Cretaceous Record of Diatom Evolution Revealed by Biomarkers in the Northwest Pacific Margin

Abstract:

Highly branched isoprenoid (HBI) alkanes and thiophenes are diagenetic derivatives of HBI alkenes produced by certain groups of diatoms, especially Rhizosolenid and Naviculoid diatoms. The occurrence of C₂₅ HBIs in geological samples has been recognized from the Upper Turonian (89.5 Ma), which regarded to indicate the appearance of rhizosolenid diatom. In the present study, we performed biomarker analysis of the Late Cretaceous marine sediments from the Yezo Group, Hokkaido, Japan, to elucidate the timing of rhizosolenid diatom expansion. Furthermore, subsequent variations in the HBI concentration are expected to reflect the variation in siliceous primary production to an extent, which have hardly been described in the northwest Pacific region due to poor preservation of diatoms.

The mudstones of Saku Formation and Haborogawa Foramtion, Yezo Group, were sampled in the Kotanbetsu area, Hokkaido, Japan. The Cretaceous Yezo Group is one of the few strata that record Cretaceous paleoceanographic changes in the northwest Pacific, and the integrated bio- and carbon isotope stratigraphy have been well established in the studied section.

The first occurrence of C₂₅ HBIs was recorded from the late Turonian mudstone samples from the horizon close to the Hitch Wood Event as suggested by the integrated bio- and carbon isotope stratigraphy. Preceding the Turonian/Coniacian boundary (Navigation Event), Hitch Wood Event dates almost comparable to the previous earliest HBI record from the equatorial Atlantic, suggesting the concurrent rise of rhizosolenid diatom production in the Pacific Ocean. The HBI concentrations subsequently increase toward the Turonian/Coniacian boundary and significantly rise during the mid- to late Santonian. These results implies rising importance of rhizosolenid diatoms in the marine production in this region, possibly associated with contemporaneous global cooling trends documented by multiple temperature records during those periods.