Tectonic control of bioalteration in modern and ancient oceanic crust as evidenced by carbon isotopes |
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| Authors: | Harald Furnes Yildirim Dilek Karlis Muehlenbachs Neil R Banerjee |
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| Institution: | Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway (email: ),;Department of Geology, Miami University, Oxford, OH 45056, USA,;Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G OE2, Canada and;Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada |
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| Abstract: | Abstract We review the carbon‐isotope data for finely disseminated carbonates from bioaltered, glassy pillow rims of basaltic lava flows from in situ slow‐ and intermediate‐spreading oceanic crust of the central Atlantic Ocean (CAO) and the Costa Rica Rift (CRR). The δ13C values of the bioaltered glassy samples from the CAO show a large range, between ?17 and +3‰ (Vienna Peedee belemnite standard), whereas those from the CRR define a much narrower range, between ?17‰ and ?7‰. This variation can be interpreted as the product of different microbial metabolisms during microbial alteration of the glass. In the present study, the generally low δ13C values (less than ?7‰) are attributed to carbonate precipitated from microbially produced CO2 during oxidation of organic matter. Positive δ13C values >0‰ likely result from lithotrophic utilization of CO2 by methanogenic Archaea that produce CH4 from H2 and CO2. High production of H2 at the slow‐spreading CAO crust may be a consequence of fault‐bounded, high‐level serpentinized peridotites near or on the sea floor, in contrast to the CRR crust, which exhibits a layer‐cake pseudostratigraphy with much less faulting and supposedly less H2 production. A comparison of the δ13C data from glassy pillow margins in two ophiolites interpreted to have formed at different spreading rates supports this interpretation. The Jurassic Mirdita ophiolite complex in Albania shows a structural architecture similar to that of the slow‐spreading CAO crust, with a similar range in δ13C values of biogenic carbonates. The Late Ordvician Solund–Stavfjord ophiolite complex in western Norway exhibits structural and geochemical evidence for evolution at an intermediate‐spreading mid‐ocean ridge and displays δ13C signatures in biogenic carbonates similar to those of the CRR. Based on the results of this comparative study, it is tentatively concluded that the spreading rate‐dependent tectonic evolution of oceanic lithosphere has a significant control on the evolution of microbial life and hence on the δ13C biosignatures preserved in disseminated biogenic carbonates in glassy, bioaltered lavas. |
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| Keywords: | bioalteration carbon isotopes Costa Rica Rift lavas microbial life mid-ocean ridges Mirdita ophiolite oceanic crust ophiolites Solund–Stavfjord ophiolite spreading rates upper oceanic crust |
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