Setting the Holocene clock using varved lake sediments in Sweden

The aim of this thesis was to study annually laminated (varved) Holocene lake sediment in Sweden, their formation and their potential as chronological and palaeoecological archives. Five lakes with continuous Holocene varved lake sediment sequences in northern (Västerbotten) and west central Sweden (Värmland) were investigated. Three of these sequences were discovered during this study, which identified the climatic and environmental prerequisites for the formation of varves and, therefore, provides a tool for finding annually laminated sediments in the Swedish boreal environment. Varve chronologies, supported by other independent dating methods, i.e. radiocarbon dating, tephra isochrones and paleomagnetic secular variations were established for two sediment sequences in Värmland. These are the longest geological records with an annual resolution known to exist in Sweden. Three mid-Holocene Icelandic tephra horizons were identified within 1-cm horizons, corresponding to c. 20 years of sediment accumulation and the varve chronologies were used to assign calendar years ages to the tephra isochrones with a precision better than ± 110 varve years. Paleomagnetic secular variation curves (both directions and intensity) presented in this thesis can be used to correlate and relatively date Holocene sedimentary sequences in Northwest and Central Europe. The accuracy and precision of the method is determined by errors associated with the varve chronologies (i.e. c. 1-2%), sampling resolution (c. 50-100 years), definition uncertainties and possible remanence lock-in effects. Comparison between the paleomagnetic secular variation curves in this study and previous obtained records from Northwest Europe suggests that no significant westwards drift of the non-dipole field has occurred in this region during the majority of the Holocene. Reconstructed virtual geomagnetic pole positions for the last c. 9000 show that the Magnetic North Pole has changed its position significantly during the Holocene. Relative palaeointensities imply that sub-millennial-scale variations in the strength of the geomagnetic dipole have taken place during the Holocene. These changes would have modulated the production of cosmogenic nuclides in combination with solar variability. The “8.2 ka cold event” was recorded as a c. 300-year long period of distinctly colder climate conditions centred at c. 7800 cal. BP in the two varved lake sediment sequences in N Sweden. Pollen influx rates indicate that the regional vegetation in this area responded rapidly (within 75 years) to the onset and end of this 300-year long cold climate anomaly and that the forest ecosystem was severely stressed

Er polvendingen nær? - sandsynligheden palæomagnetisk set

Jordens magnetfelt er aftaget 10 procent over de sidste 170 år. Nye målinger fra blandt andet den danske Ørsted-satellit viser, at de magnetiske poler nu flytter sig ca. 50 km om året. Men er det usædvanligt hurtigt? Nogle forskere mener ja. De tolker ændringerne som tegn på, at Jordens magnetfelt er på vej mod en polvending (se Peter Staunings artikel i GeologiskNyt nr. 5, 2004)

Late holocene dune activity at Sandhammaren, southern Sweden : chronology and the role of climate, vegetation, and human impact

Populärvetenskaplig sammanfattning: De begravda jordmånsbildningarna vid Sandhammaren och Skanörs ljung, södra Sverige, är lokaliserade inom kustnära dynlandskap som har bildas i response till land-och havsnivåförändringar efter den sista istiden (efter ca. 10 000 år sedan). Jordmånerna är begravda av flygsand och karakteriseras av flera organiska lager med tillhörande urlaknings-och anrikningshorisonter. De har studerats med avseende på fossila pollen och markkemi (järn, aluminium, mangan, fosfor, kol och pH) i ett försök att uppskatta graden av urlakning och att utöka kunskapen om de bildande processerna d.v.s. klimat, vegetation och mänsklig påverkan. Genom att studera fossila pollen kan man bestämma den forntida vegetationen, d.v.s. vilka trädarter och vilka arter som ingick i markvegetationen vid tillfället för jordmånsbildningen. En jordmånsbildning representerar ett klimat som är kallare och fuktigare och har en högre grundvattennivå medan en period med dynbildning representerar ett varmare och torrare klimat med en lägre grundvattennivå. En kronologi upprättades för de organiska lagren med hjälp av AMS 14C datering. De kemiska resultaten visar att jordmånsbildningarna har utvecklat karakteristiska urlakning-och anrikningshorisonter och därmed någon grad av podsolering. Jordmånsprofilerna har bildats i ett Ljungheds-landskap, med en gles trädvegetation. Ljunghedarna vid Sandhammaren har bibehållits genom betning och bränder som en del av ett traditionellt landutnyttjande, vilket inte avtog förrän i början av 1900-talet och ledde till att skogen kunde återbildades. Tiden för jordmånsbildning vid Sandhammaren är uppskattad till mellan för ca. 2000 år sedan (eventuellt tidigare, ca. 2600 år sedan) och ca. 500 år sedan med mindre sanddrift för mellan ca. 1800-1400 år sedan (e.Kr. 250-550) och för ca. 900-700 år sedan (e.Kr. 1050-1250). Den mest omfattande dynaktiviteten ägde rum före ca. 2600 år sedan och efter ca. 500 år sedan (e.Kr. 1450) och speglar troligtvis de relativt varma och torra förhållanden som rådde i slutet av tidsepoken Subboreal, (för ca. 4000-2600 år sedan), och efter den kalla och fuktiga tidsperioden, den s.k. Lilla Istiden (1500-1800-talet). Perioderna med sanddrift för mellan ca. 1800-1400 år sedan och mellan ca. 1050-1250 e.Kr. motsvaras av en period med vattennivåsänkningar i södra Sverige, för ca. 1200-1800 år sedan, respektive av en varm period under Medeltiden, för ca. 1000 år sedan. Perioder med omväxlande dyn respektive jordmånsbildning har upptäckts i flera delar av Europa och diskuterats i forskningen om regionala klimatförändringar. Denna studie visar att jordmånsbildningen vid Sandhammaren initierades av klimatförändringar, från varma och torra till kalla och fuktiga förhållanden, medan Ljungheds-vegetationen för tiden med jordmånsblidning, bibehölls av människan. Under 1700- och 1800-talet var dynaktiviteten väldigt omfattande och orsakades av en kombination av klimatförändring och mänsklig påverkan.The palaeosoils at Sandhammaren and Skanörs ljung, southern Sweden, are located within a coastal dune landscape that developed in response to Holocene land/sea changes. These buried soils are characterised by several organic layers including pedological horizons. They were studied for fossil pollen, and soil chemistry (iron, aluminium, manganese, phosphorous, carbon and pH) in order to appreciate the extent of leaching processes in the soils, and to understand the possible processes involved in their development, e.g. climate, vegetation history, and human impact. Macroscopic charcoals were selected from a series of levels in the soil profiles for AMS 14C dating. The chemical results show that the palaeosoils have developed characteristic eluviation and illuviation zones and hence some degree of podzolisation The soils developed under a Calluna heathland, with a low representation of trees. The Calluna heaths at Sandhammaren has been maintained by grazing and burning as part of a traditional land use which has only declined in the present century leading to woodland regeneration. The best age estimates for Late Holocene soil formation at Sandhammaren are between c. 2000 cal. BP (or even earlier, c. 2600 cal. BP) and c. 500 cal. BP with minor interruptions of sand drift between c.1800-1400 cal. BP (AD 250-550) and c. 900-700 cal. BP (AD 1050-1250). The most prominent dune activity took place prior to c. 2600 cal. BP and after c. 500 cal. BP (AD 1450), and probably reflect the relatively warm and dry climate conditions at the end of the Subboreal period (c. 4000-2600 BP), and after the Little Ice Age, from c. 1800 AD. The periods of minor sand drift c. 1800-1400 cal. BP and c. AD 1050-1250 may be compared with the time of low lake-levels in southern Sweden, c.1200-1800 BP, and the Medieval warm period, around AD 1000, respectively. Alternating periods of soil and dune formation have been detected in several parts of Europe and were discussed in terms of regional climate changes. This study concludes that soil formation at Sandhammaren was initiated through climatic shifts, from warm and dry to cold and wet conditions, while the maintenance of Calluna heaths during the period of soil formation was due to human impact. During the 18th and 19th centuries dune activity was very strong and due to a combination of climatic and human-impact factors

Hypoxia and cyanobacteria blooms - are they really natural features of the late Holocene history of the Baltic Sea?

During the last century (1900s) industrialized forms of agriculture and human activities have caused eutrophication of Baltic Sea waters. As a consequence, the hypoxic zone in the Baltic Sea has increased, especially during the last 50 years, and has caused severe ecosystem disturbance. Climate forcing has been proposed to be responsible for the reported trends in hypoxia (< 2 mg/l O-2) both during the last c. 100 years (since c. 1900 AD) and the Medieval Period. By contrast, investigations of the degree of anthropogenic forcing on the ecosystem on long time-scales (millennial and greater) have not been thoroughly addressed. This paper examines evidence for anthropogenic disturbance of the marine environment beyond the last century through the analysis of the human population growth, technological development and land-use changes in the drainage area. Natural environmental changes, i.e. changes in the morphology and depths of the Baltic basin and the sills, were probably the main driver for large-scale hypoxia during the early Holocene (8000-4000 cal yr BP). We show that hypoxia during the last two millennia has followed the general expansion and contraction trends in Europe and that human perturbation has been an important driver for hypoxia during that time. Hypoxia occurring during the Medieval Period coincides with a doubling of the population (from c. 4.6 to 9.5 million) in the Baltic Sea watershed, a massive reclamation of land in both established and marginal cultivated areas and significant increases in soil nutrient release. The role of climate forcing on hypoxia in the Baltic Sea has yet to be demonstrated convincingly, although it could have helped to sustain hypoxia through enhanced salt water inflows or through changes in hydrological inputs. In addition, cyanobacteria blooms are not natural features of the Baltic Sea as previously deduced, but are a consequence of enhanced phosphorus release from the seabed that occurs during hypoxia

Calendar year ages of three mid-Holocene tephra layers identified in varved lake sediments in west central Sweden

Three intermediate to rhyolitic Icelandic Holocene tephra horizons (Hekla-3, Kebister, and Hekla-4) have been identified in annually laminated (varved) lake sediments in the Province of Varmland, west central Sweden. Calendar year ages were obtained from varve counts and are supported by additional C-14 dating based on terrestrial plant macrofossils. The varve ages of the three tephras are 3295 +/- 95 cal. BP (Hekla-3), 4030 +/- 103 cal. BP (Kebister), and 4390 +/- 107 cal. BP (Hekla-4). The varve age of Hekla-3 refines the former calibrated 14 C age based on studies in the British Isles and Germany. Considering the age-error estimates associated with the varve chronology and calibrated C-14 ages, the ages of Kebister and Hekla-4 are in agreement with former studies. Thus, the age difference between Kebister and Hekla-4 is estimated to be ca 400 +/- 40 varve years (formally estimated to ca 200 C-14 years), between Hekla-3 and Hekla-4 to ca 1135 +/- 55 varve years (formally estimated to ca 1100 calibrated C-14 years) and between Hekla-3 and Kebister, 708 +/- 20 varve years

Stable lead (Pb) isotopes and concentrations - A useful independent dating tool for Baltic Sea sediments

The prehistory of the Baltic Sea has for a long time suffered from imprecise dating, due to the large uncertainties associated with bulk radiocarbon dating of Baltic Sea sediments. To constrain the timing of environmental changes in the Baltic Sea it is critical to apply new dating approaches. This study identifies lead pollution isochrones in Baltic Sea sediments, which have previously been recorded in lake sediment and peat deposits in northern Europe and ice cores from Greenland. These isochrones have formed through the deposition of atmospheric lead associated with historic lead production and silver mining in Europe, and more recently with the increased industrial emissions that peaked in the 1970's. Lead concentration and stable lead isotope analyses ((206)pb/Pb-207 ratios) reveal three distinct lead pollution horizons in the Baltic Sea, i.e. a Roman peak at 1 AD, a Medieval peak at 1200 AD and a peak in the 1970s. The new data will improve the chronological accuracy and precision of paleoenvironmental studies in the Baltic Sea, and for the first time, allow synchronization of Baltic Sea geological records within the basin and across Europe and the North Atlantic region (including Greenland). (C) 2011 Elsevier B.V. All rights reserved

Bacterial magnetite in Swedish varved lake-sediments: a potential bio-marker of environmental change

Detailed mineral magnetic measurements were carried out on three varved lake-sediment sequences in Sweden, which extend to ca. 7000 BC. The comparison of the magnetic properties of the organic rich varved lake-sediments with their respective catchment materials indicates that the magnetic properties of the sediments are dominated by relatively high concentrations of single-domain magnetite magnetosomes produced by magnetotactic bacteria. The dimictic nature of the lakes, which help form and preserve the varves due to the weakly oxic or suboxic environment, also appear to be suitable for a high degree of magnetosome preservation. The concentration of the magnetosomes in two of the sequences, as determined by mass specific magnetic measurements, exhibits a positive linear relationship with the total organic carbon content and suggests that magnetic remanence measurements reflect lake productivity, via processes of organic matter accumulation and decomposition. Further research should focus on the potential to use the fossil magnetosomes as proxy-climate/environmental change indicators and as recorders of geomagnetic field variations and behaviour. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved