LIANG Haoran, XU Guosheng, YU Qing,3), XU Fanghao,WANG Deying, and CHEN Zhiyuan,4)

1) State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology,Chengdu 610059, China

2) Bohai Oilfield Research Institute, Tianjin Branch, CNOOC Ltd., Tianjin 300459, China

3) Shanghai Branch, CNOOC Ltd., Shanghai 200335, China

4) Research Institute of Petroleum Exploration and Production, Southwest Petroleum Branch, SINOPEC, Chengdu 610041, China

Abstract Paleosalinity is vital for the paleoenvironmental reconstruction and affects the formation of source rock. The lowermiddle sections of the third member of Eocene Shahejie formation (Es3M-L) constitute the most important source rock layer in Laizhou Bay Sag. However, the paleosalinity of the depositional water in which Es3M-L submembers are deposited remains unclear. A series of integrated experiments, including major and trace elements, X-ray diffraction, total organic carbon, and Rock-Eval, was performed to reveal the paleosalinity and its relationship with organic matter (OM). Various inorganic proxies (Sr/Ba, Rb/K, B/Ga,Walker’s paleosalimeter, Adam’s paleosalimeter, and Couch’s paleosalimeter) were employed to determine the paleosalinity of samples. Prominent differences existed in the proxies. Couch’s paleosalimeter is the most reliable and qualitative approach for Laizhou Bay Sag. Samples from the lake center (depocenter) and margin showed paleosalinities from 4.92 wt‰ to 9.73 wt‰, suggesting a ubiquitous brackish (oligohaline-mesohaline) water body in the paleolake. Molybdenum enrichment in samples indicates an oxygendepleted (suboxic or anoxic) condition. The increase in salinity has a certain but non-significant positive correlation with oxygen reduction. This condition may be attributed to the weak stratification of the water column in brackish water bodies. Moreover, paleosalinity has a weak and indirect relationship with OM accumulation during the deposition of Es3M-L submembers in Laizhou Bay Sag.

Key words paleosalinity; Laizhou Bay Sag; Shahejie formation; brackish lake; major and trace elements

1 Introduction

Paleosalinity is one of the most important proxies for paleoenvironmental reconstruction (Yeet al., 2016; Heetal.,2017). Four approaches have been reported to evaluate paleosalinity in aqueous systems: 1) testing of freezingpoint temperature from primary fluid inclusions trapped in minerals (salinity of paleofluid can be calculated by the freezing-point temperature) (Bodnar, 1993; Lianget al.,2019b); 2) analysis of fossils, such as algae and ostracod assemblages (each species shows tolerance to different salinities) (Zhanget al., 2009; Yeet al., 2016); 3) organic geochemical parameters, such as gammacerane index (Moldowanet al., 1985; Tulipaniet al., 2015); and 4) analysis of inorganic geochemical proxies, including isotopes (carbon, oxygen, strontium isotopes) and major and trace elements (Rb/K, Sr/Ba) (Walker and Price, 1963; Keith and Weber, 1964; Sampeiet al., 2005; Xuet al., 2017). Among these approaches, major and trace elements have received the greatest applications in ancient lacustrine and marine systems (Poulainet al., 2015; Zhanget al., 2017a; Fuet al.,2018; Weiet al., 2018).

The concentrations or ratios of specific elements, such as strontium (Sr), barium (Ba), gallium (Ga), rubidium(Rb), potassium (K), and boron (B), are efficient indicators of paleosalinity (Yeet al., 2016). These elements exhibit unique behaviors in aqueous systems under different salinities and can be taken up by clay minerals (Degenset al.,1957; Taylor and McLennan, 1985). Element ratios (Sr/Ba,B/Ga,) are used as semi-quantitative indicators of paleosalinity, reflecting the variation in salinity indirectly (Weiet al., 2018). Boron concentration can be used to calculate the paleosalinity quantitatively on the basis of the strong linear relationship between boron and salinity in modern aqueous systems (Adamset al., 1963; Walker and Price 1963; Curtis, 1964).

The lower-middle submember of the third member of Eocene Shahejie formation (Es) (Es3M-L)is the main source rock layer in Bohai Bay Basin (BBB) (Haoet al., 2009).Previous studies on paleosalinity in Es3, BBB focused on specific sags, such as Dongying Sag, Dongpu Sag, and Langgu Sag (Li and Xiao, 1988; Guoet al., 2014; Jianget al., 2016; Weiet al., 2018). The paleosalinity of Es3in BBB varies considerably in the sag scale: Dongpu Sag(southern BBB) features a high salinity of (14 - 47) wt‰(Zhanget al., 2016), whereas Dongying Sag (middle of BBB) exhibits considerably lower values of (6 - 26) wt‰(Li and Xiao, 1988). Langgu Sag (western BBB) was determined to deposit in a freshwater lake (Diaoet al., 2014).Laizhou Bay Sag (study area) is located in eastern BBB,in which no specialized study of paleosalinity has been conducted.

In the present study, a series of inorganic geochemical proxies was employed to reveal the paleosalinity in Es3M-Lsubmembers, Laizhou Bay Sag. The different salinity proxies and the relationship between paleosalinity and organic matter (OM) were discussed. The results not only provide systematic comparison between different salinity proxies but also a clear and improved understanding of the paleosalinity of Es3M-Lsubmembers in Laizhou Bay Sag. Moreover, this work is beneficial for the study of OM accumulation in brackish lakes.

2 Geological Settings

BBB is one of the most important petroliferous basins in eastern China (Chenet al., 2017). As shown in Fig.1a,the study area (Laizhou Bay Sag) is located in the southern Bohai Sea, which is defined as the current offshore area of BBB (Wanget al., 2014). Laizhou Bay Sag is bounded by the Weibei Uplift to the south and by the Laibei Uplift to the north (Fig.1b). The eastern and western boundaries of the sag are limited by the branches of the middle Tanlu fault, a giant strike-slip fault belt in eastern China (Denget al., 2016). Laizhou Bay Sag consists of slope zones (northern, western, southern, and eastern slope zones), sub-sags (northern, northeastern, and southern sub-sags), and a central uplift zone (Fig.1b). The tectonic activities in Laizhou Bay Sag are controlled by the NNE-trending strike-slip faults and the EW-trending normal faults. Yeet al. (1985) proposed that two-stage tectonic evolution (the Paleogene syn-rift and Neogene post-rift stages) in BBB during the Cenozoic Era can be divided(Fig.2). Strata in the early and middle syn-rift phase are mainly controlled by the EW-trending normal faults, whereas the activities of NNE-trending strike-slip faults became stronger in the late stage of syn-rift (stage III) (Zhanget al.,2017b).

Fig.1 (a), Geological map of Bohai Sea (modified from the work of Xu et al., 2018); (b), detailed geological map of Laizhou Bay Sag (adopted from the work of Zhang et al., 2017b); (c), cross section exhibiting framework of strata in Laizhou Bay Sag (modified from the work of Zhang et al., 2017b).

Fig.2 Cenozoic stratigraphy of Laizhou Bay Sag (after Xu et al., 2018 and Wei et al., 2018); Symb, symbol; Fm, formation.

The Paleogene strata in the study area can be divided into Paleocene Kongdian formation, Eocene Shahejie formation, and Oligoene Dongying formation. Paleogene strata in Laizhou Bay Sag were deposited in fluvial, alluvial,deltaic, salt, or semi-deep lake facies (Allenet al., 1997;Lianget al., 2019a). Esis the most important source rock layer in the Bohai Sea (Yanget al., 2011; Niu, 2012). As shown in Fig.2, Shahejie formation can be further divided into four members (Es4, Es3, Es2, and Es1). Es3is the third member of Es(Fig.2). Laizhou Bay Sag was covered by a large-scale braided river delta from the western provenance during the deposition of Es3U, featuring thick sandstone beds (Xinet al., 2013). Semi-deep to deep lakes developed in Es3M-Lsubmembers, in which thick black shales and gray-dark mudstones were observed (Yuet al.,2009). Es3M-Lsubmembers, cha- racterized by a high OM abundance, are the main source rocks in Laizhou Bay Sag(Wanget al., 2011, 2012; Niu, 2012).

3 Samples and Methods

3.1 Sample Description

Six samples were collected from the sections with large sets of shale in Es3M-Lsubmembers, Laizhou Bay Sag.These samples were sourced from six wells that are distributed in different locations of the study area (Fig.1b) to determine the lateral variation of paleosalinity. The samples were mainly gray to black and carbonaceous shales.

3.2 Total Organic Carbon (TOC) and Rock-Eval Pyrolysis

HCl solution with a concentration of 5 wt% was used to remove the carbonate minerals in powdered samples (100 mesh). Then, the samples were digested and dried in an oven at 50℃ for 2 d. TOC was measured using a Leco CS-230 Carbon and Sulfur analyzer with 0.5% precision.

Rock-Eval pyrolysis was performed using a Rock-Eval II instrument following the standard operations of Peters(1986). Free hydrocarbons (S1) were obtained when the samples were heated below 300℃ for 3 min. The hydrocarbons generated by the cracking of kerogen (S2) were measured at 600℃. The temperature of the maximum yield of pyrolysate (Tmax) can be determined when the maximum S2yield is reached.

3.3 Mineralogy Analysis

Mineral compositions and contents can be determined by X-ray diffraction (XRD) analysis, following the detailed work of Wang and Guo (2019). Rock samples were first crushed into powder (＜ 40 μm) with an agate mortar and pestle. The experiments were conducted by a D8 Discovery X-ray diffractometer under the humidity of 35 wt%and the temperature of 24℃. Mineral percentages were calculated by the area of peaks for each mineral with the help of X’Pert High Score software. Only the powdered samples that had ＜ 5 μm fractions were measured for the clay mineral content.

3.4 Major and Trace Element Analysis

The preparation for samples followed the steps of Xieet al. (2018). Rock samples were ground to 200 mesh by using an agate mortar and pestle. Each sample was divided into two splits. In the split for major elements, the pulverized samples were placed in a muffle furnace at 1000℃for 1 h to remove the OM and carbonate, followed by lithium borate fusion and acid digestion. The major oxide contents were measured by an X-ray fluorescence spectrometer with an accuracy greater than 98%. For the split for trace elements, the samples were placed in a muffle furnace to remove water. Then, the samples were digested by a mixture of multi-acids (HCl + HClO4+ HF + HNO3) in beakers. Trace elements were tested by using inductively coupled plasma mass spectrometry. The tests of blank and standard samples were performed before the experiment for each sample to ensure accuracy. The relative standard deviations for major elements and trace elements were more than 5%.

4 Results

4.1 Bulk Geochemical Characteristics

Table 1 lists the results of TOC and Rock-Eval analyses.The samples collected from Es3M-Lsubmembers in this study showed high TOC values. The TOC of samples ranged from 4.30 wt% to 5.63 wt%. Sample L6 had a relatively low TOC value (2.86%). All the samples were in immature or low-maturity stage with Tmaxvarying from 431℃to 435℃ (Tissot and Welte, 1984). The samples were projected onto pseudo-van Krevelen diagrams of HI-OI and HI-Tmax. As shown in Fig.3, the kerogen types of these samples were Type I to Type II.

4.2 Mineral Compositions

Table 2 presents the mineral compositions of the samples. Clay minerals constituted the most abundant fraction of samples. The calcites accounted for considerable pro portions of the samples (10.5 - 44.0 wt%). Dolomites were detected in samples L4 and L5. Sample L5 featured the lowest clay fraction (20.8 wt%) but the highest calcite fraction (44.0 wt%). In addition, the samples exhibited high illite contents, whereas kaolinite and chlorite showed relatively low abundances.

4.3 Element Ratios by Weight

As shown in Table 3, the major elements of samples were predominated by SiO2, Al2O3, and CaO. SiO2accounted for the largest proportion, ranging from 33.70 wt% to 49.49 wt%. Al2O3also accounted for a considerable proportion,from 7.95 wt% to 15.66 wt%. CaO was between 5.63 wt%and 21.88 wt%. Table 3 presents the abundance of trace elements (B, Ga, Rb, Sr, Ba, Cu, Ni, Mo, Th, and U). The abundance of trace elements from different samples showed no considerable difference. The commonly used (trace or major) element ratios for paleosalinity evaluation were also calculated (Table 3). Several salinity ratios (Sr/Ca and Mg/Ca) suitable to Ostracoda or other fossil samples were not employed in the present study (Dodd and Crisp,1982; Poulainet al., 2015; Curryet al., 2016). Table 4 summarizes the classification standards of different paleosali-nity proxies.

Table 1 Bulk geochemical data of samples in Es3M-L, Laizhou Bay Sag

Fig.3 (a) HI vs. Tmax diagram of Es3M-L submember samples (after Tissot and Welte, 1984); (b) HI vs. OI diagram of Es3 samples (after Tissot and Welte, 1984). OI represents oxygen index (OI = S3 × 100 / TOC); HI refers to hydrogen index (HI = S2 ×100 / TOC).

Table 2 XRD results of samples in Es3M-L, Laizhou Bay Sag

4.3.1 Sr/Ba ratio

Barium (Ba) and strontium (Sr) exhibit different geochemical behaviors in a saline environment (Vosoughi Moradiet al., 2016). Given the elevation of water salinity, Ba precipitates from water in the form of BaSO4, whereas Sr can only be precipitated in high-salinity water (Li and Xiao, 1988; Li and Chen, 2003). Based on their selective precipitations to salinity, the Sr/Ba ratio can be used to indicate the paleosalinity of water bodies (Zhanget al.,2017b). The Sr/Ba ratios of Es3M-Lsubmember samples in Laizhou Bay Sag were between 0.08 and 0.74 (Table 3).Samples L3 (0.74) and L5 (0.64) showed values higher than 0.6, indicating a brackish water body (Tables 4 and 5).The Sr/Ba ratios of other samples, which were lower than 0.6, suggest the existence of freshwater.

Table 3 Element results of the samples in Es3, Laizhou Bay Sag

Table 4 Threshold values of paleosalinity proxyies

Table 5 Classification standard of paleosalinity (VeniceSystem, 1958)

4.3.2 Rb/K ratio

Potassium (K) abundance has an intimate relationship with the illite content (Li and Chen, 2003). Given the similar ionic radii between K and rubidium (Rb), K can be easily replaced by Rb in illite (Doyleet al., 1998). Given an increased salinity, more Rb is absorbed into the clay minerals (Yeet al., 2016). In general, the Rb/K ratio in shales has a positive relationship with the paleosalinity. The Rb/K ratios of samples in Laizhou Bay Sag were mostly equal to or lower than 0.004, suggesting a freshwater body (Tables 3 and 4). Samples L4 and L5 showed Rb/K ratios that were slightly higher than 0.004.

4.3.3 B/Ga ratio

Boron (B) is well known for its high solubility and easymigration, and it only precipitates when water evaporates continuously (Curtis, 1964). Meanwhile, Ga has a low chemical activity and is easily precipitated from the water system (Zhanget al., 2017a). Hence, the B/Ga ratio exhibits a positive covariation to paleosalinity. The B/Ga ratios of Es3M-Lsubmember samples in Laizhou Bay Sag were in the range of 1.61 - 2.81 (Table 3). The water bodytype assessment criteria of Dongying Sag proposed by Li and Xiao (1988) were adopted (Table 4). These criteria are more suitable to the study area than the others because Dongying Sag is extremely close to Laizhou Bay Sag. The B/Ga ratios of all the samples were distributed in the scope of brackish water (Table 4).

4.4 Boron-Derived Paleosalimeter

Boron has been widely used as a quantitative paleo-salinity proxy for a long time because of its strong and positive relationship with salinity in modern aqueous systems (Adamset al., 1963; Walker and Price, 1963; Couch,1971).

4.4.1 Elimination of inherited boron

The elimination of inherited boron is a prerequisite for the calculation of boron-derived paleosalimeter (Couch,1971; Zhang, 1987; Liet al., 2003). Inherited boron, defined as the boron absorbed in clay minerals before they enter paleowater, will lead to the increased inaccuracy of paleosalinity. Zhang (1987) used the crossplot of clay and boron contents to estimate the inherited boron. TheY-axis(boron content) intercept of the fitting line between clay and boron contents was considered to be the inherited boron. Yeet al. (2016) proposed another method: if boron contents of samples at the same K2O content show different values, then the sample with the lowest boron content can be used to indicate the inherited boron in this K2O content. The fitting line between K2O and the lowest boron contents can be established. TheY-axis (boron content) intercept of this fitting line is therefore the inherited boron content.

Sample quantity in the present study may be insufficient to construct a reliable fitting line when the abovementioned methods are used. The study of Zhuanget al.(2010) contains abundant data on K2O and boron contents of Paleogene mudstone in the adjacent region, Huanghekou Sag, southern Bohai Sea. Therefore, the inherited boron of Ppaleogene mudstone in the southern Bohai Sea can be estimated on the basis of Huanghekou Sag. The method of Yeet al. (2016) was employed to determine the inherited boron content. The inherited boron content was estimated as 18 μg g-1.

4.4.2 Walker’s paleosalimeter

Walker’s boron-derivedpaleo paleosalimeter is also called the ‘equivalent boron’ method (Yeet al., 2016). Boron content in illite is a direct indicator of paleosalinity (Walker,1962). The notion of ‘a(chǎn)djusted boron’ content, which reflects the boron content in illite, was proposed by Walker and Price (1963). The ‘a(chǎn)djusted boron’ (Badjust) was calculated following Eq. (1):

where Bclayrepresents the boron content in clay fraction of sample (μg g-1); K2O refers to the K2O content of sample (wt%); and 8.5 is the theoretical abundance of K2O in pure illite.

‘Equivalent boron’ content (Bequivalent) represents the adjusted boron that exists at equilibrium in illite, which contains 5 wt% K2O under the same salinity medium (Adamset al., 1963). Based on the ‘a(chǎn)djusted boron’ content, ‘equivalent boron’ (Bequivalent) can be obtained from the ‘departure curves’ (Fig.4). Then, the water body type can be estimated in accordance with the criteria established by Walker and Price (1963) (Table 4). Notably, this approach does not provide exact paleosalinity but reflects the water body type in a semi-quantitative manner (Zhanget al., 2017a).

The Badjustof samples, calculated by the major and trace element data, varied in a wide range (99.11 - 429.23 μg g-1)(Table 6). As listed in Table 6, the majority of samples had Bequivalentvalues between 50 and 150 μg g-1, identical to the Bequivalentrange of freshwater (Table 4). Only sample L5 showed a Bequivalentvalue higher than 200 μg g-1, suggesting the depositional condition of brackish water (Table 6).

4.4.3 Adam’s paleosalimeter

Adam’s method is based on the notion of ‘equivalent boron content’. Considering the significantly direct correlation between equivalent boron and depositional paleosalinity in modern Dovey Estuary sediments, Adamset al.(1963) established a regression equation to calculate paleosalinity, as shown in Eq. (2):

where Sprepresents the paleosalinity of water (wt‰) and Bequivalentrepresents the equivalent boron content (μg g-1).

Fig.4 Equivalent boron content distribution in the study area (modified from Walker and Price, 1963). Curves in this figure are the departure curves for the estimation of equivalent boron content.

Table 6 Boron paleosalimeters in Es3M-L, Laizhou Bay Sag

The Spcalculated by Adam’s method was between 0.77 and 19.82 wt‰ (Table 5). Samples L3 and L6 exhibited the lowest paleosalinity among all the samples, whereas sample L5 featured the highest salinity of 19.82 wt‰. According to the VeniceSystem (1958), all these samples represent the depositional environment of brackish water (Table 4). More precisely, brackish water can be divided into three types, namely, oligohaline, mesohaline, and polyhaline waters (Table 5). Samples L2, L3, L4, and L6 were formed in oligohaline water, and sample L1 was deposited in mesohaline water. Meanwhile, the paleosalinity of sample L5 represents a polyhaline water body (Table 5).

4.4.4 Couch’s paleosalimeter

Frederickson and Reynolds Jr. (1960) argued that all clay minerals could incorporate boron. Meanwhile, illite shows the strongest absorption capacity compared with other clay minerals. The method of Couch (1971) considers the ‘boron-up take ability’ of different clay minerals:The ‘uptake ability’ of illite is twice as that of smectite and four times as that of kaolinite. Boron concentrations can be converted into kaolinite boron (Eq. (3)). As shown in Eq. (4), paleosalinity can be obtained by using kaolinite boron based on Freundlich adsorption isotherm.

where Bkrepresents the uptaken boron content in kaolinite (μg g-1); Bclayrefers to the boron concentration of clay fraction (μg g-1); Xk, Xs, and Xirepresent the fractions of kaolinite, smectite, and illite (wt%), respectively;and Sprepresents the paleosalinity (wt‰).

Table 6 presents the paleosalinity calculated by Couch’s method. The salinity of samples ranged from 4.92 wt‰ to 9.73 wt‰, which indicates a brackish water body (Table 5). More accurately, sample L4 was formed in oligohaline water, whereas the other samples were all deposited under mesohaline water.

5 Discussion

5.1 Comparison of Paleosalinity Proxies

A series of proxies for paleosalinity estimation has been described in the previous section. Fig.5 shows the results of paleosalinity estimation. Although major differences can be observed among the results of proxies, all the proxies indicate a relatively low-salinity water body (freshwater or mixed brackish water). In addition, almost all the proxies (except Sr/Ba ratio) showed the highest salinity in sample L5, which is supposedly deposited in brackish water. The B/Ga ratio and Spfrom Adam’s and Couch’s methods exhibited similar results, as shown in Fig.5. In accordance with the criterion for water body type (Table 5),the samples calculated by these three proxies were all formed under brackish water. Meanwhile, the samples estimated by Sr/Ba, Rb/K, and Bequivalentmostly corresponded to the depositional condition of freshwater (Fig.5).Evidently, several proxies in this study may not reflect the paleosalinity objectively.

Considering the inconsistency between different proxies, more evidence should be provided to crosscheck paleosalinity. The occurrence of specific minerals can be treated as a signal of paleosalinity. Carbonate minerals in lacustrine sediments suggest intensive evaporation and an increase in water salinity (Jin and Zhu, 2006). According to Tables 2 and 3, abundant calcites and high CaO content have been detected in these samples. Dolomites were also observed in several samples. These phenomena suggest the ubiquitous existence of salinization in the paleolake.Notably, as concluded from the results on minerals, the major elements were in accordance with the B/Ga ratio and Spfrom Adam’s and Couch’s methods. The samples in the present paper are likely to deposit in a brackish lake.

The effectiveness of each proxy needs to be discussed because of the different chemical behaviors of elements.The Sr/Ba ratio for salinity estimation has been questioned in recent cases because of its inconsistency with paleosalinity and the B/Ga ratio (Zhanget al., 2017a; Weiet al.,2018; Yanget al., 2018). Sr/Ba ratio did not exhibit a simple correlation with paleosalinity in the present study (Fig.6a).The Sr/Ba ratio first decreased and then increased with the elevation of paleosalinity. Moreover, Sr/Ba ratio showed no relationship with the B/Ga ratio (r= 0.04) (Fig.6f).Sr is not only affected by water salinity but also by the content of calcites. Calcium in CaCO3can be replaced by Sr because of their similar ionic radii and valence (Heet al.,2017). Therefore, the abundant carbonates of samples in the study area may lead to the variation in Sr and mislead the indicative function for paleosalinity. Considering the uncertainty of Sr variation, the Sr/Ba ratio is supposed to be an unreliable proxy for paleosalinity, at least in Laizhou Bay Sag.

Fig.5 Comparison of different paleosalinity proxies.

Fig.6 Crossplots of proxies of Es3M-L submember samples in Laizhou Bay Sag. (a), Sp calculated by Couch’s method vs.Sr/Ba ratio; (b), Sp calculated by Couch’s method vs. B/Ga ratio; (c), Sp calculated by Couch’s method vs. Rb/K ratio; (d),Sp calculated by Couch’s method vs. Bequivalent; (e), Sp calculated by Couch’s method vs. Sp calculated by Adam’s method;(f), Sr/Ba ratio vs. B/Ga ratio; (g), Rb/K ratio vs. Sr/Cu ratio. Black solid lines in the figures represent the variation tendency of samples.

The Rb/K ratio in the present study showed a poor relationship with Sp(r= 0.37) (Fig.6c). Furthermore, the Rb/K ratio exerted more functions as a climate indicator than salinity because of its significant correlations with the Sr/Cu ratio (r= 0.98), which is a commonly used indicator for paleoclimate (Table 3; Fig.6g). Hence, the Rb/K ratio may be an inappropriate proxy for salinity estimation in Laizhou Bay Sag. Meanwhile, the B/Ga ratio has a positive correlation with Sp(r= 0.83), indicating its effectiveness to estimate the variation in salinity (Fig.6b).

Apart from the various chemical behaviors mentioned above, threshold is another key factor for determining salinity using these semi-quantitative element ratios. Diverse thresholds for the same ratio exist because of the different chemical compositions of provenance at the basin or sag scale. The diversity of thresholds will increase uncertainties. The publicly accepted threshold, such as the B/Ga ratio in the present study, may be unsuitable to each region: sediments from freshwater generally yield B/Ga ＜2.5, whereas those from saline water have higher values(＞ 5); and sediments in brackish water yield B/Ga between 2.5 and 5 (Weiet al., 2018). Several researchers argued that freshwater sediments have B/Ga values ＜ 3 (Zhanget al.,2017b). Li and Xiao (1988) proposed the thresholds of B/Ga ratio in BBB (Table 4): freshwater (＜ 1.5), brackish water (1.5 - 3), and saltwater (＞ 3). Only the threshold applicable to the study area can accurately and objectively reveal the salinity.

Paleosalimeter calculated by Walker’s, Adam’s, and Couch’s methods are all based on the unique chemical behavior of boron. Walker’s method has two drawbacks. One is the theoretical basis of the method, assuming that boron in clay fraction is only absorbed by illite (Walker and Price,1963). Limited by this principle, this approach is suitable to the samples in which illite is the sole mineral in clay fraction. Another thing is the determination of Bequivalentis a semi-quantitative estimation (Zhanget al., 2017a). The strong correlation in Fig.6d should be attributed to the clay mineral composition, because illite takes up a large part of clay minerals in these samples (Table 2). The ignorance of boron contributed by other types of clay minerals in this method will lead to an inevitable error.

Adam’s method is the continuation and improvement of Walker’s method. Quantitative results (absolute value of salinity) can be obtained by this approach. Limited by the salinities of samples in modern Dovey Estuary sediments, the valid salinity scope of the computational formula is 16 - 33 wt‰ (Adamset al., 1963). This scope is notably higher than the paleosalinity calculated in the present study (Table 6). Clearly, this method is unsuitable for Laizhou Bay Sag.

The boron-derived paleosalinity by Couch’s method is the most objective approach to date. This approach considers the boron absorption capability of different clay paleominerals. Paleosalinity can be calculated using Frederickson’s formula quantitatively. Moreover, the results of this method coincide with those from mineral composition and major elements in the study area. Therefore,Couch’s method is considered to be the most accurate and proper approach for Laizhou Bay Sag.

5.2 DeterminationofPaleosalinity in Laizhou Bay Sag inEs3M-LSubmembers

The salinity of samples in Laizhou Bay Sag calculated by Couch’s method ranged from 4.92 wt‰ to 9.73 wt‰(Table 6). Samples in the present work were collected from different areas of Laizhou Bay Sag (Fig.1). Except for sample L5, which showed a relatively high salinity (9.73 wt‰), the other five samples varied within a narrow range(4.92 - 6.64 wt‰). In the present study, six samples were collected from Es3M-Lsubmembersof Laizhou Bay Sag.The number of samples was insufficient to analyze the paleosalinity variation systematically. However, benefited by the abundant sedimentary studies of Es3M-Lsubmembersin Laizhou Bay Sag (Sunet al., 2009; Wanget al.,2012, 2018a; Xinet al., 2013; Huanget al., 2018), the lateral salinity-variation in Es3M-Lsubmemberscan be explained to a certain degree. The distribution of salinity in saline lakes follows the rules of circular distribution (Zhuet al., 2004; Jin and Zhu, 2006; Liu and Liu, 2017): Water salinity increases from the lake margin to the center of the lake as sediments change from clastics to carbonates, sulfates, or chlorides. Therefore, the highest salinity should exist at the center of saline lakes.

As a result of dextral strike-slip movement, the depocenter in Es3M-Lsubmembersgradually migrated to the east of Laizhou Bay Sag (Fig.7). Sunet al. (2009) highlighted that well LZW11-2-E was in the sedimentary center (depocenter) of Es3M-Lsubmembers. Sample L5, which was collected from well LZW-11-2-E, had the highest paleosalinity (9.73 wt‰) among all the samples (Fig.7).The other samples distributed in shore-shallow lake or the margin of semi-deep lake showed notably low salinities(4.92 - 6.64 wt‰). This finding is coincident with the circular distribution of salinity in lakes. Hence, given the unique geological position of sample L5, its paleosalinity can represent the highest salinity in paleo-Laizhou Bay lake during the deposition of Es3M-Lsubmembers. Moreover, the paleosalinities of the samples in different areas(depocenter, margin of semi-deep lake, and shore-shallow lake) varied within the scope of brackish water body (oligohaline-mesohaline water). Therefore, it can be inferred that the bottom water of the paleo-Laizhou Bay lake featured a brackish water body (oligohaline-mesohaline water)during the deposition of Es3M-Lsubmembers.

The climate and riverine flux are considered to be the major controlling factors for paleosalinity in Laizhou Bay Sag (Fig.8). Evidenced by the high Sr/Cu values and the occurrence of calcites and dolomites, paleoclimate in Laizhou Bay Sag was relatively hot and arid during the deposition of Es3M-Lsubmembers. This climate was beneficial for the continual evaporation and increased salinity of lake water. The widely developed deltas in Laizhou Bay Sag brought large amounts of freshwater by the riverine flux,which in turn diluted the water salinity. The brackish bottom water body in Es3M-Lsubmembers determines the balance of climate and riverine flux. Weak water stratification may be formed during this process (Fig.8).

Fig.7 Distribution of sedimentary facies and paleosalinity in Es3M-L, Laizhou Bay Sag; sedimentary facies were modified from the works of Sun et al. (2009), Wang et al.(2012), Xin et al. (2013), and Huang et al. (2018).

Fig.8 Sedimentary model of Laizhou Bay Sag during the deposition of Es3M-L.

5.3 Paleosalinity and Preservation of OM

Stratified water column occurs in hypersaline lakes because of gravity differentiation (Zhuet al., 2004). Water mass stratification can produce anoxic or euxinic conditions and prevent the decomposition of OM (Murphyet al.,2000). Hence, the hypersaline water body consistently exhibits excellent preservation of source rock. However, studies on the preservation of OM in brackish-lacustrine lake are limited.

Mo, U, and V are the most sensitive and effective trace elements for the estimation of redox conditions in aqueous systems (Algeo and Lyons, 2006). A poor or no-oxygen environment is beneficial for the accumulation of these elements (Wanget al., 2018b). Mo, U, and V can be used as paleoredox indicators only if the hydrogenous signal is recognizable (Xuet al., 2012). The most common method to check the hydrogenous signal is to use a crossplot for checking the correlation between the target element and aluminum (or titanium), which indicates the detrital-derived fraction (Tribovillardet al., 2006). Fig.9 shows the crossplots between redox-sensitive trace elements and aluminum. Only Mo exhibited a significant negative correlation with Al2O3(r= -0.96), suggesting that Mo accumulation is predominantly controlled by lake water (Fig.9a).No significant relationship existed between U and Al2O3(Fig.9b). Evidenced by the strong and positive relationship with Al2O3(r= 0.74), trace V in the samples was mainly influenced by detrital provenance (Fig.9b). Therefore, only Mo can be used as the redox proxy in Laizhou Bay Sag.

Fig.9 Relationship between salinity, redox proxies, and TOC. (a), Al2O3 vs. Mo; (b), Al2O3 vs. U; (c), Al2O3 vs. V; (d),EFMo vs. TOC; (e), Sp calculated by Couch’s method vs. TOC; (f), Sp calculated by Couch’s method vs. EFMo. Imaginary lines in the figures represent the variation tendency of samples. Dashed lines indicate the distribution trend of scatters.

Mo enrichment is very limited in oxic conditions because of its stable and conservative behavior in an oxygenabundant environment; the mass enrichment of Mo requires the presence of H2S, which will facilitate the conversion of molybdate to thiomolybdates (Tribovillardet al.,2008; Algeo and Tribovillard, 2009). The enrichment factor(EF) is employed to illustrate the abundance of elements(Tribovillardet al., 2006).EFcan be calculated by Eq. (5):

whereEFXrepresents the enrichment factor of element X;Xsamplerepresents the content of element X in the sample;Alsampleis the aluminum abundance in the sample; X and Al denote the average content of element X and aluminum in upper crust, North China Platform (Yanet al., 1997).

EFX＞ 3 andEFX＞ 10 correspond to detectable authigenic and substantial enrichments, respectively (Tribovillardet al. 2006). Fig.9e presents theEFMoof the samples.TheEFMovalues were higher than (or close to) 3, suggesting the authigenic Mo enrichment and oxygen depletion(suboxic or anoxic) in bottom water. As illustrated in Fig.9d,a significant positive correlation (r= 0.81) existed between TOC andEFMo. The particulate effect will lead to the abnormal increase inEFMo, considering that the Mo uptake in sediments may be influenced by the Fe-Mn cycle.According to our previous work (Lianget al., 2020), this possibility can be excluded in the third member of the Shahejie formation in Laizhou Bay Sag as the low EFMo.Therefore, the finding in Fig.9d demonstrates that the redox condition has a direct influence on the preservation of OM. With the intensification of oxygen depletion, the OM decomposition was reduced, and more OM could be retained in the sediments, leading to the increase in TOC.

A certain correlation exists between paleosalinity and TOC (r= 0.57) (Fig.9e). However, the correlation in Fig.9e showed no significance compared with that in Fig.9d. The scatters in Fig.9e showed discrete distributions, suggesting the indirect impact of salinity on TOC. Moreover, Fig.9f presents a similar phenomenon, showing evident discrete distribution of scatters. As evidenced by Fig.9e, the increased salinity in brackish lake is beneficial for the preservation of OM to a certain degree. The strong stratification of the water column can result from the increased(high) salinity and lead to a sharp decrease in oxygen content (Boehrer and Schultze, 2008). In the study of Jianget al. (2019), the high salinities ((28 - 38) wt‰) in paleo-Qaidam lake, which were associated with strong stratification of the water column, correlated well with the proxies of redox condition, and no discrete distribution of data was observed. Therefore, the discrete distributions of Sp-TOC and Sp-EFMoin Laizhou Bay Sag may be attributed to the weak stratification of the water column in lowsalinity (brackish) water bodies. Thus, redox condition is the controlling factor for the preservation of OM, whereas paleosalinity in brackish lake has an indirect influence on and weak relationship with OM preservation in Laizhou Bay Sag.

The results of this study are beneficial for the research of paleosalinity in brackish lakes. This case provides a better understanding of the relationship between salinity and OM. This research can serve as a reference for the study of lacustrine sags, which share a similar sedimentary condition with Laizhou Bay Sag.

6 Conclusions

1) A series of inorganic proxies was performed for the estimation of paleosalinity in Es3M-Lsubmembers, Laizhou Bay Sag. Prominent differences existed in these proxies.Among all the indicators, the B/Ga ratio was considered to be the most effective semi-quantitative proxy for salinity. Couch’s paleosalimeter is the most reliable proxy for quantitative salinity in Laizhou Bay Sag.

2) PaleoThe paleosalinity calculated by Couch’s paleosalimeter was between (4.92 - 9.73) wt‰. The distribution of salinity is coincident with the sedimentary environment. Samples from the lake center or margin showed paleosalinity in the scope of brackish water. The whole paleo-Laizhou Bay lake was a brackish lake (oligohalinemesohaline water) during the deposition of the Es3M-Lsubmembers.

3) Increasing the salinity will promote oxygen reduction in the water column and is beneficial for the preservation of OM. However, the correlation between salinity and preservation of OM is not significant in Laizhou Bay Sag. This finding may be attributed to the weak stratification in the brackish water body of Laizhou Bay Sag.

Acknowledgements

This study was supported by grants from the National Science and Technology Major Projects (No. 2016ZX0 5024-002-007) and the CNOOC Project (No. CCL2020TJ X0NST1271).