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Palaeoclimates

Palaeoclimates. Presented by : Cherie Forbes (MSc, Plant Conservation Unit, Botany Department, UCT) Supervisor: Lindsey Gillson , Co-supervisor: Timm Hoffman. Overview:. Introduction 1.1. What and why Palaeoclimates? 1.2. Palaeohistorical (and geological) perspective

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Palaeoclimates

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  1. Palaeoclimates Presented by: Cherie Forbes (MSc, Plant Conservation Unit, Botany Department, UCT) Supervisor: Lindsey Gillson, Co-supervisor: Timm Hoffman

  2. Overview: • Introduction 1.1. What and why Palaeoclimates? 1.2. Palaeohistorical (and geological) perspective • How we measure palaeoclimates 2.1. Proxies: oxygen isotopes (δ18O) 2.2. Sources: sediment and ice cores • Key concepts of climate variation 3.1. ‘Snowball’ earth –greenhouse and icehouse 3.2. Orbital forcing (Milankovitch cycles) – glacials and interglacials • Case study: Paleocene-Eocene Thermal Maximum (PETM, 56 mya)

  3. 1.1. What and why Palaeoclimate? • What? • Climate change in the past [‘palaeo’ = ‘old/ancient’] • another dimension to HP story • Why? • Earth hasn’t always been habitable - experienced tremendous climate change over time • Provide valuable insights into climate system, its variability, and its vulnerability • Lessonslearned from past (natural drivers) – current conservation issues (e.g. Anthropogenic climate change)

  4. 1.2. Palaeohistorical timescale

  5. 1.2. Palaeohistorical timescale

  6. 2. How we measure palaeoclimates Billions of years ago • Sources: • Dating methods (radiometric, magnetic etc) Sedimentary Rocks - 4.5 Gya Millions of years ago Sediment Cores - 200 Mya Sapropels – 22 Mya Ice cores – 800 Kya (EPICA) Thousands of years ago Speleothems – 500 Kya Tree rings – 11 Kya Coral dating - 1 Kya

  7. 2. How we measure palaeoclimates • Source + dating method + proxy data • But what is a proxy? • indirect way of measuring change in something • E.g. thermometer! • Can’t directly measure past temperature/weather/currents/weathering/biology • So we develop proxies – an indicator of processes • Common proxies used in Palaeo records: • Mg/Ca, Alkenones, TEX86, Isotopes (same atomic number, different mass e.g. δ18O, δ13C)

  8. 2.1. Proxy data: δ18O • Sea water contains a ratio of oxygen isotopes • Foraminifera (little sea creatures) living at the ocean's surface make shells from CaCO3 • Isotopic composition of the shells is a function of temperature (Colder = enriched (more 18O); Warmer = depleted (less 18O)) • interpreted in terms of changing temperature over time

  9. 2.2. Sources: Ocean sedimentary cores • Continuous ocean records going back 65 Ma • Coarse-scale sampling resolution (one sample/ 3 kyr) • Excellent archive of long-term climate change (esp. detection of ice-ages )

  10. 2.2. Sources: ice-cores • Continuous ice-core records going back 800,000 years • Fine-scale sampling resolution (annual layers in some cores) • Excellent archive of fine-scale fluctuations in climate change (esp. detection of atmospheric concentrations of CO2 and changing δ18O)

  11. 1.2. Palaeohistorical timescale

  12. 3. Key concepts: 3.1. Snowball earth (Greenhouse & Icehouse) • 'snowball' Earth (Hoffman and Schrag, 2002) • Evidenceof widespread glaciation (Kirschvink et al., 2000; Hoffman and Schrag, 2002) - Palaeomagnetic studies of equatorial carbonate deposits, prolonged drop in biological activity and formation of iron-rich rocks (formed in absence of oxygen)

  13. 3.1. Greenhouse & Icehouse • Greenhouse conditions - Earth is ice free • Icehouse conditions - polar and alpine ice-sheets Climatic megacycles during the Phanerozoic (from Huggett, 1997; Willis & McElwain, 2002)

  14. 3.1: Snowball earth (Greenhouse & Icehouse) • Transition from “Greenhouse” to “Ice House” driven by plate tectonic processes Temperature trends in the Tertiary (from Willis & McElwain, 2002)

  15. 1.2. Palaeohistorical timescale

  16. 3. Key concepts: 3.2. Orbital forcing – glacials and interglacials • Milutin Milankovitch (Serbian astrophysicist and geophysicist, born 1879–died 1958) • Improved on methods of calculating Earth’s orbital cycles and relating them to Earth’s climatic variations • affects the strength and distribution of sunlight we receive • cycles within icehouse • glacials (ice-sheets expand)and interglacials (and contract)

  17. Milankovitch Cycles 1: Eccentricity (100,000 yrs) • The orbit of the earth varies from almost circular to much more “stretched” = elliptical • This property is known as Eccentricity

  18. Milankovitch Cycles 2: Obliquity – Axial Tilt (41,000 yrs) • The seasons are caused by the tilt of the Earth’s axis • With no tilt in the Earth’s axis, there would be no seasons • The greater the angle of the tilt, the greater the difference between the seasons: • Variation in the Earth’s axial tilt (obliquity) varies from 22.1º to 24.5º with a periodicity of ~41KY.

  19. Milankovitch Cycles 3: Precession (21,000 yrs) • While the earth is spinning on its axis, it also “wobbles” like a spinning top. • The cycle takes about 16 000 years. • Note that this is separate from “tilt” (obliquity) – in the two diagrams opposite, the angle of tilt is the same, but the top is facing in a different directions because of “wobble”

  20. Milankovitch Cycles • Eccentricity (100,000 yrs) • Obliquity (41,000 yrs) • Precession (21,000 yrs) Solar forcing

  21. 3.2. Glacials and interglacials Time before 2005 (ka)

  22. Where are we now?...Icehouse, interglacial (Holocene)

  23. Where are we now?...Icehouse, interglacial (Holocene)

  24. Where are we now?...Icehouse, interglacial (Holocene) • Recent interglacial (Holocene, approximately 10,000 years BP) has been very stable – development of human civilisations

  25. 4. Case study: Paleocene-Eocene Thermal Maximum (PETM, 56 million ya) • Brief abrupt warm climatic event • frozen methane on the ocean floor melted and created a massive greenhouse gas spike • PETM = An analogy for current anthropogenic climate change??? (Zachos et al., 2001) • PETM • no humans!!! • longer time period (30 000 yr)

  26. Concluding remarks • Reconstruct past climates using a combination of different types of proxy records • Past climate change = highly variable (Ma–present) • Palaeoclimate perspective – temporal BONUS in Earth system science & why Earth habitable!!! • Challenge: Anthropogenic climate change! • PETM possibly not best analogy • SA perspective?? – West Coast Fossil Park?! • Kenya perspective?? – Lake Turkana

  27. References: • Bartoli, G. et al. Final closure of Panama and the onset of northern hemisphere glaciation. Earth Planet. Sci. Lett. 237, 3344 (2005). • Dwyer, G.S., and M.A. Chandler, 2009: Mid-Pliocene sea level and continental ice volume based on coupled benthic Mg/Ca palaeotemperatures and oxygen isotopes. Phil. Trans. Royal Soc. A, 367, 157-168, doi:10.1098/rsta.2008.0222 • Robinson, M., H.J. Dowsett, and M.A. Chandler, 2008: Pliocene role in assessing future climate impacts. Eos Trans. Amer. Geophys. U., 89, 501-502 • Zachos J, Pagani M, Sloan L, Thomas E, Billups K. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 2001; 292: 686-693. • Temperature trends in the Tertiary (from Willis & McElwain, 2002)

  28. Acknowledgments THANK YOU • Applied Centre for Climate and Earth System Science (ACCESS) • Supervisors: A/Prof Lindsey Gillson and Prof Timm Hoffman • Carl Palmer (HPW co-ordinator) and Neville Sweijd (ACCESS Operations Manager)

  29. Q1: The following are not proxies used in palaeo- research • Mg/Ca, Alkenones, TEX86, Isotopes • Mg/Ca, Alkenones, tree rings, Isotopes, sediment cores • δ18O, Mg/Ca, Alkenones, TEX86, fossil pollen, fossil charcoal • Methane, nitrogen dioxide and δ18O

  30. Q2: Which are false regarding Snowball earth? • Snowball earth is a key concept which describes the climate over a geological timescale • Snowball earth occurred over millions of years and depicted icehouse and greenhouse periods • We are in an ice-house at the moment • During greenhouse conditions the Earth has some ice-sheets in the polar and mountainous (so the alpine) regions

  31. Q3: Which of the following is false about Milankovitch cycles? • The seasons are caused by the tilt of the Earth’s axis…this property is known as Eccentricity • Our planet moves in specific ways which affects the strength and distribution of sunlight we receive • The main driver for climate change during the orbital timescale was changes of the earth’s orbit around the sun • Milankovitch cyclesare seen within icehouse as glacials and interglacials

  32. Extra slides:

  33. Proxy data: δ18O • Sea water contains a ratio of oxygen isotopes. • Foraminifera (little sea creatures) living at the ocean's surface make shells from CaCO3 • Isotopic composition of the shells is a function of temperature Colder = enriched (more 18O); Warmer = depleted (less 18O) • Shells constantly accumulating on the oceans floor in sedimentary layers • These layers provide an isotopic record that can be interpreted in terms of changing temperature over time • E.g. Layers of isotopically heavy oxygen foraminifera in marine cores indicate glacial periods

  34. Consequences of opening the Drake’s passage • Cold circumpolar ocean current around Antarctica • Warm equatorial currents prevented from reaching s. polar regions • Thermal isolation of Antarctica • Initiation of Antarctic Ice Sheet

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