EARLY CAMBRIAN OCEANIC ANOXIC EVENT IN NORTHERN GUIZHOU: BIOMARKERS AND ORGANIC CARBON ISOTOPE

Abstract

This paper first provides biomarkers of Early Cambrian black rock series in the Zunyi area, Northern Guizhou. Integrated with organic carbon isotope compositions, the significant geological events are discussed in detail. Abundant biomarkers, including n-alkanes, isoprenoid, steranes and terpanes from the Late Sinian-Early Cambrian black rock series, have been detected by GC and GC-MS. The n-alkanes are composed of nC14 - nC31 with nC18, nC19 and/or nC20 as the main peaks, and characteristic of single peak distribution. The nC17/nC31 ratios of 1.15 to 50.17 show the predominance of light hydrocarbons in the n-alkanes. OEP values vary from 0.94 to 1.11, and CPI from 0.93 to 1.13, close to equilibrium value of 1.0, indicating no obvious odd-even carbon number predominance. Pr/Ph values range between 0.24 and 0.47, and they fluctuate in the vertical section. In the m/z191 GC-MS, C30 hopane is the most abundant in the terpanes. According to the relative abundance, tricyclic terpanes are characterized by highest abundances, pentacyclotriterpanes by the second, and then tetracyclic terpanes. Minor gammacerane is presented. In the m/z217 GC-MS, the regular steranes C27-C28-C29 distributed with "V" shape, and ∑ (C27 + C28) > ∑ C29 with varying ratios between 1.25 and 1.99, whereas steranes ∑ C27/∑ C29 from 0.78 to 1.22 and rearranged steranes C27/regular steranesC27 from 0.21 to 0.47. 4-methyl steranes have been detected, but their abundance are relatively lower. The sedimentary organic matter of anomalously organic-carbon-rich laminated shales across the Zunyi area is characterized by high organic-carboD contents, ranging between 0.05 and 7.91%. The carbon isotope curve displays δ13C values of the kerogen(δ13Corg) fluctuating from -29.49 to -34.41‰ PDB with a negative excursion close to 4.3‰, which may have been associated with seal level rise, anoxic water in the bottom and scarcity of benthic fauna. On the basis of geochemical parameters of biomarkers and organic carbon isotope compositions of the black rock series in the Zunyi area, this paper discussed the maturity of rocks, quality and origin of organic matter, depositional environments and palaeoceanography. This study will provide reliable evidence for the paleogeographic reconstruction and geological events record in Early Cambrican of southern China.