The δ13C anomaly in the northeastern Atlantic

The delta(13)C of dissolved inorganic carbon was measured on samples collected at 49 degrees N in the northeast Atlantic in January 1994. Deeper than 2000 m, delta(13)C exhibits the same negative correlation versus dissolved phosphate that is observed elsewhere in the deep Atlantic. Upward from 2000 m to about 600 m, delta(13)C shifts to values more negative than expected from the correlation with nutrients at depth, which is likely due to penetration of anthropogenic CO,. From these data, the profile of the anthropogenic delta(13)C decrease is calculated by using either dissolved phosphate or apparent oxygen utilization as a proxy for the preanthropogenic delta(13)C distribution. The shape of the anthropogenic anomaly profile derived from phosphate is similar to that of the increase in dissolved inorganic carbon derived by others in the same area. The reconstruction from oxygen utilization results in a lower estimate of the anthropogenic delta(13)C decrease in the upper water column, and the vertical anomaly profile is less similar to that of the dissolved inorganic carbon increase. A C-13 budget for the atmosphere, ocean, and terrestrial biosphere indicates that within the range of probable ocean CO, uptake the ratio of delta(13)C to inorganic carbon change should be mostly influenced by the C-13 inventory change of the biosphere. However, the uncertainty in the ratio we derive prevents a strong contraint on the size of the exchangeable biosphere.