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 September 2012

Outline 1. Background 2. Objectives 3. Methods: model rationale, on-board and laboratory work 4. D357/JC068: current results 5. Conclusions/Summary 6. Future work

Global Carbon Budget [IPCC Report 2007]

Biological Carbon Pump [U.S. JGOFS]

Variability in Carbon Export Flux [Lutz et al, 2007] Flux (g C org m -2 yr -1 ) 80N 40N 0 40S

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 234 Th budgets and identify causes of those variations  Relate the 234 Th 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 234 Th-derived particle fluxes

3. Methods

Th Scavenging model  234 Th is a good proxy for estimation of particle formation, transport and dissolution  decay-product of naturally occurring conservative 238 U (t 1/2 =4.5x10 9 yr)  short-lived isotope (t 1/2 =24.1 days)  particle reactive: removed from the surface with sinking particles creating 234 Th : 238 U disequilibrium

234 Th : 238 U activity ratio  Secular equilibrium if 234 Th : 238 U =1 → No particles  234 Th-deficit if 234 Th : 238 U <1 → Uptake by particles  234 Th-excess if 234 Th : 238 U >1 → Remineralization 234 Th 238 U Activity, dpm L -1 Depth, m Th Scavenging model

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

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

3.2 Sample collection at sea  Total 234 Th – ‘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 234 Th, POC, PON, PIC, BSi  Stand Alone Pumps (SAPS)  53um and 1um mesh size fractionation  3-7 depths below the base of the mixed layer

3.3 Sample processing Th-234 co-precipitated with MnO2 SAPSFolsom Splitter RISØ low level β -counter

3.3 Sample processing  On-board  MnO2 co-precipitation of total 234 Th; spiking with 230 Th yield tracer  First 234 Th activity count (total and particulate) on RISØ low level β -counter  Splitting and filtration of SAPS samples  Land Laboratory  Background activity count after 234 Th completely decayed (~ 6 months)  Purification and quantification of 234 Th 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

4. D357/JC068 Provisional results

D357: 4 SAPS deployments at 1-3 depth below MLD 6 CTD casts at 7-20 depths (0-400m) for total 234 Th 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 depths (0- 400m) for total 234 Th 2 deep CTD casts at >1000m depth for counter calibration D357

 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 Current state of the D357/JC068 data

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

D357/JC068 Th-234 Activity profiles at the reoccupied stations EastWest

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

D357: Nitrate: Surface 500 m JC068: Nitrate+Nitrite: Surface 500 m Data: Malcolm Woodward

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

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

ug/L AMT14 subs AMT14 D357 r D357 JC068 Cruise

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

Mean 2.37±1.038 Mean 0.40±0.147 Mean 0.39±0.134 Mean 0.035±0.024

D357 Th-234 based integrated elemental fluxes at 120m

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 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

Future work. Analytical procedures  Measure POC, PON, BSi, PIC content of the SAPS samples (end of October 2012)  Complete counting of 234 Th background activity (mid-November, 2012)  Measure MnO 2 precipitation efficiency: purify 230 Th via anion-exchange chromatography and measure 229 Th : 230 Th ratio on ICP-MS (November-December, 2012)

Future work. D357/JC068 Data Analysis  Calculate vertical 234 Th activity profiles and 234 Th- derived downward biogenic and lithogenic fluxes of C, N, Si, Metals  Identify causes of spatial, temporal variability, 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)

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

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