Stable isotope and trace element stratigraphy across the Permian-Triassic transition: A redefinition of the boundary in the Velebit Mountain, Croatia


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Stable isotope and trace element stratigraphy across the Permian-Triassic transition: A redefinition of the boundary in the Velebit Mountain, Croatia
Chemical Geology
Fio K., Spangenberg J.E., Vlahovic I., Sremac J., Velic I., Mrinjek E.
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Stable isotopes of carbonates (delta(13)C(carb), delta(18)O(carb)),
organic matter (delta(13)C(org), delta(15)N(org)) and major, trace and
rare earth element (REE) compositions of marine carbonate rocks of Late
Permian to Early Triassic age were used to establish the position of the
Permian-Triassic boundary (PTB) at two continuous sections in the
Velebit Mountain, Croatia. The chosen sections - Rizvanusa and
Brezimenjaca - are composed of two lithostratigraphic units, the Upper
Permian Transitional Dolomite and the overlying Sandy Dolomite. The
contact between these units, characterized by the erosional features and
sudden occurrence of ooids and siliciclastic grains, was previously
considered as the chronostratigraphic PTB. The Sandy Dolomite is
characterized by high content of non-carbonate material (up to similar
to 30 wt.% insoluble residue), originated from erosion of the uplifted
hinterland. A relatively rich assemblage of Permian fossils (including
Geinitzina, Globivalvulina, Hemigordius, bioclasts of gastropods,
ostracods and brachiopods) was found for the first time in Sandy
Dolomite, 5 m above the lithologic boundary in the Rizvanusa section. A
rather abrupt negative delta(13)C(carb) excursion in both sections
appears in rocks showing no recognizable facies change within the Sandy
Dolomite, -2 parts per thousand at Rizvanusa and -1.2 parts per thousand
at Brezimenjaca, 11 m and 0.2 m above the lithologic contact,
respectively. This level within the lower part of the Sandy Dolomite is
proposed as the chemostratigraphic PTB. In the Rizvanusa section, the
delta(13)C(org) values decline gradually from similar to-25 parts per
thousand in the Upper Permian to similar to-29 parts per thousand in the
Lower Triassic. The first negative delta(13)C(org) excursion occurs
above the lithologic contact, within the uppermost Permian deposits, and
appears to be related to the input of terrigenous material. The release
of isotopically light microbial soil-biomass into the shallow-marine
water may explain this sudden decrease of delta(13)C(org) values below
the PTB. This would support the hypothesis that in the western Tethyan
realm the land extinction, triggering a sudden drop of woody vegetation
and related land erosion, preceded the marine extinction. The relatively
low delta(15)N(org) values at the Permian-Triassic (P-Tr) transition
level, close to approximate to 0 parts per thousand, and a secondary
negative delta(13)C(org) excursion of -0.5 parts per thousand point to
significant terrestrial input and primary contribution of cyanobacteria.
The profiles of the concentrations of redox-sensitive elements (Ce, Mn,
Fe, V), biogenic or biogenic-scavenged elements (P, Ba, Zn, V), Ce/Ce*
values, and normalized trace elements, including Ba/Al, Ba/Fe, Ti/Al,
Al/(Al + Fe + Mn) and Mn/Ti show clear excursions at the Transitional
Dolomite-Sandy Dolomite lithologic boundary and the chemostratigraphic
P-Tr boundary. The stratigraphic variations indicate a major regression
phase marking the lithologic boundary, transgressive phases in the
latest Permian and a gradual change into shallow/stagnant anoxic marine
environment towards the P-Tr boundary level and during the earliest
Triassic. (C) 2010 Elsevier B.V. All rights reserved.
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26/09/2012 15:11
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20/08/2019 15:39
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