Method for identifying moraine based on nitrate ozone isotope

Abstract

The invention discloses a method for identifying ancient tillite based on nitrate ozone isotope, which comprises the following steps: carrying out primary crushing on an ancient sedimentary rock sample to obtain 60-mesh sample particles; carrying out ultrasonic cleaning on the sample particles by using ultrapure water, drying, and carrying out secondary crushing to obtain 200-300-mesh sample powder; nitrate is extracted from the sample powder in an ultrapure water soaking mode, and supernate containing the nitrate is obtained; taking out V1 volume of supernate from the supernate containing the nitrate, determining the nitrate content in the V1 volume of supernate, and obtaining the optimal supernate sampling amount V2 of the to-be-tested sample on a machine based on the nitrate content in the V1 volume of supernate; taking out V2 volume of supernate from the remaining supernate, and testing on a machine to obtain a nitrate nitrogen and oxygen isotope data result of the sample to be tested, so as to obtain an ancient tillite identification result. According to the method, the ancient tillite is identified based on the nitrate oxygen isotope non-mass effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of isotope analysis and testing, and particularly relates to a method for identifying ancient tillite based on the triple oxygen isotopes of nitrate. Background Art

[0002] During the process of the Earth's evolutionary history, there have been multiple icehouse climates, entering the ice age, forming regional and global large glaciers. During the Early Proterozoic and Neoproterozoic, the entire Earth was completely covered by ice and snow twice, forming the "Snowball Earth". The extremely cold glacial climate caused a large-scale extinction of the Earth's organisms, and the warming of the temperature and the melting of glaciers brought a large amount of oxygen-rich and nutrient substances, resulting in a biological explosion. The latest research reveals that the formation times of the two Snowball Earths in the Paleoproterozoic and Neoproterozoic are exactly coincident with the occurrence times of the Great Oxidation Event (GOE) in the Paleoproterozoic and the Neoproterozoic Oxidation Event (NOE). The Snowball Earth event in the Proterozoic may have triggered the Great Oxygen Event and promoted the explosive evolution of the Earth's organisms.

[0003] Tillite is a kind of glacial sediment, mainly composed of diamictite, and glacial striations often develop on the surface of gravels. The glacial striations on the gravels are sometimes very difficult to discover and identify, and it is very difficult to distinguish tillite from other diamictites of different origins only based on the sediment composition and structural features. In addition, glacial sediment diamictite (tillite) is only a type of sedimentary rock in the proximal glacial deposits, and dropstones do not develop in the distal glacial deposits. Therefore, the traditional method of judging whether it is of glacial sediment origin based on whether there are glacial striations on the surface of gravels cannot be used, resulting in many tillites and glacial actions being unable to be identified, and the role of glacial actions such as the Snowball Earth in the Earth's evolution being seriously underestimated.

[0004] Atmospheric nitrate is the mineral with the largest oxygen isotope non-mass fractionation (Δ 17 O) on the Earth. The nitrogen-oxygen bond is very stable. Under normal temperature, pressure, pH value, and Eh value conditions, it does not undergo oxygen isotope exchange with the medium water, and it is the most sensitive tracer for tracing the early atmospheric photochemical reaction and oxygenation process on the Earth. The simulation experiment results show that photochemical reactions in the early Earth's oxygen-poor atmosphere can produce nitrates with oxygen isotope non-mass fractionation effects. Atmospheric nitrates can accumulate in large amounts in ice and snow during the ice age and be rapidly released after the ice and snow melt; the atmospheric nitrates in the meltwater of ice and snow can be adsorbed by clay minerals, iron and manganese oxides, etc. and be preserved in glacial sediments for a long time. Therefore, tillite can preserve atmospheric nitrates and has an obvious oxygen isotope non-mass fractionation effect.

[0005] However, due to the influence of late geological processes, modern atmospheric nitrate, and agricultural production pollution, ancient sedimentary rock samples such as tillite collected will be contaminated. If the collected ancient sedimentary rock samples are directly tested after being crushed, since there is nitrate from pollution in the test samples, the test results will surely not be able to truly reflect the original data when the ancient sedimentary rock samples such as tillite were formed. There is still no method in the existing technology that can exclude the influence of late geological processes and modern atmospheric nitrate pollution, extract trace original sedimentary nitrate from ancient sedimentary rocks such as tillite, and accurately analyze the nitrogen and oxygen isotopes of nitrate. Summary of the Invention

[0006] In view of the above analysis, the embodiments of the present invention aim to provide a method for identifying ancient tillite based on the three oxygen isotopes of nitrate to solve the above problems existing in the prior art.

[0007] The object of the present invention is achieved as follows:

[0008] A method for identifying ancient tillite based on the three oxygen isotopes of nitrate, comprising:

[0009] Step 1: Perform the first crushing on the ancient sedimentary rock sample to obtain sample particles of the first particle size; ultrasonically clean and dry the sample particles of the first particle size with ultrapure water; perform the second crushing on the cleaned and dried sample particles of the first particle size to obtain sample powder of the second particle size;

[0010] Step 2: Extract nitrate from the sample powder of the second particle size by soaking in ultrapure water to obtain a supernatant containing nitrate; wherein, the first particle size is 3 - 5 times the second particle size;

[0011] Step 3: First take out a volume V1 of the supernatant in the supernatant containing nitrate in Step 2, measure the nitrate content in the volume V1 of the supernatant, and obtain the optimal supernatant sampling volume V2 for the sample to be tested for on - machine testing based on the nitrate content in the volume V1 of the supernatant;

[0012] Step 4: Then take out a volume V2 of the supernatant from the remaining supernatant in Step 3 for on - machine testing to obtain the nitrate nitrogen and oxygen isotope data results of the sample to be tested; based on the nitrate nitrogen and oxygen isotope data results, obtain the ancient tillite identification result.

[0013] Further, the first particle size is 60 mesh, and the second particle size is 200 - 300 mesh.

[0014] Further, Step 2 is specifically: soak the sample powder of 200 - 300 mesh in ultrapure water to transfer the nitrate adsorbed in the sample powder to the ultrapure water, and obtain a supernatant containing nitrate through ultrasonic oscillation and centrifugation operations.

[0015] Further, in Step 2, the soaking time in ultrapure water is 30 ± 5 min; the ultrasonic oscillation time is 45 min; the centrifugation operation time is 15 min, and the rotation speed is 3800 r / min.

[0016] Further, in Step 3, the calculation formula for the optimal supernatant sampling volume V2 is:

[0017] V2 = (C b × V b ) / n;

[0018] wherein, V2 is the optimal supernatant sampling volume, ml; C b is the concentration of the nitrate standard sample added during the test process, mol / ml; V b is the added volume of the nitrate standard sample, ml; n is the nitrate content in the supernatant of volume V1, mol / ml.

[0019] Further, the nitrate content in the supernatant of volume V1 is determined by the bacterial denitrification method.

[0020] Further, in Step 4, determine whether the ancient sedimentary rock to be tested is a tillite according to the following principle:

[0021] When Δ 17 O ≥ 5‰, it indicates that there is an obvious 17 O anomaly of nitrate, then this sample is an ancient tillite;

[0022] When Δ 17 O < 5‰, it indicates that there is no obvious 17 O anomaly of nitrate, then this sample is not an ancient tillite.

[0023] Further, in Step 1, before the first crushing of the ancient sedimentary rock sample, it also includes a pretreatment step for the collected ancient sedimentary rock sample:

[0024] Excise the exposed surface or the surface part affected by later stages of the ancient sedimentary rock sample.

[0025] Further, the pretreatment step for the collected ancient sedimentary rock sample also includes:

[0026] Ultrasonically clean and dry the pretreated ancient sedimentary rock sample with ultrapure water.

[0027] Further, in Step 4, use a MAT253 mass spectrometer to test the supernatant of volume V2.

[0028] Compared with the prior art, the method for identifying ancient tillite based on the triple oxygen isotopes of nitrate provided by the present invention can exclude the influence of late geological processes and the pollution of modern atmospheric nitrate, extract trace amounts of original sedimentary nitrate from ancient sediments such as tillite, and accurately analyze the nitrogen and oxygen isotopes of nitrate. It is a method for identifying tillite by using the non-mass effect of the oxygen isotopes of trace nitrates in sedimentary rocks such as ancient tillite, providing important technical support and geological records for tracing large glacial events such as Snowball Earth and revealing the genetic connection between Snowball Earth and the Great Oxidation Event by using the non-mass fractionation effect of the oxygen isotopes of nitrates in sedimentary formations such as tillite, which is of great significance.

[0029] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combined solutions. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained from the content specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present specification or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only some of the embodiments recorded in the embodiments of the present specification. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0031] Figure 1 It is a schematic flow chart of the operation of the method for identifying ancient tillite based on the triple oxygen isotopes of nitrate provided by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0033] For ease of understanding the embodiments of the present application, the following will further explain and illustrate with specific embodiments in conjunction with the accompanying drawings. The embodiments do not constitute a limitation to the embodiments of the present application. In the accompanying drawings, for the purpose of clarity and / or description, the dimensions and relative dimensions of components may be exaggerated. When the exemplary embodiments can be implemented differently, the specific process sequences may be executed in a different order than described. For example, two consecutively described processes may be executed substantially simultaneously or in an order opposite to the described order. In addition, the same reference numerals denote the same components.

[0034] The terms used herein are for the purpose of describing specific embodiments and are not intended to be restrictive. As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an", and "the" are also intended to include the plural forms. In addition, when the terms "comprise" and / or "include" and their variants are used in this specification, it is stated that there are the stated features, wholes, steps, operations, components, assemblies, and / or groups thereof, but it does not exclude the existence or addition of one or more other features, wholes, steps, operations, components, assemblies, and / or groups thereof. It should also be noted that, as used herein, the terms "substantially", "about", and other similar terms are used as approximate terms rather than degree terms, so they are used to explain the inherent deviations of measured values, calculated values, and / or provided values that those of ordinary skill in the art will recognize.

[0035] Embodiment 1

[0036] Existing studies have proven that: in the early Earth's oxygen-poor atmosphere, ultraviolet photochemical reactions can produce nitric acid with an oxygen isotope non-mass fractionation effect. During ice ages such as Snowball Earth, atmospherically derived nitrates accumulate in large quantities in ice and snow, and are rapidly released after the ice and snow melt. The atmospherically derived nitrates that enter the meltwater of ice and snow can be adsorbed by clay minerals, iron, manganese, and aluminum oxides, etc. The adsorption process does not change the nitrogen and oxygen isotope composition of nitrates, and is preserved in glacial sediments such as tillite for a long time.

[0037] Based on the above theoretical basis, a specific embodiment of the present invention, as Figure 1 shown, discloses a method for identifying ancient tillite based on the triple oxygen isotope of nitrate, which is a method for identifying ancient tillite based on the non-mass effect of oxygen isotope of nitrate. By extracting atmospherically derived nitrates from sediments such as tillite, analyzing the nitrogen and oxygen isotope composition of nitrates, and identifying ancient tillite according to the non-mass effect of oxygen isotope of nitrates.

[0038] The method for identifying ancient tillite based on the triple oxygen isotope of nitrate includes the following steps 1 to 4:

[0039] Step 1: Process the collected ancient sedimentary rock samples. First, crush the ancient sedimentary rock samples for the first time to obtain sample particles of 60 mesh; ultrasonically clean and dry the 60-mesh sample particles with ultrapure water; crush the cleaned and dried 60-mesh sample particles for the second time to obtain sample powder of 200 - 300 mesh. Such processing can eliminate the influence of later geological processes and modern pollution on the test results.

[0040] Since there are often multiple samples to be tested, in Step 1, first crush the collected multiple ancient sedimentary rock samples separately to 60 mesh to obtain 60-mesh sample particles, and the 60-mesh sample particles are the best particle size of ancient sedimentary rock samples; then ultrasonically clean and dry the best particle size ancient sedimentary rock sample particles with ultrapure water; perform a second crush on the washed and dried 60-mesh best particle size ancient sedimentary rock sample particles, and further crush the 60-mesh washed and dried sample particles to 200 - 300 mesh to obtain 200 - 300-mesh sample powder of multiple samples to be tested, and the 200 - 300-mesh sample powder is the target particle size ancient sedimentary rock sample powder.

[0041] Specifically, collect representative fresh outcrop glacial tillite and other sedimentary rock samples or ancient sedimentary rock samples in the core that may contain glacial tillite. Pretreat the collected ancient sedimentary rock samples indoors. The pretreatment is to cut off the surface part of the ancient sedimentary rock samples that is exposed to the surface or affected later (for example, groundwater or surface water such as rivers also carry some atmospheric nitrate or biological / artificially synthesized non-atmospheric nitrate, and such nitrate in the water flows through sedimentary rocks such as glacial tillite, thus causing pollution to the samples to be tested). Ultrasonically clean and dry the pretreated ancient sedimentary rock samples with ultrapure water. Then crush the dried ancient sedimentary rock samples to the best particle size of 60 mesh to obtain the best particle size ancient sedimentary rock sample particles; ultrasonically clean and dry the best particle size 60-mesh ancient sedimentary rock sample particles again with ultrapure water to further remove the influence of later geological processes and modern pollution on the composition of ancient sedimentary rocks, such as removing nitrate infiltrated into the ancient sedimentary rocks through voids / fissures in later stages; after completing the re-cleaning and drying of the best particle size ancient sedimentary rock samples, further crush the re-cleaned and dried ancient sedimentary rock samples with a particle size of 60 mesh to 200 - 300 mesh to obtain the target particle size ancient sedimentary rock sample powder for nitrate extraction. Perform the above operations on all samples to obtain the target particle size ancient sedimentary rock sample powder of all samples to be tested.

[0042] In this embodiment, the optimal particle size for the initial crushing of the ancient sedimentary rock sample is determined to be 60 mesh through conditional experiments. First, the sample is crushed into different particle sizes (such as four groups with particle sizes of 1 cm, 20 mesh = 0.85 mm, 60 mesh = 0.25 mm, and 200 mesh = 0.075 mm respectively). The samples with different particle sizes are ultrasonically cleaned and dried in ultrapure water to remove the nitrates that seeped in along the voids / fissures of the sedimentary rock in the later stage. Then, it is further crushed to 200 - 300 mesh to obtain the ancient sedimentary rock sample powder with the target particle size. The nitrate content and nitrogen and oxygen isotope composition of different powder samples are measured by the bacterial denitrification method, and based on the measurement results, the optimal crushing particle size is determined to be 60 mesh.

[0043] Step 2: Extract nitrates from the sedimentary rock sample particles crushed to 200 - 300 mesh by soaking in ultrapure water to obtain the supernatant containing nitrates.

[0044] The specific extraction steps are as follows: Soak the 200 - 300 - mesh sample powder of multiple samples to be tested in ultrapure water to transfer the nitrates adsorbed in the 200 - 300 - mesh sample powder into the ultrapure water. After ultrasonic oscillation and centrifugation operations, obtain the supernatant containing nitrates of multiple samples to be tested.

[0045] Exemplarily, accurately weigh a quantitative 20 g of the ancient sedimentary rock sample powder with the target particle size and put it into a 100 - mL beaker; add 10 mL of ultrapure water to soak the 20 g of the ancient sedimentary rock sample powder with the target particle size in the 100 - mL beaker. The soaking time of the ultrapure water is 30 ± 5 min. Since the nitrate content in the ultrapure water is extremely low and nitrates are easily soluble in water, adding ultrapure water can transfer the nitrates adsorbed in the sedimentary rock sample into the ultrapure water. Then, put sedimentary rock samples such as tillite into a water bath for ultrasonic oscillation for 45 min, transfer them to a centrifuge tube for centrifugation. The centrifugation operation time is 15 min, and the rotation speed is 3800 r / min. After centrifugation, transfer the supernatant, and record the volume as V. Among them, through the cleaning of the ancient sedimentary rock sample in Step 2, a large amount of nitrate adsorbed in the ancient sedimentary rock sample to be tested can be removed, and the removal rate is up to more than 90%, and it will not affect the nitrogen and oxygen isotope composition of nitrate.

[0046] Step 3: First, take out a volume V1 of the supernatant from the supernatant obtained in Step 2, measure the nitrate content in the volume V1 of the supernatant, and obtain the optimal supernatant sampling volume V2 for the sample to be tested for machine testing based on the nitrate content in the volume V1 of the supernatant;

[0047] When testing multiple samples, take out V1 volume of supernatant from the supernatant of all samples to be tested obtained in step 2. The value of V1 for the supernatant taken out from the supernatant of all samples to be tested is equal. Measure the nitrate content in each V1 volume of supernatant, and calculate the optimal supernatant sampling volume V2 for each sample to be tested for machine testing based on the nitrate content in each V1 volume of supernatant.

[0048] The applicant found during the analysis and testing process that due to the large difference in nitrate content in each sample to be tested and being limited by the detection limit range of the instrument itself (the detection limit range includes the lowest detection limit and the highest detection limit, and within this detection limit interval is the most ideal and the test results are more accurate, otherwise it is difficult to obtain accurate test results), if the nitrate content in the supernatant is too high or too low, ideal experimental results cannot be obtained. Therefore, in order to obtain accurate and ideal experimental test results, it is necessary to pre-calculate the optimal supernatant sampling volume. The optimal supernatant sampling volume is to make the target analyte in the solution of the sample to be tested for machine testing fall within the detection limit interval of the instrument, so as to obtain accurate and precise test results.

[0049] Specifically, after obtaining the supernatant of all samples to be tested in step 2, first take out V1 volume of supernatant from the supernatant of each sample to be tested. For example, V1 = 2 mL. Use the bacterial denitrification method to measure the nitrate content n in the 2 mL supernatant taken out from all samples to be tested, and calculate the optimal supernatant sampling volume V2 based on the first measurement result.

[0050] The calculation formula for the optimal supernatant sampling volume V2 is:

[0051] V2 = (C b ×V b ) / n;

[0052] In the above formula, V2 is the optimal supernatant sampling volume, mL; C b is the concentration of the nitrate standard sample added during the test process, mol / mL; V b is the volume of the nitrate standard sample added, mL; n is the nitrate content in the V1 volume of supernatant, mol / mL. Preferably, the nitrate content in the V1 volume of supernatant is measured by the bacterial denitrification method.

[0053] Step 4: Take out another V2 volume of supernatant from the remaining supernatant in step 3 for machine testing to obtain the nitrate nitrogen and oxygen isotope data results of the sample to be tested; based on the nitrate nitrogen and oxygen isotope data results, obtain the identification result of the ancient tillite.

[0054] In this embodiment, supernatant sampling is performed twice successively. The supernatant with volume V1 in step 3 is the amount of the first supernatant sampling, and the optimal supernatant sampling amount V2 in step 4 is the amount of the second supernatant sampling.

[0055] When testing multiple samples, since the optimal supernatant sampling amount V2 of each sample is not exactly the same, therefore, in the remaining supernatant of each sample to be tested in step 3, take out the supernatant with volume V2 according to the calculation result of the optimal supernatant sampling amount V2 of each sample to be tested, perform on-machine testing on the supernatant with volume V2 to obtain the nitrate nitrogen and oxygen isotope data results of each sample to be tested; based on the nitrate nitrogen and oxygen isotope data results of the samples to be tested, obtain the identification results of ancient tillites for all samples to be tested.

[0056] Specifically, in step 3, the optimal supernatant sampling amount V2 has been calculated, and then accurately measure the respective optimal supernatant sampling amounts V2 of the samples to be tested in the remaining supernatant of each sample to be tested. Use the MAT253 mass spectrometer to test the supernatant with volume V2 of all samples to be tested to obtain the nitrate nitrogen and oxygen isotope test results of all samples to be tested. Determine whether it is a tillite according to the oxygen isotope non-mass fractionation effect of trace nitrates in the sedimentary rock to be tested, so as to identify ancient tillites according to the results, which is helpful for the research of processes such as the ancient Earth's atmospheric composition, surface chemistry, and atmospheric photochemistry.

[0057] In step 4, based on the nitrate nitrogen and oxygen isotope test results, determine whether the ancient sedimentary rock to be tested is a tillite according to the following principle:

[0058] When Δ 17 O ≥ 5‰, it indicates that there is an obvious 17 O anomaly in the nitrate, then this sample is an ancient tillite;

[0059] When Δ 17 O < 5‰, it indicates that there is no obvious 17 O anomaly in the nitrate, then this sample is not an ancient tillite.

[0060] Based on whether there is an 17 O anomaly in the nitrate in the ancient sedimentary rock, it can be used to infer the evolution process of the paleoenvironment: when there is an 17 O anomaly in the nitrate, it can be determined that ozone already existed in the ancient atmosphere, indicating that the oxygen content has reached a certain level. The paleoatmospheric composition and atmospheric evolution during the formation process of the sedimentary rock sample can be inferred, providing theoretical support for the evolution of habitable planets. When there is no 17 O anomaly in the nitrate, especially when Δ 17 O is close to 0, it indicates that the oxygen content in the paleoenvironment when the ancient sedimentary rock was formed was extremely low, which can be used as evidence for the absence or trace amount of oxygen in the ancient atmosphere.

[0061] Using the method of the embodiments of the present application, 10 ancient sedimentary rock samples collected were analyzed and tested. The test results of the nitrogen and oxygen isotopes of the supernatant of the 10 samples are shown in Table 1.

[0062] The test analysis of the nitrate nitrogen / oxygen isotope composition was carried out in the Key Laboratory of Metallogeny and Resource Evaluation, Ministry of Natural Resources, Institute of Mineral Resources, Chinese Academy of Geological Sciences, using a Thermo MAT253 mass spectrometer. Ultra-pure water (18 MΩcm -1 , Millipore, Billerica MA, USA), 100 mL beakers, adsorption container conical flasks (150 mL), and 50 mL centrifuge tubes were used throughout the experiment.

[0063] The collected samples include sedimentary rock samples such as Neoproterozoic tillite, sandstone, iron-manganese ore, dolomite, etc. and clay mineral samples such as illite, zeolite, sepiolite, etc.

[0064] Among the 13 samples shown in Table 1, 10 samples with sample numbers NO-1 to NO-10 are ancient tillite samples, and 3 samples with sample numbers NO-11 to NO-13 are non-tillite samples. The conclusion of whether the above 13 samples are tillite is known. And the identification results from the perspective of the test result data also verify that the identification of ancient tillite by the method of the present application is accurate and effective.

[0065] As can be seen from Table 1, the sampling volume of the first supernatant of the 13 samples is the same, while the sampling volume of the second supernatant is different, that is, the optimal supernatant sampling volume V2 is different, and the difference in the V2 value is obvious; from the perspective of the test result data, taking different amounts of supernatant from the same sample for testing, the obtained results are completely different. In some cases, due to the too high nitrate content in the sample exceeding the highest detection limit of the instrument, the obtained δ 17 O VSMOW (‰), δ 18 O VSMOW (‰), Δ 17 O(‰) results are completely untrustworthy (such as NO-1, NO-2, NO-3, NO-8 and NO-9). For tillite samples with too low nitrate content, taking 2 mL for testing, since it is lower than the lowest detection limit of the instrument, the obtained results are also untrustworthy (such as NO-4, NO-5 and NO-10). For samples with the optimal sampling volume close to 2 mL (that is, within the optimal detection limit range of the instrument) (such as NO-7), the two test results are consistent within the error range. Thus, it shows that the present application obtains the optimal supernatant sampling volume by testing the first supernatant, and then tests the supernatant with the optimal supernatant sampling volume, and finally the obtained results are more accurate.

[0066] Table 1 Oxygen isotope composition of tillite samples

[0067]

[0068] Compared with the prior art, the method for identifying ancient tillite based on nitrate triple oxygen isotopes provided by this embodiment has at least one of the following beneficial effects:

[0069] 1. In this application, the surface of the ancient sedimentary rock sample to be tested is first cut off, and then further crushed to 60 mesh to obtain ancient sedimentary rock sample particles with the optimal particle size. This treatment can wash away the foreign nitrate pollutants carried by fluids such as groundwater and surface water in the voids / fissures of the ancient sedimentary rock sample as much as possible, and completely separate it from the original sedimentary nitrate. Subsequently, the ancient sedimentary rock sample particles with the optimal particle size are ultrasonically cleaned again with ultrapure water and dried. After ultrasonic treatment and centrifugation with ultrapure water, the original sedimentary nitrate adsorbed in the sample can be completely separated without affecting the nitrogen and oxygen isotope composition of nitrate. The 60-mesh rock sample particles that have been washed and dried are further crushed to 200-300 mesh to obtain ancient sedimentary rock sample powders with the target particle size for multiple samples to be tested. In summary, through a series of treatments including surface cutting of the ancient sedimentary rock sample, followed by coarse particle size crushing (crushing to 60 mesh), washing and drying with ultrapure water, fine particle size crushing (crushing to 200-300 mesh), and removing the adsorbed nitrate by soaking in ultrapure water, this application can eliminate the influence of post-depositional geological processes such as tillite and the pollution of modern atmospheric origin nitrates. For example, it can effectively remove the nitrates infiltrating along the voids / fissures of sedimentary rocks such as tillite, without affecting the nitrogen and oxygen isotope composition of nitrate, which helps to obtain accurate test results.

[0070] 2. The optimal supernatant sampling volume is calculated based on the nitrate content in the supernatant of each sample to be tested. The optimal supernatant sampling volume falls within the range of the optimal detection limit of the instrument, and the nitrogen and oxygen isotope composition of the nitrate adsorbed in all samples to be tested is determined based on the optimal supernatant sampling volume. The obtained results are more accurate and have high credibility, avoiding the situation that some samples cannot obtain accurate test results due to large differences in the nitrate content in the samples.

[0071] 3. No chemical reagents such as acids and alkalis are used in the entire testing process of this application, which is economical, environmentally friendly, and low-cost. Moreover, the sedimentary rock samples after nitrate extraction can also be used for other geochemical index analyses, realizing the full utilization of precious rock samples.

[0072] 4. In the prior art, there is no literature documenting that the nitrate nitrogen and oxygen isotope composition in sedimentary rock samples such as tillite can trace the content and evolution of ancient atmospheric oxidation components. This patent first proposes that the nitrate nitrogen and oxygen isotope composition in sedimentary rock samples such as tillite can trace the content and evolution of ancient atmospheric oxidation components. The present invention is a geochemical method for identifying tillite by using the non-mass effect of trace original sedimentary nitrate oxygen isotopes in ancient sedimentary rocks such as tillite, which makes up for the deficiencies of traditional sedimentary petrology methods. It can accurately identify ancient tillite based on the non-mass effect of nitrate oxygen isotopes, and provides important technical support for revealing the genetic relationship between the snowball event and the great oxidation event by using the non-mass fractionation effect of nitrate oxygen isotopes in ancient sedimentary rocks such as tillite. It lays a solid foundation for related research on tracing the content and evolution of ancient atmospheric oxygen and other components, and provides evidence for the "habitable planet" plan.

[0073] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of this application. It should be understood that the above are only specific embodiments of this application and are not used to limit the protection scope of this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the protection scope of this application.

Claims

1. A method for identifying ancient tillites based on the triple oxygen isotope of nitrate, characterized in that, Including: Step 1: Conduct the first crushing on the ancient sedimentary rock sample to obtain sample particles with a first particle size; ultrasonically clean and dry the sample particles with the first particle size using ultrapure water; conduct the second crushing on the sample particles with the first particle size after cleaning and drying to obtain sample powder with a second particle size; Step 2: Extract nitrate from the sample powder with the second particle size by soaking in ultrapure water to obtain a supernatant containing nitrate; wherein, the first particle size is 3 - 5 times that of the second particle size; Step 3: First take out a volume V1 of the supernatant in the supernatant containing nitrate in Step 2, measure the nitrate content in the volume V1 of the supernatant, and obtain the optimal supernatant sampling volume V2 for the sample to be tested for machine testing based on the nitrate content in the volume V1 of the supernatant; Step 4: Take out a volume V2 of the supernatant from the remaining supernatant in Step 3 for machine testing to obtain the nitrate nitrogen and oxygen isotope data result of the sample to be tested; based on the nitrate nitrogen and oxygen isotope data result, obtain the identification result of the ancient tillite.

2. The method for identifying ancient tillite based on nitrate triple oxygen isotope according to claim 1, characterized in that The first particle size is 60 mesh, and the second particle size is 200 - 300 mesh.

3. The method for identifying ancient tillite based on nitrate triple oxygen isotope according to claim 2, characterized in that, Specifically, Step 2 is: soak the sample powder with a particle size of 200 - 300 mesh in ultrapure water to transfer the adsorbed nitrate in the sample powder to the ultrapure water, and obtain a supernatant containing nitrate through ultrasonic oscillation and centrifugation operations.

4. The method for identifying ancient tillite based on nitrate triple oxygen isotope according to claim 3, characterized in that, In Step 2, the soaking time of the ultrapure water is 30 ± 5 min; the ultrasonic oscillation time is 45 min; the centrifugation operation time is 15 min, and the rotation speed is 3800 r / min.

5. The method for identifying ancient tillite based on nitrate triple oxygen isotopes according to claim 1, characterized in that, In Step 3, the calculation formula for the optimal supernatant sampling volume V2 is: V2 = (C b × V b ) / n; Among them, V2 is the sampling volume of the best supernatant, ml; C b is the concentration of the nitrate standard sample added during the test process, mol / ml; V b is the added volume of the nitrate standard sample, ml; n is the nitrate content in the supernatant with a volume of V1, mol / ml.

6. The method for identifying ancient moraine rocks based on nitrate triple oxygen isotopes according to claim 5, characterized in that, The nitrate content in the volume V1 of the supernatant is measured by the bacterial denitrification method.

7. The method for identifying ancient moraine rocks based on nitrate triple oxygen isotopes according to claim 1, wherein In Step 4, determine whether the ancient sedimentary rock to be tested is tillite according to the following principle: When Δ 17 O ≥ 5‰, it indicates that there is an obvious 17 O anomaly, and then this sample is an ancient tillite; When Δ 17 O < 5‰, it indicates that there is no obvious 17 O anomaly, and thus this sample is not an ancient tillite.

8. The method for identifying ancient tillite based on nitrate triple oxygen isotope according to claim 1, wherein In Step 1, before conducting the first crushing on the ancient sedimentary rock sample, it also includes a pretreatment step for the collected ancient sedimentary rock sample: Cut off the exposed surface or the surface part affected by later stages of the ancient sedimentary rock sample.

9. The method for identifying ancient moraine rocks based on nitrate triple oxygen isotopes according to claim 8, characterized in that The pretreatment step for the collected ancient sedimentary rock sample also includes: Ultrasonically clean and dry the pretreated ancient sedimentary rock sample with ultrapure water.

10. The method for identifying ancient moraine rocks based on nitrate triple oxygen isotopes according to claim 1, wherein In Step 4, use a MAT253 mass spectrometer to test the volume V2 of the supernatant.

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