1. Cross-shelf exchange in the Beaufort Sea E. Sternberg-Bousserez 1, H. Thomas 1, P. van Beek 2 1 Department of Oceanography, Dalhousie University, Halifax, Canada 2 LEGOS, Université Paul Sabatier, Toulouse, France GEOTRACES meeting, Liège, May 2-6, 2011

2. The Canadian IPY-GEOTRACES program August 27 - September 12, 2009 Canadian research icebreaker CCGS Amundsen Beaufort Sea, Western Arctic Ocean Participating institutions: University of British Columbia (R. François, M. Maldonada, K. Orians, P. Tortell) University of Victoria (J. Cullen, D. Varela) University of Saskatchewan (C. Holmden) McGill University (A. Mucci) Dalhousie University of Newfoundland (H. Thomas, M. Kienast) Memorial University (R. Rivkin) Institute of Ocean Sciences (L. Miller) University of Portsmouth

3. The Canadian IPY-GEOTRACES program Sampling: Mackenzie River delta into the Beaufort Sea (shelf, slope and deep Canada Basin; 10 stations) Seawater, marine particles Goal: Investigate the impact of climate change on the nutrient and carbon cycles in the Arctic Ocean using a multi-tracer approach Parameters measured: physical: T, S, ice cover, light penetration chemical: nutrients, trace metals, trace gases, stable isotopes, radioisotopes biological: primary and microbial productivity, TM phytoplankton quotas

4. The Radium isotopes 3 RADIUM Four radioactive isotopes of radium occur naturally in the environment, generated as decay products from uranium and thorium in deep-sea and continental-shelf sediment: 223 Ra (half life τ 1/2 =11.4 days), 224 Ra (τ 1/2 =3.7 days), 226 Ra (τ 1/2 =1600 years) and 228 Ra (τ 1/2 =5.7 years) (Fig. 2). These isotopes have been used for decades to quantify lateral mixing processes between shelf waters and the open ocean (Koczy, 1958; Moore et al. 1980). However, until recently, radium analyses were primarily restricted to the two longer-lived Ra isotopes, which limited the utility of this tracer to time scales of months to years. A new technique for measuring 223 Ra and 224 Ra has greatly expanded the power of these tracers to include short-term mixing processes on time scales of days to weeks (Moore and Arnold, 1996; Moore, 2000). Cross-shelf carbon fluxes will be estimated using the radium quartet. Measuring their activities will allow the calculation of horizontal diffusion coefficients, as shown in Eq. 1, which in turn will allow the determination of the horizontal carbon fluxes. (1) where A is the excess activity of the isotope from unsupported coastal sources, κ is the horizontal eddy diffusion coefficient, ω is the horizontal advective velocity, and λ is the radiocative decay constant. Four naturally-occuring Ra isotopes, decay products from U and Th in deep-sea and continental-shelf sediments: 224 Ra: τ 1/2 = 3.7 d ; 223 Ra: τ 1/2 = 11.4 d ; 228 Ra: τ 1/2 = 5.7 y ; 226 Ra: τ 1/2 = 1600 y Used to quantify lateral mixing processes between shelf waters and the open ocean (time scales of months to years with the two longer-lived, days to weeks with the shorter-lived). Calculation of horizontal diffusion coefficients from the measurement of their activities, allowing the determination of horizontal fluxes. Our goal: Better understanding cross-shelf exchange in the Beaufort Sea using the radium isotopes

5. 3 RADIUM Four radioactive isotopes of radium occur naturally in the environment, generated as decay products from uranium and thorium in deep-sea and continental-shelf sediment: 223 Ra (half life τ 1/2 =11.4 days), 224 Ra (τ 1/2 =3.7 days), 226 Ra (τ 1/2 =1600 years) and 228 Ra (τ 1/2 =5.7 years) (Fig. 2). These isotopes have been used for decades to quantify lateral mixing processes between shelf waters and the open ocean (Koczy, 1958; Moore et al. 1980). However, until recently, radium analyses were primarily restricted to the two longer-lived Ra isotopes, which limited the utility of this tracer to time scales of months to years. A new technique for measuring 223 Ra and 224 Ra has greatly expanded the power of these tracers to include short-term mixing processes on time scales of days to weeks (Moore and Arnold, 1996; Moore, 2000). Cross-shelf carbon fluxes will be estimated using the radium quartet. Measuring their activities will allow the calculation of horizontal diffusion coefficients, as shown in Eq. 1, which in turn will allow the determination of the horizontal carbon fluxes. (1) where A is the excess activity of the isotope from unsupported coastal sources, κ is the horizontal eddy diffusion coefficient, ω is the horizontal advective velocity, and λ is the radiocative decay constant. The Radium isotopes  For a conservative, non-reactive tracer: A = excess activity of the isotope from unsupported coastal sources, K h = horizontal eddy diffusion coefficient, ω = horizontal advective velocity, λ = radiocative decay constant.  At steady state, if net advection can be neglected and K h is constant: A x = A 0 exp (– x √(λ/K h )) where A x is the activity at distance x from the coast  K h can thus be estimated by plotting ln A x as a function of distance: ln A x = ln A 0 exp (– x √(λ/K h )) with the slope m = √(λ/K h )

6. Sampling and Analysis Sampling:  ≈ 250 L sampled with rosette on the shelf/shelf break; ≈ 100 L offshore  Several hundred liters filtered using in situ pumps Analysis:  224 Ra: seawater filtered on MnO 2 -coated fiber at a flowrate < 0.3 L.min -1 ; measurements with  counter (RaDeCC)  226 Ra & 228 Ra: MnO 2 -fiber (onboard and ISP filtration) ashed; measurements with  spectrometer

7. S1 S1.1 S1.2 S2 L1.1 Radium-224  Higher 224 Ra xs at depth because of bottom source and short 224 Ra half-life  High 224 Ra xs offshore in surface waters (≈1 dpm.100 L -1 )  Stratification of the waters: upper 10 m separated from deeper waters by strong pycnocline (Km 0 = 50 m isobath)

8. ICEX BC 39 34 224 Ra S1 L1.1 Kadko et al., 2005: high 224 Ra xs observed offshore as well eddies, energetic wind forcing BC transect

9. 228 Ra & 226 Ra 226 Ra ≈ cte, long half-life 228 Ra decrease over shelf R 2 =0.70

10. 228 Ra Use of 228 Ra to estimate diffusion coefficient? Done in previous studies – HOWEVER: constant 228 Ra inputs? K h ≈ 230 m 2.s -1  224 Ra xs ≈ 0.17 dpm.100 L -1 at station S1…

11. 228 Ra/ 226 Ra ‘zero age water’ (SBI cruise, 2002; Kadko et al. 2005) Elapsed time since residence on the shelf: T = - ln (R/R 0 ) / T3 waters ≈ 20 yrs residence time  Beaufort shelf waters = aged waters ??? If R 0 = 0.98, T ≈ 4 yrs, in agreement with Kadko et al. 2005

12. Conclusions & Perspectives  High 224 Ra xs signal offshore still unclear  Need for shelf 228 Ra/ 226 Ra ratio closer to the MacKenzie River to better estimate water mass transport off the MacKenzie delta  Consideration of other parameters measured during the GEOTRACES cruise to help understand cross-shelf exchange  More radium data to come…