Utility of Geochemical Analysis of Trace Fossils: Case Studies Using Phycosiphon incertum from the Lower Jurassic Shallow-Marine (Higashinagano Formation, Southwest Japan) and Pliocene Deep-Marine Deposits (Shiramazu Formation, Central Japan)

Abstract

Elemental and mineralogical analyses of Phycosiphon incertum from the Lower Jurassic Higashinagano Formation and Pliocene Shiramazu Formation revealed that the core and mantle have significantly different compositions; the Al₂O₃/SiO₂ ratio, a general proxy for phyllosilicates to tectosilicates, showed a significantly (P < 0.01) higher value in the core than in the mantle. The obtained data strongly suggest that clay minerals are certainly concentrated in the core while coarse grains (i.e., quartz, feldspar) are enriched in the mantle. This is in agreement with the commonly believed interpretation of the Phycosiphon-producer; namely, it selectively ingested and excreted the clay-sized sediment grains and sorted out coarse grains. Difference in the degree of particle selection was also recognized; the tracemaker of the Higashinagano P. incertum showed greater degree of selectivity. This may be due to the differences in optimal foraging deposit-feeding activities affected by clay mineralogy of the host sediments, or in mechanistic consequences of particle selection induced by organic matter content, or in the number of other ichnotaxa recognized from each formation.

Keywords:

• Clay minerals
• Deposit feeding
• Geochemistry
• Phycosiphon incertum
• Trace fossils

ACKNOWLEDGMENTS

The author thanks Kazushige Tanabe (The University Museum, University of Tokyo) and Kazuyoshi Endo (University of Tokyo) for their supervision throughout this study, as well as Koji Seike (University of Tokyo), Kentaro Nakada (Mizuta Memorial Museum, Josai University), and Masayuki Ikeda (Ehime University) for their useful discussions both in the field and laboratory. Taichi Nozu (University of Tokyo) kindly provided helpful suggestions during the revision. I am also indebted to Hideto Yoshida and Aya Okubo (both University of Tokyo) for their assistance with EDS analysis and Akinori Karasuda (University of Tokyo) for his kind help with XRD analysis. Thanks are also due to Andreas Wetzel and an anonymous reviewer, who improved the manuscript greatly.