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

Lecture outline:1) the carbon cycle

and δ13C

2) C fractionationin the terrestrialbiosphere

3) C isotopes in the ocean

4) C isotopes in theatmosphere

Photo of a C3 leaf cross-section

green = reservoir size (1015g, 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: δ13C always reported in PDB

C3 Pathway-enzyme-mediated (RUBISCO)-RUBISCO fixes 1 O2

for every 5 CO2

-“Calvin” cycle-90% of all plants-20-30‰ fractionation

TERRESTRIAL PHOTOSYNTHESIS - theoretical calculations predict a 4.4‰ kinetic fractionation for CO2(g) moving from air through stomata to site of photosynthesis

δ13C and Photosynthesis

C4 Pathway-desert plants, some

tropical species-enzyme-mediated (PEP)-“Hatch-Slack” cycle-10% of all plants-13‰ fractionation(beggars can’t bechoosers…)

NOTE: C4 plants still execute “Calvin”cycle, but CO2 grabbing and actualcarbon fixation happening in different cells

Schoeninger and DeNiro, 1984

δ13C of living organisms: you are what you eat,plus a little bit

Why are higher trophicorganisms progressivelyhigher in δ13C?

δ13C and CO2 in soils

Why are soil CO2 andδ13C correlated?

Allison, C.E. et al., “TRENDS”, DOE, 2003.

δ13C of atmospheric CO2

What feature do they share and why?

Why do they differ?

Atmospheric biogeochemists use aglobal network of flask collectionsto track CO2 from sources to sinks

ex: most emissions are in N.H., butN-S gradient is small – thereforeN.H. must be taking up large amountof emissions

δ13C

CO2

δ13C and [CO2] for last 200 years – ice core bubbles in SipleStation, Antarctica

Suess Effectprogressive depletion of CO2

resulting from burning of isotopically light fossil fuels~1.5‰ over last century

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 mustdifferentiate between inorganic C (carbonates): typically -1‰ to +1‰ PDBand 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 • temperature low at poles, fractionation increases• surface-to-deep gradients (upwelling zones have lower δ13C(sw))

δ13C of Dissolved Inorganic Carbon (DIC) in the ocean

Phosphate and δ13C of DICin the Pacific Ocean.After Broecker and Peng, 1982 For info see Kroopnick, 1985

δ13C of DIC – vertical and meridionalgradients

ATLANTIC

PACIFIC

Kroopnick, 1985

Central Pacific DICand δ13C of DIC

What determinesthe DIC of surfaceseawater?

What determinesthe δ13C of surface DIC?

What happened here?

(benthic foraminifera)

1:1

Oceanic δ13C on glacial-interglacial timescales

Benthic foraminifera recordthe δ13C of the DIC in whichthey grow.

Can take cores from1. different depths2. different locations

and reconstruct deepwater δ13Cthrough space and time

Ninneman et al., 2002

Charles et al., 1996

Oceanic δ13C on glacial-interglacial timescales

So South Atlantic δ13Cwas lower during last glacial – NADW reduced!

Timing of δ13C shiftslook like Greenland ice!

green = reservoir size (1015g, 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: δ13C always reported in PDB

green = reservoir size (1018g)red = fluxes (1018g/yr)blue = C isotopic value

* NOTE: pre-anthropogenic valuesFigure from William White, Cornell U.

Long Term Carbon Cycle

C inputs-volcanism and

tectonics

Weathering and CO2 drawdown:

Uplift Weathering Hypothesis

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?

Osborne and Beerling, 2006

Methane Hydrates and the PETM

Zachos et al., 2005

Zachos et al., 2005

-present-daylysocline = 3700-4500m

-shoaling of lysocline to <1500mrequired~4500GtC;entirefossil fuelreservoir!

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

-5‰

Model CO2 release’s impact on δ13C and temperature

Snowball Earth Hypothesis

• Earth’s entire surface frozen over

• Evidence for 3 times, maybe more

• Earlybetween 2200Mya and 650 Mya– (Proterozoic)

• Glacial sediment deposits at tropical latitudes

• Carbonate ‘caps’ on top of glacial sediments

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

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’

Snowball Earth Hypothesis

• Major excursions in δ13C in geologic record

• Seen around world in conjunction with geologic transitions

• Crucial for acceptance of global events

• Lots of variability in marine δ13C, more than today

Decline in δ13C prior to glaciationsδ1

3C

‰ V

PD

B

δ13 C

‰ V

PD

B

Halverson et al., 2006