A method for analyzing the depth of time of El Nino-southern oscillation cycle based on annual lamination of marine and lacustrine sediments

Abstract

The present application relates to the field of sedimentology, in particular to a method for analyzing deep-time El Nino-Southern Oscillation cycle based on annual lamination of marine and lacustrine sediments. Through sampling, grinding, thin sectioning, high-definition scanning, estimating sedimentation rate, element plane distribution, principal component analysis of each element content, laser in-situ carbon and oxygen isotope testing of each annual lamination, and time domain spectrum analysis, the climate change of interannual scale in geological history period is finally analyzed. The present application combines the analysis and interpretation of element data, annual lamination thickness and annual lamination carbon and oxygen isotopes, which shows that there is correlation between each index, provides a new method for identifying and analyzing deep-time El Nino-Southern Oscillation cycle, establishes a time scale with years as a unit based on marine or lacustrine annual lamination, and carries out high-precision analysis on climate fluctuation under this scale, which fills the gap in the related art.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sedimentology, in particular to a method for analyzing deep-time El Niño-Southern Oscillation cycle based on annual lamination of lacustrine deposits. TECHNICAL BACKGROUND

[0002] El Niño-Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon that occurs between the tropical Pacific and Indian Oceans, characterized by fluctuations between the abnormally warm condition El Niño and the cold condition La Niño. Modern climate research has found that ENSO has a cycle of 2-7 years and is one of the most important driving factors of global climate fluctuations. However, current identification of ENSO phenomenon is almost entirely limited to the modern and Quaternary periods, and rarely involves the geological history period before the Quaternary. This is because in old sedimentary strata, there is currently a lack of methods for establishing a high-precision time scale at an annual scale and analyzing high-frequency climate fluctuations on this basis. Traditional radioisotope dating methods (such as U-Pb and Ar-Ar methods) have an error of up to millions of years; the smallest scale cycle that can be identified by cyclostratigraphy is the precession cycle, which is 20,000 years, and neither of them can meet the requirement of time precision at an annual scale for identifying ENSO.

[0003] In the strata before the Quaternary period, lacustrine annual lamination deposits are widely distributed, which are characterized by a relatively clear interface between the deposits of each year and the deposits of the previous year under the seasonal changes of paleoclimate, and a single annual lamination represents a one-year deposition time limit. For example, the lower part of the third member and the upper part of the fourth member of the Eocene Shahejie Formation in Jiyang Depression develop typical annual lamination: in spring and summer, algal blooms in the lake, and light-colored calcite layers are deposited; in autumn and winter, algal extinction and increased terrigenous input, and dark-colored organic-rich clay-silt layers are deposited, which are combined to form an annual lamination. Lacustrine deposits with annual lamination can provide a high-precision time scale and can be used as a good carrier for analyzing deep-time ENSO cycle.

[0004] ENSO can cause frequent extreme flooding and drought events, global temperature and sea level fluctuations, and have an important impact on the biosphere including humans. Studying the ENSO cycle in the geological history period (especially in the greenhouse period) can help us better cope with the challenges brought by future climate change.

[0005] In summary, the prior art has made some research on the ENSO cycle in the modern and Quaternary periods, but there is still a lack of relevant identification and analysis technology for old years before the Quaternary period. SUMMARY

[0006] In view of the problems in the prior art, the present application provides a method for identifying deep-time El Nino-Southern Oscillation cycles based on annual varve of marine-lacustrine sediments.

[0007] Figure 1 The specific technical solutions of the present application are as follows:

[0008] S1, sample grinding and high-definition scanning of the slice

[0009] Select the marine-lacustrine sediment core section with developmental annual varve, sample grinding and slice, the slice thickness is 0.03 mm, and no cover glass is used; high-definition scanning and photographing of the slice under a microscope are performed to obtain a whole slice photograph capable of clearly identifying the annual varve.

[0010] S2, establishment of a time scale and estimation of a deposition rate

[0011] The annual varve in the slice is counted, and the thickness of each annual varve is measured; since each annual varve represents a one-year deposition time limit, a time scale in years is established in a direction perpendicular to the annual varve, and a deposition rate is estimated according to the thickness of the annual varve.

[0012] S3, analysis of element plane distribution

[0013] An XRF micro-area X-ray fluorescence scanner is used to scan the slice to obtain the plane distribution of Si, Ca, K, Al, Fe, Ti and S;

[0014] S4, principal component analysis of the content of each element

[0015] A straight line segment is drawn in a direction perpendicular to the annual varve, and the content of Si, Ca, K, Al, Fe, Ti and S on the line segment is extracted; principal component analysis is performed on the extracted content data of each element, and a principal component capable of representing the data fluctuation is selected.

[0016] S5, carbon and oxygen isotope test to extract interannual scale climate change information

[0017] The line segment in the direction perpendicular to the annual varve is used to perform laser in-situ carbon and oxygen isotope test on the annual varve one by one to extract interannual scale climate change information.

[0018] S6, time domain spectral analysis

[0019] On the basis of the time scale established in step S2, time domain spectral analysis is performed on the annual varve thickness data, element principal component data and carbon and oxygen isotope data to identify the interannual scale climate change cycle related to El Nino-Southern Oscillation.

[0020] Preferably, in step S2, if one annual layer is composed of two or more laminae units (such as one calcite lamina unit and one clay silt lamina unit jointly compose one annual layer), the thickness of different lamina units can be further measured on the basis of measuring the thickness of the annual layer.

[0021] Preferably, in S4, the method of electron probe line scanning can be used to obtain the element content on the line segment perpendicular to the direction of the annual layer.

[0022] Further preferably, climate change (fluctuations in temperature, precipitation, etc.) will cause the amount of terrigenous input received by the sedimentary basin to change, the amount of evaporation to change, and the biological (including microorganisms such as bacteria) participating in the element cycle to flourish or die out, and ultimately to be reflected in the element changes of the sediments. Therefore, it is necessary to select characteristic elements that can reflect climate change.

[0023] The present application has the following beneficial effects:

[0024] (1) The present application shows that the thickness of marine and lacustrine annual layers, the change of characteristic element content, and the annual layer carbon and oxygen isotopes can reflect the annual scale climate change in the geological history period.

[0025] (2) The present application jointly analyzes and interprets the element data, the thickness of the annual layer, and the carbon and oxygen isotopes of the annual layer, which shows that there is a correlation between the indicators. For example, the carbon and oxygen isotope values and the thickness of the annual layer have a strong correlation, which jointly indicates the annual scale cyclic change between the warm and dry climate state and the wet and cold climate state.

[0026] (3) The present application provides a new method for identifying and analyzing the deep-time El Nino-Southern Oscillation cycle, establishes a time scale based on marine or lacustrine annual layers, and performs high-precision analysis of climate fluctuations at this scale, which fills the related technical gap. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The method flowchart provided for the embodiments of the present application;

[0028] Figure 2 The annual layer counting and thickness analysis chart;

[0029] Figure 3 The thin section XRF element scanning chart and the first and second principal component result chart;

[0030] Figure 4 The thin section XRF element principal component analysis chart;

[0031] Figure 5 The in-situ laser carbon and oxygen isotope analysis result chart of the annual layer;

[0032] Figure 6A time-domain frequency spectrum analysis result summary chart. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0034] In this embodiment, the lower part of the third member of the Eocene Shahejie Formation and the upper part of the fourth member of the Eocene Shahejie Formation in the Jiyang Depression are selected as the research object to be described in detail.

[0035] As shown in Figure 1 The method for identifying the deep-time El Nino-Southern Oscillation cycle based on the annual lamination of marine-lacustrine sediments provided by the embodiments of the present application includes the following steps:

[0036] S1, sample grinding and thin sectioning and high-definition scanning of the thin section

[0037] The sample at a depth of 3552.1 meters in Well Niu55Xian1 in the Dongying Sag of the Jiyang Depression is selected for thin sectioning, and the thin section is scanned under 50 times using a German Leica DM6M electronic microscope.

[0038] S2, establishment of a time scale and estimation of a deposition rate

[0039] The thin section scanned in step S1 is counted layer by layer for annual lamination and measured for thickness. The thin section is counted for 360 annual laminations, each annual lamination representing one year and including one bright calcite lamination unit and one dark clayey silt lamination unit, and the thickness of each annual lamination and the calcite lamination therein is measured, as shown in Figure 2 The average thickness of the annual lamination is 105.5 micrometers, and the deposition rate is 10.55 centimeters per kiloyear, obtained by dividing the deposition thickness by the deposition time.

[0040] S3, analysis of element planar distribution

[0041] Using a Bruker M4 Tornado micro-X-ray fluorescence spectrometer, high-precision XRF elemental scanning was performed on the thin slices. The thin slice samples were placed in the sample chamber, and a level was used to ensure the sample was horizontal. Elemental scanning was carried out using a silicon drift detector, with an accelerating voltage of 50 keV, a beam current of 800 mA, rhodium selected as the X-ray irradiation target material, a scanning rate of 5 milliseconds per point. The quantitative characteristics of elements such as Ca, Si, Na, K, Al, Mg, Fe, S, Ti, Mn, Sr, Ba, V, Cr, Cu, Ni, Co, Mo, etc. in the samples were determined. The scanning time for each sample was greater than 6 hours, the sampling point spacing was 10 micrometers, the elemental concentration was in wt.%, and different elements were rendered with different colors to obtain a two-dimensional distribution map of the elements, as Figure 3 shown. Subsequently, a straight line was drawn in the direction perpendicular to the laminations, as shown by the dashed line in Figure 3 . With the aid of the software system supporting the X-ray fluorescence spectrometer, the content changes of each element on this straight line were quantitatively calculated.

[0042] S4. Principal component analysis of the content of each element

[0043] Using the principal component analysis application program of Originpro v.2023b software, principal component analysis was performed on the XRF data of the Ca, Si, K, Fe, S, and Ti contents of the thin slices.

[0044] As Figure 4 shown, the contribution ratios of the first principal component PC1 and the second principal component PC2 to the data variance were 62.3% and 17.96% respectively, and the sum of the variance contribution rates of the first principal component and the second principal component reached 80.26%, indicating that the first principal component and the second principal component can effectively reflect the changes in the content of each element; PC1 was significantly negatively correlated with the Ca content and positively correlated with the Si, K, and Ti contents. This indicates that PC1 is an index that can reflect the production of carbonate minerals: when PC1 is low, the production of carbonate minerals increases, which may be related to the繁盛 of algae organisms under warm climates; when PC1 is high, the carbonate minerals precipitated under biochemical action decrease, and the proportion of terrigenous detritus (characterized by elements such as Si, K, Ti, etc.) increases. PC2 was significantly positively correlated with the Fe and S contents and had a lower correlation with the Ca and Ti contents. Fe and S are the constituent elements of pyrite (FeS2), and the enrichment of pyrite indicates a relatively strong reducing condition of the water body. This indicates that PC2 can be used as an index to reflect the changes in redox conditions.

[0045] S5. Laser in-situ carbon and oxygen isotope tests were carried out on each annual lamination one by one

[0046] The stable gas isotope ratio mass spectrometer and FLASH EA element analyzer produced by the United States Thermofisher, model Delta V Advantage, and the ND: YAG (neodymium-doped yttrium aluminum garnet crystal) laser are used to test the in-situ carbon isotope (δ13C) and oxygen isotope (δ18O) of the thin slice micritic calcite lamina. The in-situ laser carbon and oxygen isotope test requires special thin section making requirements, that is, the thickness of the thin section is 60-80 microns, single side polishing, no cover glass, and using firn glue (avoiding the use of other adhesives such as epoxy resin). The sample needs to be pretreated before testing to remove water and organic matter in the sample. During testing, each selected micritic calcite lamina is placed in the center of the field of view (under the microscope), to ensure that the bright calcite lamina is tested, not the carbonate cement in the terrigenous clastic lamina. The coherent laser beam output by the laser is focused on the selected micro area to heat and decompose the generated CO2 gas, and the carbon and oxygen isotope values of the CO2 gas are measured by the gas isotope ratio mass spectrometer. The spatial resolution of micro area measurement is 20 μm, and the carbon and oxygen isotope data are in VDPB units, as shown in Figure 5 .

[0047] S6, time domain spectral analysis

[0048] The thin section XRF element data, annual lamina thickness data, and annual lamina carbon and oxygen isotope data are subjected to time domain spectral analysis. PC1, PC2, and annual lamina thickness analysis show a significant high frequency period of 2.1-4.5 years (more than 95% confidence level), and the annual lamina carbon and oxygen isotope spectral analysis results show a period of 3.5-8.7 years and a period of 2.6-3 years, as shown in Figure 6 . Further analysis shows that the thickness of the calcite lamina unit in the annual lamina and the carbon and oxygen isotope values have a synchronous change trend (as shown in Figure 5 ), and both have a period of 2-7 years. In the warm and dry climate state, algae flourish, forming thicker calcite lamina units, and the carbon and oxygen isotopes are positively biased; in the wet and cold climate state, the growth of algae is inhibited, the calcite lamina unit is thinned, and the carbon and oxygen isotopes are negatively biased, further indicating the regional climate effect of the El Nino-Southern Oscillation in the Eocene period of Jiyang Sag.

[0049] It should be understood by those skilled in the art that the discussion of the above examples is only exemplary and is not intended to imply that the scope (including claims) of the present application is limited to these examples; under the idea of the present application, the technical features in the above examples or different examples can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above. In order to be brief, they are not provided in detail.

[0050] The present application is intended to cover all such alternatives, modifications, and variations of the application falling within the broad scope of the appended claims. Accordingly, any and all such alternatives, modifications and variations are intended to be included within the scope of the present application.

Claims

1. A method for analyzing the depth of time of El Niño-Southern Oscillation cycle based on annual lamination of marine lacustrine sediments, characterized in that, The method comprises the following steps: S1, sampling and grinding thin sections and high-definition scanning of the thin sections A core section of lacustrine sediment with developed annual laminae is selected, thin sections are sampled and ground, the thickness of the thin sections is 0.03 mm, no cover glass is used, and the thin sections are high-definition scanned under a microscope to obtain a whole photograph of the thin sections in which the annual laminae can be clearly identified; S2, establishing a time scale and estimating a deposition rate The annual laminae in the thin sections are counted, and the thickness of each annual laminae is measured; since each annual laminae represents a year of deposition time limit, a time scale in years is established in a direction perpendicular to the annual laminae, and the deposition rate is estimated according to the thickness of the annual laminae; S3, analyzing element plane distribution XRF is used to scan the thin sections to obtain the plane distribution of Si, Ca, K, Al, Fe, Ti and S elements; S4, principal component analysis of the content of each element A straight line segment is drawn in a direction perpendicular to the annual laminae, the content of Si, Ca, K, Al, Fe, Ti and S elements on the line segment is extracted, and principal component analysis is performed on the extracted content data of each element to select principal components that can represent the data volatility; S5, carbon and oxygen isotope testing to extract interannual scale climate change information The laser in-situ carbon and oxygen isotope testing is performed on the annual laminae one by one along the line segment in the direction perpendicular to the annual laminae to extract interannual scale climate change information; S6, time domain spectral analysis On the basis of the time scale established in step S2, time domain spectral analysis is performed on the annual laminae thickness data, element principal component data and carbon and oxygen isotope data to identify the interannual scale climate change cycle related to El Nino-Southern Oscillation.

2. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S1, the thin sections are high-definition scanned under 50 times using a microscope.

3. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S2, if an annual lamina is composed of multiple lamina units, the thickness of different lamina units can be further measured on the basis of measuring the thickness of the annual lamina.

4. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S2, the deposition rate is calculated by counting and measuring the thickness of the annual laminae layer by layer on the high-definition scanned thin section photograph, and the deposition thickness is divided by the deposition time to obtain the deposition rate.

5. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S3, the scanning time of each thin section is greater than 6 hours, and the sampling point spacing is 10 microns.

6. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S4, the electron probe line scanning is used to obtain the element content on the straight line segment in the direction perpendicular to the annual laminae.

7. The method for analyzing the El Nino-Southern Oscillation cycle based on the annual sediment layer analysis of marine and lacustrine deposits according to claim 1, characterized in that, In the step S5, the in-situ laser carbon and oxygen isotope testing requires that the thin section has a thickness of 60-80 microns, is single polished, does not need a cover glass, and is bonded using firn glue.

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