Magazine article Oceanus

Of the River & Time: Flowing Waters Run Deep with Evidence of Earth's Mountains and Climate

Magazine article Oceanus

Of the River & Time: Flowing Waters Run Deep with Evidence of Earth's Mountains and Climate

Article excerpt

The Fraser River in western Canada is flowing with tiny time capsules. Inside them is a fascinating history of Earth's landscape and climate.

Over four years, I traveled across the river basin to gather these time capsules from the free-flowing Fraser. I can open them to reveal a geochemical journey that starts in the atmosphere, moves through plants and minerals, and sinks to the bottom of the ocean.

It begins with carbon dioxide, a greenhouse gas in air. Raindrops soak up some of the carbon and convert it into an acid. The rain splashes the acid on mountain rocks, dissolving away tiny bits of minerals from the rocks. Carbon is also extracted from air and incorporated into plants via photosynthesis.

Along its course, the Fraser drains sheer granite slopes, dry scrubgrass plateaus, and dense cedar rainforests, picking up material and creating a cocktail of rock and plant debris. Carbon from both rock and plant remnants flows into the river and is carried into the ocean, where it eventually accumulates on the seafloor.

Radioactive atoms slowly ticking away inside rock minerals record how old the rocks are. They tell me whether the rock material in the river originated in the Rocky Mountains, where rocks are hundreds of millions of years old or, in the Coast Range, where they are mere tens of millions of years old.

By analyzing samples of water and sediments in the Fraser, I can track where materials carried by the river came from and how much carbon dioxide different regions remove from the atmosphere. I can piece together how rocks and vegetation are broken down and transported to the ocean; whether the plant-derived matter hops on and off along the route; and ultimately, how this complex process influences Earth's climate.

Telltale clues from strontium

The story begins in the highest reaches of the Fraser River's headwaters with the seasonal melting of glaciers grinding away at the Rocky Mountains and the Coast Range. Every snowflake and raindrop that falls somewhere in the Fraser River basin absorbs a tiny amount of carbon dioxide from the atmosphere. The carbon dioxide reacts with water to form carbonic acid.

The acid, in turn, reacts with carbonate and silicate minerals, chemically converting them into calcium, silica, and bicarbonate, which dissolve in water like table salt.

This process, called "chemical weathering," effectively sends dissolved remnants of rocks--my time capsules--into the Fraser.

Minerals in rocks are mostly made of geometric scaffolds of calcium, carbon, silicon, and oxygen atoms; every once in a while, however, a rogue element from the hinterlands of the periodic table will substitute for one of the more common elements and infiltrate the scaffold. Strontium is one of those rogue elements. It behaves much like the more abundant elements in the environment and comfortably resides in places where calcium fits in mineral structures. For geochemists like me, that provides a lucky break.

One particular isotope of strontium, strontium-87, is produced by the constant radioactive decay of rubidium-87 in rocks. It takes millions of years for detectable amounts of strontium-87 to build up in minerals, which makes it an excellent tool for determining the birthplace of river-borne material. When I collect a water sample in the Fraser, its strontium-87 signature tells me how much material came from older rocks in the Rockies that have accumulated more strontium-87 or from younger rocks in the Coast Range.

The carbon-climate connections

The bicarbonates produced by chemical weathering also play an important role in the big picture. When they reach the ocean, certain photosynthetic algae can use them to build hard shells. When these algae die, their shells--and the carbon in them--sink to the seafloor and become buried in sediments. Hence this slow trickle of carbon from the atmosphere-through rivers to the ocean and finally into marine sediments-extracts some carbon out of a more rapid cycle that circulates carbon among air, land, and sea. …

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