Tracking the fate of carbon in the ocean using thorium-234 Ken - - PowerPoint PPT Presentation

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Tracking the fate of carbon in the ocean using thorium-234 Ken - - PowerPoint PPT Presentation

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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

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SLIDE 1

Tracking the fate of carbon in the

  • cean 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 234Th works and history
  • 3. Examples- regional, vertical, small scale
  • 4. Summary and new advances
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SLIDE 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

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SLIDE 3

Why care about the Biological Pump?

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SLIDE 4

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
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SLIDE 5

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

  • f the biological pump?

Strength – how much flux Efficiency – how much flux attenuation

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SLIDE 6

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

  • f time

series study

Regional differences

  • why?

Bermuda Atlantic Time-Series (BATS) & Buesseler et al., Science,2007

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SLIDE 7

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

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

depth (m)

[234Th]

238U

* * * * * *

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SLIDE 8

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.

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SLIDE 9

Bhat, Krishnaswami, Lal, Rama & Moore, 1969

First measurements of 234Th in the ocean

234Th lower

near coast due to higher particle flux

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SLIDE 10

First use of 234Th as C flux tracer

 JGOFS North Atlantic Bloom Experiment

Cochran, Buesseler

Kiel March 1990 PI meeting

Buesseler et al., 1992 Deep-Sea Res.

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SLIDE 11

First use of 234Th as C flux tracer

 No, much earlier!

1976 Tsunogai & Minagawa (note C/Th ratio = 5 µM/dpm C flux @ 100m = 9 mmC/m2/d)

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SLIDE 12

234Th now widely applied in ocean sciences

Today 100’s of papers with 1000’s data points

  • Fig. from Waples et al., 2006
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SLIDE 13

Thorium-234 (dpm l-1)

1 2 3

Density

25.2 25.6 26.0 26.4 26.8

Total Chl-a (ng l-1)

500 1000

K2

~20 m ~40 m ~30 m ~180 m ~60 m ~300 m

Euphotic zone when Th < U

  • net loss of

234Th on sinking

particles

238U 234Th

Chl-a

Applications on large scales

234Th from NW Pacific

Ez = depth at base Buesseler et al., 2008, DSRI

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SLIDE 14

Thorium-234 (dpm l-1)

1 2 3

Density

25.2 25.6 26.0 26.4 26.8

Total Chl-a (ng l-1)

500 1000

K2

~20 m ~40 m ~30 m ~180 m ~60 m ~300 m

Large scale differences are well captured by 234Th

Buesseler et al., 2008, DSRI

NW Pacific 234Th/238U <1 Flux high Hawaii 234Th/238U ~1 Flux low

234Th 234Th 238U

Chl Chl

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SLIDE 15

Chlorophyll-a (g kg-1)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

NO3 + NO2 (mol kg-1)

5 10 15 20

Density

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

Euphotic zone Th<U particle loss Th>U particle remineralization

Evidence for a layered biological pump–

captured by high vertical resolution 234Th at Bermuda

234Th 238U

Chl-a deep max ~ 120m

Ez Buesseler et al., 2008

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SLIDE 16

High vertical resolution allows one to calculate % flux remineralized

Maiti, Benitez-Nelson and Buesseler, GRL, 2010

  • 20 depths

in top 200 m

  • 15% remin.

below EZ

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SLIDE 17

Magnitude of 234Th-excess is related to 3 factors

% remineralization 34Th flux remin. layer thickness

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SLIDE 18

5 10 200 400 Depth (m) 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

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SLIDE 19

Ez Ez +

100m

Examples of different remineralization patterns

Most remin. in first 100m below EZ

POC flux

Th flux

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SLIDE 20

Many now use 234Th for spatial mapping of C flux

234Th flux

C/Th POC flux South China Sea- Cai et al., 2008

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SLIDE 21

Rutgers van der Loeff et al. 2011

total 234Th

  • diss. 234Th
  • part. 234Th

Considerable spatial variability in surface export

Particulate 234Th mirrors plankton abundance Highest export associated with blooms and high particulates Lowest 234Th associated with dissolved Mn and Fe removal

38ºS 68ºS Th/U =1

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SLIDE 22

1995 Gordon Research Conference- “ThE” ratio

Some regional patterns emerge

  • high during blooms
  • esp. diatoms
  • high at polar

regions

  • low in warm

waters ThE = POC flux from 234Th/ net primary production

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SLIDE 23

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

  • variability at

scales <10km

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SLIDE 24

Henson et al., 2011

Global compilations of 234Th export now possible

Temperature effect on heterotrophic recycling

  • lower ThE in

warm waters Lower global export than suggested by

  • ther methods
  • what does this

tell us?

  • what controls

scatter?

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SLIDE 25

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
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SLIDE 26

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