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Marine snow and epipelagic suspensoids in the Reda carbonates and a pronounced mid-Ludfordian (Silurian) CIE in the axis of the Baltic Basin (Poland)

Kozłowski Wojciech

Acta Geologica Polonica

2020

Vol. 70, no. 4

529--567

The mid-Ludfordian pronounced, positive carbon isotope excursion (CIE), coincident with the Lau/kozlowskii extinction event, has been widely studied so far in shallow-water, carbonate successions, whereas its deep-water record remains insufficiently known. The aim of this research is to reconstruct the sedimentary environments and the palaeoredox conditions in the axial part of the Baltic-Podolian Basin during the event. For these purposes, the Pasłęk IG-1 core section has been examined using microfacies analysis, framboid pyrite diameter and carbon isotope measurements. The prelude to the event records an increased influx of detrital dolomite interpreted as eolian dust, coupled with a pronounced decrease in the diameter of the pyrite framboids, indicating persistent euxinic conditions across the event. The event climax is recorded as the Reda Member and consists of calcisiltites, composed of calcite microcrystals (‘sparoids’), which are interpreted as suspensoids induced by phytoplankton blooms in the hipersaturation conditions present in the epipelagic layer of the basin. Both the prelude and climax facies show lamination, interpreted as having resulted from periodical settling of marine snow, combined with hydraulic sorting within a ‘benthic flocculent layer’, which additionally may be responsible for a low organic matter preservation rate due to methanogenic decomposition. Contrary to the observed basinward CIE decline in the benthic carbonates in the basin, the Reda Member records an extremely positive CIE (up to 8.25‰). Given the pelagic origin of the sparoids, the CIE seems to record surface-water carbon isotope ratios. This points to a large carbon isotope gradient and kinetic fractionation between surface and bottom waters during the mid-Ludfordian event in a strongly stratified basin. The Reda facies-isotope anomaly is regarded as undoubtedly globally triggered, but amplified by the stratified and euxinic conditions in the partly isolated, Baltic-Podolian basin. Hence, the common interpretation of the basin record as representative for the global ocean needs to be treated with great caution.

Framboidy pirytowe jako wskaźniki paleośrodowiska

Zatoń M., Rakociński M., Marynowski L.

Przegląd Geologiczny

2008

Vol. 56, nr 2

158-164

The problemof application of pyrite framboids in reconstructions of the redox conditions is presented. The characteristics of pyrite framboids, their origin and principles of application in paleoenvironmental interpretations are given. The pyrite framboid diameter distribution as indicators of redox conditions was presented on the examples of the Middle Jurassic (Upper Bajocian-Lower Bathonian) ore-bearing clays of the Polish Jura, and Upper Devonian (Middle and Upper Famennian) dark grey and black shales from Kowala (Holy Cross Mountains). It has been shown, that during the sedimentation of the ore-bearing clays, the pyrite framboids formed in the sediment, below the oxic water column; however, some episodic oscillation around the dysoxic conditions cannot be entirely excluded. During the sedimentation of the Middle Famennian dark grey shales, the redox conditions at the sea-floor oscillated around dysoxia. In both examples, the framboids are characterized by wide range of their diameters and the presence of large, >10 "my"m, framboids. On the contrary, the Upper Famennian black shales, representing the Hangenberg event, were deposited below the euxinic conditions in the water column. It is indicated by a high frequency of small-size framboids, far below 6 "my"m in diameter, and the lack of the large ones. The overlying shales, on the other hand, show the transition to more oxic conditions. The redox conditions interpreted above are in agreement with the independent biomarker analyses and paleontological observations.