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Palaeoclimate Change SOES 3015
Carbon isotopes:
(PAW)
Lecture outline:
• 13C-Systematics
-
Carbon, its isotopes & notation
Organic carbon
Inorganic carbon
• 13C variation
-
spatially
through time
www.oceanography.ac.uk
(1) 13C-Systematics
• Carbon, its isotopes & notation
• Organic carbon
• Inorganic carbon
(i) Carbon, its isotopes & notation
Carbon: 2 stable isotopes:
12C
= 98.89%; 13C = 1.11% (abundances)
(14C is radioactive- from action of cosmic rays on 14N)
Carbon is always analysed as CO2 using “gas source” mass spec- carbonates via
drop phosphoric acid @25ºC. Corg is oxidized at ~1000ºC.
Isotope ratios expressed in same way as for oxygen vs PDB scale:
13C = [ (13C/12C) spl – (13C/12C) std
] x 1000
(13C/12C) std
We quote carbon to the same PDB (Pee Dee Belemnite) standard as for oxygen
isotopes
As for oxygen, carbon isotopes are subject to significant fractionation
during bio-chemical reactions
But the temperature dependency of carbon isotope fractionation is much less
significant than for oxygen
Therefore, the 13C of marine carbon is primarily determined by the type
(rather than temperature) of reaction
Unfortunately, the carbon cycle is a very complex so we will breakdown our
discussion into two parts:
• the organic carbon cycle
• the inorganic carbon cycle
Both are important to understanding palaeoceanographic records of 13C
measured in marine carbonates
(i) Organic carbon
Marine organisms assimilate Corg via photosynthesis, which uses dissolved
CO2:
CO2 + H2O = CH2O + O2
respiration is the reverse of this process & vital to breakdown of Corg.
In fact, the above is a simplification. More correctly, the stoichiometry of
average marine plankton is:
106CO2 + 122H2O + 16HNO3 + H3PO4= (CH2O) 106 (NH3) 16H3PO4 + 138O2
“Redfield ratio” Redfield et al. (1963)]
Key point: photosynthesis involves a significant isotopic fractionationthe carbon fixed in plant tissue is significantly enriched in 12C
some typical values for 13C Corg (PDB):
•
•
•
•
•
marine modern = -12 to –29‰ (ave.~ -20‰)
marine ancient = -25 to –30‰ (higher pCO2atms)
terrestrial C3 plants = -27‰ C4 plants = -13‰ (different photosynth. pathways)
methane hydrate = -60‰ (bacteria)
atmospheric (today) = -8‰ (pre-industrial) = -6.5‰ (combustion fossil fuel)
Vertically within the ocean you might see process of CO2 transfer via photosynthesis
and respiration referred to as the “biological pump”
recall that this can be deduced by a standard ocean nutrient profile
- see how phosphate consumed and regenerated:
Courtesy of Broecker W.S., and Peng T.H., (1982), Tracers in the Sea, Eldigo press.
13C of dissolved CO2 is determined by equilibrium w/ the large marine
bicarbonate (HCO3-) reservoir which is a major component of the inorganic pool:
The inorganic carbon pool is governed by the carbonate reactions:
CO2 (g) = CO2 (aq) ………………….P, T………………………..
(1)
CO2(aq) + H2O = H2CO3 …………………………………………..
(2)
H2CO3 = HCO3- + H+ …………….K large- v. rapid ……………
(3)
HCO3- = CO32- + H+ ……………K small- less rapid …………..
(4)
(where K = dissociation constant)
 very little of the CO2 entering sea
water remains in the gaseous formmost is rapidly converted to HCO3- and
only a small proportion of this is then
converted to CO32Thus, most of the carbon
in sea water is present as
bicarbonate HCO3- (pH of
open marine sea water
is ~ 8)
Based on data from Karl Krauskopf.
Therefore, oceans display a vertical profile
in 13C showing:
• heavy values in photic zone
(preferential 12C into Corg)
• light values deeper
(oxidation of Corg)
Courtesy of Broecker W.S., and Peng T.H., (1982), Tracers in the Sea, Eldigo press.
HCO3- & CO32- are also delivered to the oceans via riverine & hydrothermal input
(together with Ca). And precipitated as CaCO3 (in the form of bugs- our handy
substrate for measuring geochemical tracers):
Ca2+ + 2HCO3- = CaCO3 + CO2 + H2O ………………………..
(5)
Modified from: Krauskopf K. B., (1967), Introduction to Geochemistry. 721pp McGraw-Hill, New York.
Summation of relevant a values (@20ºC) means that CaCO3 precipitated in
equilibrium* w/ CO2 is enriched in 13C by ~10‰
Modern marine carbonates typically ~0 to 2‰ PDB
*As with oxygen, carbon isotopes are subject to plenty of non-equilibrium
complications- diagenesis less problematic but symbiosis, incorporation of respired
CO2 & [CO32-] effects significant.
Reprinted by permission from Macmillan Publishers Ltd:
Spero, H.J., Bijma, J., Lea, D.W., Bemis, B.E.,
Effect of seawater carbonate concentration on
Foraminiferal carbon and oxygen isotopes. Nature
v. 390, p. 497-500. Copyright (1997). Not under CC Licence
(2) Variation of oceanic 13C
Question:
Based on what you now know,
how might we use 13C to
evaluate palaeo ocean
circulation patterns?
Redrawn based on Borecker (1974) Chmical
Oceanograph, Harcourt Brace Jovanovich inc.
Redrawn from: Ocean chemistry and deep sea sediments.,
Oceanography Course Team’ (eds) prepared by Rachel James,OU and Elsevier (2005)
(i) Through time - K/T boundary “Strangelove ocean”
Que: What happens to the biological pump signal in the immediate aftermath of
the mass-extinction?
Ans: It is shut-down- total wipe-out of vertical 13C gradient as determined by
measuring planktic & benthic bugs
Reprinted by permission from Macmillan Publishing Ltd: Zachos J.C., Arthur, M.A., Dean W.E.,
Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary
boundary, Nature, v. 337, p. 61-64. Copyright (1989). Not under CC Licence.
(ii) Late Palaeocene Thermal Maximum “LPTM”
Que: What can possibly explain a large
(~2‰), rapid (~10ka), -ve excursion of
the whole ocean reservoir?
Ans: Perhaps the massive release of
methane clathrate (-60‰)- oxidised to
CO2- global warmingModern hydrate = 14,000 Gton ±?
LPTM injection =
anthropgn. injection =
1,500 Gton
2,000 Gton
best ancient analog for modern
anthropogenic experiment???
Reprinted by permission from Macmillan Publishers Ltd: Kennett, J.P., Stott, L.D.,
Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions
at the end of the Palaeocene., Nature, v. 353, p. 225-229. Copyright (1991).
Not under CC licence.
Consider the carbon cycle quantitatively
Present day exchangeable C reservoirs (<10 ka):
Organic =
Land biota =
Soil =
Dissolved marine =
3,500 Gton
500
1,500
1,500
Inorganic =
Atmosphere =
Ocean =
30,500 Gton
500
30,000
Global C cycle =
34,000 Gton
Implication: There is so much Cinorg in seawater that modest changes in marine
carbonate 13C imply significant perturbations to the carbon cycle…
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