Geochemical and palaeomagnetic characteristics of a Swedish Holocene sediment sequence from Lake Storsjön, Jämtland

Abstract: As a part of a French-Swedish collaboration project to reconstruct middle to northern European climate variations during the Holocene, a sediment sequence from Lake Storsjön, Jämtland was retrieved and analysed for geochemical and palaeomagnetic properties. The subsampled sequence, composed of one freeze core and three Niederreiter piston cores, revealed information of both climatic, environmental and anthropogenic changes over the past 10,800 years. As previously theorized by several Swedish authors, Lake Storsjön likely was covered by a complex of shifting ice-lakes before the modern lake was isolated. This study reveals the timing of the ice-lake drainage to have occurred at around 9629 cal yrs BP, portrayed as a lithostratigraphic change from laminated clays to a more organic rich clay gyttja deposit. As a response to increasing organic production in the newly isolated lake, hypoxic to anoxic conditions arose, seen in the elemental proxies of Mo/Al and Mn/Fe ratios, and resulted in sulphide reducing bacteria forming authigenic greigite of which fossil magnetosomes are recorded by the mineral magnetic assemblage. A similar situation arose when a likely more persistent ice cover during the 8.2 kyr event induced low oxygen conditions, again inducing authigenic greigite formation. The whole post-drainage interval of the sediment sequence is banded by black layering which is interpreted as paramagnetic pyritization of magnetotactic bacterial magnetite induced by low oxygen and reducing conditions, in turn caused by certain events of strong spring meltwater pulses. The black banding is inferred as being composed of a relatively coarser grain size by the elemental ratio of Zr/K which would be explained by the generally increased catchment overflow during such events. In the more recent parts of the sequence, socioeconomic crises derived from disease (Black Death) as well as climate change (Little Ace Age) are visible as a change in land use. The timing of the LIA in the area is interpreted as has been most severe during a 60 yr period between 330-270 cal yrs BP. With the onset of the industrial revolution, metal pollution has followed. Elemental concentrations of Pb, Cu, Zn and As have risen steadily over the past 130 cal yrs BP. Calculated anthropogenic Pb exhibits concentration peaks corresponding well with the long range transported Pb peaks at AD 1200, AD 1530 and AD 1975 with reference datasets from both south and north of Lake Storsjön. A large change in the physical and chemical records of Lake Storsjön is also found in the sediments deposited during the past 70 yrs as a response to the lake being used as a hydropower production reservoir. The latter however, needs to be more thoroughly examined to properly resolve the ongoing change. The sediments of Lake Storsjön were dated with both relative (palaeomagnetic secular variations, pollution Pb) and absolute (14C, 137Cs) dating techniques. The resulting age model was a mixture of the both. The age model relied heavily on the relative dating with palaeomagnetic secular variations (PSV) but was in good agreement with inserted absolutely dated material. The carriers of the natural remanent magnetisation (NRM) are interpreted as relying heavily on bacterial magnetite and have a very stable NRM throughout the sequence as inferred from its maximum angular deviation (MAD). Declination events proved more useful in constructing the early part of the age model while inclination events were used to construct the older parts. Sedimentation rates were invariable during long term climatic stability and generally increased with enhanced human activities.