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Sea provides a direct input of surface-water 14C . Additional input of 14C to the deep sea occurs by transport along isopycnal surfaces, by lateral and vertical mixing in the main thermocline of the ocean, by dissolution of carbonate skeletons, and by oxidation of organic material from sinking particles. As compared to other oceanographic markers, 14C adds a measure of time because it decays at the rate of 1% every 80 years. As a consequence, the distribution of 14C in the abyssal waters of the world ocean has been used to estimate the replacement times for Pacific, Indian and Atlantic ocean deep waters (8). The steady state of the carbon cycle was broken in the late fifties and in the sixties with the large scale testing of nuclear weapons. The 14C activity of the atmosphere in the northern hemisphere increased by about 100% and has been decreasing since the end of the nuclear tests in the atmosphere. This atmospheric 14C activity decrease is due to the homogeneization of the troposphere and to the penetration of CO2 into other carbon reservoirs, mainly the ocean and the terrestrial biosphere. The bomb produced 14C is therefore a good tracer to quantify the oceanic invasion by the atmospheric CO2 excess resulting from human activities. The conventional radiocarbon 13-counting requires the sampling of about 250 litres of sea water and chemical extraction of CO2 on board the oceanographic vessel. The same 14 C measurements can be done with 100 ml water samples poisoned with 1 ml of saturated HgC1 2 solution (9). This operation is designed to supress isotope fractionation due to respiration and photosynthesis of microorganisms, which might affect the 13C and 14 C content of the water sample. In the laboratory, the total dissolved CO2 is extracted from the sea water in a vacuum line in which the 100 ml aliquot of sea water is acidified and flushed with pure helium gas. The evolving CO 2 is trapped in liquid nitrogen and the target is prepared by catalytic reduction of this CO2 on iron powder (10). As an example of the potential of measuring oceanic 14 C by A.M.S., we present here three profiles performed in the Indian Ocean during the INDIGO-2 cruise on board of the M/S Marion Dufresne. The radiocarbon activities (Fig. 3) are expressed by means of the 6. 14 C scale (11). Stations 32 and 45 have the same locations as stations 424 and 420 of the GEOSECS expedition (12) and therefore will enable a comparison of the two sets of 6. 14C data. The three profiles exhibit the presence of bomb 14 C in the upper part of the water column, until a depth of about 1000 m where 0 14 C values are equal to the GEOSECS values and represent the so-called pre-bomb level. Following Broecker et al. (13), we have estimated the water column inventories of bomb produced 14C by integrating the area between the observed A 14 C curve and the reconstructed 6. 14C curve versus depth for pre-nuclear time. At station 28, the INDIGO integrated amount of 14C ( 1140 is 15.9 109 atoms/cm 2 , in agreement with GEOSECS data obtained at the same latitude but not at the same location (Fig. 4). At station 32, the INDIGO El lIC is 7.8 10 9 atoms/cm2 , identical with that measured at GEOSECS station 424 (114 = 7.7 10 9 atoms/cm 2 ) although the two profiles exhibit significant differences. By contrast, at equatorial station 45, the INDIGO 1 14C is 10.3 109 atoms/cm2 , twice the value measured at the same location during the GEOSECS expedition (1, 14C = 4.8 109 atoms/cm2. The column inventories for bomb radiocarbon at the stations performed during the GEOSECS expedition showed pronounced minimal values in the low latitudes (Fig. 4). Oceanic models have showed that a sizable fraction of the bomb-14C that entered the tropical ocean has been transported to the adjacent temperate zones and that the low-latitude deficit enables an equatorial upwelling component to be
DOI link for Sea provides a direct input of surface-water 14C . Additional input of 14C to the deep sea occurs by transport along isopycnal surfaces, by lateral and vertical mixing in the main thermocline of the ocean, by dissolution of carbonate skeletons, and by oxidation of organic material from sinking particles. As compared to other oceanographic markers, 14C adds a measure of time because it decays at the rate of 1% every 80 years. As a consequence, the distribution of 14C in the abyssal waters of the world ocean has been used to estimate the replacement times for Pacific, Indian and Atlantic ocean deep waters (8). The steady state of the carbon cycle was broken in the late fifties and in the sixties with the large scale testing of nuclear weapons. The 14C activity of the atmosphere in the northern hemisphere increased by about 100% and has been decreasing since the end of the nuclear tests in the atmosphere. This atmospheric 14C activity decrease is due to the homogeneization of the troposphere and to the penetration of CO2 into other carbon reservoirs, mainly the ocean and the terrestrial biosphere. The bomb produced 14C is therefore a good tracer to quantify the oceanic invasion by the atmospheric CO2 excess resulting from human activities. The conventional radiocarbon 13-counting requires the sampling of about 250 litres of sea water and chemical extraction of CO2 on board the oceanographic vessel. The same 14 C measurements can be done with 100 ml water samples poisoned with 1 ml of saturated HgC1 2 solution (9). This operation is designed to supress isotope fractionation due to respiration and photosynthesis of microorganisms, which might affect the 13C and 14 C content of the water sample. In the laboratory, the total dissolved CO2 is extracted from the sea water in a vacuum line in which the 100 ml aliquot of sea water is acidified and flushed with pure helium gas. The evolving CO 2 is trapped in liquid nitrogen and the target is prepared by catalytic reduction of this CO2 on iron powder (10). As an example of the potential of measuring oceanic 14 C by A.M.S., we present here three profiles performed in the Indian Ocean during the INDIGO-2 cruise on board of the M/S Marion Dufresne. The radiocarbon activities (Fig. 3) are expressed by means of the 6. 14 C scale (11). Stations 32 and 45 have the same locations as stations 424 and 420 of the GEOSECS expedition (12) and therefore will enable a comparison of the two sets of 6. 14C data. The three profiles exhibit the presence of bomb 14 C in the upper part of the water column, until a depth of about 1000 m where 0 14 C values are equal to the GEOSECS values and represent the so-called pre-bomb level. Following Broecker et al. (13), we have estimated the water column inventories of bomb produced 14C by integrating the area between the observed A 14 C curve and the reconstructed 6. 14C curve versus depth for pre-nuclear time. At station 28, the INDIGO integrated amount of 14C ( 1140 is 15.9 109 atoms/cm 2 , in agreement with GEOSECS data obtained at the same latitude but not at the same location (Fig. 4). At station 32, the INDIGO El lIC is 7.8 10 9 atoms/cm2 , identical with that measured at GEOSECS station 424 (114 = 7.7 10 9 atoms/cm 2 ) although the two profiles exhibit significant differences. By contrast, at equatorial station 45, the INDIGO 1 14C is 10.3 109 atoms/cm2 , twice the value measured at the same location during the GEOSECS expedition (1, 14C = 4.8 109 atoms/cm2. The column inventories for bomb radiocarbon at the stations performed during the GEOSECS expedition showed pronounced minimal values in the low latitudes (Fig. 4). Oceanic models have showed that a sizable fraction of the bomb-14C that entered the tropical ocean has been transported to the adjacent temperate zones and that the low-latitude deficit enables an equatorial upwelling component to be
Sea provides a direct input of surface-water 14C . Additional input of 14C to the deep sea occurs by transport along isopycnal surfaces, by lateral and vertical mixing in the main thermocline of the ocean, by dissolution of carbonate skeletons, and by oxidation of organic material from sinking particles. As compared to other oceanographic markers, 14C adds a measure of time because it decays at the rate of 1% every 80 years. As a consequence, the distribution of 14C in the abyssal waters of the world ocean has been used to estimate the replacement times for Pacific, Indian and Atlantic ocean deep waters (8). The steady state of the carbon cycle was broken in the late fifties and in the sixties with the large scale testing of nuclear weapons. The 14C activity of the atmosphere in the northern hemisphere increased by about 100% and has been decreasing since the end of the nuclear tests in the atmosphere. This atmospheric 14C activity decrease is due to the homogeneization of the troposphere and to the penetration of CO2 into other carbon reservoirs, mainly the ocean and the terrestrial biosphere. The bomb produced 14C is therefore a good tracer to quantify the oceanic invasion by the atmospheric CO2 excess resulting from human activities. The conventional radiocarbon 13-counting requires the sampling of about 250 litres of sea water and chemical extraction of CO2 on board the oceanographic vessel. The same 14 C measurements can be done with 100 ml water samples poisoned with 1 ml of saturated HgC1 2 solution (9). This operation is designed to supress isotope fractionation due to respiration and photosynthesis of microorganisms, which might affect the 13C and 14 C content of the water sample. In the laboratory, the total dissolved CO2 is extracted from the sea water in a vacuum line in which the 100 ml aliquot of sea water is acidified and flushed with pure helium gas. The evolving CO 2 is trapped in liquid nitrogen and the target is prepared by catalytic reduction of this CO2 on iron powder (10). As an example of the potential of measuring oceanic 14 C by A.M.S., we present here three profiles performed in the Indian Ocean during the INDIGO-2 cruise on board of the M/S Marion Dufresne. The radiocarbon activities (Fig. 3) are expressed by means of the 6. 14 C scale (11). Stations 32 and 45 have the same locations as stations 424 and 420 of the GEOSECS expedition (12) and therefore will enable a comparison of the two sets of 6. 14C data. The three profiles exhibit the presence of bomb 14 C in the upper part of the water column, until a depth of about 1000 m where 0 14 C values are equal to the GEOSECS values and represent the so-called pre-bomb level. Following Broecker et al. (13), we have estimated the water column inventories of bomb produced 14C by integrating the area between the observed A 14 C curve and the reconstructed 6. 14C curve versus depth for pre-nuclear time. At station 28, the INDIGO integrated amount of 14C ( 1140 is 15.9 109 atoms/cm 2 , in agreement with GEOSECS data obtained at the same latitude but not at the same location (Fig. 4). At station 32, the INDIGO El lIC is 7.8 10 9 atoms/cm2 , identical with that measured at GEOSECS station 424 (114 = 7.7 10 9 atoms/cm 2 ) although the two profiles exhibit significant differences. By contrast, at equatorial station 45, the INDIGO 1 14C is 10.3 109 atoms/cm2 , twice the value measured at the same location during the GEOSECS expedition (1, 14C = 4.8 109 atoms/cm2. The column inventories for bomb radiocarbon at the stations performed during the GEOSECS expedition showed pronounced minimal values in the low latitudes (Fig. 4). Oceanic models have showed that a sizable fraction of the bomb-14C that entered the tropical ocean has been transported to the adjacent temperate zones and that the low-latitude deficit enables an equatorial upwelling component to be
ABSTRACT
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