Constructing a Neoproterozoic Seawater Strontium Isotope Curve

Monday, 15 December 2014
Ying Zhou1, Graham Anthony Shields-Zhou1, Christina J Manning2, Matthew Thirlwall2, Juergen W Thurow1, Maoyan Zhu3 and Hongfei Ling4, (1)University College London, London, United Kingdom, (2)Royal Holloway University of London, Earth Sciences, Egham, United Kingdom, (3)Nanjing Institute of Geology and Palaeontology, Nanjing, China, (4)Nanjing University, Earth Sciences and Engineering, Nanjing, China
The strontium isotopic composition of seawater has varied throughout Earth history in response to the balance between Sr isotopic exchange with ocean crust and input of riverine Sr derived from continental weathering. Because of this, seawater 87Sr/86Sr highs are interpreted to reflect erosion events, related to mountain building, while 87Sr/86Sr lows are considered to result from low weathering rates or increased seafloor spreading. Seawater 87Sr/86Sr also responds to changes in the isotopic composition of material undergoing weathering. The largest ever increase in seawater 87Sr/86Sr took place sometime from approximately 900 Ma to 500 Ma, and was associated with a permanent step shift in baseline 87Sr/86Sr composition. The unprecedented size of this increase, its timing and causation remains unconstrained. This study attempts firstly to reconstruct global seawater 87Sr/86Sr trends through this increase, using well-preserved carbonate rock samples from the North China craton, calibrated against additional 87Sr/86Sr and δ13C data from Neoproterozoic samples collected from other sections around the world. Sample preparation techniques for bulk carbonate Sr isotope stratigraphy are being honed during the course of this study. Other stable isotope systems (δ13C and δ18O) and trace elements, including REE have been investigated on the same samples to identify pristine samples for Sr isotope analysis and help in interpretation. The newly obtained data from this study (mainly Huaibei group of Huaibei area), using the excellently preserved early marine calcite cements and some bulk rock samples, confirm that the carbonate strata across the Jiao-Liao-Xu-Huai stratigraphic realm of the North China Craton exhibit the moderately positive δ13C values and low 87Sr/86Sr values that are characteristic of the early Neoproterozoic (Tonian).The results help to recreate the global curve by linking negative excursions in the Shijia (Xuzhou) (Xiao et al., 2014, Precambr. Res., 246, 208-225) Formation with the global ~ 810Ma ‘Bitter Springs Anomaly’. A first attempt at a Neoproterozoic Sr isotope curve has been reconstructed using our unpublished data and published data from Halverson et al. (2007, Palaeogeog., Palaeoclim., Palaeoecol., 256, 103-129) and Sawaki et al. (2010, Precambr. Res., 179, 150-164).