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

Transcription

1 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

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

3 Why care about the Biological Pump?

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 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

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 of the biological pump? Strength how much flux Efficiency how much flux attenuation

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 of time series study Regional differences -why? Bermuda Atlantic Time-Series (BATS) & Buesseler et al., Science,2007

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

8 Carbon flux = 234 Th flux [C/ 234 Th] sinking particles Moran et al. 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

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

10 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.

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

12 234 Th now widely applied in ocean sciences Fig. from Waples et al., 2006 Today 100 s of papers with 1000 s data points

13 Applications on large scales 234 Th from NW Pacific Thorium-234 (dpm l -1 ) Th 238 U Chl-a ~20 m when Th < U - net loss of 234 Th on sinking particles Density ~30 m ~40 m Euphotic zone 26.4 ~60 m 26.8 K2 ~180 m ~300 m Ez = depth at base Total Chl-a (ng l -1 ) Buesseler et al., 2008, DSRI K2 bottle summary 234Th and ancillary.jnb July 2008

14 Large scale differences are well captured by 234 Th NW Pacific 234 Th/ 238 U <1 Flux high Thorium-234 (dpm l -1 ) Hawaii 234 Th/ 238 U ~1 Flux low 25.2 ~20 m Th 238 U ~30 m Chl 234 Th Chl Density 26.0 ~40 m 26.4 ~60 m 26.8 ~180 m K2 ~300 m Total Chl-a (ng l -1 ) Buesseler K2 bottle summary 234Th et and ancillary.jnb al., July 2008, DSRI

15 Evidence for a layered biological pump captured by high vertical resolution 234 Th at Bermuda Thorium-234 (dpm l -1 ) NO 3 + NO 2 ( mol kg -1 ) Chlorophyll-a ( g kg -1 ) Th Th<U particle loss Density U Chl-a deep max ~ 120m Ez Euphotic zone Th>U particle remineralization Buesseler et al., 2008

16 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

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

18 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 E z 100 E z 200 x = 300 Depth (m) 400 NBST CLAP 5-20 m E z T m m T Th loss = 10% (50-150m) - attenuation of POC flux always greater than 234 Th (preferential consumption of POC by heterotrophs) Carbon loss = 50%

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

20 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

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

22 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

23 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

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

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

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 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