Birds Eye View of Ocean

BCSS Ocean Observatory Helps Rewrite The History Of Seawater Composition

Mozambique based research station collaborates with German university group on a groundbreaking new study contradicting the accepted timeline of the ocean’s development in earth’s history.


BENGUERRA ISLAND: Bazaruto Center of Scientific Studies (BCSS) is excited to announce its contribution to new research which changes the timeline of the development of the ocean, as we know it.

The nine year study, led by Kiel University (CAU) and GEOMAR Helmholtz Centre for Ocean Research in Germany, challenges the elemental stability of the ocean, which has been largely accepted in the science community for the last 100 years. The study was published in Proceedings of the National Academy of Science of the USA (PNAS) on Tuesday 25 August.

The findings:

Until now, it was thought that Mg, Ca, and Sr only changed in millions of years timescales, but this new study concludes that their modern ocean variability is similar to the reconstructed increase of the elements over 20 million years (Neogene period). 

This means that many scientific studies around reconstructing the oceans and Earth history (e.g., climatology, past seawater temperature, or past seawater composition), which are based on the assumptions that these elements are stable may need revision or drastic change because they could provide erroneous results. The new findings put a word of caution on how we interpret changes in the oceans during the last 20 million years.

We found that the variability of magnesium-to-calcium and strontium-to-calcium ratios in today’s ocean waters may vary significantly from ecosystem to ecosystem. Since a largely uniform composition was previously assumed throughout the ocean, the question arises as to whether an environmental effect has been overlooked. We expect to reconcile results from other scientists using confidence intervals on the ratios change over time,” says Dr. Mario Lebrato, director of BCSS. 

The Science:

Ocean seawater is mostly composed of Ca, Mg, and Sr, beyond H2O, NaCl, O2, and CO2, with Ca, Mg, and Sr proportions being used in modern marine sciences in the form of ratios: Mg-to-Ca and Sr-to-Ca, because they reflect the Earth-ocean-atmosphere dynamic exchange of elements. These ratios are believed to change in millions of years’ time-scales, with a strong dependence on the environment and organisms because they form the base for life, which makes them of critical importance in many marine science disciplines. This is called the Principle of Constant Proportions, or the Marcet’s principle. Over the last decades, the principle has not been revisited, and therefore remains a pivotal textbook dogma in the current understanding of ancient and modern ocean chemistry. 

On geological time-scales the proportions of Mg-to-Ca and Sr-to-Ca in the ocean changed during the last 541 and 70 million years, respectively, reaching modern values. Changes in the proportions during Earth history enabled life and marine organisms to evolve until the present times. However, these elements have been considered “stable”, which facilitates ocean modelling, climate reconstructions, the calcification processes in organisms, and the understanding of global change processes.

The research process:

For the study, scientists from more than ten countries have collected and subsequently analysed 1100 samples in 14 different ecosystems from the water surface to 6000 metres depth on 79 ship expeditions worldwide over a period of nine years. 

The team worked in locations as disparate as the Poles to the Equator, including the Atlantic, the Pacific, the Indian, and the Southern Ocean. The work was also conducted in the Mediterranean Sea, the Red Sea, the Mozambique Channel, with samples collected near glaciers and icebergs. 

Bazaruto Center for Scientific Studies contributed to the study with key Indian Ocean seawater samples from its Ocean Observatory in the Mozambique Channel. BCSS provided 46 samples from five of it’s surrounding ecosystems, including coral reefs, seagrass meadows, mangrove forests, sandbanks, and the open ocean. This provided a unique combination of results to analyse different processes in the same area.

The team:

An international research team with more than 40 participants from over 20 countries, led by BCSS Chief Scientist Dr. Mario Lebrato, and working with Universities from around the globe, including Federal University of Pernambuco (Brazil), ETH (Zurich), Pacific Marine Environmental NOAA (USA) and the National Taiwan Ocean University (Taiwan).

Original publication

For further information, images or interview requests with Dr Lebrato, please contact:

Iris Uijttewaal:

Photo 1. A device used to collect deep water, called a CTD-rosette, being deployed to collect Atlantic Ocean Deep water samples in the Bermuda Atlantic Time-series Study (BATS). Photo credit: BCSS.

Photo 2. The land-ocean margin is an active zone for chemical elements transfer, in particular, aeolian particles, as well as water masses influenced by tidal action and river inputs. In this case, the land-ocean of the Bazaruto Archipelago, in the Indian Ocean, is a complex system including sand dunes, open ocean, coral reefs, and seagrass meadows, which add complexity to the ratios measured in the continental margin water masses. Photo credit: BCSS.

Photo 3. The open ocean is well-studied concerning seawater composition, yet this study finds larger variability than expected, which questions many other studies results based on the assumption of constant seawater composition. Photo credit: BCSS.

Photo 4. Coral reefs are active ecosystems where large quantities of elements are consumed and released to the ocean via processes such as calcification, deposition, accretion, and dissolution. These processes greatly impact the seawater Mg:Ca and Sr:Ca ratios. Photo credit: BCSS.

Photo 5. Chief scientist of the study, Dr. Mario Lebrato, taking seawater samples in a sailing cross-ocean expedition. Photo credit: Hugo Bayon Garcia

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