A geochemical characterisation of deep-sea floor sediments of the northern Lord Howe Rise

This study presents new information on the regional geochemical characteristics of deep-sea floor sediments (1300–2423 m water depth) on the Lord Howe Rise (deep–sea plateau) and Gifford Guyot (seamount/tablemount), remote areas off eastern Australia. The aim was to provide a coherent synthesis for a suite of geochemical data that can be used to make habitat inferences and to develop surrogates of biodiversity. Sediment characteristics analysed were mineralogy, organic carbon and nitrogen concentrations and isotopic compositions, and concentrations of major and trace elements. We also measured parameters that convey information about the reactivity of organic matter and on the bio-availability of bioactive trace elements (e.g., chlorin indices and acid-extractable elements). Surface sediments from the region were calcareous oozes that were carbon-lean (0.26±0.1%) and had moderate to high chlorin indices (0.62 – 0.97). With the exception of arsenic, inorganic element concentrations were generally low by global standards. Statistical and geochemical analyses identified two major dimensions of variation in the elemental composition: (i) the degree of mixing between the detrital sediment and carbonate end-members; and (ii) the concentrations of bio-available bioactive elements. The causes of compositional variation are discussed, and include: (i) a strong latitudinal gradient in detrital atmospheric dust deposition; (ii) size fractionation and cation exchange of dust components during transit (for non-carbonates); (iii) water depth controls on carbonate dissolution; and (iv) topographic/hydrodynamic focussing of reactive organic matter. Reactive organic matter indicators were not well correlated with water depth, and did not reflect the regional ocean productivity pattern. Basalt source signatures were not evident in the mineralogy or geochemistry of sediments overlying the seamount or in the vicinity of volcanoes.