Cross-shelf exchange in the Beaufort Sea

E. Sternberg-Bousserez1, H. Thomas1, P. van Beek2

1 Department of Oceanography, Dalhousie University, Halifax, Canada2 LEGOS, Université Paul Sabatier, Toulouse, France

GEOTRACES meeting, Liège, May 2-6, 2011

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

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 penetrationchemical: nutrients, trace metals, trace gases, stable isotopes, radioisotopes biological: primary and microbial productivity, TM phytoplankton quotas

The Radium isotopes

Four naturally-occuring Ra isotopes, decay products from U and Th in deep-sea and continental-shelf sediments:

224Ra: τ1/2 = 3.7 d ; 223Ra: τ1/2 = 11.4 d ; 228Ra: τ1/2 = 5.7 y ; 226Ra: τ1/2 = 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

The Radium isotopes

For a conservative, non-reactive tracer:

A = excess activity of the isotope from unsupported coastal sources, Kh = horizontal eddy diffusion coefficient, ω = horizontal advective velocity, λ = radiocative decay constant.

At steady state, if net advection can be neglected and Kh is constant:Ax = A0 exp (– x √(λ/Kh))

where Ax is the activity at distance x from the coast

Kh can thus be estimated by plotting ln Ax as a function of distance:ln Ax = ln A0 exp (– x √(λ/Kh))

with the slope m = √(λ/Kh)

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:

224Ra: seawater filtered on MnO2-coated fiber at a flowrate < 0.3 L.min-1; measurements with counter (RaDeCC)

226Ra & 228Ra: MnO2-fiber (onboard and ISP filtration) ashed; measurements with spectrometer

S1 S1.1

S1.2S2

L1.1

Radium-224

Higher 224Raxs at depth because of bottom source and short 224Ra half-life High 224Raxs 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)

ICEX

BC

39 34

224Ra

S1

L1.1

Kadko et al., 2005: high 224Raxs observed offshore as well

eddies, energetic wind forcing

BC transect

228Ra & 226Ra

226Ra ≈ cte, long half-life

228Ra decrease over shelf

R2=0.70

228Ra

Use of 228Ra to estimate diffusion coefficient?

Done in previous studies – HOWEVER: constant 228Ra inputs?

Kh ≈ 230 m2.s-1 224Raxs ≈ 0.17 dpm.100 L-1 at station S1…

228Ra/226Ra

‘zero age water’(SBI cruise, 2002; Kadko et al. 2005)

Elapsed time since residence on the shelf: T = - ln (R/R0) /

T3 waters ≈ 20 yrs residence time Beaufort shelf waters = aged waters ???

If R0 = 0.98, T ≈ 4 yrs, in agreement with Kadko et al. 2005

Conclusions & Perspectives

High 224Raxs signal offshore still unclear

Need for shelf 228Ra/226Ra 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…