The Origin Of The Ediacaran ?¹³C Negative Anomalies

The Ediacaran?ca.635�541 Ma?is period of the large fluctuation of carbon cycle in Earth history.Carbon isotope compositions(?¹³C)from the carbonate rocks of the Ediacaran show multiple prominent excursions,of which at least three?¹³C negative excursions have globe distribution,including the?¹³C anomaly in the early Ediacaran cap carbonate,the Shuram-Wonoka anomaly in the late Ediacaran?ca.570�550 Ma?,and the?¹³C anomaly at the Precambrian�Cambrian boundary.The Shuram anomaly is the largest?¹³C excusion in Earth history?nadir down to<-10�?with a duration of seveal millions of years?ca.2�20 Myr?.It is also well developed in Ediacaran strata in South China and variabley named as DOUNCE,EN3 or N3.The origion of the Ediacaran?¹³C anomalies,however,remains highly controversial.Prevailing hypotheses include authegenic carbonate contribution,diagenetic alteration,oxidation of large dissolved organic carbon?DOC?in the deep ocean,and methane release.Recently,it was argued that the Shuram anomaly was related to large-scale carbonatite-alkaline magmatism,based on detrital zircon geochronology.Compared to other igneous rocks,carbonatite-alkaline rocks are enriched in CO₂.The activity of carbonatite-alkaline magmatism would released a large amount of mantle-derived CO₂into the atmosphere-ocean system,changing the composition of the DIC isotope in the ocean and resulting in negative?¹³C excursion.Indeed,most of globally-correlative negative?¹³C excursions in Phanerzoic were attributed to massive volcanism.This paper conducted U-Pb dating and trace element geochemical analysis for detrital zircons extracted from two sandstone samples using the LA-ICP-MS method.The studied samples are yellowish green quartz siltstone collected from the Gaojiashan Member of the Ediacaran Dengying Formation in the Gaojiashan section,Ningqiang area,Shaanxi Province and the purplish red quartz sandstone collected from the Shamao Formation in the Fenxiang�Wangjiawan section,Yichang area,Hubei Province.In addition,we compiled data of the U-Pb age and trace element data for more than 1200detrital zircons extracted from the late Neoproterozoic-Triassic siliciclastic rocks in South China.The time of magmatic activity was obtained according to the peak range of U-Pb age of zircons,and carbonatite-alkaline magmatic zircons were screened using the"classification regression tree"?CART?analysis.The age distribution of carbonatite-alkaline zircons provides insight into the linkage between the Ediacaran?¹³C anomalies and magmatic activity.The results show that U-Pb ages of detrital zircons from the Gaojiashan Member are mostly within 781�715 Ma,with a peak age of 751 Ma.Five zirons from this sample are related to alkaline magmatism.The U-Pb ages of detrital zirons from the Shamao Formation have a wide range from Paleoproterozoic to early Paleozoic.One major peak age was identified in the early Neoproterozic?⁷61 Ma?.Three minor peak ages were observed at ca.2475 Ma,ca.910 Ma and ca.512 Ma.Among all the analyzed detrital zircons,only three has an origin of alkaline magma.Statistical analysis of more than1200 detrital zircons reveals that most ages of carbonatite alkaline are from 1000�700Ma,and 600�485 Ma,with major peak ages at 962 Ma,867 Ma,751 Ma,541 Ma,and522 Ma respectively.Three minor peaks were also identified at 2487 Ma,1824 Ma,and636 Ma.A visual observation reveals that several peak ages of arbonatite alkaline zircons are well corresponded to Ediacaran?¹³C anomalies.No peak age was along with the DOUNCE in South China,however,convincing evidence for alkalinic magmatic acitivity was obtained in the Gondwana.The match between them may indicate that the globally-correlative negative?¹³C excursions during the Ediacaran were effected by mantle-derived CO₂.This research highlights the connection between magmatism and supergene C cycle.