Ammonite concretion formation through organic decomposition in the iron reduction zone

IF 2 4区 地球科学 Q1 GEOLOGY
Yusuke Muramiya, Hidekazu Yoshida, Nagayoshi Katsuta, Ryusei Kuma, Tomoyuki Mikami
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Abstract

The ammonites in spherical carbonate concretions often preserve their original three-dimensional (3D) shell shapes and detailed fragile structures. However, the formation process of spherical ammonite concretion is not fully understood. Herein, the ammonite concretions identified in the Cretaceous (Campanian) Osoushinai Formation, Yezo Group, Japan, are examined to understand their formation process during the soft tissue decomposition after burial in marine sediments. In the Osoushinai Formation, almost all observed ammonites in concretions preserve their 3D form without phragmocone deformation. The calcite filling in the remaining body chamber of ammonites (BC1) shows that shells were buried with soft tissues. These occurrences, negative delta13C values, and the near-zero delta18O values of BC1 as well as the concretions indicate that both BC1 and concretions rapidly formed from dissolved inorganic carbon derived from organic matter, including the soft tissue of dead organisms, in the shallow part of the sediments. The increasing Fe concentration in BC1 shows that BC1 formed in the iron reduction (FeR) zone, where organic matter was decomposed owing to the activity of iron-reducing microorganisms. The similarity of the elemental and isotopic compositions of BC1 and concretions show that they concurrently formed in the FeR zone. In the Osoushinai Formation, an abundant influx of Fe(III) and intense bioturbation during the deposition of the formation promoted organic decomposition in the FeR zone, causing rapid formation of BC1 and concretions. Such rapidly formed calcite fillings and concretions protected fossils from deformation and dissolution during diagenesis to preserve their 3D form. Overall, the findings of this study provide a new insight into the relation between sedimentary environments and the fossil preservation process via rapid concretion formation.
铁还原带有机分解形成氨石凝块
球状碳酸盐固结中的菊石通常保持其原始的三维(3D)壳形状和详细的脆性结构。然而,球形菊石结核的形成过程尚不完全清楚。本文对日本野三群白垩系(坎帕系)Osoushinai组中发现的菊石结核进行了研究,以了解其埋藏于海洋沉积物中软组织分解过程中的形成过程。在Osoushinai组中,几乎所有观察到的菊石在结块中都保持了它们的三维形态,没有发生层状变形。鹦鹉螺(BC1)体腔剩余部分的方解石充填表明其壳被软组织掩埋。这些分布、δ 13c值为负、δ 18o值接近于零的特征表明,BC1和结核都是由沉积物浅部有机质(包括死亡生物的软组织)中溶解的无机碳快速形成的。BC1中Fe浓度的增加说明BC1形成于铁还原区,在此,由于铁还原微生物的活性,有机物被分解。BC1和结核的元素组成和同位素组成的相似性表明它们同时形成于FeR带。在Osoushinai组沉积过程中,丰富的Fe(III)流入和强烈的生物扰动促进了Fe带的有机分解,导致BC1的快速形成和结核。这种快速形成的方解石填充物和固结物保护了化石在成岩作用过程中免受变形和溶解,从而保持了它们的三维形态。总的来说,本研究的发现为研究沉积环境与化石保存过程之间的关系提供了新的视角。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
CiteScore
3.80
自引率
5.00%
发文量
50
审稿时长
3 months
期刊介绍: The journal is broad and international in scope and welcomes contributions that further the fundamental understanding of sedimentary processes, the origin of sedimentary deposits, the workings of sedimentary systems, and the records of earth history contained within sedimentary rocks.
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