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Akinetes From Late Paleoproterozoic Salkhan Limestone (>1600 Ma) of India: A Proxy for Understanding Life in Extreme Conditions.

ABSTRACT: Isolated elongate spore-like cells present in the >1600 Ma-old Salkhan Limestone of the Semri Group, Vindhayan Supergroup, India are considered akinetes of the heterocystous cyanobacteria. Small to large size, and young (single walled) to mature (double walled) akinetes - namely, Archaeoellipsoides bactroformis, A. conjuctivus, A. dolichos, A. elongatus, A. grandis, A. major and A. minor - found in the stromatolitic and bedded cherts have been reported in the present paper. Their role in understanding extreme environmental conditions is a subject matter of this paper. Additionally, the occurrence of doubly-terminated quartz crystals and fan-fabrics in the Salkhan Limestone indicates adverse conditions for the survival of life forms. The depositional environment of the Salkhan Limestone, Vindhyan Supergroup is suggested to be shallow marine intertidal with pulses of the intermittent hypersaline regime during which akinetes, closely resembling those of extant Nostocaceans, were formed by cyanobacteria for survival in the extreme conditions.

SUBMITTER: Sharma M

PROVIDER: S-EPMC6423410 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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Akinetes From Late Paleoproterozoic Salkhan Limestone (>1600 Ma) of India: A Proxy for Understanding Life in Extreme Conditions.

Sharma Mukund M Shukla Bandana B

Frontiers in microbiology 20190312

Isolated elongate spore-like cells present in the >1600 Ma-old Salkhan Limestone of the Semri Group, Vindhayan Supergroup, India are considered akinetes of the heterocystous cyanobacteria. Small to large size, and young (single walled) to mature (double walled) akinetes - namely, Archaeoellipsoides bactroformis, A. conjuctivus, A. dolichos, A. elongatus, A. grandis, A. major and A. minor - found in the stromatolitic and bedded cherts have been reported in the present paper. ...[more]

PMID: 30930860

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Project description:The Longwangzhuang pluton is a typical example of Paleoproterozoic A-type granite intrusions at the southern margin of the North China Craton. This pluton is composed of arfvedsonite granite and minor aegirine-augite granites. Samples from both granite types display similar zircon U-Pb ages with 207U-206Pb ages of 1612?±?19?Ma [mean square weighted deviation (MSWD)?=?0.66] and 1609?±?24?Ma (MSWD?=?0.5), respectively. The granites exhibit similar high silica (SiO2?=?71.1-73.4 wt.%), high alkaline (Na2O?+?K2O?=?8.10-9.26 wt.%, K2O/Na2O?>?1), and low Al2O3 (11.8-12.8 wt. %) contents and metaluminous to weakly peraluminous bulk chemistry. The chemical variations of the Longwangzhuang pluton suggest the effects of mineral fractionation. In addition, all samples show typical characteristics of A-type granites, such as high 10000Ga/Al ratios (4.10-7.28), high FeOtot/(FeOtot?+?MgO) ratios (0.88-0.99), and high Zr (484-1082 ppm), Ce (201-560 ppm), and Y (78-156 ppm) contents. The ?Nd(t) values and the (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t ratios of the arfvedsonite granite samples vary from -4.6 to -5.3, 15.021 to 17.349, 15.241 to 15.472, and 33.206 to 36.905, respectively, and those for the aegirine-augite granite sample amount at -0.2, 14.421, 15.175, and 33.706. The distinct and variable Nd and Pb isotope values indicate the presence of heterogeneous protoliths. Based on its geochemistry, its low initial Pb isotope ratios, and its enrichment in Nd isotopes, we infer that the Longwangzhuang A-type granite is the partial melting product of basement rocks such as the Taihua Group gneisses accompanied by some involvement of juvenile material from the mantle. Together with published data from other Paleoproterozoic A-type granite plutons exposed at the southern margin of the craton, our findings suggest that these granites had a similar origin. Furthermore, geochemically, they can be divided into two groups: A2-type, which formed earlier (~1.8-1.6 Ga), and A1-type, which formed later (~1.6-1.5 Ga). Combining this information with the variations in whole-rock Nd and zircon Hf isotopic composition at ca. 1.6 Ga, we propose that tectonic transformation from post-orogenic to anorogenic magmatism occurred at the southern margin of the North China Craton at that time.

Dataset Information

Akinetes From Late Paleoproterozoic Salkhan Limestone (>1600 Ma) of India: A Proxy for Understanding Life in Extreme Conditions.

Publications

Akinetes From Late Paleoproterozoic Salkhan Limestone (>1600 Ma) of India: A Proxy for Understanding Life in Extreme Conditions.

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