The goals of this study were the following: (1) detailed reconstruction of the Sr isotope variations for the Middle (Diener et al., 1996) and Upper Devonian sea water,
(2) study of Sr isotope trends at the Givetian/Famennian boundary, (3) comparism of the 87Sr/86Sr curves from "coeval" sequences elsewhere, and (4) correlation of the isotope curve with known or postulated bio-events. The studied interval of ~ 20 Ma was covered by 134 87Sr/86Sr measurements based on carefuly selected and screened brachiopod samples.
The studied samples originate from the Eifel Mountains and the Aachen area (borehole "Inde 1" and two quarries in Wahlheim-Friesenrath) in western Germany, island Rügen in the northeast (boreholes "Sagard 1" and "Neunkirchen 2") and from the eastern Sauerland area ("Behringhauser Tunnel") intermediate between these two regions.
The 87Sr/86Sr for Devonian sea water for the Eifel region shows values of about 0.70805 during the uppermost Emsian, decreases to ~0.7078 within about 2 Ma at the Emsian/Eifelian boundary, with subsequent minor and uniform decline to ~0.7077 for the next 7 Ma during the Eifelian and lower Givetian. The Sr isotope ratios of the upper Givetian in the Aachen region continue the above trend, but with a radiogenic shift of ~ 1x10-4 magnitude. This suggests that the beginning of the radiogenic shift is to be located in the "Muldenkerndolomit" of the Eifel Limestone Synclinorium, indicating either a sharp 87Sr rise of ~ 1.7x10-4 within < 1Ma interval or an underestimate of the overall thickness for this formation. The terminal Givetian is characterised by strong 87Sr enrichment of 3x10-4 within 4 Ma, in both the Aachen and the eastern Sauerland regions, with the steepest slope at the Givetian/Frasnian boundary. The upper Devonian (Frasnian and Famennian) portion of the curve, based on the Rügen boreholes and the "Inde 1" borehole, shows variation between 0.7080 and 0.7082, including at least four isotope "events" ("Inde 1") with a magnitude of 1-1.5x10-4. Some of these may correlate eith the postulated "lower Kellwasserkalk" and the "F/F" bio events. The Sr isotope data further suggest that the oldest samples from the transistion into the calstic "Middle" Devonian of Rügen are in fact of Upper Devonian age.
The present 87Sr/86Sr curve for the Devonian seawater supersedes in its detail the previously published observations (Burke et al., 1982) and can be thus utilized for correlation and stratigraphic purposes. Despite the fact that the curve can be reproduced in considerable detail for areas some 100 km apart, the correlation of isotopic "events" with "bio-events" at this level of resolution remains still tenuous.
Burke, W.H., Denison, R.E., Hetherington, E.A., Koepenick, R.B., Nelson, H.F. & Otto, J.B., Geology 10, 516-519 (1982).
Diener, A., Ebneth, S. & Veizer, J., Geochim. Cosmochim. Acta (1996, in press).