‘GLACIAL LAKE HARLAND' AND THE CALCIUM ISOTOPE RECORD OF THE MARINOAN GLACIATION

When the terminal Cryogenian ice sheets melted in 635 Ma, the mass of ocean water increased by ~25%. If the ice sheets ablated as fast as mountain glaciers do today (-0.5m yr^-1), meltwater production rate would have been ~15x greater than present global runoff, consuming 3 Wm^-2 of heat. It is doubtful that winds and tides would be capable of mixing the low-density meltwater into the old glacial brine on the assumed ~4 kyr timescale of deglaciation. It follows that a density-stratified ocean developed and the transgressive tracts (‘cap dolostones’) of syndeglacial sequences are chemically non-marine in origin (1). This meltwater lid is named ‘Glacial Lake Harland’ (GLH) in memory of W. Brian Harland. Its demise through ocean mixing occurred soon after the meltwater flux waned, at which time isotopic signatures of glacial deepwater would have appeared in surface waters. With this in mind, published Ca isotope records (2,3) from basal Ediacaran sequences in Brazil, Canada and Namibia are reexamined. Large secular variations in d⁴⁴Ca of ±1.0‰ record imbalance between ocean Ca input and output, as would accompany large changes in seawater pH. Cap dolostones have uniformly normal (Phanerozoic) values in all three areas, indicating Ca balance in GLH and insensitivity of d⁴⁴Ca to temperature. Overlying highstand carbonates record a 1.0‰ negative anomaly in d⁴⁴Ca, followed in the Americas by a 1.0‰ positive anomaly. Decreasing pH (carbonate dissolution) over multiple Ca residence times is needed to explain the negative Ca anomaly, which I attribute to CO₂ hysteresis during snowball glaciation. The positive anomaly indicates Ca output>input, attributed to whole-ocean warming and the lowering of atmospheric pCO₂ by postglacial silicate weathering. As the d⁴⁴Ca minima occur 10-100 m above the cap dolostone, the model implies that deposition rates remained high (<1 cm yr^-1), consistent with observed masses of sea-floor cement. The snowball hypothesis appears to provide a coherent and contextually viable explanation for the extraordinary early Ediacaran Ca isotope database (2,3). References: (1) G.A. Shields 2005, Terra Nova 17, 299-310; (2) S.A. Kasemann et al. 2005, Earth and Planetary Science Letters 231, 78-86; (3) J.C. Silva-Tamayo et al. 2010, Precambrian Research 182, 373-381.