Using multiple paleosol proxies to interpret paleoclimate change: An earliest Eocene example from Wyoming

1. Using multiple paleosol proxies to interpret paleoclimate change: An earliest Eocene example from Wyoming MARY J. KRAUS Department of Geological Sciences University of Colorado, Boulder

2. Collaborators and Acknowledgments Daniel Woody & Susan Riggins (CU Boulder) Steve Hasiotis & Jon Smith (U Kansas) Thanks to National Science Foundation for Awards to Kraus and to Hasiotis

3. Paleocene/Eocene boundary PETM Oxygen isotope data show dramatic global warming Paleocene-Eocene Thermal Maximum or PETM Short-lived: Only ~150,000 – 200,000 years Warming was rapid – analog for modern

4. No Polar Ice Caps P/E World From Blakey (2007)

5. Carbon Isotope Compilation PETM coeval with negative shift indicating that large quantities of carbon enriched in C12 were rapidly added to ocean-atmosphere reservoir After Zachos et al. 2001

6. Problem to Address • Agreement that global temperatures rose • Impact of global warming on hydrologic cycle far less certain and debated – hydrologic cycle more difficult to assess • Some models and field studies suggest greenhouse conditions accompanied by increased precipitation and more intense continental weathering – is this universal? • Need to go to a continental section to evaluate precipitation and weathering

7. Significance • Information about precipitation critical to paleontologists and paleobotanists working to understand how ancient plants and animals responded to this episode • Understanding past changes in water cycle critical to predicting how current global warming will affect the water cycle and water resources

8. Talk Organization • What was cause of greenhouse gas increase? • What caused PETM to terminate? • How did climate system react in terms of precipitation? • Focus on Bighorn Basin in Wyoming • Brief look at Colorado

9. Cause of PETM Transfer of frozen methane deposits beneath sea floor to the atmosphere where it acted as a greenhouse gas. (Tawney & Ramanujan, 2001)

10. What stopped runaway greenhouse event? • Methane rapidly oxidizes to carbon dioxide (within 10 years) • CO2 consumed by increased mass of photosynthesizing tissue • Liberated carbon dioxide taken back down into ocean again

11. 30 mi 40 km Continental Example Although studied primarily from marine cores, PETM found in a few continental sections – Bighorn Basin has best continental section in the world. Northern Locality Bighorn Mts Bighorn Basin Absaroka Range Owl Creek Mts Southeast Locality

12. Bighorn Basin • PETM interval based on isotopes from carbonate nodules and organic carbon

13. Bighorn Basin • PETM interval in fluvial deposits with excellent alluvial paleosols - seen as color bands, which are soil horizons • Found in Willwood Fm • Reds, purples due to iron oxides in B horizons

14. CIE = PETM Alluvial Paleosols at Polecat Bench • Paleosols developed primarily on slowly accumulating overbank deposits • Alternate with coarser-grained and rapidly accumulating deposits formed as channel avulsed or moved location on the floodplain

15. Bighorn Basin Climate • Plant fossils and isotopes show Mean Annual Temperature of 20o to 25oC or 68 to 77o F • Similar to Gulf Coast region today

16. Precipitation Proxies • Paleobotany – but depends on finding leaf localities and their stratigraphic distribution • Isotopes from fossil teeth – depends on finding teeth and their stratigraphic distribution • Paleosols – common and because of vertical stacking provide continuous climate record • Paleosol morphology • Soil weathering indices • Trace fossils

17. Paleosol Morphology • Matrix color and mottle colors • Presence/absence of ferruginous nodules • Presence/absence of carbonate • carbonate nodules • carbonate along root traces

18. Red Paleosol - drier Purple paleosol - wetter

19. Presence - seasonal wetness Absence - drier soil conditions Commonly, surface-water gleying leads to local depletion of iron oxides in soil channels created by roots and burrows. As shown above, burrows that were outside the areas of surface-water gleying or that were more intensely impregnated with iron oxides (note relict meniscae by arrow) are preserved and the depletion that has gone on around them makes them visible. The yellow-brown mottles have red rims indicating they represent a earlier stage of groundwater gleying. Commonly, surface-water gleying leads to local depletion of iron oxides in soil channels created by roots and burrows. As shown above, burrows that were outside the areas of surface-water gleying or that were more intensely impregnated with iron oxides (note relict meniscae by arrow) are preserved and the depletion that has gone on around them makes them visible. The yellow-brown mottles have red rims indicating they represent a earlier stage of groundwater gleying. Ferruginous nodules

20. Carbonate Nodules Powdery carbonate along root traces Commonly, surface-water gleying leads to local depletion of iron oxides in soil channels created by roots and burrows. As shown above, burrows that were outside the areas of surface-water gleying or that were more intensely impregnated with iron oxides (note relict meniscae by arrow) are preserved and the depletion that has gone on around them makes them visible. The yellow-brown mottles have red rims indicating they represent a earlier stage of groundwater gleying. Commonly, surface-water gleying leads to local depletion of iron oxides in soil channels created by roots and burrows. As shown above, burrows that were outside the areas of surface-water gleying or that were more intensely impregnated with iron oxides (note relict meniscae by arrow) are preserved and the depletion that has gone on around them makes them visible. The yellow-brown mottles have red rims indicating they represent a earlier stage of groundwater gleying. Soil carbonate - appears in two forms; both indicate drier soil moisture regime

21. Red Purple Drier Red paleosol with carbonate and no ferruginous nodules Red paleosol with no carbonate and no ferruginous nodules Paleosol Morphology Paleosol color plus different combinations of these other features allows each paleosol to be assigned a position on the paleosol spectrum below

22. Weathering Index and MAP • Chemical index of alteration (CIA) commonly used to assess paleosol weathering • Depends on precipitation: higher precipitation  higher CIA lower precipitation  lower CIA • Mean annual precipitation (MAP) can be estimated from CIA using empirical equation developed by Sheldon et al. (2002) from modern soil data • Data are major oxides from bulk samples of paleosol B horizon

23. Trace Fossils Manganiferous rhizocretions • vertical cylinders that may branch • contain manganese & iron oxides and carbon • wet conditions during formation   Shovel head for scale

24. Trace Fossils Crayfish Burrows • live mostly in open waters • burrow to escape drying out in areas of fluctuating water tables • absence means lower water tables • presence means wetter soils

25. Relatively wet based on MAP and traces Drier based on morphology, MAP, traces Relatively wet based on MAP and traces

26. PETM • Results suggest: • Soil morphology spectrum may not be best estimate of soil moisture • Pre- and post-PETM MAP ~43” • PETM MAP was ~ 23”

27. PETM • Results suggest: • Initial ~10 m of main body of CIE is sandiest interval in study section • attributed to changing wet  dry & associated large sediment flux

28. Depositional Response • Similar to pre-CIE interval • Densely spaced paleosols • Thick red paleosols • Few weak paleosols Main Body CIE • Widely spaced paleosols • Prominent avulsion intervals • More weak paleosols

29. Paleosol Density PETM Pre-PETM

30. Widely spaced and thinner paleosols- • Typical of pre- and post-CIE intervals and lowest 10 m of CIE • Suggest relatively rapid sediment accumulation rates • And relatively frequent and thick avulsion deposits to produce “pale” packages

31. Thick, well-developed red paleosols • Hallmark of PETM interval • Multiple, densely spaced, thick red paleosols • Separated by thin avulsion deposits that are worked in • So welding of different red profiles • Suggests lower sediment accumulation

32. Depositional and Pedogenic Synthesis • Pre-PETM and Post-PETM • Wetter climate • Relatively high water discharge and high sediment supply due to wet conditions • Rapid rates of sediment accumulation • Rate of sediment accumulation > rate of pedogenesis  widely spaced and non-welded paleosols

33. Depositional and Pedogenic Synthesis • Main Part of the PETM Interval • Drier climate but some wetter episodes • Lower water discharge and reduced sediment supply due to dry conditions • Slower rates of sediment accumulation • Rate of pedogenesis > rate of sediment accumulation leading to densely spaced and welded or overlapping paleosols

34. 30 mi 40 km Basinal Variations in MAP 43’’ pre & post PETM 23’’ PETM 50’’ pre and post PETM 39’’ PETM Bighorn Mts Absaroka Range Owl Creek Mts

35. ~ 23’’ ~ 43’’ Mean Annual Precipitation in North

36. Plano TX – 43” rain Northern Basin during wet periods – 43” rain Texas Northern Blackland Prairie

37. Northern Basin during dry periods – 23” rain Abilene TX – 22” rain Red Prairie of Texas

38. Colorado during PETM • Paleogene Dawson Fm has a paleosol interval ~10 m thick • Sits above an interval with somber and organic-rich fluvial deposits • Suggests up-section change to drier and warmer Farnham & Kraus, 2002

39. Conclusions • Multiple climate proxies in paleosols indicate Bighorn Basin became drier when temperatures increased in PETM • More humid climates as temperatures declined at end of PETM • May be a similar change in Colorado

40. Conclusions • Understanding this past change in precipitation important for predicting effects of current global warming on water resources particularly in Wyoming, Colorado, Texas, etc • New predictions just made for changes in precipitation with global warming into year 2040.

41. Seager et al. (2007) NOAA & Geophysical Dynamics Lab