1 Introduction

The Early Cambrian marks a unique time interval of remarkable biological innovation known as the “Cambrian explosion”, which is characterized by the rapid appearance of most phyla of modern animals and the development of a modern-like marine ecosystem (Sperling et al. 2013; Zhang and Shu 2014; Butterfield 2018). Although internal factors like genomic reorganization and ecological opportunities may have played crucial roles in triggering the biological event (Erwin et al. 2011; Mills and Canfield 2014; Butterfield 2018), the belief persists that profound changes in environmental conditions are closely associated with the diversity and decline of life during the Early Cambrian (e.g., Chen et al. 2015; He et al. 2019; Jin et al. 2016; Li et al. 2018; Wang et al. 2018a; Wei et al. 2018). One commonly cited key variable among the environmental factors is the oxygen concentration in the atmospheric-oceanic system. A rise in oxygen levels in seawater would facilitate biological innovation via producing more stable oxygenated habitats. Additionally, nutrient and food availability have also been considered as active players in early animal radiation (Cook and Shergold 1984; Brasier 1992; Sperling and Stockey 2018). The primary productivity of primary producers, which provide a fundamental food source for early animals, is ultimately regulated by nutrient availability in the ocean (Anbar and Knoll 2002; Knoll 2017). Consequently, great attention was paid to nutrient availability and its connection with macroevolution.

As a macronutrient, the availability of nitrogen in seawater is crucial for regulating primary productivity and shaping the marine ecosystem (Knoll and Follows 2016; Knoll 2017). Changes in nitrogen availability over geological time may have a profound influence on the evolution of eukaryotes (Stüeken et al. 2016; Wang et al. 2018a; Kipp et al. 2018). Due to the redox-sensitive nature of the biogeochemical cycle of nitrogen, the availabilities of different nitrogen species, such as NH4+ and NO3− in the water column, are strongly bonded to the seawater redox conditions (Stüeken et al. 2016). As fractionations of nitrogen isotopes via different metabolic pathways are variable, changes in the oceanic nitrogen cycle may potentially leave isotopic signatures in marine sedimentary records. For example, the prevalence of NH4+ assimilation by organisms tends to produce highly negative δ15N values in marine sediments (e.g., Chen et al. 2019; Higgins et al. 2012; Xu et al. 2020), while highly positive δ15N values are commonly associated with aerobic nitrogen cycles (Stüeken 2013; Cremonese et al. 2014; Ader et al. 2014; Wang et al. 2018a). Consequently, nitrogen isotope signatures in marine sediments have been used to assess the evolutionary path of marine redox states, the availability of different nitrogen species in seawater, as well as the biogeochemical nitrogen cycle within the water column in the geologic past (e.g., Ader et al. 2014; Boyle et al. 2013; Higgins et al. 2012; Koehler et al. 2017; Stüeken 2013; Stüeken et al. 2016; Wang et al. 2018b; Zerkle et al. 2008).

Given the vast potential of nitrogen isotopes in unveiling information on oceanic redox evolution and biological innovations, extensive investigation has been made on the δ15N isotopic composition of Early Cambrian sedimentary successions (e.g., Chang et al. 2022, 2019; Chen et al. 2019; Cremonese et al. 2014; Wang et al. 2018a, 2015, 2021; Xiang et al. 2018; Xu et al. 2020; Zhang et al. 2017). These studies involving a variety of sedimentary facies have delineated a clearer picture of the oceanic nitrogen cycling during the Early Cambrian. However, debates still exist with respect to the role of fixed nitrogen supply in regulating animal radiations and declines (Chang et al. 2019; Xu et al. 2020). It has been hypothesized that the expansion of the oceanic NO3−, along with the establishment of a modern-like aerobic nitrogen cycle, was related to the major waves of biological evolutions during the Ediacaran-Cambrian period (Wang et al. 2018a). However, a modern-like nitrogen cycle in a well-oxygenated ocean contradicts to the model of a redox-stratified ocean based on other geochemical indexes (e.g., trace elements, sulfur isotopes, Fe speciation) (Jin et al. 2016; Li et al. 2018, 2020). Up till now, most δ15N data of this time interval were recovered from South China. In this case, data from more continental margins offer a perspective to test the different models and their implications in boarder sense, and further, to achieve a better understanding of the global nitrogen cycle of the Early Cambrian ocean. Here we performed δ15N and δ13Corg analyses on the Lower Cambrian successions from the Shiairike section (inner ramp) and the Well Tadong 2 (deep shelf/basin) in northwestern and eastern Tarim Basin, respectively, with the aim to explore the pathways of nitrogen cycling and to unravel possible links between nitrogen availability, water-column redox during the Early Cambrian in the Tarim region.

2 Geological setting

The Tarim Block, one of the oldest continental blocks in China, was formed during the late Proterozoic (Cai et al. 2011). The Tarim Block was part of the Rodinia supercontinent during the Neoproterozoic and transitioned into a rifting stage along with the breakup of Rodinia in the late Ediacaran (Li et al. 2008; Zhang et al. 2013). The late Ediacaran-Early Cambrian represents a rift-drift transition phase of the Tarim block (Li and Powell 2001; Li et al. 2008). By the Early Cambrian, the Tarim Block has evolved into an epicontinental sea (Zhang et al. 2015). According the paleogeographic reconstruction, the Tarim Basin exhibits a transition from ramp facies into deep shelfal/basinal facies from northwest to the east (Fig. 1).

Fig. 1
figure 1

a Global paleogeography of the Early Cambrian (after Li et al. 2008). b Paleogeography of the Tarim Basin during the Early Cambrian (after Tian et al. 2018)

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Cambrian outcrops are mainly distributed in the Wushi-Keping area (northwestern margin of Tarim) and the Quruqtagh area (northeastern margin of Tarim), In the northwestern Tarim, the Lower Cambrian succession comprises the Yurtus Formation, the Xiaoerbulake Formation and the Wusonggeer Formation in ascending order. The Yurtus Formation is composed of bedded phosphatic chert and black shale in the lower and middle parts and dolomite at the top. The overlying Xiaoerbulake Formation consists of grey, granular, and algal dolomite, followed by the micritic dolomite of the Wusonggeer Formation. Previous paleontological studies have reported the presence of small shelly fossils (SSFs) and acritarch assemblage in the Yurtus Formation. The SSFs and the AHC acritarch assemblage in the basal, phosphatic chert of the Yurtus Formation has constrained the unit of Early Fortunian age, while SSFs in the carbonates of the upper Yurtus Formation indicate this unit corresponds to the lower Chiungchussuan stage (Dong et al. 2009; Qian et al. 2009; Zhu et al. 2019). Moreover, the overlying Xiaoerbulake Formation yields trilobite fossils typical of the Chiungchussuan stage (Zhou and Chen 1990; Peng et al 2012; Zhu et al 2019).

The Shiairike section near the Aksu city is located within the inner ramp facies zone during the Early Cambrian (Fig. 1b). In this section, the basal Yurtus Formation is composed of grey, bedded chert with phosphatic nodules, followed by a black, organic-rich unit known as the black rock series in Chinese literature (e.g., Yu et al. 2003; Yang et al. 2017). This unit comprises thinly bedded cherts with intercalations of black shale for its lower part and mainly black shale for the upper part. Medium-thick bedded dolomite dominates the upper Yurtus Formation, which is overlied by thickly bedded dolomite of the Xiaoerbulake Formation. 24 samples were collected from the lower part and 11 black shales were collected from the upper part (Fig. 2a; Table 1).

Fig. 2
figure 2

a Lithological column of the Yurtus Formation in the Shiairike section. b Lithological column of the Xishabulake and Xidashan Formations in Well Tadong 2

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Table 1 Analytical results of δ15N, TN, δ13Corg, TOC and K2O of samples from the Shiairike section
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At the outcrops of the Quruqtagh area in the eastern Tarim Basin, the Lower Cambrian succession comprises the Xishanbulake Formation and the Xidashan Formation in ascending order. Generally, the Xishanbulake Formation is dominated by dark cherts with phosphatic chert developed at its base. The Xidashan Formation is composed mainly of carbonates with interlayers of shale and mudstone. The basal part of the Xishanbulake Formation yields AHC acritarch assemblage and Kaiyangites (Yao et al. 2005; Dong et al. 2009), indicating it is of Early Fortunian age. The upper part of this formation yields brachiopods (Zhou 2001) and the overlying Xishanbulake Formation yields abundant trilobites, archaeocyathids, brachiopod Lingulella and sponge spicules (Zhang et al. 1983; Zhu and Lin 1983; Zhong and Hao 1990; Peng et al. 2012). Collectively, the paleontological data indicate the upper part of the Xishanbulake Formation and the Xishanbulake Formation can be correlated with the Early Chiungchussuan Stage and Chiungchussuan-Duyunian Stage, respectively (Zhu et al. 2019; 2021).

Well Tadong 2 is located within the deep shelfal/basinal facies (Fig. 1b). The well was originally drilled for gas and petroleum exploration with a total depth of over 5000 m. Based on gamma (GR) values and lithostratigraphic features, the Lower Cambrian Xishanbulake and the Xidashan Formations cover a depth range from 4974 to 4887 m (Wang et al. 2022). The subdivision of the two formations within this interval remains controversial. The boundary of the two formations was proposed to be located at 4949 m (Cai et al. 2014) or 4920 m (Wang et al. 2022) (Fig. 2b). In the present study, we adopted the first subdivision framework, as the different subdivisions can not influence our data interpretation (see Sect. 5). Differing from the outcrop sections in the Kuluketage region, the Xishanbulake Formation in Well Tadong 2 is characterized by black shale with interlayers of argillaceous limestone (Fig. 2b). The Xidashan Formation is composed of calcareous black shale with intercalation of argillaceous limestone (Fig. 2b). Drill cuttings samples were collected from the well, including 8 from the Xishanbulake Formation and 32 samples from the Xidashan Formation (Table 2; Fig. 2b).

Table 2 Analytical results of δ15N, TN, δ13Corg, TOC and K2O of samples from the Tadong 2 well
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3 Analytical methods

Before chemical analysis, all samples were crushed to small chips. After a careful examination to avoid weathered surfaces and veins, the chips were cleaned using distilled water and then ground into powders (< 200 mesh).

Organic carbon isotopic ratios measurement was performed at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Prior to analysis, sample powders were treated using 2 N HCl for 24 h. The step was repeated until all the carbonate fractions in the sample powders were removed. The residue was then rinsed using Milli-Q water several times and dried in an oven. Afterward, the carbonate-free sample powders were wrapped in a tin capsule. The carbon isotopic ratios measurement was carried out using a Delta V mass spectrometer coupled to a Flash Elemental Analyzer. Carbon isotope values are reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB). During analysis, USGS40 and UREA were used as standards for data calibration. Analytical error is within 0.5‰.

Total nitrogen (TN) and nitrogen isotopic ratio measurements were performed at the Beijing Createch Testing Technology Co. Ltd. Prior to analysis, each sample was decarbonated following the procedure described above for organic carbon isotopic analysis. The nitrogen isotopic ratios were measured on a MAT 253 plus mass spectrometer coupled to a Flash Elemental Analyzer via a Conflo IV interface. The standard B2151 was used for calibration and USG 588 was used for monitoring during the whole run. The δ15N values are reported relative to air with analytical error within 0.5‰. TN contents were obtained during isotopic measurements from the nitrogen plot area.

Total organic carbon (TOC) was analyzed at the Analytik Jena AG (Shanghai) Co. Ltd. The analysis was performed using an element analyzer (multi N/C 3100). Total organic carbon (TC) and total inorganic carbon were measured respectively, and TOC values were obtained by subtraction of total inorganic carbon (TIC) from TC.

Potassium content analysis for samples in the Shiairike section was made at the ALS Chemex (Guangzhou) Co., Ltd. The analysis was performed using a ME-XRF26d by the melting film method, calibrated against international standards. The precision was better than 5%. Potassium content analysis of samples from the Well Tadong 2 was made at the Nanjing FocuMS Technology Co., Ltd. Samples powders were digested using mixed concentrated HNO3 and HF following the method of Gao et al. (2003). The solution was then diluted and spiked with Rh. The measurement was performed on an ICP-OES (Agilent 5110) with an analytical precision of better than 5%.

4 Results

The obtained TOC, TN, δ13Corg, δ15N, and K2O data are listed in Tables 1 and 2 and the vertical variations of studied intervals in the Shiairike section and Well Tadong 2 are displayed in Figs. 3 and 4.

Fig. 3
figure 3

Chemostratigraphic column of Yurtus Formation in the Shiairike section. Cd/Mo data are from Zhu et al. (2022a)

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Fig. 4
figure 4

Chemostratigraphic column of the Xishabulake and Xidashan Formations in Well Tadong 2. Cd/Mo data are from Guo et al. (2023)

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4.1 The Shiairike section (northwestern Tarim)

The sampled interval at the Shiairike section exhibits highly variable δ15N values ranging from −6.3‰ to 4.2‰ with an average of 0.7‰. The lower, chert-dominant part (LU) shows more variabilities (−6.3‰ to 4.2‰, average = −0.2‰), with two pronounced negative excursions (Fig. 3). Comparably, δ15N variations in the upper, black shale-dominating part (UU) are more limited (−1.3‰ to 4.1‰), with a higher average value of 2.7‰. The δ15N stays positive through most of the interval with a shift towards negative values at the top (Fig. 3).

The δ13Corg values range from −37.5‰ to −31.8‰ (average = −35.1‰). Like the δ15N pattern, δ13Corg values of the LU show more variability (−37.5‰ to −31.8‰, average = − 35.1‰). The lower part of LU exhibits an overall positive excursion, with δ13Corg value increasing from ca. −37.0 ‰ to ca. −32.0 ‰ before decreasing to ca. −36.0‰. The upper part of LU shows a gentler positive excursion ranging from ca. −35.0‰ to ca. −32.0‰. In contrast, the UU is characterized by slightly lower and more stable δ13Corg values ranging from − 35.9‰ to − 34.8‰ (Table 1; Fig. 3).

TN ranges from 0.03% to 0.45% in this interval. The TN values for the LU (0.03% to 0.41%) are lower than the UU (0.27% to 0.45%), with average values of 0.07% and 0.35%, respectively. The vertical C/N pattern is similar to that of TN, with higher C/N for the UU (23.2 To 51.6, average = 38.0) than the LU (6.4 To 45.1, average = 20.3).

4.2 Well Tadong 2 (eastern Tarim)

Overall, the δ15N values for the Xishanbulake and Xidashan Formations exhibit limited variations, with all data within the range of −2‰ to 1‰ (Table 2, Fig. 4). For the Xishanbulake Formation, the δ15N ranges from −1.6‰ to 0.1‰ (average = −0.7‰), with a small negative excursion at ca. 4960 m. The δ15N values for overlying Xidashan Formation are broadly similar to that of the Xishanbulake Formation, ranging from −1.9‰ to 0.9‰ (average = − 0.9‰). Two small shifts towards more negative values can be observed in this formation, with nadirs of −1.6‰ and −1.9‰, respectively (Fig. 4).

The δ13Corg values range from −32.3‰ to −27.7‰ (average = −30.8‰) for the Xishanbulake Formation, exhibiting a small negative excursion in the lower part with a nadir of − 32.3‰ (Fig. 4). The δ13Corg values then stabilize at ca. 30.0‰ at the upper Xishanbulake Formation. The Xidashan Formation exhibit rather stable δ13Corg values ranging from −30.2‰ to −28.8‰ (average = −29.7‰) (Table 2; Fig. 4).

The TN and C/N patterns for the Xishanbulake and Xidashan Formations are similar, with lower values for the latter. The TN and C/N of the Xishanbulake Formation vary from 0.02% to 0.18% (average = 0.10%) and 24.4 to 93.6 (average = 53.5), respectively. For the Xidashan Formation, the TN and C/N vary from 0.03% to 0.11% (average = 0.06%) and 16.9 to 43.1 (average = 29.8), respectively (Table 2; Fig. 4).

5 Discussion

5.1 Evaluation of carbon and nitrogen isotopic data

The δ13Corg values of all the studied units fall within the range of typical Cambrian sedimentary successions in previous studies (e.g., Wang et al. 2015; Chang et al. 2019; Xu et al. 2020). None of the studied units exhibits significant correlations between TOC and δ13Corg values (Figs. 5a, 6a), suggesting the remineralization of organic matter during diagenesis exerted only a limited influence on the primary δ13Corg values. Nitrogen in sediments occurs as organic-bound nitrogen and clay-bound nitrogen, with the latter being an additional variable in regulating the bulk δ15N values (Ader et al. 2016; Stüeken et al. 2016). For samples in the present study, linear correlations are observed in the TOC vs. TN cross plot for the Yurtus Formation at Shiairike (Fig. 5b). Similarly, the TOC-TN cross plots exhibit good linear correlations for both the Xishanbulake and Xidashan Formations of Well Tadong 2 (Fig. 6b). These linear correlations indicate that the nitrogen mainly originated from organic matter, excluding a significant contribution of terrestrial non-primary nitrogen of the studied samples.

Fig. 5
figure 5

Cross-plots of (a) TOC versus δ13Corg, (b) TOC versus TN, (c) TOC versus C/N, (d) C/N versus δ15N, (e) TN versus δ15N, and (f) K2O versus δ15N for the Yurtus Formation in the Shiairike section

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Fig. 6
figure 6

Cross-plots of (a) TOC versus δ13Corg, (b) TOC versus TN, (c) TOC versus C/N, (d) C/N versus δ15N, (e) TN versus δ15N, and (f) K2O versus δ15N for the Xishabulake and Xidashan Formations in Well Tadong 2

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Oceanic phytoplankton is characterized by a mean C/N ratio of ca. 7 (Redfield et al. 1963). During the remineralization of organic matter, preferential loss of N would increase C/N ratios. For the Yurtus Formation at Shiairike, the samples exhibit elevated C/N ratios of 6.4 to 51.6 (average = 25.9). The C/N ratios for the Xishanbulake and Xidashan Formations of Well Tadong 2 are 24.4 to 93.6 (average = 53.5) and 16.9 to 43.1 (average = 29.8), respectively. Linear correlations between TOC and C/N values are found in all studied units (Figs. 5c, 6c). The elevated C/N ratios of the studied units, along with the linear correlations in the TOC-C/N plots, may indicate partial loss of N during diagenesis and metamorphism in all studied units. The following section will explore the influence of diagenesis and metamorphism on the primary δ15N of the studied samples.

Diagenetic processes can potentially alter the primary nitrogen isotopic signatures in sedimentary sequences (Stüeken et al. 2016). However, the changes in the primary δ15N signatures vary depending on the redox conditions. In oxic settings, an increase in δ15N values up to 4‰ is accompanied by the release and subsequent oxidation of NH4+ in porewaters during diagenesis (Freudenthal et al. 2001; Lehmann et al. 2002). Moreover, oxic diagenesis is usually characterized by δ15N values larger than 2‰ (Stüeken et al. 2016). In comparison, the isotopic fractionations in anoxic diagenetic processes are only minimal (< 2‰) (Freudenthal et al. 2001; Lehmann et al. 2002; Robinson et al. 2012). The samples from Well Tadong 2 were interpreted to have been deposited under a predominately anoxic setting based on the high enrichment of redox-sensitive elements (Guo et al. 2023). The anoxic diagenetic conditions, combined with all samples exhibiting δ15N < 2‰ (Table 2; Fig. 4), indicate that Well Tadong 2 samples are associated with anoxic diagenetic processes with minimal nitrogen isotopic alteration. Similarly, the Shiairike samples, which were deposited in anoxic to euxinic water conditions (Zhu et al. 2022a), should have been subjected to minimal influences on the primary δ15N signatures from diagenetic alteration.

Metamorphism during burial may also impose a significant effect on the primary nitrogen isotopic signatures of sedimentary successions. Thermal denitrogenation accompanied by metamorphism would lead to the preferential loss of 14N, resulting in an increase in δ15N (Ader et al. 2016). Previous studies show that the extent of δ15N value change differs among different metamorphic grades, with less than 2 ‰ for greenschist facies, 4 ‰ for amphibolite facies, and 6–10 ‰ for upper amphibolite phases (Bebout and Fogel 1992; Ader et al. 2016; Stüeken et al. 2017). According to previous studies in the Tarim Basin, the kerogen maturity of the Lower Cambrian successions from the Shiairike section and Well Tadong 2 are still in the “gas window”, indicating the rocks are below the greenschist facies (Zhang et al. 2004; Wang 2021). Hence, metamorphic alteration on the δ15N of our studied samples would be minimal.

Cross plots of δ15N-TN and δ15N-C/N also provide supporting evidence for the interpretation that diagenetic and metamorphic processes had negligible influences on the δ15N values of our samples. As both processes are characterized by the loss of nitrogen and an increase in δ15N values, negative correlations between δ15N -TN and positive correlations between δ15N-C/N would be expected for samples that subjected to significant alteration (Cremonese et al. 2014; Wang et al. 2015, 2018b; Ader et al. 2016). However, as shown in Figs. 5d, 5e and Figs. 6d, 6e, data points from all the studied units exhibit no evident δ15N-TN and δ15N-C/N correlations, indicating no significant alteration of the primary sedimentary δ15N signatures. Also, no correlation is observed between potassium abundance and δ15N (Figs. 5f; 6f), further demonstrating limited impact on nitrogen isotopes by diagenetic fluids (Zerkle et al. 2008).

5.2 Interpretation of nitrogen isotopic data

5.2.1 The Shiairike section (norhwestern Tarim)

As described in Sect. 4.1, the studied interval of the Yurtus Formation can be divided into two units according to the vertical profile in δ15N values (Fig. 3). The δ15N of the LU shows more variabilities, while the δ15N of UU is more invariant. In the following text, the δ15N signatures of the two units and their implications will be explored separately.

5.2.1.1 LU

The δ15N values for LU are highly variable, ranging from −6.3‰ to 4.2‰ (average = −0.2‰) (Fig. 3). This unit is characterized by two pronounced negative excursions in δ15N values, with nadirs of −6.3‰ and −4.8‰, respectively (Fig. 3). In addition to the highly negative values, there are also positive δ15N values exceeding +2‰ within this interval (Fig. 3). The significant variability in δ15N is similar to that of the Fortunian Liuchapo Formation at Yuanjia section in South China (δ15N: −4.3‰ to +2‰) (Wang et al. 2015), and is considered to reflect a shallow, unstable chemocline within water column during deposition, with strong upwelling regulating the nitrogen cycle within the water column (Ader et al. 2014; Wang et al. 2015). Such a scenario may also serve as a conceivable explanation for the highly variable δ15N signatures documented in the LU. The deposition of the Yurtus black rock series in northwest Tarim has been linked to a major transgression during the Early Cambrian (Yu et al. 2009; Zhang et al. 2020a; Zhu et al. 2022b). Accompanied by the transgression, widespread upwelling of deep anoxic waters was prevalent in this region, as indicated by geological and geochemical evidence including the presence of widespread phosphorite layers and nodules in the basal Yurtus Formation and the high Cd/Mo ratios typical of upwelling settings (Yu et al. 2009; Yang et al. 2017; Zhang et al. 2020a; Zhu et al. 2022a). In such circumstances, high primary productivity fueled by nutrient-rich, deep waters would cause enhanced respiratory oxygen demand in the water column (e.g., Yang et al. 2017; Zhang et al. 2020a). Meanwhile, the rise in sea level caused the impingent of anoxic waters to shallower depths (Harris et al. 2018). The combination of the two factors leads to a shoaling chemocline and further a reduction in the nitrate reservoir in shallow waters. According to such a scenario, ammonia traversing through the redox transition zone would reach the photic zone and be competitively assimilated or subjected to coupled denitrification and anammox, depending on the relative importance of the two processes (Ader et al. 2014, 2016). The former process that fractionates nitrogen isotopes by up to −27‰ would have been favored when upwelling was strong, generating negative δ15N signatures (e.g., Higgins et al. 2012; Cremonese et al. 2014; Wang et al. 2015, 2018a; Chen et al. 2019). On the other hand, the dominance of the latter would drive the δ15N signature towards positive values of > 2 ‰ due to the loss of 14N during denitrification/anammox (Papineau et al. 2009; Godfrey et al. 2013; Ader et al. 2016; Chen et al. 2019). The instability of the redox transition zone would thereby generate δ15N signatures with high variability ranging from negative to positive values (Ader et al. 2014), as observed in the LU.

Nitrogen fixation via the utilization of V- or Fe-based nitrogenase enzymes may also generate negative δ15N values in sediments (Zhang et al. 2014; Stüeken et al. 2016). This process is known to occur exclusively in Mo-depleted conditions (Zhang et al. 2014). Nevertheless, studies on Fe-rich Archean and Proterozoic oceans, which presumably have even lower aqueous Mo in seawater than the Cambrian oceans, showed no convincing evidence to suggest that alternative V and/or Fe-based nitrogenases dominated N2-fixation (Stüeken et al. 2016; Chen et al. 2019). Moreover, a previous study showed the Mo/TOC ratios for the LU were higher than that of the UU, indicating higher levels of aqueous Mo in seawaters for the LU (Zhu et al. 2022a, b). If nitrogen fixation via the utilization of V- or Fe-based nitrogenase enzymes occurred during deposition, the UU would be more prone to showing negative δ15N values. However, this study does not observe such negative δ15N values in the UU (Fig. 3). Therefore, it is unlikely that this process is responsible for the negative δ15N values observed in the current study.

The δ13Corg signatures of this interval are also consistent with a scenario with a shallow, unstable chemocline during a major transgression. Impingent of deeper, anoxic waters along with sea level rise and strong upwelling would have brought 12C-rich inorganic carbon into the photic zone (e.g., Wang et al. 2015, 2018a, b). Subsequently, assimilation of the 12C enriched carbon by organisms would produce light δ13Corg values in the sedimentary record. The variabilities in δ13Corg values, on the other hand, are reflective of the changes in the relative proportion of oxygenic photosynthetic biomass and anaerobic chemoautotrophic biomass in response to the instability in chemocline (e.g., Wang et al. 2018b).

5.2.1.2 UU

The δ15N values for the UU of the studied unit exhibit distinct characteristics in comparison to the LU. Except for the topmost interval, δ15N values of most of this unit stay relatively invariant with δ15N of > 2‰ (Fig. 3). Compared to the LU characterized by alternating anaerobic assimilation of NH4+ and denitrification/anammox, the less variable δ15N values indicate a dominance of denitrification/anammox over partial ammonia assimilation. The reason underlying this transition in nitrogen cycle may be attributed to the decline of deep water upwelling. Marine sediments from settings with strong upwelling are usually characterized by high Cd/Mo ratios (Sweere et al. 2016). Thus, the lower Cd/Mo ratios in the UU likely indicates reduced upwelling intensities compared to the LU (Fig. 3) (Zhu et al. 2022a). The cessation of intense upwelling may have resulted in a deeper and more stable chemocline, enabling an expanded pool of NO3− in shallow waters and a shift towards denitrification/anammox dominating the nitrogen cycle in surface waters. Finally, the loss of bioavailable nitrogen via denitrification/anammox would leave the residual nitrogen pool 15N-enriched, and subsequent biomass assimilating the residual NO3− in the photic zone acquired the isotopic signature with positive values δ15N of > 2‰.

The uppermost of this unit is characterized by a shift in δ15N towards lighter values within −2‰ to 1‰, which fall within the range typical of N2 fixation via Mo-based nitrogenase (Ader et al. 2014; Stüeken et al. 2016). Since N2-fixation takes place when bioavailable nitrogen (nitrate and ammonia) is limited, the presence of N2-fixation signatures may indicate a shrink of the NO3− pool in shallow waters induced by intense denitrification and/or anammox. However, as only two samples were collected for this interval, the data may not have fully recorded the nitrogen isotopic signatures. The interpretation of N2 fixation as a dominant nitrogen uptake pathway awaits further confirmation in future work.

Compared to the LU, the less variable δ13Corg signatures of this unit are consistent with the stabilization of the chemocline during deposition. The δ13Corg values are slightly higher than the highly negative values (as low as to −37.5‰) of LU, possibly due to a decline in upwelling intensities. Alternatively, the slightly higher values may reflect the enhancement of primary production relative to secondary (chemoautotrophic and/or methanotrophic) production to TOC.

5.2.2 Well Tadong 2 (eastern Tarim)

The δ15N values in both the Xishanbulake and Xidashan Formations show limited variations, ranging from −2‰ to 1‰ (Table 2; Fig. 4). The range and average δ15N (−0.7‰ and −0.9‰ for the Xishanbulake and Xidashan Formation, respectively) of the two formations are similar, indicating no major changes in nitrogen cycling during deposition. There are two possible explanations for the δ15N values throughout the studied interval. First, Mo-based N2 fixation was the major nitrogen source for the local biota. In this case, limited isotopic fractionation imparted by this process would generate nitrogen isotopic signatures close to that of atmospheric N2 (δ15N = 0‰) (Zerkle et al. 2008; Stüeken 2013; Ader et al. 2014). Second, an overall oxic seawater condition with denitrification entirely restricted to sediments could explain the near-zero δ15N values. In this scenario, quantitative denitrification of nitrate would cause no isotopic fractionation, and the isotopic composition of nitrate assimilated by biomass would be close to that of fixed nitrogen (Algeo et al. 2014; Ader et al. 2016).

Both the Xishanbulake and Xidashan Formations of the Well Tadong 2 are highly enriched in Mo and U, with high MoEF and UEF values indicative of predominately anoxic/euxinic bottom water conditions (Guo et al. 2023). Moreover, the Xishanbulake and Xidashan Formations from Well Tadong 1 with deeper water depth were also interpreted to have been deposited in anoxic bottomwater conditions (Guo 2023). The prevalence of anoxic bottom waters in the offshore sites suggests that the deep waters in the Tarim region during the Early Cambrian would have been anoxic. This indicates the seawater in the basin was likely redox-stratified similar to that of the Early Cambrian Nanhua Basin in South China (Jin et al. 2016; Li et al. 2020). A redox-stratified water column thus contradicts the scenario of a pervasively oxic ocean (i.e. the second scenario), which leaves N2 fixation as the most plausible explanation for the observed δ15N data. As the assimilation of molecular N2 into organic matter takes place at the expense of high energy cost during the breakdown of the N–N bond, it dominates the ecosystem when other nitrogen species are at low concentrations (Karl et al. 2002; Sohm et al. 2011). The modern Black Sea serves as an excellent example of such a scenario, where the extensive loss of bioavailable nitrogen (nitrate and ammonium) at the redox-transition zone hinders the accumulation of sufficient nitrate concentrations in surface waters, despite the high ammonium concentrations at depth (Ader et al. 2016). Collectively, the nitrogen isotopic signatures typical of N2 fixation are compatible with a limited oceanic NO3− reservoir in a redox-stratified ocean in the Tarim Basin during the Early Cambrian.

The small fluctuations in δ15N values in Well Tadong 2 may reflect the minor contribution of other nitrogen sources. Notably, concurrent negative excursions in δ15N and δ13Corg are observed at the lower part of the Xishanbulake Formation (Fig. 4). The δ13Corg excursion has been reported before for the Well Tadong 2, and is considered to be correlated to the BACE excursion marking the basal Cambrian, which has been widely identified in different continental margins including the Yangtze platform (Wang et al. 2022). The incursion of 12C-rich anoxic deep water into shallow waters linked to shoaling of chemocline and enhanced upwelling, accompanied by marine transgression, is invoked as the major mechanism responsible for the negative excursion (e.g., Wille et al. 2008; Chang et al. 2016; Wang et al. 2018a; Zhang et al. 2020b). Therefore, the synchronously negative excursion in both δ15N and δ13Corg may reflect the enhanced contribution of NH4+ assimilation in surface waters in response to increases in NH4+ and 12C supplies via the influx of anoxic deep waters. In this case, NH4+ assimilation, which represents a subordinate nitrogen source, would be intensified, producing a small negative excursion in δ15N values. Above the excursion, the δ15N increases to ca 1 ‰ at the top of the Xishanbulake Formation. The increase coincides with a lithology transition from shale to limestone indicative of lower sea level (Fig. 4). The sea-level fall would lead to a deepening of chemocline, subsequently suppressing the contribution of NH4+ assimilation and promoting denitrification and/or anammox, thereby causing an increase in δ15N values. (e.g., Ader et al. 2016; Hammarlund et al. 2017; Chang et al. 2019). The overlying Xidashan Formation is characterized by two negative excursions with comparable amplitude to that of the Xishanbulake Formation, while the δ13Corg values stayed invariant. Similarly, the shifts towards higher δ15N values are likely associated with lithological transitions suggestive of shallower water depths (Fig. 4). The small fluctuations can thus be readily explained by the same mechanism applied to the Xishanbulake Formation.

5.3 Implication for the Early Cambrian nitrogen cycle in the Tarim Basin

The two intervals studied in this research are not time-equivalent. The Xishanbulake and Xidashan Formations of Well Tadong 2 in eastern Tarim correspond to Fortunian to Stage 4, whereas the entire Yurtus Formation in northwestern Tarim corresponds to Fortunian to lower Stage 3 (Sect. 2). The lack of paleontological data and radiometric age constraints pose challenges in correlating these two intervals. However, valuable insights can still be gained through analysis of the nitrogen isotopic signatures of the two sections.

Our data indicate significant differences in nitrogen cycling between the near-shore and off-shore sections when the black chert-shale of the Yurtus Formation was deposited (Fig. 7). In the near-shore Shiairike section of northwestern Tarim Basin, our nitrogen isotope data demonstrated alternating anaerobic assimilation of NH4+ and denitrification/anammox in the shallow ocean during deposition of the LU. For the UU, with the reduction in upwelling intensity, nitrogen isotopic signatures featuring denitrification/anammox dominated. The nitrogen isotopic signatures of aerobic nitrogen cycling in the nearshore setting indicate at least a transient build-up of a local NO3− reservoir. By contrast, nitrogen signatures suggest persistent biological N2-fixation in the studied unit of the off-shore Well Tadong 2. As the studied interval of Well Tadong 2 encompasses the time interval of the entire Yurtus Formation at Shiairike, the persistency of N2-fixation signatures of Well Tadong 2 indicates limited NO3− in the surface waters in the off-shore section during the deposition of the Yurtus black chert-shale. The observed spatial difference in nitrogen cycles is somewhat like the Mesoproterzoic ocean, where aerobic nitrogen cycle was restricted to nearshore settings whereas anaerobic nitrogen cycle dominated offshore environments (Stüeken 2013; Koehler et al. 2017). However, unlike the Precambrian Ocean with relatively low NH4+ concentrations (Stüeken et al. 2016), deep water with high concentrations of NH4+ may have been pervasive in in the Early Cambrian Ocean due to widespread euxinic waters along continental margins. During organic matter remineralization via microbial sulfate reduction, organic nitrogen can be converted to NH4+. As sulfate is not a strong enough oxidant to oxidize NH4+, NH4+ can accumulate in euxinic settings, allowing its build-up to high concentrations at depth (e.g., Chen et al. 2019; Xu et al. 2020). In this case, strong upwelling of deep, NH4+-rich waters would lead to enhanced NH4+ assimilation in coastal areas, generating highly negative δ15N values as observed in the LU of the Shiairike section.

Fig. 7
figure 7

Schematic diagram showing nitrogen cycle and ocean redox structure during deposition of the black shale-chert of Yurtus Formation. a Corresponds to the interval during which the LU was deposited. Red line: major nitrogen-cycling pathway when upwelling was strong; blue line: nitrogen-cycling pathway when upwelling was less intensive. b Corresponds to the interval when the UU was deposited

Full size image

Based on nitrogen isotopic data from a variety of sedimentary facies in South China, it has been proposed that the Early Cambrian witnessed the waxing and waning of the oceanic NO3− pool. Notably, a modern-like aerobic nitrogen cycle in response to the expansion of an oceanic NO3− reservoir was established in most of Fortunian and early Stage 3 (Wang et al. 2018a). If a large, stable NO3− pool also existed in the Tarim Basin during the Early Cambrian, sediments deposited in deep waters would be expected to exhibit high δ15N values. Although data from Well Tadong 2 may not be fully representative of the evolution in nitrogen cycling in the deep water regime of the Tarim Basin, the available data, which show nitrogen signatures typical of N2-fixation, seems to be more consistent with a scenario involving a limited nitrated reservoir in the basin when the Xishanbulake and Xidashan Formations were deposited (Fortunian to Age 4). This interpretation is not consistent with the model of episodic expansion of a large NO3− pool proposed by Wang et al. (2018a, b). The inconsistency may be attributed to (1) spatial heterogeneity in nitrogen cycling in different basins; (2) the Early Cambrian ocean is characterized by a NO3− pool much smaller than proposed by Wang et al. (2018a, b). A recent compilation of nitrogen data from South China indicated that the build-up of NO3− reservoirs may have been only restricted to local settings during the Cambrian Age 3 (Xu et al. 2020), even during the peak of the Cambrian explosion (Chang et al. 2022). The new findings point to a scenario with a much smaller NO3− pool of the Early Cambrian ocean than that of the modern ocean; (3) It is also possible that sampling bias has led to the absence of highly positive δ15N signatures. Nonetheless, a clearer understanding of the nitrogen cycling pattern across the Tarim Basin and the global ocean during the Early Cambrian awaits future research involving a wider range of sedimentary facies from continental margins.

6 Conclusions

The present study presents paired δ15N and δ13Corg data from the Lower Cambrian Yurtus Formation of the Shiairike section in the northwest Tarim Basin (inner ramp), as well as the Xishanbulake-Xidashan Formations in the eastern Tarim Basin (deep shelf/basin). Based on the data, the following information can be obtained.

The black chert-shale unit within the Yurtus Formation is characterized by a shoaling chemocline associated with upwelling and incursion of deep, anoxic waters during a transgression. The unit documented a shift in the nitrogen cycle processes. Specifically, the lower part of the unit exhibits highly variable δ15N values, suggesting alternating dominance between anaerobic assimilation of NH4+ and denitrification/anammox, occurring against the backdrop of intense upwelling and an unstable chemocline. With the decrease in upwelling intensity, anaerobic assimilation of NH4+ weakened and denitrification/anammox began to dominate when the upper part was deposited. In contrast, the δ15N values of the Xishanbulake and Xidashan Formations in the eastern Tarim Basin indicate that N2 fixation served as the primary nitrogen source, while small fluctuations in δ15N may reflect minor contributions from other nitrogen sources.

The two units investigated in the Shiairike section and Well Tadong 2 are not temporally equivalent, as the latter encompasses a longer time interval. However, for the time interval when the black chert-shale unit of the Yurtus Formation at Shiairike was deposited, spatial variation in the nitrogen cycle was observed in these sections, which aligns with an ocean characterized by redox stratification. The accumulation of a NO3− reservoir and aerobic nitrogen cycling in seawater primarily occurred in near-shore settings, while anaerobic nitrogen cycling, dominated by N2 fixation, served as the main nitrogen uptake pathway in offshore environments.