Blog written by Larissa Roy, with information from Sahra Greve (PhD Researcher at Heidelberg University).
In the tropical paradise of Tahiti, the largest island in French Polynesia, Sahra Greve’s research journey began with the question: Can uranium isotopes be used to help us understand sea-level pressure and freshwater influences in the tropical Pacific? The answer lies beneath the surface, where Sahra spent two weeks exploring Tahiti’s submarine groundwater discharges, collecting samples to understand the unique environmental signatures of the island’s waters. These hidden freshwater springs around the Island flow from the land into the surrounding ocean and offer crucial insights into the island’s hydrology and how it interacts with marine ecosystems.
Why choose a place like Tahiti?
Tahiti is not only a beautiful tropical destination with abundant coral reefs, but its location is also fundamental to Sahra’s research. The island experiences heavy rainfall associated with trade winds, supplemented by intense and widespread rainfall events triggered by the passage of the South Pacific Convergence Zone (SPCZ). The abundant rainfall contributes to the island’s substantial groundwater discharge into the Pacific Ocean.
The isolation of Tahiti means that any variation from the known Uranium isotope value (δ234U) in seawater is likely caused by submarine groundwater discharge. This is because as the groundwater travels from the source (the precipitation point) to the sea it becomes more enriched in 234 uranium compared to 238 uranium due to the preferential release of the lighter isotope. Sahra can use this knowledge to measure how much groundwater is mixing with the surrounding seawater. This interaction will allow Sahra to identify things like how sea-level pressure influences the rate of groundwater discharge – and hopefully link this to climate variability.
The Art of Collecting Data: From Beach Sampling to Deep Dives
It is fair to say that collecting data can present certain challenges, with some areas proving more difficult to sample than others. Sahra was able to collect groundwater samples across the entire island with some adjustments – and by using some everyday items.
Beach Sampling: Groundwater often emerges at the shoreline, for this the team could use a sample bottle held up to the spring and then carefully filled with the groundwater samples.
A low-flow submarine groundwater sample is being taken at a beach source.
Sampling a submarine source while scuba diving.
Snorkelling Sampling: At this point, the samples were more difficult to get because they were underwater and it required some expertise to recognise and collect the samples. Sahra had to place a funnel over the groundwater spring to isolate the fresh water, with a bottle positioned over the funnel to collect the sample (see her homemade sampling method in the picture!). When there were waves and currents it worked against Sahra and it was challenging to hold the funnel in place.
Diving Sampling: For springs in deeper locations, diving equipment was essential. Sahra distinguished between the surrounding seawater and the groundwater spring by observing the temperature difference. The groundwater was significantly colder than the salty ocean water, and there was a lot of water discharge so she could collect samples without funnels.
The most amazing thing? The groundwater could be located as it emerged from the springs, with the colder water leaving visible trails in the surrounding seawater. This made finding these springs, which intuitively you think would be hard to find underwater, much easier!
Exciting Findings: Groundwater’s Role in the Island’s Hydrology
The initial results have been exciting. Early data suggests that there are at least two distinct δ234U signatures for the groundwater on the island of Tahiti, varying with tidal cycles. This suggests that sea level influences the amount of groundwater discharge, with higher tides reducing the flow. This observation is significant for understanding how the island’s water system responds to changing environmental conditions, including climate change and sea-level rise.
The Challenges of Fieldwork: Deep Springs, Strong Swells, and Unpredictable Weather
Despite Tahiti’s beauty, the fieldwork presented several challenges. Many of the springs were deeper than anticipated, making it difficult to dive down, stabilize the sampling funnels, and collect samples. At one point, the team members had to work together and hold each other steady underwater while they tried to locate and gather the samples.
The changes in weather conditions also played a role in making the work more difficult. During the field research, days with a large swell caused low tides to reach a level where they resembled high tides on other days, making it difficult to maintain consistent sampling conditions. Additionally, the strong swells made it impossible to carry out a planned dive, delaying the research schedule.
Experiencing Tahiti’s Unique Environment
Although access to sampling locations was often challenging, Tahiti offers remarkable landscapes and ecosystems underwater and on land. The island is a paradise filled with mountains, tropical forests, and coral reefs, and it is a fantastic place to conduct fieldwork. The frequent rain showers also create colourful rainbows, a treat while sampling along the beaches. The researchers were also able to experience some of the island’s natural wonders, such as watching whales breach from the beach and snorkelling with eagle rays and reef sharks! These moments were a reminder that this research is not just about producing scientific data, but also about preserving the delicate balance of Tahiti’s ecosystems.
The research team benefited from collaborations with local scientists who have extensive knowledge of Tahiti’s hydrology and ecosystems. Their expertise and support simplified the sampling process and provided valuable context for understanding how the results would contribute to the local understanding. Organising the fieldwork was not too difficult thanks to the help of a lot of colleagues, like Martin Köllling (MARUM, University of Bremen) who provided the first samples for measurement, and the coordinates of the submarine groundwater discharge. Then Lydie Sichoix (L’université de la Polynésie française) helped locally and gave valuable insight into the island’s hydrology. This collaboration highlighted the power of international scientific collaboration and our shared goal of protecting the environment.
The Impact of the SPP 2299 Early Career Researcher Mobility Scholarship
This groundbreaking research would not have been possible without the support of the SPP 2299 Tropical Climate Variability & Coral Reefs Early Career Researcher Mobility Scholarship that Sahra received in 2024. The scholarship enabled the research team to gain deeper insights into the study of submarine groundwater discharge, which had been a topic of interest for some time. By funding the expedition and fieldwork, the scholarship made it achievable to investigate how these groundwater systems influence the surrounding environment in more depth. Thanks to this support, the project has become a reality!
A bubbling source found at the beach.
Filtering samples back at the accommodation.
Surface water sampling off the coast.
Looking Ahead: What’s Next for the Research?
These findings are just the beginning. The research conducted in Tahiti will play a role in improving our understanding of freshwater fluxes around the island, and how this can help scientists look at variability in other locations. This project will be part of Sahra’s PhD and help her to delve further into her questions about submarine groundwater discharge and its connection to climate variability.
Ultimately, the research conducted in Tahiti not only sheds light on the island’s groundwater systems but also reinforces the importance of preserving these vulnerable ecosystems for future generations. Who knew that something as simple as a spring in the sand could unlock such fascinating insights into the natural world?
Blog written by Larissa Roy, with editing done by Jessica Hargreaves.