Astronomically forced dynamics of Late Devonian (Famennian) sea level and biotic recovery in western Junggar, Northwest China

Abstract

The Hongguleleng Formation in western Junggar, northwest China preserves a rich variety of fossils and was previously regarded as a “refugium” during the Late Devonian biotic crisis. Uncertainty in the age of the Hongguleleng Formation has persisted for a considerable time. In this study, cyclostratigraphic analysis was carried out on the Upper Devonian Bulongguoer and Wulankeshun sections from western Junggar, northwest China. Time series analysis and modeling of iron (Fe) concentration proxy data reveal variations with frequencies comparable to those of the Earth's long and short orbital eccentricity, obliquity, and precession index in both successions. Interpreted 405-kyr long orbital eccentricity cycles were used to establish floating astronomical time scales (FATs) for the two successions. From these FATs the depositional duration of the Hongguleleng Formation is calculated as 11.5 ± 0.58 Myr. The Devonian-Carboniferous boundary age of 359.3 ± 0.3 Ma was chosen as a time “anchor”, resulting in an astronomically determined age of 370.2 ± 0.66 Ma for the base of the Hongguleleng Formation. Combined with conodont biostratigraphy, this age indicates that the Hongguleleng Formation in western Junggar does not reach down to the Frasnian-Famennian boundary. Sedimentary noise modeling of the reconstructed Fe concentration time series provides an interpretation of sea-level variations in the Paleo-Asian Ocean controlled by astronomical forcing from very long orbital eccentricity cycles (g4–g3) throughout the Late Devonian period. Intensified monsoonal climates during these orbital eccentricity cycle maxima led to elevated terrigenous input and strengthened upwelling, which enhanced primary productivity in the western Junggar. We propose an “astronomical climate change” model as a driving mechanism that led to biotic recovery in the Famennian Hongguleleng Formation.