EGQSJE&G Quaternary Science JournalEGQSJE&G Quaternary Sci. J.2199-9090Copernicus PublicationsGöttingen, Germany10.5194/egqsj-71-213-2022Fluvial activity of the late-glacial to Holocene “Bergstraßenneckar” in
the Upper Rhine Graben near Heidelberg, Germany – first resultsFluvial activity of the late-glacial to Holocene BergstraßenneckarEngelMaxmax.engel@uni-heidelberg.dehttps://orcid.org/0000-0002-2271-4229HenselowskyFelixfelix.henselowsky@uni-mainz.dehttps://orcid.org/0000-0003-4145-7958RothFabianKadereitAnnetteHerzogManuelhttps://orcid.org/0000-0002-1655-4118HechtStefanLindauerSusannehttps://orcid.org/0000-0001-5363-2755BubenzerOlafhttps://orcid.org/0000-0002-3199-1156SchukraftGerdInstitute of Geography, Heidelberg University, Im Neuenheimer Feld
348, 69120 Heidelberg, GermanyInstitute of Geography, Johannes Gutenberg University Mainz,
Johann-Joachim-Becher-Weg 21, 55099 Mainz, GermanyCurt-Engelhorn-Centre Archaeometry, D6, 3, 68159 Mannheim, Germany
The term “Bergstraßenneckar” (BSN) refers to an abandoned course of the river Neckar. It flowed
in a northern direction east of the river Rhine in the eastern part of the
northern Upper Rhine Graben in southwestern Germany. The former meandering
course merged with the Rhine ca. 50 km further north of the site of the
present-day confluence near Mannheim. The palaeo-channels are still
traceable by their depressional topography, in satellite images and by the curved boundaries of
adjacent settlements and land parcels. In the plan view, satellite and
aerial images reveal a succession of meander bends, with older bends being
cut off from younger channels. Based on stratigraphic investigations of the
channel infill in the northern part of the BSN, fluvial activity is assumed
from ca. 14 500 years ago until the onset of the Holocene. We present
results of the first stratigraphic investigations at two sites in the
southern part of the BSN near Heidelberg (Rindlache, Schäffertwiesen),
together with results from granulometric, carbonate and organic content analyses, as well as
electrical resistivity tomography (ERT) measurements. The data clearly show
a change from high-energy fluvial bedload (sand, gravel) to low-energy
fluvio-limnic suspended load (organoclastic and calcareous mud) and to peat
formation. Radiocarbon dating indicates a time lag of ca. 1500 years between
the cut-off meander site (Schäffertwiesen) and the younger site
(Rindlache) that was possibly still active until the present-day confluence
near Mannheim was established and the BSN eventually became abandoned. Our
preliminary data conform with the pedo-sedimentary evidence from the
northern BSN, but slight differences in the stratigraphic pattern of the
youngest channels are identified: whilst for the younger channel
sections of the northern BSN the channel-bottom facies (sand, gravel) is
directly overlain by peat, the channel at Rindlache shows substantial
intervening mud deposition, which is interpreted as suspension load from
flooding by the new Neckar channel nearby. The study shows that more
chronostratigraphic data from channel sections of the southern BSN are
needed to better constrain the timing of the fluvial activity and to
decipher the reasons for the abandonment of the BSN. These data are also
necessary to better understand the pattern of temporary reactivation of the
BSN channels across the Holocene and their usage by humans, which can be
deduced from historical sources and archaeological data.
Kurzfassung
Mit dem Begriff Bergstraßenneckar (BSN) wird der heute inaktive
mäandrierende Lauf des Neckars bezeichnet, der im Spätglazial dem
östlichen Rand des nördlichen Oberrheingrabens folgte. Von dort
floss der BSN dem Rhein bei Trebur zu, bevor er sein Mündungsgebiet um
ca. 50 km nach Süden in den Raum Mannheim verlegte. Die morphologischen
Strukturen der verlandeten Neckarbetten sind in Satellitenbildern, im
Mikrorelief und am Verlauf von Flurgrenzen erkennbar. Auf Basis von
stratigraphischen Untersuchungen an Rinnenfüllungen des nördlichen
BSN wird die Aktivität dieses Flusslaufs von ca. 14 500 Jahren vor heute
bis zum Beginn des Holozäns angenommen. Hier präsentieren wir die
ersten stratigraphischen Untersuchungen zweier Lokalitäten im
südlichen Bereich des BSN (Rindlache, Schäffertwiesen) gemeinsam mit
granulometrischen, Organik-, Karbonat- und widerstandsgeoelektrischen Daten.
Die Ergebnisse reflektieren deutlich den Übergang von einer aktiv
durchflossenen Rinne (Sand- und Kiesfazies) hin zu Verlandung unter
fluvio-limnischen Bedingungen (organoklastische und kalkreiche
Feinkornablagerungen) mit abschließendem Torfwachstum. Die bislang
verfügbaren 14C-Daten deuten auf einen zeitlichen Versatz der
Aktivität von ca. 1500 Jahren zwischen der morphologisch älteren
Mäanderschlinge (Schäffertwiesen) und der jüngeren Lokalität
(Rindlache) hin, die möglicherweise noch zu der Zeit die Hauptrinne
bildete, als die Mündung nach Süden in den Raum Mannheim verlagert
wurde. Die hier präsentierten vorläufigen Daten sind mit der
bestehenden Chronologie am nördlichen BSN vereinbar, wenngleich auch
Unterschiede in der stratigraphischen Abfolge der Verlandungssedimente in
den zuletzt aktiven Rinnen identifiziert werden: Während im Norden die
fluviale Sand- und Kiesfazies unmittelbar von Niedermoortorf überlagert
wird, sind in den jüngeren Rinnen im Süden Feinkornablagerungen
zwischengeschaltet, die als Suspensionsfracht periodischer Überflutungen
möglicherweise ausgehend vom nur wenige Kilometer entfernten neuen
Neckarlauf interpretiert werden. Die Ergebnisse verdeutlichen, dass weitere
chrono-stratigraphische Untersuchungen an Rinnenstandorten des südlichen
BSN erforderlich sind, um den Zeitrahmen und die Abfolge der fluvialen
Aktivität besser eingrenzen und die Ursachen für die
Laufverlagerung besser definieren zu können. Zudem ergäben sich so
detailliertere Hinweise auf die temporäre Reaktivierung bestimmter
Abschnitte des BSN über das gesamte Holozän hinweg sowie auf deren
Nutzung durch den Menschen, die in historischen Quellen und durch
archäologische Daten belegt ist.
citationstatementEngel, M., Henselowsky, F., Roth, F., Kadereit, A., Herzog, M., Hecht, S., Lindauer, S., Bubenzer, O., and Schukraft, G.: Fluvial activity of the late-glacial to Holocene “Bergstraßenneckar” in
the Upper Rhine Graben near Heidelberg, Germany – first results, E&G Quaternary Sci. J., 71, 213–226, https://doi.org/10.5194/egqsj-71-213-2022, 2022.Introduction
The term “Bergstraßenneckar” (BSN) refers to an abandoned course of the river Neckar in the
eastern part of the northern Upper Rhine Graben in southwestern Germany.
Whilst the modern river Neckar flows in a western direction to connect with
the river Rhine at Mannheim after leaving the Odenwald Mountains at
Heidelberg, the BSN flowed from Heidelberg almost 50 km in a northern
direction to join the river Rhine near Trebur (Fig. 1a). The
palaeo-meanders of the BSN run parallel to the foothill zone
(“Bergstraße”) connecting the Odenwald Mountains and the Upper Rhine
Graben (Mangold, 1892; Bernhard and Hickethier, 1966; Dambeck, 2005; Dambeck
and Bos, 2002; Dambeck and Thiemeyer, 2002; Beckenbach, 2016). The
palaeo-meanders can be identified by their sinuous courses, lowered surface
and the typically curved boundaries of adjacent settlements and roads,
as well as cadastral boundaries. Satellite and aerial images reveal a
relative chronology of younger and older meander bends, with younger bends
truncating the older bends and older bends being cut off from younger
channel sections. Since early modern times, it has been contentious whether
natural or anthropogenic processes caused the Neckar to abandon the BSN
riverbed. The hypothesis of a man-made diversion of the lower Neckar from a
northern flow direction (i.e., the BSN) to a western direction towards Mannheim
(i.e., the modern Neckar) in the late Middle Ages (after 1354 CE) as a flood
protection measure (e.g. Saur, 1593; Winkelmann, 1697; Mone, 1826) was later
rejected (Mangold, 1892; Barsch and Mäusbacher, 1979, 1988). Some
palaeo-channel sections of the BSN may have still served as waterways in
historical times, particularly in the Roman period (Eckoldt, 1985; Wirth,
2011). Systematic chronological and sedimentological investigations from the
northern Upper Rhine Graben indicate drainage of the lower Neckar catchment
through the BSN during a short period between ca. 14 500 years ago and the
onset of the Holocene (see Große-Brauckmann et al., 1990; Dambeck, 2005;
Dambeck and Thiemeyer, 2002; Bos et al., 2008, 2012). This phase of fluvial
activity encompasses the formation and abandonment of different meanders, a
relative chronology of which has been established by Dambeck (2005). Here,
we aim to generate an initial chronostratigraphy of palaeo-meanders in the
southern part of the BSN near Heidelberg (hereafter: southern BSN). We
compare our results to meander activity phases as identified for the
northern BSN by Dambeck (2005), Dambeck and Bos (2002), and Dambeck and
Thiemeyer (2002) to establish working hypotheses for detailed
investigations on the southern BSN in the future.
Overview of the study area. (a) Simplified geological map of the
northern Upper Rhine Graben based on the Geological Map of Germany
1:1 000 000 (GK1000) and, for the peat deposits, the General Geological Map
of Germany 1:200 000 (GUEK200), sheets CC 6310 Frankfurt/Main – West and CC
7110 Mannheim. Data source: Bundesanstalt für Geowissenschaften und
Rohstoffe (BGR). (b) High-resolution digital elevation model emphasising
relief variation at 84–110 m NHN to highlight the active and
abandoned fluvial channels of the BSN between Heidelberg and Mannheim. RL
= Rindlache; SW = Schäffertwiesen. Data source: DGM1 of the Federal
State of Baden-Württemberg provided by Landesamt für Geologie,
Rohstoffe und Bergbau (LGBR) and established in 2000–2005. (c) Drone-based
photograph of the Rindlache site (9 November 2020). (d) Drone-based
photograph of the Schäffertwiesen site (17 January 2020).
Regional setting
Two palaeo-meanders were investigated at the field sites Rindlache (RL) and
Schäffertwiesen (SW). The sites are located 1 km apart, ca. 15 km
northwest of Heidelberg and ca. 10 km northeast of Mannheim, at the border
between the German federal states of Baden-Württemberg and Hesse near
Viernheim (Fig. 1b). The study area is part of the eastern Upper Rhine
Graben and located between the river Rhine and the eastern graben shoulder,
formed by the southern Odenwald with a Palaeozoic basement covered by (among
others) Triassic Buntsandstein sandstone (Barsch and Mäusbacher, 1979,
1988; Nickel and Fettel, 1979; Eisbacher and Fielitz, 2010). Quaternary
subsidence rates of the eastern Upper Rhine Graben near Heidelberg of
∼0.2 mm yr-1 are an order of magnitude higher compared to other
parts of the graben and lead to high sedimentation rates and a thick late
Quaternary infill (Peters and van Balen, 2007; Buness et al., 2009; Gabriel
et al., 2013).
The study sites are located north of the alluvial fan of the Neckar, which
forms where the river leaves the Odenwald and enters the surface of the last-glacial Lower Terrace inside the Upper Rhine Graben (Fig. 1b) (Barsch
and Mäusbacher, 1979, 1988). In the northernmost part of the Upper Rhine
Graben, the Lower Terrace is categorised into an upper Lower Terrace (t6,
early to middle Würm) underlying the fluvial landscape of the northern
BSN and a lower Lower Terrace (t7, late Würm) underlying the Rhine and
its floodplain (Scheer, 1978; Dambeck, 2005; Erkens et al., 2009). In the
area of the southern BSN no such distinction is made (Schottler, 1906;
Kupfahl et al., 1972; Holzhauer, 2013). The Lower Terrace around the study
sites shows varying ratios of sand and gravel, has an irregular surface, is
cut by BSN channels, and is overlain by up to several metres of BSN-related
sand- and silt-dominated flood deposits (Barsch and Mäusbacher, 1979;
Löscher, 2007). During the late Pleistocene–Holocene transition, dunes
formed on top of the silt- and sand-covered Lower Terrace (Löscher,
2007; Löscher et al., 1989) as elements of a larger regional
dune system covering substantial parts of the northern Upper Rhine Graben
(Dambeck, 2005; Holzhauer, 2013; Holzhauer et al., 2017; Pflanz et al., 2022).
The meandering course of the former BSN is reflected in the spatial
distribution of peat deposits in Fig. 1a corresponding to morphological
depressions of a depth of ∼2–4 m (Fig. 1b) (Barsch and
Mäusbacher, 1979). The two study sites are situated in the most
prominent palaeo-channels of the southern BSN (Fig. 1b) with distinct
channel morphologies of inwardly convex and outwardly concave banks, and
with diameters (half-meander path lengths sensu Howard and Hemberger, 1991) of
800–900 m. The two sites have been chosen as representative examples of (1) the presumably youngest course of the BSN (meander Rindlache) and (2) an
earlier fluvial phase (cut-off meander Schäffertwiesen) (see maps in
Mangold, 1892; Barsch and Mäusbacher, 1979) (Fig. 1b). The historical
field names indicate that the sites were formerly used for pasture (Rind= cattle; Wiese= meadow) likely due to waterlogging (Lache= marsh/swamp) caused by
a high groundwater table. The formerly high groundwater table in the Upper
Rhine Graben has lowered significantly due to major regulation measures on
the Neckar and Rhine since the early 19th century and subsequent river
incision. More recently, the intensified exploitation of drinking and
irrigation water added to groundwater level fall (see Barsch and
Mäusbacher, 1979; Dister et al., 1990).
Methods
The stratigraphy at both sites was studied using 2-D electrical resistivity
tomography (ERT) and sediment cores. ERT profiles were measured using a
GeoTom MK1E100 device with Schlumberger configuration, 100 electrodes and 1 m spacing, as in Kneisel (2003). The composite ERT profile 6–7–8–11 at
Rindlache consists of four separate profiles integrated with an overlap of
25 m (profiles 6–8) and 66 m (profiles 8 and 11), respectively. ERT profile
1–2 (112.5 m long) at Schäffertwiesen combines two separate profiles
overlapping for 37.5 m. Post-processing of ERT data comprises the
calculation of standard inversions without filtering using Res2Dinv
software. Erroneous data points, e.g. resulting from disconnected electrodes
during the measurement, were removed from the raw data prior to data
modelling.
Along the ERT profiles, sediment cores were taken using a vibracorer and two
different stainless-steel sampling tubes (Table S1 in the Supplement): (1) open percussion
gouges (ø 6 cm) (Figs. S1, S2) and (2) closed percussion gouges equipped with PVC
liner tubes (ø 5 cm) (Figs. S3–S7). Sediment cores taken in the open
gouges were documented and sampled in the field according to Ad-hoc-AG Boden (2005)
and the Munsell Soil Color Charts (Munsell Color Laboratory, 2000) (Table S2). The upper part of each core segment is prone to disturbances from
material collapsing inside the borehole or from the recovery process. These
disturbances were identified based on comparison with the lowermost part of
the overlying core segment and removed from the record. The PVC liners were
opened and the sediment documented (Munsell Color Laboratory, 2000; Ad-hoc-AG Boden, 2005) and sampled in the Laboratory for Geomorphology and Geoecology,
Institute of Geography, Heidelberg University (Table S3). One additional
core (RL01) was taken using an Edelman-type corer. At each site one core was
analysed in more detail in the laboratory to support facies interpretation.
All depths reported in the result section follow the original documentation
and correspond to the core photographs in Figs. S1–S7. Additionally,
adjusted depths of unit boundaries of the uppermost compressed metre are
given in Tables S2 and S3. Samples were dried, carefully pestled by hand and
sieved for the <2 mm fraction. Grain-size distributions of the
<2 mm fraction were measured using a laser particle sizer (Fritsch
Analysette P22) with a measurable range of 0.8–2000 µm at the
Laboratory of Sedimentology, Institute of Geosciences, Heidelberg
University. All samples were pre-treated with 10 mL H2O2 (30 %)
to remove organic carbon and Na4P2O7 (55.7 g L-1) for aggregate
dispersion. Univariate statistical measures were calculated using the Excel
sheet GRADISTAT v9.1 (Blott and Pye, 2001). Organic matter was determined by
loss-on-ignition (LOI) following a protocol slightly modified from Heiri et al. (2001). Samples of 3–5 g were combusted at 550 ∘C for 4 h in a
muffle furnace. The carbonate content was measured using the Scheibler
method according to DIN ISO 10693.
Four samples of autochthonous peat were dated by 14C accelerator mass spectrometry (AMS) at the
Curt-Engelhorn-Centre Archaeometry in Mannheim, Germany. The absence of
allochthonous root material was verified under a binocular microscope prior
to sample submission to the dating laboratory. All samples were pre-treated
with HCl, NaOH and HCl according to the acid–base–acid (ABA) method, during
which the “base” step eliminates ex situ humic acids (Wild et al., 2013). The
non-dissolved residual was then used for dating. The analysis was carried
out on a MICADAS type AMS system (Kromer et al., 2013). Results were
calibrated using CALIB 8.2 (Stuiver et al., 2022) and the IntCal20 dataset
(Reimer et al., 2020). For age interpretation, the 2σ error was
considered (Table S4). The reference date for all calibrated 14C data
is 1950 CE.
The positions of all sediment cores and ERT electrodes, as well as
topographic corrections, were determined using a Leica GS16 differential
global navigation satellite system (DGNSS) and the satellite positioning
service of the Federal State of Baden-Württemberg (SAPOS BW) in
real-time kinematic (RTK) mode (lateral error: 1–2 cm; vertical error: 2–3 cm). Elevations are given in metres above NHN (Normalhöhen-Null: official vertical datum used in
Germany signifying mean sea level in reference to Normaal Amsterdams Peil or Amsterdam Ordnance Datum) within the DHHN2016 (Deutsches Haupthöhennetz: official
German height reference system, newly levelled and introduced in 2016–2017;
AdV, 2018).
ResultsThe Rindlache siteStratigraphic record
Sediment core RL09 represents the stratigraphy of the BSN at Rindlache and
was taken on a harvested crop field in the central part of the assumed
palaeo-channel (Fig. 1b). The bottom unit of 5.45–5.20 m b.s. (below
surface) is dominated by sand of changing colour and shows only minor
amounts of silt and fine gravel (Figs. 2, S2). Between 5.20 and 4.50 m b.s., it is grain-supported, and coarser components of up to 5 cm (long axis)
contribute up to >50 %. The sorting varies. The LOI values
are very low, and the carbonate content is around 8 %. The section
4.50–2.30 m b.s. shows medium to coarse sand with lower amounts of coarser
components and with improved sorting. The organic content is equally low, and
the carbonate content decreases to 5 %–7 %. This unit is overlain by light
greyish brown mud with upward-increasing carbonate content, culminating in a
high value of >90 % in the uppermost part at 1.50 m b.s. Sand
or coarser components are absent, whereas reddish brown vertical root casts
are visible. Unfortunately, the lower boundary could not be identified due
to core loss (2.30–2.00 m b.s.); however, it is abrupt in parallel core
Gerd 1b (Fig. S3). The LOI values up to 1.36 m b.s. increase slightly to
levels of 3 %–4 %. Above a sharp contact, black peat was found, most of
which was lost during the core recovery (core loss: 1.30–1.00 m b.s.). The
LOI values reach up to >40 %. In the parallel core Gerd 1b,
peat from this unit at 1.05 m b.s. was dated to 6539–6402 cal yr BP (MAMS
43 986). The peat is overlain by dark grey, well-sorted organic-rich mud
(0.92–0.80 m b.s.) showing LOI values of 13 %, a very low carbonate
content and many fine roots. From 0.80 to 0.57 m b.s., the grey mud contains a
few terrestrial gastropod shells and shell fragments, as well as some fine vertical
roots. The carbonate content is slightly increased, whereas the LOI value is
lower. Above this unit, the clayey silt has a light yellowish-brown colour,
still containing gastropod shell fragments exhibited as higher carbonate
content. The uppermost unit (0.46–0.15 m b.s.; for decompacted values see
Table S3) is silt-dominated with increased LOI values representing the
anthropogenically turbated plough horizon.
BSN meander at Rindlache. (a) Transect ERT 6–7–8–11 crossing the
palaeo-river channel and showing the distribution of fine-grained deposits
in blue (low resistivity values) and coarse-grained deposits in green,
yellow, brown and red (intermediate to high resistivity values). (b) Synopsis of
sediment cores RL08, RL01 and RL09 with tentative facies interpretation.
For RL09, grain-size distributions, mean grain size, LOI values and
CaCO3 content are displayed. (c) Oblique view of the BSN meander in
combination with the transect ERT 6–7–8–11 and tentative facies
interpretation. A legend for the bottom drawing is provided in panel (b).
Data source: DGM1 of the Federal State of Baden-Württemberg provided by
LGBR and established in 2000–2005.
Electrical resistivity tomography (ERT)
The ERT profile 6–7–8–11 runs perpendicular to the BSN channel, indicated by
a surface depression (Fig. 2a, c). It includes the flanks on both sides and
intersects with core RL09 at 183 m horizontal distance (h.d.). In the
northwest, the ERT profile starts at the foot of a late Pleistocene dune
(Bettenberg, Fig. 2c; mapped in Barsch and Mäusbacher, 1979) at ca.
102 m NHN, running down a slightly concave slope with a small terrace
bordering the outer bank at approximately 75–95 m h.d. It reaches the
lowest elevations of the palaeo-meander channel at ca. 95 m NHN (ca. 125
to 210 m h.d.) and terminates at the inner bank of the palaeo-meander. The
root mean square error (RMSE) is 4.1 % after three iterations of data
modelling. The maximum difference in elevation across the entire profile is
6.5 m. The measured resistivity ranges between ∼10
and ∼1100Ωm, and a depth of ca. 20 m b.s. was reached
(Fig. 2a). The central channel between 125 and 210 m h.d. shows the
lowest resistivity in the uppermost 4–6 m. This is in strong contrast to
values which are an order of a magnitude higher at the southeastern end of
the ERT profile between 210 and 260 m h.d. The slope at the foot of the
Bettenberg dune also shows higher resistivities of 200–500 Ωm with
a slightly thicker wedge of low-resistivity materials on top. Between 10
and 55 m h.d. this pattern is reversed, with ca. 1 m of medium resistivities
(50–100 Ωm) at the top above very low resistivities, similar to the
palaeo-meander channel infill.
The Schäffertwiesen siteStratigraphic record
The sediment cores from Schäffertwiesen were taken along an ERT profile
oriented perpendicular to the meander channel including the outer bank and a
part of the channel (Figs. 1b, 3b, c). Sediment core SW01 was taken on the
slope of the outer bank and reaches a depth of 4 m b.s. (Fig. S4). The
lower part from 4.00 m to 1.74 m b.s. is characterised by a medium to coarse
sand matrix and varying amounts of well-rounded gravel components, the
latter mostly below 2.68 m b.s. This lowermost section is clast-supported
between 3.45 m and 3.00 m b.s. (mostly limestone of Middle Triassic
Muschelkalk, Upper Jurassic Weißjurakalk and red sandstone of Lower to
Middle Triassic Buntsandstein, as well as other limestone and quartzite
varieties). It shows increased carbonate content of up to 10 % and very low LOI values (<0.3 %). Between 2.75 m and 2.40 m b.s. some finer and darker laminae occur. From 2.40 m to 2.00 m b.s., the
core is disturbed by collapsed material. A sharp boundary separates
the sand to gravel deposits from sandy to clayey mud (1.74–0.84 m b.s.),
where LOI values increase to up to 6 %, and carbonate content reaches up
to 25 %. A thin sand layer resembling the bottom facies is intercalated
at 1.61–1.57 m b.s. Plant remains from a depth of 1.67 m b.s. were dated to
11 258–11 195 cal yr BP (MAMS 46037). The carbonate-rich mud is overlain by
peat (0.84–0.59 m b.s.) with LOI values of up to 57 % and a very low
carbonate content of <1 %. Peat-derived 14C data range from
11 079–10 722 cal yr BP (0.80 m b.s., MAMS 46036) to 7920–7701 cal yr BP
(0.60 m b.s., MAMS 46035). This peat section is separated from the
organic-rich topsoil (0.28–0.11 m b.s.; LOI values up to 24 %) by a
brownish grey sandy mud section.
BSN meander at Schäffertwiesen. (a) Transect ERT 1–2 crossing
the palaeo-river channel and showing the distribution of fine-grained
deposits in blue (low resistivity values) and coarse-grained deposits in
green, yellow, brown and red (intermediate to high resistivity values). (b) Synopsis of sediment cores SW03, SW01, SW04 and SW02 with tentative facies
interpretation. For SW04, grain-size distributions, mean grain size, LOI
values and CaCO3 content are displayed. (c) Oblique view of the BSN
meander in combination with the transect ERT 6–7–8–11 and tentative facies
interpretation. A legend for the bottom drawing is provided in panel (b).
Data source: DGM1 of the Federal State of Baden-Württemberg provided by
LGBR and established in 2000–2005.
The coarse sand and gravel unit was found in the basal parts of all cores
from Schäffertwiesen, where it varies in thickness (Fig. 3b). Its
sharp upper boundary rises from 93.11 m NHN in the west (SW03) to 93.72 m b.s. (SW01), 94.22 m b.s. (SW04) and 96.17 m b.s. (SW02) in the east. The overlying poorly sorted sandy mud from SW01 (1.74–0.84 m b.s.) was not
found in SW02 and SW04 but can be correlated with a much thicker occurrence
in the western part of the profile (SW03; 4.45–0.55 m b.s.). The
peat, however, is only present in SW04, close to the top of the sequence, in
similar thickness as observed in SW01.
Electrical resistivity tomography (ERT)
The stratigraphic correlations between the cores are reflected by the ERT
profile 1–2 at Schäffertwiesen (Fig. 3a). It has a length of 112.5 m
and an RMS error of 3.9 % after three iterations of data modelling.
Resistivity values are in the same range as in ERT profile 6–7–8–11 at
Rindlache. From its southwestern end, the profile traverses over a flat
terrace for ca. 30 m at ca. 98 m NHN before following a concave slope down
to ca. 95.5 m NHN in the lowest part of the profile, which is also where
core SW01 was taken. Between 45 m h.d. and the northeastern end, the profile
gradually rises in the form of a slightly convex slope to ca. 96.5 m NHN.
The flat part of the profile in the southwest, represented by core SW03,
shows very low resistivity values (10–40 Ωm) correlating with the
sandy mud facies. Here, resistivity only increases below ca. 5 m b.s., where
the sand and gravel deposits were encountered in SW03. Likewise, in the
topographically lowest part of the profile, corresponding to SW01, the basal
sand and gravel deposits are reflected by medium resistivity values of
around 80 Ωm, compared to 20–50 Ωm in the sandy mud and
peat of the uppermost 1.70 m of the sequence. In the northeastern part of
the profile, medium to high resistivity values reach close to the surface,
following the rising boundary between the sand and gravel unit and the peat.
DiscussionFluvial activity as reconstructed from facies patterns
The sand and gravel deposits found in the basal part of all cores from both
palaeo-meander sites consist of varying ratios of predominantly medium to
coarse sand and rounded to well-rounded gravel components. They represent
the bedload of the BSN deposited at the bottom of the formerly active
meander channel. Primary deposition of the material in pre-late-glacial
times, perhaps in a braided system in Pleniglacial times of the last-glacial
period, and subsequent reworking of these deposits in late-glacial times by a
single meandering channel is plausible. As the channels are incised into
the Lower Terrace of the Rhine, it is possible that Rhine deposits were
reactivated by the BSN. Although a quantitative petrographic
analysis to discriminate between Rhine and Neckar deposits is still pending,
the visual inspection of basal gravel components in both master cores RL09
and SW01 already shows a dominance of Muschelkalk, Weißjura and other
limestones, as well as Buntsandstein sandstone, collectively representing the
main erosional products of the Neckar catchment (Barsch and Mäusbacher,
1979; Fezer, 1997; Bibus and Rähle, 2003; Löscher, 2007; LGBR, 2021). These deposits shape the youngest part of the wide alluvial fan of
the Neckar (Löscher et al., 1980; Barsch and Mäusbacher, 1988),
which belongs to the Mannheim Formation (LGBR, 2021) and, at its northern
boundary, almost reaches the study area (Fezer, 1997; Beckenbach, 2016).
The typical sediment sequence of palaeo-meander channels of the northern BSN
(e.g. site of “Wasserbiblos” in Dambeck, 2005; Dambeck and Bos, 2002)
also starts with fluvial sands and few pebbles. The general fining-up
gradient from a sand and gravel mixture (in the southern BSN with some
clast-supported sections) to matrix-supported units and pure fluvial sands
indicates a decrease in fluvial transport capacity at the end of the phase
of fluvial activity of the BSN. This decrease is either related to lower
discharge or a thalweg shifting away from the coring site. However, it
cannot be excluded that the increasing medium sand component in the upper
part of the fining-up sequence is partially related to reactivated aeolian
processes and input during the Younger Dryas (Löscher et al., 1989;
Dambeck and Thiemeyer, 2002; Pflanz et al., 2022). The poorly sorted
greyish-brown sandy mud overlying the in-channel fluvial sands in RL09
(boundary at 2.30 m b.s.) and SW01 (boundary at 1.74 m b.s.) reflects a
distinct shift from a fluvial channel carrying bedload – until then
presumably the main active channel of the BSN – to a cut-off channel
restricted to suspension-load settling during stages of overbank flow by an
adjacent active channel (Barsch and Mäusbacher, 1979). At
Schäffertwiesen, this adjacent channel was the Rindlache channel. After
the BSN was entirely abandoned, the Rindlache site was subject to flooding
and received suspension load from a new Neckar course close by. This might
have been the current channel heading straight to the Rhine near Mannheim,
although this assumption requires verification with future research. Along
the northern BSN this type of fluvio-limnic deposition is observed for the
older meander generation before peat formation commenced, whereas at the
younger meander sites peat deposits immediately overlie the coarse-grained
channel-bottom deposits (Dambeck, 2005; Dambeck and Bos, 2002). Thus, the
sedimentary sequences at Rindlache and Schäffertwiesen both resemble the
infill of the older meander sites along the northern BSN. At some
palaeo-channel sites of the northern BSN (e.g. “Auf Esch”, “Großes
Bruch”), as well as at Rindlache, peat formation is interrupted by
organic-rich black clays that may represent a reactivation of overbank
deposition and indicate increased input of fine-grained material into the inactive fluvial system (Dambeck, 2005; Dambeck and Thiemeyer, 2002).
Dambeck and Bos (2002) and Dambeck and Thiemeyer (2002) refer to the sandy
mud overlying the fluvial channel-bottom facies at the older meander sites
as clays, silts, loam or gyttja with occasional fine sandy laminae,
depending on the site. The very high carbonate content in the uppermost part
of this fine-grained unit (>90 % in RL09) right below the
overlying peat, also referred to as calcareous gyttja along the northern BSN
(Dambeck and Bos, 2002; Bos et al., 2008), was identified as secondary
carbonate precipitation. At present, two models for the formation of this
carbonate precipitation are considered.
The first is precipitation within the sediment body at distinct substrate boundaries
in the groundwater fluctuation zone, along the capillary fringe, as
described for the so-called Rheinweiß in similar contexts (Dambeck, 2005; Holzhauer,
2013; Holzhauer et al., 2017). Being this close to the present-day land
surface, the Rheinweiß represents a relict feature. It predates the river regulation measures in the Upper Rhine Graben from 1817 CE on that led to rapid linear
incision of the Rhine and to lowering of groundwater levels by several
metres in the entire graben area (Barsch and Mäusbacher, 1979; Dister et
al., 1990).
The second is precipitation in the fluvio-limnic environment of the cut-off meander by photosynthesising Charophyceae and aquatic plants, aided by the uptake of
CO2 from bicarbonate (HCO3-) dissolved in the water (e.g.
Bohnke and Hoek, 2007). The general model, according to which carbonate
ions (CO32-) are released and attract Ca2+ ions to form
Ca2CO3 in the immediate vicinity of the photosynthesising
organisms, is described in, for example, Merz (1992).
Whilst in both the southern and the northern (site “Wasserbiblos” in
Dambeck and Bos, 2002) parts of the BSN these calcareous muds mostly date
into the Preboreal (11.7–10.3 kyr ago) (Fig. 4), they are also well
recognised to have formed earlier during the Alleröd (13.4–12.7 kyr
ago) elsewhere in Central Europe (e.g. Bohncke and Hoek, 2007; Pawłowski
et al., 2016). They may in general be associated with warmer phases of the
late-glacial to Holocene transition with more abundant (aquatic) vegetation,
shifting the carbonate balance and leading to increased carbonate
precipitation (Waldmann, 1989; Dambeck, 2005).
Assumed timeline of fluvial activity and inactivity of the
Bergstraßenneckar (BSN). All sites refer to profiles from
palaeo-channels, apart from Heißfeld (profile on the Lower Terrace
adjacent to a BSN palaeo-channel). Results from the northern BSN (upper
part) were taken from Dambeck (2005) and Bos et al. (2008). Preliminary data
and interpretation from the southern BSN (this study) are shown in the lower
part on the grey background. Wetland or temporary standing-water conditions at
Rindlache during Roman times are inferred from the partially excavated
wooden bridge across the same BSN channel ca. 1 km to the south (Wirth, 2011).
Timing of fluvial activity of the southern BSN
There are diverging assumptions regarding the timing of the fluvial activity
of the BSN. Its relatively short existence has been associated with the
late-glacial formation of the north–south-directed dune belt between
Schwetzingen and Lorsch (Fig. 1a) (e.g. Dambeck, 2005), which is assumed
to have blocked the direct connection with the Rhine between Heidelberg and
Mannheim. Yet, none of the palaeo-channels of the northern BSN are covered
by any significant drift sands, the formation of which terminated mostly
before the Older Dryas (13.6–13.4 kyr ago). Instead, drift sands were
eroded by the BSN in some places, indicating that fluvial activity postdates
the period of main aeolian activity. Initial activity of the BSN is
tentatively dated to ca. 14 500 years ago (Dambeck, 2005). At the site
“Fasanerie” near Groß-Gerau (Dambeck, 2005) and near Schwanheim
(Hoffmann and Kzyzanowski, 1984) (Fig. 1a), Laacher See tephra, now
dated to 13 006±9 cal yr BP (Reinig et al., 2021), was identified in
overbank deposits of the northern BSN. The detailed stratigraphic
investigations in the northern part of the BSN indicate a dune breach of the
Neckar towards the Rhine at approximately 12 800 to 11 500 years ago during
the Younger Dryas (12.7–11.7 kyr ago) and an end to fluvial activity of
the BSN channels at some point between ca. 11 600 and 10 120 years ago
(Haupt, 1928; Wagner, 1981; Große-Brauckmann et al., 1990; Dambeck,
2005; Dambeck and Bos, 2002; Bos et al., 2008, 2012). However, there are several
historical accounts and archaeological data pointing to the reactivation of
certain sections of the BSN by smaller tributaries draining the western
Odenwald Mountains and their use as waterways for the transport of goods, in
particular during Roman times (Eckoldt, 1985; Wirth, 2011).
For the northern BSN, two palaeo-meander generations have been classified.
Whilst the relatively younger meander generation forms a more or less
continuous course to the former mouth west of Trebur, the relatively older
meanders are morphologically detached (Kupfahl et al., 1972; Dambeck, 2005).
Based on palynostratigraphical evidence and radiocarbon data, mud deposition
in cut-off meanders of the older generation started in Alleröd times
(site “Dornheimer Lache” in Bos et al., 2008) or by the end of the Younger
Dryas (site “Wasserbiblos” in Dambeck, 2005; Dambeck and Bos, 2002; Bos et
al., 2012). Elsewhere, it is assumed that sands were blown out from the
inactive point bars of cut-off meanders to form proximal dunes on the Lower Terrace (HLfB, 1990), e.g. at the site “Heißfeld” (Dambeck, 2005; Dambeck
and Thiemeyer, 2002) at a time, during the Younger Dryas, when aeolian dunes and drift
sands of the northern Upper Rhine Graben were reactivated to a limited
extent (Löscher et al., 1989; Dambeck and Bos, 2002; Pflanz et al.,
2022). The younger meander sites show a distinct shift from fluvial sands to
peat growth roughly at the beginning of the Preboreal (Fig. 4), possibly
reflecting the abandonment of the BSN and confluence of the Neckar with the
Rhine further to the south near Mannheim (Dambeck, 2005).
The basic stratigraphic patterns of the northern and southern BSN show
striking similarities, but presently, very few radiocarbon ages are
available for the southern BSN. However, if these are used as chronometric
tie points as illustrated in Fig. 4 (lower part), the similarities become
even more obvious. As the clastic mud deposits at the site
Schäffertwiesen may date to the Younger Dryas to Preboreal period, we
assume that the coarse-grained channel-bottom deposits underneath date to
late-glacial times. The shift from mud sedimentation to peat growth may have
been induced by a denser vegetation cover at the onset of the Preboreal
(Dambeck and Bos, 2002; Bos et al., 2008) leading to reduced suspension load
during flood events and termination of the silting-up process. At the site
“Wasserbiblos”, northern BSN, sedimentation of similar silty and
calcareous mud, reflecting a change in fluvial conditions from in-channel
bedload transport and accumulation at the channel bottom to fluvio-limnic
conditions inside a cut-off meander, is also dated to the end of the
late-glacial period. A temporal overlap of changing fluvial dynamics at the
Schäffertwiesen meander (between 12 000 and 11 500 cal yr BP) at the
southern BSN and the older meander generation at the northern BSN,
represented by the site “Wasserbiblos” (Dambeck, 2005; Dambeck and Bos,
2002; Bos et al., 2012), is likely. Yet peat formation at the northern BSN
started earlier, during Preboreal times, and lasted until the end of the
Boreal or beginning of the Atlantic period (ca. 8000–7500 cal yr BP). The
peat at Rindlache continued to form until ca. 1500 years later (ca.
6500–6000 cal yr BP) (Fig. 4). The deposition of organic-rich black clays
inside the channels and as overbank fines during the Atlantic period may
represent an initial signal of anthropogenic soil erosion by Middle
Neolithic communities (Große-Brauckmann et al., 1990; Dambeck and
Thiemeyer, 2002). This interpretation is supported by the first occurrence
of Cerealia in pollen spectra of northern BSN sites as an indicator of the
introduction of agriculture and a decrease of Ulmus, which is related to the Neolithic Linear Pottery and Rössen cultures using this type of wood for
fire and construction (Bos et al., 2012). In the area of the southern BSN
people of the Late Neolithic Michelsberg (ca. 4400–3500 BCE; Lang, 1996) and/or the
end Neolithic Corded Ware ceramic cultures (ca. 2900–2350 BCE; König, 2015) may have
intensified soil erosion resulting in a subsequent increase in suspension load and the formation of the black clays.
Assuming that the Rindlache site – in contrast to the cut-off meander of
Schäffertwiesen – represented the active channel until the modern
Neckar channel was established and the BSN finally abandoned, the entire
chronostratigraphy may be offset by ca. 1500 years. Thus, it would overlap
with the chronostratigraphy of northern BSN sites representing the younger
meander generation, sensu Dambeck (2005), even though the Rindlache site shows an
interim sequence of fluvio-limnic sandy mud, which in the north is
characteristic of only older meander sites (Fig. 4).
Conclusions and outlook
Palaeo-meanders of the southern BSN are an understudied geomorphological
archive. This is all the more surprising as studies along the northern BSN
in Hesse proved to reveal detailed aspects of the late-glacial to Holocene
history of the Upper Rhine Graben riverscape (e.g. Dambeck, 2005; Dambeck
and Thiemeyer, 2002; Bos et al., 2008). The palaeo-meander channels of the
southern BSN can still be morphologically identified in the field, as well as
from satellite imagery and digital elevation models (Beckenbach, 2016),
representing a sequence of relatively older meanders which have been cut off
from younger channels. Our pilot study at the relatively older
Schäffertwiesen meander and the younger Rindlache meander shows a
stratigraphic sequence reaching from partially clast-supported
sand and gravel in-channel facies to muds and peat representing the phase
after which the meanders were cut off, and the Neckar shifted its course
entirely. The 14C data of the upper boundaries of the
peat deposits at both sites, Schäffertwiesen (cut-off meander) and
Rindlache (part of the latest course), are offset by ca. 1500 years,
reflecting the overall older age of the Schäffertwiesen sequence. In
comparison with the abandoned riverscape of the northern BSN (Dambeck,
2005), both sites studied here resemble the stratigraphic pattern of the
older meander phase with fluvio-limnic mud deposition which is vertically
confined by coarse-grained in-channel facies (below) and peat and black
clays (above). The chronology of the Schäffertwiesen site
tentatively correlates with the older meander generation, while the
Rindlache site has more of a chronological overlap with the younger meander
generation of the northern BSN, where, however, the intermittent mud is absent.
Therefore, the presence of fluvio-limnic sediments in the abandoned river
channel may be a function of flooding frequency and proximity to a still
active channel. Whilst this was the case for both of the southern sites
after the final abandonment of the BSN as they were still close to the new
Neckar course, the northern BSN channel sites were cut off from a
regular flooding regime.
Evidently, more palaeo-channel stratigraphies of the southern BSN need to be
investigated and correlated, in combination with an extended chronological
dataset of 14C ages for the organic-rich sediments and optically
stimulated luminescence ages for the sand-dominated in-channel facies, for
which no data are available to date. In particular, the palaeoenvironments of
the fluvio-limnic muds, along with any potential anthropogenic impact,
require further attention and need to be reconstructed in more
detail. Deciphering a chronology of fluvial activity in the southern BSN
domain will provide the basis for investigating the reactivation of some
reaches of and human interaction with the BSN across the Holocene, in
particular during Roman times and later historical periods, for which only
fragmented historical and archaeological information is available so far
(e.g. Eckoldt, 1985).
Data availability
All data from this study can be found in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/egqsj-71-213-2022-supplement.
Author contributions
The concept of the study was jointly developed by all authors at the
Institute of Geography, Heidelberg University, in the context of field
courses held at the two study sites. All these authors were involved in the
fieldwork. Sedimentary laboratory analyses were carried out by FR, GS and ME. FR
contributed results from his master of science thesis. Processing and analysis of ERT data were carried out by MH, FH and SH. The GIS for this project was set up by FH.
SL performed the 14C dating. A first draft of the manuscript was
written by ME, FH and AK. ME, FH, FR, AK, MH, SH, SL and OB commented on and
approved the manuscript.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Quaternary research from and inspired by the first virtual DEUQUA conference”.
Acknowledgements
We thank Peter Müller (Mannheim-Straßenheim) and the Bach family
(Heddesheim) for permission to conduct fieldwork on their properties.
Undergraduate students of the geography programmes at Heidelberg University are thanked for their engagement during field courses. Nicola Manke kindly
provided support during one field course and during the laboratory analyses.
Support and permission to use the laser particle sizer of the Sedimentology
and Marine Paleoenvironmental Dynamics research group at the Institute of
Geosciences at Heidelberg University by Andre Bahr are greatly
appreciated. Finally, we would like to thank the Landesamt für Geologie,
Rohstoffe und Bergbau (LGBR) for providing the digital elevation model DGM1
of the state of Baden-Württemberg. We are thankful for the helpful
reviews of one anonymous person and of Rainer Dambeck, who provided highly
detailed comments, plenty of ideas and improved the manuscript by
sharing his great regional expertise.
Gerd Schukraft was a driving force of the research on the
southern Bergstraßenneckar and one of the initiators back in 2019. He
sadly passed away in April 2020.
Financial support
This research was supported by funds of Heidelberg University. For the publication fee we acknowledge financial support by Deutsche Forschungsgemeinschaft within the funding programme “Open
Access Publikationskosten”, as well as by Heidelberg University.
Review statement
This paper was edited by Julia Meister and reviewed by Rainer Dambeck and one anonymous referee.
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