained based on biogenic proxies such as plant macrofossils, charcoal pieces, pollen grains, diatoms or leaf wax biomarkers (Kittel et al., 2016; Scott et al., 2016; Bliedtner et al., 2018a). Furthermore, similar to other sediment archives such as loess–paleosol and dune–paleosol sequences or colluvial deposits (Moine et al., 2008; Faust et al., 2015; New et al., 2019; Richter et al., 2019), also species distribution and/or isotopic composition of gastropods from carbonate-rich fluvial sediments were investigated for paleoecological and paleoclimatic reconstructions during the last years (Fuhrmann, 2008; Zielhofer et al., 2010; Murelaga et al., 2012; Alexandrowicz, 2013; Juřičková et al., 2013; Granai and Limondin-Lozouet, 2018).
During this study, we analyzed the species distribution of gastropods from a
Holocene fluvial sediment–paleosol sequence at the upper Alazani River in
the southeastern Caucasus that was formerly investigated by von Suchodoletz et
al. (2018b). Current carbonate contents of sediments and paleosols between
11 % and 42 % allowed for excellent preservation of gastropods throughout
the sequence. So far, Holocene malacological studies from the region mainly
focused on marine or coastal fauna (Lahijani et al., 2009; van de Velde et
al., 2019), whereas most terrestrial paleoecological studies were carried
out using pollen analyses in lakes or peat bogs (Connor and Kvavadze, 2008;
Messager et al., 2013, 2017; Joannin et al., 2014; Leroyer et al., 2016;
Connor et al., 2018). The first goal of our study was to derive reliable
independent information about possible long-lasting human activity in the
floodplain since the late Neolithic–Chalcolithic as was formerly suggested
by
The 240 km long Alazani River originates from the southern slope of the
central Greater Caucasus in eastern Georgia at ca. 2800 m a.s.l. and
finally flows into the Kura River that drains into the Caspian Sea (Fig. 1a). About 40 km from its source the Alazani joins with its right tributary
Ilto, which also originates from the southern slope of the central Greater
Caucasus at an altitude of ca. 2300 m a.s.l. After their confluence, the
river mostly runs in the Alazani thrust top basin between the southern
foothills of the Greater Caucasus in the northeast (Adamia et al., 2010) and
the Kura fold-and-thrust belt in the southwest (Kura FTB; Forte et al.,
2010). The investigated exposure at the northwestern tip of the Alazani
basin is located ca. 10 km downstream from the confluence and has a
subcatchment of ca. 1100 km
The upper Alazani basin, in which the study site is located, receives ca.
720 mm of annual precipitation, with a peak during spring and early summer that
is caused by convective events (Lydolph, 1977). Mean annual temperature is
about 12.0
The investigated fluvial sediment–paleosol sequence (42
Stratigraphical sketch of the sequence with sampling positions of gastropods; photos of Iron Age findings in the upper part of Ahb1 and late-Neolithic–Chalcolithic findings and a fireplace in the upper part of Ahb6.
The sequence mostly consists of fine-grained silty to clayey overbank
sediments (generally
For the mollusk analyses, we took 12 samples with 10 L of sediment each. These
comprised six samples from paleosols and six samples from fluvial sediment
layers of the fluvial sediment sequence (Fig. 2). The sediment was wet-sieved to obtain the fraction
Quantitative distribution of gastropods in the investigated fluvial sediment sequence and their ecological classification. For the legend of the stratigraphy please see Fig. 2.
We used an approach similar to that of Ložek (1964) and Juřičková et al. (2014) for the classification of the gastropod species into ecological groups. With regard to the general habitat, we classified the species into open-country, generalists and woodland species, and with regard to humidity requirements we distinguished wetland and aquatic species. The allocation of the taxa to the ecological groups is shown in Fig. 3.
A total of 13 gastropod species were extracted from the investigated fluvial
sediment sequence, namely
(1)
Percentages of ecological groups of gastropods, abundance and
species richness of the gastropod assemblages in the investigated fluvial
sediment sequence and comparison with a formerly derived vegetation
reconstruction based on
The quantitative and qualitative gastropod distribution across the sequence
(Figs. 3 and 5; species list in Supplement Table S1) indicated a succession
from open-country assemblages to woodland assemblages and a subsequent
reduction of woodland in the youngest layer: the lowermost three samples
(samples 12 to 10) were dominated by
The freshwater snail
If we compare the gastropod assemblages of the paleosols with those of the
non-pedogenic fluvial sediment layers, there are hardly any differences in
species composition, but the former show higher abundances (Figs. 3 and 5). Higher abundances
in the paleosols are probably caused by the longer exposure times at the
stable surfaces of the paleosols favoring shell preservation. This is a
typical phenomenon related to paleosols (Ložek, 1990). Whereas the
samples from the paleosols probably contain mainly specimens that lived at
or near the site under investigation, the samples from fluvial sediments may
also contain shells from more distant places upstream. Therefore, if there
had been forest in the upstream vicinity at the time of formation of the
deeper layers, some individuals of woodland species would probably be
present in the fluvial samples. Consequently, the congruence of the trends
seen in the samples of fluvial sediments and of paleosols indicates that the
observed succession from open-country assemblages in samples 12 to 10 and
transitional assemblages with a mix of open-country and open-woodland
species in samples 9 to 6 to assemblages dominated by forest species in
samples 5 to 2 (Figs. 3 and 5) is regional rather than restricted to the
investigated site. Species of undisturbed forest ecosystems and deadwood
specialists such as
The constant absence of the wetland species
The gastropod assemblages in our fluvial sediment sequence at the upper
Alazani River between samples 12 (Ahb6) and 10 (Ahb5) indicate that the
landscape in the surroundings of the investigated site was covered by
grassland during the Early Holocene until ca. 5.5 cal kyr BP. In samples 9
and 8 (Ahb4) a first species characteristic for open shrub to woodlands,
Map of settled regions in the southern Caucasus during different
prehistoric periods after Akhundov (2004).
As indicated by the gastropod assemblages, increased reforestation towards
an open woodland at the investigated site started within the sediment layer
between Ahb4 and Ahb3 that was deposited around 4.5 cal kyr BP (sample 7)
and lasted at least until ca. 3 cal kyr BP (sample 3, between Ahb2 and Ahb1). Unfortunately, for
this period no comparison with
A further change of the gastropod assemblages occurred in paleosol Ahb1
(position of sample 2) that had developed between ca. 3 and 1.7 cal kyr BP:
in this soil, gastropod species indicative of forests to open woodland are
found together with a significant percentage of grassland species (Fig. 3).
This indicates a change towards an even more open forest with larger
proportions of grassland. It is very likely that this was caused by resuming
human activity at this site or an intensification of land use, respectively.
Increasing human activity at this site or in its surroundings for this time
is also supported by numerous Iron Age archeological findings (potsherds,
obsidian tools and charcoal) from the upper part of Ahb1 (Fig. 2). Furthermore, from
this period burials with Scythian grave goods are known from the Alazani
valley (Mamaiashvili, 1980). However, despite a dominance of open-woodland
gastropod species, the
Wetland species of gastropods are absent from the lower part of the investigated sequence. In contrast, they occur from sample 5 upwards, i.e., since ca. 4 cal kyr BP. This indicates that the investigated site was relatively far away from the active riverbed during the Early and Middle Holocene and that the river approached towards the site during the Late Holocene (Fig. 7a and b). Morphologically young former river channels were observed up to 1.5 km north of the current river channel (Fig. 1b and c). Furthermore, the region shows a high regional tectonic activity linked with orogenesis at the southern margin of the Greater Caucasus (Forte et al., 2014), forming the base for river shifts linked with tectonic activity. Accordingly, strong earthquakes that have hit the Alaverdi Monastery located ca. 2 km east of the study site are documented for the 15th and 18th centuries CE (Gamkrelidze et al., 2013; Fig. 7a). Therefore, such a river shift towards the south during the Late Holocene was already formerly suggested by von Suchodoletz et al. (2018b) to explain strong incision of the sequence that finally led to the deactivation of the fluvial archive from ca. 1.6 cal kyr BP. The increasing presence of wetland gastropod species in the upper part of the sequence confirms this formerly suggested southern shift of the river course during the Late Holocene that was probably linked with orogenesis at the southern margin of the Greater Caucasus. However, the slowly increasing proportion of water-related gastropod species indicates that the shift of the river course must have occurred slowly rather than abruptly as was formerly suggested by von Suchodoletz et al. (2018b), i.e., in the form of preferential lateral migration (combing) as a response to lateral tilting of the floodplain (Holbrook and Schumm, 1999).
Our investigations of gastropod assemblages from a Holocene fluvial
sediment–paleosol sequence at the upper Alazani River in the southeastern
Caucasus region demonstrate that the studied site in the floodplain was free
of the natural forest vegetation during the Middle Holocene until ca. 4.5 cal kyr BP. This contrasts with a pollen-based Early to Middle Holocene
vegetation reconstruction in the neighboring Iori floodplain, showing that
this floodplain was covered with deciduous and coniferous forests from the
Late Pleistocene. Thus, our gastropod data support former
A detailed list of the gastropod species in the studied sequence can be found in Table S1 in the Supplement.
The supplement related to this article is available online at:
HvS, BH and CR designed the study. HvS carried out the fieldwork and was
the main author of the paper. CR, FW and BH determined and classified the
gastropod species. MB contributed the
The authors declare that they have no conflict of interest.
We thank Anna Skokan and Ulrich Göres (both Dresden) and Azra Khosravichenar (Leipzig) for their help during the fieldwork and Dominik Faust (Dresden) for his permanent support of the studies in eastern Georgia during the last years. We are grateful to the archeologists of the Georgian National Museum, Gia Chilingarashvili and Davit Darejanashvili, for providing scanned copies of the archeological literature while the libraries were inaccessible due to the COVID-19 pandemic. Furthermore, we thank two anonymous reviewers for their valuable comments on an early version of this paper. We acknowledge support from Leipzig University for open-access publishing.
This paper was edited by Jan-Hendrik May and reviewed by two anonymous referees.