Reconstructing historic bog iron ore deposits in the Bourtangermoor, a former raised bog in the Netherlands

Samenvatting

Bog iron ores are well-known lumps of Fe-(oxy)hydroxides (goethite, limonite) found along streams and in seasonally flooded, low-lying areas. Historic literature of the Bourtangermoor, a former raised bog in the north of the Netherlands and adjacent Germany, however, revealed a second, rare type of bog iron ores, exclusively composed of siderite (Fe-carbonate) with accessory vivianite (Fe-phosphate), minerals only stable under anaerobic conditions. In this research we compared historic literature and maps mainly from the first half of the 20th century, when parts of the bog were still intact, with present-day seepage data to allow analysis whether recent seepage could be used to reconstruct the location of historic bog iron ores in the Bourtangermoor. Our results showed two distinctly different present-day seepage patterns: one related to shallow aquifers, one to deeper aquifers. Present-day seepage from deeper aquifers was related to historic siderite bog iron ore deposits (deposits exclusively limited to the lower, early formed, still groundwater-fed part of the raised bog). Present-day seepage – deep or shallow – did not show a clear relation to historic classic-type bog iron ores (lumps of goethite and limonite). We conclude that present-day seepage patterns from deep aquifers can be used to reconstruct historic siderite bog iron ores in the Bourtangermoor. Present-day seepage patterns – deep or shallow – are however not representative for historic bog iron ores deposited along streams. Our results could be useful for wetland restoration projects as iron can contribute to nutrient-poor conditions by capturing phosphate in bog iron ores and for archaeological research regarding the potential use of bog iron ores by past societies.