Carbon isotopes in the biosphere 10/23/12 and geologic record

1. Carbon isotopes in the biosphere 10/23/12 and geologic record • Lecture outline: • the carbon cycle • and δ13C • 2) C fractionation • in the terrestrial • biosphere • C isotopes in the • ocean • C isotopes in the • atmosphere Photo of a C3 leaf cross-section

2. The Carbon Cycle green = reservoir size (1015g, Gigatons) red = fluxes (Gt/yr) blue = C isotopic value *NOTE: δ13C always reported in PDB Reservoirs and fluxes from Schlesinger, 1991; d13C from Heimann & Maier-Reimer, 1996

3. δ13C and Photosynthesis TERRESTRIAL PHOTOSYNTHESIS - theoretical calculations predict a 4.4‰ kinetic fractionation for CO2(g) moving from air through stomata to site of photosynthesis • C3 Pathway • enzyme-mediated • (RUBISCO) • -RUBISCO fixes 1 O2 • for every 5 CO2 • “Calvin” cycle • 90% of all plants • 20-30‰ fractionation

4. C4 Pathway • desert plants, some • tropical species • enzyme-mediated • (PEP) • “Hatch-Slack” cycle • 10% of all plants • 13‰ fractionation • (beggars can’t be • choosers…) NOTE: C4 plants still execute “Calvin” cycle, but CO2 grabbing and actual carbon fixation happening in different cells

6. Schoeninger and DeNiro, 1984 δ13C of living organisms: you are what you eat, plus a little bit Why are higher trophic organisms progressively higher in δ13C?

7. δ13C and CO2 in soils Why are soil CO2 and δ13C correlated?

8. δ13C of atmospheric CO2 What feature do they share and why? Why do they differ? Atmospheric biogeochemists use a global network of flask collections to track CO2 from sources to sinks ex: most emissions are in N.H., but N-S gradient is small – therefore N.H. must be taking up large amount of emissions Allison, C.E. et al., “TRENDS”, DOE, 2003.

9. δ13C and [CO2] for last 200 years – ice core bubbles in Siple Station, Antarctica d13C Suess Effect progressive depletion of CO2 resulting from burning of isotopically light fossil fuels ~1.5‰ over last century CO2

10. OCEANIC PHOTOSYNTHESIS – can utilize either CO2(g) or HCO3- +0.9‰ equil. +7-8‰ equil. • When thinking about how C isotopes move through the ocean, we must • differentiate between • inorganic C (carbonates): typically -1‰ to +1‰ PDB • and • organic C: typically -5‰ to -15‰ PDB • However, the ocean, unlike the atmosphere, is NOT well-mixed. • δ13C of marine organisms varies because: • [CO2(aq)] small in warm tropical waters, fractionation low • pH varies, and each inorganic DIC species has different a • temperature low at poles, fractionation increases • surface-to-deep gradients (upwelling zones have lower δ13C(sw))

11. d13C of Dissolved Inorganic Carbon (DIC) in the ocean Phosphate and δ13C of DIC in the Pacific Ocean. After Broecker and Peng, 1982 For info see Kroopnick, 1985

12. δ13C of DIC – vertical and meridional gradients ATLANTIC PACIFIC Kroopnick, 1985

13. What happened here? Central Pacific DIC and δ13C of DIC What determines the DIC of surface seawater? What determines the δ13C of surface DIC?

14. 1:1 (benthic foraminifera) Oceanic δ13C on glacial-interglacial timescales Benthic foraminifera record the δ13C of the DIC in which they grow. • Can take cores from • different depths • different locations • and reconstruct deepwater δ13C • through space and time

15. Oceanic δ13C on glacial- interglacial timescales Charles et al., 1996 So South Atlantic δ13C was lower during last glacial – NADW reduced! Timing of δ13C shifts look like Greenland ice! Ninneman et al., 2002

16. The Carbon Cycle green = reservoir size (1015g, Gigatons) red = fluxes (Gt/yr) blue = C isotopic value *NOTE: δ13C always reported in PDB Reservoirs and fluxes from Schlesinger, 1991; d13C from Heimann & Maier-Reimer, 1996

17. Long Term Carbon Cycle green = reservoir size (1018g) red = fluxes (1018g/yr) blue = C isotopic value * NOTE: pre-anthropogenic values Figure from William White, Cornell U.

18. C inputs-volcanism and tectonics

19. Weathering and CO2 drawdown:

20. Uplift Weathering Hypothesis

21. Evolution C4 plants • Miocene Himalayas form • Increase in weathering, drawdown of CO2 • Low CO2 conditions • Plants evolve to deal with low CO2 • C4 plants • Also more efficient in arid, hot regions • C4 plants fix more C than C3 plants amplify global decline in CO2?

23. Osborne and Beerling, 2006

24. Methane Hydrates and the PETM

26. Zachos et al., 2005

28. -present-day lysocline = 3700-4500m -shoaling of lysocline to <1500m required ~4500GtC; entire fossil fuel reservoir! Zachos et al., 2005

29. Catastrophic methane hydrate release captured in deep-sea cores? Jim Kennet, “Clathrate Gun Hypothesis”, 2002 -methane most depleted δ13C (-60‰ for biogenic, -40‰ for thermogenic) -frozen on every continental margin, but stability depends on T and P -methane is a greenhouse gas, can warm surface ocean, leading to more CH4 release, etc -can have medium-sized methane hydrate release from tectonic slope failure

30. -5‰

32. Model CO2 release’s impact on δ13C and temperature

34. Snowball Earth Hypothesis • Earth’s entire surface frozen over • Evidence for 3 times, maybe more • Earlybetween 2200Mya and 650 Mya • (Proterozoic) • Glacial sediment deposits at tropical latitudes • Carbonate ‘caps’ on top of glacial sediments

36. How did it happen? • Initial cooling + positive feedback • Supervolcano? • Orbital? (>60° ?) • Solar output? • Reduction in Greenhouse Gases? • Tropical continental position reflect more light back to space? • Feedback: albedo

37. How did we get out of it? • Plate tectonics • Volcanism—massive buildup of CO2 • And no weathering to draw it down • Massive Greenhouse following Massive Icehouse • Surge in weathering of tropical continents • Increase alkalinity • Deposition of carbonate ‘caps’

39. Snowball Earth Hypothesis • Major excursions in d13C in geologic record • Seen around world in conjunction with geologic transitions • Crucial for acceptance of global events • Lots of variability in marine d13C, more than today

41. Decline in d13C prior to glaciations d13C ‰ VPDB d13C ‰ VPDB Halverson et al., 2006