Abstract: In order to investigate the stability of carbonates in foraminifera tests under early diagenetic conditions, this study
designed simulation experiments to analyze changes in morphology, structure, and chemical composition of the tests of Nonionella decora (CF1), Ammonia aomoriensis (CF2), and Ammonia tepida (CF3) after being eroded by dissolved S^2-, dissimilatory-iron reducing bacterium (DIRB), and sulfate-reducing bacterium (SRB). Results showed that calcite in the tests of CF1 and CF2 can be dissolved by S^2- when its concentration is above 64 mmol/L; while calcite in the test of CF3 can be dissolved only when S^2- concentration is above 128 mmol/L. The dissolution occurred mainly at the place where cogwheel structure is well-developed. DIRB strongly promoted the dissolution of calcite; while such promotion was not obvious in SRB experiments. Dissolved S^2- did not change the Raman spectroscopy characteristics of calcite in foraminifera tests; while DIRB and SRB led to systematic shifts of 5~7 cm-1. Dissolved S^2-, DIRB, and SRB can promote the dissolution of carbonates in the tests and the homogenization of Mg. Besides, DIRB and SRB can promote the formation of Mg-enriched carbonates in the tests. The results suggest that DIRB is much stronger than dissolved S^2- and SRB to change the composition and structure of foraminifera tests under early diagenetic conditions. However, the effects of SRB cannot be ignored from a long-term perspective. 

Key words: foraminifer, test, early diagenesis, calcite, decomposition