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

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?

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 10-27 ppm (Kwon et al., 2010) - food source for deep sea - large variability & largely unknown

A “geochemical” view of the Biological Pump Euphotic ~50 Pg C/yr zone ~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

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

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

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.

First measurements of 234 Th in the ocean 234 Th lower near coast due to higher particle flux Bhat, Krishnaswami, Lal, Rama & Moore, 1969

First use of 234 Th as C flux tracer  JGOFS North Atlantic Bloom Experiment Cochran, Buesseler Kiel March 1990 PI meeting Buesseler et al., 1992 Deep-Sea Res.

First use of 234 Th as C flux tracer  No, much earlier! 1976 Tsunogai & Minagawa (note C/Th ratio = 5 µM/dpm C flux @ 100m = 9 mmC/m 2 /d)

234 Th now widely applied in ocean sciences Fig. from Waples et al., 2006 Today 100 ’s of papers with 1000 ’s data points

Applications on large scales 234 Th from NW Pacific Thorium-234 (dpm l -1 ) 0 1 2 3 234 Th 238 U Chl-a 25.2 ~20 m when Th < U - net loss of 25.6 ~30 m Euphotic 234 Th on sinking zone particles Density 26.0 ~40 m 26.4 ~60 m Ez 26.8 ~180 m = depth at base ~300 m K2 0 500 1000 Total Chl-a (ng l -1 ) Buesseler et al., 2008, DSRI

Large scale differences are well captured by 234 Th NW Pacific 234 Th/ 238 U <1 Hawaii 234 Th/ 238 U ~1 Flux high Flux low Thorium-234 (dpm l -1 ) 0 1 2 3 25.2 ~20 m 234 Th 238 U Chl 25.6 ~30 m Chl 234 Th Density 26.0 ~40 m 26.4 ~60 m 26.8 ~180 m ~300 m K2 0 500 1000 Total Chl-a (ng l -1 ) Buesseler et al., 2008, DSRI

Evidence for a layered biological pump – captured by high vertical resolution 234 Th at Bermuda NO 3 + NO 2 (  mol kg -1 ) Chlorophyll-a (  g kg -1 ) Thorium-234 (dpm l -1 ) 2.0 2.2 2.4 2.6 2.8 3.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 10 15 20 0 5 23.0 23.5 234 Th Th<U 24.0 Euphotic 238 U particle 24.5 zone Density loss 25.0 Chl-a 25.5 deep max ~ 120m 26.0 Ez 26.5 27.0 27.5 Buesseler et al., 2008 Th>U particle remineralization

High vertical resolution allows one to calculate % flux remineralized - 20 depths in top 200 m - 15% remin. below EZ Maiti, Benitez-Nelson and Buesseler, GRL, 2010

Magnitude of 234 Th-excess is related to 3 factors % remineralization 34 Th flux remin. layer thickness

Use of 234 Th as POC flux tracer requires both Th flux and C/Th ratio on sinking particles Th flux x POC/Th = POC flux 0 5 10 200 400 0 100 E z = x E z 200 E z Depth (m) 300 NBST 400 CLAP 5-20  m T 100 20-51  m T 100 500 51-350  m 234 Th loss = 10% Carbon loss = 50% (50-150m) - attenuation of POC flux always greater than 234 Th (preferential consumption of POC by heterotrophs)

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

Considerable spatial variability in surface export Th/U =1 Particulate 234 Th total 234 Th mirrors plankton abundance diss. 234 Th part. 234 Th Highest export associated with blooms 38ºS 68ºS and high particulates Lowest 234 Th associated with dissolved Mn and Fe removal Rutgers van der Loeff et al. 2011

1995 Gordon Research Conference- “ ThE” ratio ThE = POC flux from 234 Th/ net primary production Some regional patterns emerge - high during blooms esp. diatoms - high at polar regions - low in warm waters

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 - variability at scales <10km adapted from Buesseler et al., 2009, DSRI

Global compilations of 234 Th export now possible Temperature effect on heterotrophic recycling - lower ThE in warm waters Lower global export than suggested by other methods - what does this tell us? - what controls scatter? Henson et al., 2011

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