1. Carbon isotopes in the biosphere10/23/12 and geologic record Lecture outline: 1)the carbon cycle and  13 C 2)C fractionation in the terrestrial biosphere 3)C isotopes in the ocean 4)C isotopes in the atmosphere Photo of a C3 leaf cross-section

2. green = reservoir size (10 15 g, Gigatons) red = fluxes (Gt/yr) blue = C isotopic value Reservoirs and fluxes from Schlesinger, 1991; d13C from Heimann & Maier-Reimer, 1996 The Carbon Cycle *NOTE:  13 C always reported in PDB

3. C3 Pathway -enzyme-mediated (RUBISCO) -RUBISCO fixes 1 O 2 for every 5 CO 2 -“Calvin” cycle -90% of all plants -20-30‰ fractionation TERRESTRIAL PHOTOSYNTHESIS - theoretical calculations predict a 4.4‰ kinetic fractionation for CO 2 (g) moving from air through stomata to site of photosynthesis  13 C and Photosynthesis

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  13 C of living organisms: you are what you eat, plus a little bit Why are higher trophic organisms progressively higher in  13 C?

7.  13 C and CO 2 in soils Why are soil CO2 and  13 C correlated?

8. Allison, C.E. et al., “TRENDS”, DOE, 2003.  13 C of atmospheric CO 2 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

9.  13 C CO 2  13 C and [CO 2 ] for last 200 years – ice core bubbles in Siple Station, Antarctica Suess Effect progressive depletion of CO 2 resulting from burning of isotopically light fossil fuels ~1.5‰ over last century

10. OCEANIC PHOTOSYNTHESIS – can utilize either CO 2 (g) or HCO 3 - +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.  13 C of marine organisms varies because: 1.[CO 2 (aq)] small in warm tropical waters, fractionation low 2.pH varies, and each inorganic DIC species has different  3.temperature low at poles, fractionation increases 4.surface-to-deep gradients (upwelling zones have lower  13 C (sw) )

11.  13 C of Dissolved Inorganic Carbon (DIC) in the ocean Phosphate and  13 C of DIC in the Pacific Ocean. After Broecker and Peng, 1982 For info see Kroopnick, 1985

12.  13 C of DIC – vertical and meridional gradients ATLANTIC PACIFIC Kroopnick, 1985

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

14. (benthic foraminifera) 1:1 Oceanic  13 C on glacial-interglacial timescales Benthic foraminifera record the  13 C of the DIC in which they grow. Can take cores from 1.different depths 2.different locations and reconstruct deepwater  13 C through space and time

15. Ninneman et al., 2002 Charles et al., 1996 Oceanic  13 C on glacial- interglacial timescales So South Atlantic  13 C was lower during last glacial – NADW reduced! Timing of  13 C shifts look like Greenland ice!

16. green = reservoir size (10 15 g, Gigatons) red = fluxes (Gt/yr) blue = C isotopic value Reservoirs and fluxes from Schlesinger, 1991; d13C from Heimann & Maier-Reimer, 1996 The Carbon Cycle *NOTE:  13 C always reported in PDB

17. green = reservoir size (10 18 g) red = fluxes (10 18 g/yr) blue = C isotopic value * NOTE: pre-anthropogenic values Figure from William White, Cornell U. Long Term Carbon Cycle

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!

29. Catastrophic methane hydrate release captured in deep-sea cores? -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 Jim Kennet, “Clathrate Gun Hypothesis”, 2002

30. -5‰

32. Model CO 2 release’s impact on δ 13 C 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  13 C in geologic record Seen around world in conjunction with geologic transitions Crucial for acceptance of global events Lots of variability in marine  13 C, more than today

41. Decline in  13 C prior to glaciations  13 C ‰ VPDB Halverson et al., 2006