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Article

Lacustrine Environmental Perturbations during the Early Jurassic in the Qiangtang Basin, Northern Tibet

by
Haowei Zhang
1,2,
Jian Wang
1,2,3,*,
Mohamed Saad Ahmed
4,*,
Xiugen Fu
1,2,3 and
Lijun Shen
1,2
1
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
2
Qiangtang Institute of Sedimentary Basin, Southwest Petroleum University, Chengdu 610500, China
3
National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610050, China
4
Geology and Geophysics Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(8), 762; https://doi.org/10.3390/min14080762 (registering DOI)
Submission received: 8 May 2024 / Revised: 13 July 2024 / Accepted: 25 July 2024 / Published: 27 July 2024
(This article belongs to the Special Issue Environmental Geochemistry and Mineralogy: Application for Hydrocarbon Exploration and Ore Deposits)
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Abstract

:
The Early Jurassic was primarily a greenhouse phase in Earth’s history. Previous studies have predominantly focused on marine strata in the Tethyan Ocean, but continental records outside of Europe are still poorly understood, which has hindered a thorough understanding of its climate dynamics. To address this gap, organic, inorganic, and isotope geochemical analyses, along with mineralogical and sedimentological assessments, were conducted on samples from the Quemo Co Formation of well QZ-16 in the Qiangtang Basin (Eastern Tethys). This study aimed to investigate carbon cycle perturbations and consequences of redox conditions and paleosalinity within a lacustrine system during the Early Jurassic. The carbon isotope profile of well QZ-16 exhibited a long-term negative excursion of ca. −3.83‰ in lacustrine sediments, with relatively heavy δ13Corg values and small excursion magnitudes. Enhanced terrigenous input, as indicated by detrital proxies such as Si/Al, Ti/Al, and Zr/Al, was driven by accelerated continental weathering during the carbon isotope excursions. The presence of coarse-grained, pelitic siltstone deposits coincided with the onset of these negative carbon isotope excursions. Sr/Ba ratios (0.05–3.64, avg. 0.73) suggest a brackish to freshwater salinity regime within the third member, implying that the deposition of the Quemo Co Formation was influenced by both freshwater and seawater inputs. Most enrichment factors (VEF, ZnEF, NiEF) having a value below 1.0 and Corg/P ratios less than 50 indicate that the lacustrine environment was characterized by predominantly oxidizing conditions, particularly during the early Toarcian negative carbon isotope excursion (T-NCIE). Despite the record of the T-NCIE event, enhanced respiration in bottom and pore waters indicates that the Toarcian Oceanic Anoxic Event (T-OAE) was absent in this part of the eastern Tethys Ocean. The recorded Early Jurassic environmental settings in the Qiangtang Basin lacustrine system exhibit a close association with the T-CIE event (183 Ma), providing insights into the complex interplay between carbon cycle perturbation, climate, weathering, and biological processes during this greenhouse period.

1. Introduction

The fragmentation of the supercontinent Pangea and the birth of the Central Atlantic Ocean during the Early Jurassic were concomitant with significant shifts in atmospheric composition, global climate, and environmental conditions [1,2,3]. These disturbances are reflected in the geological record through various minor oscillations in the global carbon cycle, which are associated with both regional and global trends of warming and cooling [3,4] as well as sea-level changes [5,6,7]. However, research on the Early Jurassic climate and environmental conditions has predominantly focused on the western Tethys Ocean [1,6,7], emphasizing marine strata, with limited studies addressing Lower Jurassic terrestrial strata [8,9].
The Qiangtang Basin, located in the Tibetan Plateau (Figure 1a), represents the largest Mesozoic marine basin in China. This basin contains well-preserved strata that represent the transition from continental to marine, potentially providing valuable insights into atmospheric, climatic, and environmental changes associated with the rapid expansion of the Neo-Tethys Ocean. Paleontological, sedimentary, and carbon isotope evidence from the Qiangtang Basin indicates that this region experienced severe global environmental and climatic alterations during the Early Jurassic [10,11,12]. Notably, the Toarcian Oceanic Anoxic Event (T-OAE) has been identified in the Bilong Co and Sewa sections, exemplifying the lagoonal and shelf expressions of the Toarcian oceanic anoxic event within the Southern Qiangtang Depression [10,11,12] (Figure 1b). These findings highlight the profound implications of the significant geological phenomena of the Early Jurassic, including the T-OAE, for the Qiangtang Basin within the Eastern Tethyan domain. However, the terrestrial manifestation of these global perturbations within the Qiangtang Basin remains only partially unraveled.
The Quemo Co Formation in well QZ-16 in the North Qiangtang Depression represents Lower Jurassic lacustrine strata that document lithological transitions (Figure 1c). This includes a change in facies from red coarse-grained sediments and limestones and gypsum to dark fine-grained sediments (Figure 2). The Quemo Co Formation has been considered as representative of the early sedimentary infill of the Mesozoic Qiangtang Basin. Previous research has mainly focused on interpreting the depositional environment of the Quemo Co Formation based on sedimentological analysis [13,14]. Additionally, these studies have aimed to investigate the source and tectonic settings of the coarse-grained sandstones [15,16]. However, to date, quantitative analyses addressing the significant atmospheric and environmental disturbances according to the sudden lithological transitions within the Quemo Co Formation in the Northern Qiangtang Depression have not been undertaken. The well-preserved lacustrine sediments of the Lower Jurassic Quemo Co Formation in the Northern Qiangtang Depression provide a valuable record of the initial evolutionary stages of the broader Mesozoic Qiangtang Basin. Analyzing the geochemical compositions and sedimentological features of these sequences is crucial for understanding perturbations in the terrestrial carbon cycle during the Early Jurassic.
This study employs an integrated multi-proxy approach, combining organic, inorganic, and isotope geochemistry along with mineralogical and sedimentological characteristics to examine the lower Jurassic organic carbon-lean sediments from well QZ-16 in a lacustrine environment. The objectives of this study are to: (1) identify the record of Toarcian carbon isotope excursions in the Qiangtang Basin; (2) assess the degree of terrigenous input during the early Toarcian; and (3) investigate spatial variations in redox conditions and paleosalinity in the QZ-16 well. This study aims to elucidate the relationships between different environmental processes that occurred during the Early Jurassic. The chronological framework for the geological succession in well QZ-16 is established by integrating carbon isotope chemostratigraphy from this work with previous biostratigraphic studies [17,18,19]. This allows for a comparison between the terrestrial Qiangtang succession and temporally equivalent marine sections that exhibit analogous isotopic variations.

2. Geological Setting

The Qiangtang Basin is located in the northern Tibetan Plateau at the convergence of several major blocks, including the Tarim, Kunlun, Songpan–Ganzi, and Lhasa Blocks (Figure 1a) [13,20]. It is bounded by the southern Bangong Lake–Nujiang River suture zone to the south and the northern Hoh Xil–Jinsha River suture zone to the north. Structurally, the basin can be divided into three distinct tectonic units: the South Qiangtang Depression (SQD), the Central Uplift Belt, and the North Qiangtang Depression (NQD) (Figure 1b) [13,20].
During the Late Triassic, the North Qiangtang Depression experienced significant uplift and denudation, leading to the formation of extensive paleo-weathering crusts across this region [13,20]. This was followed by major rifting in the Late Triassic, which resulted in the deposition of volcanic and volcaniclastic rocks of the Nadi Kangri Formation atop the weathering crusts [13]. These tectonic and sedimentary events marked the inception of the broader Mesozoic Qiangtang Basin. Subsequently, seawater was able to ingress into this newly formed basin through a narrow passage in the Shuanghu area, facilitating the sedimentation of the Lower Jurassic Quemo Co Formation within the Northern Qiangtang Depression [13]. The well-preserved lacustrine sediments of the Quemo Co Formation thus provide a valuable record of the initial evolutionary stages of the Mesozoic Qiangtang Basin.
The Quemo Co Formation in the Northern Qiangtang Depression is an important Lower Jurassic sedimentary succession with a variable thickness ranging from 500 to 1950 m (Figure 3) [20]. The Quemo Co Formation in well QZ-16 consists of a thick stratigraphic succession of conglomerate, coarse- to fine-grained sandstones, siltstone, and gypsum with interbeds of muddy siltstone, silty mudstone, and micritic limestone [17]. The depositional paleoenvironments range from alluvial to fluvial environments for the first member, compared to an evaporitic setting for the gypsum-rich second member, while a shallow-lake environment with fine-grained sediments predominated for the third member (Figure 2) [13,14,21]. This study focuses on investigating the fine-grained deposits of the third member of the Quemo Co Formation (Figure 2).
The Quemo Co formation provides a valuable record of the early sedimentary history and evolution of the Qiangtang Basin. Fossil assemblages found in the Shengli River area, including bivalves such as Astarte delicate, Pleuromya sp., Chlamys sp., and Neomiodon sp., and spores and pollen like Cyathidites sp., Deltoidospora sp., and Classopollis sp., indicate an Early–Middle Jurassic age [17,18]. In addition, significant negative carbon isotope excursions (NCIEs) observed in the third member of the formation in the QK-1 well correspond with NCIEs found during the Early Jurassic T-OAE (Figure 1b) [13].

3. Samples and Methods

For this study, a total of 37 samples were collected from the QZ-16 well in the Matuo area, including 27 mudstone samples and 10 pelitic siltstone samples. The samples were collected with an average spacing of 2 m, which was reduced to 1 m in the range of 200–215 m depth (Figure 3).

3.1. Organic Carbon Isotope Analyses

A total of 18 samples from the QZ-16 well were analyzed for organic carbon isotope composition (δ13Corg) at the Northwest Institute of Eco-Environment and Resources, Lanzhou (China). An aliquot of each powdered sample was acidified with HCl to remove carbonates prior to analysis. Then, samples were washed with deionized water until a neutral pH value was reached. Next, samples were dried in an oven at 90 °C. δ13Corg analysis was conducted using an isotope ratio mass spectrometer (MAT253, Thermo-Fisher, Bremen, Germany) coupled with an elemental analyzer (Flash 2000, Thermo-Fisher, Bremen, Germany). Helium was used as the carrier gas with a 2 mL/min flow rate. Isotopic measurements were calibrated to the Chinese national standard charcoal sample GBW04407 (δ13CVPDB = −22.43‰ ± 0.07‰). Results were reported in standard δ-notation relative to the Vienna Peedee Belemnite (VPDB) standard. In this study, the analytical precision of the samples was better than ±0.05‰

3.2. Total Organic Carbon Analyses

Total organic carbon (TOC) analysis was performed on 37 samples using a TL851-6K carbon-sulfur analyzer at the Qiangtang Institute of Sedimentary Basin of Southwest Petroleum University. An aliquot of each powdered sample (200 mesh) was treated with HCl (10%) for 24 h to remove inorganic carbon fractions prior to the analysis. The analytical precision of the TOC analysis was better than +0.5 wt%.

3.3. Element Analyses

All sediment samples were air-dried, sieved, and crushed into fine 200 µm fractions prior to bulk geochemical analysis using X-ray fluorescence (XRF) techniques. Each plastic sample cup was filled with approximately 7 g of the prepared sediment, after which the cup was sealed with ultrafine film. The Hand-Held (HH) Bruker S1 Titan XRF Alloy Analyzer was used at the Qiangtang Institute of Sedimentary Basin, Southwest Petroleum University, China. This instrument comprises an X-ray tube with a rhodium (Rh) anode (4 W, 15–50 kV, 5–100 μA) and a Silicon Drift Detector (FAST SDD) with a resolution of less than 145 eV, based on the Peltier effect. Bruker® check-samples were utilized for quality assurance and quality control purposes, yielding elemental recovery rates between 90 and 100%. The HH-XRF analyzer had a detection limit of less than 0.5 g kg−1 for the measured elements.
Aluminum (Al) content is employed to normalize trace element concentrations (X/Al) in order to mitigate the effects of detrital content. For this purpose, the enrichment factor (EF) was utilized as a characteristic proxy to assess the enrichment or depletion of each element. The EF for element X was calculated using the formula:
EFX = (X/Al)sample / (X/Al)PAAS
where (X/Al)sample denotes the ratio of the trace element to Al in the sample, and (X/Al)PAAS represents the ratio of element X to Al in the post-Archean Australian shale (PAAS) [22,23]. A trace element was considered enriched if EFX > 3, and depleted if EFX < 1 [23].

4. Results

4.1. Organic Carbon Isotopes

The distribution of the δ13Corg data from the QZ-16 well is plotted in Figure 4. A prominent negative carbon isotope excursion (NCIE) of approximately 3.83‰ was recorded in the organic matter, starting at a depth of 202 m and reaching a minimum value of −26.99‰. This negative excursion spanned around 14 m of the sedimentary sequence (from 202 to 188 m depth). Two additional negative excursions were observed in the section, starting at 188 m and 176 m, respectively. These later excursions showed less abrupt decreases in δ13Corg values, reaching a minimum of −24.96‰ at 188 m and −24.92 ‰ at 170 m. This was followed by a positive shift in δ13Corg, reaching a value of −23.77‰ at 152 m.

4.2. TOC Contents

The TOC profile was significantly different from the carbon-isotope distribution (Figure 4). In the lower part of the stratigraphic column (215–200 m), a higher TOC content (>0.6 wt%) was recorded in the mudstone interval. The TOC contents experienced a slight decrease in the silty mudstone interval (∼206 m), resulting in a negative shift of about 0.32 wt%, after which the TOC gradually recovered to a higher level. Then, the TOC values sharply decreased at 202 m depth. The TOC values remained low and stable at depths of 198–152 m, with TOC contents of lower than 0.2 wt%.

4.3. Elemental Contents

Major element composition shows that SiO2 and Al2O3 were the most abundant oxides (see Supplementary Material), and were in the range from 49.8 to 72.4 wt% (60.7 wt% on average) and 12.1 to 16.6 wt% (14.5 wt% on average), respectively, while Ti and P comprised <1 wt% (Table S1). The stratigraphic variations in trace element composition (e.g., Zr, V, Zn, Ni, Sr, and Ba) are shown in Table S1. In the QZ-16 well, the EF values of V (0.48–7.68, 1.67 on average), Ni (0.23–2.33, 1.16 on average), and Zn (0.75–30.44, 3.45 on average) show that these trace elements existed in levels ranging from depleted to slightly enriched. Ratios of Si/Al, Ti/Al, and Zr/Al can be used as proxies to infer variations in detrital fluxes from fluvial and aeolian transport [24,25]. The Si/Al ratios were higher in the NCIE interval compared to the pre- and post-NCIE intervals (Figure 4). In contrast, the Si/Al ratios were lower and relatively stable in the pre- and post-NCIE intervals. The Zr/Al profile shows a similar trend to the Si/Al profile. The Ti/Al ratios show an upward increasing trend at the base of the NCIE interval, followed by relatively low ratios at the top of the NCIE interval, and finally a slight increase in the post-NCIE interval. The ratio of organic carbon (Corg) to phosphorus (P) is a useful proxy for determining the redox conditions during sediment deposition in marine and lacustrine environments [26,27]. The Corg/P molar ratio displayed a gradual upward increase at the onset of the pre-NCIE interval (Figure 4), then a decrease at the end of the pre-NCIE interval, followed by relatively lower and stable ratio in the NCIE and post-NCIE intervals (Figure 4). The Sr/Ba ratio serves as a useful proxy for tracking variations in water mass salinity and associated environmental conditions [28]. The Sr/Ba ratios ranged from 0.05 to 3.64 (0.77 on average) (Figure 4), generally showing values less than 1.0 throughout the stratigraphic column, except for one sample at a depth of 190 m.

5. Discussion

5.1. Enhanced Terrigenous Input

The temporary presence of coarser-grained sediments in marine basins or lake systems suggests that climatic influences, rather than solely local tectonic activities, control sediment supply [29]. This pattern indicates an amplified influx of clastic sediments into both marginal and deeper marine/lacustrine basins, attributed to increased runoff and/or storm-driven currents [7,30,31]. In the eastern Northern Qiangtang Depression, Lower Jurassic variations in terrigenous input are inferred from geochemical and sedimentological stratigraphic data. Specific lithogenic elements, including Al, Si, Ti, K, and Zr, serve as robust proxies to assess the role of detrital input [30,32,33]. Ti and Zr are predominantly associated with silt- and sand-sized minerals (e.g., zircon, rutile) or clay minerals [34,35,36,37], while non-biogenic Si is abundant in coarse-grained quartz and/or finer-grained silt and clay minerals [37]. Conversely, Al and K originate mainly from clay minerals, K-feldspar, and mica [37]. Consequently, Si/Al, Ti/Al, and Zr/Al ratios act as effective metrics for detrital fluxes. Shifts in K/Al, Si/Al, Ti/Al, and Zr/Al sediment ratios may reflect changes in response to the hydrological cycle due to rock hydrolysis in the source regions or changes in transport distance triggered by sea/lake level shifts [24,25]. During the rapid expansion and deepening of the lake, a landward shoreline shift and sediment transport increase fine-grained sediment prevalence compared to a decrease in the clastic components. On the contrary, a lakeward shoreline shift during lacustrine regressions enhances the coarse clastic component and elevates the clastic ratio. Considering the impact of weathering on K sediment content, this study prioritizes Si/Al, Ti/Al, and Zr/Al ratios to denote detrital fluxes.
Within the studied succession, low Ti/Al ratios in the upper interval of the NCIEs may suggest a reduction in detrital input (Figure 4). In contrast, the onset of the NCIEs is characterized by elevated Si/Al and Ti/Al ratios, indicative of increased fluvial transport of clastic materials (Figure 4). The parallel trends of Zr/Al and Si/Al (Figure 4) imply that Zr/Al is predominantly influenced by riverine runoff [29]. Therefore, Si/Al and Zr/Al ratios point to an augmented detrital supply coinciding with the onset of Early Jurassic carbon cycle perturbations. Sedimentological analysis also reveals a relative increase in silt compared to clay, corroborating the intensified terrigenous influx at that time (Figure 2a,d).
The observed increase in terrigenous sediment flux within the Qiangtang Basin during the Early Jurassic can be attributed to several factors, such as (a) regional tectonic activity leading to uplift and subsequent marine regression, (b) paleoclimatic changes, or (c) a combination of both tectonic and climatic influence [30]. However, the available evidence suggests that regional tectonism and associated sea-level fluctuations are unlikely to have been the primary drivers of the heightened detrital influx. Recent investigations indicate that the Qiangtang Basin was situated on a tectonically inactive passive continental margin in the Early Jurassic [13], making major uplift an improbable explanation for the observed increase in clastic sedimentation. Moreover, the basin is known to have undergone a marine transgression in the early Toarcian [13,19], coinciding with the global sea-level rise documented in coeval sedimentary records [25,38]. This transgressive trend directly contradicts the notion that marine regression drove the elevated detrital input at well QZ-16. Since neither regional tectonics nor sea-level changes appear to be the dominant controlling factors, the available evidence points to paleoclimatic shifts as the most likely trigger for the increased clastic influx observed during the NCIEs in the Qiangtang Basin [39].
The stratigraphic succession of the Quemo Co Formation suggests that a significant climatic transition occurred during the Early Jurassic. The second member of the Quemo Co Formation, as observed in well QZ-16, is characterized by extensive thick gypsum layers, indicative of an arid paleoclimate [13,20]. This contrasts sharply with the higher fluvial detrital proxies at the onset of the NCIEs. The increased fluvial detrital input, coupled with elevated weathering proxies [13,20], implies a transition toward a warmer and more humid climate regime. This climatic shift would have intensified riverine sediment delivery to the basin, leading to the observed increase in silt-sized sedimentation during the NCIEs. This suggests that broader climatic and environmental changes occurred on the adjacent landmasses and in the shallow marine settings surrounding the Qiangtang Basin (Figure 2a,b).
Further evidence for this climatic transition is found in the stratigraphic succession of the Quemo Co Formation across the basin. In the South Quemo Co section of the eastern NQD, the lower part of the formation displays a low chemical index of alteration (CIA) and C-value (55.04 and 0.32, respectively) [40], indicating cold and dry paleoclimatic conditions. A similar trend is observed in the southwestern NQD, where the Quemo Co Formation in the Woruo Mountain section exhibits low CIA values ranging from 58.5 to 65.7, with an average of 61.88 [13]. However, progressing towards the top of the Quemo Co Formation in the South Quemo Co section, the CIA and C-value continuously increase, peaking at 91.14 and 0.85, respectively [40]. This suggests a gradual transition to more humid or semi-humid paleoclimatic conditions. Furthermore, in the upper-middle part of the Quemo Co Formation at the Shengli River section in the southwestern NQD, the presence of a significant amount of silicified wood and plant fossils, such as Neocalamites sp., indicates warm and humid climatic conditions [18]. This is corroborated by the occurrence of palynological fossils, such as Neoraistrickia, and pteridophyte spores Cyathidites and Deltoidospora, which are indicative of tropical to subtropical moist to semi-moist climates [18].
The data indicating a reduction in TOC content concurrent with the onset of the NCIEs further supports the hypothesis that increased clastic contributions from continental sources were the primary driver behind the heightened terrigenous deposition observed in the Qiangtang Basin at this time (Figure 4). While the warm and humid climatic conditions that prevailed during the NCIEs interval in the western Tethys would generally be expected to favor organic matter production in the basin [10,29,41], the substantial influx of terrigenous clastic material appears to have diluted the organic matter content in the well QZ-16 in the Qiangtang Basin. This reduction in TOC despite the favorable climatic conditions for organic matter accumulation provides additional evidence that the climatic shift towards a warmer and more humid regime triggered the increased delivery of continental detritus to the basin [42]. The inverse relationship between TOC content and terrigenous deposition further strengthens the interpretation that the heightened terrigenous influx, rather than local paleo-productivity, was the dominant control for sedimentary characteristics during the NCIEs in the Qiangtang Basin. This additional line of evidence helps to solidify the conclusion that paleoclimatic changes were the primary driving force behind the observed sedimentary patterns in the Lower Jurassic succession of the Quemo Co Formation.

5.2. Paleoredox Conditions during the Early Toarcian

The oxygen content of the water column has significant implications for the enrichment and preservation of organic matter in sediments. In this study, the redox conditions were constrained by assessing the bulk elemental geochemistry within the Lower Jurassic strata of the northern Qiangtang Basin. Concentrations of redox-sensitive trace elements, such as V, Zn, and Ni, are widely recognized as indicators of redox state [23,43,44]. These elements exhibit greater solubility in oxygenated environments, resulting in their reduced presence in sediment deposits. Conversely, in anoxic settings, they become insoluble and are enriched in the sediment as authigenic deposits [23,43,44]. Understanding the provenance and dynamics of these trace elements is crucial for deducing paleo-redox conditions [23]. Sedimentary trace elements can be derived from detrital, authigenic, or hydrothermal sources [23]. In the absence of hydrothermal activity and given the lack of correlation between redox-sensitive trace elements (V, Zn, Ni) and aluminum (Al) (Figure 5) in the well QZ-16, especially during periods of increased fluvial input, a detrital origin for these elements is not supported, implying an authigenic source. At well QZ-16, most samples show either depleted enrichment factors (VEF, ZnEF, NiEF) below 1.0 or only modest enrichment (less than 3.0) [23,43,44]. Furthermore, intervals corresponding to the NCIEs exhibit reduced VEF and NiEF values compared to the pre-NCIEs interval, suggesting more oxic conditions at the bottom and pore waters during the NCIEs events. This indicates that the T-OAE was absent in this paleogeographic part of the eastern Tethys Ocean.
The ratio of organic carbon (Corg) to phosphorus (P) is another useful proxy for determining the redox conditions during sediment deposition in marine and lacustrine environments [26,27]. Phosphorus preserved in sediments as an authigenic mineral is characteristic of oxic to dysoxic settings, while anoxic waters promote the liberation of P into the water column via the reduction of Fe-oxyhydroxides [26]. At well QZ-16, the absence of correlation between P and Al (R = 0.079) suggests that the P has an authigenic, marine-derived origin rather than a detrital source (Figure 5). Therefore, the Corg/P ratio serves as a reliable proxy for deciphering redox conditions in this study. Geochemical investigation generally associates Corg/P ratios greater than 100 with anoxic conditions, between 50 and 100 with suboxic conditions, and less than 50 with oxic conditions [43]. Contemporary geochemical studies have cautioned that no universal elemental thresholds can be uniformly applied across all depositional systems [45,46]. In the NCIEs interval, the lower Corg/P ratios relative to the pre-NCIEs interval suggest the prevalence of fully oxic conditions in the interval from 200 to 150 m depth within the well QZ-16. Even within the pre-NCIEs interval, the Corg/P ratios indicate predominantly oxic conditions, with only brief dysoxic episodes. These observations are consistent with the findings from the elemental geochemical analyses presented earlier.

5.3. Assessing the Role of Paleosalinity

The Sr/Ba ratio serves as a useful proxy for tracking variations in water mass salinity and associated environmental conditions. Barium precipitates as barite (BaSO4) under relatively low salinity settings, whereas strontium forms the mineral celestite (SrSO4) at higher salinity levels due to the greater solubilities and solubility products (Ksp) of SrSO4 compared to BaSO4 [47,48]. Consequently, an increase in salinity promotes celestite precipitation, leading to an elevated Sr/Ba ratio. Established geochemical thresholds indicate that Sr/Ba ratios below 0.2, between 0.2 and 0.5, and above 0.5 generally denote freshwater, brackish, and marine conditions, respectively [28]. It is important to note that high calcium (Ca) content in samples may skew the bulk Sr/Ba ratio, impacting the interpretation of paleosalinity conditions. However, the low Ca concentrations (<10%) reported in the analyzed samples from well QZ-16 suggest minimal influence from carbonate-hosted Sr on the overall Sr concentrations in the sediment. The Sr/Ba ratios observed, which were predominantly below 1.0, in the third member of the Quemo Co Formation (Table S1) indicate brackish to freshwater salinity conditions. This interpretation is further supported by the identification of freshwater algae, such as Campenia sp., within the first member of the Quemo Co Formation, pointing to a freshwater depositional environment in the eastern part of the NQD during the Early Jurassic [40]. In contrast, in the Shengli River area of the southwestern NQD, the presence of marine brachiopod fossils, including Astarte and Pleuromya within the second member of the Quemo Co Formation suggests a more restricted marine setting [18]. The Sr/Ba ratios exceeding 1.0 in both the first and second members of the Quemo Co Formation in the southwestern NQD corroborate a higher-salinity, marine-based depositional environment at that time [13,40]. The observed spatial variation in aquatic environments during the deposition of the Quemo Co Formation likely reflects its location on the eastern fringe of the NQD, where it received significant freshwater inputs from terrestrial sources, in contrast to the more transgression-influenced southwestern region [13]. Thus, the deposition of the Quemo Co Formation in the NQD was likely governed by the interplay between freshwater and seawater influences.

5.4. Carbon Cycle Perturbations during the Early Jurassic

The carbon isotope ratios observed in this study varied between −23.16‰ and −26.98‰ in pelitic siltstone to mudstone at depths of 202–176 m. Despite the potential for post-depositional diagenetic alteration of sedimentary carbonates, the geochemical data from well QZ-16 suggests minimal diagenetic influence. Notably, Mg/Ca ratios in carbonates ranged from 0.1 to 0.5, indicating limited dolomitization [49,50]. Furthermore, the weak correlation between δ13Corg and TOC (R = 0.576) implies the preservation of original depositional signatures and limited diagenetic alteration (Figure 5). The preservation of primary δ13Corg patterns, coupled with strong correlations to other well-established δ13Corg records (e.g., Bilong Co [10], Nianduo [32], and Dameigou [51]), indicates that the observed trends, akin to the T-OAE, remain intact. This supports the use of δ13Corg as a reliable tool for stratigraphic correlation [10]. The study also examined the potential impact of changes in organic matter source on δ13Corg, prompted by an abrupt facies shift around 200 m depth. However, significant δ13Corg variations occurred within a consistent depositional environment, rather than across lithological boundaries, suggesting that local depositional environment and facies had a negligible effect on δ13Corg. Cross-plots of δ13Corg against the detrital proxy Ti further confirmed the independence of δ13Corg from changes in grain size and detrital input. Consequently, δ13Corg variations are independent of Ti for each microfacies. Despite high Ti values typically being associated with pelitic siltstone, δ13Corg values vary widely and do not correlate with Ti, further indicating that changes in detrital input, and thus potential shifts in terrestrial organic matter flux, have minimal impact on δ13Corg variations in well QZ-16. Collectively, these findings indicate that the δ13Corg signal from well QZ-16 has been largely preserved and unaffected by diagenesis, enabling its reliable use for stratigraphic and paleoenvironmental interpretations.
A key feature of a significant positive δ13Corg anomaly interrupted the NCIEs within the third member of well QZ-16. This has been interpreted as a hallmark in prior studies of the lower Toarcian Stage in the western Tethys [29,52,53]. The current study identifies a pronounced negative δ13Corg excursion with a modest magnitude of approximately −3.83‰ and a minimum value of −26.99‰ at a depth of 198 m (Figure 4). This pronounced negative shift in carbon isotope values is then followed by a gradual, incremental recovery in the δ13Corg profile (Figure 4). Interestingly, the NCIE interval was followed by a modest ascending trend in the δ13Corg profile with values increasing from −24.54‰ to −23.16‰ (Figure 4). These patterns observed in well QZ-16 closely mirror the NCIE signatures that have been documented in other Tethyan sedimentary successions, including the Bilong Co section (Northern Tibet) [10,11,54], Wenquan section (Northern Tibet) [55], Suobucha section (Northern Tibet) [56], Nianduo section (Southern Tibet) [32], Sichuan Basin [56], and Qaidam Basin [51] (Figure 6). These sections in various Chinese localities span a range of depositional environments, from lagoonal to shelf and lacustrine settings, and yet they all preserve evidence of the characteristic T-OAE-related NCIEs. Notably, the Bilong Co section and well QZ-16, which are both situated within the broader Qiangtang Basin, exhibit some distinct regional differences and congruent basin evolution histories [13]. Specifically, these sections tend to display relatively heavier baseline δ13Corg values and less pronounced amplitude in the CIEs [10,54]. This suggests regional variations in the expression of the T-OAE within the eastern Tethys realm compared to those in the western Tethys.
Another distinguishing feature of the NCIEs in the third member of well QZ-16 is the frequent presence of coarser-grained sedimentary deposits, including silt-sized particles (Figure 2a,d) indicative of high-energy conditions amid the generally low-energy claystone deposits. The appearance of these more energetic, coarser-grained sediments coincides stratigraphically with the most negative phase of the NCIE interval. This indicates that the sedimentological pattern observed in well QZ-16 is not unique, but rather has been documented in other marine and terrestrial settings associated with the T-OAE, including the Cardigan Bay Basin (UK) [57], Lusitanian Basin (Portugal) [7], Sichuan Basin [56], and Qaidam Basin [51] (Figure 6). The consistent appearance of these coarser clastic deposits during the most pronounced phase of the CIEs suggests a common causal mechanism linking sediment transport processes, as evidenced by elevated ratios of Si/Al, Zr/Al, and Ti/Al, and the T-OAE perturbations. This sedimentological signature is interpreted as indicative of increased fluvial transport of clastic material, likely due to intensified continental weathering.
To sum up, the NCIEs observed in the studied basin are closely connected to the global T-CIE, as evident from the geochemical and sedimentological data. However, this NCIE that characterized the T-CIE event was consistent with enhanced respiration in the water column and at the sediment-water interface, indicating the absence of T-OAE in this part of the Qiangtang Basin. Therefore, further research is crucial to refine the Toarcian chronological framework of the Quemo Co Formation within well QZ-16 in the future. Establishing a robust temporal context and high-resolution carbon isotope profile for this section will help enhance our understanding of how the Early Jurassic carbon cycle perturbations, as recorded by the NCIEs, were expressed across different settings in the broader Qiangtang Basin and eastern Tethys region.

6. Conclusions

Based on an integrated approach comprising organic, inorganic, and isotope geochemistry, alongside the assessment of mineralogical and sedimentological characteristics on samples from the QZ-16 well in the Qiangtang Basin, the following conclusions have been developed:
(1) During the Early Jurassic, the lacustrine system of the North Qiangtang Depression demonstrated environmental instabilities characterized by NCIEs. This pattern is similar to that of the T-CIE event in western and eastern Tethys regions.
(2) Within the NCIEs interval, there was a significant increase in the fluvial detritus input into the North Qiangtang Basin. This is supported by elevated detrital input proxies (Zr/Al, Ti/Al, and Sil/Al ratios) and the presence of pelitic siltstones. However, a high influx of clastic material from continental sources may have resulted in the dilution of organic matter, leading to low organic carbon burial, particularly during the T-CIE excursion interval.
(3) The studied succession from the QZ-16 well indicates well-oxygenated water column conditions due to the depleted enrichment of V, Ni, and Zn, which played a crucial role in the formation of organic matter-lean sediments. This provides evidence that the T-OAE did not occur in this part of the Qiangtang Basin.
(4) The Quemo Co Formation within the QZ-16, influenced by both freshwater and seawater, exhibits characteristics of brackish to freshwater salinity conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14080762/s1.

Author Contributions

Conceptualization: J.W.; methodology: J.W.; validation: H.Z.; formal analysis: H.Z., X.F. and L.S.; investigation: H.Z.; data curation: H.Z.; writing—original draft preparation: H.Z.; writing—review and editing: M.S.A. and J.W.; visualization: H.Z.; supervision: J.W.; project administration: H.Z. and J.W.; funding acquisition: J.W. and M.S.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 42241202, 42241203, 91955204), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019QZKK080301), Natural Science Foundation of Sichuan Province (Grant No. 2024NSFSC0833), and Researchers Supporting project number (RSP2024R455), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We thank the National Natural Science Foundation of China, the Second Tibetan Plateau Scientific Expedition, Natural Science Foundation of Sichuan Province, and the Researchers Supporting project number (RSP2024R455), King Saud University, Riyadh, Saudi Arabia. We are indebted to Zhongwei Wang for detailed discussions resulting in the improvement of the manuscript. We are also thankful to Shengqiang Zeng for providing thin sections.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geologic setting and location maps of the study area. (a) Simplified topographic map of the Tibetan Plateau [13]. (b) Tectonic framework of the Qiangtang Basin. (c) Geological map of the Quemo Co area, showing the location of the well QZ-16.
Figure 1. Geologic setting and location maps of the study area. (a) Simplified topographic map of the Tibetan Plateau [13]. (b) Tectonic framework of the Qiangtang Basin. (c) Geological map of the Quemo Co area, showing the location of the well QZ-16.
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Figure 2. Drilling-core photographs and micrographs from the well QZ-16. (a) Photograph of greyish white pelitic siltstone at 195 m depth, with blue arrows highlighting younger strata. (b) Greyish-black mudstone, 217 m. (c) Greyish-black mudstone at 220 m depth, showing strong bioturbation. (d) Cross-polarized light micrograph of pelitic siltstone at 180 m depth, revealing poor sorting and hypo-edge angle. (e) Cross-polarized light micrograph of mudstone at 217 m depth, exhibiting a completely homogeneous feature. (f) Plane-polarized light micrograph of a bivalve fragment in mudstone (indicated by yellow triangle) at 220 m depth.
Figure 2. Drilling-core photographs and micrographs from the well QZ-16. (a) Photograph of greyish white pelitic siltstone at 195 m depth, with blue arrows highlighting younger strata. (b) Greyish-black mudstone, 217 m. (c) Greyish-black mudstone at 220 m depth, showing strong bioturbation. (d) Cross-polarized light micrograph of pelitic siltstone at 180 m depth, revealing poor sorting and hypo-edge angle. (e) Cross-polarized light micrograph of mudstone at 217 m depth, exhibiting a completely homogeneous feature. (f) Plane-polarized light micrograph of a bivalve fragment in mudstone (indicated by yellow triangle) at 220 m depth.
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Figure 3. Schematic diagram showing the major lithostratigraphic unit of the Upper Triassic–Jurassic in the Qiangtang Basin (modified from Wang and Fu [13]).
Figure 3. Schematic diagram showing the major lithostratigraphic unit of the Upper Triassic–Jurassic in the Qiangtang Basin (modified from Wang and Fu [13]).
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Figure 4. Organic carbon-isotope (δ13Corg), total organic carbon (TOC) records, and geochemical proxies from the well QZ-16. Detrital influx proxies: Zr/Al × 10−4, Si/Al, and Ti/Al. Redox-sensitive proxies: Corg/P, VEF, NiEF, and ZnEF.
Figure 4. Organic carbon-isotope (δ13Corg), total organic carbon (TOC) records, and geochemical proxies from the well QZ-16. Detrital influx proxies: Zr/Al × 10−4, Si/Al, and Ti/Al. Redox-sensitive proxies: Corg/P, VEF, NiEF, and ZnEF.
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Figure 5. Cross-plots of P-Al (a), Ni-Al (b), Zn-Al (c), V-Al (d), and Ti-δ13Corg (e) within typical microfacies and TOC-δ13Corg (f) of the well QZ-16. Orange solid circles represent total samples, red solid circles represent pelitic siltstone samples, and green solid circles represent mudstone samples.
Figure 5. Cross-plots of P-Al (a), Ni-Al (b), Zn-Al (c), V-Al (d), and Ti-δ13Corg (e) within typical microfacies and TOC-δ13Corg (f) of the well QZ-16. Orange solid circles represent total samples, red solid circles represent pelitic siltstone samples, and green solid circles represent mudstone samples.
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Figure 6. Comparison between the Jurassic carbon cycle perturbations from Bilong Co [10], Sewa [55], Suobucha [56], Nianduo [32], and well QZ-16 (this study) in the Eastern Tethys.
Figure 6. Comparison between the Jurassic carbon cycle perturbations from Bilong Co [10], Sewa [55], Suobucha [56], Nianduo [32], and well QZ-16 (this study) in the Eastern Tethys.
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Zhang, H.; Wang, J.; Ahmed, M.S.; Fu, X.; Shen, L. Lacustrine Environmental Perturbations during the Early Jurassic in the Qiangtang Basin, Northern Tibet. Minerals 2024, 14, 762. https://doi.org/10.3390/min14080762

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Zhang H, Wang J, Ahmed MS, Fu X, Shen L. Lacustrine Environmental Perturbations during the Early Jurassic in the Qiangtang Basin, Northern Tibet. Minerals. 2024; 14(8):762. https://doi.org/10.3390/min14080762

Chicago/Turabian Style

Zhang, Haowei, Jian Wang, Mohamed Saad Ahmed, Xiugen Fu, and Lijun Shen. 2024. "Lacustrine Environmental Perturbations during the Early Jurassic in the Qiangtang Basin, Northern Tibet" Minerals 14, no. 8: 762. https://doi.org/10.3390/min14080762

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