The Record of Nd Isotopes in Neoproterozoic Sediments: Implications for Atmospheric Evolution,
Ocean Mixing and Global Tectonics

Stein B. Jacobsen Dept. of Earth and Planet. Sci., Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA

Gang Cheng Dept. of Earth and Planet. Sci., Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA

Alan J. Kaufman Dept. of Earth and Planet. Sci., Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA

Paul Myrow Geology Dept., Colorado College, Colorado Springs, CO 80903, USA

Many major questions relating to the evolution of Earth's oceans, atmosphere and crust can potentially be resolved by studying isotopic variations of seawater through time. The late Neoproterozoic (~750-545 Ma) is a time of very large changes in the 87Sr/86Sr and d13C isotopic records of marine carbonates. Proper interpretation of these records depend on our ability to distinguish clearly between changes in the 87Sr/86Sr of seawater, caused by variations in the total river water (erosional) flux of Sr or by variations in the 87Sr/86Sr ratio (i.e. mean source age) of this flux. The present Nd budget of the oceans is dominated by the river water flux from continental sources. Thus variations of eNd in seawater are primarily due to changes in the
Nd-isotopic composition of the continental flux to the oceans, and is, therefore, a proxy for the mean age of the continental flux into the oceans. Thus, eNd measurements on phosphates and iron-formations can be used to interpret the Sr isotope record and thereby constrain variations in the global erosion rate. By coupling the d13C and 87Sr/86Sr records of marine carbonates, we obtain constraints on organic C burial rates, and consequently inputs of O2 to the atmosphere.

A variety of primarily phosphatic sediments from the Neoproterozoic (mostly from Newfoundland and China) have been analyzed for Nd isotopes. Using these and literature values, a global average eNd curve for the late Neoproterozoic has been constructed. In general, these results show that seawater eNd changes from high values (-2 to -4) about 650 - 750 Ma to values in the range of about -15 in the latest Neoproterozoic - Early Cambrian.

The eNd record also provides constraints on paleocean evolution and mixing because of the short residence time of Nd in seawater (~1000 years). Large variations in eNd are often found between the various ocean basins through time; smaller variations are found within ocean basins. The isotopic composition of Nd in an ocean basin, in general, reflects whether it is predominantly surrounded by active (generally high eNd) vs. passive continental margins (generally low eNd). Thus, in principle, Nd isotope records may be used to distinguish between the various (controversial) models of global plate reconstructions for the Neoproterozoic. The database is not yet sufficient for a detailed analysis, but it appears that the super-continent Rodina at about 750 Ma was surrounded by primarily active margins that resulted in high eNd values in the global ocean. Later during rifting of this super-continent, the older crust was exposed to weathering and erosion such that the eNd value of the global ocean decreased to about -15. However, individual ocean basins such as the proto-Iapetus, which have high eNd -values (~ -4 to 0), must still have had margins with active volcanism.