Tidal peatlands: a diatom study in the Old Rhine estuary, The Netherlands, revealing the role of reed peat in mid-Holocene landscape transitions
Werf, K.M. van der
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During the evolution of tidal basins, the dynamic equilibrium between accommodation space and inflow may shift, leading to the closure of the tidal basin. Closure occurs commonly due to one or a combination of two mechanisms: silting up of the tidal basin as terrestrialisation outpaces sea level (rise) or closing off of the tidal basin by beach barrier development. Recently, vegetation has been put forward as a possible promotor of terrestrialisation, but its role as driver mechanism in tidal basin closure has not yet been investigated. Silting up, barrier formation and vegetation development all acted in the South-Holland tidal basin during the mid-Holocene, when the coastal area transitioned from tidal basin to peat landscape. The common palaeographical explanation at the final tipping point of tidal basin closure is that beach barriers closed and the back-barrier basin subsequent freshened, which facilitated peat formation. The basin-freshening explanation was tested, using diatoms for reconstructing salinity conditions around the lithological contact between marine clays and reed peat in the Old Rhine estuarine area within the South Holland basin. Did vegetation and peat formation play a role in “pushing out” the tidal influence? Diatom analysis from two cores showed that open connections with the sea remained until peat formation commenced under brackish conditions in an intertidal environment. Marine flooding still occurred while peat developed. There are no signs of a freshwater environment in the marine clays nor in the peat, so the beach barriers did not close off the back-barrier basin completely before peat formation started. The tidal landscape transitioned to peatland due to an interplay between silting up, beach-barrier development, and terrestrialisation by vegetation expansion. At the tipping point of the landscape transition, the development of reed fields and peat played a prominent role in pushing out marine influence of the back-barrier area. Terrestrialisation by peat formation could outpace mid-Holocene sea level rise. Importantly, peat is not only a consequence, but also a driving force of landscape transition. The fact that reed peat formed under brackish conditions is contrary to the current lithostratigraphic definition of Holland peat, that classifies all peat as fresh. Revision of this definition as to types of peat and genesis would improve reconstructions of palaeoenvironments and especially transitions of palaeolandscapes. Other proxies such as diatoms have proven to be a valuable and necessary addition to lithology in palaeogeographic reconstructions.