Controls on Sr and C Isotope Compositions of Neoproterozoic Sr-Rich Limestones of East Greenland and North China
Ian J. Fairchild, Baruch Spiro, Paul M. Herrington, Tianrui Song, 2000. "Controls on Sr and C Isotope Compositions of Neoproterozoic Sr-Rich Limestones of East Greenland and North China", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
Download citation file:
The Sr and C isotope compositions of Neoproterozoic carbonates have been argued to provide important constraints on the timing and origin of depositional events, including ice ages. We investigated the geochemical signatures of immediately preglacial Sr-rich limestones from an unusually complete section in E Greenland and compared with limestones in an analogous position in the basin stratigraphy from N China.
In some Greenland sections, slope facies grade upwards into glacimarine deposits, whereas in others slope facies are overlain by platform carbonates, then terrestrial eolian or glacial deposits without signs of erosion. The slope facies are mixed carbonate (ferroan dolomite and siderite)-siliciclastic (berthierine-quartz) shales with local limestone beds, variably disrupted by synsedimentary slumping. The shelf facies consists of mostly massive limestones with local molar-tooth structure. High Sr contents and poor fabric preservation indicate a primary mineralogy dominated by aragonite. Carbon isotopic values are negative in the slope facies whereas the overlying platform facies are isotopically positive, with mean δ13C values rising from +4.3 to +7.5‰ upwards. The current paradigm is that such a negative-to-positive anomaly reflects the termination of a glacial event elsewhere. We prefer the alternative Uiat deposition occurred across a stratified narrow ocean basin in which δ13C decreases significantly at depth. Samples from the platform facies with over 2000 ppm Sr and negligible Mn have 87Sr/86Sr ratios around 0.70635. These are rather lower than previously reported values in preglacial platformal sediments in the basin, and consistent with no increase in continental erosion prior to glacial deposition.
The Dalian area of the North China block contains a 4-km-thick, tectonically deformed Neoproterozoic shallow marine succession, at the top of which are 80 m of storm-dominated limestone facies with abundant molar-tooth structures passing up into thin peritidal and dolomitic units below an erosion surface thought to correlate with glaciation elsewhere. Sr isotope ratios correlate with Mn, Fe, and insoluble residue and inversely with Sr, indicating the diagenetic addition of heavy Sr by interaction with silicate phases. Apparently unaltered signals are found in samples with Sr > 500 ppm and Mn < 100 ppm, with 87Sr/86Sr ratios decreasing upward from 0.7069 to 0.7063 from 70 m to 20 m below the top of the limestones. Nearly all δ13C analyses lie between +1.7 and +3.3‰. These limestones correlate with preglacial carbonate-platform facies in the E Greenland-NE Svalbard basin, but cannot be tied to a specific horizon.
Whereas a Mn/Sr ratio of <2 is commonly taken to be a sufficient screening criterion for Sr Proterozoic isotope stratigraphy, increases in 87Sr/86Sr were found at Mn/Sr ratios as low as 0.2 in both regions. Availability of suitable lithologies is currently a major limitation on Sr isotope chemostratigraphic studies. However, Sr-rich Neoproterozoic limestones may achieve a 87Sr/86Sr resolution of (±0.0002. Deposition of such limestones bearing molar-tooth-type cracks ceased at around the time of a glaciation traditionally regarded as Varangian (c. 600 Ma), but which could conceivably be Sturtian (c. 740 Ma).
Figures & Tables
Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.