Corals: A marine recorder of climate variability

Corals are highly sensitive to the environmental conditions around them, varying their growth patterns and chemical composition as a result of variability in the marine system. They can be strongly affected by temperature (culminating in coral bleaching during heat waves), as well as changes in seawater chemistry (including CO2-induced ocean acidification impacting coral structure), resulting in measurable changes within their calcium carbonate structure. 

Similar to trees on land, as corals grow they produce distinct annual and seasonal bands of calcium carbonate. Within these bands, changes in the density, width and chemical composition can be analyzed to create a time series of variability through time. Since corals have been around for millions of years, there is great potential for reconstructing climate change over long time scales at very high temporal resolution.

Dating corals

Before a time series of climate variability can be developed, the coral growth bands must first be dated. Corals can be dated using radiocarbon dating, U-Th dating and/or Sr-Sr dating. 

U-Th dating is widely used for marine organisms as it offers the potential to date samples up to 500,000 years before present (cal BP) – far beyond the timespan available using the 14C dating method (ca. 40,000 cal BP). A combination of both is best since both dating methods have their drawbacks when applying them to coral samples. For example, radiocarbon dating of marine organisms can be subject to the marine reservoir effect.

Marine reservoir effect:  Radiocarbon formed in the atmosphere is dissolved in oceans in the form of CO2. Given the heterogeneity of the ocean, the 14C carbon of the surface mixing layers is different from that of the deep ocean waters. This creates a divergence between the 14C signature of marine organisms depending on their location. A correction factor can be applied, but is more reliable on sessile vs. migratory marine organisms. Read more about the Marine Reservoir Effect.

In the case of the marine reservoir effect, old carbon from the ocean can obscure the actual radiocarbon age, leading to older ages and higher uncertainties. On the other hand, U-Th dating can be less precise, and corals may make U-Th dating even more difficult when secondary sources of thorium are present within the coral structure. 

As corals grow, they take up strontium isotopes into their structure, capturing a snapshot of oceanic strontium during their lifetime. By measuring the 87Sr/86Sr ratio in the sample and placing it on the standard seawater Sr-curve the age of the sample can be determined. However, similar to radiocarbon dating, issues with homogeneity of seawater strontium make sample calibration difficult (Meknassi et al. 2018).

Despite some challenges in dating corals, multiple dating methods can be applied together as a means of pinpointing and resolving uncertainties (see Grothe et al. 2016 for details).

Isotopic tool boxes for Paleoclimate Reconstructions

Boron isotopes (δ11B) in the ocean vary as a result of changes in oceanic pH and dissolved carbon dioxide since two species of boron (B(OH)3, B(OH)4) are distributed as an artefact of pH conditions. Similar to strontium, boron isotopes are incorporated into coral structure during growth, making corals a good recorder of δ11B – and by association, pH and ocean acidification through time. For example, Gagnon et al. (2021) recently demonstrated that cold water corals continue to grow during ocean acidification but are less resilient due to low efficiency calcification. 

Oxygen isotopes (δ18O) vary in response to temperature, humidity, precipitation and other climate parameters. The skeletal δ18O in corals is a function of both SST and the isotopic composition of seawater (δ18Oseawater). Changes in atmospheric oxygen isotopic ratios impact the ocean as a result of the predictable rainout process starting from the lower latitudes and extending up to the poles, with heavy oxygen (18O) raining out more readily than light oxygen (16O). As a result, higher concentrations of 18O in the ocean are associated with periods of cooler temperatures (e.g. glacials), whereas higher concentrations of 16O (due to melting and lower latitude rainout) are associated with periods of warmer temperatures (e.g. interglacials). This connection between oxygen isotopes and rainfall variability allows corals to also record more high resolution climate events, such as changes in monsoon variability as recently demonstrated by Raj et al. (2022). 

Carbon isotopes (δ13C) vary in response to biological processes and nutrient availability. During photosynthesis, 12C is selectively removed by the zooxantellae, an algae co-existing with coral, causing an increase of δ13C in the dissolved inorganic carbon (DIC). Likewise, additional 12C-enriched CO2 is added to the water through respiration of organic matter leading to decreases the δ13C of the skeletal carbonate. These metabolic effects are responsible for variation of skeletal  δ13C and also influenced by variability in sunlight and nutrient availability (Druffel, 1997; Weinstein et al., 2016)  

Sample submission
The quantity of coral sample required depends on the analysis of interest. 

Analysis type Sample size Packaging
14C dating (AMS) 5-100mg Ziplock bags (place in Aluminum foil if sample is small or can be crushed during shipment)
U-Th dating 30-50mg Plastic or glass vials within ziplock bags
Strontium analysis 100-150mg Plastic or glass vials within ziplock bags
Boron analysis 200mg Plastic vials within ziplock bags
Oxygen and Carbon analysis 2mg Ziplock bags (place in Aluminum foil if sample is small or can be crushed during shipment)

References

Dreuffel, E.R.M., 1997: 1chemistry of corals: Proxies of past ocean chemistry, ocean circulation, and climate. Geo Proc. Natl. Acad. Sci. USA, Vol. 94, pp. 8354–836.

Gagnon, A.C., Gothmann, A.M., Branson, O., Rae, J.W. and Stewart, J.A., (2021). Controls on boron isotopes in a cold-water coral and the cost of resilience to ocean acidification. Earth and Planetary Science Letters, 554, p.116662.

Grothe, P.R., Cobb, K.M., Bush, S.L., Cheng, H., Santos, G.M., Southon, J.R., Lawrence Edwards, R., Deocampo, D.M. and Sayani, H.R., (2016). A comparison of U/T h and rapid‐screen 14 C dates from Line Island fossil corals. Geochemistry, Geophysics, Geosystems, 17(3), pp.833-845. DOI: 10.1002/2015GC005893

Meknassi, E.L., Dera, S., Cardone, G., De Rafélis, T., Brahmi, M., and Chavagnac, V., (2018). Sr isotope ratios of modern carbonate shells: Good and bad news for chemostratigraphy. Geology, 46(11), pp.1003-1006.

Raj, H., Bhushan, R., Kumar, S., Banerji, U.S., Shah, C. and Verma, S., (2022). Monsoon signature in corals from the northern Indian Ocean. Journal of Marine Systems, 226, p.103664.

  1. K. Weinstein, D.K.; Sharifi1,A.; Klaus, J. S.; Smith, T. B.; Giri, S. J.;Helmle, K. P., 2016: Coral growth, bioerosion, and secondary accretion of living orbicellid corals from mesophotic reefs in the US Virgin Islands. Mar Ecol Prog Ser, Vol. 559, pp. 45–63, DOI: 10.3354/meps11883.

Image references

Coral in ocean: https://commons.wikimedia.org/wiki/File:Pulau_Piaynemo,_Raja_Ampat.jpg

Coral cross-section: USGS (https://www2.usgs.gov/landresources/lcs/paleoclimate/archives.asp)