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The Jurassic extinction events and its relation to CO2 levels in the atmosphere : a case study on Early Jurassic fossil leaves

Santasalo, Liina LU (2013) In Examensarbeten i geologi vid Lunds universitet GEOL01 20121
Department of Geology
Abstract
The Mesozoic was a so-called “hothouse” time period in Earth’s history, with high concentrations of atmospheric CO2 and elevated temperatures, but the exact climate evolution is not fully known for the entire inter-val. The Early Jurassic for instance is believed to have been a warm period, flanked by two episodes of extremely high CO2, coinciding with environmental degradation and mass extinctions: the Triassic-Jurassic (200 Ma) and the early Toarcian (183 Ma) mass extinction events. Mass extinctions are often related to variations in climate condi-tions, which in turn are linked to the global CO2 concentration, and it is therefore important to understand the levels of CO2 before, during and after such events. Here, atmospheric CO2 levels... (More)
The Mesozoic was a so-called “hothouse” time period in Earth’s history, with high concentrations of atmospheric CO2 and elevated temperatures, but the exact climate evolution is not fully known for the entire inter-val. The Early Jurassic for instance is believed to have been a warm period, flanked by two episodes of extremely high CO2, coinciding with environmental degradation and mass extinctions: the Triassic-Jurassic (200 Ma) and the early Toarcian (183 Ma) mass extinction events. Mass extinctions are often related to variations in climate condi-tions, which in turn are linked to the global CO2 concentration, and it is therefore important to understand the levels of CO2 before, during and after such events. Here, atmospheric CO2 levels were reconstructed for the late Pliesbachian (ca.185 Ma), an Early Jurassic time period that is not well understood in terms of climate. The sto-matal proxy method of palaeo-CO2 reconstructions was applied, using fossil leaves derived from the Clarence- Moreton Basin in eastern Australia. The stomatal proxy relies on the inverse relationship between the density of plant leaf stomata and atmospheric CO2 concentrations. The ten exceptionally well-preserved fossil leaves derived from the araucarian conifer species Allocladus helgei. Two different methods of calibrations, using the nearest liv-ing equivalent species Athrotaxis cupressoides, were applied to the stomatal data and CO2 concentrations in the range 600 – 750 ppm were found. These values are high compared to present day CO2 concentration (396 ppm in May 2012), but are lower than values usually cited for this period, as interpolated between CO2 concentrations known from the flanking mass extinction events of 1000 – 2000 ppm. The results found here therefore indicate that the Pliensbachian was a cooler period than previously assumed, but that it was still a “hothouse” warm period with higher global temperatures than today. (Less)
Abstract (Swedish)
Mesozoikum anses ha utgjort en ”växthusperiod” i Jordens historia med avsevärt höga koldiox-idhalter i atmosfären samt förhöjda temperaturer, men det återstår många frågor att besvara angående klimatut-vecklingen för hela detta intervall. Tidig jura karaktäriseras till exempel av ett varmt klimat, och föregås och efter-följs av två kortare perioder med väldigt höga CO2 halter, som sammanfaller med försämrade miljöförhållanden och massutdöenden. Två massutdöenden kopplas till denna period; trias–jura massutdöendet (200 Ma) samt mass-utdöendet som inträffade under tidig toarc (183 Ma). Massutdöenden relateras ofta till klimatförändringar som i sin tur återspeglas i de globala CO2 -halterna. På grund av detta förhållande mellan temperatur och... (More)
Mesozoikum anses ha utgjort en ”växthusperiod” i Jordens historia med avsevärt höga koldiox-idhalter i atmosfären samt förhöjda temperaturer, men det återstår många frågor att besvara angående klimatut-vecklingen för hela detta intervall. Tidig jura karaktäriseras till exempel av ett varmt klimat, och föregås och efter-följs av två kortare perioder med väldigt höga CO2 halter, som sammanfaller med försämrade miljöförhållanden och massutdöenden. Två massutdöenden kopplas till denna period; trias–jura massutdöendet (200 Ma) samt mass-utdöendet som inträffade under tidig toarc (183 Ma). Massutdöenden relateras ofta till klimatförändringar som i sin tur återspeglas i de globala CO2 -halterna. På grund av detta förhållande mellan temperatur och koldioxidhalt i at-mosfären är det viktigt för oss att veta hur CO2 varierat under en längre period, före och efter dessa nämnda event. I denna studie har CO2 halter rekonstruerats för sen Pliensbach (c. 185 Ma), tidsintervall under tidig jura där det saknas kunskap angående klimatet. Stomataproxy-metoden användes för paleo-CO2-rekonstruktioner, med hjälp av fossila blad insamlade från Clarence- Moreton bassängen i östra Australien. Stomataproxy-metoden använder sig av det faktum att det föreligger ett omvänt förhållande mellan bladens stomatadensitet och atmosfärens CO2-halter. De 10 välbevarade bladfossilen härstammar från barrträdet Allocladus helgei. Två olika beräkningsmetoder har applicerats på närmaste levande släkting (NLE) Athrotaxis cupressoides. Resultaten från beräkningarna visade CO2-halter i intervallet 600-700 ppm. Dessa halter är höga jämfört med dagens CO2-halter (~400 ppm i maj -2013), men däremot lägre än CO2-halter dokumenterade i andra arbeten baserad på interpolering av CO2-halter omfat-tande massutdöendeintervallen där värden på 1000-2000 ppm uppvisats. Därmed tyder dessa resultat på att Pliens-bach var en kallare period än man tidigare ansett, men klassas dock fortfarande som en ”växthusperiod” med förhöjda halter av CO2 och högre temperatur jämfört med idag. (Less)
Please use this url to cite or link to this publication:
author
Santasalo, Liina LU
supervisor
organization
alternative title
Jurassiska massutdöende-event och dess relation till atmosfärens CO2 -halter : en fallstudie baserad på fossila jurassiska blad
course
GEOL01 20121
year
type
M2 - Bachelor Degree
subject
keywords
conifers, Pliensbachian, Australian, Early Jurassic, stomatal proxy, palaeo-CO2, koldioxidhalt, klimat, Australien, tidig Jura, Marburg subgroup, Allocladus helgei, stomata index, CO2
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
371
language
English
additional info
External supervisor: Dr.Margret Steinthorsdottir, Department of Geological Sciences, Stockholm University
id
4058125
date added to LUP
2013-09-25 11:55:56
date last changed
2013-09-25 11:55:56
@misc{4058125,
  abstract     = {The Mesozoic was a so-called “hothouse” time period in Earth’s history, with high concentrations of atmospheric CO2 and elevated temperatures, but the exact climate evolution is not fully known for the entire inter-val. The Early Jurassic for instance is believed to have been a warm period, flanked by two episodes of extremely high CO2, coinciding with environmental degradation and mass extinctions: the Triassic-Jurassic (200 Ma) and the early Toarcian (183 Ma) mass extinction events. Mass extinctions are often related to variations in climate condi-tions, which in turn are linked to the global CO2 concentration, and it is therefore important to understand the levels of CO2 before, during and after such events. Here, atmospheric CO2 levels were reconstructed for the late Pliesbachian (ca.185 Ma), an Early Jurassic time period that is not well understood in terms of climate. The sto-matal proxy method of palaeo-CO2 reconstructions was applied, using fossil leaves derived from the Clarence- Moreton Basin in eastern Australia. The stomatal proxy relies on the inverse relationship between the density of plant leaf stomata and atmospheric CO2 concentrations. The ten exceptionally well-preserved fossil leaves derived from the araucarian conifer species Allocladus helgei. Two different methods of calibrations, using the nearest liv-ing equivalent species Athrotaxis cupressoides, were applied to the stomatal data and CO2 concentrations in the range 600 – 750 ppm were found. These values are high compared to present day CO2 concentration (396 ppm in May 2012), but are lower than values usually cited for this period, as interpolated between CO2 concentrations known from the flanking mass extinction events of 1000 – 2000 ppm. The results found here therefore indicate that the Pliensbachian was a cooler period than previously assumed, but that it was still a “hothouse” warm period with higher global temperatures than today.},
  author       = {Santasalo, Liina},
  keyword      = {conifers,Pliensbachian,Australian,Early Jurassic,stomatal proxy,palaeo-CO2,koldioxidhalt,klimat,Australien,tidig Jura,Marburg subgroup,Allocladus helgei,stomata index,CO2},
  language     = {eng},
  note         = {Student Paper},
  series       = {Examensarbeten i geologi vid Lunds universitet},
  title        = {The Jurassic extinction events and its relation to CO2 levels in the atmosphere : a case study on Early Jurassic fossil leaves},
  year         = {2013},
}