Field Calibration of the δ11B-pH Proxy in Corals and Calcified Algae at a Shallow Hydrothermal Vent and Adjacent Coral Reef

Abstract:

There is increasing interest in using the boron isotopic composition (δ¹¹B) of biogenic carbonates as a proxy for seawater pH to better understand recent ocean acidification. The utility of this proxy hinges on the production of robust species-specific δ¹¹B-pH calibrations; yet, challenges remain in the interpretation of boron isotope data due to biases introduced by physiological, environmental, and analytical factors. The shallow hydrothermal vents in the Maug Islands caldera (Marianas Islands) and the adjacent coral reefs exhibit a localized gradient > 1 pH unit. This gradient was used as a natural laboratory to assess the efficacy of using skeletal δ¹¹B in a variety of corals (Porites spp., Pocillipora spp., Acropora spp.) and calcified algae (Halimeda spp. and Corallinales) as biosensors of seawater pH. Three sites were selected representing oceanic background, intermediate, and low pH zones, and direct seawater pH measurements were recorded for 3 months using SeaFETs. Corals and algae growing naturally in situ were collected from these 3 sites. In addition, corals and algae collected from a background location were stained and transplanted to these sites and allowed to grow for 3 months. Measurements of δ¹¹B in skeletal material made by multi-collector inductively coupled mass spectrometry are compared to direct seawater pH measurements to assess the sensitivity and robustness of the δ¹¹B proxy in these candidate biosensors in predicting ambient pH in the field. These data will inform ongoing efforts by the Archive of Coral Ecosystem Specimens (ACES) to collect marine carbonates for analysis and archival in the Marine Environmental Specimen Bank for broad-scale, long-term monitoring of ocean acidification and the associated impacts to coral reefs. Concurrent analyses of other trace elements, heavy metals, and isotopes in these samples will also be performed to assess their utility as biosensors for additional water chemistry parameters on coral reefs.