High resolution record of heavy metals from estuary sediments of Nankan River (Taiwan) assessed by rigorous multivariate statistical analysis

Abstract This study presents a proof-of-concept data reduction and analysis protocol that can be applied to the study of polluted sediments. Sediment cores from the Nankan River estuary are used as an example of how the protocol can be employed to quantify temporal heavy metal variability. The measurement protocol produces more detailed elemental profiles than conventional techniques using a combination of data transformation techniques and multivariate analysis. Conventional sediment analyses are used to confirm the robustness of the protocol by comparisons of heavy metal concentrations. X-ray fluorescence (XRF) core scanning provides rapid, high-resolution elemental profiles from sediment cores. The technique relies on a variety of calibration methods (ratio, additive and centred log-ratio) to transform the raw data and reduce bias caused by matrix and closed-sum effects. We further test all these calibration approaches since the transformation process is an essential step for the follow up multivariate analyses. The combination of principal component and cluster analysis objectively assesses the information implicit in the dataset. The settings in each procedure are optimized to account for the variance of the dataset. This optimization protocol explains the heavy metal trends using the sediment characteristics of the cores. Heavy metal pollution is characterized by three periods and classified by their oxidation states. We show that heavy metals have an affinity with fine-grained sediments and Mn. The interpretation is confirmed by grain size analysis and inductively coupled plasma optical emission spectrometry (ICP-OES) measurements. This study provides an impartial, cost- and time-effective protocol suitable for the analysis of other heavy metal polluted sites and further studies using sediment cores as archives.