Stable carbon isotope variations in sediment from Baffin Bay, Texas, U.S.A.: Evidence for cyclic changes in organic matter source

Baffin Bay, Texas, a drowned Pleistocene river valley, is filled with up to 20 m of exceptionally well-preserved Holocene and recent sediments. Piston cores from the upper 4 m have been described and analyzed for elemental composition and δ¹³C of the total organic carbon and carbonate. Strong cyclic patterns were observed for all of these parameters. The total organic carbon (TOC) level varied between 1% and 6%. (C/N ratios varied between 10 and 14. δ¹³C showed a cyclicity with depth and a slight shift toward more positive values with depths. These variations are interpreted as being due to different relative inputs of seagrass (δ¹³C−≈ 10‰) and phytoplankton (δ¹³C−20‰.) to the sediment. The cores contained fine-grained carbonate and some massive dolomite. δ¹³C of the carbonate varied between −3% and + 1‰ except for two more positive values. The lack of a trend toward light carbonate with depth was taken to mean that little (< 5%) CO₂ from the oxidation of organic matter is present in the carbonates. Overall the changes in the relative intensity of these sources is thought to reflect regional climatic and weathering cycles.