Abstract
Laser Raman microspectroscopy was used as a microchemical analysis technique to characterize the wall chemistry of organic-walled microfossils (acritarchs and prasinophytes) extracted from Proterozoic (Tonian: ca. 900 Myr) and early Palaeozoic (Silurian: ca. 420 Myr) marine sediments in the Volta Basin of Ghana, and the Ghadamis Basin of Libya, respectively. Raman spectra of Proterozoic acritarchs show spectral features characteristic of kerogenous compounds at ~1350 and ~1600 cm−1, consistently with previously published reports. In addition, spectra from prasinophyte algae from the Silurian sample also show an interesting spectral feature at ~1707 cm−1indicative of carbonyl moieties.Broadly speaking, shape and position of Raman bands appear to depend on the nature of the specimen considered, suggesting that laser micro-Raman analysis can potentially be used to establish phylogenetic relationships (high-rank taxonomy) among the main groups of pre-Cambrian to Palaeozoic palynomorphs.
Highlights
Introduction and scope of studyAcritarchs are a taxonomically informal group of morphologically complex organic-walled microfossils (10–200+ μm) which are found in large abundance in fine-grained marine sediments of Proterozoic (2500–542 Myr ago [9,12,14,16]) and Palaeozoic (542–251 Myr ago [4]) age
The present study is part of a larger project aiming at elucidating the nature, biological affinities, and palaeoecological role of the major groups of pre-Cambrian to Palaeozoic palynomorphs, involving different and complementary analytical techniques; in this paper we present preliminary results on micro-Raman spectroscopy on exceptionally well preserved organic-walled microfossils from two assemblages of differing geological age (Silurian and early Neoproterozoic) with the aims of (1) improving the existing database of microchemical analyses on single-specimens of acritarchs of Neoproterozoic to Palaeozoic age and compare the results vis-à-vis the existing literature data; (2) highlighting possible effects of kerogen degradation through geological times; (3) contributing to assess the potential of microchemical characterization for taxonomic assignment of microfossils and/or enigmatic carbonaceous structures in sedimentary to metamorphic rocks
Thermal maturity of organic matter was assessed by observation of kerogen and microfossils colour and comparison with published tables of colour-index of thermal maturity [17] which resulted in an estimated early mature organic matter for the study samples
Summary
Introduction and scope of studyAcritarchs are a taxonomically informal group of morphologically complex organic-walled microfossils (10–200+ μm) which are found in large abundance (commonly in the order of 105 specimens per gram of rock) in fine-grained marine sediments of Proterozoic (2500–542 Myr ago [9,12,14,16]) and Palaeozoic (542–251 Myr ago [4]) age. Among the various techniques newly applied to the study of acritarch palaeobiology, micro-Raman spectroscopy holds considerable potential as a non-intrusive and non-destructive analytical technique capable of providing crucial information about the microchemical structure of organic-walled microfossils [1,21]. Recent previous studies on micro-Raman spectroscopy on Archean to Proterozoic fossil material have been principally applied (a) to demonstrate the correlation between optically discernible morphologies and kerogenous composition of permineralized fossil microorganisms, especially in situ [20,21,22], (b) as a means to show evidence of biogenicity of microfossil-like structure in Earth’s oldest microfossils and – potentially – in extraterrestrial material [2,21], (c) to tentatively establish phylogenetic relationships between Proterozoic acritarchs and dinoflagellates [1]
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