[M-IS17] Aqua planetology

[MIS17-P02] Evaluation for formational mechanism of spherical carbonate concretions on Earth and comparison with Martian spherical nodules

*Watanabe Hayao¹, Hitoshi Hasegawa¹, Minoru Ikehara³, Saki Asai², Ryusei Kuma², Hidekazu Yoshida² (1.Faculty of Science and Technology, Kochi University, 2.Nagoya University, 3.Center for Advanced Marine Core Research, Kochi University )

Keywords:concretion, formation mechanism, Mars

Concretion is a hard, compacted material found in sedimentary rock, which generally formed by the precipitation of carbonate minerals, such as calcite, dolomite, siderite, within the spaces between clastic particles. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. It normally forms several cm in size, although some forms much larger several meters in size. Some of the spherical carbonate concretions contains body fossil and trace fossil and thus it has been thought that precipitation of carbonate is related to decay of organic matter and formed by biogenic origin (Yoshida et al., 2015, 2018a). Nevertheless, some concretions do not contain obvious fossil remains but form large size by burial diagenesis (Loyd et al., 2014; Muramiya et al., 2020) or methane sheep origin (Maeyama et al., 2020). In addition, some carbonate concretions in non-marine deposits are formed by evaporation of groundwater in vadoce zone by inorganic process due to the arid climatic setting (Hasegawa et al., 2009). On the other hand, spherical concretion-like objects are also found on Mars, such as hematite spherules (blueberries) in Meridiani Planum (Chan et al., 2004), which are interpreted to be formed by intercalation of precursor carbonate concretions and acidic water infiltrations (Yoshida et al., 2018). Recently discovered spherical shape nodules in Gale crater (Stack et al., 2014; Wiens et al., 2017) also suggest that the carbonate concretions may have been a precursor. However, formation mechanisms of these spherical nodules remain largely uncertain. In order to evaluate the origin of these Martian spherical nodules, the present study aimed to examine various types of spherical concretions on Earth and to verify how we can distinguish between those formed by biogenic and inorganic processes.
In this study, a variety of concretion samples are collected from several areas with different geological age and depositional environment, such as (1) Upper Cretaceous Yezo Group in Haboro area, Hokkaido, (2) Miocene Morozaki Group in Chita Peninsula, Aichi, (3) Miocene Misaki Group in Tosashimizu area, Kochi, (4) Miocene Yatsuo Formation, Toyama, (5) Lower Jurassic Navajo Sandstone in Utah, USA, (6) Upper Cretaceous Djadokhta Formation in southern Mongolia. Polished surface of these concretion samples are observed by Reflected fluorescence microscope and elemental mapping obtained by X-ray analytical microscope (XGT-5000). On the basis of the internal textures, fossil contents, and elemental distributions, we categorized them into 3 types as follows. Type 1 is characterized by body fossil remains with phosphorous concentration surrounding them. Type 2 is characterized by occurrence of trace fossils and pellets, with formation of pyrite minerals. Type 3 does not have clear nuclei and signs of biogenic influences. For better understanding of causal mechanisms of different types, detail stable carbon and oxygen isotope analysis are now ongoing.