Eolian dust influx and massive whitings during the kozlowski/Lau Event: carbonate hypersaturation as a possible driver of the mid-Ludfordian Carbon Isotope Excursion

摘要

The mid-Ludfordian Carbon Isotope Excursion (CIE) is one of the biggest perturbation in the isotope and facies record in the Palaeozoic. However, its causes still remain unknown. In the periplatform setting of the Baltica palaeocontinent the event interval contains rock-forming minute calcite crystals, interpreted here as suspension-originated analogues of recent whitings and calcite rafts. The grains were preferentially formed around picocyanobacterial filaments, which are preserved as tubes inside. An event-related, 9 m-thick, massive calcisiltite bed points to the persistent hypersaturated state conditions (HSS) during the increasing limb of the CIE. The studied interval contains also a significant admixture of minute-sized detrital dolomite grains, appear for the first time several meters below and reach peak abundance immediately below the calcisiltite interval. The dolomite grains show consistent δ~(13)C values (?0‰) across the CIE. Their record is decoupled from the bulk-rock C-isotope record and thus indicates overwhelmingly pre-sedimentary formation and extrabasinal provenance of the dolomite. The size and provenance of the grains, along with their incorporation into surface water precipitates and formation of specific very thin laminas with internal density gradation of grains (dolomite- quartz couplets), suggest the eolian delivery of the grains to the basin. The dust influx pulses seem to have triggered the calcite precipitation events. Furthermore, the size-frequency distribution of the pyrite framboids across the studied interval shows that the progressing dust influx was followed by persistent euxinia occurring right before and during the increasing limb of the CIE. In the proposed model, globally enhanced eolian delivery of iron during the assumed glaciation promoted high net pyrite formation. The resulting “sulphate to bicarbonate exchange”, a hypothetical dissolution of eolian carbonate dust particles in deep basins, a concurrent bicarbonate supply from the carbonates emerged due to the sea-level fall, and a parallel cessation of the carbonate factory (exposed carbonate platform tops), raised the alkalinity and formed the global carbonate hypersaturated state (HSS) of the ocean. The processes needed largescale invasion of the CO_(2), consumed in the ocean by bicarbonate formation processes. It is hypothesized that during the subsequent initial transgression, the HSS was discharged by massive carbonate precipitation. The rapid “carbonate reflux” returned the earlier uptaken CO_(2) to the atmosphere by fast degassing. Its fast transfer resulted in the globally synchronized, kinetic, residual, ~(13)C_(DIC)-enrichment of water pools localized over individual carbonate platforms. During “carbonate reflux”, the local CIE amplitude depended on the carbonate production rate, water column thickness and pool residence time.