CARBONIFEROUS ISOTOPE STRATIGRAPHY FROM THE SOUTHERN URALS-NEW DATA FOR THE TRANSITION FROM THE LATE PALEOZOIC GREENHOUSE TO THE PERMO-CARBONIFEROUS ICEHOUSE

Brachiopod samples from two sections (Sikaza and Akyn) in the Southern Urals were analysed for their d¹⁸O, d¹³C and ⁸⁷Sr/⁸⁶Sr isotopes, with the enclosing rocks measured for d¹³C of the total organic carbon (TOC). The Sikaza section covers the uppermost Devonian to the upper Tournaisian, the Askyn covers the Serpukhovian to Moscovian.

The ⁸⁷Sr/⁸⁶Sr data from the Askyn section support a nearly uninterrupted sedimentation across the mid Carboniferous boundary (MCB), as indicated by the excellent agreement with the data from the GSSP reference section at Arrow Canyon, Nevada, USA.

The d¹⁸O values at Askyn are characterised by a + 2 per mil shift at the MCB, attaining a maximum value of +1 ‰ in the lower Bashkirian. This steep increase is followed by a gradual decline to lower values in the Moscovian. The trend inversely mimics the second order sea level record, and probably reflects a glacial control via ice mass effects.

The d¹³C_carb values from the Askyn section show a significant increase of more than +7 ‰ across the MCB, with unusually low values (0 to -2 ‰) in the upper Serpukhovian. In contrast, the upper Serpukhovian d¹³C_TOC are atypically high, ranging from -19 to -22 ‰, but returning to normal values for marine phytoplankton in the Bashkirian. This unusual carbon isotope signature most likely reflects restricted environmental conditions.

The most important isotope feature from the Tournaisian samples of the Sikaza section is a significant d¹⁸O shift in the late Tournaisian (Tn3 a). This positive isotope excursion may hint at short lived glaciation in the lower Carboniferous, significantly predating the onset of the major Permo-Carboniferous glacial phase at the MCB.