STRONTIUM ISOTOPE STRATIGRAPHY OF THE MIDDLE DEVONIAN - BRACHIOPODS AND CONODONTS

摘要

A set of 145 strontium isotope ratios, based on skeletal components of Emsian to Frasnian age, reveals a detailed structure of the seawater Sr-87/Sr-86 curve that can ultimately serve as a tool for ''high resolution isotope stratigraphy'' in the Devonian. Brachiopod and conodont samples were collected in the Eifel region of Germany, the area around the global stratotype for the Middle Devonian, with an apparent average temporal resolution in the 10(5) y range. Preservation of the brachiopod shell material has been assessed by optical microscopy, SEM, and cathodoluminescence and only the better preserved internal (''secondary'') layer of the shell has been utilized for strontium isotope measurements. The Sr-87/Sr-86 curve shows a short decline from 0.7081 to 0.7078 through the late Emsian, followed by short-term fluctuations with an amplitude of up to 10(-4) around a mean of similar to 0.7078 during the Eifelian and Givetian, and a rise from late Givetian into the Late Devonian. The above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records, due to erosional and nondepositional events, but a direct confirmation for such a scenario can be claimed only for the latest Eifelian otomari-event. Insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes. Conodont skeletal elements, even if well preserved at CAI < 2.5, yielded results that are mostly enriched in radiogenic Sr-87, if compared to coeval brachiopods. Although this enrichment is usually within the 10(-5) range, conodonts, despite their superior stratigraphic resolution, should be utilized only as a material of second choice in cases where brachiopods are rare or absent. [References: 82]