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University of Washington Geosecs North Atlantic carbon-14 results
Earth and Planetary Science Letters, 23 (1974) 8 7 - 9 0 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
t_Ld
UNIVERSITY OF WASHINGTON GEOSECS NORTH ATLANTIC CARBON-14 RESULTS MINZE STUIVER and STEPHEN W. ROBINSON Department o f Geological Sciences and Quaternary Research Center, University o f Washington, Seattle, Wash. (USA) Received March 11, 1974 Revised version received June 11, 1974 The 14C activities of North Atlantic water at Geosecs stations 5, 27, and 29 are reported. The influence of the correction for isotope fractionation on the 14C results is discussed, and experimental procedures are given.
The Geosecs 14C laboratory at the University of Washington, housed in the new Quaternary Research Center, became operational in May 1973. During the first ten months of operation the 14C activities of 120 samples were measured. Thirty three of these analyses are reported here to complete the 14C distribution picture of the Northern Atlantic. The standard laboratory treatment of the samples is as follows: The sodium carbonate solution received from the shipboard operations is transferred to a glass reaction vessel in a nitrogen atmosphere and acidified with hydrochloric acid. During acidification, the released CO2 is carried by a flow of nitrogen through a section of the purification system. Treatment in this section consists of washing in a potassium permanganate solution, drying in a water trap (-80°C), reaction with silver at 375°C, drying in a second water trap and CO 2 condensation in two vessels cooled with liquid nitrogen. The carrier nitrogen escapes through a check valve to the atmosphere. Prior to acidification, the reaction vessel and per- ' manganate solution are flushed with a flow of nitrogen to clean this part of the system from any residual CO 2. All other sections of the purification system are evacuated to a pressure less than 0.1 micron. During sample release through acidification, the pressure inside the section of the purification system in use is always larger than 1 atm, thus preventing any possible addition of atmosphere CO 2 through accidental leakage. Non-condensable gases are removed from the CO 2 collected in the condensation vessels by evacuation at liqGEOSECS Publication No. 29.
uid nitrogen temperatures to pressures of less than 0.1 micron. The CO 2 gas is subsequently passed through a small copper oven at 450°C, and measured with regard to yield. The above method provided samples suitable for counting, but with marginal purity. The critical factor for pure samples appears to be contact time with the hot copper. Originally we passed the sample a few more times through the copper oven to obtain the required purity, but at present all samples are brought routinely into a 500°C copper oven, attached at top and bottom to a 5-liter CO 2 reservoir. The thermal recirculation of CO 2 in this system provides pure sampies in a few hours. The sample activities are determined with two counters with backgrounds of 0.9 and 1.5 counts per minute. Zero age counting activity in these counters is 4 2 - 4 3 counts per minute. As discussed in the calibration paper in this issue [1], the accuracy of the 14C analyses is 4%0, or better. The 13C determinations are made with a Nuclide RMS-60 isotope ratio mass spectrometer with internal precision of 0.1%o. Intercalibration experiments, as discussed in the aforementioned paper [1 ], indicate a difference in 13C ratio of about 0.20o between laboratories. The sample activities are compared to age corrected 95% activity of oxalic acid, normalized to a/513 C ratio of-19.0% o. As discussed previously [1] age correction of the oxalic acid to 1950 A.D. is desirable for geochemical measurements. The measured 14C activity of the sample AS, is compared to the age corrected
88
M. STUIVER AND S.W. ROBINSON A first approximation of this equation results in:
(8-c/
÷
A14C = ~14C - (2813C + 50) 1+ 1000 ]
o o
-
~
I 0
i
I -10
i
I
~
-ao
a O 3 (%,)
I -30
h
I
I
-40
Fig. 1. Differencesbetween calculated ~14C and AC 14Cactivities versus ~ 13Cvalues for a sample with ~ 14C = 0. The A terms are explained in the text. and 13C normalized oxalic acid activity A O . The deviation from standard activity 814C in per mil is defined as (As/Ao-1) 1000. Such a 514C term does not take into account isotope fractionation effects. The 513C values that are used for the isotope fractionation correction are given in per mil deviation from the PDB standard: 813C = (Rs/RpDB±I) 1000. The 13C/12C ratios of sample and standard are respectively R~ and RpD B . The ~C baseline for 14C work is - 2 5 % o with regard to PDB. Thus no correction for isotope fractionation is needed for materials with R s ( - 2 5 ) = 0.975RpD B. Normally the 14C fractionation is given by the square of the 13C fractionation factors. In this instance the 13C corrected radiocarbon activity of the sample is:
ASN = AS [RS(_25)/Rs ] 2 2 F/
613C\
=As(O.975RpDB) /[_[1+
72
=As 0"9752/( 1+ 613C~21-0~] Substitution of AC 14C = (AsN/A0-1) 1000 and 514C = (ASIA0 - 1 ) 1000 yields: Ac14C=1000[(1+~14C~ 0"9752 -1] 1000] (1 + 813C1~)2
(1)
This last term is routinely used for isotope fractionation corrections. Thus the generally accepted A14C value, due to the approximations made, is not entirely correct. A comparison between the correct h C value and A, as a function of the 13C ratio is given in Fig. 1 for sample activities equal to the age corrected oxalic acid activity (814C = 0). In addition to the error introduced by approximating, the assumption that the 14C fractionation (f14) equals the square of the 13C fractionation factor (f13) is not entirely correct. Consideration of the vibrational partition function [2] of CO 2 appear to suggest values o f b closer to 1.9 for fl 4 =f~13-With b -~1.9, the differences between A and AC are around 2 - 3 % 0 (as a second-order approximation):
Ac--A: (1+ ~ ) [ 0 . 9 7 5 b'X 1"000- 950
+(2-0"975bb)813c +O'975bb
~513C 2
Geosecs samples have 813 C ratios around 0°/oo relative to PDB [3] and 14C activities not too much different from oxalic acid, and thus all reported A14C TABLE 1 Geosecs North Atlantic 14C results, station 5 (location 56°54'N, 42°47'W; collected 1-3 August 1972) Sample No.
Depth (m)
QL-200 QL-202 QL-201 QL-207 QL-205 QL-206 QL-204
307 1230 2052 2672 2877 3082 3287
A14C(%o) -3.1 * -42.4" -58.4* -52.2** -56.1 ** -43.2** -36.0**
Depth to bottom: 3390 m * Also measured by Miami. ** Values quoted in Miami list were preliminary; values given here are final. ~14C values have been derived through use of 95% NBS oxalic acid activity corrected for 14C decay from 1950 A.D. to year Of sampling. Accuracy of 14C results: ± 4°/oo, IBCresuits ± 0.1%o.
UNIVERSITY OF WASHINGTON GEOSECS NORTH ATLANTIC 14C RESULTS TABLE 2 Geosecs North Atlantic 14Cresults, station 27 (location 41°59'N,42°01'W;coUected 12, 13 September 1972)
89
STATION
27
41" 59'N
42"01'W
0
Sample No.
Depth (m)
A 14C(%o)
QL-225 QL-227 QL-228 QL-229 QL-208 QL-209 QL-210 QL-211 QL-212 QL-219 QL-220 QL-221 QL-223 QL-224 QL-213 QL-214 QL-215 QL-216 QL-217 QL-218
10 120 225 320 425 530 635 740 860 1096 1245 1394 1693 1893 2543 3162 3778 4396 4653 4859
123.2 104.6 67.3 56.3 50.0 12.5 -4.4,-3.2 -47.9 -42.5 -41.0 -40.7 -50.2 -57.7 -57.4 -74.8 -77.9 -84.4 -86.9 - 82.4 -66.2
1000
~e4
2000 I'hi
=E "1"
I-n 3O00 LU Q
4000
Depth to bottom: 4867 m
I
-120
A 14C values have been derived through use of 95% NBS oxalic acid activity corrected for 14C decay from 1950 A.D. to year of sampling. Accuracy of 14C results: +-4%0,13C resuits -+0.1%o.
TABLE 3 Geosecs North Atlantic 14C results, station 29 (location 35°58'N, 47°01'W; collected 18 September 1972) Sample No.
Depth (m)
QL-230 QL-231 QL-232 QL-233 QL-234 QL-235
2123 2653 3182 3712 4241 4639
h 14C(% o) -62.7 -74.6,-78.2 -79.2 -86.5 -84.3 -88.2
Depth to bottom 4993 m AI4c values have been derived through use of 95% NBS oxalic acid corrected for 14C decay from 1950 A.D. to year of sampling. Accuracy of 14C results: +-4%0, 13C results +_0.1% o.
-80
I
-40
I
0
I
+40 +80 & 14C (%0)
I
+120 +160
Fig. 2. &I4c values as a function of depth at station 27. B = bottom. values are too low by about 0 . 6 ° o due to the approximation made in deriving eq. 2. This is a small error, and does not cause internal inconsistancies in the ocean water values because 13C and 14C ratios do not vary widely. However, the value o f b is not necessarily 2.0, and larger differences o f A - A c than 0.6°/oo are possible. The value o f b is not precisely known, and for the moment our 14C results are reported using eq. 1. The 14C results for stations 5, 27 and 29 are given in Tables I - 3 and are plotted for stations 27 and 29 in Figs. 2 and 3. A correction for contamination by residual CO~ in the NaOH solution (1.3mmol with 14C = +300 7oo) has been applied to all data. There is no detectable difference in 14C activity for 2 0 0 0 4500 m North Atlantic Deep Water between 36 ° and 42°N. Arctic Bottom Water appears to penetrate to at least 36°N in 1972. For an overall discussion of the profiles and characteristics o f the major water masses the reader is referred to the paper by 0stlund et al. in this issue [4].
90
M. STUIVER AND S.W. ROBINSON STATION
29
35"58'N i
47"01' W
i
Acknowledgement
i
This work has been supported by Grant No. GX28166, from the International Decade of Ocean Exploration, National Science Foundation. tOO0
References
nr hi I".~ 2 0 0 0
,,,
1 M. Stuiver, S. Robinson, H.G. (}stlund and H.G. Dorsey, ]4C Calibration between the University of Washington and the University of Miami Geosecs Laboratories, Earth Planet. Sci. Lett. 23 (1974) 65, this issue. 2 W.S. Broecker and V.M. Oversby, Chemical Equilibria in the Earth (McGraw Hill, 1971). 3 P. Kroopnick, R.F. Weiss and H. Craig, Total CO2, laC, and dissolved oxygen 180 at Geosecs II'in the North Atlantic, Earth Planet. Sci. Lett. 16 (1972) 103. 4 H.G. Ostlund, H.G. Dorsey and C.G. Rooth, Geosecs North Atlantic radiocarbon and tritium results, Earth Planet. Sci. Lett. 23 (1974) 69, this issue.
v
-1fit. uJ 3 0 0 0 (3
4OOO
MW
B
-120
-80
-4'0
0
+40
+80
+120 +160
Z~ t4C (%°)
Fig. 3. A14C values as a function of depth at station 29. B = bottom.
t_Ld
UNIVERSITY OF WASHINGTON GEOSECS NORTH ATLANTIC CARBON-14 RESULTS MINZE STUIVER and STEPHEN W. ROBINSON Department o f Geological Sciences and Quaternary Research Center, University o f Washington, Seattle, Wash. (USA) Received March 11, 1974 Revised version received June 11, 1974 The 14C activities of North Atlantic water at Geosecs stations 5, 27, and 29 are reported. The influence of the correction for isotope fractionation on the 14C results is discussed, and experimental procedures are given.
The Geosecs 14C laboratory at the University of Washington, housed in the new Quaternary Research Center, became operational in May 1973. During the first ten months of operation the 14C activities of 120 samples were measured. Thirty three of these analyses are reported here to complete the 14C distribution picture of the Northern Atlantic. The standard laboratory treatment of the samples is as follows: The sodium carbonate solution received from the shipboard operations is transferred to a glass reaction vessel in a nitrogen atmosphere and acidified with hydrochloric acid. During acidification, the released CO2 is carried by a flow of nitrogen through a section of the purification system. Treatment in this section consists of washing in a potassium permanganate solution, drying in a water trap (-80°C), reaction with silver at 375°C, drying in a second water trap and CO 2 condensation in two vessels cooled with liquid nitrogen. The carrier nitrogen escapes through a check valve to the atmosphere. Prior to acidification, the reaction vessel and per- ' manganate solution are flushed with a flow of nitrogen to clean this part of the system from any residual CO 2. All other sections of the purification system are evacuated to a pressure less than 0.1 micron. During sample release through acidification, the pressure inside the section of the purification system in use is always larger than 1 atm, thus preventing any possible addition of atmosphere CO 2 through accidental leakage. Non-condensable gases are removed from the CO 2 collected in the condensation vessels by evacuation at liqGEOSECS Publication No. 29.
uid nitrogen temperatures to pressures of less than 0.1 micron. The CO 2 gas is subsequently passed through a small copper oven at 450°C, and measured with regard to yield. The above method provided samples suitable for counting, but with marginal purity. The critical factor for pure samples appears to be contact time with the hot copper. Originally we passed the sample a few more times through the copper oven to obtain the required purity, but at present all samples are brought routinely into a 500°C copper oven, attached at top and bottom to a 5-liter CO 2 reservoir. The thermal recirculation of CO 2 in this system provides pure sampies in a few hours. The sample activities are determined with two counters with backgrounds of 0.9 and 1.5 counts per minute. Zero age counting activity in these counters is 4 2 - 4 3 counts per minute. As discussed in the calibration paper in this issue [1], the accuracy of the 14C analyses is 4%0, or better. The 13C determinations are made with a Nuclide RMS-60 isotope ratio mass spectrometer with internal precision of 0.1%o. Intercalibration experiments, as discussed in the aforementioned paper [1 ], indicate a difference in 13C ratio of about 0.20o between laboratories. The sample activities are compared to age corrected 95% activity of oxalic acid, normalized to a/513 C ratio of-19.0% o. As discussed previously [1] age correction of the oxalic acid to 1950 A.D. is desirable for geochemical measurements. The measured 14C activity of the sample AS, is compared to the age corrected
88
M. STUIVER AND S.W. ROBINSON A first approximation of this equation results in:
(8-c/
÷
A14C = ~14C - (2813C + 50) 1+ 1000 ]
o o
-
~
I 0
i
I -10
i
I
~
-ao
a O 3 (%,)
I -30
h
I
I
-40
Fig. 1. Differencesbetween calculated ~14C and AC 14Cactivities versus ~ 13Cvalues for a sample with ~ 14C = 0. The A terms are explained in the text. and 13C normalized oxalic acid activity A O . The deviation from standard activity 814C in per mil is defined as (As/Ao-1) 1000. Such a 514C term does not take into account isotope fractionation effects. The 513C values that are used for the isotope fractionation correction are given in per mil deviation from the PDB standard: 813C = (Rs/RpDB±I) 1000. The 13C/12C ratios of sample and standard are respectively R~ and RpD B . The ~C baseline for 14C work is - 2 5 % o with regard to PDB. Thus no correction for isotope fractionation is needed for materials with R s ( - 2 5 ) = 0.975RpD B. Normally the 14C fractionation is given by the square of the 13C fractionation factors. In this instance the 13C corrected radiocarbon activity of the sample is:
ASN = AS [RS(_25)/Rs ] 2 2 F/
613C\
=As(O.975RpDB) /[_[1+
72
=As 0"9752/( 1+ 613C~21-0~] Substitution of AC 14C = (AsN/A0-1) 1000 and 514C = (ASIA0 - 1 ) 1000 yields: Ac14C=1000[(1+~14C~ 0"9752 -1] 1000] (1 + 813C1~)2
(1)
This last term is routinely used for isotope fractionation corrections. Thus the generally accepted A14C value, due to the approximations made, is not entirely correct. A comparison between the correct h C value and A, as a function of the 13C ratio is given in Fig. 1 for sample activities equal to the age corrected oxalic acid activity (814C = 0). In addition to the error introduced by approximating, the assumption that the 14C fractionation (f14) equals the square of the 13C fractionation factor (f13) is not entirely correct. Consideration of the vibrational partition function [2] of CO 2 appear to suggest values o f b closer to 1.9 for fl 4 =f~13-With b -~1.9, the differences between A and AC are around 2 - 3 % 0 (as a second-order approximation):
Ac--A: (1+ ~ ) [ 0 . 9 7 5 b'X 1"000- 950
+(2-0"975bb)813c +O'975bb
~513C 2
Geosecs samples have 813 C ratios around 0°/oo relative to PDB [3] and 14C activities not too much different from oxalic acid, and thus all reported A14C TABLE 1 Geosecs North Atlantic 14C results, station 5 (location 56°54'N, 42°47'W; collected 1-3 August 1972) Sample No.
Depth (m)
QL-200 QL-202 QL-201 QL-207 QL-205 QL-206 QL-204
307 1230 2052 2672 2877 3082 3287
A14C(%o) -3.1 * -42.4" -58.4* -52.2** -56.1 ** -43.2** -36.0**
Depth to bottom: 3390 m * Also measured by Miami. ** Values quoted in Miami list were preliminary; values given here are final. ~14C values have been derived through use of 95% NBS oxalic acid activity corrected for 14C decay from 1950 A.D. to year Of sampling. Accuracy of 14C results: ± 4°/oo, IBCresuits ± 0.1%o.
UNIVERSITY OF WASHINGTON GEOSECS NORTH ATLANTIC 14C RESULTS TABLE 2 Geosecs North Atlantic 14Cresults, station 27 (location 41°59'N,42°01'W;coUected 12, 13 September 1972)
89
STATION
27
41" 59'N
42"01'W
0
Sample No.
Depth (m)
A 14C(%o)
QL-225 QL-227 QL-228 QL-229 QL-208 QL-209 QL-210 QL-211 QL-212 QL-219 QL-220 QL-221 QL-223 QL-224 QL-213 QL-214 QL-215 QL-216 QL-217 QL-218
10 120 225 320 425 530 635 740 860 1096 1245 1394 1693 1893 2543 3162 3778 4396 4653 4859
123.2 104.6 67.3 56.3 50.0 12.5 -4.4,-3.2 -47.9 -42.5 -41.0 -40.7 -50.2 -57.7 -57.4 -74.8 -77.9 -84.4 -86.9 - 82.4 -66.2
1000
~e4
2000 I'hi
=E "1"
I-n 3O00 LU Q
4000
Depth to bottom: 4867 m
I
-120
A 14C values have been derived through use of 95% NBS oxalic acid activity corrected for 14C decay from 1950 A.D. to year of sampling. Accuracy of 14C results: +-4%0,13C resuits -+0.1%o.
TABLE 3 Geosecs North Atlantic 14C results, station 29 (location 35°58'N, 47°01'W; collected 18 September 1972) Sample No.
Depth (m)
QL-230 QL-231 QL-232 QL-233 QL-234 QL-235
2123 2653 3182 3712 4241 4639
h 14C(% o) -62.7 -74.6,-78.2 -79.2 -86.5 -84.3 -88.2
Depth to bottom 4993 m AI4c values have been derived through use of 95% NBS oxalic acid corrected for 14C decay from 1950 A.D. to year of sampling. Accuracy of 14C results: +-4%0, 13C results +_0.1% o.
-80
I
-40
I
0
I
+40 +80 & 14C (%0)
I
+120 +160
Fig. 2. &I4c values as a function of depth at station 27. B = bottom. values are too low by about 0 . 6 ° o due to the approximation made in deriving eq. 2. This is a small error, and does not cause internal inconsistancies in the ocean water values because 13C and 14C ratios do not vary widely. However, the value o f b is not necessarily 2.0, and larger differences o f A - A c than 0.6°/oo are possible. The value o f b is not precisely known, and for the moment our 14C results are reported using eq. 1. The 14C results for stations 5, 27 and 29 are given in Tables I - 3 and are plotted for stations 27 and 29 in Figs. 2 and 3. A correction for contamination by residual CO~ in the NaOH solution (1.3mmol with 14C = +300 7oo) has been applied to all data. There is no detectable difference in 14C activity for 2 0 0 0 4500 m North Atlantic Deep Water between 36 ° and 42°N. Arctic Bottom Water appears to penetrate to at least 36°N in 1972. For an overall discussion of the profiles and characteristics o f the major water masses the reader is referred to the paper by 0stlund et al. in this issue [4].
90
M. STUIVER AND S.W. ROBINSON STATION
29
35"58'N i
47"01' W
i
Acknowledgement
i
This work has been supported by Grant No. GX28166, from the International Decade of Ocean Exploration, National Science Foundation. tOO0
References
nr hi I".~ 2 0 0 0
,,,
1 M. Stuiver, S. Robinson, H.G. (}stlund and H.G. Dorsey, ]4C Calibration between the University of Washington and the University of Miami Geosecs Laboratories, Earth Planet. Sci. Lett. 23 (1974) 65, this issue. 2 W.S. Broecker and V.M. Oversby, Chemical Equilibria in the Earth (McGraw Hill, 1971). 3 P. Kroopnick, R.F. Weiss and H. Craig, Total CO2, laC, and dissolved oxygen 180 at Geosecs II'in the North Atlantic, Earth Planet. Sci. Lett. 16 (1972) 103. 4 H.G. Ostlund, H.G. Dorsey and C.G. Rooth, Geosecs North Atlantic radiocarbon and tritium results, Earth Planet. Sci. Lett. 23 (1974) 69, this issue.
v
-1fit. uJ 3 0 0 0 (3
4OOO
MW
B
-120
-80
-4'0
0
+40
+80
+120 +160
Z~ t4C (%°)
Fig. 3. A14C values as a function of depth at station 29. B = bottom.