BIOLOGICAL MARINE CYCLES IN THE EARLY CAMBRIAN CONSTRAINED BY NITROGEN AND ORGANIC CARBON ISOTOPES FROM SOUTH CHINA

Marine life on the Earth and associated biological cycles have developed significantly over the last 4 billion years coupled with environmental evolution. The Pre-cambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet, when chemical and physical conditions trigged a biodiversity “explosion”. Nevertheless, much work still needs to be done to separate cause from effect and constrain biogeochemical feedbacks during this transition. The geology of the Yangtze Platform (South China) offers a wonderful cross-section of this period, characterized by continuous and unaltered successions which are ideally suited to high-resolution paleomarine investigations. The study of δ¹⁵N and d¹³C isotopes in organic matter in sediments from Xiaotan Section (Yunnan) and their fluctuations with time provide insight into variations in primary productivity and ecological developments. This outcrop, well recording the geological history by high sedimentation rates, presents several lithologies: carbonates, cherts, phosphorites, sandstones and siltstones alternating in nearly 600 meters thickness. Organic carbon isotope values vary between -36‰ and -21‰, tightly following variations in the δ¹³Ccarb record throughout the section. The base of Cambrian is characterised by a significant drop from -35‰ to -25‰ coeval to the δ¹³Ccarb drop observed in this and other sections worldwide. Later on in the Dahai memeber, values increase again up to -20‰ then recovering previous values, showing again direct interactions between DOC and DIC pools. From the base of Zhongycun values stabilize mostly around -30‰ till end section, testifying a period of relative paucity. Accordingly, nitrogen isotopes signals ranging from +9‰ and -1‰ up to the Dahai member testify changes in nitrogen fixation/denitrification ratios and chemocline fluctuations in the water column, coeval to the period of carbon isotopes major variations. Up in the section also nitrogen shows a stable and smoother trend with values between 0‰ and +5‰, a probable indication of higher equilibrium in marine biochemistry and nutrient cycles. The overall variability in δ¹⁵N closely follows that observed from recent marine sediments, which argues for the achievement of modern biogeochemical conditions during the Pc-C transition.