期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：17
页码：5337-5341
DOI：10.1073/pnas.1422887112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe snowball Earth hypothesis predicts that the entire Earth was covered with ice. Snowball Earth events were suggested to have occurred several times during the Precambrian. Classic paleo-thermometers (e.g., 18O/16O in marine carbonates) are not available from snowball Earth episodes, and only a few reconstructions of 18O/16O in ancient meteoric water exist. Here we present a novel approach to reconstruct the 18O/16O composition of ancient meteoric waters using the triple oxygen isotopic composition (17O/16O and 18O/16O) of hydrothermally altered rocks. The inferred 18O/16O for waters that precipitated at (sub)tropical paleo-latitudes on a Paleoproterozoic ([~]2.4 gigayears ago) snowball Earth are extremely low. Today, similar compositions are observed only in central Antarctica. The oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition (17O/16O, 18O/16O) of Proterozoic rocks to reconstruct the 18O/16O ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [[~]2.3-2.4 gigayears ago (Ga)] snowball Earth, {delta}18O = -43 {+/-} 3{per thousand} is estimated for pristine meteoric waters that precipitated at low paleo-latitudes ([≤]35{degrees}N). Today, such low 18O/16O values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme 18O depletion. For a Neoproterozoic ([~]0.6-0.7 Ga) snowball Earth, higher meltwater {delta}18O estimates of -21 {+/-} 3{per thousand} imply less extreme climate conditions at similar paleo-latitudes ([≤]35{degrees}N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how 17O/16O measurements provide information beyond traditional 18O/16O measurements, even though all fractionation processes are purely mass dependent.
