A method for identifying ancient moraines based on nitrate trioxygen isotopes

Abstract

The present invention discloses a method for identifying ancient glacial till based on nitrate trioxygen isotopes, comprising the following steps: performing a first crushing on an ancient sedimentary rock sample to obtain 60-mesh sample particles; ultrasonically cleaning and drying the sample particles with ultrapure water, and performing a second crushing to obtain a 200-300-mesh sample powder; extracting nitrate from the sample powder by soaking in ultrapure water to obtain a supernatant containing nitrate; first taking out V1 volume of supernatant from the supernatant containing nitrate, measuring the nitrate content in V1 volume of supernatant, and obtaining an optimal supernatant sampling volume V2 for testing the sample on a machine based on the nitrate content in V1 volume of supernatant; taking out V2 volume of supernatant from the remaining supernatant for testing on a machine, obtaining nitrate nitrogen and oxygen isotope data results of the sample to be tested, and then obtaining an ancient glacial till identification result. The present invention realizes the identification of ancient glacial till based on the non-mass effect of nitrate oxygen isotopes.

Description

Technical Field

[0001] The present invention belongs to the technical field of isotope analysis and testing, and in particular relates to a method for identifying ancient moraines based on nitrate trioxygen isotopes. Background Art

[0002] Throughout Earth's evolutionary history, icehouse climates have occurred numerous times, leading to ice ages and the formation of regional and global glaciers. During the Early Proterozoic and Neoproterozoic eras, the entire Earth was completely covered in ice and snow, creating a "Snowball Earth." Extremely cold glacial climates caused mass extinctions on Earth, but warming temperatures and melting glaciers brought a flood of oxygen-rich, nutrient-rich waters, triggering a biological explosion. Recent research reveals that the formation of these two "Snowball Earths" during the Paleoproterozoic and Neoproterozoic periods coincides with the Paleoproterozoic Oxidation Event (GOE) and the Neoproterozoic Oxidation Event (NOE). The Proterozoic Snowball Event may have triggered the Great Oxygenation Event, fueling the explosive evolution of life on Earth.

[0003] Moraine is a glacial deposit, mainly composed of conglomerate rocks, and glacial scratches often develop on the surface of the gravel. Glacial scratches on gravel are sometimes difficult to find and identify, and it is difficult to distinguish moraine from conglomerate rocks of other origins based solely on the sediment composition and structural structure. In addition, glacially deposited conglomerate rocks (moraine) are only a type of sedimentary rock deposited near the source of the glacier. Falling rocks are not developed in the sediments of the distant source of the glacier. Therefore, the traditional method of judging whether there are glacial scratches on the surface of the gravel cannot be used to determine whether it is of glacial deposition origin. As a result, many moraines and glacial processes cannot be identified, resulting in the serious underestimation of the role of glacial processes such as the Snowball Earth in the evolution of the Earth.

[0004] Atmospheric nitrate is the result of the mass-independent fractionation of oxygen isotopes on Earth (Δ 17 O) is the largest mineral with a very stable nitrogen-oxygen bond. Under normal temperature, pressure, pH, and Eh conditions, it does not exchange oxygen isotopes with the surrounding water. This makes it the most sensitive tracer for photochemical reactions and oxygenation in Earth's early atmosphere. Simulation experiments indicate that photochemical reactions in Earth's early oxygen-depleted atmosphere can produce nitrates with a mass-independent oxygen isotope fractionation effect. Atmospheric nitrates can accumulate in large quantities within ice and snow during glacial periods and be rapidly released upon melting. Atmospheric nitrates in meltwater can be adsorbed by clay minerals, iron-manganese oxides, and other minerals, preserving them for long periods in glacial sediments. Therefore, glacial till can preserve atmospheric nitrates with a significant mass-independent oxygen isotope fractionation effect.

[0005] However, due to the influence of later geological processes, modern atmospheric nitrates, and agricultural pollution, collected ancient sedimentary rock samples can be contaminated. If these ancient sedimentary rock samples are directly tested after being crushed, the test results will inevitably not truly reflect the original data of the ancient sedimentary rock samples at the time of their formation due to the presence of nitrates introduced by the contamination. The existing technology does not yet have a method that can eliminate the influence of later geological processes and modern atmospheric nitrate contamination, extract trace amounts of original sedimentary nitrate from ancient sedimentary rocks such as glacial moraines, and accurately analyze the nitrogen and oxygen isotopes of the nitrates. 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 moraines based on nitrate trioxygen isotopes, so as to solve the above problems existing in the prior art.

[0007] The object of the present invention is achieved like this:

[0008] A method for identifying ancient moraines based on nitrate trioxygen isotopes, comprising:

[0009] Step 1: The ancient sedimentary rock sample is crushed for the first time to obtain sample particles of a first particle size; the sample particles of the first particle size are ultrasonically cleaned with ultrapure water and dried; the washed and dried sample particles of the first particle size are crushed for the second time to obtain sample powder of a second particle size;

[0010] Step 2: Extracting 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 V1 volume of supernatant from the supernatant containing nitrate in step 2, measure the nitrate content in V1 volume of supernatant, and obtain the optimal supernatant sampling volume V2 for testing the sample on the machine based on the nitrate content in V1 volume of supernatant;

[0012] Step 4: Take out V2 volume of supernatant from the remaining supernatant in step 3 for testing on the machine 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 moraine identification results.

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

[0014] Furthermore, step 2 specifically comprises: soaking the sample powder of 200-300 mesh in ultrapure water to transfer the nitrate adsorbed in the sample powder into the ultrapure water, and performing ultrasonic vibration and centrifugation to obtain a supernatant containing nitrate.

[0015] Furthermore, in step 2, the ultrapure water immersion time is 30±5 min; the ultrasonic vibration time is 45 min; the centrifugal operation time is 15 min, and the rotation speed is 3800 r / min.

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

[0017] V2=(C b ×V b ) / n;

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

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

[0020] Furthermore, in step 4, whether the ancient sedimentary rock to be tested is moraine is determined according to the following principles:

[0021] When Δ 17 When O≥5‰, it indicates that there is obvious presence of nitrate. 17 If O is abnormal, the sample is an ancient moraine;

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

[0023] Furthermore, in step 1, before the ancient sedimentary rock sample is crushed for the first time, a pre-processing step is also included for the collected ancient sedimentary rock sample:

[0024] Excision of an ancient sedimentary rock sample to expose the surface or the surface portion that has been subjected to later influences.

[0025] Furthermore, the pre-processing step of the collected ancient sedimentary rock samples also includes:

[0026] The pretreated ancient sedimentary rock samples were ultrasonically cleaned with ultrapure water and dried.

[0027] Furthermore, in step 4, the V2 volume supernatant was tested using a MAT253 mass spectrometer.

[0028] Compared with the existing technology, the method for identifying ancient glacial till based on nitrate oxygen isotopes provided by the present invention can eliminate the influence of later geological actions and nitrate pollution of modern atmospheric origin, extract trace original sedimentary nitrate from ancient sediments such as glacial till, and accurately analyze nitrate nitrogen and oxygen isotopes. It is a method for identifying glacial till by utilizing the non-mass effect of trace nitrate oxygen isotopes in sedimentary rocks such as ancient glacial till, and provides important technical support and geological records for utilizing the non-mass fractionation effect of nitrate oxygen isotopes in sedimentary structures such as glacial till to trace large glacial events such as the Snowball Earth and reveal the causal relationship between the Snowball event and the Great Oxidation Event, which is of great significance.

[0029] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of the present invention will be described in the following description, and some advantages will become apparent from the description or be learned through practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0031] Figure 1 Schematic diagram of the operating process of the method for identifying ancient moraines based on nitrate trioxygen isotopes provided by the present invention. DETAILED DESCRIPTION

[0032] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0033] To facilitate understanding of the embodiments of the present application, the following will be further explained with reference to specific embodiments in conjunction with the accompanying drawings. The embodiments do not constitute a limitation of the embodiments of the present application. In the accompanying drawings, the sizes and relative sizes of the components may be exaggerated for clarity and / or descriptive purposes. When the exemplary embodiments can be implemented differently, the specific process sequence may be performed in an order different from that described. For example, two consecutively described processes may be performed substantially simultaneously or in an order opposite to that described. In addition, the same reference numerals represent 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 "one (kind, person)" and "said (the)" are also intended to include plural forms. In addition, when the terms "comprise" and / or "include" and their variations are used in this specification, the features, integral bodies, steps, operations, parts, assemblies and / or their groups stated are explained, but the presence or addition of one or more other features, integral bodies, steps, operations, parts, assemblies and / or their groups is not excluded. It should also be noted that, as used herein, the terms "substantially", "approximately" and other similar terms are used as approximate terms and not as degree terms, so that they are used to explain the inherent deviations of the measured values, calculated values ​​and / or the values ​​provided that will be recognized by those of ordinary skill in the art.

[0035] Example 1

[0036] Studies have shown that ultraviolet photochemical reactions in the oxygen-depleted atmosphere of early Earth could produce nitric acid with non-mass fractionation of oxygen isotopes. During ice ages like the Snowball Earth, atmospheric nitrates accumulated in large quantities within ice and snow, and were rapidly released upon melting. These atmospheric nitrates, once absorbed into meltwater, were adsorbed by clay minerals and iron, manganese, and aluminum oxides. This adsorption process did not alter the nitrogen and oxygen isotope compositions of the nitrates, allowing them to be preserved for long periods in glacial sediments such as moraines.

[0037] Based on the above theoretical basis, a specific embodiment of the present invention is as follows: Figure 1 As shown, a method for identifying ancient glacial till based on nitrate oxygen isotopes is disclosed. It is a method for identifying ancient glacial till based on the non-mass effect of nitrate oxygen isotopes. The method extracts atmospheric nitrate from sedimentary rocks such as glacial till, analyzes the nitrogen and oxygen isotope composition of nitrate, and identifies ancient glacial till based on the non-mass effect of nitrate oxygen isotopes.

[0038] The method for identifying ancient moraines based on nitrate trioxygen isotopes 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 60-mesh sample particles; ultrasonically clean the 60-mesh sample particles with ultrapure water and dry them; and crush the cleaned and dried 60-mesh sample particles for a second time to obtain 200-300-mesh sample powder. This treatment can eliminate the influence of later geological effects and modern pollution on the test results.

[0040] Since there are often multiple samples to be tested, in step 1, the multiple collected ancient sedimentary rock samples are first crushed to 60 mesh to obtain 60-mesh sample particles, which are the optimal particle size ancient sedimentary rock sample particles; then the optimal particle size ancient sedimentary rock sample particles are ultrasonically cleaned and dried with ultrapure water; the washed and dried 60-mesh optimal particle size ancient sedimentary rock sample particles are secondarily crushed, and the 60-mesh washed and dried sample particles are further crushed to 200-300 mesh to obtain 200-300 mesh sample powders of multiple samples to be tested, and the 200-300 mesh sample powders are the target particle size ancient sedimentary rock sample powders.

[0041] Specifically, representative fresh outcrop moraine and other sedimentary rock samples or ancient sedimentary rock samples that may contain moraine in the core are collected, and the collected ancient sedimentary rock samples are pretreated indoors. The pretreatment is carried out by removing the exposed surface of the ancient sedimentary rock samples or the surface part that has been affected by later periods (for example, groundwater or surface water such as rivers also carry some atmospheric nitrates or biological / artificially synthesized non-atmospheric nitrates. Such nitrates in the water flow through sedimentary rocks such as moraine, thereby causing contamination to the samples to be tested). The pretreated ancient sedimentary rock samples are ultrasonically cleaned with ultrapure water and dried. The dried ancient sedimentary rock sample is then crushed to an optimal particle size of 60 mesh to obtain the optimal particle size ancient sedimentary rock sample particles; the optimal particle size 60 mesh ancient sedimentary rock sample particles are ultrasonically cleaned and dried again with ultrapure water to further remove the effects of later geological processes and modern pollution on the ancient sedimentary rock composition, such as removing nitrates that have infiltrated from the pores / cracks of the sedimentary rock in the later period; after the optimal particle size ancient sedimentary rock sample is cleaned and dried again, the cleaned and dried ancient sedimentary rock sample with a particle size of 60 mesh is further crushed to 200-300 mesh to obtain the target particle size ancient sedimentary rock sample powder for nitrate extraction. The above operation is performed on all samples to obtain the target particle size ancient sedimentary rock sample powder for all samples to be tested.

[0042] In this example, a conditional experiment determined that the optimal particle size for initial crushing of ancient sedimentary rock samples was 60 mesh. The samples were first crushed into different particle sizes (e.g., four groups of particle sizes: 1 cm, 20 mesh = 0.85 mm, 60 mesh = 0.25 mm, and 200 mesh = 0.075 mm). The samples of varying particle sizes were ultrasonically cleaned and dried in ultrapure water to remove nitrates that had infiltrated through the pores and fissures of the sedimentary rock. The samples were then further crushed to 200-300 mesh to obtain the target particle size of the ancient sedimentary rock sample powder. The nitrate content and nitrogen and oxygen isotope composition of the different powder samples were determined using bacterial denitrification. Based on the measurement results, the optimal crushing particle size was determined to be 60 mesh.

[0043] Step 2: Nitrate is extracted from the sedimentary rock sample particles crushed to 200-300 meshes by soaking in ultrapure water to obtain a supernatant containing nitrate.

[0044] The specific extraction steps are: soaking 200-300 mesh sample powders of multiple samples to be tested in ultrapure water, transferring the nitrate adsorbed in the 200-300 mesh sample powders into the ultrapure water, and performing ultrasonic vibration and centrifugation to obtain the supernatant containing nitrate of the multiple samples to be tested.

[0045] For example, 20 g of a quantitative ancient sedimentary rock sample powder of the target particle size is accurately weighed and placed in a 100 mL beaker; 10 ml of ultrapure water is added to the 100 mL beaker containing 20 g of the ancient sedimentary rock sample powder of the target particle size for soaking, and the ultrapure water soaking time is 30±5 min. Since the nitrate content in ultrapure water is extremely low and nitrate is easily soluble in water, the addition of ultrapure water can transfer the nitrate adsorbed in the sedimentary rock sample to the ultrapure water. Then, the sedimentary rock sample such as moraine is placed in a water bath and ultrasonically shaken for 45 minutes, transferred to a centrifuge tube for centrifugation, and the centrifugation operation time is 15 minutes at a speed of 3800 r / min. After the centrifugation is completed, the supernatant is transferred and the volume is recorded as V. Among them, by cleaning 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, with a removal rate of more than 90%, and will not affect the nitrogen and oxygen isotope composition of the nitrate.

[0046] Step 3: first take out V1 volume of supernatant from the supernatant obtained in step 2, determine the nitrate content in V1 volume of supernatant, and obtain the optimal supernatant sampling volume V2 for testing the sample on the machine based on the nitrate content in V1 volume of supernatant;

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

[0048] During the analysis and testing process, the applicant found that due to the large differences in the nitrate content in each sample to be tested, and limited by the detection limit range of the instrument itself (the detection limit range includes the lowest detection limit and the highest detection limit. It is ideal within this detection limit range, 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 calculate the optimal supernatant sampling volume in advance. The optimal supernatant sampling volume is to make the target detection object in the test solution of the sample to be tested fall within the detection limit range of the instrument, so as to obtain accurate and precise test results.

[0049] Specifically, after obtaining the supernatant of all the samples to be tested in step 2, V1 volume of supernatant is first taken out from the supernatant of each sample to be tested, for example, V1 = 2 mL, and the nitrate content n in the 2 mL supernatant taken out from all the samples to be tested is determined by the bacterial denitrification method, and the optimal supernatant sampling volume V2 is calculated 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 The concentration of nitrate standard sample added during the test, mol / ml; V b is the amount of nitrate standard sample added, ml; n is the nitrate content in the supernatant volume V1, mol / ml. Preferably, the nitrate content in the supernatant volume V1 is determined by bacterial denitrification method.

[0053] Step 4: Take out V2 volume of supernatant from the remaining supernatant in step 3 for testing on the machine 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 moraine identification results.

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

[0055] When testing multiple samples, since the optimal supernatant sampling volume V2 of each sample is not exactly the same, in step 3, V2 volume of supernatant is taken out from the remaining supernatant of each sample to be tested according to the calculation result of the optimal supernatant sampling volume V2 of each sample to be tested, and the V2 volume of supernatant is tested on the machine 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, the ancient moraine identification results of all samples to be tested are obtained.

[0056] Specifically, the optimal supernatant sampling volume V2 has been calculated in step 3. Then, the optimal supernatant sampling volume V2 of each sample to be tested is accurately measured from the remaining supernatant of each sample to be tested. The V2 volume supernatant of all samples to be tested is tested using a MAT253 mass spectrometer to obtain the nitrate nitrogen and oxygen isotope test results of all samples to be tested. Based on the non-mass fractionation effect of oxygen isotopes of trace nitrate in the sedimentary rock to be tested, it is determined whether it is glacial till. Therefore, ancient glacial till can be identified based on the results, which is helpful for the study of ancient Earth atmospheric composition, surface chemistry, atmospheric photochemistry and other processes.

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

[0058] When Δ 17 When O≥5‰, it indicates that there is obvious presence of nitrate. 17 If O is abnormal, the sample is an ancient moraine;

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

[0060] Based on the presence of nitrates in ancient sedimentary rocks 17 O anomaly can be used to infer the evolution of the ancient environment: when nitrate exists 17 O anomalies can be used to determine that ozone existed in the ancient atmosphere, indicating that the oxygen content has reached a certain level. The ancient atmospheric composition and atmospheric evolution during the formation of sedimentary rock samples can be inferred, providing theoretical support for the evolution of habitable planets. When nitrate does not exist, 17 O abnormal, especially Δ 17 When O is close to 0, it indicates that the oxygen content in the ancient environment when the ancient sedimentary rocks were formed was extremely low, which can be used as evidence that there was no oxygen or only a trace amount of oxygen in the ancient atmosphere.

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

[0062] The nitrogen / oxygen isotope composition of nitrate was analyzed in the Key Laboratory of Mineralization and Resource Evaluation, Ministry of Natural Resources, Institute of Mineral Resources, Chinese Academy of Geological Sciences, using a Thermo MAT253 mass spectrometer. Ultrapure water (18 MΩ cm) was used throughout the experiment. -1 , Millipore, Billerica MA, USA), 100 mL beaker, adsorption vessel conical flask (150 mL), 50 mL centrifuge tube.

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

[0064] Of the 13 samples shown in Table 1, 10 samples numbered NO-1 to NO-10 are ancient moraine samples, and 3 samples numbered NO-11 to NO-13 are non-moraine samples. Whether these 13 samples are moraine is known. The identification results from the test data also verify that the identification method of ancient moraine is accurate and effective.

[0065] As can be seen from Table 1, the first supernatant sampling volume of the 13 samples was the same, while the second supernatant sampling volume was different, that is, the optimal supernatant sampling volume V2 was different, and the V2 values ​​were significantly different. From the test results, it can be seen that different amounts of supernatant were taken from the same sample to test, and the results were completely different. This was partly because the nitrate content of the sample was too high, exceeding the maximum detection limit of the instrument, and the obtained δ 17 O VSMOW (‰), δ 18 O VSMOW (‰), Δ 17 The results of O(‰) are completely unreliable (such as NO-1, NO-2, NO-3, NO-8 and NO-9). For moraine samples with too low nitrate content, 2 mL was taken for testing. Since it was lower than the minimum detection limit of the instrument, the results obtained were also unreliable (such as NO-4, NO-5 and NO-10). For samples with an optimal sampling volume close to 2 mL (i.e., within the optimal detection limit of the instrument) (such as NO-7), the two test results were consistent within the error range. Thus, it shows that the application obtains the optimal supernatant sampling volume by first testing the supernatant, and then tests the supernatant with the optimal supernatant sampling volume, and the final result obtained is more accurate.

[0066] Table 1 Oxygen isotope composition of tillite samples

[0067]

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

[0069] 1. This application first cuts off the surface of the ancient sedimentary rock sample to be tested, and then further crushes it to 60 mesh to obtain the optimal particle size of the ancient sedimentary rock sample particles. This treatment can clean out the foreign nitrate pollutants carried by groundwater, surface water and other fluids in the voids / cracks of the ancient sedimentary rock sample to the greatest extent possible, and completely separate them from the original sedimentary nitrate; then, the optimal particle size of the ancient sedimentary rock sample particles are ultrasonically cleaned and dried again with ultrapure water. After ultrapure water ultrasound and centrifugation, the original sedimentary nitrate adsorbed in the sample can be completely separated without affecting the nitrogen and oxygen isotope composition of the nitrate ion; the washed and dried 60-mesh rock sample particles are further crushed to 200-300 mesh to obtain the target particle size of multiple samples to be tested. Ancient sedimentary rock sample powder. In summary, this application first cuts off the surface of the ancient sedimentary rock sample, and then performs a series of treatments including coarse-grained crushing (crushing to 60 mesh), ultrapure water washing and drying, fine-grained crushing (crushing to 200-300 mesh), and ultrapure water soaking to remove adsorbed nitrate. This can eliminate the influence of post-depositional geological actions such as moraine and the pollution of nitrates of modern atmospheric origin. For example, it can effectively remove nitrates that later infiltrated along the voids / cracks of sedimentary rocks such as moraine, will not affect the nitrogen and oxygen isotope composition of nitrate, and help 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 optimal detection limit range of the instrument. The nitrogen and oxygen isotope composition of the adsorbed nitrate in all samples to be tested is determined based on the optimal supernatant sampling volume. The results obtained are more accurate and have high credibility, avoiding the inability to obtain accurate test results for some samples due to large differences in nitrate content in the samples.

[0071] 3. The entire testing process of this application does not require acid, alkali or other chemical reagents, is economical, environmentally friendly and low-cost, and the sedimentary rock samples after nitrate extraction can also be used for analysis of other geochemical indicators, thus achieving full utilization of precious rock samples.

[0072] 4. No prior art documents document that the nitrogen and oxygen isotope composition of nitrate in sedimentary rock samples, such as moraine, can trace the content and evolution of ancient atmospheric oxidizing components. This patent proposes for the first time that the nitrogen and oxygen isotope composition of nitrate in sedimentary rock samples, such as moraine, can trace the content and evolution of ancient atmospheric oxidizing components. The present invention is a geochemical method for identifying moraine using the non-mass effect of oxygen isotopes of trace amounts of original sedimentary nitrate in ancient sedimentary rocks, such as moraine. This method overcomes the shortcomings of traditional sedimentary petrology methods and can accurately identify ancient moraine based on the non-mass effect of nitrate oxygen isotopes. It also provides important technical support for revealing the causal connection between the Snowball Event and the Great Oxidation Event using the non-mass fractionation effect of oxygen isotopes of nitrate in ancient sedimentary rocks, such as moraine. This method 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" project.

[0073] The specific implementation methods described above further illustrate the purpose, technical solutions and beneficial effects of this application. It should be understood that the above description is only the specific implementation methods of this application and is not intended to limit the scope of protection of this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application should be included in the scope of protection of this application.

Claims

1. A method for identifying ancient moraines based on nitrate trioxygen isotopes, characterized in that: include: Step 1: The ancient sedimentary rock sample is crushed for the first time to obtain sample particles of a first particle size; the sample particles of the first particle size are ultrasonically cleaned with ultrapure water and dried; the washed and dried sample particles of the first particle size are crushed for the second time to obtain sample powder of a second particle size; Step 2: Extracting 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; Step 3: first take out V1 volume of supernatant from the supernatant containing nitrate in step 2, measure the nitrate content in V1 volume of supernatant, and obtain the optimal supernatant sampling volume V2 for testing the sample on the machine based on the nitrate content in V1 volume of supernatant; Step 4: Take out V2 volume of supernatant from the remaining supernatant in step 3 for testing on the machine 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 moraine identification results.

2. The method for identifying ancient moraines based on nitrate trioxygen isotopes 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 moraines based on nitrate trioxygen isotopes according to claim 2, characterized in that: Step 2 specifically comprises: soaking the 200-300 mesh sample powder in ultrapure water to transfer the nitrate adsorbed in the sample powder into the ultrapure water, and performing ultrasonic vibration and centrifugation to obtain a supernatant containing nitrate.

4. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 3, characterized in that: In step 2, the ultrapure water immersion time is 30±5 min; the ultrasonic vibration time is 45 min; the centrifugal operation time is 15 min, and the rotation speed is 3800 r / min.

5. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 1, characterized in that: In step 3, the calculation formula for the optimal supernatant sampling volume V2 is: <h2 style=";text-align:left;direction:ltr">V2=(C<h2 style=";text-align:left;direction:ltr"> b <h2 style=";text-align:left;direction:ltr"> ×V<h2 style=";text-align:left;direction:ltr"> b <h2 style=";text-align:left;direction:ltr"> ) / n; Among them, V2 is the optimal supernatant sampling volume, ml; C b The concentration of nitrate standard sample added during the test, mol / ml; V b is the amount of nitrate standard sample added, ml; n is the nitrate content in the supernatant volume V1, mol / ml.

6. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 5, characterized in that: The nitrate content in the supernatant of volume V1 was determined by bacterial denitrification method.

7. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 1, characterized in that: In step 4, determine whether the ancient sedimentary rock to be tested is moraine according to the following principles: When Δ 17 When O≥5‰, it indicates that there is obvious presence of nitrate. 17 If O is abnormal, the sample is an ancient moraine; When Δ 17 When O<5‰, it indicates that there is no obvious nitrate 17 O is abnormal, then this sample is not an ancient tillite.

8. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 1, characterized in that: In step 1, before the ancient sedimentary rock sample is crushed for the first time, the collected ancient sedimentary rock sample is also pre-processed: Excision of an ancient sedimentary rock sample to expose the surface or the surface portion that has been subjected to later influences.

9. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 8, characterized in that: The pre-processing steps for the collected ancient sedimentary rock samples also include: The pretreated ancient sedimentary rock samples were ultrasonically cleaned with ultrapure water and dried.

10. The method for identifying ancient moraine based on nitrate trioxygen isotopes according to claim 1, characterized in that: In step 4, the supernatant of volume V2 was tested using a MAT253 mass spectrometer.

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