SOURCES OF DISSOLVED INORGANIC CARBON IN TWO SMALL STREAMS WITH DIFFERENT BEDROCK GEOLOGY: INSIGHTS FROM CARBON ISOTOPES

Radiocarbon natural abundances (Delta C-14) are being increasingly used to trace carbon cycling in stream ecosystems. To understand the ultimate sources of carbon, we determined the stable carbon isotope ratios (delta C-13) and Delta C-14 values of dissolved inorganic and organic carbon (DIC and DOC, respectively) and of particulate organic carbon (POC) in two small streams in central Japan, one of which flows over limestone bedrock (Seri) and the other does not (Fudoji). Investigations over four seasons revealed that the Delta C-14 values of the DIC (from -238 parts per thousand to -174 parts per thousand for Seri and -23 parts per thousand to +10 parts per thousand for Fudoji) were less variable than those of the other carbon fractions (DOC: from -400 parts per thousand to -138 parts per thousand for Seri and -2 parts per thousand to +103 parts per thousand for Fudoji; POC: from -164 parts per thousand to -60 parts per thousand for Seri and -55 parts per thousand to +37 parts per thousand for Fudoji). Based on mass balance calculations using the d13C and Delta C-14 values, the proportions of carbon in the DIC originated from (1) atmospheric CO2 were 47% to 57% for Seri and 74% to 90% for Fudoji, (2) organic matter degradation were 29% to 35% for Seri and 4% to 21% for Fudoji, and (3) carbonate rock were 14% to 22% for Seri and 4% to 6% for Fudoji. We compared the results with previous studies that had been conducted in larger rivers and showed that in small streams, the dissolution of atmospheric CO2 and weathering of carbonate rock are more important factors in the carbon cycling than the biological degradation of organic matter.