1. Open Ocean CDOM Production and Flux
Norm Nelson, Dave Siegel, Stéphane Maritorena Chantal Swan, Craig Carlson UC Santa Barbara
ACE Ocean Productivity and Carbon Cycle (OPCC) Workshop
UCSB, June 2011

2. Outline What is CDOM and why should we care Remote sensing of CDOM
CDOM Dynamics in the global ocean

3. CDOM What and Why
CDOM is dissolved (passes 0.2 m filter) organic matter that absorbs light.
CDOM has a major impact upon ocean color -- influences retrieval of chlorophyll, penetration of PAR and UV to depth. This enables remote sensing of surface CDOM.
CDOM is produced by all kinds of heterotrophic activity and is destroyed primarily by solar radiation.
CDOM is found in measurable (if you’re careful) quantities at all depths everywhere in the ocean.
CDOM is not correlated to DOC abundance in the open sea.

4. Zooplankton CDOM Production
Example spectra for controls vs. plankton

5. Solar Bleaching of CDOM
daCDOM/dt (measured)
E0*ācdom
Time
Time
Time
Chantal Swan, UCSB
E0*ācdom*(o=325nm)
E0*ācdom*(o=325nm)
Time
Time

6. CDOM What and Why (2)
CDOM is also an primary sensitizer of photochemical reactions involving climate-relevant trace gases (CO2, CO, OCS, DMS)
CDOM is an indicator of terrestrial runoff and riverine input to the ocean
Alas: In the open ocean we can’t ascribe carbon content to CDOM (yet?) nor do we know much about the identities of the chromophores

7. Photochemical CO production from space

8. CDOM from Ocean Color

9. CDOM Optics and Remote Sensing
CDOM absorption spectrum is distinct from phytoplankton absorption
Ocean color reflectance spectra can be inverted to retrieve absorption by CDOM and particles and particulate backscattering
CDOM
Particles

10. Mean Global Surface CDOM Distribution From SeaWiFS as acdm(443 nm, m-1) Garver-Siegel-Maritorena model
acdom (443 nm, m-1)
Siegel et al. [2005] JGR -- Nelson et al. [2010] GRL

11. CDOM from Ocean Color
Matchup with NOMAD data (IOCCG IOP report; Lee et al. 2006)
Model-data fits are pretty good – though not excellent
GSM01 is optimized for all 3 retrievals (CHL, CDM, BBP)

12. Global Dynamics of CDOM

13. Example CDOM Profiles

14. CDOM and AOU Distribution
Atlantic
Pacific
Indian
CDOM and AOU Distribution
[Nelson et al. 2010]

15. CDOM / AOU Correlation

16. CDOM Dynamics Summarized
CDOM is produced primarily as a function of remineralization (terrestrial inputs are local, only a few taxa of autotrophs produce CDOM)
CDOM is destroyed primarily by photolysis (we don’t observe labile DOM as it’s consumed too rapidly by microbes)
Time scales for CDOM production / destruction are comparable to time scales for ocean circulation (otherwise the ocean would be yellow)
Observed CDOM distribution results from a balance between source/sink processes and circulation

17. CDOM Dynamics - N. Pacific / Indian
Weak ventilation in northern basin
Particle flux leads to CDOM accumulation Eq
Biological
Physical
Bleaching N

18. CDOM Dynamics - N/S Atlantic
Strong ventilation in subarctic basin
Higher surface CDOM signal transmitted to deep Eq
Biological
Physical
Bleaching
N or S

19. CDOM Dynamics - S Pacific/Indian
Strong ventilation in Southern Ocean
Lower surface CDOM signal transmitted to deep Eq
Biological
Physical
Bleaching S

20. Summary / Conclusions
CDOM is a remotely sensible semiconservative tracer, produced by heterotrophs and destroyed by photolysis
The relationship between CDOM and oxygen (as AOU) in the deep sea is modulated by circulation processes
CDOM assessment is important to do ocean color right, and is useful in its own right

21. Extra slides

22. CDOM Optics and Remote Sensing
Remote sensing reflectance spectrum can be inverted to retrieve inherent optical properties (absorption and backscattering spectra) of the surface water (mixed layer to ~ 60m).
Absorption spectra can be deconvolved into particle absorption and CDOM+detritus absorption spectra (which we call CDM) given some assumptions about the shape of the component spectra.
Garver-Siegel-Maritorena model (GSM) uses shape functions determined using a global optimization of available global open ocean field data.

23. Seasonal CDOM Cycle CDM Seasonal changes at most latitudes
Lower in summer
Reduced in tropics
Higher towards poles
Hemispheric asymmetry
%CDM

24. Surface CDOM & SeaWiFS r2 = 0.65; N = 111 slope = 1.16
Siegel et al. [2005] JGR
A20
A22
Overall, there was good correspondence between the GSM01 estimated CDM and the in situ CDOM absorption coefficient at 443 nm (r2 = 0.65; N = 111). The linear regression slope was 1.16, with an intercept of m-1, indicating approximately a 15% overestimation of CDOM absorption by the GSM algorithm.
A16N

25. a*cdom(325) a*cdom = CDOM / DOC (units m2g-1)
Upper layers bleaching & production signals
a*cdom increases w/ depth & age
CDOM “abundance” changes less than the DOC decline -- CDOM is refractory DOM
New
Bleaching
Aging
Nelson et al. [2007] DSR-I

26. Regressions between age and CDOM
T ~ 10y
T ~ 50y
T > 200 y
P < 0.025
Nelson et al. [2007] DSR-I

27. Trends in CDOM spectral characteristics - N. Atl.
Nelson et al. [2007] DSR-I