Peat Archives: The Key To Unravel The Influence Of Peat Compaction On Delta Evolution

Peat is most compressible of all natural soils. Therefore, compaction of peat layers leads to substantial amounts of land subsidence. In Holocene deltas, which are often densely populated, subsidence due to peat compaction is caused by anthropogenic processes such as loading by construction works and groundwater level lowering, but also, a significant amount of compaction is caused by natural processes. However, there is a lack of knowledge concerning effects of peat compaction on the evolution of deltas, which is mainly due to a lack of field data and suitable methods to quantify peat compaction in fluvial-deltaic settings on Holocene time scales (100-103 years). Therefore, new field methods to measure the amount of peat compaction in fens and swamps are developed. These methods are based on 1) dry bulk density and organic matter content measurements in vertical peat profiles, 2) 14C dating and 3) detailed analysis of sediment sequence stratigraphy based on logged boreholes. A new coring device has been developed to collect sufficiently large samples of uncompacted (modern) peat, without disturbing the internal structure. We present data obtained with these methods from the Rhine-Meuse delta (The Netherlands), the Cumberland Marshes (Canada) and the Biebrza National Park (Poland). Values of up to about 50 % of compaction (percentage of the initial thickness) are observed at sites where the peat layer is relatively thick, has a high organic matter content and is overlain by a meters thick sediment overburden. It is hypothesized that accommodation space created by peat compaction is an important mechanism controlling the development of natural levees of low gradient rivers. Peat compaction underneath natural levees, and crevasse splay deposits, fixes river channels and prevents renewed avulsion, arguable the most important process controlling sediment distribution in deltas. Herewith, we contradict the hypothesis stating that subsidence due to peat compaction creates changes in cross-valley gradient which possibly drives avulsion. Avulsions in peatlands are controlled by other mechanisms than peat compaction. Results of this ongoing study are of high value for predicting sites of severe subsidence due to peat compaction and to determine its influence of delta evolution.