The salt industry on the Danish Kattegat island of Læsø (1150-1652): Hypersaline source, climatic dependence, and environmental impact

Around 1,700 ruins of Medieval salt production huts have been identified on the island of Læsø. It has been uncertain what the production's raw material was, how important the production was, when and why the industry expanded and declined, and what the industry's natural causes and environmental effects were. It is shown that present-day settings of hypersaline groundwater accumulations are comparable to the palaeo-settings of the Medieval salt production and that up to 135 production huts were simultaneously active during more than 400 years from the production began around 1150-1200 until the industry culminated around 1585 and completely ceased only a few decades later. The growth and decline in number of active production huts seems clearly related to climatically induced control of the length of accessible coastal stretches where hypersaline groundwater could be formed. During the 17th century's reinforcement of the Little Ice Age the length of productive stretches fell to less than 15% of the length they had around 1585. Thus, the industry's fast decline after 1585 and the complete cessation in 1652 was not only caused by shortage of fuel as hitherto believed, but more importantly caused by climatic cooling and lack of hypersaline water.