Cyclostratigraphic calibration of the Miaolingian Series (Middle Cambrian, Southern Scandinavia, Sweden)

Abstract

<p>The Cambrian Period recorded critical evolutionary events and geochemical changes. These changes, such as the “Cambrian Explosion” (Briggs, 2015; Peng <em>et al.</em>, 2020) and the “Cambrian Substrate Revolution” (Mángano & Buatois, 2017; Peng <em>et al.</em>, 2020) can persist for many millions of years or can be a short carbon isotopic excursion or anoxic event. Despite the significance of this period for the history of life on Earth, it features a remarkably poorly defined time scale owing to 1) the paucity of high-precision radioisotope age data, 2) the generalized endemism (especially during the lower Cambrian) and 3) the lack of well-preserved exposures.</p><p>Recent advances in time-series methods for identifying Milankovitch cycles have accelerated the refinement of the Phanerozoic GTS and the invariant set of periods for the Earth’s orbital eccentricity for at least the last 600 Ma have allowed for the building of high-resolution floating astronomical time scales (ATS) for Mesozoic and Paleozoic sequences.</p><p>A crucial issue in unraveling Milankovitch cyclicity in Paleozoic successions is the selection of suitable sedimentary sequences, which are able to record orbitally-forced climatic cycles continuously. A recent cyclostratigraphic study by Zhao <em>et al.</em> (2022) on the middle and upper portion of the Albjära-1 drill core confirmed the record of such cycles in a time interval that extends from the lower Guzhangian to the Lower Ordovician. In this study, we conducted a high-resolution (1 mm) XRF core scanning on the lower portion (27 m) of the Albjära-1 drill-core to assess Milankovitch cyclicity recorded by variations in detrital input proxies and built a floating ATS for the middle Wuliuan-lower Guzhangian interval. Our ATS is in stratigraphic continuity with Zhao <em>et al.’s</em> (2022) ATS, thus allowing us to use the U/Pb absolute age anchor below the Cambrian-Ordovician boundary (486.78 ± 0.53 Ma) and expand their ATS to the middle Wuliuan.</p><p>The core recovery is close to 100%. The first 5 m are characterized by sandy limestone of the Gislöv Formation, and the overlying 22 m consist of deep-water black shales of the Alum Shale Formation, from which 151 samples were taken each 15 cm for δ<sup>13</sup>C<sub>org</sub> analysis.</p><p>The combination of both δ<sup>13</sup>C<sub>org</sub> and XRF elemental analyses allows for precise integration of the ATS in the global Cambrian geochemical framework and provides better insight into the timing and origin of geochemical fluctuations during the studied time interval.</p><p> </p><p>REFERENCES</p><p>Briggs, D. E. G. (2015). The Cambrian explosion. <em>Current Biology</em>,<em> 25 </em>(19), R864-R868. https://doi.org/10.1016/j.cub.2015.04.047</p><p>Mángano, M. G., & Buatois, L. A. (2017). The Cambrian revolutions: Trace-fossil record, timing, links and geobiological impact. <em>Earth-Science Reviews</em>,<em>173</em>, 96-108. https://doi.org/10.1016/j.earscirev.2017.08.009</p><p>Peng, S.-C., Babcock, L. E., & Ahlberg, P. (2020). The Cambrian Period. In F. Gradstein, J. G. Ogg, M. D. Schmitz, & G. M. Ogg (Eds.), <em>Geological Time Scale 2020</em> (Vol. 2, pp. 565-629). Elsevier. https://doi.org/10.1016/B978-0-12-824360-2.00019-X</p><p>Zhao, Z., Thibault, N.R., Dahl, T.W., Schovsbo, N.H., Sørensen, A.L., Rasmussen, C.M.Ø., and Nielsen, A.T. (2022). Synchronizing rock clocks in the late Cambrian. Nature Communications, 13, 1-11. https://doi.org/10.1038/s41467-022-29651-4</p>