1. The relationship between trace element characteristics and depositional environment
During the deposition process, there is a complex geochemical balance between sediment and water medium. The exchange of elements between sediments and the adsorption of certain elements by sediments, etc. In addition to being related to the properties of the elements themselves, this exchange and adsorption is also affected by the physical and chemical conditions of the sedimentary medium. The water media in different sedimentary environments have different physical and chemical conditions, which provides a basis for the use of sediment trace elements and their contents. Paleoenvironmental analysis provides a theoretical basis.
The formation of primary carbonate rocks and clay minerals in closed salt lakes is closely related to the chemical properties of the lake water. From the low-stand system tract of the lake to the high-stand system tract, the clay minerals, mudstone and clay minerals in the lake mudstone The trace element composition in mud salt is very sensitive to hydrological changes in the depositional environment, so it can effectively reveal changes in lake level. The geochemical indicators and environmental parameters that are most sensitive to changes in the sedimentary environment were selected for this study, such as B, Sr, Ba, Ti, Fe, P, Mn content and Mg/Ca (Chen Jun et al., 1997, 1998, 1999) and Sr/Cu The distribution characteristics and change patterns of the ratio in the longitudinal and transverse directions are used to reveal their relationship with changes in lake level.
2. The relationship between the boron element content and its ratio changes and the environment
Previous studies have believed that there is a linear relationship between the B content in the water body and the salinity in the water body, that is, the water body salt The higher the degree, the greater the B content, and the more B ions adsorbed by the sediment (Yang Rui, 1993). For mud shale formations at different layers or different regional locations in continental salt lakes, if the B content is high (average greater than 135 μg·g-1), it indicates that the depositional environment is an arid-semi-arid salt lake depositional environment; if the B content Low or normal (average less than or equal to 135 μg·g-1), indicating that the mud shale was deposited in a relatively humid salt lake depositional environment. However, when the deposition area is far away from the center of the salt lake, it can also represent an arid-semi-arid salt lake depositional environment. This is mainly due to its low salinity, which results in low B content. Therefore, B content can be used to analyze the depositional environment.
3. The relationship between Rb and Sr content and the environment
In nature, Rb and Sr are mostly dispersed in potassium- and calcium-containing minerals in the form of isomorphs. Among them, Rb mainly exists in the silicate component, and its chemical properties are relatively stable. Sr mainly exists in the carbonate component and has more active chemical properties. Rb has a large ionic radius and high surface potential energy. During weathering, Rb is easily adsorbed by negatively charged clay and enriched in fine-grained sediments such as clay. The ionic radius of Sr is between K and Ca, so Sr exists in calcium- and potassium-containing minerals, but mainly occurs in calcium-containing minerals. Since Rb-rich minerals are more stable than Sr-rich minerals, as the degree of weathering increases, Sr in the formation is more easily dissociated from the mineral than Rb and is leached out. Therefore, changes in the Rb/Sr ratio in the formation mainly depend on the degree of Sr loss. Research shows that the Rb/Sr ratio is positively correlated with the degree of weathering, and the degree of weathering leaching is controlled by changes in the climate environment, especially temperature and precipitation. That is, in a warm and humid environment, there is a lot of precipitation and strong weathering, and the leaching of Sr The greater the degree, the higher the Rb/Sr ratio; conversely, in a dry and cool climate environment, the Rb/Sr ratio is low. Therefore, the Rb/Sr ratio, which is an indicator of weathering degree, can be used as one of the indicators to demonstrate changes in paleoclimate and paleoenvironment.
The Rb/Sr ratio change curve in sediments can reflect changes in provenance and depositional environment. Dasch (1969) conducted a detailed study on the migration patterns of Rb and Sr in various parent rocks under weathering conditions, and pointed out that the Rb/Sr ratio can indicate the weathering intensity of the parent rock; Gallet et al. (1996) conducted a study on Luochuan Studies on the distribution of Rb/Sr ratios in loess profiles have found that this ratio can clearly identify paleosol stratigraphic units; studies by Chen Jun et al. (1997, 1998, 1999) have shown that the Rb/Sr value can indicate the wind resistance of loess accumulations. The Rb/Sr curve reflects the climate fluctuation information since 2.5 Ma and its coupling relationship with regional and even global climate changes.
Existing mineralogical studies have shown that changes in particle size will not cause significant differentiation of calcium- and potassium-containing minerals. Therefore, the material composition of the original rock has become the main factor restricting the changes in the content of Rb and Sr elements in the sedimentary strata, and the source is closely related to the sedimentary environment. Different sedimentary environments may bring different sources, and changes in material sources must Will cause changes in the Rb/Sr ratio. Sr, which has similar geochemical properties to the main element Ca, will be enriched in marine sediments as the Ca content increases when the climate is warm and the sea level is high. The biological effects of organisms will also cause Sr to be obtained. With different degrees of enrichment, the Rb/Sr ratio at this time will be characterized by low values; when the climate is humid and cold and the sea level is low, continental chemical weathering is strengthened, and Sr as a trace component mainly exists in detrital minerals (long-term stone, clay minerals and dolomite), so the Rb/Sr ratio in fluvial and lacustrine sediments is relatively high.
4. The relationship between Ti, P, Mn, Cu, Ca and the environment
Previous studies believe that changes in the content of elements such as Ti reflect the degree of addition of terrestrial substances. The higher the value, the richer the terrigenous content, indicating a warm and humid climate background. Similar to Sr, high P content in sedimentary rocks indicates a climate background of an arid, hot, high-salinity environment, while low content indicates a humid climate background. Mn often exists stably in the form of Mn2 in lake water. Only when the lake water evaporates strongly and Mn2 is saturated, will it precipitate in large amounts, thus showing high values ??in sedimentary rocks. The Sr/Cu and Mg/Ca ratios are also very sensitive to changes in paleoclimate. Generally, the Sr/Cu ratio between 1 and 10 indicates a warm and humid climate, while greater than 10 indicates a hot and dry climate; similarly, a high value of the Mg/Ca ratio indicates a hot and dry climate, and a low value indicates a warm and humid climate.
5. The relationship between Ba, Sr and the environment
Ba and Sr have very similar chemical properties and cannot form independent minerals. Under certain conditions, the two are often combined regularly. It is known that Sr mainly interacts with Ca and Ba more often with K to produce isomorphic substitutions. Like Sr, Ba can also form soluble barium salts and enter the water. Ba in the water is easily adsorbed by hydrolyzed sediments. At the same time, the solubility of barium salts, especially barium sulfate, is very low, so under the action of evaporation, neutralization or the increase of sulfate anions, barium sulfate will first precipitate. It can be seen that Ba is more stable than Sr in nature. Sr is easy to migrate but Ba is difficult to be leached.
Generally, low Sr content indicates a humid climate background, and conversely, a dry climate background. Because the Sr/Ba value gradually increases away from the lake (sea) shore, the Sr/Ba value can qualitatively reflect the ancient salinity of the medium. The Sr/Ba ratio greater than 1 is a salty (marine) water medium, and the Sr/Ba ratio less than 1 is a freshwater medium. When fresh water and salty lake water (seawater) are mixed, the Ba2 in the freshwater combines with the salty lake water (seawater) to form BaSO4 precipitation, and Sr-SO4 has a large solubility, and it can continue to migrate to the center of the salt lake (the far sea), passing Biological effects precipitate.
6. The relationship between the Mg/Ca ratio and the environment
It has been confirmed by a large number of studies that changes in the Mg/Ca ratio are affected by climate or environmental factors. Roberts et al. (1998) studied the high-precision Mg/Ca ratio of secondary chemical sediments in Holocene caves in northern Scotland and found that changes in the Mg/Ca ratio within the year were caused by temperature changes. Kathleen R. Johnson et al. (2006) studied high-resolution stalagmites from Monk Cave in China and speculated that they mainly reflect changes in seasonal precipitation. Wang Xinzhong (2005) studied the changes in the Mg/Ca ratio in modern dripping water and also believed that the Mg/Ca ratio in cave sediments can reflect changes in external dry and wet conditions. Li Bin et al. (2000) studied stalagmites in Guilin and concluded that when the atmospheric circulation system does not change significantly and the karst hydrogeological conditions are relatively similar, the Mg/Ca ratio mainly depends on changes in ambient temperature, while when the atmospheric circulation system changes significantly When changes occur (such as significant changes in glacial and interglacial periods), it mainly depends on changes in precipitation conditions.