Pitfalls of normalization of marine geochemical data using a common divisor

Normalization of trace element contents of sediment through division by the content of an immobile element – usually aluminum – is common practice in marine geochemical studies. This might appear to be a simple way of correcting for dilution by sedimentary phases barren of a particular trace element – for instance carbonates or silica – and for comparison with its content in a standard clay or shale. The purpose of this paper is to revive the awareness of the pitfalls of this practice by giving numerical examples of the often-unexpected results of normalization. Taking a statistical perspective, it is first shown that uncorrelated variables acquire spurious correlations when normalized. But normalization can also increase, decrease, change sign of, or even blur the correlations between unmodified variables. Next, a number of realistic scenarios are worked out to show that the correlations between normalized element contents still suffer from the closure effect. Only in a few simple cases it is possible to extract, from the normalized data, realistic estimates of trace element contents in distinct sedimentary phases, such as organic matter. This is ascertained from data on copper contents in sediment from the Black Sea and Arabian Sea. When the coefficient of variation (i.e. standard deviation divided by the mean) of the aluminum data is relatively low and much lower than for the original values of the trace elements, comparisons of correlations can equally well or even better be made on the basis of the unmodified values. Then, the comparison of trace element data with their values for standard shales is discussed. The inherent problem is that the composition of the commonly used standard shale and, consequently, the reference values of normalized elements are not necessarily representative of the local or regional sediments in the study area. Also, this method falls short of identifying the processes responsible for enrichment or diminution in trace element contents. Finally, consideration is given to some aspects of a proper use of Al normalization