Sources and transformation processes of pheopigments: Stable carbon and hydrogen isotopic evidence from Lake Haruna, Japan

Abstract

Pheopigments such as pheophytin a, pyropheophytin a and steryl chlorin esters are abundant chlorophyll transformation products in many oceanic and lacustrine sedimentary environments. They have been widely used as direct indicators of phytoplankton and zooplankton activity in aquatic environments. In this study, we evaluate the sources and transformation processes of pheopigments in a sediment core from Lake Haruna (Japan), based on stable carbon (δ 13C) and hydrogen (δD) isotopic composition of phytol and sterols as side chains of pheopigments. The isotopic compositions of phytol from sedimentary pheophytin a (δ 13C = −29.2‰ to −27.6‰, δD = −388‰ to −376‰) and some sterols from steryl chlorin esters (e.g. 24-methycholesta-5,22-dien-3β-ol; δ 13C = −29.4‰ to −27.9‰, δD = −340‰ to −325‰) are close to those of phytol (δ 13C = −31.7‰ to −27.0‰, δD = −385‰ to −371‰) and corresponding sterols (e.g. 24-methycholesta-5,22-dien-3β-ol; δ 13C = −28.4‰ to −28.1‰, δD = −335‰ to −320‰) from modern phytoplankton. The isotopic signatures are consistent with an origin exclusively from aquatic organisms. In contrast, the isotopic compositions of phytol from pyropheophytin a (δ 13C = −31.7‰ to −29.1‰, δD = −379‰ to −341‰) and other sterols from steryl chlorin esters (e.g. 24-ethylcholest-5-en-3β-ol; δ 13C = −33.2‰ to −30.2‰, δD = −298‰ to −270‰) vary throughout the core, with values ranging between those from modern phytoplankton (e.g. 24-ethylcholest-5-en-3β-ol; δ 13C = −28.7‰ to −28.1‰, δD = −348‰ to −345‰) and terrestrial C 3 higher plants (δ 13C = −41.5‰ to −31.7‰, δD = −352‰ to −295‰ for phytol; δ 13C = −38.6‰ to −28.0‰, δD = −283‰ to −225‰ for 24-ethylcholest-5-en-3β-ol). These results suggest that terrestrially derived phytol and sterols potentially contribute to the side chain of pyropheophytin a and steryl chlorin esters, respectively.