Modern silica sinter deposits from an island-arc setting and their potential for fossilizing plants

Abstract

Silica sinters deposited by hot-spring activities form Lagerstätten with numerous three-dimensional, cell-preserved fossils. The formation processes and depositional facies of silica sinters have been documented in large-scale geothermal provinces, including mantle-plume hotspots and mid-ocean ridges. Silica sinters in these areas preserve plants and microbes which are adapted to survive under geothermal stress, and which live inside the hot-spring system. Most plants are, however, intolerant to this specific environment. They are growing outside the hot springs, their chance for being embedded and preserved in the silica sinters is thus limited. Paleodiversity estimates and paleoecological reconstructions of past silica sinter Lagerstätten are therefore considered to be ambiguous. Here, we present a new depositional facies model of silica sinter from a forested island-arc setting, and discuss the taphonomy of richly preserved plants from these settings. At Nakabusa Hot Springs in central Japan, numerous small seep points emerge on densely forested slopes, where they form silica sinters. These sinters are characterized by the incorporation of abundant modern plants and insects from the adjoining forests, in addition to temperature-specific microbes. Because narrow channels flow down steep slopes with little disturbance of the forests, a distinctive bio- and lithofacies has developed that richly preserves the present-day native vegetation. The facies model presented here can serve as a valuable modern analog for better understanding the depositional processes of plant-rich fossil silica sinters. It may further help to understand the factors controlling the fossilization of land-based vegetation, thereby improving its relevance for interpretations throughout the geologic record.