Silicon isotopic systematics of deep-sea sponge grounds in the North Atlantic

Abstract

Reconstruction of silica cycling in the oceans is key to a thorough understanding of past climates because of the inherent links between the biogeochemistry of silicifiers and sequestration of organic carbon. Diatoms are one of the most important phytoplankton groups in determining export production from surface waters, and rely largely on upwelling deeper waters as a source of dissolved silicon, an essential nutrient for their growth. Quantification of changes in deep water dissolved silicon concentrations in the past allows a more robust understanding of changes in surface nutrient supply and whole-ocean silicon cycling, but cannot be achieved using surfacederived geochemical archives. In the last few years, there has been increasing focus on the use of geochemical archives in siliceous skeletal elements, or spicules, from seafloor-dwelling sponges to fill this gap. The stable silicon isotopic composition of spicules has been shown to be a function of ambient dissolved silicon, providing a potential archive for past changes in bottom water nutrients. However, biomineralisation processes impact silicon isotope fractionation and silica formed by atypical processes (derived from carnivorous sponges, hypersilicified spicules, and giant basal spicules) result in anomalous geochemical signatures. Furthermore, there is considerable scatter in the calibration between spicule silicon isotopes and dissolved silicon in seawater, even when the atypical groups have been removed. Here, we explore this variability further, by examining aggregation and assemblage-level differences in isotopic fractionation, using silicon isotopic measurements of specimens from two monospecific sponge groups (Pheronema carpenteri and Vazella pourtalesi), and one mixed-species population (genus Geodia) from the North Atlantic. Our new data reveal that variability within the monospecific aggregations is less than mixed-species assemblage, pointing towards a genetic control in isotopic fractionation. However, there is still variability within the monospecific aggregations, which cannot be explained by macroscale environmental differences: such variability is likely a reflection of the physiological health of the individuals, or highly localised heterogeneities in sponge habitats. Other challenges remain in the interpretation of spicule silicon isotopes as proxies for dissolved silicon changes through time, especially when investigating periods of Earth history that extend back considerably further than the residence time of dissolved silicon in the oceans. Despite all the questions still surrounding the use of sponge silicon isotopes in palaeoceanographic applications, they are still the only known archive of bottom water dissolved silicon. Continued efforts to understanding sponge biomineralisation and to incorporate silicon isotopes into oceanic models will help to improve further the reliability of the archive.

Constraints on Earth System Functioning at the Paleocene‐Eocene Thermal Maximum From the Marine Silicon Cycle

“…This would result in dSi‐depleted North Pacific deepwater and dSi‐enriched North Atlantic deep/intermediate water. Sponge spicule Si isotope ratios are well suited to substantiate this, given the relationship between their magnitude of Si isotope fractionation and the dSi concentration of the water they grow in (Hendry et al, 2019; Hendry & Robinson, 2012; Wille et al, 2010). This takes the form of an inverse relationship that asymptotes at a maximum fractionation of approximately −5‰ at high [Si] (Hendry et al, 2019; Hendry & Robinson, 2012).…”

“…The light fraction containing bSi was then wet sieved at 53 μm to isolate intact radiolarian tests and sponge spicules that were handpicked under a light microscope. The radiolarians were well preserved and represent a bulk community (Fontorbe et al, 2016), whereas only monoaxonic spicules were picked to avoid variability associated with varying morphologies (Cassarino et al, 2018; Hendry et al, 2015; Hendry et al, 2019). Microscopy shows no evidence for silica conversion to, or overgrowth by, clay minerals.…”

“…Radiolarians live mostly in the upper few 100 m (Botlovskoy 2017; Suzuki & Not, 2015) but do not bloom, so their δ 30 Si is considered more of a passive tracer of water mass δ 30 Si. Sponge spicules have the useful characteristic that their Si isotope fractionation is related to ambient dSi concentrations, making them a proxy for bottom water dSi concentrations (hereafter [Si]) (Hendry et al, 2019).…”

The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

“…Silicon isotope variations are denoted throughout by δ 30 Si, reported relative to a known standard (NIST quartz standard NBS28 RM8546) according to Equation 1: acidified with 50 µl of in-house distilled 8M nitric acid and diluted with 1ml of water before the dissolved silica was purified by cation exchange chromatography (Georg et al, 2006;Hendry et al, 2019).…”

“…In contrast, fractionation in benthic sponge spicules shows considerable variability, with an empirical non-linear relationship to ambient DSi concentrations (Hendry et al, 2010;Wille et al, 2010). These data are derived from geochemical approaches that have been applied in a range of palaeoceanographic settings, and we do not yet have a full understanding of the mechanism or the biochemical pathway(s) responsible for Si isotopic fractionation (Hendry et al, 2019). A fuller understanding of this phenomenon will require systematic comparative studies across diverse silicifiers.…”