Zoned spherical concretions from Atsuta Formation, Japan: a record of rapid geochemical shifts in early diagenesis

Introduction

Carbonate concretions form at shallow burial depths (Raiswell, 1971), with most bicarbonate ions derived from the anaerobic oxidation of organic matter (Claypool and Kaplan, 1974). Specifically, spherical carbonate concretions are proposed to grow concentrically through the diffusion-driven outward migration of the carbonate supersaturation front (Yoshida et al., 2018). Consequently, the center-to-edge isotopic and geochemical profiles of spherical carbonate concretions provide valuable records of pore-water evolution. Zoned spherical concretions, which exhibit distinct zonation, can potentially preserve traces of multiple diagenetic processes. This presents a unique opportunity to reconstruct the temporal and spatial evolution of the pore-water environment at higher resolution. This study presents a detailed analysis of zoned spherical concretions from the Miocene marine sediments of the Atsuta Formation, Japan, to elucidate their formation processes.

Results and Discussion

Well-preserved zoned spherical concretions from the Atsuta Formation are subdivided into two distinct zones: a spherical nucleus (Inner-Concretion) and an outer crust (Outer-Concretion) (Fig. 1). Its spherical shape and the presence of fossilized mud shrimp claws at the center suggest a concentric growth mode. Both the Inner- and Outer-Concretion are inferred to have formed rapidly at shallow burial depths, based on the following evidence:

• Fossilized fecal pellets and mud shrimp claws were found in both the Inner- and Outer-Concretions, exhibiting no signs of compaction.
• The carbonate content was 83.0 ± 0.5 wt% in the Inner-Concretion and 79.4 ± 0.1 wt% in the Outer-Concretion, indicating precipitation within porous sediments prior to significant compaction.
• The average stable oxygen isotope ratios (δ¹⁸O) were +0.63 ± 0.77‰ (n = 76) in the Inner-Concretion and +0.92 ± 0.48‰ (n = 44) in the Outer-Concretion, suggesting the absence of isotopic fractionation effects related to burial depth.

Detailed measurements of stable carbon isotope ratios (δ¹³C) (n = 129) revealed characteristic trends in each zone, with abrupt changes at their boundaries. The δ¹³C values in the Inner-Concretion steadily increased from -15‰ near the center to +10‰ toward the edge, reflecting isotopic fractionation associated with enhanced methanogenesis. In contrast, δ¹³C in the Outer-Concretion was approximately -15‰ and relatively constant. The presence of pyrite throughout the concretion suggests that at least the Outer-Concretion formed within the sulfate reduction zone.

These findings collectively indicate a shift in dominant organic matter degradation processes from methanogenesis to sulfate reduction, a trend that deviates from the classical geochemical sequence of sedimentary environments (Berner, 1981). A plausible hypothesis is that methanogenesis occurred entirely within the sulfate reduction zone, potentially as a localized microenvironment around organic matter during the formation of the Inner-Concretion.