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Landscape evolution and human impact in the Schwetzinger Hardt environment since the Last Glacial Maximum
The Schwetzinger Hardt and the surrounding flood plains of the Kinzig-Murg-Rinne and Leimbach river systems form two very different and distinctive landscape units. Partially, basic aspects of landscape evolution have not been understood or been scientifically investigated at all.
The study gives attention to these problems and discusses the landscape evolution of the northern parts of the middle Upper Rhine Graben segment since the LGM with the widespread aeolian sand deposits of the Schwetzinger Hardt as a central part of the investigation area. Three main subject areas are focussed, which need to be revised or have not been investigated sufficiently:
- Evolution and age dating of the aeolian sands and sand dunes
- Evolution and interrelations of the northern parts of the Kinzig-Murg-Rinne river system
- human impact on relief and soils with special regard to the transient areas of the landscape units
The PhD project uses a multi-method approach with investigation of soil profiles in pits and encountered by percussion drilling for the achievement of soil and sediment related data. Furthermore, pedochemical investigations and heavy mineral analyses are being conducted. Two-dimensional data of sedimentological constitutions could be acquired by the extensive application of Earth Resistivity Tomography (ERT). The combined application of field methods generates valid data with a precise mapping of the subsurface structures. Transformation to a higher level of analysis and interpretation is achieved by utilization of various dating methods like radio carbon dating and implicated Optical Stimulated Luminescence (OSL) datings and a high-resolution DEM.
Based on a comprehensive discussion of the current state of research concerning the regional landscape evolution of the northern Upper Rhine Valley the results are attended in catenary coherences for each working area in the landcape units investigated in this study. The findings are transferred to a higher aggregated level of spatiotemporal regard for interpretation of coherences of the landscape evolution. The achieved results bridge gaps in the state of research while raising new questions and focussing in on unresolved problems of investigation on landscape evolution in southern Germany.
Subsequent fundamental results could be achieved according to the three main subject areas.
1. Virtually all aeolian sand deposits in the investigation area were initially formed in the Pre-Allerød. All central sand dunes are covered with LST- and silt-bearing Upper Beds formed in the Younger Dryas cooling event on completely consolidated dunes. Thus, the formation of the widespread parabolic dune ridges has to be related to the Older and Oldest Dryas. The very short periods of possible aeolian activity in these stadials lead to the conclusion of a considerably older main phase of initial forming of the aeolian sand deposits towards the last phase of the LGM (18–16 ka BP), as long as the formation took place on slightly elevated parts of the lower terrace which could no longer be affected by high flood events. However, continued formation and differentiation of parabolic dunes in the Oldest and Older Dryas is hardly to be doubted. Remobilisation of aeolian sand deposits within the Younger Dryas were limited to distal dunes adjacent to active channels of the Kinzig-Murg-Rinne river system or channels according to the Neckar alluvial fan which affected the dunes by lateral erosion. Accompanied by the quick climatic change to more cold and distinctive arid conditions in the second half of the Younger Dryas, the reactivated dunes migrated rapidly across the inactivated channels nearby with migration rates of 15 m up to approximately 200 m in particular.
Furthermore, the apparent problem of aeolian activity in the Younger Dryas cooling contrasts with inconsistent palynological findings that suggest a perennial vegetation cover with open forest. Based on the methodological approach and results of this work, solifluction processes due to short-term periglacial environmental conditions are supposed.
2. The northern part of the Kinzig-Murg-Rinne river system is composed of two isochronously formed river arms (the “Leimener arm” and “Mingolsheimer arm”). In contrast to the assumptions of former studies, the Leimener arm as well as the Mingolsheimer arm seem to have existed up to the transition from the Atlantic to the Subboreal with strongly reduced discharge in case of the Leimener arm, however. Based on consistent radiocarbon and palynological datings conducted on peats derived from the abandoned river branches the assumption of former studies of a synchronous inactivation of the complete northern parts of the Kinzig-Murg-Rinne river system could be confirmed. This event can be associated with the break-through of the Murg river to the Rhine in the area of Rauental nearby Rastatt south of Karlsruhe at the end of the Atlantic. Furthermore, the contemporaneous existence of both river arms refers to the discussion of river bed transformations during the Late Glacial. Closely spaced, the intensively meandering Mingolsheimer arm existed synchronously with the multi-channel and slightly sinuous river bed of the Leimener arm in the northern parts of the Kinzig-Murg-Rinne. The controversial issue concerning a continuation of the Kinzig-Murg-Rinne river system north of the Neckar alluvial fan while receiving the full discharge of the Odenwald Neckar could be negated. In fact, the Kinzig-Murg-Rinne obtained at least episodically discharges from several fluvial channels in the southern part of the Neckar alluvial fan and had its outlet to the recent floodplain of the Rhine amongst Ketsch and Mannheim-Rheinau during the whole Lateglacial and Holocene.
3. The results concerning character and scale of the human impact on relief and soils of the investigation area showed extensive patterns, which are mainly affected by complex, high to late medieval sequences and interfingerings of pre-, syn- and postsedimentary flood plain deposits and remobilised aeolian sands in the border zones of the main landscape units. As expected, the evidence of excessive medieval impact on the Schwetzinger Hardt under formation of partly thick colluvial layers and dune remobilisations was detected adjacent to the medieval nuclei of settlements. However, immense and small-scale variations of this obvious spatial pattern were determined, which certainly refer to differences in land use history and intensity. In contrast, even the steepest leeward slopes of dunes in the central parts of the Schwetzinger Hardt are barely affected by soil erosion and colluviation although there is evidence of extensive medieval land use in the central parts.
The extent of sedimentation of loess-bearing floodplain deposits deriving from the catchment areas of the small Leimbach and Kraich streams exceeded what has been expected in the course of the preliminary studies. Due to the fact that virtually all early medieval or older flood plain levels were buried by floodplain fines, some of the underlying high to late medieval flood flows must have occurred with a catastrophic extent. In some areas, the correspondent fines were deposited even on the lower terrace distant to the actual floodplain, which hadn´t been affected by fluvial processes since the LGM. The dimensions and intensity of soil erosion and colluviation on the lower terrace were unattended. Despite generally low amounts of local relief, truncated soil profiles and correlate, partly thick colluvial layers in small depressions were frequently encountered.