Rapid recovery of Ediacaran oceans in the aftermath of the Marinoan glaciation

Abstract

Abstract. The termination of Cryogenian glaciations would have undoubtedly impacted the chemistry of Neoproterozoic oceans, with possible consequences for life; but the extent and duration of this impact are poorly constrained. In this study, we use the lithium (Li) isotope composition of Ediacaran cap dolostones from South Australia (Nuccaleena Formation) and China (Doushantuo Fm) to investigate changes in ocean chemistry that followed the Marinoan deglaciation. The effect of diagenesis was evaluated and while the Nuccaleena Fm is likely to have preserved the primary composition of cap dolostone deposition, the offset in Li isotope ratios observed for the Doushantuo Fm could possibly reflect partial overprinting by diagenetic fluids. The Li isotope composition of Ediacaran seawater was estimated and we suggest it was similar to that of late Cenozoic oceans for most of the cap dolostone deposition. Using a box model for the oceanic Li cycle, we show that at the onset of deglaciation, the supply of riverine Li to the oceans was up to 50 times the modern flux. The modelled riverine Li isotope composition suggests that continents resembled modern high-latitude regions during this time. This episode was short-lived (up to 1 Myr) and the subsequent supply of riverine Li was similar to modern conditions, both in flux and isotope composition, for the whole duration of cap dolostone deposition. These results suggest that Ediacaran oceans and continents rapidly recovered from the Marinoan glaciation to reach environmental conditions similar to the late Cenozoic. From the standpoint of the Li oceanic budget, the Ediacaran oceans in which complex lifeforms emerged may have not been that different from our modern oceans.

Lithium isotope constraints on the plumeworld hypothesis for the Marinoan Snowball Earth

“…1b). Together with the previously published δ 7 Li data from the Daotuo (DT) section in Guizhou Province [30], these sections represent a transect from inner shelf to slope facies (Fig. 1b) [31].…”

Response to Referee #2

“…Nevertheless, it is possible to provide evidences that support the applicability of the protocol in Taylor et al (2019) to this study: in our dataset, there is no relationship between δ 7 Li and several proxies for silicate contamination in the leaching solution: Li/Ca, Al/Ca or Al/Mg (see Fig . 1 for the Nuccaleena Formation).…”

“…Referee 2: the seawater data in Table A1 (Column calibration using seawater samples in Taylor et al (2018)) look not convincing, they scatter between 28.8 and 32.0 ‰Ṫhis is a big spread considering that it should be 31.1 ‰ and that seawater has an easy to deal with matrix.…”

Response to Referee #1

“…Referee 1: the manuscript heavily relies on the results of a study from the same group (Taylor et al 2018), seemingly submitted to the Earth Surface Dynamics journal in 2018…”

“…Our response: Experimental investigation of Li isotope fractionation between a range of natural carbonate chemistry (pure CaCO 3 , CaM gCO 3 , M gCO 3 etc.) has been done for calcium carbonate (Marriott et al 2004a, b), and results for Ca-Mg and Mg carbonates are presented in Taylor et al (2019Taylor et al ( ) (cp-2018.…”