Unraveling Ancient Sea Level Secrets

Sea Level Secrets in the Middle of the Ocean

Sep. 24 2017

During this expedition we have studied both Penguin Bank in Hawaii and Palmyra Atoll, 1000 miles south of Hawaii. Dr. Ken Rubin has been exploring Penguin Bank for over a decade, yet Palmyra has not been studied geologically at the depths between 100 and 200 meters. Even with the distance between the two regions, both have important similarities – they are far from the continental tectonic plates allowing for freedom from many geological complications when studying the past.

Dr. Ken Rubin looking over dive notes as he reviews samples collected from the seafloor.

Historical records of coral
Dr. Rubin and his team collect samples of ancient coral that relate to around 20,000 – 8,000 years ago. These coral samples contain material that enables scientists to calculate how old they are – in this case the team use isotope dating specially using Uranium and Thorium methods.  As corals develop and grow they utilize materials in their surrounding environments that ultimately form the hard physical structures we are familiar with. During this process of development the basic materials they ingest contain amounts of Uranium. Once the physical structures of a coral has developed it no longer ingests these materials and the amount of Uranium is “locked in.”

Through a natural process of Uranium decaying into Thorium we can at present day determine this ratio and calculate the age of these corals. Scientists can date these corals by comparing two samples to each other with precision around 25 years. Using specific species of corals, the age of each sample is then matched with the depth of the water at which the sample was found to link a relationship of when the coral was growing near the sea level surface.

Spatial variability and models
One of the unique aspects of sea level rise that scientists have come to understand is that over time the rate of rise is not a linear constant, nor does it occur at the same rate everywhere on the planet. As sea level rises the rate at which it takes place is different depending on your geographic location. A further complicating factor is that sea level rises in episodic events – large rises can take place over a brief period of time. “Geological records tend to smooth out short time scale events” says Dr. Rubin. Society is most interested in how changes impact the present day. Dr. Rubin is interested in using geological records to precisely measure these “human scale” time periods in order to have a high resolution data set that modelers can use.

ROV SuBastian coming back aboard after a successful dive at Kingman Reef.

The data in our models is based upon the best observational science, and this type of science happens on an iterative basis. We observe a phenomenon and work to create a model of this observation. Then we make hypotheses based on the model output using new observations to validate or modify the model, repeating this cycle. In order to enable a model that can accurately predict sea level rise for us as a society we need to collect the best data to base this upon. Having a variety of locations around the globe that we can observe, or not observe, the sea level rise variability only furthers the quality of the model. Adding Palmyra to this list was a major factor in making this lengthy transit of nearly 1000 miles.

As Falkor heads back to Hawaii, Dr. Rubin and his team will take their coral samples from Palmyra and Penguin Bank to the lab. Once onshore, they will use isotope equipment to find the ages of the coral and begin to piece together this site wide analysis to help modelers refine seal level predictions that will impact current and future generations.

PI Ken Rubin examining samples collected by ROV SuBastian off Penguin Bank.

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