1. Estimating particle export using the Thorium-234 approach C/Th ratios
Ken O. Buesseler
Woods Hole Oceanographic Institution
Idea is to set the stage & get certain images in your mind as we move in to workshop mode
Think about what C/Th examples we have- new/unpublished/syntheses that might be useful for the group
2004 Thorium-234 workshop

2. C/Th ratio vs. size, depth,
Outline
 Introduction
What is the “234Th approach?”
Concept of C/Th ratio
How did we get here?
 C/Th variability- selected examples
C/Th ratio vs. size, depth,
region, collection device
 How do we know it works?
Comparisons with other methods

3. Carbon flux = 234Th flux  [C/234Th]sinking particles
Thtot/U
234Th flux 1000
sinking
particles
C/Th :4
Empirical Approach
Don’t need to know anything about process (but of course this is short sighted! C/Th tells you something about system)
C flux  1/4

4. Carbon flux = 234Th flux  [C/234Th]sinking particles
Thtot/U
234Th flux
sinking
particles
C/Th : :4
Thorium accociated with “recently produced, surface active, pollysaccharide-rich macromolecules” Quigley et al OK, but does not limit empirical use of this tracer
Trap = 3 days; filter = one point depth/time
C flux  1/  2/4

5. Carbon flux = 234Th flux  [C/234Th]sinking particles
Thtot/U
234Th flux
sinking
particles
C/Th : :4 1:2
Thorium accociated with “recently produced, surface active, polysaccharide-rich macromolecules” Quigley et al OK, but does not limit empirical use of this tracer
Trap = 3 days; filter = one point depth/time
C flux  1/  2/  1/2
Assumptions
- measure C/Th representative of sinking particles
- sinking particles are only/major Th loss
- sinking particles average over same time/space as flux

6. Carbon flux = 234Th flux  [C/234Th]sinking particles
POC/234Th
Empirical approach
Must use site and depth appropriate ratio
Decrease w/depth- due to C loss; increase in relative ratio acid polysaccharides which bind w/Th on surfaces
Not appropriate to estimate 100m flux, from characteristics of surface derived particles
i.e. variability in C/Th tells you nothing about accuracy/uncertainty in flux proxy
Similar to Redfield Ratio; C/Chl
Th flux on particles

7. How did we get here? Bhat et al., ‘69 & many others………..
What about using C/Th ratio and 234Th flux to determine sinking C export?
 1st? JGOFS North Atlantic Bloom Experiment
Kiel March 1990 PI meeting
Claudia already reviewed history
Kiel- personal story Kirk & myself (not in original proposal) asked for C flux
but then at SCOR meeting met Tsunogai abstract! C/Th = 5 uM/dpm & C 100m = 9 mMC/m2/d

8. How did we get here? Bhat et al., ‘69 & many others………..
What about using C/Th ratio and 234Th flux to determine sinking C export?
 1st? JGOFS NABE
Kiel March 1990 PI meeting
DSRI 1992 Buesseler et al.
 No, much earlier!
1976 Tsunogai & Minagawa
(note C/Th ratio = 5 mM/dpm
C 100m = 9 mmC/m2/d)
Claudia already reviewed history
Kiel- personal story Kirk & myself (not in original proposal) asked for C flux
but then at SCOR meeting met Tsunogai abstract! C/Th = 5 uM/dpm & C 100m = 9 mMC/m2/d

9. Early concepts on controls on C/Th ratio
more to the story-C/Th tells you something about particle cycling & process that determine Th scavenging
Acid Polysaccharides appear important- Santschi talk to follow
Barg, Lee, Lal et al. ‘93
Santschi, Quigley, Honeyman, Baskaran et al
Burd, Jackson, Moran et al.
Usbeck et al.
Michaels & Buesseler, ONR SIGMA proposal 1990

10. Early concepts on controls on C/Th ratio
C/Th story more complicated
- surface chemistry
- ligands/APS & colloids
- adsorption/aggregation
Barg, Lee, Lal et al.
Santschi, Quigley, Honeyman, Baskaran et al.
Burd, Jackson, Moran et al.
Usbeck et al.
more to the story-C/Th tells you something about particle cycling & process that determine Th scavenging
Acid Polysaccharides appear important- Santschi talk to follow
Barg, Lee, Lal et al. ‘93
Santschi, Quigley, Honeyman, Baskaran et al
Burd, Jackson, Moran et al.
Usbeck et al.
Michaels & Buesseler, ONR SIGMA proposal 1990

11. How does ratio vary w/size? - no simple relationship
What if bottle POC to high? Often GFF from bottle used for 1um POC and pumps for larger size classes
Diatoms in So. Ocean- primary producers, actively growing and exported intact w/o grazing
Smaller cells/grazing, lower C/Th in other sites
Guo et al., 2002
Savoye, Trull, Buesseler unpub.
- watch out for comparisons of bottle POC (high) for 0.7um GFF
and pumps for Th
- also swimmer C/Th high- not passive sinking material
- colloidal C/Th can be troublesome (sorptive losses)

12. How does ratio vary w/depth?
- decreases with depth
0 m
Depth (m)
SOFeX- at depth, C/Th 1 vs. 53 um particle difference <factor of two
Pump 1um 234Th & Bishop 1 um POC for Charette et al
200 m
PAPA
Coast
POC/234Th (mM/dpm)
Charette et al. 1999
POC/234Th (mM/dpm)
SOFeX, Buesseler et al. 2004

13. How does ratio vary w/region?
- C/Th higher in high latitudes
& coastal settings
Example from Ken’s SMP data on www
Range of data sets, from n=2 to many years data at one site
Trap = pump < bottle
Murray, Dunne, Charette, Buesseler, Benitez-Nelson, Coppola, Fowler, Schmidt, Amiel, Cochran, Sarin, Shimmield, Moran, & more

14. How does ratio vary w/collection device?
- is one method “best”?
- what does range tell us?
- more work is needed
400
mM/dpm
Gustafsson et al. unpub. data Gullmar Fjord
new methods- SPLITT; NBST; tube to tube variability; bottles vs. pumps
Good news is not huge differences at any one site
Variability between sampling periods, i.e. particle source term varies, so expect biases to vary
BATS unpub. Data
- Watch out for difference
in methods
e.g. trap C vs. filter C
- Don’t expect single offset
Cochran et al MedFlux

15. Variations in C/Th are measured
- What does this mean for accuracy of empirical method?
look for oceanographic consistency
direct comparisons to other methods (budgets & traps)
- few data sets with appropriate space/time scales
- hard to measure particle export by difference
- comparison to other particle flux collectors- traps

16. - consistency of oceanographic trends
ThE < 5%
EQPAC- example of low export efficiency
small cells, grazing controls & communities in “steady state”

17. - how does 234Th approach compare to other methods?
Example of what we can/should do as group?
Caveats w/each method & with space/time integral- 234Th no worse/better than others
- watch out for averaging differences
space/time/depth
& lack of budget constraints

18. - how does 234Th approach compare to other methods?
APF
Buesseler et al., 2003
- example w/complete annual cycle

19. - example w/complete annual cycle
APF
APF
Buesseler et al., 2003
- example w/complete annual cycle
Nelson et al., 2003

20. Carbon fluxes determined from 234Th approach
1. show oceanographic consistency
2. generally agree with other methods
3. advantages over other methods
SOFeX- at depth, C/Th 1 vs. 53 um particle difference <factor of two

21. Carbon fluxes determined from 234Th approach
1. show oceanographic consistency
2. generally agree with other methods
3. advantages over other methods
Errors arise from both Th flux prediction & C/Th
- need to quantify Th flux uncertainty (next talk!)
- local C/Th variability at a specific site/depth < factor 2
(differences between particle size, filter vs. traps, etc.)
- assumptions need to be clearly stated in each case
e.g. steady state, character of sinking particles,
spatial vs. temporal change, role of physical
loss/supply

22. ? Other examples of X/Y ratio variability in ocean sciences? Depth (m)20 40 60 80
100 5 10 15
Depth (m) ?
Found this data set- NOAA PMEL/AOML….
POC/234Th

23. Wanninkhof, 2003
Considerable research & debate on wind speed vs. gas transfer velocity relationship-
but exact processes responsible for variability remains poorly understood

24. However we still use: F = k ∆ pCO2
to calculate global CO2 flux from site/time specific transfer velocity, k, and >1,400,000 measurements of surface ocean CO2
Wanninkhof, 2003
Like C/Th- many processes control ratio- gas transfer changes w/winds, bubble injection, surface films, physics, chemistry
Make progress- community consensus important! Reason for this meeting
Considerable research & debate on wind speed vs. gas transfer velocity relationship-
but exact processes responsible for variability remains poorly understood
Takahashi et al., 2002

25. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

26. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
C/Th ratio is variable- w/size, depth, region-
- fine for application as empirical tool
- progress needed in sampling sinking particles
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

27. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
C/Th ratio is variable- w/size, depth, region-
- fine for application as empirical tool
- progress needed in sampling sinking particles
Evaluate accuracy by comparison to other methods
- proper space/time integration important-
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

28. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
C/Th ratio is variable- w/size, depth, region-
- fine for application as empirical tool
- progress needed in sampling sinking particles
Evaluate accuracy by comparison to other methods
- proper space/time integration important-
Expand applications to other elements
- PON, bSi, HOC’s, TM
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

29. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
C/Th ratio is variable- w/size, depth, region-
- fine for application as empirical tool
- progress needed in sampling sinking particles
Evaluate accuracy by comparison to other methods
- proper space/time integration important-
Expand applications to other elements
- PON, bSi, HOC’s, TM
Better understanding of 234Th speciation needed
- aggregation & scavenging; ligands & complexation
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

30. C/Th ratios- status report
We have made HUGE advances in our application of 234Th to determine upper ocean carbon flux since 1990 (oops, 1976!)
C/Th ratio is variable- w/size, depth, region-
- fine for application as empirical tool
- progress needed in sampling sinking particles
Evaluate accuracy by comparison to other methods
- proper space/time integration important-
Expand applications to other elements
- PON, bSi, HOC’s, TM
Better understanding of 234Th speciation needed
- aggregation & scavenging; ligands & complexation
Learn from CO2 community
- similar accuracy issues & unknowns
- community workshops, organized field efforts,
standardized protocols; synthesis of larger data sets
Can’t imagine more progress in many other fields in last 15 years!
4L method- 18 profiles in SOFeX- 28 days
Acid Polysacharide ligands
14C PProd- standardized method- argue over what it means, but helps to have one method

31. Thorium-234 approach & C/Th ratios
- many to thank & credit for ideas, inspiration, challenges…
Decided to take EndNote ref list and scramble first authors names to create this movie credit
NASA, NOAA, DOE, ONR

34. Carbon flux = 234Th flux  [C/234Th]sinking particles
Thtot/U
234Th flux
sinking
particles
C/Th / /4 1/2
C flux x 1/ x 2/ x 1/2
assumptions- measure C/Th representative of sinking particles
- sinking particles are only/major Th loss
- sinking particles average over same time/space as flux

35. EQPAC- 140W- boreal spring cruises
- how does 234Th approach compare to other methods?
EQPAC- 140W- boreal spring cruises
Method
Export C
(mM C/m2/d)
Reference
Comments
234Th
1-4
(range 10N - 10S)
Buesseler et al. ‘95
0-100m; C/Th pumps, varies with lat/long & season
3 (equator)
Bacon et al. ‘96
0-100m; C/Th pumps
6 (equator)
Murray et al. ‘96
0-100m; C/Th traps
tCO2 & DOC
<5
(1N - 1S budget)
Feely/Peltzer et al.
0-100m; by difference
New prod 15N
<6-12
McCarthy et al.
Flux at 100m
Traps
4 - 36
(equator)
Murray, Dunne et al.
Flux at 100m; PITS uncorrected (overcollect- 234Th) XX
Hernes et al.
Flux at 120m; IRS trap

36. - how does 234Th approach compare to other methods?
Ross Sea- JGOFS
Method
Export C
(mM C/m2/d)
Reference
Comments
234Th
(two stations)
Buesseler et al. ‘00
0-100m; summer to fall Orca & Minke; C/Th pumps
Del-POC
150
Smith et al ‘00
0-50m; avg. for summer 40
Gardner et al. ‘00
0-100m; summer to fall
Net Comm Prod 41
Sweeney et al ‘01
0-200m; summer to fall
Traps
1.5
Collier et al. ‘00
Flux at 200m; (undercollect-Th/Pa)

37. North Atlantic Bloom Experiment
- how does 234Th approach compare to other methods?
North Atlantic Bloom Experiment
Method
Export C
(mM C/m2/d)
Reference
Comments
234Th
Buesseler et al. ‘92
0-150m; varies with time and depth
tCO2 70
Goyet et al.
0-125m; avg. for bloom
Del-NO3 & O2
30-40
Bender et al. ‘92
Flux at 50m; early bloom only
Traps
7-13
Martin et al. ‘92
Flux at 150m; invariant over bloom

38. How does ratio vary w/region?
- C/Th higher in high lats & coastal
(100m >50 or 70 mm)
(90 to 200m traps)
(50-100m >.5-1 mm)
Different time integration
Different number- single pt vs. whole year
Murray, Dunne, Charette, Buesseler, Benitez-Nelson, Coppola, Fowler, Schmidt, Amiel, Cochran, Sarin, Shimmield, Moran, …….

39. How does ratio vary w/collection device?
- is one method “best”?
- if not, what does range tell us?
- more work is needed
new methods- SPLITT; NBST; tube to tube variability; bottles vs. pumps
BATS unpub. data
- watch out for difference in methods
e.g. trap C vs. filter C