Sources, deposition rates and decomposition of organic carbon in recent sediment of Sachem Head Harbor, Long Island Sound

The vertical distributions of excess ²¹⁰Pb and fall out ^{239, 240}Pu imply a uniform sedimentation rate of 1·4-1·6 cm year^-1 from 0 to 105-110 cm. This sediment accumulation rate is compatible with sulfate reduction rate data from this location. Below 70 cm only 'aged' refractory carbon is present (C_R = 1·8% C) with an age of approximately 2400 years. This phase is present in a number of locations across Long Island Sound. Planktonic carbon (C_P) is present above the 60-67 cm horizon. A value of 1·0 for A_P (¹⁴C activity) at 32-37 cm was taken, A_P = 1·285 was used for contemporary plankton. This was obtained by correcting the measured A_P of a plankton tow sample for admixed refractory carbon. These values were then used to calculate C_R, C_P and C_F (fossil carbon) at 32-37 cm and 6-12 cm. The only values compatible with the known sulfate reduction rate data are C_R equal to pre 60-67 cm levels (1·6-1·8% C), C_F being 0·3% C at both depths, and C_P decreasing with depth from 0·3 to 0·4% C at 6-12 cm to close to zero at 32-37 cm.