Biomarkers in the rock outcrop of the Kazusa Group reveal palaeoenvironments of the Kuroshio region

Hiroto Kajita,Atsushi Suzuki,Toshihiro Yoshimura,Ayumi Maeda,Naohiko Ohkouchi,Hodaka Kawahata,Masayuki Utsunomiya

doi:10.1038/s43247-021-00154-2

Abstract

Long-chain alkenones and n-alkanes preserved in marine and lake sediment cores are widely used to reconstruct palaeoenvironments. However, applying this technique to exposed sedimentary rock sequences is relatively challenging due to the potential for the diagenetic alteration of organic biomarkers. Here, we extract long-chain alkenones and n-alkanes from an exposed outcrop of the Kazusa Group in central Japan, one of the most continuous sedimentary successions in the world, covering almost the entire Pleistocene. We find that the alkenone unsaturation ratio and average chain length of n-alkanes appears to reflect the glacial-interglacial changes in sea surface temperature and terrestrial climate, respectively. Alkenone-based sea surface temperatures between 1.1 and 1.0 million years ago concur with foraminiferal Mg/Ca-based temperature estimates and may reflect an intrusion of the Kuroshio Current. We suggest that the preservation of these biomarkers in the Kazusa Group demonstrates its potential to provide a detailed palaeoenvironmental record.

Highlights

Long-chain alkenones and n-alkanes preserved in marine and lake sediment cores are widely used to reconstruct palaeoenvironments
Long-chain alkenones (LCAs) are C35–C42 unsaturated methyl and ethyl ketones with 2–4 double bonds and feature unique lipid biomarkers produced by Isochrysidales haptoaplhkyetneosnlievsin(gUiK3n70 )suisrfsatcreonwgaltyercso1r.rTehlaeteddegwreitehotfhuengsraotuwrtahtiotenminpeCra37ture in culture experiments and is widely used as one of the most quantitative and well-established proxies for sea surface temperature (SST)[2,3,4]
The middle to lower part of the Kazusa Group is represented by successions of submarine fan deposits associated with deep-sea, basin-plain, and slope deposits accumulated during the Pleistocene, which is characterised by continuous and well exposed sedimentary successions with a high sedimentation rate[13,14]

Summary

Introduction

Long-chain alkenones and n-alkanes preserved in marine and lake sediment cores are widely used to reconstruct palaeoenvironments. B Surface distribution of the Otadai Formation (blue-coloured) in the Kazusa Group (grey-coloured) on the Boso Peninsula[26,27], showing the locations of the survey sites in this study (red triangles) and the TR-3 core (open square).

Results

Conclusion