Photos of the Long Island Sound

Research & Monitoring

Status and Trends: LISS Environmental Indicators

Sediment Delta 13C

Johan Varekamp, Wesleyan University.

δ13C: Carbon Isotopes
δ13C pm core depth (cm)
1997 -3.42 0
1996 -3.36 0.1
1992 -2.34 0.8
1988 -3.1 2.8
1984 -3.18 5.3
1975 -5.67 7.8
1965 -4.42 10.3
1955 -2.49 12.8
1940 -2.23 15.3
1910 -3.41 17.8
1880 -0.01 20.3
1850 -2.65 22.8
1837 -1.3 25.3
1820 -1.28 27.8
1754 -1.61 30.3
1695 -1.88 33.3
1652 -1.44 35.3
1594 -1.77 37.8
1530 -1.08 40.3
1462 -0.71 42.8
1382 -2.15 45.5
1311 -1.57 47.8
1229 -0.71 50.3
1141 -0.94 52.8
1049 -1.26 55.3
951 -1.99 57.8


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.

what are historical/legacy indicators?

Some of our indicators are categorized as either historical or legacy indicators. The Carbon Isotopic indicator is a historical indicator. This type of indicator provides perspective on past environmental conditions in Long Island. It is not measured periodically like our other indicators. Many indicators of this type use sediment core samples to measure how conditions in the Sound have changed over hundreds to thousands of years.

A legacy indicator is another type of indicator that is no longer actively tracked by the Long Island Sound Study. This is often due to lack of funding or termination of field sampling programs. A legacy indicator might also be replaced by a more effective indicator or may not need to be updated because the goal has been met. We continue to present these legacy indicators because they can still provide baseline information on conditions in the Sound if the sampling programs resume.

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