Neoproterozoic Variations in the C- and Sr-Isotopic Composition of Seawater: Stratigraphic and Biogeochemical Implications

Alan J. Kaufman Department of Earth and Planetary Sciences, Harvard University,

20 Oxford Street, Cambridge, MA 02138, USA

Stein B. Jacobsen Department of Earth and Planetary Sciences, Harvard University,

20 Oxford Street, Cambridge, MA 02138, USA

The recent proliferation of stratigraphic studies of d13C and 87Sr/86Sr in carbonates and co-existing organic matter in later Neoproterozoic and basal Cambrian successions (ca. 850-530 Ma) indicates a strong oscillating trend in the isotopic composition of surface seawater. Integrated with the growing fossil record, C and Sr isotope chemostratigraphy facilitates the intra- and inter-basinal correlation of later Neoproterozoic successions. Results of studies on well-preserved microsamples from shallow marine carbonates -- formed in isotopic equilibrium with seawater -- from thick carbonate-dominated strata in Svalbard/East Greenland, northern Siberia, Arctic Canada, western USA, Oman, and Namibia are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian
(pre-Varanger and post-Sturtian), and (4) the early Cryogenian (pre-Sturtian). For most Neoproterozoic intervals combined C and Sr isotope stratigraphy provide a much better tool for correlation than do fossils or available radiometric ages. In a number of well-documented cases, isotope chemostratigraphy allows for formation level (and possibly even member level) correlations between widely separated basins.

Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. For example, comparison of the Neoproterozoic isotope records with those of the Cenozoic permit investigation of the general relationship between mountain building events and continental glaciation. While ice ages mark both the Neoproterozoic and Cenozoic, different stratigraphic relationships between the strong increase in 87Sr/86Sr and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for Neoproterozoic ice
ages. Furthermore, these integrated studies suggest that strong negative shifts in the C-isotopic composition of seawater are associated with oceanic mixing events during (at least) four distinct episodes of Neoproterozoic continental glaciation. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic carbon and a concomitant rise of atmospheric O2.