Historical Trends in pH and Carbonate Biogeochemistry on the Belize Mesoamerican Barrier Reef System

Abstract

Coral reefs are important ecosystems that are increasingly negatively impacted by human activities. Understanding which anthropogenic stressors play the most significant role in their decline is vital for the accurate prediction of future trends in coral reef health and for effective mitigation of these threats. Here we present annually resolved boron and carbon isotope measurements of two cores capturing the past 90 years of growth of the tropical reef‐building coral Siderastrea siderea from the Belize Mesoamerican Barrier Reef System. The pairing of these two isotope systems allows us to parse the reconstructed pH change into relative changes in net ecosystem productivity and net ecosystem calcification between the two locations. This approach reveals that the relationship between seawater pH and coral calcification, at both a colony and ecosystem level, is complex and cannot simply be modeled as linear or even positive. This study also underscores both the utility of coupled δ11B‐δ13C measurements in tracing past biogeochemical cycling in coral reefs and the complexity of this cycling relative to the open ocean.

Automation of boron chromatographic purification for δ11B analysis of coral aragonite

“…In order to validate the accuracy of δ 11 B values using the prepFAST‐MC method, several reference materials and samples have been processed including a biogenic coral reference material (JCp‐1 24 : δ 11 B = 24.3 ± 0.18‰ (2sd), see Foster et al, 16 Table S1 (supporting information), and other literature reports 6,25‐27 ), seawater (δ 11 B = 39.6 ± 0.2‰ 28 ), matrix‐free boric acid NIST SRM 951 (δ 11 B = 0‰ 29 ), and samples of corals Siderastrea siderea (hereafter S. siderea ) from the forereef of the Mesoamerican barrier reef system (core FR‐02, Belize 3,7 ). The sample size, B/Ca ratio 30 (when applicable), indicative of boron content in a given biogenic carbonate, and the long‐term average δ 11 B values of these samples and reference materials are summarized in Table 1 .…”

“…One way to reconstruct environmental parameters further back in time is to use indirect measurement of pH using, for example, the boron isotope ratio pH proxy in corals. This method has been used to capture existing records of surface pH change 5 and to extend the historical pH record (e.g 6,7 …”

Three decades of degradation lead to diminished impacts of severe hurricanes on Caribbean reefs

“…, Fowell et al. ). Tropical storms represent one of the best‐known threats to coral reefs, and their long history of study suggests their effects should now be predictable.…”

“…Even though the community structure of most coral reefs is now unlike that of a few decades ago, many of the disturbances to which they are exposed remain unchanged, although they have been augmented by rising temperature and declining seawater pH (Hoegh-Guldberg et al 2007, Fowell et al 2018. Tropical storms represent one of the best-known threats to coral reefs, and their long history of study suggests their effects should now be predictable.…”

The effect of matrix interferences on in situ boron isotope analysis by laser ablation multi‐collector inductively coupled plasma mass spectrometry

“…Boron isotope analysis has become a key geochemical tool in the Earth and Planetary sciences . For example, it is employed as a palaeo‐pH proxy for seawater in studies concerned with ocean acidification and to reconstruct past variations in atmospheric CO 2 , as a tracer for fluid–rock interactions and mixing processes in the ocean‐crust–mantle system, and to investigate the timing of condensation and accretion in cosmochemistry . This broad utility stems from there being two stable isotopes, 10 B and 11 B, with a relatively large mass difference leading to large natural isotope variations (δ 11 B values of −100‰ to +60‰).…”