Evidence of circadian metabolism by photosynthetic microorganism in neoarchean and palaeoproterozoic banded iron formations

Abstract

Banded iron formations (BIF) are chemical depositions happened only at a particular evolutionary stage of Earth (3.8-1.8 Ga) characterized by dichotomous silica-rich and iron-rich bands with hierarchical structures of thickness and composition. The forming mechanism of these bands has long been a fascinating question and the explanations are still highly controversial. Macroband, mesoband and microband are three distinct scales of bands recognized in BIFs each represents geological episodes at different time scales and/or with varied sedimentary rhythmites. Previous studies indicated that in the mesobands, there are microbands (aftbanding) with complex structures which were suggested to be annual or even diurnal varves deposited as the result of the combined temperatures and geochemical processes. The Palaeoproterozoic BIFs represent the peak deposition through the history of BIF. The Kuruman Iron Formation is in the Transvaal Supergroup of Northern Cape Province, South Africa, with an age of 2460 Myr; The Abitibi BIF is embedded in the uppermost section of the Hunter Mine Group of bimodal volcanic complex in the Abitibi greenstone belt with an age of 2728-Myr. The centimeter-sized specimens were investigated by electron microscopic methods. Hitachi S-4800 FEG SEM in the secondary electron mode at low voltage (3-5 kV) was used for surface structure observation, and the backscatter electron mode was used for density induced texture differences. The structure of ultrafine hematite was characterized by Philips Tecnai G2 20 S-TWIN STEM by selected area electron diffraction technique and the chemical composition was analyzed by equipped electron dispersive X-ray spectroscopy. Hematite in most BIFs experienced only low-grade metamorphism is characterized by its extremely fine crystal size (4-6 nm) which indicated very early sedimentary or diagenetic features because the transformation of amorphous ferrihydrite to hematite takes time much shorter than geological time scale. There is no microband of hematite can be observed, but there are hematite breccias of a few tens micrometers or larger in sizes made of much finner laminated structures. We observed 17-26 nm nanobands in hematite and suggest them to be the evidence of diurnal deposition. The 4-12 micrometers microbands of chert and jasper were observed and we suggest them to be evidences of annual depositions. The thickness of hematite nanobands, if assumed to be diurnal precipitation, equals to an annual depositional of about 18.6 μm or 18.6 meter per million years of deposition. The thinnest microbands of magnetite and ferro-dolomite of 20-40 micrometers appear cyclically in the matrix of chert, implying their episodic precipitating natures. However, their easy growths upon continuous supply of materials during the early diagenetic and/or lithification processes make it hard to determine if their thinnest bands were annual deposition or not. These depositional rates are consistent with the integrated depositional rates calculated from the geochronological methods. The records of diurnal hematite and annual depositions of chert or jasper in BIFs make it possible to measure the short-term processes, such as oceanographic, climate and ecological cycles. The clear bandings can be measured or even counted to year to less than millennium time scales that is essential to the reconstruction of climate and ecology processes in the shallow-water environments of Neoarchean to Palaeoproterozoic eras.