Late paleozoic climate revealed by coral fossil patterns

However, this method requires well-preserved, detailed subjects. The object of this paper is the coral fossil unearthed in the Shiqiantan Formation in Xinjiang Province. Only two intact samples were selected from the same batch of approximately 3100 coral fossils. The samples have very clear growth lines and can be used to study paleoclimate by measuring the growth increments. Previous records of paleoclimatic reconstructions were held by ice sheets, and the oldest record is 5 million years old [6]. However, the fossils used in our paper remarkably exceed this record. The coral fossils in this paper were from the Shiqiantan Formation in Xinjiang Prov- ince obtained approximately 310 million years ago, a typical sedimentary layer from the Late Carboniferous, which preserved a diverse assemblage of coral fossils. Research conducted by Wang [7] supports that coral fossils in the Shiqiantan Formation are predominantly found in shallow marine environments. This conclusion is based on the sensitivity of corals to environmental conditions and the geological context of the area [8]. However, subsequent to the deposition of the Shichantan Formation, corals and other shallow marine organisms gradually decreased and eventually became extinct in the Junggar region, suggesting a marine regression during the late stages of deposition, resulting in the transfor- mation of the Junggar area from a shallow sea to a terrestrial environment. Materials and methods This paper uses coral fossils unearthed in Shiqiantan in Xinjiang Province and belong to

warm-water monomer corals [9]. The research on the specific structure and growth mecha- nism of the monomer coral is unavailable, but this ancient species of coral still exists. Thus, this paper starts with the relevant content of modern corals. The basic structure of modern coral skeleton includes calice, wall, septum, basal plate, dissepiment, columella, and spine, as shown in Fig 1. Very fast-growing specimens are required to study climate variations through the measure- ment of growth line width; otherwise, the variation in width will not be distinguishable. There- fore, this paper starts with the growth pattern of hermatypic coral, as shown in Fig 2. Hermatypic coral can efficiently use Ca2+ and dissolved inorganic carbon in seawater and rapidly precipitate aragonite calcium carbonate (CaCO3) skeletons, which are the main com- ponents of coral reefs. Coral calcification is affected by light intensity. Eyal G. studied Euphylliaparadivisa [11] and found calcification increases dramatically with increasing light intensity in shallow-light- treated corals. This finding is consistent with the following calcification equation: Ca2þ þ 2HCO 3$ 2Photon CaCO3 þ H2CO3 ð1Þ According to reaction kinetics, the reaction rate depends on the square of light intensity. The photons of the calcification equation are catalysts, and the study of Eyal G. Cohen I pointed out the minimum saturation irradiance. Thus, measurements of the width of the growth line can demonstrate the changes in day length or solar radiation. Although the fossil

coral at Shiqiantan is not the type of hermatypic coral, the basic principle of light-enhanced calcification in coral species should not have changed, and the calcification deposits should fol- low the calcification equation. Therefore, the growth line width of coral fossils in the Shiqian- tan Formation in Xinjiang Province can reflect the changes in ancient day length or solar radiation. Growth ridges on fossil epitheca are a fundamental skeletal structure in tabulates rugosans and scleractinians. They are not only the basic materials of the exterior of the corallum but also the raw materials in various guises as septa and dissepiments, as shown in Fig 3. PLOS ONE Late paleozoic climate revealed PLOS ONE | https://doi.org/10.1371/journal.pone.0290127 August 15, 2023 2 / 13