Methyltrimethyltridecylchromans (MTTCs) in lacustrine sediments in the northern Bohai Bay Basin, China

Abstract

Although methyltrimethyltridecylchromans (MTTCs) have been identified and studied for over 30 years in a large number of sediment extracts and crude oils, their origins and significance for palaeosalinity are still debated. Here, the geochemical features of MTTCs in 133 organic-rich lacustrine sediments obtained from the northern Bohai Bay Basin, China, are studied in detail. The sediment samples are in an immature to early mature stage and primarily enriched in Types I and II organic matter, which were deposited in lacustrine environments with relatively weakly stratified water columns and anoxic to suboxic conditions. The development of photic zone euxinia (PZE) in the water column was indicated by the occurrence of aryl isoprenoids. MTTCs were detected in all samples and characterized by low abundances of δ-MTTC and high abundances of α-MTTC. Variations in the relative abundance of MTTCs and calculated MTTCI values, suggest that selected lacustrine sediments were deposited in normal to semi-saline environments. A ternary diagram of mono-, di- and trimethyl-MTTCs was established, and shows potential for application as an indicator of changes in palaeosalinity. Variations in the abundance of MTTCs can effectively indicate palaeosalinity changes in lacustrine environments.