Preservation of environmental sulfur isotope records in maritime peats: a test of baseline pre-anthropogenic signal and diagenetic effects in a mid-Pleistocene peat

Different sulfur fractions in a mid-Pleistocene peat from Fugla Ness, Shetland Isles, UK, were separated and analysed isotopically in order to establish whether an unequivocal pre-anthropogenic baseline sulfur isotopic signal is preserved, or whether syn-depositional or subsequent diagenetic effects obscure any such information. Such a test is vital if peats are to be used as historical archives for the extent and timing of anthropogenic sulfur deposition by recording changing sulfur isotopic composition as anthropogenic sulfur emissions increased. Sulfidic sulfur isotopic compositions in the peat are highly variable (δ34S from −10.4‰ to +58.6‰), largely reflecting post-depositional diagenetic effects. Organic sulfur isotopic compositions, however, are much less variable (δ34S from 3.3‰ to 15.4‰). The Fugla Ness peat would have been subject to a maritime-dominated sulfate input and its organic sulfur isotopic compositions generally parallel those of modern peat bog flora in pristine maritime settings. Peat samples from clay-peat interbeds show a bias to more 34S-depleted isotopic compositions as a result of diagenetic addition of sulfur to the organic pool. This only has a significant effect in a small fraction of samples analyzed; to a large extent, the organic sulfur isotopic composition shows no evidence of significant diagenetic alteration.