The sediment core LZ01 (119.8413°E, 37.8316°N; Figure 1 ) was collected in 2013 using a rigging drill. The sediment core, measuring 70.1 meters in length, was obtained at a water depth of 17.82 meters. The average core recovery rate was 86.8%. The core was split lengthwise, described visually, and subsequently sub-sampled in the laboratory. A total of 261 subsamples were taken from the core LZ01 at 25–30 cm intervals for grain size and elemental analyses. The samples were treated and tested in accordance with the methodology described by Xu et al. (2015). The grain size properties were calculated in accordance with the formula proposed by Folk and Ward (1957). The quality of the elemental analysis was evaluated using parallel samples and the national sediment standard GBW07309 (GSD-9), which was analyzed for every 10 unknown samples. The relative error of the parallel samples was determined to be less than 5%.

The selection of gastropods and unbroken shells, including the peat layers, for accelerator mass spectrometry (AMS) 14C dating (Stuiver et al., 2020) is presented in Table 1 . Nine samples from core LZ01 were collected for optically stimulated luminescence (OSL) dating of quartz, employing a Daybreak 2200 thermoluminescence/OSL reader ( Table 2 ). The concentrations of K, U, and Th were determined through inductively coupled plasma mass spectrometry and subsequently converted into dose rates, with the requisite data sourced from Aitken (1998) and Marsh et al. (2002).

The general stratigraphic framework of core LZ01 was identified through a process of correlation involving the application of AMS¹⁴C and OSL ages, sedimentary structures, sedimentary facies, grain size, and comparison with other well-studied cores from adjacent areas. The core was subdivided into four units, designated DU1–DU4, in descending order ( Figures 2 , 3 ).

DU 1 (70.10–51.87 m) was primarily composed of dark gray, dark yellow-gray, earthy yellow silt, and clayey silt, interspersed with clayey bands and silt lenses. Calcium nodules and brownish-yellow rust spots are visible, indicating moderate biological disturbance. Locally, there are gray-yellow to yellow-gray fine to medium sands, with average sorting. The sediment exhibits cross-bedding and contains minor clayey bands, along with occasional black spots and rust stains. The sediment composition is predominantly silt, with an average content of 54.86%, followed by sand at an average of 32.92%, with a maximum content reaching 94.23%. Clay content is the least, ranging from 0% to 37.85%, with an average of 12.21%. The average grain size varies significantly, ranging from 3.05 to 7.55 Φ, indicating considerable changes in the sedimentary environment, with an average of 5.17 Φ. The OSL dating for this unit yields relatively young and unreliable ages. Comparisons with surrounding boreholes suggest that the sedimentary environment corresponds to a coastal-marine transitional setting prior to the Late Pleistocene, which maybe formed during marine isotope stage (MIS) 6-7.

DU 2 (51.87–19.03 m) consists mainly of dark gray, dark yellow-gray, dark gray, and earthy yellow clayey silt and silt, with locally dense interlayers and irregular silt to fine sand lenses. A few brown rust spots are visible, along with intact small gastropods. The central area features yellow-gray fine to medium sand, with average sorting and cross-bedding, interspersed with irregular clayey bands. Shell layers are observed at depths of approximately 31.6 m, 33.6 m, 40.8 m, and around 51.8 m, containing abundant shell fragments and intact small gastropods. The sediments are primarily silt and sand, with average contents of 46.59% and 44.16%, respectively, while clay content is minimal, ranging from 0% to 35.63%, with an average of 9.25%. The average grain size also exhibits significant variability, ranging from 3.05 to 7.51 Φ, indicating substantial changes in the sedimentary environment, with an average of 4.69 Φ. The AMS ¹⁴C dating of four intact gastropods from the top to the bottom of this unit yields ages greater than 43,500 years. The OSL dating data is similarly young and unreliable. Comparisons with surrounding boreholes indicate that the OSL ages for the corresponding segment of borehole WFZK07 range from 34.7 ± ka to greater than 110 ka. The sedimentary environment of this unit is interpreted as a coastal to shallow marine setting, corresponding to the sedimentary layers formed during the Cangzhou and Xianxian transgressions, which formed during MIS 3-5.

The DU 3 unit (19.03–10.66 m) primarily consisted of dark gray, dark yellow-gray, yellow-gray, and ochre-colored silt to fine to medium sand, with generally poor sorting. It exhibits no distinct layering, interspersed with clay-rich bands, and occasionally contains black carbon spots and a small amount of brown rust stains. The sediment composition is predominantly silt and sand, with silt content ranging from 15.60% to 62.68%, averaging 48.21%. Sand content varies between 18.48% and 81.70%, with an average of 39.95%. Clay content is minimal, averaging 11.84%. The average grain size ranges from 3.47 to 5.91 Φ, with a mean of 4.95 Φ. Three OSL dating results from the top to the bottom of this unit range from 16.6 ± 2 ka to 25.1 ± 3 ka. The sedimentary environment of this unit is interpreted as a terrestrial river deposition during the Last Glacial Maximum (MIS 2).

The DU 4 unit (10.66–0 m) was divided into two subunits DU 4-2 and DU 4-1. DU 4-2 (10.66-6.80 m) mainly comprised dark gray silt-fine sand, with clayey banding, shells, and gastropods. The bottom layers were characterized by peat and shell fragments, with AMS¹⁴C ages of 9649 cal yr BP and 9459 cal yr BP, respectively. The mean grain size ranged from 4.46 to 5.98 Φ, with an average of 4.94 Φ. This peat layer is a marker of the Holocene bottom of the Bohai Sea. The other two AMS¹⁴C ages of the gastropods were 9102 cal yr BP and 9159 cal yr BP. Therefore, this subunit was interpreted as a transgressive deposit formed during the Early Holocene. DU 4-1 (6.80-0 m) was characterized by dark gray clayey silt-silt, with a silt-fine sandy lens. The mean grain size ranged from 4.15 to 7.17 Φ, with an average of 5.90 Φ. No dating data were available for this subunit. From the sedimentary characteristics and stratigraphic correlation with cores BH1302 and WFZK07 in the adjacent area ( Figure 3 ), DU 4-1 was interpreted as shallow marine deposits during the Middle-Late Holocene.

The mean Cu, Pb, Zn, Cr, Cd, As, and Hg concentrations in the LZ01 core were 17.36, 19.94, 52.73, 51.41, 0.08, 7.24, and 0.008 mg/kg, respectively ( Table 3 ). The average concentrations of all metal(loid)s were lower than those in the Yellow River estuary (Liu et al., 2023), Dongying coast (Wu et al., 2022), Laizhou Bay (Duan et al., 2021), Southern Shandong Peninsula (Liu et al., 2015), and Coastal Shandong Peninsula (Li et al., 2013). The average concentrations of all metal(loid)s were below the element baseline in Shandong Peninsula soil (CNEMC, 1990).

Figure 4 illustrates the vertical profiles of the major and trace elements and the mean grain size (Mz) in the LZ01 sediment core. In addition to arsenic, all other metal(loid)s exhibited comparable distribution patterns, which were predominantly influenced by mean grain size. All metal(loid)s demonstrated the highest average concentrations in unit DU 4, which formed during the Holocene ( Table 3 ).

The results of the study, expressed as enrichment factors (EFs), are presented in Figure 5 . The EFs of Cu, Pb, Zn, Cr, Cd, As, and Hg ranged from 0.06 to 2.34, 0.47 to 1.35, 0.62 to 1.47, 0.51 to 1.38, 0.62 to 2.02, 0.29 to 2.47, and 0.11 to 1.27, with mean values of 0.83, 0.92, 0.97, 0.92, 1.10, 0.92, 0.49, respectively ( Table 4 ). The EF values decreased in the following order: Cd > Zn > Pb = Cr = As > Cu > Hg. The mean EF for all metal(loid)s was found to be below 1.5, indicating that these metal(loid)s were not subject to anthropogenic enrichment. The EF values for Cu, Cd, and As exceeded 1.5 in 2.68%, 11.26%, and 8.70% of the samples, respectively ( Figures 5 , 6A ).

The I_geo values for Cu, Pb, Zn, Cr, Cd, As, and Hg in the sediments exhibited a range of -5.17 to 0.28, -2 to -0.22, -1.94 to 0.11, -2.12 to -0.23, -1.66 to 0.43, -2.63 to 0.41, and -4.15 to -0.37, with mean values of -1.33, -0.98, -0.93, -0.99, -0.76, -1.11, and -2.02, respectively ( Table 4 ). The mean degree of pollution was highest for Cd, followed by Zn, Pb, Cr, As, Cu, and Hg. The mean I_geo values for all metal(loid)s were less than zero, indicating that the sediments were not contaminated by these metal(loid)s. The I_geo values for Cu, Zn, Cd, and As exceeded zero in 2.30%, 1.92%, 8.56%, and 5.93% of the samples, respectively ( Figures 7 , 6B ).

The vertical distribution of E r i was similar to that of I_geo ( Figure 8 ). Except for Cd and Hg, the concentrations of all the other metal(loid)s were considered to present a low potential ecological risk ( E r i < 40) in all units ( Table 4 ). For Cd, the sediments from units DU4, DU2, and DU1 were at moderate potential risk in a certain layer. For Hg, the sediments from units DU2 and DU1 were at moderate potential risk occasionally. The results of the comprehensive potential ecological risk index (E_R ) showed that almost all metal(loid)s in the core sediments were at a low ecological risk (E_R < 105) ( Figure 8 ). Only a few layers in DU4, DU2, and DU1 showed a moderate ecological risk, appearing in MIS1, MIS3-5, and MIS 6-7. Overall, the cumulative concentrations and potential ecological risk coefficients of the metal(loid)s in Laizhou Bay were low.

The Pearson’s correlation coefficients between the concentrations of metal(loid)s (Cu, Pb, Zn, Cr, Cd, As, and Hg), selected major elements (Al, Ti, Mn and Fe), and Mz of the sediments are presented in Table 5 . With the exception of As, Al demonstrated a strong correlation with the remaining elements (r = 0.709–0.939) and Mz (r = 0.861). Mz exhibited a moderate correlation with As and a positive correlation with the other elements. With the exception of As, all the remaining elements displayed a positive correlation with each other (1.0 > r ≥ 0.55).

In order to ascertain the sources of metal(loid)s in the environment, a principal components analysis (PCA) was conducted using the statistical software package SPSS. For the core sediments, the Kaiser-Meyer-Olkin (KMO) measure was 0.92, and the Bartlett’s test yielded a value of 0, indicating that the heavy metal data in this study are suitable for PCA analysis (Waykar and Petare, 2016). The loadings are presented in Table 6 . Two principal components with an eigenvalue greater than 0.9 were extracted, accounting for 82.46% of the total variance ( Figure 9 ). Principal component 1 (PC1) contributed 74.67% of the total variance and included all metal(loid)s and Mz, with high loadings for Al (0.92), Ti (0.88), Mn (0.90), Fe (0.97), Cu (0.88), Pb (0.74), Zn (0.97), Cr (0.94), Cd (0.87), Hg (0.78), and Mz (0.89). This indicates that these metal(loid)s have the same sources. The significant positive correlations between Mz (Al, Ti, Mn, and Fe) and the metal(loid)s listed in Table 5 indicate that metal(loid)s are predominantly associated with fine terrigenous sediments (particularly clay minerals) via preferential exchange and/or adsorption (Larrose et al., 2010). Aluminum oxide (Al₂O₃) is a product of continental differentiation and is relatively stable in the crust. Consequently, aluminum is regarded as a stable element in the transition from continents to oceans (Diagomanolin et al., 2004; Yuan et al., 2018), and is employed as an indicator of terrigenous components in the ocean and of terrigenous origin. The correlation between aluminum and metal(loid)s was found to be positive and moderate. In light of the combined results of EF and Igeo, it can be inferred that the metal(loid)s are derived primarily from crustal materials or natural weathering. Therefore, PC1 was identified as a natural source. PC2 accounted for 7.79% of the total variance, with a high loading for As (0.82). The EF and I_geo values of As exceeded 1.5 and zero in 8.70% and 5.93% of the samples, respectively. These occurrences were observed in a few layers within units DU 4, DU 2, and DU 1. In the context of historical times, it can be stated that there was no evidence of human activity. It can therefore be inferred that PC2 represents the change in sedimentary environment (Xiang et al., 2023).

The elemental background value refers to the content of chemical elements and compounds in soil that has not been influenced by human contamination. However, few accessible areas on Earth’s surface have not been affected by human activities, and determining appropriate sampling sites is difficult. Therefore, the use of drill core sediments from historical periods is a better approach for obtaining background elemental values. When assessing trace element pollution using surface sediments from the Laizhou Bay, the selection of different background values often leads to incomparable evaluation results (Xu et al., 2021; Zheng et al., 2022). In the present study, we used drill core sediments to determine the average content of trace elements in DU 4-2, including Cu (16.59 mg/kg), Pb (18.01 mg/kg), Zn (51.31 mg/kg), Cr (50.83 mg/kg), Cd (0.07 mg/kg), As (6.63 mg/kg), and Hg (0.01 mg/kg), which can be considered as background values for the Laizhou Bay region. The DU 4-2 unit was formed during the Early Holocene, when Laizhou Bay was submerged in seawater and the sea level was stable. The sediment source and depositional environment were similar to those of the present day, and no human activity occurred during this period. Therefore, the average values of trace elements in DU 4-2 can be considered as elemental background values.

The sources and variations in heavy metal content in sediments are closely related to terrestrial inputs, sea-level changes, provenance shifts, and dynamic depositional environments (Wang, 2020). The DU 1 sedimentary unit formed during the MIS 6-7 period, which was a coastal-marine transitional setting. The rise of the Miao Island has impeded the exchange of water between the outer sea and the Bohai Sea. During periods of high sea level, the primary factors influencing the sedimentary environment in Laizhou Bay remain fluvial processes, with marine processes playing a secondary role. The study area is located at the terminal point of fluvial material transport, where sediment particles are relatively fine, likely dominated by sediments from the Yellow River, which exhibit higher concentrations of heavy metal elements. Entering Marine Isotope Stages (MIS) 3-5, the subsidence of the Miao Island uplift effectively ceased, resulting in a structural configuration of the Bohai Sea that is largely consistent with the present day. Sea level changes synchronized with the outer sea led to the formation of unit DU 2. The source of sediments in the study area may have shifted, now primarily consisting of near-source fluvial material from the southern shore of Laizhou Bay, characterized by coarser particle sizes and reduced concentrations of heavy metal elements in the sediments. The DU 3 sedimentary unit formed during the MIS2 period when the sea level was 120 m lower than it is today. The study area was exposed to the seabed and experienced fluvial deposition from small rivers and the Yellow River along the southern coast of Laizhou Bay (Yuan et al., 2023). The sediments were coarse grained. The heavy metal content in most of the sediments in the study area showed a strong correlation with grain size, indicating that metal(loid)s were mainly included/adsorbed into fine-grained sediments. Therefore, the heavy metal content of DU 3 was relatively low. During the Holocene, the sea level gradually increased and the study area was submerged by seawater. It was difficult for sediment from small rivers along the southern coast of Laizhou Bay to reach the study area, and the sediment in the borehole was mainly from the Yellow River with a fine grain size. The heavy metal content then began to increase.