Magnitude, variability and controls on the ratio of particle export to primary production in the upper ocean

Transcription

1 Magnitude, variability and controls on the ratio of particle export to primary production in the upper ocean Ken O. Buesseler Woods Hole Oceanographic Institution Talk Outline: What is the relationship between particulate export and primary production? short background Methods for primary productionproduction- satellites & 14C Methods for exportexport- traps; thoriumthorium-234; others What we have learned from JGOFS regarding shallow POC flux & primary production - magnitude; variability; controls 1

2 2 SeaWiFS derived global primary production (Behrenfeld & Falkowski, 1997) How do we get from here? To here.? C flux to seafloor (Jahnke,, 1996) Hypothetical bloom dynamics Primary Production POC Export export/production ratio - varies within bloom - varies between food webs Time lag between onset of primary production and POC export

3 3 Methods for primary production- 14 C incubations (in-situ vs. deck; TM & bottle issues) satellite derived products (surface Chl-a; O 2 balance (in bottles; regional balance) a; Pb opt ; etc) Methods for (shallow) POC export- sediment traps (swimmers; hydrodynamics) thorium-234 ( Th flux model; POC/Th ratio issues) particle counting (optical methods; in situ filtration) geochemical balance (inverse modeling; regional budgets) Shallow sediment traps - accuracy issues Swimmers Horizontal flow (hydrodynamics) Preservation (solubilization in collection tubes)

4 4 Shallow trap flux vs. primary production Bermuda Atlantic Time-Series (BATS) 8 trap flux at 15m 2% 2% No single relationship between production and export 1 primary production (all units mg C m - 2 d - 1 ) = Laws et al. 2 depth (m) Thorium-234 approach for estimating particle export * [ 234 Th] * * * * * 238 U half-life life = 24.1 days source = 238 U parent is conservative sinks = attachment to sinking particles and decay Calculate 234 Th flux from the measured 234 Th activities d 234 Th/dt = ( 238 U Th) * l - P Th + V where λ = decay rate; P Th = 234 Th export flux; V = sum of advection & diffusion low 234 Th = high flux need to consider non-steady state and physical transport

5 5 Thorium-234 approach for estimating POC export (& PON, biogenic Si, PAH s, PCB s) POC export = 234 Th flux [POC/ 234 Th ] sinking particles POC/ 234 Th highest in surface water POC/ 234 Th high in blooms (esp. diatoms/high latitudes) issues remaining regarding best methods to collect particles See references by: Buesseler, Bacon, Benitez-Nelson, Cai, Charette,, Cochran, Coppola, Dunne, Guo, Gustafsson,, Hall, Langone,, Miller, Moran, Murray, Pates, Roy-Barman, Rutgers vd Loeff, Santschi, Sarin,, Shimmield, Smith, Wei & more.. JGOFS North Atlantic Bloom Experiment 234 Th (dpm l -1 ) Th Nitrate Silicate April 25 May 5 May 15 May 25 Storm Nitrate or Silicate (mm l -1 ) POC Export (mm m -2 d -1 ) 5 25 Prim. Prod. avg. upper 35m POC export at 35m April 25 May 5 May 15 May Primary Production (mm m -2 d -1 )

6 6 POC flux vs. primary production POC flux derived from 234 Th (mmole C m -2 d -1 ) "ThE" = 5% 1% Primary Production (mmole C m -2 d -1 ) 2% NABE NABE POC export rates variable, but high during bloom ThE = 2-5% Note: ThE = POC export (from 234 Th)/Primary Production (from 14 C) POC flux (mm m -2 d -1 ) (from 234 Th- Buesseler et al.) JGOFS Equatorial Pacific Process Study Prim. Prod. POC Export EqPac 125 West (non El Nino) degrees North Latitude Primary Production (mm m -2 d -1 ) (Chavez et. al.)

7 7 POC flux vs. primary production POC flux derived from 234 Th (mmole C m -2 d -1 ) "ThE" = 5% 1% Primary Production (mmole C m -2 d -1 ) 2% NABE EqPac: all data BATS: March - October EQPAC POC export rates low Bermuda export also generally low (watch out for eddies!) ThE = 2-1% 2 High C flux (low thorium-234) associated with bloom of large diatoms during Monsoon Garrison et al. Buesseler et al.

8 8 POC flux vs. primary production POC flux derived from 234 Th (mmole C m -2 d -1 ) "ThE" = 5% 1% Primary Production (mmole C m -2 d -1 ) 2% NABE EqPac: all data BATS: March - October Arabian Sea: Jan. - July Arabian Sea: Aug./Sept. Arabian Sea POC export variable Low export, except at end of SW Monsoon ThE = 2-25% 2 25% Diatom link AESOPS 4 cruises Oct. March Examine production & export balance manuscript by: Buesseler, Barber, Dickson, Hiscock, Moore, Sambrottoaccepted DSRII

9 9 5 Primary production Particulate organic carbon export 5 Degrees Latitude South ice APF 26-Oct 15-Nov 5-Dec 25-Dec 14-Jan 3-Feb 23-Feb 15-Mar =1 mmcm -2 d -1 PProd; =5 mmcm -2 d -1 POC Primary Production highest in December all latitudes 26-Oct 15-Nov 5-Dec 25-Dec 14-Jan 3-Feb 23-Feb 15-Mar POC flux increases Jan/Feb 6-65 S and Feb/Mar S - See production & export time lag - Large spatial and temporal variability Use satellite Chlorophyll to extrapolate seasonal patterns of production (calibration to ship Chl-a measurements OK) APF ice Note ice edge blooms SeaWiFS data along 17W (1 lat x 3 long weekly bins)

10 1 Seasonal primary productivity model (Behrenfeld & Falkowski) Measured and calculated PProd agree See 2x higher PProd than traditional B&F would predict- higher P^Bopt used here Missed SIZ bloom peaks on ship PProd (mm/m 2 /d) PProd (mm/m 2 /d) PProd (mm/m 2 /d) PProd (mm/m 2 /d) PProd (mm/m 2 /d) PProd (mm/m 2 /d) APF average Ice % average Ice % average Ice % 5 6-Oct 26-Oct 15-Nov 5-Dec 25-Dec 14-Jan 5-55 S S S S S S 3-Feb 23-Feb 15-Mar % ice coverage % ice coverage % ice coverage Calculate seasonal balance of Production and export from ship measurements and extrapolate using satellite based PProd model Primary Production Particulate Organic Carbon flux (mol Carbon m -2 y -1 ) APF 6 Latitude South 55 5 Primary Production deceases towards south Particle export/pprod = 16-64% Especially high in south Despite low PProd, shallow POC flux is relatively high i.e. biological pump is very efficient!

11 11 POC flux vs. primary production POC flux derived from 234 Th (mmole C m -2 d -1 ) "ThE" = 5% 1% Primary Production (mmole C m -2 d -1 ) 2% NABE EqPac: all data BATS: March - October Arabian Sea: Jan. - July Arabian Sea: Aug./Sept. High Latitudes Southern Ocean High POC export, even at low PProd ThE = 15 to >5% Diatom link Can we parameterize export flux from the upper ocean? POC flux Suess et al. '8 Berger et al. '87 Pace et al. '87 (? Bishop/Karl et al. '96?) Primary Production 1m Export = Primary Production ß z NO SeaWiFS PProd from Behrenfeld AND Laws temp. dependent food web model (Laws et al., )?

12 12.8 POC flux/primary Production Laws et al. AESOPS seasonal avg Temperature For 17 W from Subantarctic Zone to Ross Sea- see different relationship export ratio vs. temperature than used in Laws et al. If extrapolated to Southern Ocean (>5 S) Export ratio = 35% (vs. 28% in Laws et al.) POC flux = 1.8 Gt C/yr (vs..62 in Laws et al.) or 1 Gt C/yr in Schlitzer POC flux/primary Production AESOPS seasonal avg Si (um).7 POC flux/primary Production AESOPS seasonal avg Avg. Mixed Layer (m) POC flux/primary Production Fv/Fm (Feb/Mar only) In Southern Ocean- highest export ratios associated with: coldest, southernmost high Si waters, low photosynthetic eff.; shallow MLZ s; ; both centric diatoms & Phaeocystis blooms

13 13 Remember, what we think we know about particle export differs depending upon methodology - often Shallow Traps Thorium-234 approach Upper Ocean Export efficiencies (Export/Prim. Prod) NABE EqPAC Ross Sea 234 Th-derived High (1-3%) Low (1-1%) High (25->75%) Traps Low High Low Advances will depend upon uniform and consistent methodologies Conclusions- particle export/production efficiency Hard to measure directly - severely data limited; ratios vary widely Important not to mix methods - apples vs. oranges Can see regional &/or food web controls - role of diatoms in shallow flux Seasonal & temporal dynamics important - integration across season critical Large uncertainties remain in shallow POC flux budgets - both Pprod & export in So. Ocean may be underestimated Challenge to find simple parameterization for export efficiency that works globally

14 14 Future To further constrain the ocean C sink, a better understanding of export rates and foodweb/export controls will be needed - time-series studies - lagrangian style studies (FeEx) - regional comparisons & ecosystem studies - use (inverse) models to provide scales & hypothesis to test Remineralization rates of C:N:P:Si:Fe important - studies of twilight zone processes between 1-1m 1m important Improved field methods will help - neutrally buoyant traps; 2L Th-234 Southern Ocean US JGOFS PFZ = Polar Frontal Zone (boundary where N- flowing waters sink; strong?temp) POOZ = Permanently Open Ocean Zone (varies in extentnarrow along 17 W) SIZ = Seasonal Ice Zone (area = Antarctic continent; short growth season; melt water effects) Buesseler, Barber, Dickson, Hiscock, Moore, Sambrotto- accepted

15 15 SeaWiFS data along 17W (55-7 S) ice Cruise track = white lines Note in So. Ocean considerable cloud cover (black) Use weekly averages to extrapolate to seasonal avg. (5-75 S) ice Note ice edge blooms POC flux (mm C m -2 d -1 ) JGOFS Arabian Sea Process Study Jan. - July Aug./Sept. 5% 1% Primary Production (mmole C m -2 d -1 ) 2% 1m Th surface flux (dpm/m -2 at 25m) 12 Aug./Sept. 8 March Diatiom pigments (Fucoxanthin -5m, mg m -2 ;Bidigare, Goericke, et. al.)

16 16 Summary of JGOFS results: No single export:production relationship JGOFS 234 Th studies provide significant insights Export efficiency (ThE ratio) <5% to >5% Efficiency of biological pump tied to foodweb diatom cycle important Significant time lag between production and export seasonal/episodic variability So where does this leave us wrt models and JGOFS synthesis? 5-59 S; N-PFZ & C-SAZ Always low Si Iron <.2 nm Generally deeper MLZ Smaller phytoplankton High photo efficiency Low particle flux S; PFZ & S-PFZ High Si front moves south Iron starts >.2 nm & decreases Shallow spring MLZ Centric diatom bloom moves S High photo efficiency High POC and highest bsi flux S; S-ACC & N-RS High Si always Iron <.2 nm Shallow spring MLZ Ice edge species only Low photo efficiency High POC flux