7.8−7.4 ka environment of diatom−rich layer and relative sea level change in the south plain of the Haihe River, Tianjin

This paper is the result of marine geological survey engineering.

Objective Studying the Holocene paleoenvironment and sea−level changes is of great practical significance for predicting the modern geological environment evolution trend.

Methods This paper analyzes the diatoms and AMS ¹⁴C dating data from three 30 m deep drilling cores collected near Chenier V of northern Cangzhou, Bohai Bay.

Results The diatoms are rare in these drilling cores, and there is only one diatom rich layer in each core, which is composed of peat and humus gray clay. The thickness of the layer is 10 cm to 60 cm, and thinner from seaward to landward. 7473 cal a BP, site of DC01 changed from a sea water influenced swamp environment to a fresh water swamp environment, indicating a relative sea−level lower than −6.37 m; 7513 cal a BP, site of QX02 was transformed from saltmarsh environment (zone Ⅰ) to sea water influenced shallow swamp, then to salt marsh environment (zone Ⅲ), The elevation of the boundary between zone Ⅰ and zone Ⅱ indicating a relative sea−level of −6.68 m. 7836 cal a BP, site of QX01 was transformed from a freshwater swamp (zone Ⅰ) to a salt marsh environment (zone Ⅱ). The boundary between zone Ⅰ and zone Ⅱ indicating a relative sea−level of −7.68 m.

Conclusions Relative sea−level raised −1 m over a period of about 350 years from 7.8 to 7.4 cal ka BP and the sea water affected the area to the west of the Chenier V. The salt marshes and freshwater swamps were the main environmental types near the transgression maximum in the Early−Mid Holocene. Under the background of modern sea−level rise, marshification is a major environmental problem that modern coastal areas must face.