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Geochemistry of the new Permian-Triassic boundary section at Sitarička Glavica, Jadar block, Serbia
Chemical Geology ( IF 3.362 ) Pub Date : 2020-06-01 , DOI: 10.1016/j.chemgeo.2020.119696
Michael E. Brookfield; Jeremy Williams; Alan G. Stebbins

The new Permian-Triassic boundary section at Sitarička Glavica is unusual for the Alpine-Dinaride area, in that it is dominantly detrital in the early Triassic. It shows a change from late Permian limestones and shales with a diverse algal-brachiopod-echinoid-molluscan biota deposited in quieter normal marine water (unit 1), into similar sediments, with coarser calcisiltites and a reduced fauna (unit 2), into a post-extinction early Triassic calcareous micaceous siltstones and silty limestones (unit 3) with only molluscan fossils. The major element ratios to Al, reported previously, change little through the section, but with slightly higher ratios just above the extinction horizon (except for K) at the top of unit 2, and show moderate chemical weathering, and change to slightly less mature sediments above. Trace element ratios to Al show little consistent change, except for higher values of most ratios at the contact of units 1 and 2, where slight but temporary increases in TOC, δ13Corg and δ15Norg, and C/N, and decrease in δ34Spyrite occur. Low HFSE (Zr, Nb, REE, Th, U, Ta) values and ratios to other elements indicate that the resistant mineral containing these elements were somehow removed prior to deposition in residual lags by preferential wind erosion. Paleoredox indicators are inconsistent but anoxia seems present in unit 1 and 2 with mostly oxic conditions in unit 3 above. The geochemistry of this section does not show the concentrations of Ni and other elements attributed to Siberian Trap sources and evidence of volcanic input to the sediments is confined to below and above the extinction horizon. The main geochemical changes occur at the top of unit 1, corresponding with a faunal diversity decrease, but not extinction, and at the +5 metre horizon, which seems to- have no faunal changes associated with it. The carbon and other organic-related elements and isotopes indicate a terrestrial input of freshwater and organic matter just before the main negative spike in carbon isotopes, which greatly reduced the diversity of the fauna, and was shorty followed by an input of intermediate volcanic ash. The actual extinction and paleontological Permian-Triassic boundary occurred at the top of unit 2, and has little geochemical changes associated with it. These events can be recognized in other Alpine-Dinaride sections -. All Alpine-Dinaride sections also show a latest Permian ostracod-dominated horizon with a negative spike in organic carbon and sulphur isotopes that indicate an input of freshwater and land plant material, reducing the surface waters to almost brackish, followed by a return to normal salinities with the return of a reduced latest Permian fauna, accompanied by a gradual negative shift in carbonate carbon isotopes accompanied by input of intermediate volcanic material. The main extinction is accompanied by a drop in the difference between carbon and organic carbon isotopes, indicating less land plant input; but otherwise there is little geochemical change at this horizon.
更新日期:2020-06-18

 

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