Orbital forcing of ice sheets during snowball Earth

Ross N Mitchell,Yebo Liu,Adam R Nordsvan,Thomas M Gernon,Chuang Xuan,Xiaofang He,Uwe Kirscher,Grant M Cox,Xu Liu

doi:10.1038/s41467-021-24439-4

Abstract

The snowball Earth hypothesis—that a runaway ice-albedo feedback can cause global glaciation—seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and “cap” carbonates. One of the most significant challenges to snowball Earth has been sedimentological cyclicity that has been taken to imply more climate dynamics than expected when the ocean is completely covered in ice. However, recent climate models suggest that as atmospheric CO2 accumulates, the snowball climate system becomes sensitive to orbital forcing. Here we show the presence of nearly all Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth. These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. Orbital glacial advance and retreat cycles provide a simple mechanism to reconcile both the sedimentary dynamics and the enigmatic survival of multicellular life during snowball Earth.

Highlights

The snowball Earth hypothesis—that a runaway ice-albedo feedback can cause global glaciation—seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and “cap” carbonates
We show that banded iron formation (BIF)9,10—wellstratified sedimentary rock containing abundant iron oxide—is ideally suited to cyclostratigraphic analysis because it is closely associated with glacial diamictites[9,10,11,12], and deposited at a relatively constant rate when averaged over multiple depositional cycles as evidenced by the rhythmic banding[13,14] on decimeter and centimeter (Fig. 1d; Supplementary Fig. 1c) and millimeter scales (Supplementary Fig. 2)
The occurrence of BIF in Neoproterozoic glacial deposits was initially conceived as a “last gasp” for global glaciation[15] and thought to represent oxidation of hydrothermal iron accumulated within a snowball ocean after the re-emergence of open-ocean conditions[6,15]

Summary

Introduction

The snowball Earth hypothesis—that a runaway ice-albedo feedback can cause global glaciation—seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and “cap” carbonates. We show the presence of most Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. While evidence for orbital forcing has been suggested in Cryogenian glacial successions[5], evidence for multiple, internally consistent timescales of orbital forcing has not been demonstrated This is likely because most glacial deposition occurs irregularly[7], forming glacial diamictites (that is, lithified sediments comprising chaotic mixtures of a wide range of clast- and grain-size distribution) only rarely preserving stratification. The occurrence of BIF in Neoproterozoic glacial deposits was initially conceived as a “last gasp” for global glaciation[15] and thought to represent oxidation of hydrothermal iron accumulated within a snowball ocean after the re-emergence of open-ocean conditions[6,15]. BIF deposited during snowball Earth has been interpreted as the product of hydrothermal derived iron mixing with oxidized subglacial meltwater[16]

Methods

Results

Conclusion