Arabian Journal of Geosciences (2018) 11: 584 https://doi.org/10.1007/s12517-018-3922-7

ORIGINAL PAPER

Carbonate formation mechanism in paleosol sediments of the coastal zone,

Shuhuan Du1 & Baosheng Li2 & Rong Xiang1 & Dongfeng Niu3 & Yuejun Si4

Received: 6 April 2017 /Accepted: 17 September 2018 /Published online: 2 October 2018 # Saudi Society for Geosciences 2018

Abstract The Bohai Sea coastal zone of China consists of the Pacific Ocean to the east and Eurasia to the west; hence, this region is influenced by both the ocean and continental landmasses. The carbonate formation mechanism of eolian sediment within this area is poorly understood. The loess-paleosol sediments of the stratigraphical section (MDS) contain a record of carbonate changes in this region during the last interglacial period. New insights into regional carbonate formation mechanisms since the last interglacial period were obtained by analyzing the ages of various sedimentary facies in combination with proxy paleocli- matic indices (including average grain size, standard deviation, CaCO3 content, and clay minerals), as well as via foraminiferal analysis. The results led to three principal findings: (1) The carbonate content change in the MDS was neither controlled by grain size nor affected by minerals. The carbonate change controlled by precipitate leaching in the Loess Plateau region cannot explain the eolian sediments within the Bohai Sea coastal zone. (2) Various subsections contain obvious carbonate content changes caused by foraminiferal deposits atop the eolian sediments, which were deposited by storm surges during a high sea-level period. This increased the carbonate content in the eolian sediments and restricted the carbonate content in the Bohai Sea coastal zone. (3) Newly detected foraminifera of the last interglacial period on the eolian sediment of the MDS were the main source of carbonate content, providing a new understanding of the carbonate formation mechanism in eolian sediments different from that of the Loess Plateau.

Keywords Bohai Sea coastal zone . Eolian sediments . Carbonate content . Sea level change . Foraminiferal fossils . Formation mechanism

Introduction

Electronic supplementary material The online version of this article Carbonate has been used as an important indicator in (https://doi.org/10.1007/s12517-018-3922-7) contains supplementary material, which is available to authorized users. Quaternary climatic research since the early 1960s. In these studies, deep-sea sediments of the glacial (interglacial) period * Shuhuan Du that contain high (low during the interglacial) carbonate con- [email protected] tent were referred to as Pacific-type sediments, whereas those sediments of the glacial (interglacial) period with low (high 1 CAS Key Laboratory of Ocean and Marginal Sea Geology, South during the interglacial) carbonate content were known as China Sea Institute of Oceanology, Guangzhou 510301, People’s Atlantic-type sediments (Elderfield 2002;Haysetal.1969). Republic of China Both types indicate that carbonate content is altered by climat- 2 Department of Geography, South China Normal University, ic change. The importance of carbonate indicators is apparent Guangzhou 510631, People’sRepublicofChina in deep-sea and continental sediments, particularly in eolian 3 Physical Science and Technology School, Lingnan Normal sediments such as the loess of China. Carbonate in this loess is University, Zhanjiang 524048, People’s Republic of China a result of primary and secondary processes. Native sediments 4 School of Geography and Planning, Guangxi Teachers Education of the desert and Gobi regions include detrital carbonate, University, Nanning 530001, People’sRepublicofChina 584 Page 2 of 12 Arab J Geosci (2018) 11: 584 whereas a secondary carbonate process that occurs after the Geological setting weathering of the loess deposit formation (i.e., migration and deposition) is influenced mainly by precipitation (Zhao 1993). The geological structure in the study area belongs to East Carbonates in eolian loess sediments within the region of the South Liaoning land; the bedrock is mainly East Asian monsoon have been used both as indicators of Sinian quartzite with some phyllite and slate. Hills in the inner ancient environmental change and for reconstruction of envi- islands have an elevation of approximately 100 m, and there ronmental evolution and changes (Diao and Wen 1995;Liu are mostly gravel beaches along the coast (Cao et al. 1993). 1985;Zhao1999, 2002; Xie et al. 2003). This index has also The climate in the Miaodao Archipelago is a warm temperate been applied effectively to other eolian sediments such as zone monsoon climate, with 200 to 220 frost-free days and an loess and desert transitional zones (Wang et al. 2012; Guo average annual temperature of 11–12 °C. The coldest month is et al. 2015) and even applied in the Bohai Sea coastal zone, January with an average temperature from − 3to− 4 °C, and where sediments have been classified as eolian (Cao et al. the most thermidor is August with an average temperature of 1987, 1993;Li1987) in the same latitudinal zone of China. 24–26 °C. The average annual rainfall is 700–800 mm; ap- Wang et al. (2007) studied the magnetic and carbonate con- proximately 81% of the annual precipitation occurs during the tents of loess sediments within the coastal zone and reported summer and autumn and 19% during the winter and spring four previous climatic fluctuations in this area. However, it (Shandong Compilation Committee 1990). was noted that because the carbonate levels in the sediment The section of the MDS used in this study is at 37° 56′ eluviations varied significantly, they could not be used to in- 31.9″ N, 120° 40′ 35.9″ E, at an elevation of 17 m above sea dicate paleoclimatic and environmental changes. The CaCO3 level in the northwest region of the Miaodao Archipelago in content in loess regions is affected mainly by eluviation. the Bohai Sea (Fig. 1). At this site, approximately 6.5 m of Under this mechanism, the effect of leaching during cold Quaternary sediments rests on an unconformity atop a weath- and dry periods is weak, which is reflected in the high ered granitoid crust (Fig. 1). In this study, we only discuss the

CaCO3 content of the loess. Conversely, leaching has a strong subsection MDS5, with sedimentary sequences with depths effect during warm and humid paleosol periods is represented ranging from 3.4 to 6.3 m and a total of five identified layers by the low CaCO3 content in the loess (Liu 1985;Lv1981). as follows: MDS5a, MDS5b, MDS5c, MDS5d, and MDS5e. However, this theoretical model is not applicable to carbon- MDS5a, MDS5c, and MDD5e are three paleosols, and ates in coastal eolian sediments. The coastal zone is a sensitive MDS5b and MDS5d are eolian sand (Fig. 2). area at the junction of land and sea; however, carbonate mech- anism of eolian sediments is poorly understood. Thus, a new carbonate formation mechanism for coastal zones is required Methods and materials to explain both the regional CaCO3 index and the carbonate index in coastal zones. Optically stimulated luminescence age test The Bohai Sea coastal zone consists of the Pacific Ocean to the east and the Eurasian continent to the west. Thus, this Using optically stimulated luminescence (OSL), this study region is influenced significantly by both the ocean and the obtained five sets of age data from four layers in the MDS continent and it is highly sensitive to climatic change. The that represent the last interglacial period. The OSL procedure loess-paleosol sequences in the Bohai Sea coastal zone are was conducted using the Daybreak 2200 OSL reader at the an important eolian archive and, along with those on the edge OSL Laboratory of the Institute of Hydrogeology and of the Asian Gobi Desert and the Chinese Loess Plateau as Environmental Geology, Chinese Academy of Geological part of the East Asian eolian dust accumulation system, are Sciences, and the 1100B OSL instrument at the Isotope and also important for global particulate research; therefore, the OSL Dating Laboratory of the Department of Earth Sciences, stratigraphic record can be used to extract information regard- Sun Yat-Sen University (Daybreak Nuclear and Medical ing the features of climate, sea-level change, and the evolu- Systems, Inc., USA). The tubes of the OSL samples were tionary process of the East Asian monsoon. The Miaodao opened under subdued red light in the laboratory. The sample stratigraphical section (MDS) in the Bohai Sea coastal zone, surface at both ends of the tube was scraped away for mea- in which sediments have accumulated since the last intergla- surements of the moisture content, radionuclide concentration, cial period, was selected as the study area for this research. In and other proxies. Only the central part of the sediment was this study, we present and discuss the chronology established used for OSL dating to avoid any incidental exposure to light for the MDS based on OSL measurements of sediments in during sampling and transportation. combination with data showing the CaCO3 content, grain size, The samples were treated with 10% HCl and 30% H2O2 to mineralogy, and foraminifera. Based on our results, we sug- remove carbonates and organics respectively; this procedure gest a new explanation for the carbonate formation mecha- took approximately 15 to 20 days. Quartz minerals, extracted nism in sediments of the Bohai Sea coastal zone. using heavy-liquid separation with sodium polytungstate, Arab J Geosci (2018) 11: 584 Page 3 of 12 584

10°140E′ 12°200E′ 134 ° 40′ E 120°15′ E 120°56′ E 121°37E′ ′ 47° 30 N a b 38° 30 N CHINA ea

Bohai s 40° 00′ N

LZ908 B Changdao MDS5 section

Miaodao Archipelago 32° 30′ N 37° 42 N Fig. 1 Location of the Miaodao stratigraphical section (MDS) and Changdao section were subsequently treated with 40% hydrofluoric acid for ap- silicone oil. This process involved preheating the samples to proximately 60 min to etch the grain surfaces and to dissolve 260 °C for 10 s before measuring the natural and regenerative any remaining feldspar minerals. The grain-size fraction of 4– doses of radiation, in addition to preheating to 220 °C for 10 s 11 μm was extracted by wet sieving. The resulting quartz before measuring each test dose of radiation. Each grain was grains were washed with 10% HCl to remove fluoride precip- optically stimulated for 2 s at 125 °C using a focused green itates. Quartz grain purity was checked by infrared stimulation laser beam. At the end of each measurement cycle, the grains at 830 nm; quartz grains with obvious infrared stimulated were optically stimulated for 60 s using blue light-emitting luminescence signals were retreated with H2SiF6 to avoid diodes while being held at 280 °C (Jacobs et al. 2006; equivalent dose underestimation (Lai et al. 2008). The Wintle and Murray 2006). During the dose recovery test, pretreated grains were then mounted on the 0.1-cm-diameter 220 °C was a suitable temperature. The dose rate for the central parts of 9.7-mm-diameter stainless steel disks using grain-size fraction of 4–11 μmisbetween3and4Gyka−1

Fig. 2 Sequence and timescale of the Miaodao stratigraphical section since the last interglacial period 584 Page 4 of 12 Arab J Geosci (2018) 11: 584

(Lu et al. 2007). The calculation of the contribution of the (Type 3070; Rigaku International Corp., Tokyo, Japan) in the cosmic dose rate was based on Prescott and Hutton (1994). Cold and Arid Regions Environment and Engineering Research Institute of the Chinese Academy of Sciences. The Proxy paleoclimate indices samples were initially dried, heated to 80 °C for 24 h, then ground, sieved through a 200-mesh screen, and further ground A total of 146 samples were analyzed to determine grain size into small pieces 30 μm in diameter for analysis. The national and CaCO3 and foraminiferal contents from the top of the standard sediments, GSD9 and GSS1, were used as controls section downward through the last interglacial MDS5 subsec- for the test results. The relative deviation and errors were both tion. In addition, some other samples were collected for min- less than 5%. In this study, the Rb/Sr ratio served as the proxy eral and geochemistry analyses. paleoclimatic index to discuss the climatic environmental A Malvern Mastersizer 2000 M laser grain-size analyzer evolution. with a measuring range of from 0.02 to 2000 μm was used for the grain-size analysis following the procedures used for loess grains reported by Lu and An (1997); the data were obtained Results via computer processing. The mean grain size (Mz; Φ)andσ of the MDS samples were calculated by using a formula de- Chronology veloped by Folk and Ward (1957), where Mz = (Φ16 + Φ50 + Φ84) 3 and σ =(Φ84 − Φ16 4 + (Φ95 − Φ56.6). The test results and relevant parameters of the OSL procedure

The CaCO3 content in each sample was analyzed three to are listed in Table 1. Figure 2 shows the relationship between four times using a Bascomb calcimeter in the sediment labo- the ages and variations by depth. The linear regression corre- ratory of the Department of Geography, South China Normal lation coefficient (r) between the ages and the depth was 0.99. University; the average standard deviation was 0.18 wt% A sediment chronology was obtained using the OSL ages as a

(Bascomb 1961). The volumes of CO2 for a certain number general framework with which to calibrate the MDS segment. of samples with excess HCL were calculated using the vol- The top and base of MDS5 were correlated with the marine umes and temperatures of the produced CO2 according to the isotope stage (MIS) 4/5 (71.2 ± 2.2 ka) and MIS 5/6 (129.1 ± equation PV = nRT. Finally, the carbonate contents were 2.6 ka) boundaries, respectively (Martinson et al. 1987). The calculated. upper limit of MDS5 lacked testing data and, hence, the com- Heavy mineral analysis was carried out at the Langfang plete MDS5 timescale during the last interglacial period was Integrity Geological Services Ltd. First, 0.2 kg of air-dried established using known ages and segmentation sedimenta- samples was weighed for screening, separation, and micro- tion rates. The upper limit of the MDS5 was near the boundary scopic identification and, then, quantitative analyses were per- between MDS4 and MDS5a. The OSL dating age of the upper formed on 400–600 grains. Finally, the test results were to middle portion of MDS5a was 82.1 ± 3.5 ka, and the OSL conformed with the data of the regional reconnaissance survey dating age above MDS4 was 55.8 ± 1.0 ka. Using an average phase of the natural heavy-concentrate assay of the Ministry sedimentary rate of 0.453 cm/ky during this period, an esti- of Land and Resources (DZ/T0130.9-2006). mated age of 71.2 ka was determined for the upper limit of Clay minerals < 2 μm in diameter were separated accord- MDS5. For the lower limit of the sequence, at the interface ing to Stoke’s settling velocity principle after removing car- between MDS5e14 and MDS6, the dating age was determined bonate and organic matter with 10% H2O2 and 0.5 N HCl, as 129.1 ± 2.6 ka. As the results show, the time boundary respectively. Clay mineralogy determinations were performed determined for MDS5 in this study differs slightly from that at the Guangzhou Institute of Geochemistry, Chinese for MIS5. However, considering that the dating errors of the Academy of Sciences, using standard X-ray diffraction OSL procedure were between 1.0 and 5.0 ka, the ages deter- (XRD) at 40 kV with 25 mA CuKα radiation using a D8 mined for MDS5 in this study were largely consistent with the ADVANCE diffractometer. The clay minerals were identified period from 73 to 132 ka for MIS5. This period is generally mainly by the position of the (001) series of basal reflections known as the last interglacial period. on the XRD diagram of the ethylene glycol salvation. To compare climatic changes in MDS5 to the divisional For the foraminiferal analyses, 50 g dry weight of each scheme from marine cores of the MIS5 section and subsec- sample was collected. The specimens were weighed and tions (Martinson et al. 1987), MDS5 was divided into five soaked in distilled water for ~ 24 h for disaggregation before stratigraphic segments based both on known OSL ages and being washed over a 150-μm sieve and subject to select flo- on those calculated from the regression lines for the sedimen- tation via carbon tetrachloride. Foraminiferal fractions < tation rates (Fig. 2). Results were obtained as follows: 150 μmwerecounted. MDS5a, 71.2–89.8 ka; MDS5b, 89.8–98.9 ka; MDS5c, Forty-four samples at 8-cm intervals were selected for geo- 98.9–105.7 ka; MDS5d, 105.7–115.2 ka; MDS5e, 115.2– chemical analyses using an X-ray fluorescence spectrometer 129.1 ka. Arab J Geosci (2018) 11: 584 Page 5 of 12 584

Table 1 OSL ages of some horizons in the Miaodao section (MDS) and their analytical data

Sample No. Depth (m) U (ppm) Th (ppm) K (%) Total dose (Gy/ka) Annual dose Water content (%) Dating (ka) (m Gy)*

MDS4- SY 03 3.0 1.4 9.1 1.8 187.7 ± 3.4 3.4 ± 0.3 4.5 55.8 ± 1.0 MDS5a-10G521 3.6 1.2 7.0 1.9 246.9 ± 3.5 3.01 7.6 82.1 ± 3.5 MDS5d-10G522 5.3 1.0 6.7 1.9 337.7 ± 4.9 3.03 6.7 111.4 ± 5.0 MDS5e- SY 04 5.4 1.5 10.6 1.9 404.4 ± 7.1 3.5 ± 0.3 11.2 115.2 ± 2.4 MDS5e- SY 05 6.3 1.3 7.9 1.3 309.5 ± 5.0 2.4 ± 0.2 11.7 129.4 ± 2.6

*Eight aliquots were measured for each sample, and the final dose was the average of the eight doses

Grain size generally finer (4.9ϕ) than those of the eolian particles (3.7ϕ). The formation of the sedimentary facies is a re- The results of the Mz analysis for MDS5 are shown in flection of its sources and is related to biochemical Fig. 3(a) including the high and low values. Low values weathering, which is closely linked to climatic change. are apparent in the MDS5b (3.7ϕ) and MDS5d (3.7ϕ) Thus, the observed changes in the sedimentary facies subsections, and high values are apparent in the MDS5a and Mz represent different climatic environments. This (4.6ϕ), MDS5c (3.9ϕ), and MDS5e (5.6ϕ) subsections result corresponds with the findings of studies on the (Fig. 3(a)). The Mz results reflect the average diameter Loess Plateau that showed that Mz is a good proxy index of the sediment grains and transport force strength. Two for representing climatic change during the East Asian sedimentary facies deposited in MDS5 include the winter and summer monsoons (An et al. 1991;Ding paleosol (MDS5a, MDS5c, and MDS5e) and eolian sand et al. 1992). The standard deviation (σ)oftheMDS5 (MDS5b, MDS5d). Between these two sedimentary fa- (Fig. 3(b)) grain sizes ranges from 1.6 to 2.5, with an cies, the paleosol is composed of fine to very fine sand/ average of 2.0. The σ results closely reflect the sediment silty sand with clay, while the eolian sand is composed of grain separation degree, which can distinguish different silty fine sand. The sizes of the paleosol particles are environments of deposition (Barusseau and Braud 2014;

Fig. 3 Vertical variations of Mz (Φ), standard deviation (σ), Rb/Sr ratio, and CaCOK contents (%) in the MDS5 segment 584 Page 6 of 12 Arab J Geosci (2018) 11: 584

Clarke et al. 2014;Kalińska-Nartišaetal.2017; Zhang the loess develops with coarser Mz and greater CaCO3 content et al. 2015). (Liu 1985; Liang et al. 2014).

The MDS5e has small Mz and low CaCO3 content and, therefore, it appears to be a good example for the study of Rb/Sr ratio loess carbonate. Although the eolian deposits in MDS5b and MDS5d have coarser Mz and low CaCO content, the The Rb/Sr ratio has been positively correlated with weathering 3 paleosol in MDS5a had the greatest CaCO content in MDS5. intensity (Dhuime et al. 2015; Liu et al. 2014). Moreover, 3 Chen et al. (1998) adopted the Rb/Sr ratio to indicate climatic variations during the formational periods of loess and Mineralogy paleosols on the Loess Plateau. This ratio can accurately indi- cate the intensity of the winter and summer monsoons (Pang The clay minerals in MDS5 are shown in Table 2. The quartz et al. 2001). In MDS5, MDS5e (paleosol) had the highest Rb/ contents in the clay minerals of the different sections are sim- Sr ratio (Fig. 3(c)), indicating a formational interval with a ilar, with an average value of 18.8%. Ca minerals such as strong degree of chemical weathering. calcite differ in content among the sections. The highest cal- cite content (17.7%) was in MDS5a, followed by MDS5d (3.9%) and MDS5b (2.5%), while MDS5c and MDS5e had CaCO content 3 no calcite. The light minerals, with a specific gravity of less than 2.8 The CaCO content in MDS5 is shown in Fig. 3(d), ranging 3 and generally light in color, on average comprised 99.9% of from 0.5 to 9.2%, with an average content of 2.4%. MDS5a the detrital minerals. The detrital minerals in order of abun- had the highest mean content at 5.1% and the lowest mean dance were quartz (33.1%), silicate minerals containing Ca content 1.0% was in MDS5e; the other subsections MDS5b, such as potash feldspar (20.6%), and heavy minerals with MDS5c, and MDS5d had a content mostly of 2%. trace distributions. Of the heavy minerals, silicate minerals The bivariate plot of Mz, Rb/Sr, σ, and CaCO of MDS5 3 containing Ca such as epidote were the most common, follow- are shown in Fig. 4. It can be seen that the correlation coeffi- ed by hornblende, pyroxene, and garnet. Those containing Ca cient (r)betweenσ, Mz, Rb/Sr, and CaCO is 0.6, 0.04, and − 3 carbonate minerals such as calcite had the lowest percentage. 0.85, respectively. These (r) values show a weak correlation In general, the Mohs hardness of carbonate minerals was rel- between these parameters. These results differ from findings atively low, i.e., a value of 3 for calcite. Thus, these minerals on the Loess Plateau. In the loess-paleosol sequences of Loess are easily weathered and leached under the appropriate condi- Plateau, Mz has a good correlation with CaCO content 3 tions, leading to the migration of Ca2+.However,silicatemin- (Liang et al. 2014;Liu1985). The changes in mean grain size erals containing Ca have higher hardness numbers and greater are controlled by the East Asian monsoon. When the summer resistance to physical weathering. Therefore, they provide a monsoon prevails, the climate is relatively warm and humid, lower carbonate contribution in eolian sediments, as is evi- and paleosol deposits are thin with small Mz. High precipita- denced by their absence among the studied sections. tion leads to greater carbonate leaching and migration. During the winter monsoon, the climate is relatively cold and dry, and Foraminiferal analysis

The examination of 146 samples in MDS5 resulted in the identification of 139 foraminifera in 15 samples, all concen- trated in MDS5a. Of these, 79.8% were eight genera and nine species of benthic foraminifera including Ammonia beccarii, Ammonia convexidorsa, Guembelitria vivans, Cribrononion incertum, Elphidium magellanicum, Cassidulina carinata Silvestri, Pseudononionella variabilis Zheng, Pseudononion sp. A, and Rosalina vilarbodeana d’Orbigny (Table 3, Fig. 5). The Pseudononion sp. A was most abundant at 33.1% and it was identified in 11 of the 15 samples of benthic foraminifera. It was followed in abundance by G. vivans (12.2%, 8/15), A. beccarii (10.8%, 6/15), and E. magellanicum (7.9%, 5/15); the other species comprised < 5% of the total. Eight samples containing planktonic foraminifera show Fig. 4 Scatter plot of Mz (Φ), σ, Rb/Sr ratio, and CaCO3 (%) in the MDS5 segment Globigerina bulloides had a content of 20.1%. All the Arab J Geosci (2018) 11: 584 Page 7 of 12 584

Table 2 Mineralogy of MDS5 segment Sample No. Quartz Potash feldspar Soda feldspar Illite Montmorillonite Kaolinite Calcite

MDS5a-07 18 6.2 13 4.9 26.8 13.4 17.7 MDS5b-20 16.3 4.7 11.3 9.9 44.4 11 2.5 MDS5c-18 21.5 6.9 13.8 11.4 34.9 11.5 – MDS5d-17 18.9 6 14.5 9.2 36.2 11.3 3.9 MDS5e-24 19.4 6.4 14.9 14.6 35 9.7 –

individual foraminiferal specimens within the sediments were subsections. However, MDS5a has the highest average

<100μm in diameter, and a small number had broken shells. CaCO3 content (Fig. 3) and MDS5e, a paleosol with the highest Mz, σ, and Rb/Sr ratio values, has the lowest CaCO3 content. The influence of climate on the same continental Discussion eolian deposit formed during the last interglacial period is inconsistent. The correlation between Mz and carbonate con- The few carbonate studies that have been conducted in the tent is r = 0.03 (Fig. 4), indicating that grain size had little Bohai Sea coastal zone in China have indicated that carbonate effect on the carbonate content in the MDS5 subsection. fluctuates significantly in eolian sediments because of eluvia- tion; therefore, this index should be considered carefully when Mineralogy used as an indicator of climatic change (Li 2009;Wangetal. 2007; Wang 2010). Throughout the Bohai Sea coastal zone, The minerals in MDS5 were analyzed to examine the carbon- the changes in carbonate characteristics are similar. The car- ate formation mechanism. The results show that the detrital bonate change curve in the Changdao section shows similar content of each section is similar, consisting mainly of light tendencies in variation as that in the Miaodao section, both of minerals and weathered detritus with an absence of Ca car- which are at the same latitude (Wang et al. 2007; Fig. 6). This bonate minerals such as calcite. Silicate minerals are mainly suggests that the carbonate characteristics in the Bohai Sea those containing Ca such as anorthites, which constitute near- coastal zone sediments are regionally representative and, thus, ly 25% of the total in MDS5b, MDS5c, and MDS5d. This their formation mechanism should be consistent. However, value is 17.6% in MDS5e, which contains lower carbonate this explanation applies only to the loess carbonate model; content than that of the other sections. The lower CaCO3 con- the actual formation mechanism of the carbonate in the sedi- tent in MDS5b–MDS5e might be related to the mineral hard- ments of the Bohai Sea coastal zone is poorly understood. ness of carbonate minerals. As previously discussed, calcite with a Mohs hardness of 3 is easily weathered and leaching 2+ Grain size, Rb/Sr ratio, and CaCO3 under appropriate conditions leads to Ca migration. Therefore, the carbonate in the eolian sediments of the To determine the actual cause of the carbonate formation in Bohai Sea coastal zone is restricted partly by the effect of the Bohai Sea coastal zone sediments, the grain size (Mz), precipitation leaching, which might explain the lowest carbon- standard deviation (σ), and Rb/Sr ratio of the sediments were ate content in MDS5e. Each section containing silicate min- studied. The Mz and σ results reflect the average diameter and erals, including Ca, has a smaller carbonate contribution. separation degree of the sediment grains, and the Rb/Sr ratio Although the hardness of silicate minerals containing Ca is indicates the intensity of chemical weathering which has been high at 6–6.5, rocks containing these minerals can be split used in various studies to infer the intensity of weathering easily under the effects of chemical weathering. These min- (Armstrong-Altrin et al. 2014, 2015, 2017). Moreover, be- erals have a high Ca content of > 25% and a strong resistance cause they are affected by pedogenesis, the results for these to physical weathering. However, the indication of weak parameters also represent the extent of the warm and humid chemical weathering during the period illustrates an additional climate. The sedimentary facies of MDS5a are a paleosol, and possibility as to why MDS5a–MDS5d reflect greater eolian the Mz of 4.6ϕ indicates that warm and humid conditions sand activity. The warm and humid climate during the time of were significantly stronger than those during the formation MDS5e, stronger chemical weathering, and greater leaching of the eolian sand at Mz 3.7ϕ in MDS5b, the sandy paleosol led to the low carbonate content reflected by the anorthites. at 3.9ϕ in MDS5c, and the eolian sand at 3.7 ϕ in MDS5d. If The results of the clay mineral analysis, as shown in Table 2, the carbonate sediments were controlled mainly by precipita- indicate that MDS5a has more calcite, which might have af- tion leaching, as reflected in the particle size, the CaCO3 con- fected the change in carbonate content. However, the number tent of MDS5a should be less than that in the other three of clay minerals in that subsection is highly limited; therefore, 584 ae8o 12 of 8 Page

Table 3 Results of foraminife identified in MDS5

Sample Foraminifer species No. Pseudononion Guembeliria Ammonia Cassidulina carinata Elphidium Cribrononion Rosalina Ammonia Pseudononionella Globigerina sp. A vivans beccarii Silvestri magellanicum incertum vilarbodeana convexidorsa variabilis Zheng bulloides

MDS5a-01 + + + ------MDS5a-02 - + ------MDS5a-03 - - - - + + - - - - MDS5a-07 + ------MDS5a-10 - - - - - + - - - - MDS5a-11 + - - - - - + - - + MDS5a-17 + - - - - - + - - - MDS5a-18 + + ------+ MDS5a-21 - - - - + - - - - - MDS5a-22 + + + + + + - + + + MDS5a-24 + + + - - - - + - + MDS5a-25 + + + + - - - - - + MDS5a-26 + - + + - - - - - + MDS5a-28 + + - + + - - - - + MDS5a-33 + + + + + - + - - +

B+^ means existence; B-^ means inexistence rbJGoc 21)1:584 11: (2018) Geosci J Arab Arab J Geosci (2018) 11: 584 Page 9 of 12 584

Fig. 5 Some benthic foraminifera species identified in MDS5

a discussion of the carbonate change in the sediment based on carbonate content of 8.4%, whereas samples from the adjacent minerals appears inappropriate. MDDS5a-12, which are marked by the absence of foraminif- era, have a lower carbonate content of 4.9%. Therefore, the Biogenic carbonate presence of marine calcareous fossils or shell fragments obvi- ously improves the carbonate content in the sediments; how- The change in carbonate content in the MDS is not controlled ever, the percentage remains lower than the value of 28% by grain size, nor is it affected by minerals; therefore, the reported from the LZ908 core of Bay (Zhao et al. mechanism for loess is not applicable to the eolian sediments 2011). The same eolian sediments from the last interglacial in the Bohai Sea coastal zone. Considering its special geo- period have distinct differences in marine carbonate biology, graphical position as a coastal zone, sea-level change could which explains the difference in carbonate contents in the influence the source material. Thus, the carbonate formation MDS. Approximately 20 m offshore, the MDS is approxi- mechanism in the Bohai Sea coastal zone should have a new mately 6.5 m thick and has an elevation of 17 m. According index. This indicator can be found in the marine constituents, to field investigations and laboratory analyses, the sediment in including shell debris and foraminifera, in some layers of the the MDS is composed of continuous continental eolian de- MDS formed during the last interglacial period. This marine posits, which excludes the transgression effect on the sedi- carbonate is concentrated mainly in MDS5a, which has the mentary facies caused by biogenic carbonate. The foraminif- highest carbonate content. Non-uniform foraminifera occur in era detected in the sediments indicates the effects of sea level the MDS5a sediment, in which the discovery of foraminiferal fluctuations under specific weather events. Moreover, some shells corresponds to the carbonate content. For example, the marine calcareous organisms are present, and specific climatic samples of MDS5a-11, which contain foraminifera, have a conditions were favorable for storm-surge deposits. These 584 Page 10 of 12 Arab J Geosci (2018) 11: 584

Fig. 6 Carbonate content of the Miaodao and Changdao sections factors play important roles in both increasing the carbonate 100 cal ka BP and remained at a level of 4.4 m, which is the content in the eolian sediments and restricting the carbonate second-highest level of the last 140 cal ka BP. The RWL then changes in the Bohai Sea coastal zone. dropped to ~ 2 m at 97 cal ka BP; a situation that persisted for 6 ka, apart from a slight increase at 94 cal ka BP. The highest Sea level and CaCO3 RWL in the reconstructed series occurred between 89 and 79 cal ka BP, during which maximal values reached nearly Changes in the relative positions of sea and land surfaces are 5 m and the average level was > 4 m (Yi et al. 2012). The indicative of vertical land movement, changes in ocean vol- Bohai Sea level responded to a trend in global sea-level ume, and, as in most cases, a combination of both factors change during the last interglacial period. However, tectonic (Lambeck and Chappell 2001). Because the MIS6 isostatic movements approximately 100–80 cal ka BP in the Bohai Sea rebound led to high sea-level stands during the late MIS5 might have resulted in uplift of the surrounding mountains and (Muhs et al. 2002), several high stands of relative sea level subsidence of the depositional basin (Xu et al. 2005). This have been detected in the western North Atlantic Ocean would have caused the highest sea level in MIS5a in the (Potter and Lambeck 2004) and northeastern North Carolina, Bohai Sea and it implies a relationship between sea-level fluc- USA (Parham et al. 2007). Lambeck et al. (2002)highlighted tuations and regional tectonic activities. that global sea levels during MIS5e were 5 m higher than Although sea-level change did not appear to affect the sed- current levels, whereas sea level during MIS5b and MIS5d imentary facies directly, sea-level oscillations caused redistri- might have been as much as 40–60 m lower than current bution of coastal sediments within the region, in turn, resulting levels. During the major interstadials of MIS5a and MIS5c, in differences in carbonate content that reflect the sea-level sea levels were higher than during adjacent periods by approx- changes. Declining sea levels led to the deposition and emer- imately 20–30 m (Lambeck et al. 2002). The MDS in the gence of a large coastal shelf. In contrast, rising sea levels Miaodao Archipelago in the Bohai Sea coastal zone also ex- were likely partly responsible for transporting shelf sediments perienced sea-level oscillations during the last interglacial pe- to the continental landmass via wave activity. These sea-level riod. Yi et al. (2012) analyzed sediment grain sizes to establish oscillation processes caused differential sedimentation within fluctuations in the level of the southern Bohai Sea since the the study area. When the climate cooled during the MDS5b Late Pleistocene (140 ka). They reported the following se- and MDS5d periods, sea levels dropped and the continental quence. At the beginning of the Late Pleistocene, the reference shelf was exposed near the surface. Species such as forami- water level (RWL) in the southern Bohai Sea rose quickly to nifera were unable to survive, and loose sediments provided 2.9 m relative to present values, then dropped and remained < an abundant source of sand for eolian transport and deposi- 2.0 m. The RWL rose again from 110 to approximately tion. Therefore, no foraminifera fossils are identified in Arab J Geosci (2018) 11: 584 Page 11 of 12 584 subsections MDS5b and MDS5d. The highest sea level, pre- provide a new explanation for carbonate formation in eo- sented in MDS5a, is approximately 5 m higher than the cur- lian sediments within the Bohai Sea coastal zone. rent level. MDS5a is rich in marine biology carbonates includ- ing shell debris and foraminifera, which could have been Funding information This work was supported by the National Natural transported by storm surges and deposited in the section (Du Science Foundation of China under Grant (Nos. 41206036 and 41471159), Guangdong Province Science and Technology Project (No. et al. 2016). Thus, such organisms within the sediment aug- 2015A020216015), and Youth talent forefront project of South China Sea mented the level of carbonate content. Institute of Academy of Sciences (No. SQ201306). Cheng et al. (1995) found some foraminiferal and radiolarian fossils with extensively broken shells in sediments of the Bohai coastal zone that were deposited References during the last glacial period. 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