Stable silicon isotope composition of sponge spicules from the Schultz Massif Seamount, Arctic

The stable silicon isotopic composition of siliceous sponge skeletal elements, spicules, is a potential archive of past dissolved silicon (silicic acid, or DSi) concentrations in bottom waters. Several field-based studies have shown that there is a non-linear relationship between the concentration of ambient DSi and both the isotopic composition (denoted by d30Si) of spicules and apparent isotopic fractionation by sponges during growth. There is considerable scatter in the calibration, with some studies highlighting variation within an individual sponge, and between individuals in both monospecific and more diverse populations. When reconstructing past DSi, it is only possible to differentiate spicules by their morphology, which in many cases will not be taxonomically diagnostic. However, there has yet to be a systematic study of core top and downcore d30Si measurements from different spicule types. Here we address that gap using spicules extracted from two shallow sediment cores from the Schultz Massif Seamount between the Norwegian and Greenland Seas collected on R/V G.O.Sars expedition GS2016109A. Sediments were sliced at 1cm intervals, washed and dried, and spicules hand-picked out and sorted by morphological type. The spicules were dissolved and analysed for silicon isotopic composition using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS).