نقش نشانگر های رسوب شناختی و ژئوشیمیایی در شناسایی شرایط اکسیداسیون-احیا در بخش جنوبی حوضه خزر جنوبی

نوع مقاله : مقاله پژوهشی

نویسندگان

1 مدیریت زمین‌شناسی دریایی، سازمان زمین‌شناسی و اکتشافات معدنی کشور، تهران

2 گروه زمین‌شناسی، دانشگاه بوعلی‌سینا، همدان

3 شرکت مهندسین مشاور ژئوتکنیک زمین‌کاوان جنوب، تهران

چکیده

یک روش تحلیلی شاخص (نشانگر­های ژئوشیمیایی (کربن آلی کل و میزان فلزات کمیاب حـساس به شرایـط اکسـیداسیون-احیا) و نشان­گرهای رسوب­شناختی) برای شناخت شرایط اکسیداسیون-احیا در بخش جنوبی حوضه خزر جنوبی مورد استفاده قرار گرفته است. بر پایه بررسی­های رسوب­شناختی، رسوبات گل­پشتیبان در بخش­های دور از منشا سکوی قاره فراوان هستند، درحالی­که تناوبی از دبریت­ها و رسوبات پلاژیک-همی­پلاژیک، بافت­های مهم در شیب و بخش­های نزدیک به منشا دشت حوضه هستند. چندین پارامتر (مانند اکسیژن محلول زیرین، کربن آلی کل، محتوی زیستی، پلت­های مدفوعی و فلزات کمیاب) پیشنهاد می­دهند که این رسوبات در محیط­هایی با شرایط متفاوت اکسیداسیون-احیا نهشته شده­اند: 1- محیط­های سکو و شیب قاره (رسوبات با شرایط اکسیدان) و 2- محیط­های دشت حوضه و حوضه­های درون­شیبی (رسوبات با شرایط کم اکسیژن). یافته­های حاصل از این پژوهش نشان می­دهد که رسوبات دشت حوضه و حوضه­های درون­شیبی در زون کمینه اکسیژن بر جای گذاشته شده­اند. رسوبات کم اکسیژن دارای مقادیر بیش­تری از فلزات کمیاب (وانادیم، روی، کروم، مس و نیکل)، کربن آلی کل، پلوئید­های گلی و شامل مواد آلی نوع II هستند، درحالی­که رسوبات اکسیدان مقادیر بیش­تری از موجودات ساکن در بین رسوبات (مانند استراکودا و گاستروپودا)، پلت­های مدفوعی و مواد آلی نوع III و مخلوط II/III را دارند. مواد آلی نوع III نشان­دهنده آورد مواد آلی با منشا قاره­ای (خشکی­زی) به رسوبات سکو و شیب قاره می­باشد. میزان حفظ­شدگی ضعیف مواد آلی و زیست­آشفتگی از تجمع مواد آلی در رسوبات اکسیدان پیشگیری می­کند. این پژوهش نـشان مـی­دهـد که مـیزان حفظ­شدگی مواد آلی در بخش جنوبی حوضه خزر جنوبی بوسیله چندین پارامتر مانند اکسیژن محلول زیرین، نوع مواد آلی و محتوی زیستی کنترل می­شود.

کلیدواژه‌ها


عنوان مقاله [English]

The role of sedimentological and geochemical indicators in the recognition of redox conditions in the southern part of the South Caspian Basin

نویسندگان [English]

  • reza behbahani 1
  • gh hosseinyar 1
  • A. Karimkhani 1
  • H. Mohseni 2
  • Z. Atashmard 3
1 Dept., of marine geology, Geological survey of Iran, Tehran
2 Dept., of geology, Bu-Ali sina university, Hamadan, Iran
3 Zamin Kavan Geotechnical Consulting Engineers, Tehran, Iran
چکیده [English]

Sediment constituents, total organic carbon contents, redox-sensitive trace-metals concentrations, and organic matter (OM) types have been analyzed on fifteen sediment cores (73cm-165cm long) taken from the southern part of the South Caspian Basin (SPSCB). 257 samples for sedimentological studies (particles size analysis, mineralogy and biota contents) and 120 samples for organic-inorganic geochemistry studies (TOC, S2 and RSTM concentrations) were analyzed. Sediments on the distal parts of shelf are mud dominated, while slope and proximal parts of the basin plain comprise interbedded debrite, and pelagic-hemipelagic sediments. Several factors (e.g. bottom dissolved oxygen (BDO), TOC, biota content, faecal pellets, and RSTM) suggest that these sediments were deposited under markedly different redox conditions, namely: (1) Shelf and slope settings (oxic sediments), and (2) intraslope basins and basin plain settings (dysoxic sediments). Observations suggest that basin plain and intraslope basins sediments were deposited under oxygen-depleted (oxygen minimum zone) conditions. The dysoxic sediments are enriched in RSTM (V, Zn, Cr, Cu, Ni), with high TOC values, with mud peloids, and contain type II OM, while the sediments of the oxic sediments are enriched in infaunal organisms, faecal pellets, and contain types III and mixture II/III OM.

کلیدواژه‌ها [English]

  • Dysoxic sediments
  • Oxygen minimum zone
  • Sediment cores
  • Redox-sensitive trace-metals
  • South Caspian Basin
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