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

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

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 CO 2 by 200 ppm - increase remineralization depth by 24 m decreases atmos. CO 2 by ppm (Kwon et al., 2010) - food source for deep sea - large variability & largely unknown

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

A “geochemical” view of the Biological Pump Euphotic zone Twilight zone ~50 Pg C/yr ~5-10 Pg C/yr <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

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

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

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

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

Calculate 234 Th flux from measured 234 Th concentration  234 Th/  t = ( 238 U Th)  - P Th + V where  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 Thorium-234 approach for particle export natural radionuclide half-life = 24.1 days source = 238 U parent is conservative sinks = attachment to sinking particles and decay depth(m) [ 234 Th] 238 U      

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

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

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

Carbon flux = 234 Th flux  [C/ 234 Th] sinking particles POC/ 234 Th highest in surface water POC/ 234 Th 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.

234 Th loss = 10% (50-150m) EzEz T 100 Carbon loss = 50% EzEz T 100 x = Th flux x POC/Th = POC flux Use of 234 Th as POC flux tracer requires both Th flux and C/Th ratio on sinking particles - attenuation of POC flux always greater than 234 Th (preferential consumption of POC by heterotrophs) EzEz

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

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

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

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

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 234 Th 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 CO 2