The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform

An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallowmarine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust’-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing δ13C positive values as high as+4.6‰and least-altered 87Sr/86Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals

Extreme geomagnetic reversal frequency during the Middle Cambrian as revealed by the magnetostratigraphy of the Khorbusuonka section (northeastern Siberia)

© 2019 Elsevier B.V. We present new magnetostratigraphic results obtained for the Drumian stage (504.5–500.5 Ma; Epoch 3/Middle Cambrian) from the Khorbusuonka sedimentary section in northeastern Siberia. They complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time. Magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions. Thermal demagnetization makes it possible to distinguish two magnetization components. The low unblocking temperature (20 reversals per Myr during the Drumian to ∼1.5 reversals per Myr during the Furongian/Upper Cambrian. Such a sharp decrease is consistent with a transition at a ∼1-Myr timescale, probably caused by threshold effects in core processes, between two geodynamo modes, one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per Myr, and the other marked by hyperactivity of the reversing process, with reversal rates >15 reversals per Myr

Extreme geomagnetic reversal frequency during the Middle Cambrian as revealed by the magnetostratigraphy of the Khorbusuonka section (northeastern Siberia)

© 2019 Elsevier B.V. We present new magnetostratigraphic results obtained for the Drumian stage (504.5–500.5 Ma; Epoch 3/Middle Cambrian) from the Khorbusuonka sedimentary section in northeastern Siberia. They complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time. Magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions. Thermal demagnetization makes it possible to distinguish two magnetization components. The low unblocking temperature (20 reversals per Myr during the Drumian to ∼1.5 reversals per Myr during the Furongian/Upper Cambrian. Such a sharp decrease is consistent with a transition at a ∼1-Myr timescale, probably caused by threshold effects in core processes, between two geodynamo modes, one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per Myr, and the other marked by hyperactivity of the reversing process, with reversal rates >15 reversals per Myr

BRACHIOPODS FROM THE BYRD GROUP (CAMBRIAN SERIES 2, STAGE 4) CENTRAL TRANSANTARCTIC MOUNTAINS, EAST ANTARCTICA: BIOSTRATIGRAPHY, PHYLOGENY AND SYSTEMATICS

Brachiopods from Cambrian Series 2, Stage 4 carbonate strata of the Byrd Group in the Central Transantarctic Mountains, East Antarctica, are described for the first time. These include six lingulate, one paterinate, and one rhynchonelliform taxa, including the new lingulate brachiopod Plicarmus wildi gen. et sp. nov. The biostratigraphy correlates closely to the brachiopods recently reported from the Xinji Formation (Shuiyu section) in North China, as well as brachiopods recovered from the Dailyatia odyssei Zone across the Arrowie Basin of South Australia. These findings also support the previously identified close palaeobiogeography of these regions. The first unambiguous example of the acrotretid brachiopod Eohadrotreta zhenbaensis Li and Holmer outside South China is also identified in the context of its ontogenetic stages. Well preserved specimens of the acrotheloid Schizopholis yorkensis (Holmer and Ushatinskaya in Gravestock et al.) facilitates a new reconstruction of its musculature and visceral region. This data is synthesised into a new cladistic analysis that resolves Acrotheloidea as a well-supported monophyletic clade and supports previous hypotheses of a morphocline in acrotheloid evolution

Chapter 19 Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity

<p>Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through <em>c.</em> 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation. </p

Himalayan Cambrian brachiopods

A synoptic analysis of previously published material and new finds reveals that Himalayan Cambrian brachiopods can be referred to 18 genera, of which 17 are considered herein. These contain 20 taxa assigned to species, of which five are new: Eohadrotreta haydeni, Aphelotreta khemangarensis, Hadrotreta timchristiorum, Prototreta? sumnaensis and Amictocracens? brocki. Well-preserved topotype material from the classic Parahio Valley section is described for three species that have not previously been illustrated photographically. A biostratigraphical scheme based on Cambrian brachiopod occurrence is integrated with that recently proposed for trilobites. Regional correlations based on shared taxa within and across Himalayan lithotectonic belts demonstrate that erosion of Cambrian rocks in the Kurgiakh orogeny in the Parahio Valley removed little of the Parahio Formation and that all of the fossil-bearing lithotectonic zones share similar late early Cambrian brachiopods, suggesting regional faunal continuity at the time. Palaeobiogeographical analysis of the Cambrian brachiopod faunas from the Himalaya shows that they occupied a marginal position of the Australasian segment of Gondwana