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Radium-228: Application to thermocline mixing studies
EARTH AND PLANETARY SCIENCE LETTERS 16 (1972) 421-422. NORTH-HOLLAND PUBLISHING COMPANY
RADIUM-228: APPLICATION TO THERMOCLINE MIXING STUDIES Willard S. MOORE U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland, USA Received 20 April 1972 Revised version received 17 July 1972 Radium-228 is easily measurable in surface ocean waters but its concentration decreases abruptly in the thermocline. This strong gradient should prove useful in studies of the exchange rate of water through the main thermocline. Radium-228 should complement cosmic-ray-produced and fallout isotopes which have been applied to such studies.
1. Introduction Fallout and cosmic-ray-produced isotopes have been used to estimate the rate of mixing through the main ocean thermocline. All of these isotopes are introduced intermittently primarily at the ocean-atmosphere interface. The transitory nature of their distributions has led to difficulty in directly applying concentration gradients to mixing rates. The dual origins of 14C and 3H make their distributions especially complicated. This report shows that unsupported radium-228, a natural 232Th-series isotope present in ocean surface waters, also offers promise in such mixing rate studies. Radium-228 has a 6.7 year half-life and is easily measurable in surface waters; typical values in the open Atlantic range from 1 - 3.5 dpm/100 kg [ 1]. The apparent source of this 228Ra is diffusion from 232Thbearing nearshore and continental shelf sediments [ 1].
2. Experimental techniques A detailed profile of large volume (600 1) samples was collected from the USC&GSS Discoverer within 25 miles of 10°N 44°W in February 1969. The sampling and analytical procedures have been described in detail by Moore [1]. The water was raised to the surface in a Niskin Bag and pumped into a fiberglass tank. Barium nitrate was added to co-precipitate radium sulfate after a 20 liter split of the sample had
been reserved for 226Ra measurements. The BaSO4, containing 70 - 90% of the radium in the water sample, was returned to the laboratory where it was purified and converted to the chloride form. Thorium was quantitatively extracted and the sample was aged 20 months to allow growth of 228Th, an alpha-emitting daughter of 228Ra. The sample was then spiked with 230Th and, after purification, the 228Th/23°Th activity ratio was measured in an alpha spectrometer. The exact recovery of radium from the water was determined by comparing the 226Ra content of the barium concentrate with the 226Ra present in the 20 liter split of the original sample.
3. Results and discussion The large volume water profile collected from the surface to 1025 m in the equatorial Atlantic Ocean revealed a rapid decrease of radium-228 in the top 225 m of the water column. Below 300 m 228Ra activity was negligible (table 1). The abrupt disappearance occurs near the top of the pycnocline in this area (fig. 1) and closely resembles profiles of tritium in the equatorial Atlantic [2]. If a steady-state system, negligible advective movements and particle settling of 228Ra are assumed, the concentration as a function of depth is C 2 = C 1 exp ( - [)t228/Dz] 1/2z) ,
W.S. Moore, Radium-228: Application to thermocline mixing studies
422
eddy diffusion. Solving this equation between 0 and 150 m givesDz = 0.1 cm2/sec and between 1 5 0 - 2 2 5 m , D z = 0.5 cm2/sec. Of course to adequately describe the vertical mixing in this area, several profdes should be collected with more samples in the upper 300 m. The purpose of this study was not to characterize the mixing in this area as much as it was to investigate the distribution of 228Ra in the thermocline so that future studies could better plan sampling depths.
Table 1 Radium-228 profile at 10°N. 44°W. Depth (m)
0 150 225 375 600 1025
228Ra/226Ra
228Ra
Activity ratio
(dpm/100 kg)
0.20 0.014 0.007 < 0.003 0.004 < 0.003
1.8 0.13 0.07 < 0.03 0.04 < 0.03
-*0.0l -+ 0.003 -+ 0.003 + 0.003
-+0.1 +- 0.03 -+ 0.03 +- 0.03
A reagent blank equal to 0.2 dpm 226Ra and 0.06 dpm 228Ra per sample has been subtracted from these results.
The abrupt decrease of 228Ra in the pycnocline confirms low vertical mixing rates in this area [2]. Because it is a natural system, the 228Ra distribution in the ocean should approach steady state. Although there are limited data on the time-variability of 228Ra in the ocean, the consistency of growth rate measurements on massive coral heads using 228Ra and 90Sr [3] lend support to a steady-state hypothesis. Ocean mixing models based on such a system are inherently simpler than those based on transitory species.
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Acknowledgements
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4. Conclusions
d,
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,~
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Fig. 1. 228Ra concentration and density as a function of depth at 10°N 44°W. The heavy dashed line between the surface and 150 m corresponds to a vertical eddy diffusion coefficient (VEDC) of 0.1 cm2/sec. The light dashed line between 50 and 150 m corresponds to a VEDC o f 0.05 cm2/sec. There is no data in this region to suggest which curve the 228Ra distribution follows. From 150 to 225 m the heavy dashed line corresponds to a VEDC of 0.5 cm2/sec.
where C 1 and C 2 are concentrations at two depths, z the depth interval, )k228 the decay constant of 228Ra (3.3 X 10-9 sec-1), andD z the coefficient of vertical
I thank Feder Ostapoff of ESSA (now NOAA) and the officers and crew of the USC&GSS Discoverer for their assistance in sample collections. Financial support for sample collections was provided by AEC contract (30-1) 3629 to the State University of New York at Stony Brook. Professor D. Lal of the Tata Institute of Fundamental Research provided hospitality during most of the measurements. The comments of Peter Vogt, B.L.K. Somayajulu, and S. Krishnaswamy served to improve the manuscript.
References [1 ] W.S. Moore, Oceanic Concentrations of Radium-228, Earth Planet. Sci. Letters 6 (1969) 437. [2] H.G. Ostlund, M.O. Rinket and C. Rooth, Tritium in the equatorial Atlantic Current System, J. Geophys. Res. 74 (1969) 4535. [3] W.S. Moore, S. Krishnaswami and S. Bhat, Radiometric Determinations of Coral Growth Rates, in: Thomas Goreau Memorial Volume, E. Graham, Ed. (University of Miami, Miami), in press.
RADIUM-228: APPLICATION TO THERMOCLINE MIXING STUDIES Willard S. MOORE U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland, USA Received 20 April 1972 Revised version received 17 July 1972 Radium-228 is easily measurable in surface ocean waters but its concentration decreases abruptly in the thermocline. This strong gradient should prove useful in studies of the exchange rate of water through the main thermocline. Radium-228 should complement cosmic-ray-produced and fallout isotopes which have been applied to such studies.
1. Introduction Fallout and cosmic-ray-produced isotopes have been used to estimate the rate of mixing through the main ocean thermocline. All of these isotopes are introduced intermittently primarily at the ocean-atmosphere interface. The transitory nature of their distributions has led to difficulty in directly applying concentration gradients to mixing rates. The dual origins of 14C and 3H make their distributions especially complicated. This report shows that unsupported radium-228, a natural 232Th-series isotope present in ocean surface waters, also offers promise in such mixing rate studies. Radium-228 has a 6.7 year half-life and is easily measurable in surface waters; typical values in the open Atlantic range from 1 - 3.5 dpm/100 kg [ 1]. The apparent source of this 228Ra is diffusion from 232Thbearing nearshore and continental shelf sediments [ 1].
2. Experimental techniques A detailed profile of large volume (600 1) samples was collected from the USC&GSS Discoverer within 25 miles of 10°N 44°W in February 1969. The sampling and analytical procedures have been described in detail by Moore [1]. The water was raised to the surface in a Niskin Bag and pumped into a fiberglass tank. Barium nitrate was added to co-precipitate radium sulfate after a 20 liter split of the sample had
been reserved for 226Ra measurements. The BaSO4, containing 70 - 90% of the radium in the water sample, was returned to the laboratory where it was purified and converted to the chloride form. Thorium was quantitatively extracted and the sample was aged 20 months to allow growth of 228Th, an alpha-emitting daughter of 228Ra. The sample was then spiked with 230Th and, after purification, the 228Th/23°Th activity ratio was measured in an alpha spectrometer. The exact recovery of radium from the water was determined by comparing the 226Ra content of the barium concentrate with the 226Ra present in the 20 liter split of the original sample.
3. Results and discussion The large volume water profile collected from the surface to 1025 m in the equatorial Atlantic Ocean revealed a rapid decrease of radium-228 in the top 225 m of the water column. Below 300 m 228Ra activity was negligible (table 1). The abrupt disappearance occurs near the top of the pycnocline in this area (fig. 1) and closely resembles profiles of tritium in the equatorial Atlantic [2]. If a steady-state system, negligible advective movements and particle settling of 228Ra are assumed, the concentration as a function of depth is C 2 = C 1 exp ( - [)t228/Dz] 1/2z) ,
W.S. Moore, Radium-228: Application to thermocline mixing studies
422
eddy diffusion. Solving this equation between 0 and 150 m givesDz = 0.1 cm2/sec and between 1 5 0 - 2 2 5 m , D z = 0.5 cm2/sec. Of course to adequately describe the vertical mixing in this area, several profdes should be collected with more samples in the upper 300 m. The purpose of this study was not to characterize the mixing in this area as much as it was to investigate the distribution of 228Ra in the thermocline so that future studies could better plan sampling depths.
Table 1 Radium-228 profile at 10°N. 44°W. Depth (m)
0 150 225 375 600 1025
228Ra/226Ra
228Ra
Activity ratio
(dpm/100 kg)
0.20 0.014 0.007 < 0.003 0.004 < 0.003
1.8 0.13 0.07 < 0.03 0.04 < 0.03
-*0.0l -+ 0.003 -+ 0.003 + 0.003
-+0.1 +- 0.03 -+ 0.03 +- 0.03
A reagent blank equal to 0.2 dpm 226Ra and 0.06 dpm 228Ra per sample has been subtracted from these results.
The abrupt decrease of 228Ra in the pycnocline confirms low vertical mixing rates in this area [2]. Because it is a natural system, the 228Ra distribution in the ocean should approach steady state. Although there are limited data on the time-variability of 228Ra in the ocean, the consistency of growth rate measurements on massive coral heads using 228Ra and 90Sr [3] lend support to a steady-state hypothesis. Ocean mixing models based on such a system are inherently simpler than those based on transitory species.
O"t 24
0
I l
I00
.
25
.
.
.
26
27
.
. ~' -'O'~
~
~ ' ~
i I
0 - ~
a)
E
30C
l
Acknowledgements
400
soo
4. Conclusions
d,
o.~ Ro 2 2 8
,5 (dpm/lOOk ~})
,~
2,0
Fig. 1. 228Ra concentration and density as a function of depth at 10°N 44°W. The heavy dashed line between the surface and 150 m corresponds to a vertical eddy diffusion coefficient (VEDC) of 0.1 cm2/sec. The light dashed line between 50 and 150 m corresponds to a VEDC o f 0.05 cm2/sec. There is no data in this region to suggest which curve the 228Ra distribution follows. From 150 to 225 m the heavy dashed line corresponds to a VEDC of 0.5 cm2/sec.
where C 1 and C 2 are concentrations at two depths, z the depth interval, )k228 the decay constant of 228Ra (3.3 X 10-9 sec-1), andD z the coefficient of vertical
I thank Feder Ostapoff of ESSA (now NOAA) and the officers and crew of the USC&GSS Discoverer for their assistance in sample collections. Financial support for sample collections was provided by AEC contract (30-1) 3629 to the State University of New York at Stony Brook. Professor D. Lal of the Tata Institute of Fundamental Research provided hospitality during most of the measurements. The comments of Peter Vogt, B.L.K. Somayajulu, and S. Krishnaswamy served to improve the manuscript.
References [1 ] W.S. Moore, Oceanic Concentrations of Radium-228, Earth Planet. Sci. Letters 6 (1969) 437. [2] H.G. Ostlund, M.O. Rinket and C. Rooth, Tritium in the equatorial Atlantic Current System, J. Geophys. Res. 74 (1969) 4535. [3] W.S. Moore, S. Krishnaswami and S. Bhat, Radiometric Determinations of Coral Growth Rates, in: Thomas Goreau Memorial Volume, E. Graham, Ed. (University of Miami, Miami), in press.