The Delta 13C (δ13C) value is the ratio of the stable (non-radioactive) isotopes of carbon in a sample. This value is used by environmental scientists to determine the sources of organic matter in a sample. Organic matter from phytoplankton, seaweed and land plants all have a different δ13C value. For this indicator, δ13C was measured in the shells of long-dead organisms as a way to find out about the sources of carbon in bottom waters of Long Island Sound during the time period that the organisms were alive. Scientists are also able to use δ13C values to detect hypoxia that may have occurred in the past. Using this method, scientists can recreate a historical record of organic matter and hypoxia in Long Island Sound.
Very low δ13C values are a proxy for hypoxic conditions in Long Island Sound bottom waters. In modern Long Island Sound, a decrease in δ13C is observed along the same east-west gradient as the annual summertime decrease in dissolved oxygen of bottom waters.
The chart above depicts δ13C values over the last 1000 years in Long Island Sound. Sharp declines in δ13C were observed beginning approximately 150 years ago, and were not observed prior to that time in the historical record. This data suggests that problems with hypoxia in the Sound may date back to the 19th century.