Outline 1. Background- the biological pump & why we care 2. How 234 Th works and history

1. Tracking the fate of carbon in the ocean using thorium-234Ken BuesselerDept. of Marine Chemistry and GeochemistryWoods Hole Oceanographic Institution Outline 1. Background- the biological pump & why we care 2. How 234Th works and history 3. Examples- regional, vertical, small scale 4. Summary and new advances

2. The “Biological Pump” Combined biological processes which transfer organic matter and associated elements to depth - pathway for rapid C sequestration - flux decreases with depth -

3. Why care about the Biological Pump? - sinking particles provide a rapid link between surface and deep ocean - important for material transfer, as many elements “hitch a ride” - impact on global carbon cycle and climate - turning off bio pump would increase atmospheric CO2 by 200 ppm - increase remineralization depth by 24 m decreases atmos. CO2 by 10-27 ppm (Kwon et al., 2010) - food source for deep sea - large variability & largely unknown

4. A “geochemical” view of the Biological Pump Euphotic zone ~50 Pg C/yr ~5-10 Pg C/yr Twilight zone <1 Pg C/yr What controls the strength & efficiency of the biological pump? Strength – how much flux Efficiency – how much flux attenuation

5. A “geochemical” view of the Biological Pump Euphotic zone ~50 Pg C/yr ~5-10 Pg C/yr Twilight zone <1 Pg C/yr Variability poorly understood even after 20 years of time series study Regional differences-why? Bermuda Atlantic Time-Series (BATS) & Buesseler et al., Science,2007

6. NBST – neutrally buoyant sediment trap follows its local water parcel which is aimed to eliminate hydrodynamic collection issues Surface tethered sediment trap follows water motions (+ surface drag) integrated over the length of the tether Deep – bottom moored sediment trap trap is fixed to the bottom & water parcels flow past it

7. NBST 500 m – S=50 m/d – Dep 2 source funnels collection funnels Siegel et al. DSR-1 [2008]

8. NBST 500 m – VERTIGO - Hawaii 100 m/d 200 m/d 50 m/d Siegel et al. DSR-1 [2008] Source & collection funnels are 0 to 40 km from NBST Funnel displacements & directions vary w/ sinking speed

9. Sediment Trap Sampling of Export • Needs integration time of 2-5 days • Issues with … • Local hydrodynamics (flows within the trap) • Swimmers (zooplankton - both + & -) • Preservation of samples (poison yes or no) • Remote hydrodynamics (source funnels) • Sorting by sinking rate (w/ different source times) • Get samples to analyze in the lab

10. Thorium-234 approach for particle export [234Th] natural radionuclide half-life = 24.1 days source = 238U parent is conservative sinks = attachment to sinking particles and decay * * * * depth (m) * 238U * • Calculate 234Th flux from measured 234Th concentration • d234Th/dt = (238U - 234Th) *l - PTh + V • where l = decay rate; PTh = 234Th export flux; V = sum of advection & diffusion • low 234Th = high flux • need to consider non-steady state and physical transport

11. Applications on large scales 234Th from NW Pacific 234Th 238U Chl-a when Th < U - net loss of 234Th on sinking particles Euphotic zone Ez = depth at base Buesseler et al., 2008, DSRI

12. Large scale differences are well captured by 234Th NW Pacific 234Th/238U <1 Flux high Hawaii 234Th/238U ~1 Flux low 234Th 238U Chl Chl 234Th Buesseler et al., 2008, DSRI

13. Evidence for a layered biological pump– captured by high vertical resolution 234Th at Bermuda 234Th Th<U particle loss 238U Euphotic zone Chl-a deep max ~ 120m Ez Buesseler et al., 2008 Th>U particle remineralization

14. Carbon flux = 234Th flux  [C/234Th]sinking particles • POC/234Th highest in surface water • POC/234Th high in blooms (esp. large diatoms & high latitudes) • Issues remain regarding best methods to collect particles for C/Th • Must use site and depth appropriate ratio • exact processes responsible for variability remain poorly understood Moran et al.

15. Use of 234Th as POC flux tracer requires both Th flux and C/Th ratio on sinking particles Th flux x POC/Th = POC flux Ez Ez = x Ez T100 T100 234Th loss = 10% (50-150m) Carbon loss = 50% - attenuation of POC flux always greater than 234Th (preferential consumption of POC by heterotrophs)

16. Examples of different remineralization patterns Most remin. in first 100m below EZ Ez Ez + 100m POC flux Th flux

17. Many now use 234Th for spatial mapping of C flux 234Th flux C/Th POC flux South China Sea- Cai et al., 2008

18. But what controls spatial variability in export? - in subtropical N Pacific, ThE = 0-32% • Why? • - food web • bacteria • zooplankton • - physical processes • aggregation • - particle type/bio • TEP • ballast • physical variability at scales <10km adapted from Buesseler et al., 2009, DSRI

19. Summary- We’ve come a long way! Methods- from 1000 to 4 liters High resolution brings better quantification of: - euphotic zone export - vertical processes & remineralization below Ez - regional averages - mesoscale (& submeso?) variability Making progress on controls of export & flux attenuation - not just primary production - scale dependent (time/space) - physics- aggregation - food web- temperature, community structure - particle type- ballast, stickiness, size

20. New Advances Models - moving from steady state to non-steady state - include direct estimates of physical transport - 3D times series now possible Best to combine 234Th with sediment traps, particle filtration, cameras, bioptics , nutrient/C budgets Applications beyond C to N, Si, trace metals, organics Important to understand controls on biological pump in a changing climate - will biological pump increase/decrease in strength and efficiency? - significant impacts on atmospheric CO2