Mg and Sr Isotopes in Cap Dolostone: Implications for Oceanic Mixing after a Neoproterozoic Snowball Earth Event

Abstract

The snowball Earth (SBE) describes a state of the Earth’s climate with global or near-global ice cover. The cap dolostone at the base of the Ediacaran successions serves as useful archives for studying environmental change during the Marinoan Snowball Earth deglaciation in Neoproterozoic. The characteristic compositions in dolomite provide critical information on continental weathering and coastal water mixing after glacial retreat. However, valid methods for pristine dolomite separation remain challenging. In this study, four selected cap dolostone samples from the base of the Ediacaran Lantian Formation were used for establishing a new 3-step leaching method, to remove the secondary calcite and other impurities before determination of δ26Mg and 87Sr/86Sr in dolomite. Non-destructive Raman, X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to examine the distribution of dolomite and minor calcite/silicate in each sample. Micro-drill powders before each extraction procedure were examined in weight loss and mineralogical compositions, as well as the chemicals in the leaching solutions. Potential diagenetic artifacts were evaluated using Sr/Ca, Mn/Sr, 87Sr/86Sr and δ26Mg in solutions. By applying a simple two-end member mixing between the seawater and the silicate sources (R2 = 0.48, n = 23), the down-core variations of δ26Mg and 87Sr/86Sr in cap dolostone can be used to gain a better understand of the temporal weathering intensity changes, as well as the coastal oceanic mixing processes, after the Marinoan deglaciation.