Tracking the fate of carbon in the ocean using thorium-234

Ken BuesselerDept. of Marine Chemistry and

GeochemistryWoods Hole Oceanographic Institution

Outline1. Background- the biological pump & why we care2. How 234Th works and history3. Examples- regional, vertical, small scale4. 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 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

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 fluxEfficiency – 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 NBSTFunnel 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 234Th flux from measured 234Th concentration

234Th/t = (238U - 234Th) - PTh + Vwhere decay rate; PTh = 234Th export flux; V = sum of advection & diffusion

• low 234Th = 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 = 238U parent is conservative

sinks = attachment to sinking particles and decay

depthdepth(m)(m)

[234Th]

238U

Euphoticzone

when Th < U- net loss of 234Th on sinking particles

238U234Th Chl-a

Applications on large scales 234Th from NW Pacific

Ez = depth at base

Buesseler et al., 2008, DSRI

Large scale differences are well captured by 234Th

Buesseler et al., 2008, DSRI

NW Pacific 234Th/238U <1Flux high

Hawaii 234Th/238U ~1Flux low

234Th234Th

238U Chl Chl

Chlorophyll-a (g kg-1)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

NO3 + NO2 (mol kg-1)

0 5 10

15

20

Den

sity

23.0

23.5

24.0

24.5

25.0

25.5

26.0

26.5

27.0

27.5

Thorium-234 (dpm l-1)

2.0

2.2

2.4

2.6

2.8

3.0

Euphoticzone

Th<Uparticleloss

Th>Uparticleremineralization

Evidence for a layered biological pump–captured by high vertical resolution 234Th at Bermuda

234Th

238U

Chl-adeep max ~ 120m

Ez

Buesseler et al., 2008

Carbon flux = 234Th flux [C/234Th]sinking

particles

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.

0 5 10 200 400

Dep

th (

m)0

100

200

300

400

500

5-20 m

20-51 m

51-350 m

CLAP

NBST

234Th loss = 10%(50-150m)

Ez

T100

Carbon loss = 50%

Ez

T100

x =

Th flux x POC/Th = POC flux

Use of 234Th as POC flux tracer requires both Th flux and C/Th ratio on sinking particles

- attenuation of POC flux always greater than 234Th(preferential consumption of POC by heterotrophs)

Ez

Ez

Ez +100m

Examples of different remineralization patterns

Most remin. in first 100m below EZ

POCflux

Thflux

Many now use 234Th for spatial mapping of C flux

234Th 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

bacteriazooplankton

- physical processesaggregation

- particle type/bioTEPballast

-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 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