Peat compaction in deltas : implications for Holocene delta evolution

Abstract

Many deltas contain substantial amounts of peat, which is the most compressible soil type. Therefore, peat compaction potentially leads to high amounts of subsidence in deltas. The main objective of this research was to quantify subsidence due to peat compaction in Holocene fluvial-deltaic settings and to evaluate effects of peat compaction on delta evolution. For this purpose, field research has been carried out in the Cumberland Marshes (Canada), the Rhine-Meuse delta (The Netherlands) and the Biebrza National Park (Poland). Additionally, a new numerical peat compaction model that is calibrated with an extensive field dataset has been developed. The amount and rate of subsidence due to natural peat compaction is highly variable in space and time, mainly depending on (1) the effective stress (2) the organic-matter content of peat, (3) the thickness of the peat layer, and to a lesser extent (4) the plant species composition of peat. High-organic thick peat layers may accumulate during thousands of years without significant compaction. Most compaction occurs within decades to a few centuries after a substantial increase of the effective stress, for example caused by fluvial sedimentation on top of a peat layer, or by groundwater table lowering. Subsidence rates, due peat compaction, may be as high as 15 mm/yr. Human-induced peat compaction and oxidation may even lead to subsidence rates on the order of a few cm/yr. In deltaic environments experiencing a low sediment supply, accommodation space may be filled with peat. Subsequent compaction of this peat revitalizes existing accommodation space for increased fluvial deposition and/or peat formation. In this way, compaction-induced accommodation space increases the sediment trap efficiency, by which thick fluvial sediment layers may develop and delta propagation may be slowed down. In the Rhine-Meuse delta, subsidence due to compaction of up to ~3 m has occurred. Locally, up to ~40% of the total Holocene accommodation space has been (re)created by peat compaction. Peat compaction may locally lead to the formation of thick natural levees or crevasse-splay deposits. Generally, differential compaction initially does not increase cross-valley gradients on a floodplain, which could initiate avulsion, because compaction-induced accommodation space will usually be filled by fluvial deposition or peat formation. Moreover, regional gradients are low in peatlands, which inhibits a crevasse splay to evolve into an avulsion, as is usually the mechanism of avulsion initiation. Only if the maximum peat compaction potential at a certain location has been reached, while at a nearby location on the floodplain rates of subsidence due to peat compaction are still relatively high, differential peat compaction may lead to gradient advantages. This may affect spatial sedimentation patterns on a floodplain. Because peat compaction distorts the original stratigraphy, and through its control on regional and local sedimentation patterns, it influences the alluvial architecture of deltaic sequences, which is important knowledge for e.g. the exploration of natural resources. In populated deltas, subsidence due to peat compaction may seriously increase flood risks. Therefore, peat compaction, as well as the alluvial sequence composition, should be taken into account in delta management strategies.