Formula omitted] isotopic evolution of Lower Carboniferous seawater: Dinantian of western Europe

Abstract

The chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system. The original seawater signature can be measured only indirectly. For the Palaeozoic, samples with the greatest utility for such an approach are the secondary layers of articulate brachiopods. For a correct interpretation of isotope data, samples that have suffered alteration by diagenetic processes must be avoided. Trace element analysis is frequently utilized as an analytical tool. The present study deals with the delineation of a detailed 87 Sr 86 Sr - curve for the Dinantian. This, in turn, enables us to search for causative factors of the observed isotopic oscillations. A total of 175 Lower Carboniferous (Dinantian) brachiopod shells from a variety of western European locations have been analysed for their 87 Sr 86 Sr ratios. For 152 of these shells, quantitative geochemical tests on diagenetic alteration, based on trace element repartitioning during recrystallisation, have been performed. The concentrations of Ca, Mg, Sr, Mn and Fe were routinely measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), analysing the diluted phosphoric acid that remained after carbonate dissolution for stable isotope gas preparation. In addition, 71 samples have been analysed by proton-induced X-ray emission spectroscopy (micro-PIXE). Despite large differences in the analysed masses (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 related to inhomogeneities of the shells (e.g. punctae). Of these shells 73 have Sr and Mn contents of > 600 ppm and < 350 ppm, respectively, close to the values in modern brachiopods; 78 samples are characterised by either Sr depletion or Mn enrichment, but only 1 sample shows both of this features simultaneously. A partial alteration of the trace element signal, whether Sr loss or Mn gain, does not automatically indicate an unacceptable Sr isotope signal. This, complemented by the good preservation of microstructures, suggests that diagenetic recrystallisation (if any) was mostly achieved within a relatively closed diagenetic system. In addition, the Sr isotopic composition of diagenetic fluids was likely to have been buffered at close to marine values, because the shells were embedded in marine limestones within the thick carbonate sequences. The Sr isotope record of the Dinantian seawater is characterised by a decline in 87 Sr 86 Sr ratio from 0.7082 at the Devonian/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 geochronological and correlation tool, particularly for the Hastarian to lower Chadian interval, where the attainable resolution is ∼1 Ma. This is better than the resolution available by biostratigraphy. The higher-order wiggles, for the most part, can be generated by changing the riverine flux of Sr or its isotopic ratio within reasonable geological scenarios. Some ‘spikes’, however, are too large and too short to be explained by the above phenomena. These apparent spikes may reflect, instead, the fact that their duration has been underestimated due to unreliable geochronology and/or the presence of unrecognised hiatuses.