Assessment of purification and contamination correction methods for analysing the oxygen isotope composition from biogenic silica

Mineral particles and other contaminants cause a serious and undesirable shift in δ18O values of biogenic silica and should be removed prior to isotope analysis. This problem is particularly significant in the fraction containing clay particles and therefore the fraction < 10 μm is generally avoided. Here, we present an updated preparation protocol for the < 10 μm fraction based on known methods, developed using test material from short cores of Lake El'gygytgyn, NE Russia. New contamination assessment and correction techniques are discussed. Best results (low initial weight/high precision and throughput rate) for the < 10 μm fraction were achieved using geochemical mass-balancing and determining the “percentage of contamination” by analysing the sample's and end-member's Al2O3 with improved Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Energy-Dispersive X-ray Spectroscopy (EDS) protocols or by determining the factor for calculating the percentage of contamination from the Al2O3 percentage of clay assemblages. Despite using small initial weights for ICP-OES (10 mg) and EDS analysis (< 0.5 mg) a sufficiently high precision (for low Al2O3 percentages between 0.1 and 1.5%) could be achieved.For the > 10 μm fraction, point-counting under the light microscope was the preferred option. The different size-fractions from Lake El'gygytgyn samples show a distinct and significantly different species composition (average counts: < 10 μm: > 90% Cyclotella ocellata complex; > 10 μm: > 60% Pliocaenicus seczkinae). Differences in the oxygen isotope composition of these different size fractions could be explained by a species-effect on the δ18O values. However, this species-effect disappeared after the correction (avg δ18O < 10 μm: 19.8 ± 0.6‰; > 10 μm: 19.8 ± 0.4‰). By comparing these δ18O values with additional recent water samples, an average 1000 ln α of 39.6‰ for Lake El'gygytgyn sub-recent diatoms was calculated. This “tool-kit” provides support for the presented improved contamination assessment and corrections and allows utilising smaller initial weight (app. 10 mg) and small fractions (< 10 μm) for gaining corrected, higher resolution δ18O records.

► Contamination causes unwanted shifts in δ18O of biogenic silica.
► A tool-kit was developed to tackle contamination and correct the δ18O values.
► Contamination in < 10 μm fractions should be assessed by SEM-supported EDS or ICP-OES.
► Corrections are valid for samples showing up to 15% contamination or ~ 2.5% Al2O3.
► Only after contamination correction, no isotope diatom species-effect was found.