Field testing cellulose-water oxygen isotope relations in periphyton for paleohydrological reconstructions

The oxygen isotope composition (δ 18 O) of aquatic cellulose extracted from lake sediment cores has long been used to reconstruct past hydrological responses to climate variation and anthropogenic (e.g. mining, agriculture, forestry) disturbance. Application of this method depends upon a constant oxygen isotope fractionation factor between aquatic cellulose and lake water, often reported as α 18 O cell-lw = 1.028. Prior field-based assessments of α 18 O cell-lw , however, commonly relied on one-time measures of lake water δ 18 O, which introduces uncertainties because values may not represent lake water δ 18 O at the time of cellulose synthesis. We field-tested the use of α 18 O cell-lw = 1.028 across a broad hydrological gradient of lakes within the Peace-Athabasca Delta (PAD), west-central Canada, where we constrained cellulose production by periphyton on artificial substrates to bracketed measurements of lake water δ 18 O collected in late May 2015 and late June 2015. Cellulose-inferred lake water δ 18 O, calculated using α 18 O cell-lw of 1.028 ± 0.0015, corresponded with the range of measured lake water δ 18 O in 36 of 52 (75%) of our study lakes. Agreement improved to 86% for ‘clean cellulose’ samples that appeared to be of higher quality (i.e. those with minimal minerogenic material). Mean apparent α 18 O cell-lw calculated for clean cellulose samples, using both May lake water δ 18 O (1.0286) and the May–June mean lake water δ 18 O (1.0275), did not differ significantly from 1.028, but was significantly, albeit slightly, below 1.028 using June lake water δ 18 O (1.0264). Closer correspondence of apparent α 18 O cell-lw , calculated using May and May–June average lake water δ 18 O, suggested rapid and early colonization and cellulose production by periphyton. For samples that were a mixture of cellulose and minerogenic material, mean apparent α 18 O cell-lw was significantly below 1.028 for these corresponding lake water δ 18 O measurements, but differences were small and would potentially lead to under-estimation of cellulose-inferred lake water δ 18 O by only 1–2‰ in sediment stratigraphic records. Overall, strong agreement observed between the cellulose-inferred and directly measured lake water δ 18 O supports application of α 18 O cell-lw = 1.028 ± 0.0015 and lends confidence to paleohydrological reconstructions using this method in lakes of the PAD and elsewhere.