渤海湾贝壳堤:特征、成因与分类OA

This paper is a comprehensive study about the Shelly Ridges in Bohai Bay. [Objective]The shelly ridge(shelly chenier)and inter-ridge lowland system,as an important geological complex,records the shoreline positions and sea-land changes along the western and southwestern coasts of the Bohai Bay since the mid-Holocene.Previous studies have largely identified the spatio-temporal distribution of the shelly ridges,explained their formation mechanisms,and classified them into the"beach-ridge and strandplain"system.However,these fundamental understandings have seen little development or enrichment over the past few decades.On the other hand,current destruction has led to the degradation of shelly ridges and significant changes in the morphology of inter-ridge lowlands,causing research on the Bohai Bay shelly ridges,along with numerous unresolved issues,to gradually fade from public attention of the researhers concerned.In light of this,this paper systematically studies the shelly ridges of Bohai Bay,comprehensively investigates their basic characteristics and formation mechanisms,and attempts to provide a more reasonable classification,with the aim of offering insights for future forecasting of coastal development and sedimentology research.[Methods]This paper collects and references international research on two major non-deltaic systems:the"beach-ridge and strandplain"and the"barrier-island and lagoon",as well as the transition between these two major systems,two case studies on cheniers and barrier islands,and erosional slopes(scarps).At the same time,our another paper(with detailed appendices)(Shang et al.,2025,this issue)specifically discusses all available origional data of the cross-sections of the shelly ridges in Bohai Bay,including the first-hand field records and photographs,the material composition of the ridges and related muddy sediments,micro-geomorphology,and 14C chronological data.Detailed spatio-temporal distribution maps of shelly ridges and inter-ridge lowlands were drawn,and comparisons between ridges and climatic events were made tentatively.This represents the most comprehensive and detailed summary of previous and our own research on the shelly ridges to date.Building on these essential preparatory works,this paper further explores the basic characteristics of shelly ridges and inter-ridge lowlands.[Results]The findings include:(1)The tidal flat first experienced transgressive erosion over several decades to centuries,forming a slope at the depth of the intertidal to upper subtidal zone,which then became the base for shelly ridge accumulation.(2)After a period of relatively mild environmental conditions during the initial formation of the shelly ridges,the middle part of the ridges can be divided into progradational and erosional types,with the latter becoming the dominant type of the young ridges(i.e.,Ridges II-3 and I)over the past 2 ka.Numerous erosional surfaces within the ridges indicate a continuous transgressive process from～2 ka to the present,enriching the previously proposed"Neo-Transgression"hypothesis(Wang et al.,2023).(3)Based on the residence time corrections(Li et al.,2015;Shang et al.,2016;Li et al.,2021),the reworked 14C ages were systematically revised as much as possible,obtaining the start and end times for each ridge:V:6.27-5.53 ka,IV:5.05-4.07 ka,IV-I:4.85-4.74 ka,III:3.3-3.1 ka,II-1:2.11-1.82 ka,II-2:1.59-1.2 ka,II-3:1.45 ka-?,and I:1.1-0.08 ka.(4)The boundary conditions for the formation of shelly ridges and inter-ridge lowlands,including ESL(Eustatic Sea Level),GIA(Glacial Isostatic Adjustment),L1(neotectonics),L2(self-compaction)and L4(vertical accretion),were discussed.Semi-quantitative analysis is shown that under the slowing rise of ESL by～5.4-3.5 m since～7 ka(Lambeck et al.,2014;Peltier et al.,2015),the interplay between GIA+L4 and L1+L2 is essentially a"zero-sum"process,resulting in the western Bohai Bay basically remaining at the intertidal depths since～7 ka,favoring the formation of quasi-barrier island-type shelly ridges and the long-term maintenance of quasi-lagoon states in the inter-ridge lowlands.Conversely,the significant elevation increase in the western Bohai Bay is likely due to soil erosion from agricultural intensification starting around 3 ka,which postdates the appearance of shelly ridges and inter-ridge lowlands at the intertidal depths.(5)It was demonstrated that the mid-Holocene sea water influence's furthest extent,and the maximum transgressive boundary,and the appearance of the oldest shelly ridge(Ridge V)in Bohai Bay and Louisiana's chenier(LCW)are all separated by lowlands tens of kilometers wide and a time span of～0.5-4 ka,even similatly in Nayarit.This may be a universal phenomenon,reflecting slightly larger accommodation space during the mid-Holocene,causing the"temporo-spatial lag"in the appearance of the first ridges,which has a causal relationship with the"transgressive depositional lag"in neritic sea during the Early-middle Holocene rapid rise of sea level(Jiang et al.,2022).(6)The spatial scale of shelly ridge plain and chenier plain is～100-200 km long,～30-100 km wide,and～5 m thick,with a total volume of～15-100 km3,spanning 3 000-6 000 a.Shelly ridges and cheniers are a quasi-barrier island-type special category of the linear sand bodies in coastal zones,forming an important coastal composite body together with the quasi-lagoons behind them.[Conclusions]Based on the above,this paper proposes a sub-concept of"quasi barrier-island and lagoon"between the"beach-ridge and strandplain"and"barrier-island and lagoon"systems.The shelly ridges of Bohai Bay and the cheniers of Louisiana,belong to this new sub-category.This tentative new sedimentological classification scheme and the large-scale spatio-temporal dimensions of shelly ridges may provide new insights for oil and gas exploration.In the 21st century,accelerated sea level rise and reduced sediment supply have increased coastal accommodation space,causing both natural and artificial shorelines(seawalls,sluices,reclaimed land,docks,aquaculture ponds,and waterfronts)to inevitably enter a"barrier island"mode,as evidenced by the transgression since the past 2 ka.On the other hand,research on shelly ridges can provide references for the study of newly emerging natural shell beaches+Spartina alterniflora flats+oyster-barnacle flats composite geoecological systems in Bohai Bay and the protection and restoration of natural shelly ridge segments.The Supplement related to this article is available in the same issue.