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‰ (relative to PDB), being enriched in 13C relative to diploptene (−31.6 to −26.3‰) by 5–9‰. 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 ( −31 to −29‰), indicating a common source derived from soil components (terrestrial plants or bacteria in soils).