Paleoceanography

1. Paleoceanography

2. Paleoceanography • Paleoceanography - The study of ocean history • Many sub disciplines • Requires indirect or direct sampling • Studies of ocean sediments and what they tell us • Began in 1930’s and 40’ • Kullenberg invented piston corer which allowed longer sediment records. First used in 1947

3. Paleoceanography • Information contained in sediment core • Changes in type of sediment (terrestrial, biogenic, etc) • Changes in species of planktonic and benthic organisms through time • So what? (see transfer function analysis) • Changes in geochemistry of shells of organisms • Huh? (see oxygen isotopes)

4. Paleotemperature • Many marine organisms have a narrow T range • most widespread deposits on seafloor are carbonate ooze • mostly composed of foraminifera tests • climate indicators par excellence • good agreement between faunal and climatic zonation

5. Paleotemperature • Transfer function analysis • simple methodology • a temperature estimate is given based on the assemblage of forams present in a sample • calibration set is used consisting of modern sea floor assemblages vs. surface T • most abundant + next most+ next etc. with given T optimum for each species

6. Paleotemperature • Application • CLIMAP (Climate/Long-range Investigation, Mapping and Prediction) group used complicated TFA to estimate SST for Last Glacial Maximum • Looked at core data from 18 kyr • How? Oxygen isotope stratigraphy • polar front extended from New York to Iberian peninsula; today south of greenland

8. Paleotemperature • Uses and limitations • method may be used for any variables that show correlation with plankton abundance • salinity shows no correlation • calibration set based on surface samples; avg. of last 2-3000 yrs. • differential dissolution and selective preservation; delicate species gone • old samples, evolutionary changes in T optima

9. Oxygen Isotopes • Harold Urey 1947 • Determined that oxygen isotopes in a shell may be used to calculate the T˚ of the water it precipitated from • Won the Nobel Prize in chemistry • Epstein, 1953, developed the formula • T = 16.5 - 4.3(s - w) + 0.14(s - w)2 • A function of T˚ and isotopic ratio in the water

10. What do we measure? • Delta notation • (x) = (Rx -Rstd)/Rstd x 103 • X = sample • std = standard • R=18O/16O • SMOW = standard mean ocean water • PDB = Pee Dee Belemnite, Cretaceous from South Carolina

11. Oxygen Isotopes and Deep Sea Sediments • Record of the temperature and seawater chemistry in their shells • Accumulate slowly over time • This could be good

12. Oxygen Isotopes and Deep Sea Sediments • Shell chemistry may be used as proxy for past climates • How?

13. Oxygen Isotopes • Emiliani, 1955, collected piston cores in the Caribbean and N. Atlantic and looked at the oxygen isotopic composition of forams • Wrote paper in the Journal of Geology, Pleistocene Temperatures • He thought that the change was all temperature due to glacial/interglacial episodes • Slightly more complicated, however

14. Oxygen Isotopes • The problem is, that ratio in the shells reflects T˚ AND the ratio in S.W. • Sometimes called “the curse of Harold Urey” • How might the ratio of S.W. change?

15. Isotopic Fractionation • Partitioning of isotopes between two substances w/different isotopic ratios • May occur during a phase change • Preferentially incorporate the lighter isotope • Fractionation is a temperature dependent process • Less fractionation at high temperature • Low T˚, high fractionation • Biological mediation can alter thermodynamic fractionation

16. Oxygen Isotopes • Evaporation of seawater preferentially takes lighter isotopes • This leaves the ocean relatively heavier • Today, most rainwater runs back into the ocean, so the ratio stays relatively constant • However, during glacial times, the rain is stored on land as ice

18. Oxygen Isotopes • Shackleton, 1967, argued that the record was all ice volume based on his use of benthic forams • How can we separate the the temperature signal from the S.W. ratio changes? • Compare benthic and planktonic in same core • In fact, it is about 2/3 ice volume, 1/3 temperature change in the deep sea isotopic record

19. Oxygen Isotope stratigraphy

20. Oxygen Isotopes • Cool • Oxygen isotopes in forams record glacial and interglacial cycles, or ice ages, through time • What causes the cycles?

21. Orbital Variations • James Croll, Scottish geologist developed an astronomical theory of glaciations (1864-1867) • Milutin Milankovitch, Serbian astronomer/mathematician calculated all planetary perturbations and how they affect incoming solar radiation

22. Orbital Variations • These are often called Milankovitch cycles, but sometimes Croll-Milankovitch cycles • Since these variations affect the amount of solar radiation reaching the Earth, they should therefore play a role in climate • Right? • There are three main orbital variations

23. Eccentricity • Change in shape of orbit • Occurs on a 100,000-year cycle • Affects the average solar energy reaching the Earth

24. Obliquity • Axial tilt changes from about 22 to 25 degrees • Has a cycle of about 41,000 years • Affects the amount of heat each hemisphere receives at high latitude

25. Precession of the equinoxes • Occurs because of wobble in the earth’s rotation and changes in ellipse • Alters where the earth is at certain times of year w/respect to sun • Shifts the dates of the winter and summer solstices clockwise around the orbit • Has cycle of 23,000 and 19,000 years

26. Test this idea (that ice ages are controlled by orbital variations) • Let’s match orbital variations to the continuous deep sea record of 18O • Hays, Imbrie and Shackleton (1967) Variations in the Earth’s Orbit: Pacemaker of the Ice Ages

27. Look closely at the wiggles

29. Ice Ages mechanism found? • Not so fast, my friend! • Questions/Problems • Orbital variations extend into the past, but glacial/interglacial cycles only occur at certain times in Earth history • The 100 kyr signal appears the strongest in the record, but the change in solar radiation is the weakest • Do CO2 and greenhouse gases play a role? • And what about climate change more rapid than orbital variations?

30. Rapid climate change • Ice core data indicates changes in climate can be much more rapid than Milankovitch cycles • For instance the Younger Dryas • Wait, first explain ice cores.

31. Ice Core Data

32. Ice Core Data: how it works • Gases are trapped in ice. carbon dioxide, methane, etc. • The ice accumulates in layers that may be counted back in time • Measures of atm concentrations may be obtained • Geochemistry of ice tells us temperature • Much higher resolution than deep sea sediment cores OK?

33. The Younger Dryas Event

34. Younger Dryas

35. Younger Dryas • Younger Dryas is name of arctic plant • Evidence indicates rapid shift back to a more glacial climate during a time when climate should be warming • What could cause this?

36. Meltwater events • Evidence from deep sea sediments indicates meltwater pulses occurring during the glacial retreat • How does this cause Younger Dryas cooling?

37. Remember the Oceanic Conveyor System?

38. Ocean plays a role • Temperature and salinity of water drive thermohaline circulation (conveyor) • Meltwater pulses disrupt the conveyer system, therefore no warm water north • Climate becomes colder (Younger Dryas event) • Arctic warming after event was 7˚ in about 50 years! • Wow! • Any studies that show the connection between ocean circulation and glacial/interglacial cycles?

39. Caribbean Paleoceanography • Late Quaternary Paleoceanographic Changes in the Western Caribbean Sea as Recorded in Intermediate Depth Periplatform Sediments of the Serranilla Basin • Duncan,D.S., Hine, A.C, and Droxler, A.W. • Changes in ocean sediments can tell us about changes in ocean currents, which may help us understand how the ocean and glaciations are connected

47. Once you get your core interpreted for oxygen isotope stratigraphy, much data may be compared

50. Conclusions • Evidence for milder glacial/interglacial cycles in the Serranilla Basin during stages 7-13 include: • continuous presence of G. menardii in the Caribbean • poor metastable carbonate preservation (due to continued entrainment of AAIW into Caribbean, continuous NADW formation) • The record from the Serranilla Basin supports the link between Southern Ocean Water and NADW formation, and provides evidence from the tropics of the importance of global circulation changes in affecting glacial/interglacial cycles