干旱区生态环境知识资源中心

A succession that includes the Permian-Triassic transition was penetrated in the David 1 Borehole (Israel), located on the Levant margin sector of the northwestern Gondwanan plate. This is a fully marine section with a complex depositional interplay of distal siliciclastic and ramp carbonate sediment. Foraminifera distribution, sedimentary microfacies, delta C-13 excursions and e-log (gamma-ray; resistivity) data were analyzed. The local P-T boundary is placed at the first appearance of Triassic taxa, but there is a 90 m interval of overlapping faunas where the Permian taxa are small species atypical of the underlying Permian strata, and some specimens are clearly reworked. Carbonate strata were deposited in mid-to outer-ramp environments under varying conditions of sea level, storm regime, and siliciclastic (mostly silty) influx. Sea level, tracked by facies shifts, follows a low-order cyclic pattern. Two higher orders of cyclicity are also present: a short-term oscillation in storm frequency in the local depositional basin and a high-order mode in which clastic supply alternates with clastic-free carbonates, interpreted as reflecting humid-arid climate shifts of a remote, continental source region. The last mode lies within the Milankovich band of frequencies. Negative delta C-13 excursions, glauconite, and pyrite-rich horizons indicate hypoxic or anoxic conditions and reflect perturbations of the global carbon system in the oceans during this time. Although lithification of the latest Permian carbonates in the transitional interval was suppressed, the state of preservation of the carbonate components is not indicative of deposition under conditions of oceanic acidification.