The influence of invertebrate faecal material on compositional heterogeneity, diagenesis and trace metal distribution in the Ogeechee River estuary, Georgia, USA

Bioturbating organisms contribute significantly to elemental cycling in sediments through burrowing, grazing, organic matter and altering porewater chemical conditions. In the process, organisms are known to produce copious amounts of faecal material at high rates, sometimes in excess of 1 000 000 kg day(-1) in a 1 km(2) area. Material from three organisms (Arenicola marina, Callichirus major and Diopatra cuprea) was collected from two locations, Raccoon Key and Steamboat Pass, in the Ogeechee River estuary, Georgia, USA, to explore how faecal production affects organic carbon cycling and clay mineralogies. The individual organisms' feeding strategies played a strong role in the extent to which they concentrate organic matter and lead to the formation of clay minerals. The faecal material of filter feeding Diopatra cuprea and selective deposit feeding Callichirus major contains up 45.8% and 47.3% kaolinite, respectively, while kaolinite is below detection limits in the surrounding matrix. By contrast, the non-selective deposit-feeder Arenicola marina does not appear to form or concentrate clay minerals. Callichirus major increases organic carbon contents at Raccoon Key by 68-fold and Diopatra cuprea increases total organic carbon (w/w%) by 119-fold and 32-fold at Raccoon Key and Steamboat Pass, respectively. Arenicola marina, in contrast, does not noticeably concentrate organic matter in its faecal material, most likely as a consequence of non-selective deposit-feeding. Potentiometric titration data was used to explore surface reactivity and metal sorption. Diopatra cuprea and Callichirus major faecal material has strong metal binding affinities relative to the surrounding matrix, thereby increasing the potential for trace metals to be sequestered into the sedimentary record. The widespread occurrence of invertebrate faecal material enriched in clay minerals and organic matter likely has a significant influence on organic matter, grain size and trace metal distribution in estuarine sediments.