Seasonal variations of dissolved inorganic carbon and δ13C of surface waters: application of a modified gas evolution technique

Abstract

Seasonal concentrations and δ 13C of dissolved inorganic carbon (DIC) in a river-tributary system in Kalamazoo, southwest Michigan, USA, have been measured using a modified gas evolution technique. The technique makes use of evacuated glass septum tubes pre-loaded with phosphoric acid and a magnetic stir bar. Water samples are injected into these septum tubes in the field, which eliminates problems associated with CO 2 loss/gain during sample storage and transfer to the vacuum line during DIC extraction. Using this technique, a precision of 1% and 0.1‰ can be achieved for DIC concentrations and δ 13C DIC measurements, respectively. As this technique provides reliable measurements of DIC concentrations and carbon isotope ratios, it was used to evaluate the processes that control DIC in the river-tributary system. Results of DIC concentration and δ 13C DIC measurements of water samples from the river-tributary system show that the DIC pool is mostly dominated by groundwater. The DIC concentrations and δ 13C DIC are within the ranges measured for the most isotopically evolved groundwater in this region. Seasonal variations superimposed on the baseline values are attributed to secondary processes such as CO 2 invasion from the atmosphere, enhanced recharge from lakes and biological activities of photosynthesis, respiration, and decay. With the onset of spring, there is a concurrent increase in the DIC concentration and δ 13C DIC of these streams. A simultaneous increase in concentration and 13C enrichment of the riverine DIC pool is consistent with CO 2 invasion and recharge from lakes. During the summer, biological activity is the predominant control on shifts in the DIC pool. Although photosynthesis, respiration and decay occur during this time, decreases in the DIC concentration and increases in the 13C DIC indicates CO 2 removal from the pool by photosynthesis. In the late summer-early fall, photosynthesis declines and respiration and decay cause an increase in the DIC concentration and a decreases in the δ 13C DIC. When biological activity decreases significantly during the late fall and winter months, the river and most of its tributaries approach a baseline DIC concentration and δ 13C DIC similar to that of isotopically fully evolved groundwater in the Kalamazoo area. Although this holds true for tributaries and the main river, the timing and magnitude of shifts from background DIC is different for individual streams. The magnitude of shifts in the DIC concentration and δ 13C DIC is most pronounced in tributary streams because of the short residence time of water in these streams. This study shows that DIC concentration and δ 13C DIC measurements can be successfully used to evaluate the timing and dominance of the major processes that influence DIC in a riverine system.