Through the Sands of Time: Giant Clams as Paleoclimate Timekeepers

When you hear “giant clam,” what first comes to mind?

Perhaps you imagine a whimsical cartoon character with exaggerated features, a large clamshell sporting a playful smile and large, expressive eyes. Or maybe your mind takes a darker turn, imagining a colossal creature lurking in the depths of the ocean, capable of swallowing you whole.

In reality, these giant bivalves native to the shallow waters of the Indo-Pacific region are in fact, not vicious creatures. Giant clams (Tridacna) can live to be over 100 years old and engage in a fascinating symbiotic relationship with tiny photosynthetic plankton called zooxanthellae. Like corals, giant clams provide a safe haven for the zooxanthellae inside their shells, while the zooxanthellae use sunlight to produce energy for the clam. However, this delicate partnership relies on a very particular equilibrium of water temperature, light availability, and nutrient concentration. If the surrounding water becomes too warm, zooxanthellae are ejected from the clam leaving it vulnerable to starvation. Similarly, if there is not enough light available in the clam’s habitat the zooxanthellae will struggle to photosynthesize.

Giant clam (T. derasa) shell from a 1955 Palau expedition courtesy of ANSP Malacology Collection

Marine organisms with shells made of calcium carbonate, like giant clams and corals, encounter formidable obstacles in the face of increasing acidity in the ocean. The ocean’s critical role in regulating the Earth’s climate by absorbing carbon dioxide from the atmosphere helps to slow the impacts of global warming on Earth, but as CO2 dissolves in seawater, it reacts with water to form carbonic acid making the oceans more acidic. The elevated acidity of the ocean accelerates the dissolution of calcium carbonate and reduces aragonite saturation, posing a significant threat to these organisms. Measuring stable carbon and oxygen isotopes in giant clam fossils can be one of the most accurate proxies for reconstructing past ocean conditions and in turn, reveal the changes caused by anthropogenic (human-caused) climate change.

Giant clam (T. derasa) shell from a 1955 Palau expedition courtesy of ANSP Malacology Collection

So, what exactly are oxygen and carbon isotopes, and how can they help us understand ocean acidification and past ocean conditions? 

Let’s start with carbon. Carbon exists in nature as two stable isotopes: carbon-12 (12C) and carbon-13 (13C), as well as radiocarbon (carbon-14 or 14C), a radioactive isotope used for carbon dating. Photosynthetic organisms like algae and plants preferentially take up carbon dioxide from the atmosphere, and they have a slight preference for carbon-12 over carbon-13 during this process. When these organisms die and their remains settle in the ocean, the carbon they contain becomes part of the marine ecosystem. By analyzing the ratio of carbon-13 to carbon-12 in giant clam fossils, scientists can determine the predominant carbon sources present during the clam’s shell formation. This helps differentiate between carbon dioxide from human activities and natural processes, as each source has its own distinct isotopic signature. Natural processes that release carbon dioxide, such as volcanic eruptions and decomposing biomass, are all part of the Earth’s natural carbon cycle. The carbon cycle maintains an equilibrium that helps to regulate the climate of ecosystems. When humans burn fossil fuels, we rapidly add carbon dioxide to the atmosphere that would have otherwise remained buried in a deep sink for many more years, therefore upsetting the balance of the carbon cycle.

Oxygen has three stable isotopes: oxygen-16 (16O), oxygen-17 (17O), and oxygen-18 (18O). The ratio of 18O to 16O in seawater can vary depending on factors such as sea surface temperature (SST) and the amount of ice present on Earth. When water evaporates, molecules containing the heavier 18O isotope tend to remain in the liquid phase, while the lighter 16O isotope is more likely to evaporate. This preferential evaporation process creates a relationship between the isotopic composition of water and environmental conditions.

Slab of Tridacna shell shown above used for isotope sampling (Photo: Alice Andrews)

Analyzing oxygen isotopes in giant clam fossils helps determine changes in SST and global ice volume during the clam’s shell formation. That is, when there is a larger volume of ice on earth, a larger ratio of 18O to 16O will be measured in aragonite shells as the oxygen of the seawater incorporated into the shells is enriched with 18O. Conversely, when the volume of ice on earth decreases  due to planetary warming, oxygen isotope measurements in aragonite shells will show a lower ratio of 18O to 16O due to the influx of 16O in the freshwater melt. Additionally, freshwater input from rivers or high levels of precipitation can impact regional oxygen isotope measurements, decreasing the ratio of 18O to 16O.

Giant clams are particularly useful for studying ocean acidification because they can live for over 100 years and the isotopes in their shells are less affected by diagenesis, the process of post-depositional alteration, compared to corals. And similar to corals, giant clams are accurate climate proxies for very specific locations as they tend to stay in one place for their entire life.

A trend towards a more acidic ocean has serious consequences for marine life.

For example, organisms that rely on calcium carbonate, such as corals, oysters, and sea urchins, may find it harder to build their shells and skeletons as carbonate ions become less abundant with a more acidic ocean. This could lead to a decline in populations of these species, with cascading effects on the entire ecosystem. Additionally, the increased acidity of the oceans can affect the development of fish and invertebrate larvae leaving them more vulnerable to predators.

Giant clams serve as remarkable timekeepers, allowing us to unlock the secrets of the past and shed light on the effects of ocean acidification caused by anthropogenic carbon emissions. By understanding the changes occurring in our oceans, we can work towards preserving these fragile ecosystems for future generations. Next time you hear “giant clam,” remember that every part of our world is impacted by our collective actions as humans, and that it is urgent that we all contribute to mitigating and adapting to the climate crisis we currently face. Ultimately, in the search for solutions we must work with nature, not against it.

This piece was written by Alice Andrews and originally appeared on the Student Blog for Penn’s Center for Science, Sustainability & the Media.

Alice Andrews is a junior at the University of Pennsylvania studying Environmental Studies with a concentration in sustainability and environmental management and a minor in Italian culture. She approached Michael E. Mann, PhD, at Penn in the fall with the interest of conducting undergraduate ocean acidification research under his mentorship. They connected with the Academy of Natural Sciences of Drexel University to do a collaboration involving work with giant clams (Tridacna) where they met Michelle Gannon, PhD, a passionate researcher in the Biogeochemistry lab in the Academy’s Patrick Center for Environmental Research. Gannon’s work mainly surrounds stable isotopes and giant clams, and her expertise has been invaluable to Andrews’s work. This summer, her research involves further examining the relationship between Tridacna isotopes and past ocean conditions, particularly ocean acidification.

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