This paper is the result of Coast Geo-Environment.
[Objective] Based on combination between the global sea level history and the reconstruction of the observed local Holocene relative sea level change, this research is aimed at revealing that the sea level change is the most important driving force controlling the land-sea change and morphological pattern transition and future coastal trend in the muddy coast of Bohai Bay, China. [Methods] Modelling effort has reconstructed high-resolution history of the global sea level change since the LGM (e. g., Lambeck et al., 2014; Peltier et al., 2015), and summarized contributions of various elements to the sea level change (e.g., Horton et al., 2018). On the other hand, as most coastal geologists, we scrutinized primarily the local and regional elements, which affected the relative sea level (RSL) behaviors in Bohai Bay coast, with quantitative or semi-quantitative estimates (Li et al., 2021; Wang, 2022) and more detailed case studies (Fan et al., 2005; Wang et al., 2020; Tian et al., in review). [Results] The aforementioned approaches indicated global mean sea level (GMSL) rise was ~9mm/a during 10～7 ka while the rate of the local relative mean sea (RMSL) was ~5.6 mm/a, contemporaneously. Such rapid rises resulted shoreline recession worldwide, including Bohai Sea region with possible inundation of a number of beach ridges. However, after ~7ka, rise of the ice-equivalent sea level (ESL) decreased to ~0.64 mm/a; after 5 ka, the rise even stopped. As to Bohai Bay, the RMSL rise was ~0.46 mm/a after 7 ka and was only ~0.18 mm/a since 5.5 ka. Both global and regional sea level changes show obvious two-stage-rise characteristics with a turning-point around 7 ka, before and after which the rise changed greatly with an order of magnitude. Overlapping on the remarkable deceleration is an essential offset
between the regional isostatic uplift and the local subsidence of neotectonics and sediment self-compaction. Thus, the triple influences, caused by slowing rise of sea level, isostatic uplift and local subsidence, created a nearly ~7 ka long morphological pattern, in which a cyclicity evolution between barrier-typed shelly cheniers and lagoon/saltmarsh lowlands existed. During the last 150 years from 1870 CE, GMSL rised quickly with an average rate of ~1.7mm/a and even quicker in the last decade (IPCC AR6, 2021; IPCC AR6 SYN, 2023). By the same time, entire protruded sectors of muddy lowland (with upper part of intertidal flat), ~1～
3 km in the front of Chenier I, were fully eroded away and, consequently, the 1870 CE-shoreline was retreated to the Chenier I, an old shoreline ended before the 1870's (Wang et al., 2002, 2010). Therefore, we think that there must be a reasonable causality between the global temperature-sea level rise and shoreline recession in our study area. [Conclusions] Following predictions of ~6～8 mm / a rise of global sea level in the 21 century (IPCC AR5, 2013; IPCC AR6 SYN, 2023), this will increase the local accommodation space and resulting RSL rise (sea level rise and local subsidence). Consequently, a number of coastal responses will follow such as shoreline retreat, tidal flat erosion and salt wetland deterioration. Finally, the local morphological pattern could plausibly return from the 7ka-lasted barrier-typed Chenier-Lagoon System to the Beach Ridge-Coastal Lowland System estimated during the late Pleistocene to early Holocene.
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