Enhanced Continental Weathering and Intense Upwelling Drove the Deposition of Organic-Rich Shales in the Late Permian Dalong Formation, South China

Abstract

Marine black shales are important to geologists, because they are not only potential sources and reservoir rocks for shale gas/oil, but also, their deposition could influence the climatic and oceanic environments. Here, a detailed study of the shales in the Dalong Formation in South China was conducted to understand the changes in continental weathering and upwelling and their influences on organic matter accumulation in the late Permian. The results revealed that the deposition of the Dalong and Daye Formations could be divided into five stages, with the highest TOC values (>2%) being observed in stages 2 and 4, intermediate TOCs (~1% to 2%) being observed in stages 1 and 3, and the lowest TOC values (<1%) being observed in stage 5. This study attributed the enhanced organic matter accumulation in stages 2 and 4 to enhanced continental weathering (high CIA values and δ²⁶Mg values) and intense upwelling (high Mo/TOC ratios and low δ¹³C_org and Co_EF × Mn_EF values), both of which contributed to high primary productivity and increased anoxia of the bottom waters, further leading to the accumulation of organic matter. Overall, both enhanced continental weathering and upwelling contributed to the development of anoxia, even euxinia, of the seawater and further triggered an end-Permian mass extinction (EPME).

1. Introduction

The deposition of organic-rich shales is of significant interest to geologists globally, as the enrichment of organic matter in those rocks is crucial for shale gas formation and climatic variations [1,2]. Since organic-rich shales are characterized by the massive accumulation of organic matter and large amounts of pores in organic matter and inorganic minerals, they are typically regarded as hydrocarbon sources, reservoirs, and cap rocks of shale gas [3]. In addition, changes in climate and oceanic environments are strongly influenced by carbon cycling between the atmosphere and ocean; as a result, the accumulation of organic matter in shales could influence global biogeochemical cycling (for example, carbon) and further affect changes in pCO₂, climate, and biogenic evolution [2,4].

Recently, the deposition and diagenesis of black shale have attracted significant interest from sedimentary and petroleum geologists [5,6,7]. However, the mechanisms of organic matter preservation and accumulation remain a subject of debate, primarily focused on whether marine shales and mudstones with large amounts of organic matter are driven by an extremely high flux of biomass to the bottom waters or by the expansion of anoxic conditions in seawater [6,7,8,9]. The former suggests that sediments’ high organic matter content results from higher primary productivity in the surface ocean [10,11]. In contrast, the latter emphasizes the significance of anoxic water conditions, as they would contribute to the preservation of organic matter in marine sediments, further enhancing organic matter accumulation [12,13].

During the Permian, the sedimentary environment in the western Hubei region was dominated by carbonate rock deposition, which evolved from the marine–terrestrial transitional phase in the early Permian to carbonate sedimentation in the middle Permian and coastal–continental shelf sedimentation in the late Permian. In the late Permian, anoxia of seawater and climate changes were observed in the middle Yangtze region in South China, which was accompanied by a wide distribution of black shales named the Dalong Formation [7,8,9]. The Upper Permian Dalong Formation in the middle Yangtze region is predominantly characterized by organic-rich siliceous shales with proper maturity (0.6 to 1.8%), making it a significant source of rocks in South China [14,15]. Recently, numerous studies have proposed the significance of geological events (e.g., enhanced continental weathering and intense upwelling) on the deposition of organic-rich shales [16]. Therefore, several crucial geological events that occurred in the late Permian have been suggested as the factors controlling the deposition of black shales in the Dalong Formation, such as a warming climate [17,18], intense upwelling [19], and tectonism [20].

In particular, upwelling and continental weathering have been widely suggested to drive organic matter accumulation during geological times [2,16,21]. First, upwelling zones are considered a significant depositional setting for marine source rocks, as they can provide nutrient-rich cold waters and accelerate primary productivity [4,12], which could further contribute to organic matter accumulation [19]. Second, continental weathering rates primarily include chemical and physical weathering, with enhanced weathering rates resulting in high nutrient transportation and primary productivity [21,22,23]. In this case, numerous studies attributed the enrichment of organic matter to enhanced continental weathering [21,24]. Typically, weathering proxies in mudstones primarily include the chemical alteration index (CIA), chemical weathering index (CIW), plagioclase alteration index (PIA) [25,26,27], or lithium (Li) [23,28] and magnesium (Mg) [26,29,30,31] isotopic compositions in shales.

In this study, detailed investigations of the shales in the Dalong Formation were conducted to understand the weathering trends (employing weathering indices in shales and δ²⁶Mg in silicates), as well as the intensity of upwelling (employing key element concentrations and their ratios in shales), in the late Permian. Building on this, this study elucidated and determined the changes in the paleoclimates and hydrographic conditions, as well as their influences on the primary productivity and redox water conditions. Ultimately, this study elucidated the formation mechanism of organic-rich shales and provided insights into the end-Permian mass extinction (EPME).

2. Geological Setting

The South China Craton was formed by the collision of the Yangtze and the Cathaysia blocks along the Jiangnan Orogenic Belt during the Neoproterozoic. In the late Permian, the South China Craton was situated at low paleolatitudes and was located in the eastern Paleo-Tethys Ocean, with Kangchen oldland in the paleo south and Cathaysia oldland in the paleo north (Figure 1a). During this period, a series of geological events occurred, including the expansion of the Paleo-Tethys Ocean, the eruption of the Emeishan Large Igneous Province, and the basement fault reactivation [32,33,34,35,36]. In the late Permian Changhsingian, the South China Craton was mainly composed of deep-water basin deposits in the northern and southwestern parts, shallow carbonate platform deposits in the middle part, and continental and shallow marine clastic deposits in the western and southeastern parts (Figure 1a). The western Hubei Basin in South China is one of the most significant basins in the middle Yangtze Block, with the evolution of the western Hubei Basin being associated with large-scale transgressions and regressions [32]. The limestone of the Wuchiapingian Formation formed during the transgression periods, indicating a relatively low sea level [33]. The western Hubei Basin offered ample space for sediments in the late Permian, resulting in the deposition of significant amounts of siliceous shales and siliceous limestones. These rocks with high quartz contents are known as the Upper Permian Dalong Formation [20,34].

Most areas of the western Hubei region were characterized by intra-platform basins and the marginal facies of platform basin. The Permian black rock series in the study area mainly occur in the Guadalupian Gufeng Formation, the Lopingian Wujiaping Formation, and the Dalong Formation (Figure 1c). In the Sichuan Basin, they are represented by the unconformity surface at the top of the Maokou Formation, the Longtan Formation, and the Changhsingia Formation, respectively (Figure 1c). The Gufeng Formation in the western Hubei area is in conformable contact with the underlying Maokou/Qixia Formation (Figure 1c). Also, the Wujiaping Formation is in contact with the underlying Dalong Formation and Daye Formation (Figure 1c).

The Shuanghe (SH) section, situated within the Shuanghe village, western Hubei Province, South China, was chosen as the study area (Figure 1b). From the middle to late Permian, the western Hubei Province was an intra-platform rift basin. During this interval, black siliceous rocks, known as the Gufeng Formation, were widely deposited, overlying the Maokou limestones (Figure 1c). Subsequently, the widespread distribution of limestones overlying the Gufeng Formation suggests shallower waters during this period. These rocks are typically known as the Wuchiapingian Formation (Figure 1c), with a thickness of about 30 m. The Dalong Formation is primarily characterized by siliceous and argillaceous shales (Figure 2a,b). These black shales represent the initiation of the formation of the western Hubei Basin, indicating deeper waters. Finally, the interbedded yellowish-gray shales and limestones were deposited on top of the Dalong Formation and are known as the Daye Formation (Figure 2c–e). The Dalong Formation can be divided into three parts, from the bottom to the top. In the lower Dalong Formation, most of the shales are dominated by yellowish-brown rocks with low organic matter contents (Figure 2a,f,h). In the middle Dalong Formation, the majority of the shales are characterized by black shales with high quartz contents and high organic matter contents (Figure 2b,g). In the upper Dalong Formation, the shales are characterized by black to gray-white rocks (Figure 2e).

3. Materials and Methods

3.1. TOC and δ¹³C_org

Before the analysis of the total organic matter (TOC) and δ¹³C_org values, all the rocks were treated with 2N HCl to remove the carbonate minerals and were treated with Milli-Q water to remove the HCl. The TOC values were analyzed using Elementar Vario EL (Elementar, Langenselbold, Germany), with analytical precision that is better than 0.1%. The δ¹³C_org was measured using Finnigan MAT-253 (Thermo Fisher Scientific, Dreieich, Germany), with analytical precision that is better than 0.1%.

3.2. Major and Trace Elements

The melting method was used for the pretreatment for the major element analysis, with the cosolvent being lithium fluoride, lithium metaborate, and lithium tetraborate and the oxidant being ammonium nitrate and lithium bromide. The melting temperature was set as 1050 °C and the time at 15 min. An X-ray fluorescence spectrometer (XRF) was used for the analysis of major elements, with the standard curve using Chinese national standard material (GBW07101-14: Ultrabasic Rocks, National Standardization Administration of China, Beijing, China) and the relative standard deviation (RSD) being less than 2%. The analytical precision was better than 10%.

The CIA was calculated as follows [37]:

CIA = Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O) × 100

(1)

In the equation, CaO* represents the CaO content in the silicate, and thus it is calculated following the method proposed by McLennan (1993) [38]: CaO* is initially corrected using the P₂O₅ contents (CaO − 10/3 × P₂O₅), and if the calculated value exceeds the Na₂O content, the CaO* value is set as the Na₂O value; otherwise, the CaO* value is set as the CaO content. Due to the possible addition of K₂O into weathered samples during the K-metasomatism, a correction of CIA values (CIA_corr) was proposed as follows [39]:

CIA_corr = Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O*) × 100

(2)

In Equation (2), the K₂O* is calculated as follows:

(K₂O*) = {m × [(Al₂O₃)] + m × [(CaO*) + (Na₂O)]}/(1 − m)

(3)

In Equation (3), the m is calculated as follows:

m = (K₂O)/[(Al₂O₃) + (CaO*) + (Na₂O) + (K₂O)]

(4)

In order to eliminate the influence of K₂O, Harnois (1988) proposed the CIW proxy [25].

CIW = Al₂O₃/(Al₂O₃ + CaO* + Na₂O) × 100)

(5)

Fedo et al. (1995) defined the PIA proxy as an important weathering intensity [39], which is calculated as follows:

PIA = (Al₂O₃ − K₂O)/(Al₂O₃ − K₂O + CaO* + Na₂O) × 100

(6)

The biogenic Si (Si_bio) concentrations [40] were calculated as:

Si_bio = Si_tot − [Al_tot × (Si/Al_background)]

(7)

where Si_tot and Ti_tot indicate the total Si and Ti contents in shales, and Si/Al_background represents a background value of 3.11 [41].

The trace element was analyzed using an Agilent 7900 ICP-MS (Agilent, Tokyo, Japan) at Wuhan Sample Solution Analytical Technology Co., Ltd. First, fresh and fine-grained shale samples were powdered into 0.075 mm diameter pellets, which were placed in an oven at 105 °C for ~12 h. Second, about 50 mg of samples was dissolved with HNO₃ and HF. Third, HNO₃, Milli-Q water, and internal standard solution were added. Fourth, the addition of 2% HNO₃ was used to dilute the final solution. The analytical precision was better than 5%.

The enrichment factors of Mo (Mo_EF), U(U_EF), and V(V_EF) were calculated as X_EF = [(X/Al)_sample/(X/Al)_PAAS]. In the equation, X represents the Mo, U, and V concentrations, and Al represents the Al concentrations. PAAS means the elemental concentrations of post-Archean average shale (PAAS) [42].

3.3. Mg Isotopes in Silicate

In order to analyze the Mg isotopes in silicates (δ²⁶Mg_silicate), 2N HCl was used to remove the carbonate minerals. The Mg isotopes analysis mainly involved the extraction of Mg and measurement of Mg isotopes. The extraction of Mg included a two-step acid leaching approach, which was described in detail by Hu et al. (2023) [43]. The elemental concentrations of pure Mg solution and matrix solution were analyzed after the two-column ion exchange process to monitor the recovery and purification. Our method yielded low matrix elements (<0.2%) and a high Mg recovery of >95%.

The values for magnesium isotopes in silicates (δ²⁶Mg_silicate) were analyzed using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany). The measured δ²⁶Mg_silicate were normalized to the international Mg isotope standard (DSM3) using the following equation: δ^xMg_sample = [(δ^xMg/²⁴Mg)sample/(δ^xMg/²⁴Mg)_DSM3 − 1] × 1000. Here, x means 25 and 26.

3.4. The Mineralogical Compositions

The mineralogical compositions of the bulk rocks were subjected to XRD analysis using a Rigaku automated powder diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation (40 kV, 40 mA). The diffracted beam was measured with a step size of 0.017°. Jade software (Jade 9.0) was used to interpret the raw diffraction data and determine the mineral contents. The minerals (e.g., quartz, feldspar, clay) that were contained in the samples were identified by comparing the X-ray spectrum with a database (Joint Committee on Power Diffraction Standards-International Center for Diffraction Data (JCPDS-ICDD).

4. Results

4.1. TOC and δ¹³C_org Values

The analytical results in terms of TOC and δ¹³C_org values are presented in

Table 1. The major elemental concentrations (%) and Si_bio, TOC (%), δ¹³C_org (‰), CIA, CIA_corr, CIW, PIA, δ²⁶Mg_silicate (‰), and δ²⁵Mg_silicates (‰) values in the Dalong and Daye Formations of Shuanghe section.

Samples	Height (m)	Formation	TOC	δ13Corg	δ26Mgsilicate	δ25Mgsilicate	SiO2	Al2O3	MgO	Na2O	K2O	P2O5	TiO2	CaO	TFe2O3	MnO	CIA	CIAcorr	CIW	PIA	Sibio
SH-06	3.7	Dalong	1.52	−24.4	1.51	0.79	56.57	21.27	0.95	0.13	2.45	2.00	1.98	0.64	2.17	0.00	84.0	84.9	93.9	93.1	13.6
SH-12	8.1	Dalong	1.77	−25.8	1.08	0.56	84.48	5.95	0.48	0.12	1.06	0.06	0.24	0.04	2.31	0.00	80.7	82.7	95.7	94.7	72.5
SH-16	12.6	Dalong	1.03	−25.9	/	/	93.51	1.41	0.13	0.09	0.22	0.04	0.07	0.05	0.91	0.00	74.6	75.7	85.4	82.9	90.7
SH-18	14.1	Dalong	0.11	−25.4	/	/	75.39	10.48	0.75	0.13	1.28	0.25	0.32	0.09	6.21	0.05	85.6	86.6	96.5	96.0	54.2
SH-21	16.9	Dalong	12.47	−27.3	1.03	0.54	69.04	8.83	0.88	0.13	2.05	0.06	0.41	0.03	0.70	0.00	78.0	81.0	97.0	96.1	51.2
SH-25	19.9	Dalong	9.87	−27.2	1.13	0.58	67.36	10.71	0.75	0.03	1.94	0.07	0.41	0.00	2.41	0.01	83.3	85.5	99.5	99.4	45.7
SH-29	23.8	Dalong	8.58	−27.7	/	/	74.87	8.37	0.70	0.24	2.00	0.03	0.35	0.01	0.79	0.00	76.5	79.4	95.4	93.9	58.0
SH-31	25.8	Dalong	7.23	−27.4	1.40	0.73	80.15	5.87	0.45	0.40	1.28	0.05	0.23	0.02	0.83	0.00	73.7	75.9	89.4	86.5	68.3
SH-34	27.8	Dalong	2.96	−26.7	/	/	87.06	4.00	0.27	0.20	0.68	0.02	0.19	0.01	1.59	0.01	78.6	80.2	92.0	90.4	79.0
SH-36	31.1	Dalong	1.42	−26.6	1.09	0.56	92.68	2.50	0.15	0.27	0.33	0.01	0.12	0.01	0.24	0.00	75.3	76.0	84.3	82.2	87.6
SH-40	37.9	Dalong	6.50	−27.5	1.42	0.74	82.41	4.39	0.38	0.14	0.84	0.02	0.20	0.01	1.71	0.01	79.1	81.2	94.6	93.3	73.5
SH-43	42.8	Dalong	9.06	−27.4	/	/	80.86	4.37	0.43	0.08	0.88	0.03	0.28	0.02	0.42	0.01	79.5	81.9	96.3	95.3	72.0
SH-46	48.4	Dalong	10.64	−26.8	0.70	0.36	58.53	12.20	0.79	0.62	2.63	0.17	0.40	0.03	6.27	0.01	75.7	77.9	91.9	89.7	33.9
SH-47	51.4	Daye	0.26	−26.2	0.53	0.28	39.05	8.98	1.26	0.34	2.15	0.23	0.33	23.80	2.55	0.18	72.2	74.6	88.9	85.6	20.9
SH-48	54.4	Daye	0.65	−26.9	/	/	50.06	12.67	1.47	0.63	2.83	0.16	0.46	13.10	4.00	0.11	71.1	73.0	85.9	82.2	24.5
SH-49	57.4	Daye	0.52	−26.5	0.41	0.21	57.50	11.97	2.10	0.67	2.77	0.11	0.42	9.33	3.46	0.09	69.6	71.5	84.4	80.2	33.3
SH-50	60.4	Daye	0.48	−25.8	/	/	66.26	11.82	1.45	0.56	2.70	0.14	0.39	6.07	2.74	0.04	71.3	73.2	86.6	82.9	42.4

Shuanghe: SH; chemical alteration index: CIA; correction of chemical alteration index: CIA_corr; chemical weathering index: CIW; plagioclase alteration index: PIA; biogenic Si: Si_bio.

. In the SH section, the TOC values are highly variable (ranging from 0.11% to 12.47%; average = 4.41%). This study divided the TOC values in the SH section into five stages, with stages 1 (0.1% to 1.8%) and 3 (1.4% to 3.0%) showing intermediate values, stages 2 (7.2% to 12.5%) and 4 (6.5% to 10.6%) showing high values, and stage 5 (0.3% to 0.7%) showing low values. In the SH section, the δ¹³C_org values range from −27.7‰ to −24.4‰, with an average value of −26.6‰. The δ¹³C_org values in the middle Dalong Formation (stages 2, 3, and 4) are significantly lower, varying between −27.7 ‰ and −26.6‰. In contrast, the δ¹³C_org values in stages 1 (−24.4‰ to −25.8‰) and 5 (−25.8‰ to −26.9‰) show high values.

4.2. Major Elements

The results for the major elements are listed in Table 1. In the SH section, the samples primarily comprise SiO₂ and Al₂O₃, with significantly higher SiO₂ contents (ranging from 39.1% to 93.5%, average = 71.5%) relative to Al₂O₃ contents (ranging from 1.4% to 21.3%, average = 8.6%). Na₂O and K₂O are related to clay minerals, with the contents varying from 0.03% to 0.7% (average = 0.3%) and 0.2% to 2.8% (average = 1.7%), respectively. CaO and MgO are primarily associated with carbonate minerals, with contents varying from 0% to 23.8% (average = 3.1%) and 0.1% to 2.1% (average = 0.8%), respectively. Fe₂O₃ is primarily related to pyrite, with the contents varying from 0.2% to 6.3% (average = 2.3%). The remaining major elements, including P₂O₅, MnO, and TiO₂, exhibit low concentrations, i.e., below 2%.

The calculated weathering proxies, including CIA, CIA_corr, CIW, and PIA, ranged from 69.6 to 85.6, 71.5 to 86.6, 84.3 to 99.5, and 80.2 to 99.4, respectively. The biogenic Si contents (Si_bio) in the Shuanghe section varied between 13.6% and 90.7%.

4.3. Trace Elements

The results for the trace elements are presented in

Table 2. The main trace (Sc, V, Ni, Zr, Mo, Th, U) elemental concentrations (ppm) and V/Al (ppm/%), Mo/Al (ppm/%), U/Al (ppm/%). U_EF, Mo_EF, V_EF, C_org/P (mol/mol), Mo/TOC (ppm/ppm), Co_EF × Mn_EF, and Ni/Al₂O₃ (ppm/%) values in the Dalong and Daye Formations of Shuanghe section.

Samples	Height (m)	Formation	V/Al	Mo/Al	U/Al	UEF	MoEF	VEF	Corg/P	Mo/U	Mo/TOC	CoEF × MnEF	Ni/Al2O3	Sc	V	Ni	Zr	Mo	Th	U
SH-06	3.7	Dalong	14.4	0.0	0.5	2.4	0.4	1.7	4.5	0.1	0.5	0.0	1.1	28.5	284.8	24.2	582.5	0.8	11.6	9.4
SH-12	8.1	Dalong	86.4	1.4	2.6	15.0	15.2	11.3	177.5	0.5	5.1	0.1	16.3	5.8	535.0	97.1	140.6	8.9	3.5	16.4
SH-16	12.6	Dalong	218.1	6.4	7.1	41.7	70.8	29.8	148.4	0.9	9.6	0.4	27.2	1.4	333.8	38.4	17.3	9.9	0.5	10.9
SH-18	14.1	Dalong	7.7	0.6	1.0	5.5	6.5	1.0	2.6	0.6	62.3	1.7	30.0	13.8	81.2	314.3	133.0	6.7	6.4	10.6
SH-21	16.9	Dalong	159.0	0.5	2.2	10.6	4.6	18.2	1211.6	0.2	0.3	0.0	13.5	11.2	1278.3	118.8	188.0	4.0	7.0	17.3
SH-25	19.9	Dalong	102.0	1.8	1.0	5.1	16.8	11.7	849.2	1.8	1.8	0.0	9.8	11.0	996.5	104.5	115.6	17.8	7.1	10.0
SH-29	23.8	Dalong	125.9	4.2	1.0	5.2	40.9	15.2	1616.9	4.2	3.9	0.0	9.6	7.7	1009.5	80.4	118.0	33.9	4.8	8.0
SH-31	25.8	Dalong	136.6	4.2	1.1	6.0	41.5	16.9	887.3	3.7	3.3	0.0	9.9	5.2	788.8	57.9	68.4	24.0	3.6	6.5
SH-34	27.8	Dalong	22.3	0.6	1.2	6.6	5.9	2.9	953.1	0.5	0.8	0.2	18.3	3.0	93.1	73.4	49.3	2.3	2.6	4.9
SH-36	31.1	Dalong	61.5	3.3	1.4	8.0	36.7	8.4	694.0	2.5	6.4	0.0	4.8	3.2	167.3	12.1	38.8	9.1	2.0	3.7
SH-40	37.9	Dalong	226.7	35.7	2.0	10.7	359.3	28.4	2275.8	18.0	23.9	0.2	22.4	4.8	989.2	98.4	51.1	155.6	2.9	8.7
SH-43	42.8	Dalong	218.9	23.4	4.2	22.3	230.5	26.8	1730.1	5.5	11.0	0.0	10.9	4.4	931.8	47.7	79.0	99.6	5.3	18.0
SH-46	48.4	Dalong	100.7	19.2	2.8	13.4	170.0	11.1	369.9	6.8	19.3	0.1	17.3	14.0	1074.9	211.1	139.4	204.9	9.0	30.1
SH-47	51.4	Daye	10.1	0.4	0.8	3.7	3.2	1.0	6.7	0.5	11.0	3.8	5.9	9.3	71.8	53.4	68.7	2.9	8.0	6.0
SH-48	54.4	Daye	11.3	0.4	0.4	2.0	3.8	1.2	24.5	1.0	7.4	1.8	6.3	12.3	123.4	79.9	98.3	4.8	11.4	4.6
SH-49	57.4	Daye	8.8	0.1	0.3	1.4	0.5	1.0	28.8	0.2	1.1	1.2	3.9	11.5	95.1	46.6	98.4	0.6	10.2	3.1
SH-50	60.4	Daye	9.1	0.0	0.4	1.9	0.5	1.1	19.9	0.1	1.2	0.3	4.4	11.7	103.4	52.2	101.8	0.6	10.9	4.1

Shuanghe: SH; enrichment factors of Mo: Mo_EF; enrichment factors of U: U_EF; enrichment factors of Mn: Mn_EF; enrichment factors of Co: Co_EF.

. The Zr, Hf, La, and Sc concentrations are typically considered indicators of detrital flux or the type of source rocks. In the SH section, the Zr and Hf contents vary between 17.3 ppm and 582.5 ppm and between 0.46 ppm and 13.2 ppm, respectively. The La and Sc contents vary between 6.1 ppm and 223.4 ppm and between 1.4 ppm and 28.5 ppm, respectively.

Redox-sensitive trace metals, such as Mo, U, and V, tend to exhibit different behaviors under oxidizing and reducing conditions. In the SH section, the Mo, U, and V contents vary between 0.6 ppm and 204.9 ppm, between 3.1 ppm and 30.0 ppm (average = 9.8 ppm), and between 71.8 ppm and 1278.3 ppm (average = 507 ppm), respectively.

4.4. The Mineralogical Compositions

Four shales were analyzed in this study: SH-06, SH-18, SH-21, and SH-47 (

Table 3. The mineralogical compositions of bulk rocks in the Dalong and Daye Formations.

Samples	Height (m)	Quartz (%)	Illite (%)	Albite (%)	Pyrite (%)	Calcite (%)	Anatase (%)	Siderite (%)
SH-06	3.7	48.6	45	0	0.8	0	5	0.6
SH-18	14.1	88.9	11.1	0	0	0	0	0
SH-21	16.9	92.1	7.2	0.8	0	0	0	0
SH-47	51.4	41.8	10.2	3.5	2	42.5	0	0

Shuanghe: SH.

). All the shales are mainly composed of quartz (41.8% to 92.1%), calcite (0% to 42.5%), and illite (7.2% to 45.0%), with minor contents of pyrite (0% to 2.0%), albite (0% to 3.5%), anatase (0% to 5.0%), and siderite (0% to 0.6%).

5. Discussion

5.1. Evaluation of the Chemical Weathering Index

Traditionally, Zr, Ti, Al, and their ratios are used to evaluate the lithology of parental rocks and tectonic settings [44]. The discrimination diagrams indicate that all the samples are located near the regions that are associated with felsic to intermediate volcanic rocks (Figure 3a–c). The geochemical diagrams indicate different sources for the studied clastic rocks (intermediate to felsic). However, we found that several volcanic tuffs are developed throughout the Dalong Formation, indicating intense volcanism during this period. In this case, we believe that some heavy minerals (such as zircon) in the shales could be related to the volcanic eruption [7,8,33]. In addition, the volcanic nature and source have been studied for decades due to possible relationships with PTB mass extinction events and the Pangea supercontinent assemblage, and all of these studies suggested that the tuffs are sourced from felsic volcanism in the continental arc background [33]. Therefore, we suggest that the source of the studied shales should be felsic igneous rocks. Additionally, the types of source rocks are based on A-CN-K diagrams. The ternary diagrams indicate that the samples from one section (SH section) converge to a point (granodiorite; Figure 3d). These comprehensive observations strongly support the inference that late Permian shales are derived from a consistent protolith source. Therefore, the observed variations in weathering indicators during the Dalong and Daye periods are unlikely to be attributable to variations in the types of source rocks.

The addition of K₂O during diagenesis can influence the accuracy of the CIA values, as it causes the samples to undergo less weathering than they actually undergo. The CIA_corr values are calculated to prevent the influence of potassium metasomatism. In this study, a strong positive correlation between CIA_corr and CIA suggested that the correction of CIA maintained the temporal weathering trend (Figure 4a). Furthermore, the variations in weathering proxies might be associated with multiple sedimentary cycles before burial, thereby influencing the actual weathering experience of the protolith in source areas. The Al₂O₃/SiO₂ ratio serves as a proxy for grain size, with fine particles exhibiting high Al₂O₃/SiO₂ ratios and coarse components exhibiting low Al₂O₃/SiO₂ ratios. There was no significant relationship between CIA_corr and Al₂O₃/SiO₂, indicating a negligible influence of the grain size on the variations in weathering proxies (Figure 4b). As CIA_corr was positively correlated with the other weathering proxies (CIW, PIA) (Figure 4c,d), CIA_corr should be a reliable indicator of the continental weathering trend throughout the Dalong and Daye Formations.

5.2. Evaluation of the δ²⁶Mg Values

5.2.1. Influence of Authigenic Clay Minerals

Reverse weathering, which involves the formation of marine authigenic clays [28,45], is widely recognized as one of the major sinks for Mg from seawater. It also influences the δ²⁶Mg values in sediments, as it preferentially removes ²⁶Mg over ²⁴Mg [45,46]. The reverse weathering reaction requires reactive sources of Si (considered to be predominantly derived from opal in modern oceans), reactive sources of Al, cations (for example, Mg²⁺, K⁺, Na⁺), and alkalinity [45]. Recently, several studies have suggested that intense reverse weathering may have also occurred during the late Permian and early Triassic periods [28] and during the early Cenozoic [47]. Therefore, evaluating the potential influence of reverse weathering or the formation of authigenic clay minerals during early diagenetic processes on the δ²⁶Mg values of shales is crucial.

The formation of authigenic clay minerals could increase the Mg content in the clastic component and affect its Mg isotopic composition [45,46]. A majority of the authigenic clay minerals are rich in Mg, with Mg-rich minerals predominantly including clinochlorite, chrysochlorite, and saponite [45]. However, the clay minerals in the SH section are characterized primarily by illite, which is not considered authigenic [20]. Therefore, authigenic clay mineral formation does not interfere with the weathering record of a sample and can be excluded.

5.2.2. Influences of Mineralogy, Granularity, and Diagenesis

The marine sedimentary rocks in the SH section are characterized primarily by detrital materials (quartz, clay, and feldspar) [20]. These shales contain varying illite and smectite contents, with the illite contents ranging from 10% to 77% and the smectite contents ranging from 0% to 4% [20]. The changes in the δ²⁶Mg_silicate content of shales that are reported in this study are unlikely to be influenced by a change in the clay mineralogy, as there is no correlation between clay minerals and δ²⁶Mg_silicate or between illite and δ²⁶Mg_silicate.

The clastic rocks contain unweathered compositions (for example, terrestrial quartz with a high Si content), as well as weathered compositions (for example, clay minerals with high Al content). The former originates from primary rocks and has low δ²⁶Mg values, whereas the latter has relatively high δ²⁶Mg values. However, no correlation exists between δ²⁶Mg_silicate and the Al₂O₃/SiO₂ ratio (Figure 4e), indicating the minimal influence of granularity on the variations in the δ²⁶Mg_silicate values.

Generally, late diagenesis has a significant effect on the mineral composition and evolution of mudstones [5,48]. For example, the conversion of smectite to illite slowly releases a large proportion of SiO₂ into the porewater, and small-grain-size authigenic quartz is formed when the SiO₂ concentration in the porewater reaches saturation [5,48]. During illitization, K and Al are incorporated into smectite, while Mg, Na, and Si are released into the porewater. However, this process does not involve the incorporation of seawater Mg, resulting in the rocks preserving their primary δ²⁶Mg values. The δ²⁶Mg_silicate values likely indicate a continental weathering trend from the Dalong period to the Daye period because of the positive correlation between the δ²⁶Mg_silicate and CIA_corr values (Figure 4f).

5.3. Continental Weathering Trend in the Late Permian

5.3.1. Fluctuations in CIA_corr Values During the Late Permian

Weathering proxies, including the CIA and CIA_corr, have been used to analyze weathering trends throughout different geological periods [1,27,49]. Regions that experience high chemical weathering intensities are characterized by elevated CIA and CIA_corr values. In contrast, areas with reduced chemical weathering intensity are characterized by low CIA and CIA_corr values.

Within the Yangtze Block, CIA and CIA_corr exhibit relatively high values from the lower to middle Dalong Formation (stages 1 to stage 4), indicating a high chemical weathering intensity (Figure 5). These high weathering proxies can also be observed in other sections of South China [50], North China [51,52], and Australia [53]. This intense chemical weathering corresponds to intense volcanism and an increase in temperature, which was followed by a global cooling climate during the early Wuchiapingian [54,55].

The weathering proxies (CIA and CIA_corr) indicate a rapid decrease from the upper Dalong Formation to the lower Daye Formation (stage 5), suggesting an obvious decrease in the chemical weathering intensity during this period (Figure 5). These variations in chemical weathering intensity could be frequently observed in other sections [50,51,52], suggesting that weathering trends based on CIA and CIA_corr during the late Permian are deemed reliable and consistent across global boreholes and sections. However, this decrease in chemical weathering intensity is inconsistent with the traditional view that high temperatures result in high chemical weathering intensities [27]. This anomalous weathering trend was accompanied by an extremely high denudation rate under a hot and arid climate [56], where preweathered soils were gradually removed, leading to a significant amount of unweathered fresh rocks in the deposits [50,57]. Furthermore, this intensive erosion rate, driven by a hot and arid climate, is believed to have persisted until the earliest Triassic [57,58].

5.3.2. Fluctuations in δ²⁶Mg_silicate Values During the Late Permian

Magnesium is a fluid-mobile major element within the crust and has three isotopes (²⁴Mg, ²⁵Mg, and ²⁶Mg) that can undergo fractionation [59]. The sedimentary rocks are characterized by variable δ²⁶Mg values, with clastic rocks exhibiting significantly higher δ²⁶Mg values than carbonates do [60,61].

Typically, Mg in marine rocks is primarily present in unweathered fragments, weathered products, and authigenic clays [30,45]. Previous studies have widely accepted that δ²⁶Mg could be used as a reliable weathering proxy [30,62,63] because of its association with Mg isotope fractionation (up to ~2‰) during secondary clay formation and chemical weathering [62,64]. In such a case, the weathering products could retain heavy Mg isotopes [29,30]. During the initial chemical weathering process, isotopically light Mg is dissolved and transported into porewater or rivers, resulting in weathering residues with high δ²⁶Mg values [64]. With increasing chemical weathering, the δ²⁶Mg values of the weathering residues are expected to increase due to the preferential dissolution of hornblende and ²⁴Mg, along with the retention of heavy Mg isotopes [29,30]. When primary rocks experience high-intensity chemical weathering, enhanced secondary clay formation could contribute to Mg isotope fractionation, resulting in the enrichment of ²⁶Mg in secondary clay. Several studies have reported weathering-induced variations in the δ²⁶Mg values of rocks. For example, Teng et al. (2010) reported an increase in δ²⁶Mg from about –0.22‰ in unweathered diabase to 0.65‰ in weathered rocks [30].

Given the covariation of the CIA and δ²⁶Mg_silicate values in the late Permian, the variation in δ²⁶Mg_silicate is strongly associated with the variation in chemical weathering intensity (Figure 5). Frequent high values of δ²⁶Mg_silicate are observed in the low and middle Dalong Formation (stage 1 to stage 4), with stages 2 and 4 exhibiting peak δ²⁶Mg_silicate values (Figure 5). These high values probably indicate enhanced and intense chemical weathering in the past. The high δ²⁶Mg_silicate values subsequently and rapidly decrease from the Dalong Formation to the Daye Formation (stage 5), indicating significant waning of chemical weathering (Figure 5). Similarly to the explanations for the changes in the CIA and CIA_corr values, the high erosion rate and weak chemical weathering intensity during hot and arid climates in the latest Permian and early Triassic [57,58] could have contributed to the increased erosion of unweathered rocks with low δ²⁶Mg_silicate values.

5.4. The Hydrographic Conditions

The topography of the seafloor, terrestrial input, and water circulation in marine systems could be affected by the hydrographic background (open ocean or restricted ocean) [65,66]. In particular, upwelling in the open ocean can provide nutrient-rich cold waters and accelerate primary productivity, thereby leading to the deposition of ancient source rocks [4,12]. The significantly high opal accumulation is typically attributed to intensified upwelling and is associated with siliceous export productivity [67]. Recently, numerous studies have investigated the Dalong shales of the Sichuan Basin via geochemical analyses. These studies focused on the source of quartz in organic-rich shales, indicating that biogenic quartz can account for a majority of the quartz in the Dalong Formation [20,68]. Overall, the extremely high biogenic quartz content could indicate intense upwelling in the late Permian.

Generally, Mo/TOC and Co_EF × Mn_EF have been proposed as valuable proxies to distinguish upwelling environments from restricted settings [65,66]. Therefore, the Mo/TOC and Co_EF × Mn_EF methods have been widely employed to analyze the hydrodynamic environments of Paleozoic marine shales [9,69]. Generally, high Mo/TOC and low Co_EF × Mn_EF (<1) values suggest sediment deposition under upwelling settings, whereas low Mo/TOC and high Co_EF × Mn_EF (>1) values indicate more restricted settings [65,66]. In the SH section, the sediments in the Dalong Formation exhibit obvious changes in both the Mo/TOC and Co_EF × Mn_EF values (Figure 5), with stage 4 showing continuously high Mo/TOC and low Co_EF × Mn_EF values and stage 5 showing persistently low Mo/TOC and high Co_EF × Mn_EF values. This observation indicates that the deposition of shales in stages 1, 3, and 5 was characterized by weak upwelling in the restricted ocean, whereas the deposition of shales in stages 2 and 4 was characterized by intense upwelling in the open ocean.

Commonly, δ¹³C (δ¹³C_carb or δ¹³C_org) values are strongly associated with the geological carbon cycle in the atmosphere and ocean [6,45]. However, several geological events, such as the upwelling intensity, volcanism, and burial of organic matter, could also influence variations in δ¹³C_org [6,45,70]. Previous studies have suggested the occurrence of intense volcanism at the Wuchiapingian–Changhsingian boundary or during the latest Permian [24,52] instead of the middle Dalong period. Therefore, the negative δ¹³C_org excursion should not be attributed to the addition of volcanic -¹²CO₂ (Figure 5). The ¹²C-enriched CO₂ derived from ¹³C-depleted recycled organic matter is continuously replenished from depth, whereas the supply of ¹²C remains effectively limited in an open system with high organic matter preservation [71,72,73]. The sequestration of this depleted carbon into the surface ocean can result in sedimentary carbonate and biomass that are similarly depleted. This study revealed that significant negative δ¹³C_org excursions occurred during stages 2 and 4, which corresponded to two peaks in the TOC content (Figure 5). This negative relationship between TOC and δ¹³C_org indicated that the variation in δ¹³C_org was not influenced by continuous organic matter burial because of the preferential incorporation of ¹²C into the organic matter. Instead, the decrease in δ¹³C_org (stages 2 and 4) can likely be attributed to the influence of upwelling on marine deposition. Moreover, upwelling led to continuously recycled ¹²C-enriched CO_2aq and seafloor sediments, resulting in high surface water nutrients and high primary productivity.

Briefly, the deposition of shales in stages 2 and 4 was characterized by intense upwelling in the open ocean. However, the deposition of shales in stages 1, 3, and 5 was characterized by weak upwelling in the restricted ocean. In particular, the restriction of the ocean in the latest Permian and early Triassic was likely caused by the hot and arid climate, given that high temperatures resulted in weak ocean cycling.

5.5. Variations in Primary Productivity and Redox Water Conditions

5.5.1. Evaluation of Primary Productivity

Ba, P, Si, Cu, and Ni are all important nutrient elements for marine phytoplankton and other organisms [74,75]. However, Ba is readily released into the water column by sulfate reduction in strongly reducing environments [74]. Moreover, the burial efficiency of P is influenced by a series of depositional and diagenetic processes (for example, redox water conditions), which could affect the preservation of P in marine sediments [75,76]. Additionally, P can be released into porewater and tends to diffuse into the overlying water column under reducing conditions [75,77]. In this case, neither Ba nor P in sediments is suitable for indicating primary productivity during fluctuations in redox conditions.

The biogenic Si (Si_bio) content, which is estimated by subtracting the Si of a detrital origin from the total Si content, is widely employed as a geochemical proxy for paleo-productivity reconstruction [6,68]. Additionally, the Ni content that is normalized by Al₂O₃ (Ni/Al₂O₃) can be used to indicate primary productivity in the surface ocean. In the SH section, obvious changes in primary productivity can be observed throughout the Dalong and Daye Formations (Figure 6). In stages 1, 3, and 4, the shales have high Sibio and Ni/Al₂O₃ values, suggesting high primary productivity. In stage 2 and stage 5, the shales have low Sibio and Ni/Al₂O₃ values, suggesting low primary productivity (Figure 6).

5.5.2. Evaluation of Redox States

On the basis of the dissolved O₂ levels, the redox states of the water columns were classified as oxic (~>2 mL/L), dysoxic (~0.2–2 mL/L), suboxic (~0–0.2 mL/L), or anoxic ferruginous and euxinic (~0 mL/L) [78]. The redox state of seawater during the deposition of shales is typically determined by analyzing multiple redox-sensitive elements (e.g., U, Mo, V) [65,78,79]. These elements are generally less susceptible to detrital material and are easily incorporated into sediments that undergo deposition under anoxic water conditions [78]. Therefore, the contents and ratios of these redox-sensitive elements can serve as reliable indicators of redox states [78]. Previous studies have analyzed the redox conditions in the Yangtze paleo-ocean in the late Permian and early Triassic on the basis of redox-sensitive elements and Mo-U–S isotopes [4,80,81]. The results revealed a significant expansion of the global area of anoxic seafloor from the latest Permian to the early Triassic [4,19,81,82].

Mo, U, and V enrichment in sediments has been widely used for the reconstruction of the redox state of water columns [81,83,84]. This is attributed to the fact that these elements can be released into sediments under anoxic water conditions [74,78,79]. Generally, high Mo_EF (~26) and U_EF (~2.4) values can indicate permanently euxinic conditions, intermediate Mo_EF (~5) and U_EF (~2.3) values represent anoxic ferruginous water conditions, and low Mo_EF (~1.1) and U_EF (~1.2) values represent oxic water conditions [79]. Additionally, high Mo/U ratios (~>4.6) in marine sediments could indicate euxinic conditions, whereas extremely low Mo/U ratios (~<0.46) suggest oxic water conditions [79]. The enhanced dissolution of Mn oxides can result in the precipitation of V₂O₃ or hydroxide in sediments. In this case, the sediments that are deposited in sulfidic settings would have high V contents [85]. High V_EF (~1.6) values can indicate permanently euxinic conditions, intermediate V_EF (~1.2) values represent anoxic ferruginous water conditions, and low V_EF (~1.1) values represent oxic water conditions [79].

In the SH section, high-frequency redox fluctuations can be observed throughout the Dalong and Daye Formations (Figure 6). In stage 1, the shales have intermediate C_org/P, Mo_EF, U_EF, and V_EF values and low Mo/U ratios, indicating oxic–anoxic water conditions. In stage 2 and stage 4, the shales have high Mo_EF, U_EF, V_EF, C_org/P, and Mo/U ratios, indicating anoxic–euxinic water conditions. In stage 3, the shales have low Mo_EF, Mo/U, and V_EF values and high C_org/P and U_EF ratios, indicating oxic–anoxic water conditions. In stage 5, the shales have persistently low C_org/P, Mo_EF, U_EF, Mo/U, and V_EF values, indicating oxic water conditions (Figure 6).

Recently, the enrichment of V, U, and Mo has also been used to investigate the depositional environments (e.g., normal oxic, within and beneath perennial oxygen-minimum zones: P-OMZ) [86]. In these studies, the high enrichment of V, U, and Mo is caused by the large flux of reactive solid phases (e.g., organic matter, Fe/Mn oxides) with high water renewal rates in the open ocean. According to the results of these studies, the enrichment of V (V/Al > 46 μg g⁻¹/%), U (U/Al > 5 μg g⁻¹/%), and Mo (Mo/Al > 5 μg g⁻¹/%) is strong evidence for sediments depositing within the anoxic core of a perennial OMZ environment [86]. In contrast, low enrichment of V(V/Al < 23 μg g⁻¹/%), U(U/Al < 1 μg g⁻¹/%), and Mo (Mo/Al < 0.4 μg g⁻¹/%) can indicate sediment deposition in normal oxic environments [86]. The majority of shales in stages 1 and 5 are characterized by low enrichment of V, U, and Mo, suggesting that they are deposited in oxic water conditions (Figure 7a,b). The shales in stages 2–4 are characterized by high enrichment of V and intermediate enrichment of U and Mo, indicating that they are deposited beneath P-OMZ or euxinic conditions (Figure 7a,b). This study suggests that the extremely high enrichment of V and Mo in the shales of stages 2 and 4 are related to two explanations: (1) the extremely high organic matter contents can absorb V and Mo and preserve them in sediments that are deposited in anoxic water conditions; (2) intense upwelling during these periods provides abundant trace elements for the enrichment of V and Mo in sediments.

5.6. Factors Influencing the Accumulation of Organic Matter in the Dalong Formation

The mechanisms behind the enrichment of organic matter and silica in the Dalong Formation have recently been debated. For example, intense upwelling, restricted water circulation, intense tectonism, and weathering of volcanic ash have all been proposed as contributing factors to the accumulation of organic matter and silica [8,17,19,20].

This study suggested that the coupling of intense upwelling and enhanced continental weathering influenced the deposition of organic-rich shales in the Dalong Formation (Figure 8). The Dalong period followed the late Paleozoic glaciation, specifically from the late Capitanian to the late Wuchiapingian [27,54]. It has been proposed that this glaciation was induced by the enhanced weathering of a significant volume of mafic rocks from the Central Atlantic Igneous Province or Deccan Traps [87]. From Wuchiapingian to Changhsingin, the end of glaciation and increased sea surface temperature led to increased runoff and further contributed to enhanced continental weathering in stages 1 and 3. This enhanced continental weathering, supported by increased CIA and δ²⁶Mg_silicate values, further promoted high primary productivity. However, these periods are also characterized by restricted oceans, which led to the limited transportation of nutrients and anoxic waters into the surface ocean. In this case, the bottom waters were characterized by oxic–anoxic conditions during these stages, resulting in intermediate TOC values in the shales.

In stages 2 and 4, enhanced continental weathering and intense upwelling can be observed. First, the enhanced weathering upwelling induced high primary productivity and further contributed to the consumption of oxygen in the bottom waters. Second, upwelling could also transport anoxic waters onto the shelf. The resulting high primary productivity and anoxic water conditions should be considered primary factors influencing the enrichment of organic matter in the shales of the middle Dalong Formation (Figure 8).

In stage 5, both weak continental weathering and weak upwelling were observed. These processes would have led to either limited transportation of nutrients from continental or deep waters or less anoxic waters from deep oceans to the shelf. Therefore, both the low primary productivity and ocean oxygenation fail to provide enough organic matter flux and optimal conditions for organic matter burial in the shales of the upper Dalong and Daye Formations (Figure 8).

5.7. Implications for the End-Permian Mass Extinction

The end-Permian mass extinction (EPME) was the greatest extinction event in the Phanerozoic [88]. The expansion of anoxia that was caused by a warming climate and intense volcanism has long been proposed as the main factor controlling the EPME [89,90]. In this study, we observed high-frequency redox fluctuations in the late Permian and early Triassic, which likely affected the marine ecosystem. In particular, euxinia in seawater, a widely accepted killing mechanism for mass extinctions in the Phanerozoic [89,90,91], has been observed in the deposition of shales in the Dalong Formation. These euxinic water conditions were likely caused by enhanced continental weathering and intense upwelling in the late Permian, as the former could have provided enough SO₄^2– into the ocean for the release of H₂S, and the latter could have transported abundant euxinic waters onto the shelf [91].

6. Conclusions

Comprehensive geochemical investigations were conducted to analyze the continental weathering trends and the intensity of upwelling in the Shuanghe section of the upper Yangtze region during the late Permian. This study, therefore, aimed to determine the mechanism of organic matter accumulation in Dalong black shales.

(1)

Five stages can be identified on the basis of TOC and environmental variations, with the highest TOC values (>2%) being observed in stages 2 and 4, intermediate TOC (~1% to 2%) values being observed in stages 1 and 3, and the lowest TOC values (<1%) being observed in stage 5.

(2)

The deposition of organic-rich shales (stages 2 and 4) in the late Permian Dalong Formation is related to the enhanced continental weathering and intense upwelling. In stages 2 and 4, the enhanced continental weathering (high CIA_corr and δ²⁶Mg_silicate values) and intense upwelling (high Mo/TOC ratios, low δ¹³C_org and Co_EF × Mn_EF values) contributed to the high primary productivity (high Si_bio and Ni/Al₂O₃) and anoxic water conditions (high C_org/P and Mo/U ratios and high Mo_EF, U_EF, and V_EF values), which is in accordance with the high TOC values in shales (>2%).

(3)

Both the enhanced continental weathering and upwelling contributed to high-frequency redox fluctuations in the late Permian and early Triassic, in which case the development of anoxia/euxinia of seawater further triggered the EPME.