A TEMPORALLY CONSTRAINED CHEMOSTRATIGRAPHIC RECORD OF THE LATE NEOPROTEROZOIC AND INITIATION OF THE STURTIAN SNOWBALL EARTH FROM THE UPPER TAMBIEN GROUP IN ETHIOPIA

The snowball Earth hypothesis holds that ice sheets reached sea level in the tropics for millions of years. A critical Snowball Earth prediction is that due to the ice albedo feedback, the initiation of pan-glacial conditions should occur rapidly. Some have argued that glacial onset may be the cause or consequence of the negative Islay δ¹³C anomaly. Here we test these predictions with high precision U-Pb geochronology from a mixed shallow water siliciclastic-carbonate succession from the upper Tambien Group in northern Ethiopia.

A U-Pb TIMS age of ~735 Ma for a tuffaceous horizon located directly above the nadir of a negative δ¹³C values interpreted to correlate to the Islay anomaly agrees with previous Re-Os age constraints, and confirms that this isotope anomaly precedes the Sturtian glaciation by ~18 Ma. This age difference means any causal relationship is highly unlikely. Two dates from tuffaceous horizons 73 and 80m below the glacigenic diamictite at the top of the Tambien Group suggest a ~717 Ma start for Sturtian glaciation. This age closely agrees with the only other high precision age constraint for the onset of the Sturtian glaciation, which comes from Laurentia. These data are consistent with a globally synchronous start to snowball conditions

The origin of the Islay anomaly and implications of longterm positive and negative trends in δ¹³C can be better elucidated with δ⁴⁴Ca measurements. δ⁴⁴Ca in carbonates is interpreted to record changes in mineralogy and diagenesis. Our data from the Tambien Group show that periods of invariant positive and negative δ¹³C values in the Tonian are associated with light δ⁴⁴Ca and may indicate deposition as aragonite.