Insights into the Phaeozem Formation from the Fluvial Parent Material

doi:10.21203/rs.3.rs-1738450/v1

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Insights into the Phaeozem Formation from the Fluvial Parent Material

https://doi.org/10.21203/rs.3.rs-1738450/v1

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The formation and evolution of phaeozems have always been an area of interest because the phaeozems play an important role in two regions dubbed as "European Bread Basket" and "Northeast China Storehouse." As the phaeozems contain a large proportion of parent materials and are sensitive to human intervention, their formation and evolution have been restricted. Due to the homogeneity of loess parent material, there has been extensive research on the formation of surface phaeozems from the loess parent material. However, phaeozems developed from key fluvial parent material have so far received little attention. Therefore, key information on the formation of phaeozems is missing. In this context, this study applies geochemical tests to determine rare earth elements and major and trace elements in the deep phaeozem layer from the Arongqi borehole in the phaeozems of Northeastern China. Further, the sporopollen fossil tests were conducted in the HT-1-4-2 borehole, and the distribution characteristics of the deep phaeozem layer were analyzed using geostatistics. The results from this study indicate that rare earth data from phaeozems in Arongqi's deep layer shows a comparatively high total concentration of rare earth elements, with a mean value of 141.45 mg.kg^− 1. The chondrite normalization curve demonstrates that the distribution of light rare earth elements shows a positive slope. Further, Eu shows a trend with a slight negative anomaly, while Ce shows a trend with an evident negative anomaly. Ce > -0.1 (mean value 3.9) indicates a reducing environment with hypoxia. The distribution of rare earth elements indicates that phaeozems contain transportable parent materials with trace elements Th/U > 2 (mean value 2.88), Sr/Ba < 0.6 (mean value 0.38), 0 < Rb/Sr < 0.6 (mean value 0.26), Sr /Cu > 10 (mean value 30.98), and climate index at mean value of 0.23. The presence of trace elements indicated freshwater habitats and a dry-hot to semi-dry-hot climate. The sporopollen fossils are mainly comprised of shrubs, coniferous forests, and terrestrial herbs. The presence of Ephedra and Polygonaceae indicates that the environment was hot and arid at that time. Moreover, the presence of several ferns observed in the vertical profile indicates an intermittent warm and humid climate between droughts. Thus, it can be concluded that the deep phaeozems in Arongqi developed in the floodplains of Arun River under the dry-hot-semi-dry-hot climatic conditions.

Phaeozems

Fluvial deposits

Dry-hot-semi-dry-hot climatic conditions

Phaeozems of Northeast China

According to the World Reference Base for Soil Resources^[1], phaeozems are soils characterized by soft surfaces, limited distribution, and dark layers above a transition or clay layer. Globally, phaeozems are mostly found in four key locations^[2]. Out of these four locations, the Ukrainian and Northeast Chinese phaeozems are known as "European Bread Basket" and "Northeast China Storehouse" respectively^[3–4].

Phaeozems are soils commonly distributed in the Manchuan and Mangang areas of Northeast China. This region has an altitude of 250–300 m and forms a clear boundary with chernozem. Phaeozems showed a crescent-shaped distribution pattern and are scattered throughout the piedmont and intermountain depressions of Daxing'anling, Xiaoxing'anling, and Changbai Mountains. Loess is the primary parent material of these phaeozems. As there are different types of soils, these phaeozems are referred to as typical black soils (phaeozems and chernozems)^[5]. and narrow sense black soils (solely referred to as phaeozems)^[6]. Traditionally, haeozems form a different class of soils because of the existence of a calcareous layer^[7].

The formation of phaeozems has always been a contentious issue in phaeozem research. Scientists have conducted extensive research studies and held fruitful discussions on this topic. Previous studies have primarily focused on the collection and characterization of phaeozems from mature loess parent material found on the surface and in ancient ruins^[2,8−16]. However, there is a lack of studies on the formation of phaeozems from other parent materials. Recent studies have discovered that there are several types of phaeozem parent materials, particularly river parent material, which is the primary parent material of phaeozems in small watersheds of Northeast China^[17]. However, studies focusing on the formation of this parent material are still lacking. Although scientists have reached a consensus on the classification and division of phaeozems, the complex formation of Chinese phaeozems is yet to be understood. Therefore, it is necessary to have a thorough understanding of the formation of phaeozems, especially from the river parent material, and to investigate the climatic and environmental factors influencing phaeozems.

At present, studies on the formation of phaeozems mostly depend on the vertical changes in major and trace elements of the soil that indicates soil evolution^{[2,15,18−19]}. However, these studies have limitations because they consider only a single method and insufficient unilateral interpretation. Therefore, the dearth of information on major and trace elements in soil necessitates the need for in-depth study on soil elemental analysis. In recent years, sporopollen and rare earth element analytical techniques extensively employed in geology to analyze climate and environment have yielded positive results^{[2,14,20−21]}. Studying sporopollen fossils helps to know about the parent plant through examination of disseminated spore pollens preserved in the deposit. It is also useful for deducing the vegetation law and environmental feedback during this geological epoch^[22]. Due to their widespread distribution and high stability in the stratum, rare earth elements are commonly employed in environmental and geological research, primarily as indicators and as tracers in petrogenesis, environmental pollution, and determination of the origin of the deposit among others. The difference in the concentrations of rare earth elements, in addition to the combination of different elements, can accurately reflect the provenance and the environmental quality and are thus regarded as significant geochemical markers of the environment^[23–24]. Therefore, sporopollen and rare earth element analytical techniques can be employed to aid in the determination of major and trace elements involved in the formation and evolution of phaeozems.

This study examines a subclass of phaeozems. The black humus layer of phaeozems is 40 to 120 cm deep, whereas the bottom transition layer is 150 cm thick. The elevation ranges from 325 to 375 m, with the soil profile not yet entirely established. The area is characterized by a layer of soil deposited in the lower part of Arun River, Arongqi, located on the eastern slope of Daxinganling, known as a fluvial deposit. The top half of the area contains a layer of phaeozems formed from the river parent material, whereas the phaeozems in the lower part of the fluvial deposit are referred to as deep phaeozems. Taking these features into consideration, we carried out sampling and evaluation of the major, trace, and rare earth elements. The sporopollen fossils of deep phaeozems were evaluated based on the rare earth elements, major and trace geochemical characteristics, sporopollen data, and drill core display results. The study examined the provenance features and formation of this layer of deep phaeozems before comparing the differences in the formation and evolution of phaeozems derived from various parent materials.

2.1 Study area

The study area is located in the Arun River basin of Arongqi City ,As shown in Fig. 1(a), Inner Mongolia Autonomous Region, China, having the coordinates 123°22′34″ to 123°30′30″E and 48°04′15″ to 48°10′11″N. It is located in the southern part of Arongqi City. Arun River runs through the north and south, with the terrain steep in the northwest and descending gently in a stepwise manner in the southeast. The elevation of the area is 198–1149 m, with a mean elevation of 350 m. It is the transition belt between the northeastern slope of the Greater Khingan Mountains and Songnen Plain. The area is spanned by the Volhui, Nanling, and Duolun Mountains, which are branches of Daxinganling. The area is a low-middle mountain-hill diffuse post landscape and a transition belt between forest edge and grassland, with a forest coverage rate of 54.2%. The annual average temperature in the area is 22.5°C. The yearly precipitation is 430–450 mm, but annual evaporation can reach up to 1500 mm, i.e., three to four times the annual rainfall. The groundwater level is 57 m, and the annual average sunshine is 2791 h. The weather in the area falls under a typical mid-temperate continental semi-humid climate. The soil type in the area is predominantly phaeozem. The landform in the Arun Valley is distributed in such a way that the first-level terrace lies above the second-level terrace, and a small proportion of chernozem meadow soil is spread throughout the area. The soil belongs to the phaeozems from Songnen Plain in Northeastern China^[25–26]. Soil pH is weakly acidic with rich organic matter content, which is conducive to the growth of vegetation. The vegetation in the area is mainly oak and willow, with a low cover of shrub willow, elm and artificial pine, and larch^[27–28].

2.2 Geological features

The bedrock in the study area is composed of the Cretaceous Guanghua Formation (K1gn) rhyolite, rhyolite tuff, and dacite volcanic rock, the Longjiangzu Formation (K1l) andesitic and andesite tuff, and the Ganhe Formation (K1g) basaltic andesitic and basalt^[29–30]. The types of Quaternary Holocene deposits in the study area can be divided into four types; namely, flood alluvial, swamp, slope flood alluvial, and alluvial. The geomorphology of Holocene flood alluvium (Qp1pal) is characterized by class II accumulation terraces, with an earthy yellow medium-coarse sandy gravel layer, a sandy clay layer, clayey sandy alluvium, and flood alluvial sandy gravel. The Holocene slope flood deposit (Qh1dpl) is distributed in pre- and inter-hill areas and characterized by sandy gravel and sandy gravel-bearing sub-sandy soil. The Holocene swamp deposits (Qh2fl) are primarily found on low and high rocks on both the sides of the river valley and are dominated by dark gray, gray-black clay, gravelly clay, sandy clay, and traces of muddy silt and muddy fine sand. The Holocene alluvium (Qh3al) is predominantly found in the modern Arun River bed. The river bed consists of brownish-yellow and earthy-yellow loose gravels, grayish-white coarse sandy gravelly cobbles, muddy silt, sub-clay, and sub-sand deposits^[29–30].

2.3 Phaeozem characteristics

The surface phaeozems are mostly distributed as second-order terraces, first-order terraces, and high floodplain geomorphic units. The thickness of the surface phaeozems varies according to the location. The thickness of the phaeozems also varies within different geomorphic units. Phaeozems in the second-order terraces are 80–120 cm thick, but those in the high floodplains are about 40 cm thick. The parent material of the surface phaeozems is a fluvial deposit. The floodplains have a gently undulating slope, and the soil is used for agriculture and construction purposes^[17,29−30]. Fluvial deposition accounts for the bottom portion of the surface phaeozems in the study area. The deep phaeozems investigated in this study are located at the bottom of this deposited layer. Figure 1c illustrates the soil profile. The deep phaeozems are located in the parent material layer beneath the surface phaeozems. The borehole deposits indicate that the deep phaeozems are mostly found on both the banks of Arun River in first-order terraces and high floodplain landform units. The lower part of the deep phaeozems consists of the Arun River deposit, while the upper part consists of a fluvial deposit, with a sharp contact boundary between the two. The thickness of deep phaeozems varies with depth and with varying burial depths. It was discovered that the deep phaeozem is dispersed between 2.2 and 5 m below the surface, with a mean depth of 2.6 m and a thickness of 30– 200 cm, and that burial depths are deeper in the northwest and shallower in the southeast. The phaeozem layer is thick in the southwest and thin in the northeast. The deep phaeozems are dark gray-black, with fine grain size and a high percentage of clay components and occasionally contain well-rounded gravels. The soil profile is not completely developed and is significantly different from the fluvial deposit^[17,29−30]. A typical phaeozem profile observed within the borehole can be described as follows: Ah layer (humic layer): this layer is found in all boreholes and has a thickness of 30–200 cm. It is a dark, damp, and medium soil with a granular structure. It is relatively loose and contains massive woody roots; Abs layer (transition layer): it is exclusively found in some boreholes with a thickness between 10 and 30 cm. It is moist, brown-black clay loam with enormous structure, has evident woody roots, and exhibits an erratic pattern of leaching rust; and C layer (parent material): this layer is thick, dark gray sandy clay that is separated into sections according to the Foth's soil ontogeny^[31], with the deep phaeozem section representing the juvenile soil-mature soil stage.

2.4 Sample collection and analysis

Thirty-two boreholes in the study region were sampled for deep-layered phaeozems, and the top 20 cm of the deep phaeozem layer was collected for examination. Figure 1(b) illustrates the sampling borehole location. A total of 32 samples were collected using the quartering method, after which they were ground, sieved, bagged, and sent to the experimental facility for testing and analysis. Sixteen elements; namely, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, a total of 14 rare earth elements and major trace elements; namely, Fe, Mn, Cr, Ni, V, Co, Ca, Mg, Sr, Th, U, Ba, K, Na, Rb, and Cu were determined in the samples using the inductively coupled plasma mass spectrometry (ICP-MS) following GB/T14506.30-2010 method. The samples were analyzed by Northeast Mineral Resources Supervision and Testing Center, Ministry of Natural Resources. The deep phaeozem layer and the lower strata of the black 1-4-2 hole were sampled, and the sporopollen fossils were studied based on the drilling layer division. 200 g samples were collected at every 15 cm interval. Eight samples were taken from 1–4 borehole at sampling depths of 1.5–2.7 m and depths of 1.5–2.55 m for deep-layered phaeozems, as shown in Fig. 1(c). Sporopollen samples were investigated by the Sporopollen Laboratory of Shenyang Normal University (Key Laboratory of Paleontology Evolution in Northeast Asia, Ministry of Natural Resources) using the conventional sporopollen analysis methods.

Data sorting and analysis and plotting of graphs were conducted using Excel 2016, SPSS 16.0, Coreldraw2019, etc.

2.5 Statistical analysis

The equations (1) to (3) were used to conduct standardized computation on chondrite analytical results to accurately analyze the concentrations of rare earth elements in the deeper layers. The computation method for analyzing the concentrations of rare earth elements is represented by the equations (4) to (6) ^[26,32−35].

(La/Sm)N = (Lasample/Lameteorolite)/(Smsample/Smmeteorolite) (1)

(Gd/Yb)N = (Gdsample/Gdmeteorite)/(Ybsample/Ybmeteorolite) (2)

(La/Yb)N = (Lasample/Lameteorite)/(Ybsample/Ybmeteorolite) (3)

Ce = 2(Ce)N/[(La)N + (Pr)N] (4)

Eu = 2(Eu)N/[(Sm)N + (Gd)N] (5)

Ceanom = lg[3CeN/(2LaN + NdN)] (6)

3.1 Composition and spatial distribution of rare earth elements in the deep phaeozems

The concentration values of rare earth elements in the deep phaeozem samples from the study area are shown in Table 1. The total concentration of rare earth elements (∑REE) is 96.52– 187.58 mg·kg^− 1, with a mean value of 141.45 mg·kg^− 1, which is higher than that of surface rare earth elements in Inner Mongolia, China. Mean ∑REE is also higher than the soil background value (132.95 mg·kg^− 1) and deposits in mainland China^[36–37]. However, the mean ∑REE was found to be lower than the Chinese soil REE (187.6 mg·kg^− 1)^[38], North American shale REE (187.0 mg·kg^− 1), and crustal REE (188.8 mg·kg^− 1) The standard deviation of ∑REE is found to be 26.46 mg·kg^− 1, while the coefficient of variation is found to be 0.16, indicating weak mutation and minimal variational change. The mean value of rare earth elements in deep phaeozems follows the trend Ce > La > Nd > Pr > Sm > Gd > Dy > Yb > Er > Eu > Ho > Tb > Tm > Lu, indicating that Ce, La, and Nd are dominant in the samples. The distribution of all the rare earth elements is shown in Fig. 2, which conforms to the Oddo-Harkins rule, i.e., concentrations of even-numbered elements are significantly higher than those of the two adjacent odd-numbered elements^[39]. The comparative results and distribution laws indicate that the deep phaeozems are shell-derived products^[40]. Except for Eu, Gd, Tb, Dy, and Tm, all the other elements are normally distributed. The variation coefficients of rare earth elements in the deep phaeozems are in the order Eu > La > Ho > Gd = Sm = Ce = Nd > Tb = Er = Pr = Dy = Yb = Lu > Tm. All the variation coefficients are found to be less than 0.25. The spatial distribution of individual rare earth elements is evenly compared.

Table 1

Concentrations of rare earth elements in the deep phaeozem samples from Arongqi (mg·kg^− 1)
Element (10^− 6)	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy
Min	17.3	35.1	5.05	21.6	4.14	1.2	4.4	0.8	2.66
Max	43.9	81	9.86	40.9	7.74	2.12	6.44	1.03	5.25
Average	28.57	57.01	7.21	28.07	5.07	1.21	4.16	0.67	3.58
CV	0.19	0.17	0.16	0.17	0.17	0.22	0.17	0.16	0.16
Element (10^− 6)	Ho	Er	Tm	Yb	Lu	REE	LREE	HREE	L/H
Min	0.49	1.47	0.22	1.59	0.24	96.52	84.51	10.59	7.04
Max	1.13	3.1	0.45	2.93	0.43	187.58	169	20.66	10.55
Average	0.78	2.2	0.34	2.24	0.33	141.45	127.15	14.31	8.89
CV	0.18	0.16	0.15	0.16	0.16
Annotation: LREE = La + Ce + Pr + Nd + Sm + Eu; HREE = Gd + Tb + Dy + Ho + Er + Tm + Yb + Lu; L/H = LREE/ HREE; ΣREE = LREE + HREE

The concentration of light rare earth elements (LREE) varied between 84.51 and 169 mg.kg^− 1, with a mean of 127.15 mg.kg^− 1, which is less than that of LREE (143.40 mg.kg^− 1) in the Chinese soils^[38]. The concentration of heavy rare earth elements (HREE) ranged from 10.59 to 20.66 mg·kg^− 1, with a mean value of 14.31 mg·kg^− 1, which is lower than that of HREE(37.20 mg·kg^− 1) in the Chinese soils^[38]. The LREE/HREE ratio is calculated to be 7.04 to 10.55, with a mean value of 8.90, which is greater than that of North American shale (7.47)^[41]. It indicates that there is a clear differentiation between LREE and HREE in the deep phaeozems of Arongqi, and the LREE are relatively enriched. Thus, this study demonstrates that the LREE and HREE migrate at different rates during soil formation. The HREE are more likely to form bicarbonates and organic complexes and undergo leaching. On the contrary, the LREE are more easily adsorbed by clay and undergo accumulation in the soil. Therefore, there is relative enrichment of LREE and relative loss of HREE in the phaeozems^[40,42−43]. This implies that chemical weathering occurs concurrently with the formation of deep phaeozems in the study area of Arongqi. This conclusion is further corroborated by analyzing the data on loess as parent material for surface phaeozems^[44]. LREE/HREE ratio of deep phaeozems is significantly lower than that of the topsoil in southern China^[38], indicating poor weathering. This finding is consistent with the distribution of rare earth elements (i.e., light in the north and heavy in the south) in China^[37].

The Kriging interpolation method was deployed to represent the ∑REE, LREE, and HREE concentrations in deep phaeozems more directly. As shown in Fig. 3(a)REE, the high concentrations of rare earth elements in the deep phaeozems are found in the northern and southern regions of the study area, with the maximum value exceeding 185 mg.kg^− 1 in the northern area. The lowest concentrations in the northwest and middle indicate high concentration values on both sides and a gradually decreasing concentration trend in the middle. The distribution of LREE in the deep phaeozems is similar to that of rare earth elements, i.e., a high value towards the sides and a steadily decreasing trend in the middle part, as shown in Fig. 3(b)LREE. High concentrations of rare earth elements are observed in both the northern and southern parts of the study area, with the highest value of > 160 mg.kg^− 1 in the northern region. High concentrations of HREE are observed in the middle and northern parts of the study area, with the highest concentration exceeding 10.2 mg.kg^− 1 and gradually decreasing concentrations observed from north to south, as shown in Fig. 3(c)HREE. The distribution of LREE and HREE has a strong correlation, while the correlation of rare earth elements with HREE is found to be weak. This implies that LREE dominates the ∑REE, and substantial geographical heterogeneity is observed between the LREE and HREE.

3.2 Concentration distribution of major and trace elements in the deep Phaeozems

The concentrations of trace elements in the deep phaeozem samples are shown in Table 2. The mean concentrations of Cr, Ni, V, Sr, Fe, Mn, and Ba in the samples are all higher than those previously found in the Chinese soil^[45]. On the contrary, the mean concentrations of Co, Ca, Mg, Th, U, B, K, Na, Pb, Cu, and Rb are all lower than those previously found in the Chinese soils^[45]. The variation coefficients of Rb, Cu, Pb, Na, K, Ba, B, U, Th, and Mn are in the range of 0.25 to 0.5, showing a relatively even distribution. The variation coefficients of V, Co, Ca, Mg, and Sr ranged from 0.5 to 1.0, indicating uneven distribution, whereas the variation coefficients of Fe, Cr, and Ni are all greater than 1.0, indicating extremely uneven distribution. It can be observed that the geochemistry of deep phaeozems is relatively stable and not significantly influenced by the exogenous components. This feature of deep phaeozems differs from the surface phaeozems, which have a larger variation coefficient^[46].

The trace elemental concentrations in the deep phaeozems also differ greatly from those in the typical Neolithic buried phaeozems^[47]. Ca, Cu, Fe, Mn, K, Rb, V, Cr, Ni, and Co concentrations in the typical Neolithic phaeozems are higher than those in the deep phaeozems from the study area. However, Sr is present in lower concentrations in the deep phaeozems. In general, anthropogenic activities increased the concentrations of Ca, Cu, and other elements in the surface phaeozems. The elemental composition of the deep phaeozems from the study area is substantially different from that of the typical Neolithic phaeozems. Moreover, the elemental concentrations are greater than those found in the normal soil profiles. As indicated previously, the constituents in the deep phaeozems differ significantly from those found in the ordinary Neolithic phaeozems^[47]. Further, no historical artifacts have been discovered in the deep phaeozems. Therefore, the formation of deep phaeozems rules out the possibility of ancient human activity and is consistent with the variation in the elemental composition observed in the study.

Table 2

Concentrations of major and trace elements in mudstone of the deep phaeozem samples from Arongqi (mg·kg^− 1)
Element (10^− 6)	Fe	Mn	Cr	Ni	V	Co	Ca	Mg
Min	698.83	495.77	9.84	1.97	26	3.5	2598.96	0.14
Max	19986.63	2099.3	178.64	124.5	187.08	43.45	36306.9	3.71
Average	3094.52	931.15	27.54	16.2	74.9	12.6	12455.95	0.89
CV	1.1	0.4	1.13	1.43	0.65	0.72	0.63	0.8
Element (10^− 6)	Sr	Th	U	Ba	K	Na	Cu	Rb
Min	133.71	2.45	1.06	65.94	2281.91	172.42	4.98	16.07
Max	1100	11.15	6.45	1500	20126.49	2804.97	19.09	138.08
Average	383.9	5.75	2.52	894.7	14199.47	1257.47	13.25	80.86
CV	0.57	0.35	0.48	0.38	0.3	0.41	0.26	0.38

3.3 Characteristics of sporopollen fossils in the deep phaeozems

A total of 175 sporopollen samples, 61 angiosperm sporopollen samples, 99 gymnosperm sporopollen samples, 13 fern spores, and 2 fungus spores were identified in 8 samples collected from black 1–4 borehole. As shown in Fig. 4, a total of 13 plant families were observed in the samples; namely, Conifers: Pinaceae (Fig. 4a), Taxod iaceae (Fig. 4b), Cupressaceae (Fig. 4c), shrub plants based on the pollen of Ephedra (Fig. 4d), terrestrial herbaceous plants based on the pollen of Compositae (Fig. 4e), Gramineae (Fig. 4f), Asclepiadaceae (Fig. 4g), Polygonaceae (Fig. 4h), Ranunculaceae (Fig. 4i), Caryophyllaceae (Fig. 4j), ferns based on the pollen of Gleicheniaceae (Fig. 4k), and Polypodiaceae (Fig. 4l). The pollens of the coniferous plant Ephedraceae Willowherbaceae, the terrestrial herbaceous plant Gramineae and Asteraceae, and the fern Polypodiaceae were found in large quantities. Other types of spores and pollens were present in small quantities or were sporadically observed. Ephedraceae and Polygonaceae plant families represent the arid environment, indicating that the climate was relatively hot and arid. The repeated appearances of ferns and terrestrial herbs in the collected samples are indicative of the prevalence of intermittent warm and humid climates during the droughts.

4.1 Distribution of rare earth elements and provenance tracing in the deep phaeozems

The relative abundance of rare earth elements in the soil varies during the soil formation due to the combined influence of soil-forming parent material, climatic conditions, humus, and clay components^[48]. Chondrite is among the initial materials on earth. Therefore, it neither contains light nor heavy rare earth elements and hence can be used as a reference for determining the degree of fractionation in the samples. The data on the rare earth elements in phaeozems was normalized using chondrite for accounting for either odd or even numbers of elements influencing the relative abundance of rare earth elements^[49], as shown in Table 3. The distribution of rare earth elements in the phaeozems of Arongqi is represented in Fig. 5. The distribution pattern indicates that all the sample points show a right-sloping pattern, and this pattern is consistent with that previously observed in the Chinese soils^[38], North American shales (Haskin and Paster, 1979), and the Earth's crust, implying that the source of deep phaeozems is terrestrial. The slope of the curve from La to Eu is steeper, whereas the slope of the curve from Eu to Lu is gentle. Ce has a distinct valley shape, while Eu has a weak valley shape, both of which show negative anomalies. The distribution of rare earth elements in the deep phaeozems shows that LREE is relatively abundant and exhibit typical terrestrial rare earth characteristics^[50–51].

(La/Yb)_N method is used to characterize the degree of differentiation between light and heavy rare earths. A ratio greater than one implies enrichment of LREE, whereas a ratio less than one suggests depletion of HREE^[52]. As observed from Table 3, the highest value of (La/Yb)_N is 10.41, the minimum value is 5.92, and the average value is 7.58, all of which are greater than 1, implying that the LREE and HREE are well differentiated in the deep phaeozems of Arongqi. (La/Sm)_N is a coefficient reflecting the degree of fractionation of LREE^[53], and (La/Yb)_N is a coefficient reflecting the degree of fractionation of HREE. It can be observed from Table 3 that the (La/Sm)_N values of the deep phaeozems range from 2.54 to 4.89 with a mean value of 3.54, and there is a huge variation in the fractionation of LREE. The (La/Sm)_N value ranges between 0.90 and 1.63, with a mean of 1.14, indicating that the fractionation of HREE in the deep phaeozems is not significantly different. However, the fractionation of LREE is more pronounced than that of HREE in the deep phaeozems samples.

The analysis of rare earth elements is an important way of deposit tracing and can reliably maintain the geochemical information about the originating place^[54]. The rare earth element standards are frequently used to characterize the provenance quality. Numerous studies have demonstrated that the deposits with the same provenance frequently exhibit comparable rare earth element distribution curves^[53]. A low similarity in the rare earth model curves is observed with a wide fluctuation in the distribution of LREE. Moreover, sometimes, dissimilar model curves are observed, and characteristics of multiple provenances become evident. Studies have demonstrated that when the parent rock is a medium-acid felsic rock such as granite, ∑REE of deposits is relatively high, the Eu negative anomaly is evident, and the LREE/HREE ratio is high. When the parent rock is basic, ∑REE is relatively low, the negative Eu anomaly is not evident, and the HREE are relatively abundant^[55–56]. Based on this characteristic, it is hypothesized that the pedogenic parent rock in the deep phaeozems contains granite components. This can be verified by analyzing the provenance properties of deep phaeozems using the La/Yb-ΣREE map and La/Yb-ΣREE, as shown in Fig. 6. The deep phaeozem samples were concentrated in granite and alkaline basalt areas, proving the previous theory that the deep phaeozems may have granite components. However, granite and alkaline basalt rocks were not discovered in the bottom bedrock drilling and surrounding mountains^[29], indicating that the bottom and surrounding bedrock were not the origins of phaeozems. The rare earth elements and types of volcanic rocks from the Longjiang Formation (K11) and Guanghua Formation (K1gn) do not conform to the provenance properties^[29]. Figure 6 demonstrates that the material source for deep phaeozems is a mixture of rocks, and the findings agree with the REE distribution pattern for the deep phaeozems. Therefore, it can be stated that the parent material in the deep phaeozems is transportable. This is consistent with the inference that the accumulation-type parent material of phaeozems gets deposited on the surface in the study area^[29].

4.2 δCe, δEu anomaly, and anomaly index (Ce_anom) in the deep phaeozems

Deep phaeozems differ from the surface-layer phaeozems in some aspects, such as minimal human interference during the formation of deep phaeozems and the rigid environment surrounding the deep phaeozems. The rare earth elements, also well-known as environmental response factors, react to the environmental changes throughout the soil formation process^[40]. The variable rare earth elements, Eu and Ce, are susceptible to redox reactions in specific natural environmental conditions and thus, exhibit geochemical properties distinct from those of other rare earth elements. δCe and δEu are often used to reflect upon the soil-forming environment^[43]. The δCe and δEu values represent the anomalies of Ce and Eu, respectively. δCe and δEu < 1 indicate negative anomalies, while δCe and δEu > 1 indicate positive anomalies^[42,57]. Under oxidizing conditions, Ce³⁺ is easily oxidized to more stable Ce⁴⁺. Thus, Ce is separated from other trivalent rare earth elements, showing a positive δCe anomaly. On the contrary, under reducing conditions, δCe shows a negative anomaly as Eu³⁺ is reduced to Eu²⁺. Eu element mainly exists in the form of Eu³⁺; therefore, the reducing environment results in the loss of Eu, leading to δEu negative anomaly^[57]. As shown in Table 3, both δCe and δEu in the deep phaeozems show negative anomalies, implying a depletion of Ce and Eu in the deep phaeozems under reducing conditions in the presence of water.

The anomaly index (Ce_anom) method can also be deployed to understand the environment surrounding the deep phaeozems^[32]. Several research and verification studies have highlighted that this index is a good indicator of paleoenvironment restoration^[57]. Ce_anom >-0.1 indicates an anoxic reducing environment, whereas Ce_anom < -0.1 indicates an oxidizing environment. As shown in Table 3, the Ce_anom values for the deep phaeozem samples range from 3.49 to 4.20, with a mean of 3.90. This value is much greater than − 0.1, implying anoxic reducing conditions in the deep layer, and is consistent with the inferences drawn based on δCe and δEu data described above.

Table 3

Different parameters related to rare earth elements in the deep phaeozem samples from Arongqi
	REE	LREE	HREE	L/H	La/Yb	La/Sm	Gd/Yb	δCe	δEu	Ce_anom
Min	283.32	207.17	66.35	2.71	5.92	2.54	0.9	0.36	0.51	3.49
Max	535.62	405.63	129.99	3.87	10.41	4.89	1.63	0.46	1.03	4.2
Average	391	298.71	92.29	3.24	7.58	3.54	1.14	0.41	0.86	3.9

4.3 Salinity of water bodies formed by the deep phaeozems

Sr/Ba and Th/U were analyzed to characterize the salinity of water bodies formed by the deep phaeozems. Both Sr and Ba exist in water in the form of soluble bicarbonate, sulfate, and chloride, and Sr has a stronger migration ability than Ba. When the water salinity starts increasing, Ba precipitates, and further, when the salinity reaches a particular level, Sr starts precipitating, Sr/Ba increases, and Sr/Ba is positively correlated with the water salinity. According to one study, Sr/Ba in the freshwater environment is less than 0.6, Sr/Ba in brackish water is between 0.6 and 1, and Sr/Ba in salty water is greater than 1. Th/U can also be used as a major means for the measurement of paleosalinity. Th is easily adsorbed by the clay minerals, and U is easily leached or oxidized. Th/U is less than 2 in saltwater habitats, whereas it is higher in freshwater environments^[58–60]. The Th/U and Sr/Ba ratios are shown in Fig. 7. All the samples have a Th/U value greater than 2, with a mean value of 2.87. Most of the Sr/Ba values are less than 0.6, except Sr/Ba values of the deep phaeozems at HT1-6 and HT0-2, which are between 0.6 and 1 with a mean value of 0.38. Both the Th/U and Sr/Ba values indicate that the freshwater water body is formed by the deep phaeozems. A comprehensive analysis in conjunction with the investigation of types of borehole deposits further suggests that the water body formed by the deep phaeozems is Arun riverine system.

4.4 Paleoclimate records of the deep phaeozems

The concentration and composition of trace elements in the deposits can provide valuable information about paleoclimate. Therefore, several characteristic elements are frequently used for the reconstruction of the paleoclimate^[59–62]. At present, the widely used element indices are Rb/Sr^[63], climate index (CI), and Sr/Cu^[61]. Rb has a relatively strong geochemical connection under weathering conditions. Sr is easily oxidized and leached out. Strong weathering leads to the leaching of Sr under warm and humid climatic conditions. However, Sr leaching is minimal because of weak weathering under arid climatic conditions. A lower Rb/Sr value implies a dry climate, and a higher Rb/Sr value indicates a warmer and humid climate. Sr is a typical dry-prone element, and a high Sr content indicates a dry climate. Cu is a typical humidity-prone element, and a high Cu content indicates a humid climate. Sr/Cu < 10 represents a humid climate, and Sr/Cu > 10 represents a dry and hot climate^[61,63]. High concentrations of elements in the deposits, especially Ni, Zn, A1, Fe, and Co, indicate a humid and warm climate. Under dry conditions, a significant proportion of Sr, Ca, Mg, and Na undergo precipitation because of the loss of water by evaporation and increased alkalinity. Thus, higher Sr, Ca, Mg, and Na indicate an arid climate. CI is used to describe the changes in the climate system. Lower CI implies a dry climate, while higher CI indicates a humid one^[61]. CI provides an accurate description of both modern and paleoclimates^[39,64]and can be calculated by Eq. 7^[61]. The climate patterns based on the values of CI are shown in Table 4.

CI = (Fe + Mn + Cr + Ni + V + Co)/(Ca + Mg + Sr + Ba + K + Na) (7)

Table 4

Climate patterns based on the climate index
Climate index	< 0.2	0.2–0.4	0.4–0.6	0.6–0.8	0.8–1.0
Type of Paleoclimate	dry heat	dry heat	Semi dry heat-Semi humid	Semi humid	Humid

Rb/Sr values in the deep phaeozems of Arongqi are shown in Table 5 and Fig. 8. The maximum and minimum values of Rb/Sr are 0.53 and 0.04, respectively, with a mean value of 0.23. The statistics reveal that the Rb/Sr ratio is 0.44 when CI is 0.2, the ratio is 0.47 when CI lies in the range of 0.2–0.4, and the ratio is 0.09 when CI lies in the range of 0.4–0.6. When CI is in the range of 0–0.4, the Rb/Sr ratio is 0.91, and it indicates an arid climate, as shown in Fig. 8. Sr/Cu ratio in the deep phaeozems of Arongqi is much greater than 10. The minimum and maximum values of Sr/Cu are 10.52 and 84.36, respectively, with a mean value of 31.02. Sr/Cu value indicates that the recorded climate in the deep phaeozems of Arongqi was arid. Table 5 and Fig. 8 show that the CI of the deep phaeozems ranges from 0.58 to 0.05, with a mean value of 0.22. The statistics show that the proportion of CI less than 0.2 is 0.56, the proportion of CI in the range 0.2–0.4 is 0.31, and the proportion of CI in the range 0.4–0.6 is 0.13. The proportion of CI in the range 0–0.4 is 0.87, with a mean value of 0.29, implying a dry-hot-semi-dry-hot climate. Rb/Sr, Sr/Cu, and CI all indicate that the climate recorded in the deep phaeozems of Arongqi was dry-hot-semi-dry-hot.

Table 5

Rb/Sr and climate index (CI) in the deep phaeozems of Arongqi
Proportion	0–0.2	0.2–0.4	0.4–0.6	0.6–0.8	0.8–1.0
Rb/Sr	0.44	0.47	0.09	0	0
CI	0.56	0.31	0.13	0	0

4.5 Formation of the phaeozems from various parent materials, including fluvial deposits

The salinity of water bodies was recorded in the deep phaeozems based on the contact boundary between river deposits and deep phaeozems in quaternary boreholes. Moreover, the source of materials, climate information, and the formation of deep phaeozems were also similarly discussed. The study concluded that the deep phaeozems were formed from an early riverine marsh ecosystem under an arid climate system.

The formation of phaeozems has been discussed for a long time, and several hypotheses have been put forward, including the anthropogenic origin of phaeozems from the loess parent material at Neolithic sites^[65–67], natural origin of surface phaeozems from the loess parent material^[68–70], the origin of phaeozems from the zeolite basalt residues^[71], and origin of phaeozems from the volcano deposits^[72]. However, there is no evidence that the phaeozems of anthropogenic origin exist in Neolithic sites of Northeast China; therefore, only phaeozems of natural origin were considered in this study. The phaeozems are formed as a result of the interactions of several elements, and the underlying mechanism is complex. As described previously, other elements also play a greater part in the formation of phaeozems from river parent material deposits under arid conditions. The environment and mechanism of phaeozem formation are determined by the combined effect of many factors, with each factor playing a different role. Therefore, this study considers the phaeozems of natural origin and explores multiple formation mechanisms of phaeozems derived from various parent materials. As shown in Table 6, the phaeozems formed from different parent materials are compared to understand the formation and evolution of phaeozems and the major controlling factors determining the formation of phaeozems.

Table 6

Formation of phaeozems from different parent materials
Phaeozem formation	Loess parent material[69]	Fluvial deposit parent material	Basalt Residual parent material [71]	Andesitic residual parent materials [72]
Based on the type of parent material	Loess	Fluvial deposit	Basalt (weathered basalt)	Andesitic materials
Modern distributed vegetation	Modern forest grassland landscape	Modern grassland landscape	Modern forest landscape	Modern forest landscape
Elevation (m)	220–300	320–350	850–1150	Around 500
Physiographic or Topographic position	Hill, mountain root	river terraces	Hill slope	Above the island
Climate zone	Temperate	Temperate	Tropical	Tropical
Phaeozem records vegetation characteristics	Xerophytic, mesophytic, hygrophytic, aquatic	Xerophytic, hygrophytic, aquatic
Phaeozem records climatic characteristics	Warm and wet	Arid - semi-arid	Hot and humid	Hot and humid
Characteristics of soil formation	Warm climate and humid	Arid climate	Presence of zeolite minerals	Coral organic matter
Factors controlling soil formation in this study	Coniferous forests, broad-leaved forests (deciduous), shrubs and terrestrial herbs, ferns	Coniferous forests, shrubs and terrestrial herbs, ferns	forest	Modern agricultural landscape

4.5.1 Zonal mechanism

Parent materials, namely, fluvial deposits and loess^[69], are concentrated in the warm and moist regions of the horizontal distribution zone. A previous study^[2]demonstrates that the phaeozems are extensively dispersed across latitudes between 30° and 54°, with the highest distribution at 46°. According to that study, the temperate continental monsoon climate, characterized by long, cold, and dry winters as well as short, hot, and rainy summers, is conducive to the production and accumulation of organic matter in the environment. The current conditions are a result of the climatic conditions in the region. However, climate does not play a major role in the preservation and distribution of phaeozems^[69]. The phaeozems formed from basalt deposits and volcanic deposits as parent materials are concentrated in the tropics^[69,73−74], because of the strong chemical weathering and the presence of special minerals in the tropics. It is important to note that climate does not play a crucial part in this process^[71–72]. The vertical zone mechanism is demonstrated by the distribution of phaeozems from different parent materials and the distribution geomorphology of phaeozems. All the phaeozems are situated on the mountain slopes or intermountain valleys with relatively elevated topography. The topography, climate, landform, and hydrology are found to be distinct at different altitudes. The phaeozems from the loess parent material are dispersed between 220 and 300 m, whereas those from the basalt residual parent material are distributed between 850 and 1150 m in the mountains. Based on this topography, the climate is vertically differentiated, and the vertical climate prevails. The topographical features such as inter-mountain depressions at the base of slopes are conducive to the storage of water from precipitation. Climate, geomorphology, and hydrology determine different environmental conditions at different altitudes. A combination of all these factors defines the occurrence, development, and growth of phaeozems at various altitudes.

4.5.2 Commonality in formation mechanism

The factors controlling the formation of phaeozems from diverse parent materials are extraordinarily complicated. Generally, the formation is the combined result of several distinct mechanisms, of which one or two are prominent.

According to the previous studies^[44,69], the climate is warm and humid, and the degree of chemical weathering is low during the formation of the phaeozems from the loess parent material. If the conditions are warm and humid, grassland vegetation ensures an adequate source of organic matter and chemical weathering. The low degree of weathering causes mineral and organic matter in the soil to decompose slowly, thus providing a stable environment for the formation of phaeozems. Although the phaeozems from zeolite basalt parent material are formed in a tropical climate, the high degree of chemical weathering in a tropical climate is unfavorable for the formation of phaeozems. The presence of zeolite not only prevents the conversion of montmorillonite to kaolin but also maintains the alkali saturation level in the environment. This allows the preservation and accumulation of organic matter, thus leading to the formation of phaeozems in tropical climates^[69,71]. The formation of phaeozems from volcanic deposits takes place because the clay minerals in volcanic deposits are unfavorable to soil infiltration. This helps in the storage of soil physical and chemical indicators and ensures the presence of organic matter. Thus, phaeozems were formed due to the combined effects of multiple factors^[72]. In this study, the formation of phaeozems from river parent material is due to the swampy environment that ensures the source of organic matter and to the arid climate, which is the major contributing factor. In comparison of this study with the previous studies, it can be stated that the formation of phaeozems from various parent materials is a result of adequate sources of organic matter with common properties and a favorable environment for the preservation of organic matter. A comparative study also helps to verify the finding that the phaeozems are formed in warm and humid regions^[44,69]; however, this may not be entirely accurate. The factors controlling the formation of phaeozems from various parent materials vary, and several mechanisms play essential roles in regulating the organic matter content.

(1) The concentration of rare earth elements (including Y) in the deep phaeozems ranges from 113.78 to 213.47 mg·kg^− 1, with a mean value of 160.79 mg·kg^− 1. There is an enrichment of light rare earth elements, while the heavy rare earth elements are depleted. The concentrations of elements follow the trend: Ce > La > Nd > Y > Pr > Sm > Gd > Dy > Yb > Er > Eu > Ho > Tb > Tm > Lu, which conforms to the Oddo-Harkins rule. The distribution pattern of rare earth elements is comparable to the patterns observed in the North American shale, crust, and Chinese soil. All the samples show a right-dipping pattern, with relative enrichment of light rare earth elements and depletion of heavy rare earth elements. The samples show slight negative δEu anomalies and evident negative δCe anomalies. The (La/ The Sm)N and (Gd/Yb)N values reflect a lower degree of fractionation of heavy rare earth elements and a higher degree of fractionation of light rare earth elements, indicating that the deep phaeozems display common terrigenous properties.

(2) The La/Yb-REE diagram and REE distribution curve of the samples show that the parent material of the deep phaeozems is primarily derived from alkaline basalt and granite, which have various provenances and are accumulation-type parent materials. The Ce_anom and δCe values of the samples from the study area indicate that the environment surrounding drilled phaeozems has an anoxic reducing environment.

(3) The salinity index of water bodies reveals that the majority of samples with Sr/Ba > 0.6 (mean: 0.38) have Th/U > 2 (mean: 2.88), indicating that the water bodies formed by the deep phaeozems are terrestrial freshwater ecosystems. Rb/Sr between 0–0.6 (mean: 0.26), Sr/Cu > 10 (mean: 30.98), and climate index (mean: 0.23) indicate that the deep phaeozems had dry-hot-semi-dry hot climate

(4) The sporopollen fossils are mainly comprised of shrubs, coniferous forests, and terrestrial herbs. Ephedra and Polygonaceae show that the environment at that time was generally hot and arid, with the formation of numerous vertical ferns and intermittent warm and humid conditions between the dry intervals.

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No competing interests reported.

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Insights into the Phaeozem Formation from the Fluvial Parent Material

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Study area

2.2 Geological features

2.3 Phaeozem characteristics

2.4 Sample collection and analysis

2.5 Statistical analysis

3. Results

3.1 Composition and spatial distribution of rare earth elements in the deep phaeozems

3.2 Concentration distribution of major and trace elements in the deep Phaeozems

3.3 Characteristics of sporopollen fossils in the deep phaeozems

4. Discussion

4.1 Distribution of rare earth elements and provenance tracing in the deep phaeozems

4.2 δCe, δEu anomaly, and anomaly index (Ce_anom) in the deep phaeozems

4.3 Salinity of water bodies formed by the deep phaeozems

4.4 Paleoclimate records of the deep phaeozems

4.5 Formation of the phaeozems from various parent materials, including fluvial deposits

4.5.1 Zonal mechanism

4.5.2 Commonality in formation mechanism

5. Conclusions

References

Additional Declarations

Status:

Version 1

Insights into the Phaeozem Formation from the Fluvial Parent Material

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Study area

2.2 Geological features

2.3 Phaeozem characteristics

2.4 Sample collection and analysis

2.5 Statistical analysis

3. Results

3.1 Composition and spatial distribution of rare earth elements in the deep phaeozems

3.2 Concentration distribution of major and trace elements in the deep Phaeozems

3.3 Characteristics of sporopollen fossils in the deep phaeozems

4. Discussion

4.1 Distribution of rare earth elements and provenance tracing in the deep phaeozems

4.2 δCe, δEu anomaly, and anomaly index (Ceanom) in the deep phaeozems

4.3 Salinity of water bodies formed by the deep phaeozems

4.4 Paleoclimate records of the deep phaeozems

4.5 Formation of the phaeozems from various parent materials, including fluvial deposits

4.5.1 Zonal mechanism

4.5.2 Commonality in formation mechanism

5. Conclusions

References

Additional Declarations

Status:

Version 1

4.2 δCe, δEu anomaly, and anomaly index (Ce_anom) in the deep phaeozems