Mercury chemostratigraphy across the Silurian–Devonian boundary event in the Prague Basin

Laura Marcela Caicedo Florez

Master's thesis

Abstract:

The evolution of global biodiversity and its relationship to environmental change over geological time is a major goal of Earth and Life Sciences research. The increase in global biodiversity during the Phanerozoic was often interrupted by global extinction events which are not fully understood. In the last decade, evidence has emerged of intense Large Igneous Province (LIP) volcanic activity having caused changes in global climate, biosphere, and global extinction events. Extinctions near the Silurian-Devonian boundary are well documented worldwide, and they have contributed to the reduction of Paleozoic global marine biodiversity. This critical period has been characterized by many global events: climate oscillations, eustatic sea-level changes, anoxic/hypoxic events in seawater, and instability of the global carbon cycle, but the possible link between these extinctions and intense volcanic activity has never been analyzed. Mercury enrichments in sedimentary rocks are often used as diagnostic fingerprints of major volcanic eruptions and as possible links to the Phanerozoic mass extinctions. In geological history, Hg was dominantly emitted via volcanism in the atmosphere–land-ocean cycle before sinking to the seafloor. I report (for the first time) the mercury (Hg) chemostratigraphic record across the Silurian-Devonian boundary interval focusing on the analysis of Hg, major and trace elements contents from two sections of different depositional environments of the continental shelf sediments of the Prague Basin: the Opatřilka section representing the nearshore environment, and the Klonk section, characterized by offshore deposition which was approved by the International Commission on Stratigraphy as the Global Boundary Stratotype Section and Point (GSSP). The present study is based on 83 rock samples collected from these sections and analyzed for their total sulfur, total carbon, CO2, and major and trace element contents, including mercury. Main lithological types were also characterized by microfacial analysis. Mercury content differs in both sections, ranging from 1 up to 18 ppb in the Opatřilka section (except for one sample showing 230 ppb), and from 11 up to 87 ppb in the Klonk section. Data obtained from statistical correlation analyses showed that Hg exhibits positive correlations with most of the major and trace elements, except those that are components of the carbonate fraction of the rock (Ca, Sr, and CO2) which shows that the Hg content in the sections mainly depends on the detritic component of the rock as well as by total organic carbon and total sulfur contents. Significant differences were found between sections but the correlations between the detrital component and total organic carbon and total sulfur do not allow us to distinguish whether mercury is controlled by TOC and TS only or also by detritus. A significant finding of this work is the observation of a Hg peak (ca. 230 ppb) in the lower part of the Silurian Monograptus transgrediens graptolite Biozone at the Opatřilka section. The Hg peak cannot be explained as of detrital or TOC origin. It is unclear if the Hg anomaly represents an indication of extensive volcanic activity or Hg accumulation resulting from diagenetic/hydrothermal mercury mobilization. Further investigations of the anomaly are needed. It should be noted this thesis is the first application of Hg chemostratigraphy in the classic Barrandian area.

Keywords

Prague Basin, Volcanism, GSSP, Extinctions, Silurian–Devonian boundary, Mercury chemostratigraphy

Language used: English

Date on which the thesis was submitted / produced: 8. 12. 2021

Thesis defence

• Supervisor: prof. Dr. RNDr. Jiří Frýda

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Master programme / field:
Geology / Environmental Geosciences