The Boron Geochemistry of Biogenic Silica: Insights from Marine Sponges and Diatoms

Abstract

This thesis investigates whether the boron geochemistry of biogenic silica is linked to seawater pH or other environmental variables, and assesses its potential as a tool for palaeoceanographic reconstructions. Biogenic silica is an especially important palaeoceanographic archive in the Southern Ocean, where carbonate preservation is poor, however to date there is little published data on the boron geochemistry of biogenic silica. This study presents new datasets of boron concentration (B/Si) and isotope composition (d11B) of siliceous sponges, and the B/Si ratio values of diatoms.

Sponge B/Si ratio values range from 2.12 to 5.63 (±0.23, 2σ) mmol/mol, and do not show any significant correlation with seawater pH or other environmental variables. The sponge-seawater B/Si partition coefficients (KD) range from 1.22 x 10-5 to 1.13 x 10-3. The relatively narrow range in sponge B/Si ratio values, and the lack of correlation with environmental conditions, indicate boron uptake is closely regulated by sponges.

A new method for sample preparation and analysis of boron isotopes by positive ion thermal ionisation mass spectrometry (PTIMS) was developed in lieu of any published procedures for 11B/10B measurement in biogenic silica. The analytical precision of NIST SRM 951 boric acid was ± 0.0007 (2se, n=10), and for SRM 951 standards processed through the entire chemical procedure it was ± 0.0017 (2se, n=24), or 0.42‰. The precision for the sponge standard SP150 was ± 0.0022 (2se, n=18), or 0.53‰, and the external reproducibility was 2.2‰ (2σ).

Sponge values are relatively heavy compared to other marine boron reservoirs, and ranged from +5.8 to +24.5‰ (±2.2, 2σ). values do not correlate with seawater pH, but correlate well with seawater Si(OH)4 concentrations (R2 = 0.71, p<0.0001). This indicates that boron isotope fractionation is related to silica uptake during spicule formation. A model for boron uptake and incorporation is proposed that accounts for fractionation during transport from seawater to the sclerocyte, and subsequent Raleigh fractionation during silica polymerisation.

Diatom B/Si ratio values, determined from experimental cultures of Thalassiosira pseudonana, display a negative, nonlinear correlation with [Si(OH)4], with B/Si values ranging from 0.32 to 8.64 (±0.02, 2σmean). This correlation likely arises because boron uptake by diatoms is relatively constant, therefore B/Si reflects silica uptake and deposition rate, which is a function of substrate (Si(OH)4) concentration. Diatom B/Si ratios therefore demonstrate promising potential as a palaeoceanographic proxy for seawater [Si(OH)4]. These findings were used to interpret qualitative variations in seawater Si(OH)4 concentrations from the B/Si ratios in diatom sediment from a marine sediment core from the Southern Ocean (E33-22), which ranged from 0.33 to 0.69 (±0.2, 2se) mmol/mol. The B/Si record indicates an increase in seawater silica concentration during the last glaciation, consistent with previous palaeoceanographic reconstructions. Show more