Tracking the fate of carbon in the ocean using thorium-234 Ken Buesseler Dept. of Marine Chemistry...
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Transcript of Tracking the fate of carbon in the ocean using thorium-234 Ken Buesseler Dept. of Marine Chemistry...
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