Genesis of soils from Holocene tidal deposits at the North Sea coast

- Pedogenesis from tidal sediments is predetermined by geogenic preconditions.
- Geogenic preconditions initiate different development pathways.
- Four main pathways are distinguished at the Southern North Sea coast.
- Pathways are mainly controlled by sulfur cycling and supply of calcareous sediments.
- Pathways are applicable to other humid coastal areas.

The Southern North Sea coast has served as an exemplification to analyse and discuss the genesis of soils from Holocene tidal deposits. The accumulation wedge of Holocene tidal deposits at the North Sea coast is artificially embanked for the most part. In the enclosed area (hinterland) a variety of different soils developed. Disparate models attempting to explain the genesis of these soils deduce genesis of hinterland soils from recent foreland conditions, disregarding that geogenic preconditions have fundamentally changed. By consolidating and reviewing established literature we show that differences in geogenic preconditions are the initiating key for a feasible model. Describing geogenic preconditions to infer pedogenic processes, we develop a model comprising four different development pathways. High settling velocities and regular flooding, as present in the foreland, cause high inputs of mineral sediments and carbonate, moderate sulfur dynamics and thus the development of calcareous soils. Synsedimentary decalcification prevails in irregularly flooded areas due to enduring low sedimentation rates, low carbonate inputs and moderate sulfur dynamics under alternating redox states, as mainly proposed for the unobstructed landscape. High contents of soil organic matter (SOM) facilitate intensive sulfur dynamics creating potential acid sulfate soils (PASS), which become extreme acid upon oxidation (actual acid sulfate soils (AASS)). A great gradient in depositional conditions, occurring at unobstructed plain coasts, enables the deposition of deflocculated fine-grained sediments which promotes the formation of very compact soils with low water conductivities. Although this proposed model is based mainly on conditions known from the German North Sea coast, it is shown that shaping geogenic and pedogenic processes are applicable to other humid coastal regions around the world.