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

Robygd, Joakim LU (2015) In Dissertations in Geology at Lund University GEOR02 20142
Department of Geology
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... (More)
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. (Less)
Popular Abstract (Swedish)
Som en del i ett fransk-svensk sammarbete i syfte att rekonstruera holocena klimatförändringar i centrala och norra Europa, togs en sedimentsekvens upp från Storsjön, Jämtland och analyserades för geokemiska och paleomagnetiska egenskaper. Den provtagna delen av sekvensen, bestående av en fryskärna samt tre Niederreiter-kärnor, tydliggjorde förändringar orsakade av klimat samt antropogen påverkan de senaste 10800 åren. Det har tidigare spekulerats i att Storsjön efter deglaciationen var täckt av en serie issjöar innan den moderna sjöbassängen etablerades. Den här studien avslöjar att det sista issjö-stadiet avslutades omkring 9629 år före nutid. Tappningen av issjö-stadiet är synligt i sedimentsekvensen som en litostratigrafisk förändring... (More)
Som en del i ett fransk-svensk sammarbete i syfte att rekonstruera holocena klimatförändringar i centrala och norra Europa, togs en sedimentsekvens upp från Storsjön, Jämtland och analyserades för geokemiska och paleomagnetiska egenskaper. Den provtagna delen av sekvensen, bestående av en fryskärna samt tre Niederreiter-kärnor, tydliggjorde förändringar orsakade av klimat samt antropogen påverkan de senaste 10800 åren. Det har tidigare spekulerats i att Storsjön efter deglaciationen var täckt av en serie issjöar innan den moderna sjöbassängen etablerades. Den här studien avslöjar att det sista issjö-stadiet avslutades omkring 9629 år före nutid. Tappningen av issjö-stadiet är synligt i sedimentsekvensen som en litostratigrafisk förändring från laminerade leror till en mer organisk lerig gyttja avsättning. I respons till sagd förändring uppstod hypoxiska till anoxiska förhållanden, återspeglat i de geokemiska elementar-kvoterna Mo/Al och Mn/Fe. De låga syreförhållandena gav upphov till sulfidreducerande bakteriell nedbrytning av organiskt material och gynnade formationen av autogen greigit av magnetotaktiska bakterier. Dessa fossila magnetosomer reflekteras tydligt i den mineralmagnetiska sammansättningen samt i koncentrationen av svavel. En liknande situation uppstod då förmodade förlängda istäcken minskade syretillgången i bottenvattnet under den så kallade 8.2 kiloanno händelsen där årsmedeltemperaturen sänktes avsevärt under en period. Under denna period tolkas den magnetiska sammansättningen åter att bestå av autogen greigit från magnetotaktiska bakterier. Under hela perioden av lerig gyttja avsättning så är svarta band utspridda genom sediment sekvensen. Dessa band tolkas som pyritisation av magnetotaktisk magnetit som uppstått i respons till kortare intervall av minskad syretillförsel. Anledningen till det minskade syret tros vara episoder av intensifierad snösmältning och avrinning. Denna tolkning stöds vidare av en härleden ökad minerogen kornstorlekssammansättning av de svarta banden från elementarkvoten av Zr/K. I den senare delen av sekvensen är socioekonomiska kriser härrörande från sjukdom (Digerdöden) samt klimatförändring (lilla istiden) synliga som härledda förändringar i markanvändning i avrinningsområdet. Lilla istiden är bedömd att ha varit mest förödande för jordbruket mellan 330-270 år före nutid. Den industriella revolutionen har bevisligen medfört föroreningar av tungmetaller. Koncentrationer av Pb, Cu, Zn och As har stigit kontinuerligt över de senaste 130 åren. Beräknat föroreningsbly sammanfaller väl med koncentrationstoppar av långtransporterade blyföroreningar vid år 1200, 1500 och 1975 från referensdataset både söder och norr om undersökningsområdet. En stor förändring i de geokemiska och paleomagnetiska egenskaperna syns under de senaste 70 åren som en respons till utbyggnationen av en kraftverksdam men kommer kräva en mer detaljerad studie för att utröna exakta konsekvenser av de förändrade förhållandena.
Sedimenten daterades med både relativa (paleomagnetiska sekulära variationer, föroreningsbly) samt absoluta (14C, 137CS) dateringsmetoder. Den slutgiltiga åldermodellen bestod av en kombination av de båda. Åldersmodellen är till stor del uppbyggd av PSV datering men insatta absoluta datum passade väl in i det interpolerade relativa dateringarna. Bärarna av NRM tolkas bestå till stor del av bakteriell magnetit och har en väldigt stabil NRM signal genom hela sekvensen härledet från de låga MAD-värdena. Deklination användes först och främst till uppbyggnaden av den senare delen av åldersmodellen och inklination till den tidigare delen. Sedimentationshastigheter visade låg variation under härledd klimatisk stabilitet och ökade generellt med ökad mänsklig aktivitet i området. (Less)
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author
Robygd, Joakim LU
supervisor
organization
course
GEOR02 20142
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Jämtland, geochemistry, palaeomagnetism, PSV, sedimentation rate, Holocene development, geochronology, climate change
publication/series
Dissertations in Geology at Lund University
report number
424
language
English
id
5038403
date added to LUP
2015-01-30 13:40:36
date last changed
2015-01-30 13:40:36
@misc{5038403,
  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.},
  author       = {Robygd, Joakim},
  keyword      = {Jämtland,geochemistry,palaeomagnetism,PSV,sedimentation rate,Holocene development,geochronology,climate change},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Geochemical and palaeomagnetic characteristics of a Swedish Holocene sediment sequence from Lake Storsjön, Jämtland},
  year         = {2015},
}