Centennial to millennial-scale changes in thermocline ventilation in the Arabian Sea: insights from the pteropod preservation record

Authors

  • Arun Deo Singh Micropaleontology and Oceanography Laboratory, Department of Geology, Institute of Science, Banaras Hindu University, Varanasi 221 005, India

DOI:

https://doi.org/10.54991/jop.2021.18

Keywords:

Pteropods, Monsoon, ACD, OMZ, Thermocline ventilation, SAMW–AAIW

Abstract

The Arabian Sea hosts one of the three thickest oxygen minimum zones (OMZs) of the world ocean. Mid–depth oxygen depletion profoundly influences the chemistry of thermocline waters (HCO3ˉ, CO3 and pH), which in turn significantly influences the preservation state of carbonates. The carbonate preservation is primarily controlled by the degree of saturation level of seawater with respect to the calcite and aragonite. The seawater in OMZ is undersaturated with respect to the aragonite (a metastable polymorph of CaCO3). Pteropod test being aragonitic in composition is therefore highly susceptible to the dissolution and dissolves completely below the aragonite compensation depth (ACD). Because of the current condition of intense OMZ due to high primary productivity, enhanced respiration of sinking organic carbon and reduced thermocline circulation; the ACD is shallow, lying in the middle of the OMZ. Hence, the preservation record of pteropods in seafloor sediment archives past changes in thermocline oxygen condition, carbonate chemistry, the ACD and OMZ intensity. High resolution records of various pteropod preservation indices (total pteropod abundance, transparent Limacina inflata abundance, fragmentation index) in a sediment core from the lower OMZ of the Indian margin (off Goa) enabled to investigate aragonite preservation/dissolution events and their links with the changes in ACD and OMZ intensity in the eastern Arabian Sea during the last 70 kyr BP. The proxy records reveal centennial to millennial scale changes in aragonite preservation condition in concert with Northern Hemisphere climatic events (Dansgaard–Oeschger (D–O) cycles and Heinrich events). The pteropod preservation spikes apparently correspond to the Northern Hemisphere cold events (D–O stadials and Heinrich events). Whereas, the pteropod tests were either poorly preserved or completely dissolved during the warm phases of D–O cycles (interstadials). The aragonite preservation events are attributed to the low monsoon induced productivity combined with the increased thermocline ventilation by Subantarctic Mode and Antarctic Intermediate Waters (SAMW–AAIW) resulting a weak OMZ and deeper ACD. The novel proxies (abundances of Globorotalia menardii, a planktic foraminifera and Styliola subula, a pteropod species) are used to gain better insights into the variability of thermocline ventilation and OMZ intensity through time.

सारांश

अरब सागर विश्व महासागर के तीन सबसे मोटे ऑक्सीजन न्यूनतम क्षेत्रों (ओएमजेड) में से एक की मेजबानी करता है। मध्य-गहराई ऑक्सीजन की कमी थर्मोकलाइन जल (HCO3ˉ, CO32ˉ और pH) के रसायन को गंभीरता से प्रभावित करती है, जो बदले में कार्बोनेट के संरक्षण की स्थिति को महत्वपूर्ण रूप से प्रभावित करती है। कार्बोनेट संरक्षण प्राथमिक रूप से केल्साइट और ऐरोगोनाईट के संबंध में समुद्री जल के संतृप्ति स्तर की डिग्री द्वारा नियंत्रित किया जाता है। ऐरोगोनाईट (CaCO3 का एक मेटास्टेबल पॉलीमॉर्फ) के संबंध में ओएमजेड में समुद्री जल असंतृप्त है। संघटन में ऐरोगोनाईट होने के कारण टेरोपोड टैस्ट विघटन के लिए अतिसंवेदनशील है और ऐरोगोनाईट कम्पनसेशन (एसीडी) के नीचे पूरी तरह से घुल जाता है। उच्च प्राथमिक उत्पादकता होने से तीव्र ओएमजेड की वर्तमान स्थिति के कारण, डूबने वाले कार्बनिक कार्बन की बढ़ी हुई श्वसन और कम थर्मोकलाइन परिसंचरण से; एसीडी उथला है, ओएमजेड के मध्य में पड़ा हुआ है। अतः समुद्र-तल अवसादों में टेरोपोड्स का संरक्षण रिकॉर्ड थर्मोकलाइन ऑक्सीजन की स्थिति, कार्बोनेट रसायन, एसीडी और ओएमजेड तीव्रता में परिवर्तन को अभिलेखित करता है। भारतीय मार्जिन (गोवा के बाहर) के निचले ओएमजेड से एक अवसाद कोर में विभिन्न टेरोपोड संरक्षण सूचकांक के उच्च रिज़ॉल्यूशन रिकॉर्ड (कुल टेरोपोड प्रचुरता, पारदर्शी लिमासिना इन्फ्लेटा प्रचुरता, विखंडन सूचकांक) पूर्वी अरब सागर में पिछले 70 kyr BP के दौरान अर्गोनाइट संरक्षण / विघटन घटनाओं और एसीडी और ओएमजेड की तीव्रता में परिवर्तन से उनके लिंक के अन्वेषण करने में सक्षम बनाता है। प्रॉक्सी रिकॉर्ड उत्तरी गोलार्ध की जलवायु घटनाओं (डांसगार्ड-ओशगेर (डी-ओ) चक्र और हेनरिक घटनाएँ) में सामंजस्य के साथ ऐरोगोनाईट संरक्षण की स्थिति में शताब्दी से सहस्राब्दी पैमाने पर परिवर्तन प्रकट करते हैं। टेरोपोड संरक्षण स्पाइक्स स्पष्ट रूप से उत्तरी गोलार्ध की शीत घटनाओं (डी-ओ स्टैडियल्स और हेनरिक इवेंट्स) के अनुरूप हैं। जबकि, डी-ओ चक्र (इंटरस्टेडियल्स) के ऊष्म चरणों के दौरान टेरोपोड टैस्ट या तो खराब रूप से संरक्षित थे या पूर्णतः घुल गए थे। 

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2021-09-10

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Deo Singh, A. (2021). Centennial to millennial-scale changes in thermocline ventilation in the Arabian Sea: insights from the pteropod preservation record. Journal of Palaeosciences, 70((1-2), 253–266. https://doi.org/10.54991/jop.2021.18