The Shuram excursion: A response to climate extremes at the dawn of animal life

Abstract

The Ediacaran-aged Shuram excursion was the last and largest of the Neoproterozoic negative carbon isotope anomalies. Recognized in stratigraphic successions around the globe, it precedes diverse evidence for macroscopic, multicellular life, and follows the Cryogenian global glaciations and Ediacaran Gaskiers glaciation. Hypotheses for the cause of the Shuram excursion can be broadly grouped into those that argue for post-depositional diagenetic alter-1

Deep‐water first occurrences of Ediacara biota prior to the Shuram carbon isotope excursion in the Wernecke Mountains, Yukon, Canada

“…Second, recent geochemical temperature records for the ca. 590-550 Ma period suggest that while the Shuram CIE interval itself may have been associated with substantial warming, the temperatures prior to and during the appearance of the Avalon assemblage were not actually that hot (Bergmann, Osburn, et al, 2022, in agreement with the worldwide occurrences of glaciogenic sedimentological evidence as seen in Hoffman and Li (2009) and Wang et al (2023)). This is especially true when compared to other times in the Tonian or Cambro-Ordovician where much hotter tropical temperatures of ~35-45°C have been suggested (Bergmann, Boekelheide, et al, 2022;Goldberg et al, 2021;Hearing et al, 2018).…”

“…Second, recent geochemical temperature records for the ca. 590–550 Ma period suggest that while the Shuram CIE interval itself may have been associated with substantial warming, the temperatures prior to and during the appearance of the Avalon assemblage were not actually that hot (Bergmann, Osburn, et al., 2022, in agreement with the worldwide occurrences of glaciogenic sedimentological evidence as seen in Hoffman and Li (2009) and Wang et al. (2023)).…”

“…It may simply be that the cold, deep sea was the only habitat in the middle Ediacaran where animals could depress their metabolic rates sufficiently to meet the limited available food supply, recognizing that the decrease in organismal food demand with depth must be sharper than the attenuation of food supply with depth. While this “Greenland shark” strategy may explain some of the paleontological pattern, it also cannot explain why Avalon assemblage organisms are not found in polar regions, which, given available temperature records (Bergmann, Osburn, et al., 2022) suggest that similarly cold temperatures to the deep sea (and concomitantly low metabolic rates) would also have been found at high latitudes. Again, uncertainties in Ediacaran paleogeographic reconstructions also confound this counterpoint.…”

Natural sampling and aliasing of marine geochemical signals

“…However, although widely recognised as coincident with major sea level rise (Busch et al, 2022), and therefore considered to be condensed at its base (Grotzinger et al, 2011), the Shuram excursion is still frequently described as being a "rapid" event, followed by a "gradual" recovery. Loose mixing of temporal and spatial terminology is commonplace, but if believed can lead potentially to unrealistically catastrophic interpretations (Bergmann et al, 2022) of what might have been relatively gradual system change. Indeed the shape of the Shuram anomaly varies greatly due to variable sedimentation rates even within the same sedimentary basin (Lu et al, 2013) and with the presence of unconformities (e.g., Husson et al, 2015b).…”

Temperature: a key driver of Earth's habitability over the last billion years

“…While the δ 13 C values of Neoproterozoic dolomite and limestone often diverge, we note the significant variability in the record itself during this time interval. Some large carbon isotope excursions have a temporal mineralogical signal across them (47,48). Dolomite and limestone are often forming in shallow and deeper water environments in the Neoproterozoic, respectively (40).…”