The origin of the precession-scale cycles in the ~2.45 Ga Joffre Member, Brockman iron formation, Western Australia

Abstract

It was recently shown that astronomical climate forcing through quasi-periodic variations of the eccentricity of Earth’s orbit played a dominant role in the deposition of Neoarchean and Paleoproterozoic banded iron formations (BIFs). The presence of eccentricity points at the climatic precession cycle as main astronomical driver. Therefore, it is crucial to study geochemical alternations on the precession scale in order to understand the climatic and depositional processes. Here we investigated regular precession-scale variations previously described as Knox cyclothems from two selected drill-core intervals of the ~2.5 Ga Joffre Member BIF (Brockman Iron Fm, NW Australia) by using stable isotope analyses of the organic matter and bulk carbonate content along with detailed mineralogical facies descriptions and high-resolution XRF analyses. Our results revealed systematic variations of the weight percentages of the carbonate and organic carbon and the stable isotope signatures of the carbonates and organic carbon over the Knox cyclothem. The carbonate facies are characterized by a higher weight percentage of organic carbon and carbonate and a heavier stable isotope signature with respect to the oxide and chert facies. We interpreted these results as evidence for original fluctuations in the organic matter during the deposition of BIF. These fluctuations directly controlled the intensity of subsequent diagenetic sedimentary reworking via dissimilatory iron reduction. In the presence of abundant Fe(III)-oxyhydroxides, organic matter was near quantitatively oxidized and partly incorporated into siderite. In the absence of abundant Fe(III)-oxyhydroxides, DIR quantitatively reduced the Fe(III), which triggered the precipitation of a large amount of carbonates. The fluctuations of the organic carbon in the BIF sediment were presumably regulated through changes in bioproductivity in the water column, which varied systematically through the episodic input of terrestrial material and associated bio-limiting nutrients. The input of terrestrial material into the oceans was on its turn regulated by a climate-sensitive (monsoonal) system that operated on the precession scale. Therefore, we have demonstrated that climate directly influenced BIF deposition. This reinforces the idea of BIFs as archives for climate variability on the early Earth.