4:15 PM - 4:30 PM
[HCG22-09] Classification of spherical carbonate concretions on Earth: Toward understanding the origin of spherical concretions on Mars
Keywords:Spherical concretion, Formation mechanism, Earth, Mars, Biological origin
Spherical concretions of various sizes and compositions are found in sedimentary rock on Earth. Several formation mechanisms are proposed, including (i) the decay origin of biological remains immediately after deposition (Yoshida et al., 2015), (ii) manganese reduction and sulfate reduction in the early diagenetic process, (Loyd et al., 2014; Liu et al., 2019), (iii) deeply buried methanogenic and thermocatalytic origin (Dale et al., 2014). On the other hand, spherical concretions have also been discovered by NASA's landing rovers (Opportunity and Curiosity) in the strata of the Meridiani Planum (Chan et al., 2004; Yoshida et al., 2018) and Gale Crater (Stack et al., 2014; Wiens et al., 2017; Sun et al., 2019) on Mars. Since most of the spherical concretions on Earth are formed with the involvement of organisms, if the Martian spherical concretions contain biogenic material, they may preserve traces of life on ancient Mars and thus important for exploring the habitability of extraterrestrial life. In this study, we performed elemental mapping and isotope ratio analysis in order to understand formational mechanism of spherical concretions collected from various strata on Earth.
In this study, a variety of concretion samples are collected from several areas with different geological age and depositional environment, such as (1) Miocene Yatsuo Group, Toyama, (2) Upper Cretaceous Yezo Group in Haboro area, Hokkaido, (3) Miocene Misaki Group in Tatsukushi area, Kochi. Internal elemental distributions were analyzed by XGT-7200 and EPMA, and carbon and oxygen isotope ratio were analyzed by Thermo Fisher Scientific GasBench II system.
On the basis of the results of elemental and isotopic distributions, the concretions are classified into three types according to the differences in formation and diagenetic processes. Type I is enriched in phosphorus and has a very light carbon isotope ratio (-20‰ to -17‰). The tusk-shell concretion of the Yatsuo Formation is this type, which is thought to have formed in association with the decay of biogenic remains immediately after deposition (Hall & Savrda, 2008; Yoshida et al., 2015). Type II is enriched in phosphorus in the center and enriched in Mn, Fe, and S in the outside, with lighter carbon isotopic ratios (-15‰ to -8‰) toward the center. The ammonoid concretions of the Yezo Group are this type, with the inner part formed by decay of biogenic remains and the outer part formed by manganese and sulfate reduction during the early diagenetic process (Loyd et al., 2014; Liu et al., 2019). Type III is enriched in Mn, Fe, and S, and has relatively light carbon isotopic ratios (-10‰ to -5‰), but there is no marked difference between the central and rim parts. This is the case for the concretion of the Tatsukushi area, which was probably formed under the influence of manganese and sulfate reduction.
Although there are limited studies documenting the formational mechanisms of the Martian concretions, those of Gale crater have a variety of shapes, including spherical, flat, irregular, and dendrite, which are thought to be formed by burial diagenetic process (Sun et al., 2019). In addition, lighter sulfur isotopes (-10‰ ~ -20‰) and carbon isotopes (-30‰ ~ -40‰) were reported in some horizons of Gale crater, suggesting that sulfate-reducing and methanogenic bacteria likely existed on ancient Mars (Franz et al., 2017; House et al., 2022). However, isotopic measurements have not yet been conducted on the concretionary zone. On the other hand, elemental compositions have been measured for several concretions (Sun et al., 2019), and one concretion show a high concentration of P, although causal mechanism of such high P concretion are largely unclear.
In this study, a variety of concretion samples are collected from several areas with different geological age and depositional environment, such as (1) Miocene Yatsuo Group, Toyama, (2) Upper Cretaceous Yezo Group in Haboro area, Hokkaido, (3) Miocene Misaki Group in Tatsukushi area, Kochi. Internal elemental distributions were analyzed by XGT-7200 and EPMA, and carbon and oxygen isotope ratio were analyzed by Thermo Fisher Scientific GasBench II system.
On the basis of the results of elemental and isotopic distributions, the concretions are classified into three types according to the differences in formation and diagenetic processes. Type I is enriched in phosphorus and has a very light carbon isotope ratio (-20‰ to -17‰). The tusk-shell concretion of the Yatsuo Formation is this type, which is thought to have formed in association with the decay of biogenic remains immediately after deposition (Hall & Savrda, 2008; Yoshida et al., 2015). Type II is enriched in phosphorus in the center and enriched in Mn, Fe, and S in the outside, with lighter carbon isotopic ratios (-15‰ to -8‰) toward the center. The ammonoid concretions of the Yezo Group are this type, with the inner part formed by decay of biogenic remains and the outer part formed by manganese and sulfate reduction during the early diagenetic process (Loyd et al., 2014; Liu et al., 2019). Type III is enriched in Mn, Fe, and S, and has relatively light carbon isotopic ratios (-10‰ to -5‰), but there is no marked difference between the central and rim parts. This is the case for the concretion of the Tatsukushi area, which was probably formed under the influence of manganese and sulfate reduction.
Although there are limited studies documenting the formational mechanisms of the Martian concretions, those of Gale crater have a variety of shapes, including spherical, flat, irregular, and dendrite, which are thought to be formed by burial diagenetic process (Sun et al., 2019). In addition, lighter sulfur isotopes (-10‰ ~ -20‰) and carbon isotopes (-30‰ ~ -40‰) were reported in some horizons of Gale crater, suggesting that sulfate-reducing and methanogenic bacteria likely existed on ancient Mars (Franz et al., 2017; House et al., 2022). However, isotopic measurements have not yet been conducted on the concretionary zone. On the other hand, elemental compositions have been measured for several concretions (Sun et al., 2019), and one concretion show a high concentration of P, although causal mechanism of such high P concretion are largely unclear.