New Delhi: A new study shows that the history of alteration in ocean chemistry has been recorded in volcanic rocks of island arcs for more than 2 billion years. These arcs that are developed by volcanic action along the subduction boundaries where two tectonic plates join are critical in the construction of Earth’s continental crust. Described in the journal Proceedings of the National Academy of Sciences, the study reveals how changes in ocean chemistry have been translated into island arc rocks through complex subduction dynamics.
The research is done by Caltech graduate student Amanda Bednarick and relates seawater strontium chemistry to strontium isotopic compositions in island arc magmas. The present results indicate that the hydrothermal exchange of oceanic crust with seawater results in a certain chemical signature within the subducted oceanic crust. As these crustal slabs descend into the mantle, seawater-modified strontium is also returned to the mantle to interact with mantle materials to produce magmas that contain oceanic geochemical signals when they emerge in island arc settings. These traces are now discernable in rocks from such island chains as the Aleutians.
Archaeological Changes in the Strontium Isotopes Related to Ocean Chemistry
Analysing the data on strontium isotopes, Bednarick and her colleagues noted that strontium content in the island arc basalts reflects changes in seawater chemistry during the history. The strontium isotope ratios in arc rocks rose sharply 600 million years ago, correlating with documented fluctuations in oceanic and atmospheric conditions in the late Neoproterozoic epoch. The researchers’ model associates these isotope shifts to fluctuations in the ocean’s ratio of radiogenic strontium to non radiogenic strontium, which are affected by continental and mantle inputs.
A Legacy of Radiogenic Isotope Studies
The study is based on the pioneering isotopic work of co-author Donald DePaolo, who first established techniques for determining neodymium and strontium isotopes in volcanic rocks. Today, careful assembly of data accumulated over thousands of years has enabled Bednarick and co-authors to explain how the chemistry of Earth’s ancient oceans has been preserved in volcanic rocks for millennia. “It is quite shocking that Earth’s systems are so interrelated,” says Bednarick.
Future Forecast from Past Oceans Below
In future work, Bednarick intends to study ophiolites, which are sections of hydrothermally altered oceanic crust that got incorporated in continents, to better reconstruct the chemistry of the oceans from over a billion years ago. Her work demonstrates how the ocean and volcanic activities are very connected and presents fresh insights into the formation and climatic development of the Earth.
This study stresses the planet’s primordial interconnectedness and demonstrates how volcanic rocks provide a glimpse into the Earth’s past ocean composition that informs tectonic and geochemical histories.
