SULFUR ISOTOPE SYSTEMATICS OF CARBONATE ASSOCIATED SULFATE (CAS) OF RIBBON ROCKS FROM THE MIDDLE CAMBRIAN XUZHUANG FORMATION, NORTH CHINA: EFFECT OF EARLY DIAGENESIS ON CAS DATA

Carbonate associated sulfate (CAS) is a trace amount of sulfate that incorporates into mineral lattice during carbonate precipitation. Sulfur isotopic composition of CAS (δ³⁴S_CAS) is regarded as recording seawater sulfate isotopic composition (δ³⁴S_sw), and has been widely used in the study of marine sulfur cycle in paleoceans. However, its implication in chemostratigraphic correlation has been hampered by the wide-range of variation in δ³⁴S_CAS. In order to test whether δ³⁴S_CAS would be affected in early diagenesis, in this study, we measured sulfur isotopic compositions of CAS extracted from the ribbon rock samples collected from the middle Cambrian Xuzhuang and Zhangxia formations in North China. Ribbon rock consists of alternating limestone and marlstone layers. It is widely accepted that the limestone and marlstone layers have undergone differential diagenesis with preferential calcite precipitation in the limestone layer and compaction in the marlstone layer. δ³⁴S_CAS of the limestone layers (δ³⁴S_lst) are exclusively higher than that of the marlstone layers (δ³⁴S_CAS), but CAS concentrations are lower in the limestone layers. Such isotopic difference between the limestone and marlstone layers cannot be attributed to contamination from pyrite oxidation, nor could it result from rapid oscillation of seawater sulfate isotope compositions. We suggest that higher δ³⁴S_lst is best interpreted as diagenetic calcite precipitation of the limestone layers within microbial sulfate reduction (MSR) zone, in which porewater sulfate has lower concentration but higher δ³⁴S than seawater sulfate, while calcite precipitation is inhibited by clay minerals in the marlstone layers. Our study indicate that δ³⁴S_CAS could be modified due to diagenetic calcite formation within MSR zone. In order to reconstruct δ³⁴S_sw by using CAS data, rigorous diagenetic evaluation is required, and carbonate containing substantial amount of diagenetic carbonate minerals should be avoided.