Strontium isotope composition of the lower proterozoic carbonate concretions: The Zaonega Formation, Southeast Karelia

Abstract

The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite con cretions contain some dolomite admixture (Mg/Ca = 0.011-0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3-2.1). The Sr content is as much as 385-505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA frac tion). In individual calcite samples, discrepancy between the measured 87 Sr/ 86 Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008-0.0033. The initial 87 Sr/ 86 Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with "mild" reductive con ditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87 Sr from the associated siliciclastic minerals, resulting in growth of the initial 87 Sr/ 86 Sr in concretions up to 0.7108-0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. There fore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87 Sr/ 86 Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.

Hydrothermal dedolomitisation of carbonate rocks of the Paleoproterozoic Zaonega Formation, NW Russia — Implications for the preservation of primary C isotope signals

“…Additionally, it is unlikely that the oxidation of CH 4 , (e.g., through anaerobic oxidation of methane, AOM) was the sole source of DIC because none of the obtained δ 13 C carb values in calcite-rich carbonates is lower than -17‰. Typical AOM derived carbonates show δ 13 C carb values well below -20‰ (e.g., Campbell, 2006 Kuznetsov et al (2012) for the Zaonega Formation.…”

“…This has led to the interpretation that the transition from the 13 C-rich carbonates to those with values near 0‰ in the middle part of the Formation denotes the end of the Lomagundi-Jatuli Event (Melezhik et al, 2015). The 13 C-depleted carbonate rocks in the upper part of the Formation were originally interpreted as concretions (Kuznetsov et al, 2012;Melezhik et al, 1999) but more recent studies have shown that they are variably altered sedimentary layers Melezhik et al, 2015). Interpretations of such low δ 13 C carb values are quite variable and include: incorporation of C derived from anaerobic methane oxidation (Yudovich et al, 1991), diagenetic recrystallisation and incorporation of C derived from oxidised organic matter (Krupenik et al, 2011), and recycling of organic matter involving microbial and/or thermal reduction of sulphates and Fe-and Mn-oxides (Melezhik et al, 2015).…”

Isotope and elemental geochemistry of black shale‐hosted fossiliferous concretions from the Cretaceous Santana Formation fossil Lagerstätte (Brazil)

“…As reported by Peryt & Anczkiewicz () based on analyses of Miocene evaporites, substantial inflow of continental waters during brine formation has the potential to shift the Sr‐isotope ratio of marine gypsum and anhydrite towards non‐marine, more radiogenic values. Alternatively, ascending fluids may have been modified by interaction with terrigenous siliciclastics and/or continental crust (Meister et al ., ; Kuznetsov et al ., ). In the Araripe Basin, the concretion‐bearing deposits are superimposed on a heterogeneous lithological succession made up of terrigenous and marginal marine siliciclastics, which cover a complex suite of Proterozoic crystalline basement (Martill, ).…”

An Akouemma hemisphaeria Organic Macrofossils Colony Hosting Biodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) dated at 2.2 Ga

“…The formation of silico-carbonate "abiotic" nodules by interactions between sediments and microorganisms lead to the formation of the accretion laminae observed in these nodules and in the stromatolites, from the Proterozoic [22][23][24] to modern analogues [25]; and microbially mediated calcification can be traced back for at least the Proterozoic [26][27][28][29][30]. Additionally, these "abiotic" concretions typically develop from a central point and present often a zonal concentric structure; they may contain concentric and lateral accretion laminae [31], characterized by variations in the isotopic and geochemical compositions of the different growth zones [32].…”