نوع مقاله : مقاله پژوهشی

نویسنده

ژئومورفولوژی، گروه میراث طبیعی، پژوهشگاه میراث فرهنگی و گردشگری

چکیده

در این پژوهش، به منظور مطالعات ژئوشیمیایی رسوبات رودخانه‏ای، افق‏های سیمانی کربناته، و خاک‏های دیرینه حوضه آبخیز رودخانه سقز شانزده نمونه از محل‏های مختلف نمونه‏برداری شد. تجزیه عناصر با دستگاه ICP-MS و روش هضم 4 اسیدی، به‌لحاظ عناصر اصلی، در نمونه‏های کربناتی (چه نمونه‏های کربنات پدوژنیک چه نمونه‌های سیمانی کربناتی) نشان داد مقدار درصد عنصر Ca نسبت به سایر عناصر بیشتر است که در واقع کلسیتی بودن این نمونه‌ها را از نظر نوع سیمان کربناتی نشان می‏دهد. به منظور بررسی فرایندهای پدوژنیک بر اساس نسبت‏های عناصر اصلی و ردیاب در نمونه‏های رسوب آبرفتی متأثر از پدوژنیک (خاک قدیمی)، از رابطه‏های نسبت‌های عناصر اصلی (  ) و آبشویی (Ba/Sr) و به منظور درک شدت هوازدگی در پروفیل خاک قدیمی از شاخص‌های CIA، CIW، و ƩBases/Al استفاده شد. میانگین شاخص   معادل 35/0 به دست آمد که بسیار پایین است و شور شدن کم نمونه‏های مورد بررسی از پروفیل خاک قدیمی سقز را نشان می‌دهد. میانگین شاخص Ba/Sr معادل 74/0 به دست آمد. با توجه به این میانگین، مقدار Sr در نمونه‏ها بیشتر از Ba بوده و آبشویی در این پروفیل کمتر اتفاق افتاده است. میانگین CIA محاسبه‌شده 5/66 بود. این مقدار با CIA خاک‏های قدیمی مشابه آلفی‏سول (آرجیلی‏سول) بازالت در گروه بازالت سیلابی رودخانه کلمبیا، تحت شرایط دمایی که مقادیر CIA افق B را به 60 تا 70 رسانده است، همخوانی دارد. میانگین شاخص ƩBases/Al 96/0 بود. با توجه به مقدار 5/0 از نسبت ƩBases/Al، که یک خط تقسیم مهم آماری میان آلفی‏سول و آلتی‏سول است، افق خاک قدیمی مشابه آلفی‏سول‏هاست. پس، بر اساس این شاخص ژئوشیمیایی، کربنات پدوژنیک در شرایط آب‌وهوایی سرد و خشک پلیستوسن پایانی تحول پیدا کرده است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Geochemistry analysis of river sediments, carbonate cemented and paleosols horizons and the implications for provenance and chemical weathering (Case Study: Saqqez River)

نویسنده [English]

  • Khe bat Derafshi

Geomorphology, Natural Heritage Department, Cultural Heritage and Tourism Research Institute

چکیده [English]

Introduction
In theory, the bulk composition of the sediment may provide a way of determining the provenance of this material if there is sufficient diversity in the bedrock compositions. However, sediment compositions may be changed from the original bedrock as a result of transport and chemical weathering. The geochemistry of bedload and suspended sediments has been used by many workers to determine the provenance, weathering, and tectonics of several river basins. Geochemical studies of sediments of aquatic bodies such as rivers can be an effective step to find sediment source, elements distribution pattern and climatic conditions at the time of formation of cementitious carbonates between gravelly sediments and riverine sands.
 
Methodology
In this study, 16 samples were collected from different sites of the study profile in one of the main tributaries of Saqqez River to geochemistry studies of river sediments, cement carbonates and paleosols horizons of the river. The samples include 2 samples of pedogenic carbonate horizon, 7 samples of paleosols horizon (fine-grained alluvial sediments affected by pedogenic processes), 2 samples of coarse-grained alluvial unconsolidated sediments, 4 samples of alluvial consolidated sediments (conglomerate) and one sample of basaltic bedrock. After crushing the samples, for alluvial samples (conglomerates), their carbonates were separated by binocular microscope; then processing the samples in laboratory, they were all filtered to less than 63 microns (screening at 200 meshes); the samples were further sent to Actlabs Laboratory, Canada, for ICP-MS analysis and 4-acid digestion (Ut4).
 
Results and discussion
The graph for transition metals shows that carbonate cements in conglomerate sediments as well as pedogenic carbonates (paleosols horizon) have a small amount of these elements; only Zn and Cu have been reported in one sample. But for other samples, the amount of reported transition metals, especially Ni, is higher. The basaltic andesite bedrock has similar geochemical properties with pedogenic alluvial sediment samples in terms of transition metals. The elements of Zr, Hf, Nb, Mo and W are part of the HFS (high-field intensity) elements reported in the geochemistry analysis; in the case of carbonate cement samples, conglomerate sediments have higher Mo than another element. This is most abundant in pedogenic alluvial sediment samples including Zr and then Mo. The basaltic andesite bedrock in terms of HFS elements, such as basic elements and transition metals, exhibits geochemical properties similar to pedogenic alluvial sediments. Geochemistry analyzes reported for alkaline earth elements indicates that carbonate samples (pedogenic and cement carbonate) contain a small amount of these elements. Whereas the abundance of alkali elements is more in alluvial sediment samples; the elements of Sr, Cs, Ba and Li have been reported in these samples. The geochemical properties of the basaltic andesite bedrock sample in term of alkali elements are similar to the pedogenic alluvial sediment samples; however, the Sr and Ba values ​​in bedrock have been reported more than other samples due to the absence of leaching the elements in the bedrock sample. Geochemistry analyzes of rare earth elements show that carbonate cement samples of conglomerate sediments generally contain small amounts of La, Ce, Nd, Sm, Gd, Tb, Yb, Lu and Eu; pedogenic carbonate samples is also contains a small amount of these rare earth elements. In alluvial sedimentary samples affected by pedogenic and pedogenic carbonate samples, the amounts of these elements increased and Nd, Eu and La were most abundant. In relation to heavy metals and actinides, elements of Tl, Pb, Th, U and Bi have been reported; conglomerate sediments with higher carbonate cement content large amounts of the element Pb. In pedogenic carbonates, the elements U and Bi are slightly higher than other elements. In the case of alluvial sediments affected by pedogenic and paleosols, the amount of heavy metals is approximately the same. The geochemical report of heavy metal elements in the basaltic andesite bedrock sample are similar to those of the pedogenic affected sediments, except that the bedrock has a smaller number of elements, especially Bi and Tl.
 
Conclusion
One of the major element ratios is (K+Na) / Al that the average of this index in the 11 studied samples is equal to 0.35, which is very low and indicating low salinity of the studied samples. Leaching was another ratio of tracer element using Ba/Sr; the mean of this indicator was 0.74 for the 11 investigated samples. Higher values of this, ​​indicate more leaching because Sr is significantly more soluble than Ba ​​and is more easily washed under most conditions. According to the calculated average for this index in the studied samples of the paleosols profile of Saqqez River basin, the Sr content in the samples was higher than Ba ​​and the leaching is less frequent in this profile. Acidification is another ratio of trace element that uses La/Ce, Sm/Nd or U/Th ratios. The CIA, CIW and ƩBases/Al indices have been used to understand the weathering intensity in the paleosols profile of the Saqqez River basin based on pedogenic alluvial sediment samples. The average CIA calculated for the 9 samples under study is 66.5; this value is similar to the CIA of Alfisol-like paleosols (Argillisols) on tholeiitic basalts of the Columbia River Flood Basalt group under temperate conditions gave rise to B horizon CIA values of 60–70 C. The mean of ƩBases/Al index is 0.96 in the samples collected from the paleosols horizon. Given a value of 0.5 of the Bases/Al ratios, which is a statistically significant dividing line between alfisol (>0.5; dystric Argillisols) and Ultisol (<0.5; eutric Argillisols), the paleosols horizon or alluvial sediment affected by pedogenic processes, is similar to aliphosols. Based on weathering indices derived from geochemical element ratios in pedogenic-influenced alluvial sediment samples, the calculated average of CIA was 66.5 for the 9 samples, which is similar to CIA of Alfisol-like (Argillisols) and Histosol-like paleosols.

کلیدواژه‌ها [English]

  • Geochemistry
  • Alluvial sediments
  • Provenance
  • Chemical weathering
  • Saqqez River