Recent sedimentary hopanoids in the northwestern Pacific alongside the Japanese Islands - Their concentrations and carbon isotopic compositions

Two hopanoids, 17β(H),21β(H)-hop-22(29)-ene (diploptene) and 17β(H),21β(H)-bishomohopanoic acid (ββC₃₂-HA), are the most abundant among the pentacyclic triterpenoids found in Recent sediments of the Pacific Ocean alongside the Japanese Islands. The concentration of diploptene normalized to organic carbon content is higher in sediments where C₃₇ polyunsaturated alkene (a biomarker of Haptophytes) is in high concentration, suggesting that the diploptene may be associated with the accumulation of marine organic matter. In open marine settings, δ¹³C values of ββC₃₂-HA range from -23.8 to -19.4 parts per thousand (relative to PDB), being enriched in ¹³C relative to diploptene (-31.6 to -26.3 parts per thousand) by 5-9 parts per thousand. The isotopic difference indicates the presence of at least partially different sources for the two hopanoids. While diploptene is derived from cyanobacteria and chemotrophic bacteria in the water column or sediment, bishomohopanoic acid may be produced mainly by heterotrophs in the sediment using marine organic matter. In contrast, the δ¹³C values of the two hopanoids from river and bay sediments are similar (approximately -31 to -29 parts per thousand), indicating a common source derived from soil components (terrestrial plants or bacteria in soils).