No abstract

A large perturbation in atmospheric CO 2 and O 2 or bioproductivity will result in a drastic pulse of 17 O change in atmospheric O 2 , as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphereatmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO 2 concentrations proposed by the "snowball' Earth hypothesis produce a typical MOSD duration of less than 10 6 y and a magnitude of 17 O depletion reaching approximately −35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO 2 and low O 2 concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate 17 O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O 2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event. (3,(6)(7)(8)(9). Available data show that for the past 750 My, tropospheric O 2 has had a small magnitude of 17 O depletion, except for one unusual episode: the immediate aftermath of the Marinoan glacial meltdown at 635 Mya, when the Δ 17 O O2 probably reached values as negative as ∼−40‰ (6), compared with the −0.34‰ in today's atmospheric O 2 (10). An ultra-high pCO 2 condition was proposed to explain this Marinoan Oxygen-17 Depletion (MOSD) event (3, 6), which is consistent with the "snowball" Earth hypothesis that argues for a completely icecovered globe lasting several million years (11,12). However, it is not known if there are other potential atmosphere-biosphere scenarios that might produce distinctively negative Δ 17 O O2 , such as a condition in which pCO 2 is moderate but pO 2 and/or biosphere O 2 flux are low, as proposed recently in an effort to reconcile organic and inorganic carbon isotope data from the postglacial cap carbonates (13). It therefore is imperative to examine the Δ 17 O O2 variability among geologically reasonable scenarios. Previous modeling studies on Δ 17 O O2 vary the ratio of pO 2 /pCO 2 , but with a fixed O 2 residence time, or focus on steady state without considering dynamic evolution of pO 2 , pCO 2 , or O 2 production fluxes (3, 13) or consider only minor Δ 17 O O2 difference between recent glacial and interglacial periods (14). In addition, a critical piece of information, the evolution of pO 2 before and after the meltdown of the most severe global glaciation in Earth history, w...

The element oxygen has three stable isotopes: 16 O, 17 O, and 18 O. For a defined process, a change in 18 O/ 16 O scales with the corresponding change in 17 O/ 16 O, or the fractionation factors 18 α and 17 α have a relationship of θ = ln 17 α/ln 18 α, in which the triple oxygen isotope exponent θ is relatively fixed but does vary with reaction path, temperature, and species involved. When the small variation is of interest, the distinction of three concepts-θ, S (a slope through data points in δ 17 O-δ 18 O space), and C (an arbitrary referencing number for the degree of 17 O deviation)-becomes important. Triple oxygen isotope variations can be measured by modern instruments and thus offer an additional line of information on the underlying reaction processes and conditions. Analytical methods and Earth science applications have recently been developed for air oxygen, carbon dioxide,

No abstract