An assessment of aluminum and iron in normalisation and enrichment procedures for environmental assessment of marine sediment

Abstract

An increasing use of Al and Fe as normalising elements in assessment of anthropogenic change (metal enrichment) in marine environments has prompted the current study. This investigation was facilitated by a unique, large data set comprising total- and size-normalised sedimentary metals data from 41 estuaries (n = 3494) in central New South Wales, Australia. Total sediment Al and Fe concentrations varied substantially in these estuaries, even for samples comprising 100% fine fraction (ff), due to changes in the clay mineral mix and variable clay mineral chemistry. Normalising sedimentary metals to 100% ff produced variable results, however this approach improved markedly when samples with <10% ff was removed. The technique of normalising sedimentary metals by adjusting total Al to 50,000 μg/g considered to represent 100% ff was critically assessed. Results showed that total Al for 100% ff varied between estuaries and the most appropriate Al concentration for normalisation was the intercept value of the trend line in total Al vs ff bivariate plots and the 100% ff axis. It is recommended that, in the absence of size-normalised metals data, this intercept value be used as the normalising Al concentration, which should be determined locally for 100% ff. Iron was also assessed as a normalising element using the same approach and provided similar results, but was less attractive due to diagenetic alteration. The post-extraction normalisation (PEN) method provided valid, normalised metals data capable of estimating enrichment (magnitude of anthropogenic change) in marine environments.