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By Katsiaryna Pabortsava, Dr. Patrick Martin, Dr. Richard Sanders, Prof. Richard Lampitt

Longitudinal variability of export fluxes of particulate matter based on 234 Th budgets in the Atlantic Sector of the Southern Ocean along 40S [GEOTRACES Cruises D357 and JC068]. By Katsiaryna Pabortsava, Dr. Patrick Martin, Dr. Richard Sanders, Prof. Richard Lampitt JC068 Post-Cruise Meeting

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By Katsiaryna Pabortsava, Dr. Patrick Martin, Dr. Richard Sanders, Prof. Richard Lampitt

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  1. Longitudinal variability of export fluxes of particulate matter based on 234Th budgets in the Atlantic Sector of the Southern Ocean along 40S [GEOTRACES Cruises D357 and JC068] By Katsiaryna Pabortsava, Dr. Patrick Martin, Dr. Richard Sanders, Prof. Richard Lampitt JC068 Post-Cruise Meeting 10-11 September 2012

  2. Outline • Background • Objectives • Methods: model rationale, on-board and laboratory work • D357/JC068: current results • Conclusions/Summary • Future work

  3. Global Carbon Budget [IPCC Report 2007]

  4. Biological Carbon Pump [U.S. JGOFS]

  5. Variability in Carbon Export Flux 80N 40N 0 40S [Lutz et al , 2007] 0.05 0.01 1 10 50 Flux (g Corg m-2 yr-1)

  6. 2. Objectives • Verify Lutz et al 2007 Carbon Flux estimates in the Southern Atlantic Ocean • Quantify longitudinal variations in downward fluxes of particulate organic Carbon (POC), Nitrogen (PON), inorganic Carbon (PIC), biogenic Silica (BSi) , and Micronutrient Metals based on the measured 234Th budgets and identify causes of those variations • Relate the 234Th derived particle fluxes with • Micronutrients metal fluxes and their concentration profiles • Upward fluxes via mixing • Explore the relationships between nutrient cycling, phytoplankton community structure and variability of the 234Th-derived particle fluxes

  7. 3. Methods

  8. 3.1234Th Scavenging model • 234Th is a good proxy for estimation of particle formation, transport and dissolution • decay-product of naturally occurring conservative 238U (t1/2=4.5x109 yr) • short-lived isotope (t1/2=24.1 days) • particle reactive: removed from the surface with sinking particles creating 234Th : 238U disequilibrium

  9. 3.1234Th Scavenging model 234Th : 238U activity ratio • Secular equilibrium if234Th : 238U =1 • No particles • 234Th-deficit if 234Th : 238U <1 • Uptake by particles • 234Th-excess if 234Th : 238U >1 • Remineralization Activity,dpm L-1 0 234Th 238U Depth,m 500

  10. 3.1234Th Scavenging model • Total 234Th activity, dpm l-1 d-1: • Net loss of 234Th via sinking particles: • Integrated 234Th flux from surface to depth z at steady state: Flux Credits: F.A.C. Le Moigne

  11. 234Th Scavenging model • Chen et. al 1986 Uranium-salinity relationship : • Measure concentration of 234Th and POC on sinking particles: Flux Credits: F.A.C. Le Moigne

  12. 3.2 Sample collection at sea • Total 234Th – ‘small’ volume technique • From a CTD rosette: 4L water collected at different horizons from surface to depth • Higher sampling resolution in the upper 100m • Particulate 234Th, POC, PON, PIC, BSi • Stand Alone Pumps (SAPS) • 53um and 1um mesh size fractionation • 3-7 depths below the base of the mixed layer

  13. 3.3 Sample processing Th-234 co-precipitated with MnO2 RISØ low level β-counter SAPS Folsom Splitter

  14. 3.3 Sample processing • On-board • MnO2 co-precipitation of total 234Th; spiking with 230Th yield tracer • First 234Th activity count (total and particulate) on RISØ low level β-counter • Splitting and filtration of SAPS samples • Land Laboratory • Background activity count after 234Th completely decayed (~ 6 months) • Purification and quantification of 234Th recovery via anion-exchange chromatography and ICP-MS analysis • POC/PON analysis • PIC content from Ca measurements on ICP-OES • BSi content from Si measurement via wet alkaline digestion

  15. 4. D357/JC068 Provisional results

  16. D357: • 4 SAPS deployments at 1-3 depth below MLD • 6 CTD casts at 7-20 depths (0-400m) for total 234Th • 2 CTD casts are reoccupation stations • 1 deep CTD cast at >1000m depth for counter calibration • JC068: • 4 SAPS deployments at 3-7 depths below MLD • 10 CTD casts at 10-14 depths (0-400m) for total 234Th • 2 deep CTD casts at >1000m depth for counter calibration D357

  17. Current state of the D357/JC068 data • D357: All laboratory analyses and data processing were completed by Dr. Patrick Martin • JC068: • First Th-234 activity count = complete • Th-234 background activity count = in progress • Particulate (SAPS) samples are prepared for analyses

  18. D357/JC068 Th-234 Activity profiles East • JC068 Data • Background 234Th activity: 0.30±0.01cpm • Precipitation efficiency: 95±1% Mid-Ocean Ridge West

  19. D357/JC068 Th-234 Activity profiles at the reoccupied stations East West

  20. D357/JC068 Fluorescence and Nutrients profiles at the reoccupied stations Data: M.Woodward

  21. D357/JC068 Th-234 flux at 100m Th-234 flux dpm/m2/day AMT14 CRUISE D357 D357 reoc JC068 AMT14 AMT14 subs AMT14 AMT14 *AMT14 data from Thomalla et al 2006

  22. Cruise AMT14 subs AMT14 D357 r D357 JC068 ug/L

  23. Cruise AMT14 subs AMT14 D357 r D357 JC068 Data: M. Woodward uM

  24. Mean POC:Th-234 @120m: 2.37±1.038 Mean PON:Th-234 @120m: 0.40±0.147 Mean PIC:Th-234 @120m: 0.39±0.134 Mean BSi:Th-234 @120m: 0.035±0.024

  25. D357 Th-234 based integrated elemental fluxes at 120m

  26. Summary • Variations in depth-integrated Th-234 fluxes (D357 and JC068) and particle fluxes (D357) were observed along 40S transect • Depth-integrated Th-234 fluxes along 40S were significantly higher than the Th-234 fluxes measured to the North of the transect (in oligotrophic S. Atllantic (data from Thomalla et. al. 2006) ) • Lutz et. al. 2007 carbon export model in the southern ocean is realistic • Th-234-derived depth-integrated particle fluxes at 120m during D357 decreased away from the African shore westwards. • Temporal variations in depth-integrated Th-234 fluxes were observed at the reoccupied stations • Elemental measurements of the JC068 data have to be completed and data thoroughly analyzed to identify the processes responsible for the observed spatial and temporal variations

  27. Future work. Analytical procedures • Measure POC, PON, BSi, PIC content of the SAPS samples (end of October 2012) • Complete counting of 234Th background activity (mid-November, 2012) • Measure MnO2 precipitation efficiency: purify 230Th via anion-exchange chromatography andmeasure229Th : 230Th ratio on ICP-MS (November-December, 2012)

  28. Future work. D357/JC068 Data Analysis • Calculate vertical 234Th activity profiles and 234Th-derived downward biogenic and lithogenic fluxes of C, N, Si, Metals • Identify causes of spatial, temporalvariability, and variability with depth (incorporate D357 data) in particle fluxes along 40S transect • Composition of fluxes – what drives the carbon export in the area? • Compare derived carbon exports along the transect with model outputs (Lutz et al 2007)

  29. Acknowledgements • Dr. Gideon Henderson • Dr. Walter Geibert • Dr. F.A.C. Le Moigne • NMF technicians of JC068 • Science crew of JC068 • K. Kouvaris

  30. Thank you for your attention!

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