During the Devonian period, major continents/blocks were located in the southern hemisphere (e.g. Stampfli et al. 2002, 2013; Scotese 2014), with a major mountain range running on the northeastern margin of Laurussia to Gondwana: the Appalachian range (Fig. 1) (DeSantis 2010, Scotese 2014). This period of time is marked by one of the most significant modifications in palaeobiogeography with the precipitated end of the strong faunal endemism of the Emsian-Eifelian and the initiation of the Givetian-Frasnian cosmopolitanism (Oliver & Pedder 1979b). During the early Emsian to late Eifelian interval, three distinct faunal assemblages allow the definition of three different marine realms (Oliver & Pedder 1979a, 1979b, 1994; May 1995, 1997b), separated by various barriers. The Malvinokaffric Realm (MKR), located along the margins of Gondwana (Fig. 1), is relatively poor in corals and it is characterized by cold-water species (Oliver 1990;

<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>

<p>During the Middle Cambrian, western Utah (USA) occupied the deepest part of the House Range embayment along the western edge of North America and accumulated deep-water facies dominated by cm-scale, cyclically interbedded limestone-marl layers (‘rhythmites’) of the Marjum Formation. The studied section is ~500 m thick and spans the lower Drumian-lower Guzhangian. The rhythmites (cm-scale) are packaged into repeated 2-5 m-thick parasequences and larger scale eustatic sequences both characterized by more carbonate-dominated rhythmites alternating with more siliciclastic-dominated rhythmites. Previous studies of the Marjum rhythmites suggest millennial-scale interpretation for the cm-scale rhythmites, corresponding to wetter vs drier climate oscillations influencing the relative abundance of siliciclastic vs carbonate accumulation in the deep-water setting (Elrick and Hinnov, 1996; Elrick and Hinnov, 2007).</p><p>We conducted a high-resolution study of a representative 35 m-thick interval of the Marjum Formation to build a floating astrochronology assuming the meter-scale parasequences represent orbital-scale frequencies modulated by the larger scale sequences (based on biostratigraphically controlled sediment accumulation rates). To capture orbital-scale frequencies and to avoid aliasing related to millennial-scale limestone-marl lithological changes, 350 rock samples were collected at minimum 10-cm-interval exclusively in the limestone layers of rhythmite couplets. Major elements (e.g., Ti, Al, K, Si), TOC and δ<sup>13</sup>C were measured on each sample to infer on paleoenvironmental and paleoclimatic conditions. To estimate the duration of the sampled interval, multiple spectral techniques will be applied on the paleoclimatic proxies using amplitude modulation, frequency ratio as well as the 405 kyr long eccentricity as a starting point for the calibration of the time scale. Having an astronomically calibrated time scale for this interval will provide 1) independent evidence assessing the millennial-scale paleoclimatic origin of the cm-scale rhythmites, and 2) refine the Cambrian time scale to better constraint the origin and rate of the Cambrian explosion events.</p><p>Elrick, M. and Hinnov, L.A., 1996. Millennial-scale climate origins for stratification in Cambrian and Devonian deep-water rhythmites, western USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 123(1-4): 353-372.</p><p>Elrick, M. and Hinnov, L.A., 2007. Millennial-scale paleoclimate cycles recorded in widespread Palaeozoic deeper water rhythmites of North America. Palaeogeography, Palaeoclimatology, Palaeoecology, 243(3-4): 348-372.</p>

<p>The Cambrian Explosion is a fundamental turning point in the evolution of life that occurred during the Cambrian Period (~541 to ~485 million years ago), which involved the origination and explosive radiation of all major animal phyla. The bursts of evolution characterizing this period appear concurrently with major modifications to the physico-chemical conditions of the world’s oceans, and are recorded in critical fossil localities where soft-tissues are exceptionally well preserved, including Lagerstätten such as the Burgess Shale and Chengjiang Biota. As a result of the severe lack of biostratigraphically-correlatable fossils (due to widespread endemism during the Cambrian) and sparse high-precision radioisotopic dates, the Cambrian time scale remains among the least defined stage of all the Phanerozoic Eon, with a minimum uncertainty of ±2 million years at its stage boundaries. The absence of a high-resolution geological time scale for the Cambrian Explosion hampers our ability to robustly address widely debated questions concerning the origins and rates of the evolutionary and ecological events, their relationship with paleoceanographic conditions, their responses to astronomically-forced climate change, including from Milankovitch “grand” cycles, and whether these events were globally synchronous.</p><p>Using an integrated set of geophysical/chemical proxies with advanced time series techniques on selected stratigraphic sections, this project aims at (1) Generate sets of high- resolution geophysical and geochemical stratigraphic proxies enabling to capture Milankovitch forcing within the selected sedimentary records, (2) Building a high-resolution time scale to improve our knowledges on the timing of major Cambrian evolutionary milestones and geochemical changes and (3) Determine the relationships between Cambrian evolutionary and ecological events with the paleoceanographic changes and Milankovitch cycles.</p>

<p>Of the many biocrises that occurred during the Devonian period, the most studied ones are the end-Devonian Kelwasser event (Frasnian-Famennian Boundary) and the Hangenberg event (Devonian-Carboniferous Boundary). However, the Middle Devonian crisis are receiving an increasing attention. The crisis occurring immediately before the Eifelian-Givetian Boundary (<em>ensensis</em> conodont zone) is known as the Kačák event. It has essentially been identified by time-specific lithofacies in deep-environment settings where pelagic faunas (conodonts, ammonoids, dacryoconaridids) suffered extinctions. On the Belgian neritic carbonate shelf system the Kačák event has been identified, in the lower part of the Hanonet Formation where a complex faunal turnover took place but with few changes in the depositional settings as known in deeper environments. The event is also recognised on a palaeobiological base as the pre-crisis Old World Realm fauna-dominated assemblages are suddenly facing the invasion of East American Realm fauna such as siphonophrentid and heliophillid rugose corals. After the crisis, these corals are very uncommon and the Old World Realm assemblages become largely dominant again. This palaeobiological criterion is proposed to help, especially when the typical pelagic guides are missing, in the identification of the Kačák event.</p>