Sedimentary pyrites and C/S ratios of mud sediments on the East China Sea inner shelf indicate late Pleistocene-Holocene environmental evolution

Sedimentary pyrite and the ratio between organic carbon and pyrite sulfur (C/S ratio) have been widely used as paleosalinity indicators to distinguish between marine and freshwater environments. However, in unsteady marine environments with strong physical reworking, the formation of pyrite is limited, leading to a high C/S ratio, as is the case in freshwater environments; thus, whether C/S ratios can identify unsteady marine environments in sedimentary strata is still a controversial topic. To calculate the availability of these classic indicators in unsteady environments, we analysed multiple indicators of core sediments obtained from the inner shelf of the East China Sea. Our results show that there are no pyrite aggregates in the sediments deposited under terrestrial conditions before 13.1 ka. During this period, the C/S ratios are higher than 2.8, representing the characteristics of sediment deposited in freshwater environments, which is also supported by low Sr/Ba ratios. When the core site was initially affected by seawater intrusion at approximately 13.1 ka, pyrite aggregates appeared for the first time, accompanied by a rapid decrease in C/S ratios, indicating a brackish tidal flat environment. We find that the C/S ratio overlaps between tidal flat and inner shelf environments, but the relatively high content of pyrite aggregates and fine-grained sediments in the shelf sediments can effectively separate these two environments. Our results further reveal that the mud depocentre initially developed at approximately 7.5 ka when the sea level reached its highstand. These new findings suggest that geochemical indicators should always be used in conjunction with sedimentary and paleoecological evidence to prevent biased conclusions.

Automated summary

Sedimentary pyrite and C/S ratio have been widely used as indicators for distinguishing between marine and freshwater environments. However, in unsteady marine environments, pyrite formation is limited, resulting in high C/S ratios similar to freshwater environments. By analyzing core sediments from the East China Sea, we found that sediments deposited before 13.1 ka were from freshwater environments, while sediments after 13.1 ka indicated a brackish tidal flat environment. Although there is an overlap in C/S ratios between tidal flat and inner shelf environments, the presence of pyrite aggregates and fine-grained sediments in inner shelf sediments effectively distinguishes these two environments. Additionally, the development of the mud depocentre coincided with a highstand sea level at approximately 7.5 ka.