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The Carbon Isotope Fluctuations across the Lower–Middle Jurassic Boundary and the Paleoclimate Changes
Author(s) -
YI Haisheng,
XIA Guoqing,
LI Gaojie,
YI Fan,
ZHANG Shuai,
CAI Zhanhu,
JIAO Haijing,
WU Chihua
Publication year - 2019
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.13789
Subject(s) - chinese academy of sciences , sedimentary rock , geology , china , geochemistry , library science , archaeology , geography , computer science
Exiubu section located at the Amdo area of Qiangtang Basin, where the Lower Jurassic Quse Formation and Middle Jurassic Sewa Formation deposited continuously with late Toarcian to Aalenian ammonites at the J1–J2 boundary (Yin Jiarun et al., 2006). We collected 60 samples with a mean spacing of 2 m at the boundary, and these samples are black shales, calcareous mudstones, marls and micrites. One hundred and twenty carbonates samples have been carried out for carbon and oxygen isotopes of carbonate contents, and sixty samples for organic carbon contents and isotopes. Fig. 1 is the carbon isotopes and climate changes curves at the JI–J2 boundary of Exiubu section, Amdo area of Tibet. To visually express the change process of the analysis data, the original data is smoothed by Moving Median (Appendix 1). According to the simultaneous analysis of 60 samples of organic carbon (δCORG) and inorganic carbon (δCCAL) results, the δCORG varies between −22.3‰ and −25.3‰ (PDB) with an average value of −24.05‰ (PDB), and the δCCAL varies from 2.1‰ to −5.3‰ with an average value of −1.12‰ (Fig. 1), which shows the positive excursions observed in the δCORG records in the upper part (0–60 cm depth) of Quse Formation (Lower Jurassic) and the negative excursions in the Sewa Formation (60–120 m depth); especially at the JI–J2 boundary, the δCORG values gradually decline and the Δ value is 3‰ (Δ=3‰ ). Furthermore, the change of the inorganic carbon δCORG of in the J1–J2 profile boundary is similar to that of δCCAL. The top of Quse Formation (Lower Jurassic) exhibits carbon-isotopic values much richer than Sewa Formation (Middle Jurassic), and at the 30–70 m depth, the δCCAL records reach the maximum positive carbonisotope excursion of 1.8‰.