Earth’s Carbonate-Silicate Cycle: Will the Brightening Sun Disrupt It Enough Over the Next Billion Years to Destroy Life on Earth? Maybe or Maybe it Will Take Longer

     The earth undergoes several different geochemical cycles such as a carbon cycle and a nitrogen cycle. In geology, one of the most important cycles is the carbonate-silicate cycle. It is a long-term cycle that takes millions of years to complete. Wikipedia describes it as follows:

‘The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism.”

When weathered rocks are buried the CO2 in them is removed from the atmosphere. It returns to the atmosphere via volcanism. Thus, this cycle is a long-term CO2 regulator, often referred to as the Earth’s thermostat or buffering system. The carbonate-silicate cycle can be considered a branch of the carbon cycle, the inorganic branch as distinguished from the organic carbon branch of the cycle. It cycles over a much longer time period than organic carbon does.

     Carbonic acid (H2CO3) is a weak acid that occurs in rainwater and over time dissolves both carbonate rocks and silicate rocks. The main reactions of the carbonate-silicate cycle are shown below:

“99.6% of all carbon on Earth (equating to roughly 108 billion tons of carbon) is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in the form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere.”

Both biology and tectonics affect weathering rates and atmospheric CO2. The carbonate-silicate cycle is considered to be insensitive since it allows for large temperature swings in the Earth’s history. All planets with water may all have some sort of carbonate-silicate cycle.

     A 2019 article about the cycle by James Kasting at Chicago University notes:

“Most silicate weathering is thought to occur on the continents today, but seafloor weathering (and reverse weathering) may have been equally important earlier in Earth’s history.”

He also explains the carbonate-silicate  cycle as follows:

“The by-products of silicate weathering include calcium and magnesium ions (Ca2+ and Mg2+), bicarbonate ions (HCO3−), and dissolved silica (SiO2). These dissolved products are carried by streams and rivers down to the ocean where various organisms use them to make shells of calcium carbonate (CaCO3) or silica. Today, much of this carbonate precipitation is carried out by organisms that live in the surface ocean, such as the planktonic foraminifera. During the Precambrian, this function was performed primarily by benthic, mat-forming organisms, creating stromatolites. But carbonate would precipitate anyway, even on an abiotic planet, as the products of weathering—specifically, alkalinity ≅ [HCO3−] + 2[CO32−]—accumulated in the ocean. When organisms such as foraminifera die, they sink into the deep ocean. The deep ocean is slightly more acidic than the surface ocean, and so most of the carbonate redissolves. A portion of it is preserved, however, and forms carbonate sediments that coat parts of the seafloor. When this seafloor is subducted, some of the carbonate is scraped off, but some of it is carried down to great depths. There, the heat and pressure cause calcium and magnesium to recombine with silica (which by this time is the mineral quartz), reforming Ca/Mg silicates and releasing gaseous CO2. This CO2 is restored to the atmosphere by volcanism. Ignoring Mg, the entire cycle can be represented by the reaction

 CaSiO3 + CO2 ↔ CaCO3 + SiO2

     New research suggests that life may have more than 1 billion years left on Earth, perhaps 1.6 billion years or more. A September 2024 paper described by Phys.org notes that other feedbacks could take place, extending life on Earth. It is a strange prediction, I suppose, since it is difficult to predict what will happen the next day, let alone the 365 billion days into the future it would take to make up 1 billion years. According to phys.org:

But if Earth's biosphere has a much longer lifespan than thought, that affects the hard steps model.

"A longer future lifespan for the complex biosphere may also provide weak statistical evidence that there were fewer 'hard steps' in the evolution of intelligent life than previously estimated and that the origin of life was not one of those hard steps," the authors conclude.

If that's the case, then exoplanet habitability could be less rare than thought.

     I’m not sure what I wanted to accomplish with this post. Perhaps I just wanted to introduce the carbonate-silicate cycle, associated feedback cycles, and some possible (far-off) future implications.  

 

References:

Life might thrive on the surface of Earth for an extra billion years. Evan Gough. Phys.org. September 20, 2024. Life might thrive on the surface of Earth for an extra billion years (msn.com)

Carbonate-silicate cycle. Wikipedia. Carbonate–silicate cycle - Wikipedia

The Goldilocks Planet? How Silicate Weathering Maintains Earth “Just Right.” James F. Kasting. Elements. Volume 15, Number 4, August 2019. Geoscience World. Kasting_2019.pdf (uchicago.edu)