Assessing the magnitude of anthropogenic ocean acidification using the boron isotopic composition of corals and coralline algae

Gavin Foster, Toby Tyrrell, Dr Branwen Williams (Claremont Colleges, US), Dr Justin Ries (University of North Carolina, US)

Rationale

Global warming is not the only consequence of rising levels of carbon dioxide (CO2) in the atmosphere. Half of human emissions of CO2 are mixed into the ocean creating carbonic acid, which raises the hydrogen ion (H+) concentration and causes the oceans to become more acidic (so far, a pH drop of ~0.1 unit). This has resulted in a significant reorganisation of the ocean carbonate system, causing the concentration of CO3=, and hence the saturation state of calcium carbonate (CaCO3), to drop. Determining exactly how marine organisms that build their skeletons from CaCO3, such as corals, are impacted by this change is imperative, particularly since by the end of the 21st century pH is expected to drop by a further 0.3-0.5 units as the oceans continue to absorb humanity’s growing emissions of CO2. One way to improve our understanding of how marine calcifiers will be affected by future ocean acidification is to reconstruct how they have already responded to historic acidification (e.g. De’ath et al., 2009). However, the utility of this approach is limited at present because seawater pH measurements are restricted to the last 20 years or so. The aim of this proposal is to use the boron isotope-pH proxy as a means to directly quantify ocean acidification over the recent past (ca. 100 yrs) for several regions deemed particularly high risk (e.g. the high latitudes)

Methodology

The boron isotopic composition (11B) of marine carbonate, such as coral, is pH dependent (e.g. Krief et al., 2010). The skeletons and shells of marine carbonates are therefore not only recorders of the impacts of post-industrial acidification on marine calcification (e.g. via analysis of skeleton growth bands; e.g., Castillo et al., 2011), but also, via boron isotopes, monitors of the pH of the seawater in which they grew. Extended records of 11B in coral skeletons may therefore serve as multi-century, high-resolution archives of palaeo-seawater pH. The initial focus of this project will be to use established techniques (e.g. Krief et al., 2010 and references therein) to measure 11B across century-long cores from a well-calibrated species of reef-building coral from the Belize Barrier Reef (http://www.youtube.com/watch?v=nvy7hFhiMZU), already the subject of considerable study (e.g. Castillo et al., 2011). During the early stages of the project, culturing efforts will also be initiated with Dr. Ries on encrusting tropical coralline algae as a means to investigate the utility of these algae as recorders of palaeo-pH (and other environmental variables). Unlike coral, coralline algae has a wide geographic distribution and these experiments, combined with a modern coralline algae calibration set from high latitudes (with Dr. Williams), will be used to explore the 11B-pH relationship within these cosmopolitan species. The latter portions of the project will be focused on applying this calibration to reconstruct the magnitude of ocean acidification at both high and low latitudes using historic samples of coralline algae (e.g. Hertzinger et al., 2011). This project offers opportunities to participate in field work in Belize and for extended stays with US supervisors.

Training

The successful candidate will join a large and active research group specialising in geochemistry and enroll in the NOC Graduate School where they will receive specialist training in oral and written presentation skills, and state of the art analytical skills in a world class geochemical facility. They will also receive training in methods of coralline algae culturing and field-based reef-coring. These skills are all in high demand in both academia and industry, and will thus provide the student with excellent future employment prospects.

Wider Implications

Understanding the impact of ocean acidification on marine calcifiers is of great importance and is the subject of considerable investigation both in the UK and world wide. On going research at Southampton is central to the UK effort (e.g. http://www.nerc.ac.uk/research/programmes/oceanacidification/) and the student will become an active member of the multi-disciplinary Ocean Acidification research group (http://www.noc.soton.ac.uk/obe/OA/index.php) at NOCS.

Background Reading

Castillo, K. D., Ries, J. B. & Weiss, J. M. Declining coral skeletal extension for forereef colonies of Siderastrea siderea on the Mesoamerican Barrier Reef system, Southern Belize. Plos one, 6, e14615 (2011).
De’ath, G., Lough, J. M. & Fabricius, K. E. Declining coral calcification on the Great Barrier Reef. Science 323, 116-119 (2009).
Hertzinger, S. et al. High-resolution analysis of trace elements in crustose coralline algae from the North Atlantic and North Pacific by laser ablation ICP-MS. Palaeogeography Palaeoclimatology Palaeoecology 302, 81-94 (2011).
Krief, S. et al. Physiological and isotopic responses of scleractinian corals to ocean acidification. Geochimica et Cosmochimica Acta 74, 4988-5001(2010).

For enquiries about this project, please contact Gavin.Foster(at)noc.soton.ac.uk.

University of Southampton. Job opportunity.

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