1. Dissolved Organic Matter distribution, reactivity, and composition

2. 2013 CMORE Agouron Repeta Lecture 1
Typical 1D profile of dissolved organic carbon in the ocean
TOC (µM)
Often measured as TOC
Surface values typically µM
Deep water 40 µM
(implies some unknown feedback/
control of DOC values)
Global inventory about 660 GT C
Data from Peltzer and Hayward (1996) DSR
2013 CMORE Agouron Repeta Lecture 1

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Global Dissolved Organic carbon (< 30 m)
Hansell et al., GBC 2012
2013 CMORE Agouron Repeta Lecture 1

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Atlantic
A16
Pacific P
P16
2013 CMORE Agouron Repeta Lecture 1
From Dennis Hansell CLIVAR data set

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2013 CMORE Agouron Repeta Lecture 1

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Global circulation and the distribution of DOC
Hansell et al. Oceanography 2009
On a global scale, carbon is sensitive to water mass transport. See it very clearly in the N Atlantic where NADW formation can be seen N of 40oN, and we can see loss of DOC in the deep pacific. Assuming steady state, then there is a 30% loss of carbon between the deep basins, and a lot of that loss occurs in Pacific north of the equator. This is interesting bc of the radiocarbon age of DOC is very old- some deep process can remove old radiocarbon.
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How do we measure carbon fluxes in DOC ?
We can measure processes that cycle carbon through DOC on very short time scales using bacterial production and carbon demand measuremetns, and over longer timescales by following the changes in DOC concentration along isopycnal surfaces. But we can also use natural abundance radiocarnon, as a built in clock. Radiocarbon is produced by spallation reactions, the collision of cosmic rays (high energy protons and neutrons from outside the solar system, produced by supernova) with N-14. A neutron collides with N-14 and ejects a proton, making C-14. The atmospheric carbon is about 1 ppt of C-14. When radiocarbon decays it ejects an electron and returns to N-14. beta decay. Half life of about 5730 year. Most C-14 is produced at an altitude of km above the surface. Rate of synthesis depends on geomagnetic field, and cosmic ray flux. Latitudinally dependand.
2013 CMORE Agouron Repeta Lecture 1

8. National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS)
C-14 tracer measurements use >106x more C-14 than natural abundance!!!!!
2013 CMORE Agouron Repeta Lecture 1

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History of radiocarbon in the atmosphere and ocean
McNichol and Aluwihare, Chem Rev. 2007
2013 CMORE Agouron Repeta Lecture 1

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Top “pre bomb” 14C distribution as calculated from GEOSECS alkalinity data. Bottom, measured 14C activity by GEOSECS in the 1970s. Based on these pictures, what would you predict the DOC in the deep ocean to be?
2013 CMORE Agouron Repeta Lecture 1
McNichol and Aluwihare 2007

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DOC cycling via DO14C
UV photooxidation
Deep water DIC values are years (Craig et al.)
Depth 14C(‰) Age
1880m ‰ ybp
1920m ‰ ybp
Williams, Oeschger, and Kinney; Nature v224 (1969)
2013 CMORE Agouron Repeta Lecture 1

12. Peter M. Williams and Ellen Druffel;
Radiocarbon in the Atlantic and Pacific Oceans
Peter M. Williams and Ellen Druffel;
Nature 1987, JGR 1992

13. Radiocarbon in the Atlantic and Pacific Oceans
Sargasso suspended POC
Pacific suspended POC
Pacific sinking POC

14. Radiocarbon in the Atlantic and Pacific Oceans
DIC 14C in surface waters
of the Atlantic and Pacific
have similar isotopic values.
Sargasso suspended POC
Pacific suspended POC
Pacific sinking POC

15. Radiocarbon in the Atlantic and Pacific Oceans
DIC 14C in surface waters
of the Atlantic and Pacific
have similar isotopic values.
DOC is always older than DIC
(by 2-3 kyrs in surface water)
DIC POC DOC
Sargasso suspended POC
Pacific suspended POC
Pacific sinking POC
Pacific suspended POC
Pacific sinking POC
Sargasso suspended POC

16. Radiocarbon in the Atlantic and Pacific Oceans
DIC 14C in surface waters
of the Atlantic and Pacific
have similar isotopic values.
DOC is always older than DIC
(by 2-3 kyrs in surface water)
14C of DIC and DOC
is about the same in the
deep Atlantic and Pacific
Sargasso suspended POC
Pacific suspended POC
Pacific sinking POC
Pacific suspended POC
Pacific sinking POC
Sargasso suspended POC

17. Radiocarbon in the Atlantic and Pacific Oceans
DIC 14C in surface waters
of the Atlantic and Pacific
have similar isotopic values.
DOC is always older than DIC
(by 2-3 kyrs in surface water)
14C of DIC and DOC
is about the same in the
deep Atlantic and Pacific
Deep ocean values of DOC
are equal to a radiocarbon
age of 4-6 kyrs
Either there is a source of
“old” DOC, or DOC persists
for several ocean mixing
cycles.

18. Radiocarbon based models of DOC cycling in the water column
Old (low radiocarbon), non-reactive DOC
New, semi-reactive
DOC
Atlantic surface water
14Ccalc = ‰
14Cobs = ‰
Pacific surface water
14Ccalc = ‰
14Cobs = ‰
No microbial carbon cycling in the deep ocean???
Williams and Druffel, 1987; Beaupre and Druffel 2009
2013 CMORE Agouron Repeta Lecture 1

19. Cycling of reactive and semi-labile DOC by phytoplankton and bacteria
DOC”reactive”
DOC”semi-labile”
CO2
Heterotrophic bacteria
Autotrophic
microbes
What DOC plots don’t show you is one of the most important aspects of DOC cycling, and that is the microbial loop.
Bacterial growth and respiration is thought to be fueled by very reactive DOC that has a residence time in seawater of hour to days.
2013 CMORE Agouron Repeta Lecture 1

20. at best very slow removal of DOM in the deep sea
Under this perspective, DOM is produced and rendered recalcitrant by marine microbes.
Combined with the two component model of radiocarbon and deep sea DOC, it suggests no or
at best very slow removal of DOM in the deep sea
2013 CMORE Agouron Repeta Lecture 1

21. LIW Depth (m) 40 mM 34 mM Very rapid degradation of DOC in the
Mediterranean Sea. If DOC is refractory,
why does this happen?
LIW
Depth (m)
40 mM
34 mM
C. Sanitelli et al. 2013
2013 CMORE Agouron Repeta Lecture 1

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Southern Ocean DOC-14
data suggests the two
component model
cannot be correct.
2013 CMORE Agouron Repeta Lecture 1

23. Could there be a transfer of carbon from the surface water
into the deep ocean ?
Depth
[DOC]
The two component model only requires a
conservation of mass and isotopic abundance,
but does not stipulate what the isotopic/mass
distribution of DOC is within the old (deep)
fraction of DOC. A number of mass/isotope
mixes can constitute the old fraction of DOC.
UV photooxidation
Smaller amount of
very old DOC,
and a larger amount of
younger DOC
2013 CMORE Agouron Repeta Lecture 1

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