Sedimentation and Preservation of Organic Matter in an Estuary, Niigata Plain, Central Japan

Abstract

The development of an estuary and the accumulation of organic matter in it are strongly influenced by variations in sea level and river discharge related to climate changes. We examined the relationship between development of a Holocene estuary and the sedimentation of organic matter. The study estuary, on the Niigata Plain, central Honshu, Japan, is wave-dominated and microtidal, with a barrier island and bay-head delta. We took two 50-m-long cores, one from an area adjacent to major river discharge and another from the basin margin. Estuarine environmental and sea-level changes were reconstructed using facies analysis. Bottom conditions, which affect the preservation of organic matter, were also examined by means of facies and geochemical analyses, including the TS (total sulfur) content and C/S (carbon to sulfur) ratio. Organic constituents of the estuarine sediments were examined with reflected-light fluorescent microscopy and geochemical analyses, including TOC (total organic carbon), C/N (carbon to nitrogen) and stable carbon isotope ratio (δ 13 C) of organic matter. Late transgressive and highstand estuaries are especially suitable for the preservation of organic matter, because they are enclosed by a barrier island that promotes anoxic bottom conditions. Nearly all of the organic matter preserved in this estuary is terrestrial origin, and a part of it is preserved as NFA (nonfluorescent amorphous organic matter), which forms under anoxic conditions. The NFA formed during the highstand stage in the basin-margin area, whereas an even greater abundance of coarse- grained organic matter accumulated during the late transgressive stage in the area with major river input. During the late highstand stage, coarse-grained terrigenous organic matter, including vitrinite and cutinite, was deposited in both the marginal area and the river- mouth area.