Scientists who study ocean composition consider the impact of two main sources from which chemicals are transferred between solid Earth and the oceans: rivers and mid-ocean ridges made up of underwater volcanic mountain ranges. In a broad sense, water is evaporated from the ocean and returned in surface waters, bringing fresh magnesium and sulfate from the continents. Persistent heat from shallow magma reservoirs simultaneously produces hydrothermal fluids that remove magnesium and sulfate from seawater and add fresh calcium from the ocean crust.
New research from scientists within the Earth and Environmental Sciences Area at Berkeley Lab and UC Berkeley indicates that changes in the composition of seawater during the past 500 million years may have previously unrecognized effects on the composition of hydrothermal fluids flowing back into the oceans throughout millions of years. As a result, scientists may have previously overestimated the amount of weathering and erosion – the removal of material from land – needed from rivers to change the ocean’s composition over geologic time.
The research team proposed a new model for hydrothermal fluids that takes into account the effects of magnesium, calcium, strontium and sulfate concentration variations in ancient seawater in their paper published last week in PNAS Early Edition. Authors Michael Antonelli of UC Berkeley and EESA’s Nicholas Pester, Shaun Brown and Don DePaolo assert that distinctions in the chemical composition of ancient seawater would have affected the exchange of chemicals between seawater and oceanic basalt in the mid-ocean ridges.
Scientists may have previously overestimated the amount of weathering and erosion – the removal of material from land – needed from rivers to change the ocean’s composition over geologic time.
Accurately accounting for chemical concentrations within seawater over millions of years is a key component for estimating weathering and erosion rates in the past. The authors developed a model that takes into account variations in past seawater chemistry for estimating the amounts of seawater-derived calcium and strontium in hydrothermal fluids throughout time. They calibrated their model to modern hydrothermal systems in order to apply it to ancient systems.
Lead author Michael Antonelli explains, “When estimating the amount of erosion and weathering throughout time, scientists have generally assumed that the composition of hydrothermal fluids over the past 500 million years was equivalent to the composition of hydrothermal fluids in modern times. Existing scientific models have not acknowledged possible differences in hydrothermal fluid compositions through time, which could have important implications for the interpretation of ancient seawater chemical records.”
Assuming that hydrothermal chemical compositions were constant may have led scientists to overestimate the degree of weathering through geologic time in previous models – which is important because weathering rates can be directly linked to changes in atmospheric composition and surface temperatures in the geologic past.”
Their results suggest that modern hydrothermal fluids are atypical due to low calcium and strontium relative to magnesium and sulfate in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician Periods, hydrothermal fluids had more seawater-derived strontium and calcium.
This study comes at a pivotal time in the history of our oceans – as seawater composition undergoes change at an unprecedented rate. Knowledge that seawater chemistry has influence over the exchange of elements between Earth and the oceans through hydrothermal circulation could impact scientific attempts to constrain global weathering rates over time.