Origin and composition of settling iron aggregates in oligotrophic Sombre Lake, Signy Island, Antarctica

Abstract

Sediment traps were deployed in an oligotrophic, seasonally anoxic maritime Antarctic lake for 15 months . Immediately after the onset of the inflow in spring many iron oxyhydroxide aggregates were collected in the traps . Image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to examine the aggregates .The aggregates consisted of primary particles that persisted in the aggregates . The mean diameter of the aggregates was constant with depth . The aggregates consisted predominantly of iron, phosphorus and oxygen but calcium was also an important constituent . Significant concentrations of manganese and sodium were also detected . The molar ratio Fe :P remained constant at 4 :1 as did the ratio Fe :Ca at 52 :1 . The concentration of iron, phosphorus and calcium in the aggregates increased with depth, whilst the concentration of manganese decreased with depth in parallel with a gradient of increasing anoxia .The stable water column formed under ice cover and the temporal and spatial data provide evidence that the Fe :P and Fe :Ca ratios are constant and characteristic of the aggregates, whilst the overall composition of the aggregates is more dynamic and dependant on redox conditions and water chemistry .