Deposition and early diagenesis of terrestrial plant-derived terpenoids in deep-sea sediments in the northwestern Pacific ocean off Japan

Abstract

Terrestrial plant-derived terpenoids (TPTs) in deep-sea surface to subsurface sediments of the northwestern Pacific were investigated at two sites along the Kuroshio Current and in the gyre over the Nishishichitou Ridge following a latitudinal transect at 138°36′E (Sites LM3 and LM5P), as well as the Japan Trench off the Sanriku coast of northeastern Japan (Site LM8). Cadalene, oleanene, and retene were detected as geoterpenoids in all cores. Possible sources of these geoterpenoids are soil materials, which were attached to clay minerals with or without early diagenetic alteration of precursor bioterpenoids. Friedelin and dehydroabietic acid (DAA) were identified as major functionalized TPTs, or bioterpenoids, in all core samples. Concentrations of TPTs in the core-top samples from all study sites varied with distance from land, sediment lithology, and upland vegetation. Bioterpenoids in the TPTs are relatively abundant at all sites. These results suggest that TPT biomarkers are mainly transported to the open ocean as bioterpenoids. The ratio of friedelin to DAA in core-top samples clearly varied (2.53 in LM3, 1.12 in LM5P, and 0.95 in LM8) depending on upland vegetation, and therefore the bioterpenoid ratio is proposed to be a (paleo)vegetation indicator. Vertical profiles of TPT concentrations showed pronounced variability. The concentrations of oleanene and retene generally decreased with depth in all cores, indicating that these compounds are labile in surface to subsurface sediments. However, the cadalene concentration increased or decreased slightly with depth in cores from the Nishishichitou Ridge, indicating that cadalene is a refractory compound. The DAA concentration decreased markedly with depth, and the gradients were similar in all cores. Thus, DAA was effectively decomposed and behaved as a typical labile compound. On the other hand, the friedelin concentration increased slightly with depth in the core collected off the Sanriku coast. Thus, friedelin appeared to behave as a refractory compound in surface to subsurface sediments despite being a bioterpenoid.