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Holocene climate and environmental dynamics on the Tristan da Cunha island group, South Atlantic

Ljung, Karl LU (2007) In Lundqua thesis 60.
Abstract (Swedish)
Popular Abstract in Swedish

Efter den senaste istiden har klimatet varit dynamiskt med både stora och snabba klimatförändringar, om än inte av samma storlek som under istiden. Hur utbredda och vilka effekter dessa klimatförändringar har haft är dock inte helt kartlagt. Målet med denna avhandling är att i detalj rekonstruera de klimat- och miljöförändringar som har påverkat ögruppen Tristan da Cunha i Sydatlanten och vad som har orsakat dessa förändringar, under de senaste 10 700 åren.



Lagerföljder från mossar och sjösediment har analyserats med avseende på innehåll av pollen och sporer, mineralmagnetiska och geokemiska parametrar, och daterats i detalj med kol-14 metoden. Sammantaget har dessa analyser... (More)
Popular Abstract in Swedish

Efter den senaste istiden har klimatet varit dynamiskt med både stora och snabba klimatförändringar, om än inte av samma storlek som under istiden. Hur utbredda och vilka effekter dessa klimatförändringar har haft är dock inte helt kartlagt. Målet med denna avhandling är att i detalj rekonstruera de klimat- och miljöförändringar som har påverkat ögruppen Tristan da Cunha i Sydatlanten och vad som har orsakat dessa förändringar, under de senaste 10 700 åren.



Lagerföljder från mossar och sjösediment har analyserats med avseende på innehåll av pollen och sporer, mineralmagnetiska och geokemiska parametrar, och daterats i detalj med kol-14 metoden. Sammantaget har dessa analyser gett en god bild hur klimatet och miljön har varierat på öarna.



Den längsta lagerföljden sträcker sig tillbaka 10 700 år och visar att klimatet, framförallt nederbörden, har varierat stort. Mellan 10 700 och 8600 år för nu avsattes torv. Torvavsättningen följdes av deposition av gyttja vilket visar att grundvatten ytan höjdes och lokalen förändrades från en mosse till en sjö. Den mest troliga orsaken till denna förändring är att nederbörden ökade till följd en förskjutning norrut av västvindsbältet som för med sig nederbörd. Förskjutningen av västvindsbältet var troligen relaterad till storskalig omorganisation av atmosfärens cirkulationsmönster över Sydatlanten.



Flera kortare perioder med ytterligare ökad nederbörd på följde denna efter förändring. Dessa perioder sammanfaller med kallare förhållanden och ökad deponering av minerogent material från isberg i Nordatlanten. Förmodligen orsakade förändringar i Nordatlanten störningar i Atlantens cirkulation vilket fick till följd att mindre varmt ytvatten transporterades från Syd- till Nordatlanten. Detta gjorde att Sydatlanten värmdes upp och nederbörden ökade.



En av dessa perioder med ökad nederbörd sammanfaller med en kall perioden som registrerats i borrkärnor från Grönlands inlandsis omkring 8200 år sedan, det så kallade 8.2 eventet. Under denna period dränerades stora mängder glacialt sötvatten från Nordamerika till Nordatlanten vilket störde Atlantens cirkulation. Det ledde till att transporten av varmt ytvatten från Sydatlanten minskade och nederbörden i Sydatlanten ökade. Dessa resultat stöds också av en simulering med en numerisk klimatmodell.



Tristan da Cunha upptäcktes 1506 av Portugisiska upptäcktsresanden, och genom detaljerade pollenanalyser har människans påverkan på vegetation rekonstrueras. Omkring 1550 uppträder Plantago lanceolata i pollen diagrammet. Denna ört förekommer inte naturligt på Tristan da Cunha utan introducerade, troligen omedvetet, av de första upptäckarna. I mitten av sextonhundratalet började ön få mera regelbundna besök vilket orsakade ökad markerosion och introducering av fler nya arter. På artonhundratalet etablerades den permanenta bosättningen och det mesta av skogen på öns lågland röjdes och ersattes av betesmark. (Less)
Abstract
It has been shown that the present interglacial has been climatically dynamic with both rapid and severe climatic fluctuations. The spatial and temporal differences of these changes are however not fully resolved. The aim of this thesis is to present a detail high resolution record of past climate changes from the central South Atlantic.



Peat and lacustrine sediment sequences were retrieved from the Tristan da Cunha island group in the central South Atlantic. These sequences were analysed with a multiproxy approach including pollen analysis, measurements of total carbon, nitrogen and sulphur, mineral magnetic measurements and detailed radiocarbon dating. In addition tree-ring widths from two trees are ... (More)
It has been shown that the present interglacial has been climatically dynamic with both rapid and severe climatic fluctuations. The spatial and temporal differences of these changes are however not fully resolved. The aim of this thesis is to present a detail high resolution record of past climate changes from the central South Atlantic.



Peat and lacustrine sediment sequences were retrieved from the Tristan da Cunha island group in the central South Atlantic. These sequences were analysed with a multiproxy approach including pollen analysis, measurements of total carbon, nitrogen and sulphur, mineral magnetic measurements and detailed radiocarbon dating. In addition tree-ring widths from two trees are presented.



The oldest sediment sequence extends back to 10 700 cal. yrs BP and the analyses show that the climate varied considerably during this time. Peat was accumulating from 10 700 cal. yrs BP to around 8600 cal. yrs BP when it was replaced by gyttja deposition. This change implies a substantial increase in effective humidity probably by precipitation. The cause of this change was probably a northward shift of the Southern Hemisphere westerlies, related to a large scale reorganization of the circulation system. This was probably caused by cooling of the Southern Ocean and the Atlantic sector of Antarctica, which caused expansion of sea ice and northward movement of the westerlies.



Several periods with increased catchment erosion and further increased precipitation follow this transition. These periods were probably caused by increased air humidity as a response to higher sea surface temperatures. In the North Atlantic conditions were cool with increased deposition of ice rafted debris. Probably, these changes was caused by weaker meridional overturning circulation and less northward transport of warm shallow water in the Atlantic, which caused higher sea surface temperatures in the central South Atlantic and cooler conditions in the North Atlantic.



One period of increased precipitation and sea surface temperature is contemporaneous with the 8.2 ka event in the Greenland ice cores and is probably the South Atlantic response to the weaker circulation induced by freshwater forcing in the North Atlantic.



Tristan da Cunha was discovered in 1506 AD by Portuguese explorers and their arrival is clearly seen in the pollen diagram. The introduced species Plantago lanceolata appears around 1550 AD. The seal hunting and establishment of the permanent settlement caused introduction of more species and clearing of the lowland forest. (Less)
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author
supervisor
opponent
  • Professor Walker, Michael, Department of Archaeology, University of Wales, Lampeter, Ceredigion
organization
publishing date
type
Thesis
publication status
published
subject
keywords
palynologi, Geology, climate change, pollen analysis, Palaeobotany, phylogeny, palynology, Paleobotanik, fylogeni, vegetation change, gyttja, peat, Holocene, Quaternary, South Atlantic, Tristan da Cunha, physical geography, Geologi, fysisk geografi
in
Lundqua thesis
volume
60
pages
28 pages
publisher
Quaternary Sciences, Department of Geology, Lund University
defense location
Pangea, Centrum för Geobiosfärsvetenskap, Geologiska Inst, Kvartärgeologi, Sölveg. 12, Lunds Universitet,
defense date
2007-09-14 14:15
external identifiers
  • scopus:57349106572
ISSN
0281-3033
ISBN
978-91-86746-94-0
language
English
LU publication?
yes
id
ff8165dd-b045-4904-ae0a-f005b3abd9ec (old id 548953)
date added to LUP
2007-09-05 10:32:40
date last changed
2017-01-01 06:39:03
@phdthesis{ff8165dd-b045-4904-ae0a-f005b3abd9ec,
  abstract     = {It has been shown that the present interglacial has been climatically dynamic with both rapid and severe climatic fluctuations. The spatial and temporal differences of these changes are however not fully resolved. The aim of this thesis is to present a detail high resolution record of past climate changes from the central South Atlantic.<br/><br>
<br/><br>
Peat and lacustrine sediment sequences were retrieved from the Tristan da Cunha island group in the central South Atlantic. These sequences were analysed with a multiproxy approach including pollen analysis, measurements of total carbon, nitrogen and sulphur, mineral magnetic measurements and detailed radiocarbon dating. In addition tree-ring widths from two trees are presented.<br/><br>
<br/><br>
The oldest sediment sequence extends back to 10 700 cal. yrs BP and the analyses show that the climate varied considerably during this time. Peat was accumulating from 10 700 cal. yrs BP to around 8600 cal. yrs BP when it was replaced by gyttja deposition. This change implies a substantial increase in effective humidity probably by precipitation. The cause of this change was probably a northward shift of the Southern Hemisphere westerlies, related to a large scale reorganization of the circulation system. This was probably caused by cooling of the Southern Ocean and the Atlantic sector of Antarctica, which caused expansion of sea ice and northward movement of the westerlies.<br/><br>
<br/><br>
Several periods with increased catchment erosion and further increased precipitation follow this transition. These periods were probably caused by increased air humidity as a response to higher sea surface temperatures. In the North Atlantic conditions were cool with increased deposition of ice rafted debris. Probably, these changes was caused by weaker meridional overturning circulation and less northward transport of warm shallow water in the Atlantic, which caused higher sea surface temperatures in the central South Atlantic and cooler conditions in the North Atlantic.<br/><br>
<br/><br>
One period of increased precipitation and sea surface temperature is contemporaneous with the 8.2 ka event in the Greenland ice cores and is probably the South Atlantic response to the weaker circulation induced by freshwater forcing in the North Atlantic.<br/><br>
<br/><br>
Tristan da Cunha was discovered in 1506 AD by Portuguese explorers and their arrival is clearly seen in the pollen diagram. The introduced species Plantago lanceolata appears around 1550 AD. The seal hunting and establishment of the permanent settlement caused introduction of more species and clearing of the lowland forest.},
  author       = {Ljung, Karl},
  isbn         = {978-91-86746-94-0},
  issn         = {0281-3033},
  keyword      = {palynologi,Geology,climate change,pollen analysis,Palaeobotany,phylogeny,palynology,Paleobotanik,fylogeni,vegetation change,gyttja,peat,Holocene,Quaternary,South Atlantic,Tristan da Cunha,physical geography,Geologi,fysisk geografi},
  language     = {eng},
  pages        = {28},
  publisher    = {Quaternary Sciences, Department of Geology, Lund University},
  school       = {Lund University},
  series       = {Lundqua thesis},
  title        = {Holocene climate and environmental dynamics on the Tristan da Cunha island group, South Atlantic},
  volume       = {60},
  year         = {2007},
}