DOLOMITIZATION TRUNCATES THE EARLIEST EDIACARAN NEGATIVE δ13CCARB EXCURSION

The Maieberg Formation (Otavi Group) of northern Namibia offers an unusually expanded carbonate record of earliest Ediacaran time and is the type example of a global negative δ¹³C excursion, the Maieberg CIE (carbon isotope excursion). The formation presents a complete depositional sequence comprised of three members: M1, a basal transgressive dolopelarenite (aka ‘cap’ dolostone), deposited above storm wave-base diachronously during syndeglacial regression and subsequent profound marine inundation; M2, a middle limestone rhythmite deposited mainly below storm wave-base during maximum post-GIA flooding; and M3, a regressive dolostone member that filled accommodation created by tectonic subsidence and erosion during the preceding Marinoan glaciation. The formation is 250-400 m thick on the platform, while isotopically correlative strata taper to 40 m distally on the foreslope. The negative CIE encompasses the entire formation, but its nadir at -5.5‰ PDB coincides with M2. However, where M2 is locally dolomitized, the nadir is 3‰ heavier than normal and is indistinguishable from M1 and M3. Moreover, the isotopically light ‘tail’ in the upper part of M1 also vanishes where the M2 is dolomitized, yet Mg:Ca data show that the light ‘tail’ is not correlated with calcite content. Isotopic enrichments in C and CAS in areas of the platform where M2 is dolomitized were previously ascribed to restriction and lateral gradients in platform waters (Hurtgen et al., EPSL 245: 551-570, 2006), but CIE truncation associated with M2 dolomitization on the unrestricted foreslope casts doubt on this explanation. Isotopic alteration during prolonged burial diagenesis (Bold et al., this meeting) trumps this objection. Whatever the mechanism, it evidently altered pre-existing dolomite (the M1 ‘tail’) as well as the adjacent limestone. Quantitatively variable expression of other CIE’s may be related to alteration during burial diagenesis, particularly in areas like Namibia where widespread chemical remagnetization suggests long-lived groundwater flows driven by orogenic topography.