Impact of skeletal heterogeneity and treatment method on interpretation of environmental variability from the proteinaceous skeletons of deep-sea gorgonian octocorals

Abstract The stable isotope geochemistry of gorgonian octocoral skeletons facilitates detailed time series reconstructions of nutrient biogeochemistry. However, comparisons among reconstructions from different locations require realistic estimates of the uncertainty surrounding each measured geochemical value. Here, we determine quantitative uncertainties related to 1) standard skeletal pretreatment in preparation for stable isotopic analysis and 2) biological variability associated with a heterogeneous isotopic composition of the gorgonin skeleton. We found that the 5% HCl pretreatment required for the δ 13 C measurements does not significantly impact the δ 15 N values of the skeleton nor the reproducibility of the δ 15 N measurements. In contrast, while 5% HCl pretreatment significantly altered bulk δ 13 C values via removal of CaCO 3 , it did not change amino acid δ 13 C values in the organic skeleton. We found that the variance of repeat measurements of skeleton samples formed contemporaneously and homogenized skeleton for both δ 13 C and δ 15 N exceeded that of instrumental uncertainty of an acetanilide standard. This indicates that instrumental uncertainty underestimates the true precision of an isotopic measurement of the organic skeleton. Furthermore, measurements of contemporaneous skeleton around the circumference of an octocoral colony yielded variability exceeding that of homogenized skeleton. Based on these results, we find that 1) both δ 13 C and δ 15 N values can be measured simultaneously in pretreated skeleton, 2) growth bands should be homogenized prior to analysis, and 3) reported error should include uncertainty due to biological effects determined from repeat analysis of homogenized skeleton and not just instrument error to reduce false significant differences. Our results present an important protocol for processing proteinaceous octocoral skeletons and propagating uncertainty to more accurately reconstruct nutrient dynamics from proteinaceous deep-sea octocoral skeletons.