Neodymium isotopic composition of Cambrian–Ordovician biogenic apatite in the Baltoscandian Basin: implications for palaeogeographical evolution and patterns of biodiversity

Abstract

Biogenic apatite preserved in 148 samples of conodonts and organophosphatic-shelled brachiopods from Cambrian through Ordovician successions of the Baltoscandian Basin (Baltica Plate) preserves a sensitive record of early Palaeozoic sea-water chemistry interpreted via neodymium isotope ratios. Consistent ε Nd (t) values of −9.6 to −8.3 for Lower to Middle Cambrian samples suggest no significant lateral or temporal variation across the region. Average Upper Cambrian values are −7.2 to −7.7. Sedimentary analysis suggests that the influence of continental weathering from Baltica as a major source of radiogenic Nd was negligible. Ordovician samples show a rise to −5 to −6 in the early Arenig, early-mid Llanvirn and late Caradoc. Sea-water mixing from the southeast Iapetus Ocean was a constant factor throughout Cambrian-Ordovician times. The rise reflects erosion of obducted volcanic arc complexes along the Caledonian margin, and probably also relates to pollution of the Baltica sector of Iapetus from the approaching Avalonia Plate. Patterns of evolutionary biodiversity and palaeobiogeographical linkages support the geochemical signatures in interpreting the tectonic history of the region. Extinction of lingulate brachiopod faunas in the Tremadoc, followed by subsequent recovery and emergence of benthic assemblages typical of the Ordovician Evolutionary Fauna in the Billingen-early Volkhov regional stages coincide with significant changes in geochemical characteristics of water masses across the Baltoscandian basin. The early and mid Ordovician (Arenig to Llandeilo) brachiopod faunas of the North Estonian Confacies Belt are characterized by high endemism and low turnover rates, whereas increased immigration resulted in the extinction of a number of local lineages in the late Llanvirn. From the mid Caradoc to mid Ashgill, when Baltica was drifting on a course to collide eventually with Avalonia and gradually approach Laurentia, brachiopod assemblages were characterized by higher turnover rates. At the same time they gradually became more cosmopolitan and less influenced by the invasion of new faunas.

First report of craniide brachiopods in the Palaeozoic of Iran (Pseudocrania, Ordovician), and Early to Mid‐Ordovician biogeography of the Craniida

“…A cicatrix attachment developed on the umbonal area of the ventral valve shows that the animal attached to hard grounds by cementation immediately after settlement, but Orthisocrania at least soon transferred to an ambitopic (free‐lying) mode of life, which makes this taxon unique among craniides. The Mid‐Ordovician was a time of increasing endemism and biogeographical isolation of Baltican faunas (Sturesson et al. 2005).…”

Nd and Sr isotopic composition of ancient iron‐made artifacts and ores from northwest Russia

“…The Sr and Nd isotopic composition of springs, groundwaters, and rivers are governed by the isotopic ratios of these elements in the rocks of the catchment area (e.g., Möller, Dulski, & Gerstenberg, ). Early Paleozoic limestone and dolomite exposed on the EEP in the Ordovician glint (a scarp several meter high) from Lake Ladoga to Estonia have 143 Nd/ 144 Nd ratios of 0.51195–0.51247 (S. Felitsyn, Sturesson, Popov, & Holmer, ; Sturesson et al, ), and 87 Sr/ 86 Sr ratios of 0.7075–0.7085 (Denison et al, ; Veizer et al, ). In contrast, the 143 Nd/ 144 Nd ratios of Precambrian rocks in the Baltic craton range from 0.510678 to 0.511354 (Nazarova, Bibikova, & Kostitsyn, ).…”

“…In the areas where ancient Precambrian rocks predominate (like the Baltic craton), continental waters have low 143 Nd/ 144 Nd ratios (<0.5115) and high 87 Sr/ 86 Sr ratios up to 0.943 (Fisher & Strueber, ; Goldstein & Jacobsen, ). To the south in the EEP, where the isotopic composition of water is strongly influenced by Ordovician carbonate rocks, 87 Sr/ 86 Sr varies from 0.7085 to 0.7092 (Denison, Koepnick, Burke, & Hetherington, ) and 143 Nd/ 144 Nd is about 0.5120 (Sturesson, Popov, Holmer, & Felitsyn, ). Therefore, data suggest that iron deposits in the northern and southern parts of the Vodskaya pyatina could also have contrasting Nd and Sr isotopic compositions.…”

Reconstruction of currents in the Mid-Ordovician–Early Silurian central Baltic Basin using geochemical and mineralogical indicators

“…3C). The brachiopod taxa of the Avalonian affi nity reveal a decline in the Pirgu Stage compared to the previous Kukruse-Rakvere interval (Sturesson et al, 2005) implying that the current did not pass by Avalonia before reaching the Baltic Basin. The evidence supports the north-to-south orientation of the current and its presumed near-equatorial source in the time corresponding to the Jonstorp Formation.…”