Iron occurrence in soils and sediments of a coastal catchment A multivariate approach using self organising maps S.C. Löhr a, ⁎, M. Grigorescu a , J.H. Hodgkinson b , M.E. Cox a , S.J. Fraser b a Discipline of Biogeoscience, Faculty of Science and Technology, Queensland University of Technology, Brisbane, Australia b CSIRO Exploration and Mining, Brisbane, Australia abstract article info Article history: Received 5 September 2009 Received in revised form 21 January 2010 Accepted 20 February 2010 Available online 24 March 2010 Keywords: Iron geochemistry Multivariate data analysis Self organising maps Pine plantation Water-logging Organic complexation The processes controlling the distribution and phases of Fe in soils and sediments of a forested coastal catchment in southeast Queensland, Australia are identified. The physicochemical attributes of 120 spatially distributed soil and sediment samples, along with geomorphic, geological and land-use characteristics were analysed using Kohonen's self organising maps (SOM) methodology. The SOM-based data analysis approach permits the analysis of complex multivariate datasets that are not well suited to traditional statistical methods. Across the catchment readily extractable Fe concentrations are low overall, despite a high proportion of Fe- concretions in many soils. However, two processes associated with elevated Fe occurrences are identified. Firstly, seasonal water-logging leads to the accumulation of Fe in clay-rich soils on lower slope positions. Mottling and the presence of lepidocrocite indicate cyclical redox conditions. Secondly, a high proportion of total readily extractable Fe is present in organically complexed form in the stream sediments. It is suggested that an accumulation of Fe-rich sediment and organic detritus in streams permits the release of Fe under anoxic conditions, which is then bound by organic material. Contrary to the findings of previous studies, vegetation type does not affect Fe; there is no major difference between the concentrations of Fe of soils under pine plantation and native vegetation. Landform, however, does play an important role. Local depressions on gentle-sloped, low-lying terrain and associated with high topographic wetness indices were found to enhance seasonal redox processes. The groupings obtained from the SOM analyses were internally consistent and can be regarded as ‘process groups’. The results show that SOM is a tool that can aid in the interpretation of complex datasets and help identify geochemical processes operating on a catchment scale. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Iron (Fe) is commonly a limiting nutrient in the marine environ- ment (Rose and Waite, 2003). Recent studies in terrestrial and marine systems have identified the negative impacts of increased Fe fluxes from coastal catchments on marine ecosystems (Ahern et al., 2007). For example, increased fluxes of terrestrial Fe from coastal catchments have been linked to toxic cyanobacteria (Lyngbya) blooms in the near- shore waters of Queensland, Australia (Albert et al., 2005). Some studies have linked the increased Fe exports to extensive pine plantation forestry in coastal catchments (e.g. Ahern et al., 2006). However, the relative impact of a wider range of environmental factors on the chemistry of Fe in such catchments has not been assessed. This study aims to identify the factors that influence the concen- tration and distribution of readily extractable Fe in the surficial soils and sediments of a forested coastal catchment. Particular consideration is given to the influence of catchment morphology, geology, land-use and soil physicochemical properties on the distribution, concentration and form of Fe. This work is part of a larger research project that aims to describe the processes influencing Fe chemistry and assess potential links to plantation forestry in this coastal catchment. 1.1. Possible controlling factors A number of variables are known to affect the form and distribution of iron in the near-surface environment within which it is widespread. Land-use and vegetation type can be important factors influencing the availability and mobility of Fe in forest soils. Vegetation that leads to a build-up of soil organic matter typically results in humic complexes of Al and Fe. Consequently, much of the Al and Fe in surface horizons that are high in organic matter is associated with the organic fraction of the soil (Harsh et al., 2002). Furthermore, the production of soluble organic acids can strongly complex Al and Fe, and assist in their transport from the surface to lower horizons (Jansen et al., 2003, 2004, 2005). Geoderma 156 (2010) 253–266 ⁎ Corresponding author. E-mail address: s.loehr@qut.edu.au (S.C. Löhr). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.02.025 Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma