SR-87 SR-86 ISOTOPIC EVOLUTION OF LOWER CARBONIFEROUS SEAWATER - DINANTIAN OF WESTERN-EUROPE/

The chemical and isotopic evolution of ocean water mirrors the dynamic s of the earth system.:The original seawater signature can be measured only indirectly. For the Palaeozoic, samples with the greatest utilit y for such an approach are the secondary layers of articulate brachiop ods. For a correct interpretation of isotope data, samples that have s uffered alteration by diagenetic processes must be avoided. Trace elem ent analysis is frequently utilized as an analytical tool. The present study deals with the delineation of a detailed Sr-87/Sr-86-curve for the Dinantian. This, in turn, enables us to search for causative facto rs of the observed isotopic oscillations. A total of 175 Lower Carboni ferous (Dinantian) brachiopod shells from a variety of western Europea n locations have been analysed for their Sr-87/Sr-86 ratios. For 152 o f these shells, quantitative geochemical tests on diagenetic alteratio n, based on trace element repartitioning during recrystallisation, hav e been performed. The concentrations of Ca, Mg, Sr, Mn and Fe were rou tinely measured by inductively coupled plasma-atomic emission spectros copy (ICP-AES), analysing the diluted phosphoric acid that remained af ter carbonate dissolution for stable isotope gas preparation. In addit ion, 71 samples have been analysed by proton-induced X-ray emission sp ectroscopy (micro-PIXE). Despite large differences in the analysed mas ses (3-6 mg for ICP-AES; as low as 10 ng for PIXE) both methods yield comparable data for the Sr and Mn contents. Any discrepancies can be r elated to inhomogeneities of the shells (e.g. punctae). Of these shell s 73 have Sr and Mn contents of > 600 ppm and < 350 ppm, respectively, close to the values in modern brachiopods; 78 samples are characteris ed by either Sr depletion or Mn enrichment, but only 1 sample shows bo th of this features simultaneously. A partial alteration of the trace element signal, whether Sr loss or Mn gain, does not automatically ind icate an unacceptable Sr isotope signal. This, complemented by the goo d preservation of microstructures, suggests that diagenetic recrystall isation (if any) was mostly achieved within a relatively closed diagen etic system. In addition, the Sr isotopic composition of diagenetic fl uids was likely to have been buffered at close to marine values, becau se the shells were embedded in marine limestones within the thick carb onate sequences. The Sr isotope record of the Dinantian seawater is ch aracterised by a decline in Sr-87/Sr-86 ratio from 0.7082 at the Devon ian/Carboniferous transition to 0.7076 in the mid-Visean. Superimposed on this trend are higher-order fluctuations with a periodicity in the Ma range. The Dinantian seawater curve may potentially serve as a geo chronoiogical and correlation tool, particularly for the Hastarian to lower Chadian interval, where the attainable resolution is similar to 1 Ma. This is better than the resolution available by biostratigraphy. The higher-order wiggles, for the most part, can be generated by chan ging the riverine flux of Sr or its isotopic ratio within reasonable g eological scenarios. Some 'spikes', however, are too large and too sho rt to be explained by the above phenomena. These apparent spikes may r eflect, instead, the fact that their duration has been underestimated due to unreliable geochronology and/or the presence of unrecognised hi atuses.