Wetlands and lakes formation and evolution on the Lower Danube Floodplain during Middle and Late Holocene

Abstract

River floodplains are amongst the most dynamic features which are continuously changing their shape and structure in relation to climate, vegetation, human pressure and base-level variability. In this study, we focus on the lower reach of Danube (upstream of its delta) and investigate the formation and transformations of wetlands and lakes during the last 8000 years using a multi-proxy methodology. We identified two main stages of floodplain landscape changes which are divided by a major shift in the regime of the most important controlling factor of this region which is the sea level. The transgressive stage covers the 8000-5500 BP period with sustained sea level rise (6-3 mm/yr) when floodplain dynamics acted to adapt to the ever-changing base level. Consequently, successive transitions between wet and dry conditions have been recorded as intertwined (thick) coarse-grained and (thinner) fine-grained sedimentary units. The latter stage (5500 BP - present) begins once with abrupt sea-level rise deceleration which fostered the expansion of persistent wetlands and lakes within meta-stable floodplain. The depositional evidence of this stage is composed of fine-grained sediments and peat. The transition between the two floodplain stages corresponds to the blooming phase of lakes, marshes and secondary channels which altogether persisted until the 20th century when during the communist period large floodplain areas were subject to drainage and desiccation works and their transformation into arable lands.

Introduction

The river floodplains landscape in lower reaches and deltas around the world witnessed dramatic change throughout their youngest geological history (Posamentier and Vail, 1988; Wright and Marriott, 1993; Aslan et al., 1999; Bloom and Tornquist, 2000; Hori and Saito, 2007). Floodplain dynamics and sedimentation is controlled by the balance between sea-level, sediment supply and accommodation space (Stanley and Warne, 1994; Smith et al., 2011; Törnqvist and Hijma, 2012), which disturbance translates in changing river sediment transport capacity and deposition pattern. In recent years, multidisciplinary studies and multi-proxy approaches combined with absolute dating of sedimentary sequences in the lower reaches of river systems have been conducted adding to our understanding of river floodplain development under the forcing base level changes or human induced sediment load oscillations. Thoroughly investigated river floodplains sedimentation and deltas development are discussed in relation with the Holocene sea level rise rate: Rhine-Meuse (Törnqvist, 1994; Berendsen and Stouthamer, 2000; Hijma and Cohen, 2011, 2019), Mississippi (Roberts, 1997; Aslan and Autin, 1999; Chamberlain et al., 2018; Törnqvist et al., 2020); Rhone (Arnaud-Fassetta, 2002; Vella et al., 2005), Po (Amorosi and Milli, 2001; Campo et al., 2016; Amorosi et al., 2017), Danube (Panin2003; Vespremeanu-Stroe et al., 2017, Nowacki et al., 2019), Eufrat (Morozova, 2005).

In the case of the lower Danube, the poor knowledge on the Holocene floodplain sedimentation history contrasts with the current state-of-the-art of the Danube delta chrono-stratigraphy. The inferred Danube delta evolution scenarios (Panin, 2003; Giosan et al., 2006; Vepremeanu-Stroe et al., 2013, 2017) highlight the river solid discharge, nearshore circulation and sea-level in shaping the deltaic system at different times.

Throughout the early Holocene the semi-enclosed Black Sea basin was subject to major sea level changes related to climate variability (i.e. post Younger Dryas transition) and re-connection to the World Ocean (9400 BP, Yanchilina et al., 2017; Ankidinova et al., 2020). Post re-connection, the Black Sea level rose in accordance with the general eustatism with average rates of 10–12 mm/yr until the 8500-8400 BP sea-level jump associated with Agassiz Lake drainage (Harrison et al., 2018). After that, sea level rise decelerated gradually from 6 to 3 mm/yr (8000-5500 BP) to less than 2 mm/yr during the second half of the Holocene, becoming almost stable in the last two millennia, as shown for the southern Danube delta (Vespremeanu-Stroe et al., 2013) and for the western Mediterranean Sea (Vacchi et al., 2016.

At the same time, in some regions of the world (predominantly in the Middle East, Nile Valley but also in Southern and Central Europe) human activities transformed the landscape through deforestation and land use change to agriculture (Turney and Brown, 2007; Forenbaher and Miracle, 2006) mobilizing large volumes of sediment which perturbed the solid discharge of the rivers. Sediment delivery increased first in Eneolithic and Bronze Age (ca 6500-3000 BP) but even much more in Antiquity (e.g. Danube reached the maximum sediment load during 2100–1800 BP interval, Vespremeanu-Stroe et al., 2017) when river floodplains and deltas experienced accelerated aggradation and progradation rates (Simeoni and Corbeau, 2009; Maselli and Tricardi, 2013; Vespremeanu-Stroe et al., 2013, 2017) and, more recently, during and after the industrial revolution (19–20th centuries) disturbing the river sediment discharge with consequences on erosion and sedimentation).

This study aims to constrain the sedimentary pattern of the Danube Lower Valley floodplain delta based on four sediment cores retrieved from the 70 km long floodplain sector upstream of the delta apex which intercepted representative floodbasin sequences. Newly obtained multi-proxy data (i.e. grain size; loss on ignition; magnetic susceptibility; microfauna and ¹⁴C ages) inform on the floodplain depositional pattern of the Lower Danube Valley. The results of this study contribute to the understanding of the floodplain accumulation dynamics under the influence of seal level changes during the Middle and Late Holocene.

The Lower Danube Floodplain between Galaţi and Tulcea (Fig. 1) extends over approximately 900 km², upstream of the Danube Delta (which has a total surface of 5480 km²). Holocene fluvial deposits onlap much coarser sediments deposited by the Danube mainly in Last Glacial Maximum. The topography has a low gradient gently sloping laterally from the river levees. The annual water and sedimentary discharge at Tulcea are ~205 × 10⁹ m³ (equivalent to ca 6500 m³/s) and 6470 kg/s, respectively (Bondar and Iordache, 2017). Today, many dams exist on the Danube and its tributaries, built during the 20th century and having resulted in significant decrease of the sedimentary discharge. Nowadays, mean annual solid discharge is of 20.4 × 10⁶ t (1990–2015) which is roughly three times smaller than in semi-natural condition when the annual average was 62.7 × 10⁶ t (1840–1970) (Preoteasa et al., 2016).

At the beginning of the 20th century the landscape of wetlands with multiple and interconnected channels, lakes and swamps was transformed through artificial dykes, desiccation works and irrigation channels being thus fully reclaimed for agricultural purpose (Giosan et al., 2013).

The local geology consists of older Hercinic roots of the Cimmerian orogen with numerous Paleozoic magmatic bodies, emerged as granitoid massifs bordering the right river bank to the south, and Sarmatian limestone covered 5–20 m thick Quaternary loess blanket (Seghedi and Oaie, 2018) bordering the left riverbank to the north. The roughly W-E oriented Danube river course sector downstream of Galați moulds the tectonic region of North Dobrogea Orogen.

The local climate is temperate-dry being characterised by mean annual temperatures of 10–11 °C (−2 °C in January, 23 °C in July) and total annual precipitation of 400–450 mm with a higher potential evapotranspiration (~700 mm) which induce a hydric deficit of 200–300 mm (Posea, 2005).

Our study sites includes three former lakes and wetlands in the Lower Danube floodplain which have been cored for this study. Two of them (i.e. Brateș lake and Sireasa deltaic wetland) have been drained during the last century.

Brateş Lake is located in the Western part of the study area and nowadays has a surface of ~20 km², which is much smaller than the original (undisturbed) surface of ~100 km² from the 18th and 19th centuries (Țuțuianu et al., 2018). It is surrounded by natural vegetation (Phragmites, Typha and Salix alba prevail) and agricultural land. The maximum water depth is 3 m. Our core was retrieved from the SE part of the former lake (Fig. 1).

Parcheş-Somova Lakes Complex, locally called “the upper delta” offers a pristine landscape preserving its natural landscape in the middle of the study area, immediately upstream of Danube delta. This area comprises a network of lakes and swamps interconnected by small canals. Our corings were made in two representative places: Parcheş (PAR) core is located in a swampy area, close to a small channel and Somova (SMV) core is located in the southern extremity of a circular lake with a surface of only ~0.6 km² (see Fig. 1 for details).

Sireasa is a polder derived from a former wetland situated near the apex of delta. Nowadays this wetland is completely drained and rendered to agricultural use. Historical maps of the region (1898, 1923) depict about 25 small lakes separated by numerous channels. Sireasa (SIR) core is placed in-between two former lakes.