Study On Paleoceanographic Environment Of Late-Carboniferous Reefs In Southern Guizhou Province, China

Reefs preserve and record the life and geographical conditions of in situ organisms, and their formation and developmental history reflects the evolutionary process of reef-building organisms and their surrounding environments. The process of studying the evolution of reefs for paleoceanographic, paleoclimatological and paleoenvironmental information has been regarded as a heightened and intensified research field to both modern marine biologists and geologists. Unfortunately, when using only general observation and description, on reefs it is sometimes impossible to reveal specific paleoceanographic environments. As a result, several advanced testing capabilities based on biology, marine ecology and geochemistry are used in this paper to precisely extract ocean environmental information. Using these methods, the paleoceanographic environment of late-Carboniferous reef strata in Southern Guizhou province is reconstructed completely and systematically. It greatly extends the study scope and depth for organic reefs of the south Guizhou region.Despite the fact that the Carboniferous is considered a time dominated by phylloid algae, southern Guizhou Province had, at that time, developed relatively better organic reefs. The number and types of reefs are substantial in this region, and more importantly, a large-sized coral Fomitchevella skeletal framework reef occurred in the Triticites zone-the lower stratum of Maping Formation. The development of metazoan framework reefs in south Guizhou has become a feature of Carboniferous reefs in China, and also a bright spot all over the world. Organic reefs represent the initial ecological pattern in shallow marine carbonate settings, and they are products of organism-environment interactions, therefore reefs are good subjects of oceanic paleoenvironmental research. South Guizhou of China is thus regarded as an ideal place for studying late Carboniferous paleoceanographic environments."Today is the key to the past." By comparing ancient coral reefs with their modern counterparts, we can infer growth conditions of ancient reefs. With the "past inferred from present" Uniformitarianism philosophy of geology, several factors that affect reef development in south Guizhou are studied systematically, including a series of single factors about physical environment such as sea level fluctuations, hydrodynamic energy, substrate types or properties, water depth and illumination, water temperature, salinity and seawater chemistry such as Mg/Ca molar ratio, calcite saturation level, water quality, nutrient availability and biological metabolic products. All of these factors have a great effect on phylloid algae, coral Fomitchevella and other reef-building organisms. In this context, the substratum types, illumination intensity and water temperature have been studied especially closely.Two models have been proposed to describe recruitment patterns of reef-building organisms. In the "lottery" model, recruitment occurs randomly when space becomes available, whereas in the "deterministic" model, the larval selectivity for appropriate substrata is important in determining spatial patterns during recruitment. The recruitment patterns of phylloid algae, Fomitchevella, Ivanovia cf. manchurica and Antheria in south Guizhou support the "deterministic" model, and in many cases these organisms settle on biogenic hard substrata.The skeleton of the fast-growing Fomitchevella is porous, so it has a high specific surface area and could absorb large quantities of nutrients from running water. A higher hydrodynamic regime at a large-scale coral reef core could increase the frequency of nutrient absorbing as compared to low-energy environments such as the reef margin, and could therefore obtain more amount of organic material in shorter amounts of time, which helps skeleton-growing and allows for rapid expansion of the entire colony. Longitudinal distribution of Carboniferous reef-building organisms in south Guizhou follows certain rules, which reflect the characteristics of biological types varied with water depth. Perhaps the differences in light intensity from deep to shallow waters controlled the morphological changes of phylloid algae and coral colonies. For example, phylloid algal morphology varied from dispersed single-cup to binder-leaf shape, and then to closely packed acervate-pups. It is the same with the rugose coral Fomitchevella, which adjusted its structure and morphology when the amount of light changes.Temperature has long been regarded as a principal control on the formation of reefs. Paleo-oceanic seawater temperature can be deduced from the stable oxygen isotope ratio of articulated fossil brachiopod shells that are composed of low-magnesium calcite. Isotopic exchange equilibrium between calcite shells of marine organisms and ambient seawater is an important prerequisite for reliable paleotemperature reconstruction. The secretion rate of adult brachiopod shells in shallow water of low latitudes is slower than that of juveniles, so biological fractionation of oxygen isotope takes place less readily in adult shells. Adult shells are therefore more appropriate for carbon and oxygen isotopic sampling and more reliable for interpretation of paleoenvironment. Paleosalinity reconstruction can be attained by oxygen and carbon isotopic compositions (δ18Oand δ13C) of brachiopod shells, as it has more to do with δ13C values.Brachiopod faunas experienced a series of long geological processes after death and did not always keep their original isotopic composition, which reduces their environmental significance during interpretation. The biominerals usually have poor preservation and need to be removed due to alteration caused by diagenesis during analysis of the sample, especially in those shells before the Mesozoic era. By far, the preservation state of brachiopod shells is usually tested by conventional selection criteria such as microscopy, cathodoluninescence and trace elemental analysis. If there is no obvious erosion or recrystallization and no secondary minerals in shell microstructure, and nonluminance in the bright cathode-ray, and trace elemental contents are also comparable with that of modern brachiopods-that is to say, those shells have low Fe, Mn and high Sr, Na-contents are regarded as well-preserved. Trace elements might be absorbed into mineral lattice and distributed in ringlike growth bands of shells during shell growth. Elemental concentrations might differ with changed external conditions such as seasonal alternation, but in terms of the same band, elemental distribution in an A-B shell transect should be symmetrical about center line S-S’because they belong to the same historical stage.δ18O values of brachiopod shells vary from-4.980%o to- 3.249%o (PDB) in the late Carboniferous reefs of South Guizhou, China. Using a standard conversion formula, these values indicate temperatures ranging from24.39to34.21℃, which shows that a warm to hot climate with minimum effect from the glaciation of Gondwanaland in the southern hemisphere. The high temperature means high water CaCO3saturation, and it is favourable toward benthic skeletal calcification and marine carbonate precipitation.δ13C value of brachiopod shell is high and varies from4.271%o to5.793%o (PDB). The data reflects that large quantities of organic carbon were buried and marine biological productivity was high. Z value represents salinity and varies from134.79to137.44, reflecting that at that time, the marine environment was normal and the fluctuations in seawater salinity were very small.Sedimentary environments remained stable over the long periods of time during the process of reef development in the study area. Heavy metal concentrations were low, and water quality was safe and clean because terrestrial detrital materials were almost entirely prevented from going to the reef area at the platform margin.