Possible Fe Isotope Fractionation During Microbiological Processing in Ancient and Modern Marine Environments

Abstract

Eight iron (Fe) isotopic compositions of iron deposits in biofilms and granules found in two recent burrowing marine invertebrates (the sea urchin ­Echinocardium cordatum and the bivalve Montacuta ferruginosa) were obtained by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). δ56Fe values ranged between –1.78‰ and −0.74‰. The lightest δ56Fe is ­associated with the iron granules in the intestinal wall of E. cordatum and may be due to the abiotic oxidation of source Fe(II) with an isotopic composition reflecting that of light reduced Fe in sediment porewater. This lightest value could represent the best value for the pristine value. Fe in the biofilms was typically heavier by up to +1‰, mean ∼ +0.7‰. These results are compared with Fe isotopic composition of 17 Jurassic limestones from the Rosso Ammonitico Veronese (Italy) containing red and gray hemipelagic facies. The red facies show clear evidence of iron bacteria and fungi, which are interpreted as a possible equivalent of the iron microbial communities associated with the recent organisms. Pronounced Fe isotope fractionation was observed in the Jurassic red hardground levels and in the more condensed red facies where bacteria and fungi lived and have accumulated, with values ­typically lighter by −1‰ than the gray facies where microorganisms were absent. This fractio­nation probably involved the passive accumulation of originally light porewater Fe in the exopolymeric substances (EPS) produced by filamentous bacteria, thereby favoring heavier Fe isotopes. Alternating stages of oxidation Fe(II)/Fe(III) occurred near the sediment/water interfaces as a consequence of microenvironmental changes in the marine porewaters and caused the red/gray facies interlayering. The comparison of the Fe isotopic compositions of the “biominerals” in the recent organisms and in the iron minerals of the red and gray Jurassic facies suggests an isotopic biofractionation of at least ∼+0.7‰. Both studied organisms (the sea urchin and the bivalve) thrive in similar microenvironmental conditions as the ­microorganisms of the condensed red facies. Their Fe isotope compositions are the same, as is the range of the probable biofractionation.