Reassessing the possibility of marine incursion into a paleo-lake under the constraints of a chronological framework: Implications for lake salinization mechanisms

The issue of marine incursion into paleo-lakes has been a topic of considerable debate, which has led to ambiguity about the processes of lake salinization. A case in point is the decades-long debate over the salinization mechanism of paleo-lakes in the Eocene Bohai Bay Basin of eastern China. Despite the longstanding preference for the marine incursion model as an explanation, there is currently no comprehensive and convincing chain of evidence validating the occurrence of marine incursions. The present study conducted cyclostratigraphic analyses on lacustrine sedimentary sequences from the Dongying Depression in the Bohai Bay Basin, established an astronomical time scale for the middle−late Eocene, and reconstructed continuous and objective lake-level records. The possibility of marine incursions was reassessed by examining the relationships between paleo-lake salinity and lake-level and sea-level variations through isochronous comparison. The results indicate that the lake-level changes at 2.4 m.y. cycles in the Dongying Depression were in-phase with global sea-level changes during the middle−late Eocene. However, these changes were not completely synchronous on millennial time scales, but rather exhibited an antiphase relationship in most periods. Concurrently, high lake water salinity was predominantly observed during periods of low lake level. These phenomena are incongruous with the conventional characteristics of marine incursion. Nevertheless, they still imply interactions between lake and sea, which we suggest are primarily caused by the subsurface seepage of seawater through the barrier separating the sea and direct modulation of the groundwater table by sea-level fluctuations. We subsequently propose a new mechanism for the salinization of paleo-lakes in the Dongying Depression. This mechanism emphasizes the combined contribution of terrigenous supply and seawater seepage as sources of salt ions, and incorporates fluctuations in climate as a crucial factor in maintaining perennial salinization. A primary process of this mechanism occurs during periods of arid climate, when enhanced evaporation results in the concentration of lake water and a significant drop in lake level, causing a difference in elevation between the ocean surface and the lake. This creates a hydraulic gradient sufficient to drive seawater seepage through the barrier, thereby further accelerating lake salinization. The proposed model offers a potential resolution to the long-standing debate surrounding the marine incursion hypothesis and empirical geological evidence for the study of lake salinization in analogous geological settings.