Stable isotopes of oxygen and hydrogen in meteoric water during the Cryogenian Period

摘要

We measured delta O-18 and delta H-2 values of muscovite and carbonate mineral separates from metamorphosed carbonate -bearing mudstone layers in late Tonian to early Cryogenian strata, including Sturtian glacial deposits, which were deposited in a coastal setting at an approximate paleolatitude of 30-35 degrees S and now crop out on Islay and the Garvellach Islands, Scotland. From these values, we calculated delta O-18 and delta H-2 values of meteoric water that equilibrated with clay at diagenetic conditions which we infer were reached shortly after deposition (i.e. before the end of the Cryogenian Period) because sediment accumulation was rapid due to fast subsidence at that time. This calculation required removal of the effects of exchange with reservoir rocks, metamorphic volatilization and mixing with metamorphic fluids on delta O-18 and delta H-2 values. The values we calculated for meteoric water fall within the 2 sigma ranges delta O-18 = 1 to -4 parts per thousand and delta H-2 = 0 to -23-parts per thousand, respectively. These ranges are similar to present day values at equivalent latitudes. This finding is consistent with sediment accumulation in the Cryogenian Period having occurred in a climate similar to present day (Ice Age) conditions. This conclusion is not at odds with the Snowball Earth hypothesis because one of its predictions is that sediment accumulation occurred as the climate warmed at the end of panglaciation, a prediction supported by sedimentological evidence of multiple glacial advances and retreats in our study area and elsewhere.