Red coralline algae assessed as marine pH proxies using 11B MAS NMR

Area of Science:

• Geochemistry
• Paleoceanography
• Marine Biology

Background:

• Biogenic carbonates are used to reconstruct past ocean pH using boron isotopes.
• This method assumes only borate, not boric acid, is incorporated into the carbonate structure.
• Red coralline algae are valuable paleoenvironmental indicators due to their distribution and growth patterns.

Purpose of the Study:

• To investigate the boron speciation in the red coralline alga Lithothamnion glaciale.
• To assess the validity of using boron isotopes in coralline algae for accurate pH reconstruction.
• To identify necessary steps for improving the reliability of coralline algae as paleo-pH proxies.

Main Methods:

• High-field Nuclear Magnetic Resonance (NMR) spectroscopy was used to analyze boron composition.
• Bulk samples of Lithothamnion glaciale were analyzed for boron speciation.
• The presence and proportion of borate and boric acid were quantified.

Main Results:

• Bulk analysis revealed that thirty percent of the boron in Lithothamnion glaciale was present as boric acid.
• This finding indicates a significant deviation from the assumption of pure borate incorporation.
• The presence of boric acid complicates direct pH reconstruction from boron isotopes.

Conclusions:

• The assumption of exclusive borate incorporation in coralline algae is not universally valid.
• Future pH reconstructions require species-specific determination of boron composition and distribution.
• Site-selective boron analyses are needed to validate coralline algae as reliable paleo-pH proxies.