CARBON ISOTOPIC COMPOSITIONS AND EVOLUTION IN THE DEVONIAN MARINE CARBONATE ROCKS

The Devonian carbonate rocks in Liujing,Hengxian,Guangxi exhibit the following trend of carbon isotopic compositions:δ¹³C became progressively heavier from the Lochko-vian to Pragian;but obviously lighter during the Emsian;heavier once again during the Eifelian resulting in a remarkable positive excursion during the Givetian;the heaviest near the Frasnian-Famennian boundary,and subsequently lighter once again.This secular trend is positively correlated with (second-order) sea-level changes during the Devonian, implying the genetic affiliation with sea-level changes and relevant changes in climates and CO₂ contents in the atmosphere and oceans.The evolutionary patterns of carbon isotopic compositions near the Frasnian-Famennian boundary suggests that the mass extinction events near the boundary were probably caused by the accumulated effects of the eustatic fluctuations and periodic extension of anoxic environments.