Scientist Stirs up Hot Theory on Ocean Chemistry
His calculations, however, have led to a startling conclusion: only once before has seawater had the same composition that it does today, and that was 550 million years ago.
"Today is an exceptional time, according to this model," Hardie said. Present-day seawater began emerging about 24 million years ago.
More importantly, Hardie believes he has discovered the key chemical mechanism behind the changes and has developed a model to actually predict the seawater composition at any given time in Earth's distant past. The age of dinosaurs, for example, or far more ancient times, long before vertebrate life walked the planet.
The research will be detailed in a scientific paper in the March issue of Geology, published by the Geological Society of America.
The basis for radical changes in seawater chemistry revolves around a surprisingly simple idea: massive volumes of water continually feed into the oceans from two major sources, rivers and hydrothermal vents on the ocean floors.
It's an effect that can be likened to two garden hoses gushing water into a pool. But the volume of water released by the two sources fluctuates; the ultra-briny hydrothermal source increases and decreases over time, as does the river water flow. The hot "hydrothermal brines" issue from midocean ridges, regions of active volcanism. Those waters are 100 times more salty than river waters. So just a slight increase or decrease in the proportion of water feeding into the oceans from midocean ridges brings about radical changes in seawater composition, Hardie said.
This revelation leads to the heart of his hypothesis.
Over tens of millions of years, those fluctuations cause a general change in the ocean water concentrations of magnesium, calcium, potassium and sulfates, salts formed by the evaporation of seawater.
The engine driving this changing seawater chemistry is the formation of new crust on the seafloor. As hot volcanic magma pushes up from the gaping midocean ridges and cools, forming new crust, seawater passes through the spreading seafloor. This circulating seawater reaches temperatures of more than 400 degrees centigrade (750 degrees Fahrenheit) and is vented in large plumes called gray and black smokers, which are loaded with heavy metals such as lead, zinc and iron.
The faster the rate of crustal formation, the more heat is produced, and the greater the amount of heated seawater (hydrothermal brine) that is pumped into the ocean from midocean vents. When seawater comes into contact with volcanic rock, the concentrations of magnesium and sulfates are reduced. So, increasing the amount of hydrothermal brines results in less magnesium and sulfate, bringing about lower ratios of magnesium to calcium and sulfate to chloride, another type of salt. But just the opposite is happening today: midocean ridge activity is unusually slow, leading to high ratios of magnesium to calcium and sulfate to chloride.
In contrast, the Cretaceous period, when dinosaurs ruled the Earth, was marked by more volcanism. The ocean waters were warmer, and they were rich in calcium, rather than magnesium, Hardie said. These mineralogical changes are recorded in ancient limestones and salt deposits.
"The only other time, in my predictions, that you had anything like modern seawater was back about 550 million years ago," Hardie said. That was during the early Cambrian period, a time from which some of the earliest known vertebrate fossils have emerged. Hardie has produced a hypothetical record of the changing seawater composition spanning hundreds of millions of years. From that model, he has calculated that the current period began about 24 million years ago.
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