[M-IS19] Biogeosciences of cold seeps, mud volcanoes, and hydrothermal vents

[MIS19-P06] Incipient formation of carbonate concretions observed in a deep sediment core from the northeastern Japan Sea offshore Joetsu

*Yusuke Miyajima¹, Satoshi Furota¹, Yuki Ota², Kiyofumi Suzuki³, Akira Ijiri⁴, Hideyoshi Yoshioka¹, Atsuko Amano⁵, Hiroshi A Takahashi⁶, Tomo Aoyagi², Tomoyuki Hori², Mikio Satoh¹ (1.Research Institute for Geo-Resources and Environment, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 3.Energy Process Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 4.Graduate School of Maritime Sciences, Kobe University, 5.Research Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 6.Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:Authigenic carbonate, Concretion, Japan Sea, Early diagenesis

Authigenic carbonates precipitated in marine sediments have been a major carbon sink throughout Earth’s history (Schrag et al., 2013). Authigenic carbonates are formed inorganically by increases in alkalinity resulting from microbial degradation of organic matter, including methane oxidation, sulfate reduction, and methanogenesis (e.g., Raiswell & Fisher, 2000). Carbonate concretions, often observed as isolated bodies in ancient marine sedimentary rocks on land, have been considered products of these early diagenesis and carbonate authigenesis. However, the mechanisms of carbonate concretion formation, such as the growth rates and depths of formation, are not fully understood. Modern analogues would provide valuable insights into the incipient formation of carbonate concretions, but have rarely been reported. Here we report carbonate-rich layers observed in a sediment core from the northeastern Japan Sea offshore of Joetsu, which may represent snapshots of the incipient concretionary growth. The sediment core was collected from a reference site C8008A during Expedition 809 of D/V Chikyu in September 2022. Semi- to fully-consolidated carbonate-rich layers and isolated spherical bodies, which were 2 to 10 cm thick and lighter in color than the surrounding mud, were observed in 11 sections from 10–139 m below seafloor (bsf) of the core. Carbonate content increased from 10–32 wt% in the surrounding mud to 31–61 wt% in the carbonate-rich layers. The carbonate-rich layers showed a 0–12% decrease in porosity and a few micrometers decreases in pore sizes relative to the surrounding mud. Smear-slide observations showed that acicular calcite crystals of ~10 μm in length were major constituents of the carbonate-rich layers. Element analysis using an X-ray fluorescence scanner showed elevated calcium (Ca) intensities in the carbonate-rich layers and gradual decreases in Ca toward the surrounding mud. The gradual decreases in the Ca intensity were observed at a thickness of ~2–3 cm, which is an order of magnitude thicker than the “reaction front” previously recognized by a steep change in Ca concentration in consolidated carbonate concretions (Yoshida et al., 2015). These results suggest that the carbonate-rich layers and spherical bodies were formed over months, with slower growth rates than those estimated for carbonate concretions exposed on land. Future investigations will elucidate the inorganic carbon source, organic carbon content, pore fluid geochemistry, and microbial communities in the carbonate-rich layers and help to develop a precise model for the early processes of concretion formation. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).