در مطالعۀ حاضر سعی شد روند تغییرات اقلیمی در زمان تشکیل نهشتههای کربناتۀ سازند داریان به سن آپتین در زون چینخورده - راندۀ زاگرس با استفاده از دادههای ژئوشیمیایی (شامل دادههای ایزوتوپی و عنصری) بررسی شود. بهمنظور مطالعۀ نوسانات دمای دیرینه از روند تغییرات ایزوتوپ اکسیژن (شاخص حساس به دما) استفاده شد. نتایج به دست آمده وجود یک دوره گرمشدن در ابتدای آپتین پیشین و دو دوره سردشدن (یکی در انتهای آپتین پیشین و دیگری در آپتین پسین) را نشان دادند. انطباق روند تغییرات نسبت استرانسیوم به کلسیم، ایزوتوپ پایدار کربن و روند نوسانات دمایی در این نهشتهها و مقایسه آن با سایر مطالعهها نشان میدهد ورود دیاکسیدکربن به سیستم جوی - اقیانوسی از طریق فعالیتهای آتشفشانی زیردریایی مهمترین سازوکار افزایش دماست؛ در مقابل، خروج این گاز گلخانهای در اثر خروج کربن از طریق دفن مواد آلی در نهشتهها عامل اصلی کاهش دما در نظر گرفته میشود. دادههای حاصل همانند نتایج بهدستآمده در سایر نقاط دنیا نشان میدهند طی تشکیل نهشتههای کربناتۀ سازند داریان، شرایط گلخانهای در تمام بازۀ زمانی آپتین و به تبع آن دورۀ کرتاسه غالب نبوده و پیوستگی شرایط گلخانهای توسط دورههایی از سردشدگی و کاهش دما قطع شده است.
عنوان مقاله [English]
Investigation of palaeotemperature changes trend in Aptian–Albian carbonate deposits of the Zagros Fold-Thrust Belt based on oxygen isotope data
In this study, geochemical (including elemental and stable isotope) data were used for study of climatic changes during the deposition of Aptian carbonate of the Dariyan Formation in the Zagros Fold-Thrust Belt. In order to study palaeotemperature fluctuations, oxygen stable isotope was used as a temperature sensitive proxy. Results of this study show an increasing of temperature at the beginning of early Aptian and two cooling episodes during late early Aptian and late Aptian in the studied deposits. Correlation of Sr/Ca ratio, carbon stable isotope values and temperature fluctuations trend in these deposits with other studies reveal that influx of carbon dioxide in atmospheric-oceanic system via sub-marine volcanism activities is main mechanism for increasing temperature. On the contrary, carbon outlet via burial of organic matter in deposits regarded as most important factor in decreasing temperature. Obtained data are in accordance with other studies and show that during deposition of the Dariyan carbonate deposits greenhouse condition was not dominated throughout the Aptian age , and cooling episodes occurred in some intervals.
Keywords: Oxygen Isotope, Palaeotemperature, Dariyan Formation, Aptian–Albian, Zagros.
The assumption that the Cretaceous had greenhouse climate, with no ice-sheets has been challenged with evidence for short-term cooling and possible formation of ice caps (Maurer et al. 2012), for example, during the Aptian and according to the different palaeotemperature indices such as oxygen–isotope ratios, calcareous nannofossils and palynomorphs, relatively high increase in temperature during the OAE1a event in different parts of the Earth has been shown (Jenkyns 2018). On the other hand, this interval is accompanied with extension of Low Stand System Tract (LST) deposits and dropstones in high latitude which show cooling episodes (Jenkyns et al. 2012; Lorenzen et al. 2013). The Dariyan Formation in Kuh-e-Sefid section with Aptian–Albian age (Moosavizadeh et al. 2015), has been documented evidences of cool and warm episodes. This section located in the Zagros Fold-Thrust Belt and in the Interior Fars Province (Farzipour‐Saein et al. 2009) and contains Hauterivian–Cenomanian deposits. In the studied section this formation divided into three informal units: lower Dariyan Formation that contains thin bedded limestone with benthic foraminifera which is led to pelagic black limestone, Kazhdumi Tongue that comprises pelagic shale and upper Dariyan Formation which contains medium- to thick-bedded orbitolina rich limestone (James and Wynd 1965). In this study oxygen isotope ratio and its fluctuation trends was used for investigation of palaeotemperature fluctuations during deposition of the Dariyan Formation.
Material & Methods
The Dariyan Formation with 300 meter thickness was measured and 283 samples were collected based on facies variations. The classifications of Dunham (1962) and Embry & Klovan (1971) were used for facies description. Fifty-seven samples were selected for oxygen isotopic and elemental analysis. These samples were analyzed by atomic absorption spectrophotometry (Shimatzu AA-670/670G) for Sr, Na, Mn and Ca (with ±5 ppm standard deviation) content in the Geochemistry Laboratory of the Ferdowsi University of Mashhad, Iran. For this, 2 g of powdered bulk samples was dissolved by cold acetic acid (3 %) and were analyzed. Stable carbon and oxygen isotope measurements were also performed on same samples at the Stable Isotope Laboratory of Kansas University. The obtained results are expressed in the standard VPDB and the standard deviation on the measurements is ±0.1‰.
Discussion of Results & Conclusions
Elemental and isotopic analysis of the Dariyan Formation deposits and cross plots of these data release that these deposits subjected to a burial diagenetic realm in a clos to semi-closed system and data are close to primary values. Therefore these data can be used as relatively unaltered values for palaeotemperature reconstruction. The stable oxygen isotope trend divided into 10 segments (Bottini et al. 2015) and were used for investigation of palaeotemperature changes. These trends show an increasing in temperature (negative excursion) during early Aptian that can be attributed to increasing carbon-dioxide in atmospheric-oceanic system via submarine volcanisms (Large Igneous Province: LIP). This event is simultaneously with high fertility level of oceanic water due to entrance of nutrient ions in the oceans. Consequently, oxygen isotope trend shows two positive excursion in the late early Aptian and the late Aptian that interpreted as decreasing in temperature. The most important factor for these cooling episodes is carbon outlet from oceanic system via burial of organic matter in deposits which led to formation of black shale and black limestone in the formation and Aptian deposits in many region on Earth. Deposition of glacial sediments in high latitudes and expansion of LST sediments in low latitudes confirm decreasing in temprature during late early Aptian and late Aptian. These evidences documented in the Dariyan Formation as exposure surfaces and karstification in carbonate rocks in shallow part of basin and deposition of LST sediments in deep parts. On the other hand, increasing continental weathering and transportation of organic carbon to sedimentary basin has been suggested as other mechanism for temprature decreasing. Generally these data are in accordance with other studies and show that greenhouse condition was not dominated throughout the Cretaceous, and cooling episodes occurred in some intervals.