The δS (pronounced delta 34 S) value is a standardized method for reporting measurements of the ratio of two stable isotopes of sulfur, S:S, in a sample against the equivalent ratio in a known reference standard. Presently, the most commonly used standard is Vienna-Canyon Diablo Troilite (VCDT). Results are reported as variations from the standard ratio in parts per thousand, per mil or per mille, using the ‰ symbol. Heavy and light sulfur isotopes fractionate at different rates and the resulting δS values, recorded in marine sulfate or sedimentary sulfides, have been studied and interpreted as records of the changing sulfur cycle throughout the earth's history.