Stable (δ13C, δ18O) and strontium () isotopes through the Paleocene at Gebel Aweina, eastern Tethyan region

Abstract

Abstract Stable ( δ 13 C, δ 18 O) and strontium ( 87 Sr 86 Sr ) isotopic records have been established through the Paleocene section at Gebel Aweina, in the Eastern Desert, Egypt. The section has been formed in a shelf-upper bathyal environment and, spans the interval from planktonic foraminiferal Zones P1d to P6a or P6b. Integrated evaluation of isotopic results and scanning electron microscope observations suggest that, strontium and whole-rock δ 13 C records reflect primarily original isotopic trends, whereas δ 18 O and foraminiferal δ 13 C values have been severely affected by diagenesis. The whole-rock δ 13 C record exhibits a continuous increase of 4‰ from the early Paleocene to middle late Paleocene. The increase is followed by a stepwise δ 13 C decrease of the same magnitude through the late late Paleocene and onwards. The gross trends of this δ 13 C record have earlier been registered in many coeval deep-sea sections, but there are also some significant disparities. At Aweina, for example, the Paleocene carbon isotopic changes have a higher amplitude. In the deep sea, the mid-to-late Paleocene δ 13 C increase has been interpreted to reflect high biological productivity and/or changes in the global carbon mass balance. The δ 13 C decrease in the upper part of the Aweina section can be identified as the Paleocene-Eocene transitional long-term (> 10 3 kyr) negative δ 13 C shift, which may reflect a major reorganisation in global oceanic circulation patterns. The well-known latest Paleocene, deep-sea, benthic foraminiferal extinction event has been found in the upper part of the Aweina section, and coincides with a distinct and rapid δ 13 C decrease of 1.5‰ The benthic extinction event and the associated δ 13 C anomaly thus represent important stratigraphic markers also in more shallow environments. The Gebel Aweina δ 13 C record shows considerably lower values compared to the global δ 13 C record during early and early late Paleocene and in the latest Paleocene. In the early late Paleocene (foraminiferal Zones P3a to the base of P4) the Aweina δ 13 C values increase rapidly by 1.5‰, and converge with the global record; thereafter the two records covary up to the benthic extinction event, after which they diverge. The convergence between the records may reflect sea-level rise and more open water-mass exchange between the eastern Tethys and the world ocean. The similarity of pilot δ 13 C records from Greece and Tunisia with the Aweina record support this interpretation. Diagenetic processes, however, cannot be ruled out, and further studies are required to determine the significance of the 13 C depletions in the eastern Tethyan sediments. The 87 Sr 86 Sr ratios through the Paleocene at Gebel Aweina are remarkably invariable close to a value of 0.7077. This is the same trend that has been reported in some earlier detailed studies conducted on well-preserved calcitic material from the Paleocene, indicating original Sr-isotopic signatures at Gebel Aweina. The homogeneity of Sr-isotopic ratios throughout the Paleocene contrasts with the prominent changes observed in the carbon isotope record, indicating a decoupling of the mechanisms regulating the mass fluxes of these elements. The invariable Sr-isotopic ratios suggest stable conditions as regards to relative changes in mass fluxes of riverine and/or hydrothermal Sr during the Paleocene.