Whose CO₂ is it Anyway? Ocean Fizz or Smokestack Blame

Guest essay by Jennifer Marohasy (reposted from her website by request) with addendum by Anthony.

Picture this: it’s a hot day, and you grab a soda can that’s been in the sun. You crack it open—psssht—and CO₂ fizzes out, tickling your nose, maybe spraying your shirt if you’re slow. It’s a tiny chaos, a burst you can’t control. Now imagine that fizz across the ocean’s sun-warmed surface, covering 71% of Earth, bubbling CO₂ into the air we breathe. Wild, right? A bit mad. I reckon it’s a missing piece of the climate puzzle.

The IPCC pins it all on smokestacks—11 billion tonnes of carbon a year from fossil fuels. Even skeptics like the CO₂ Coalition echo this, leaning on guys like Ferdinand Engelbeen who do their maths by the consensus numbers on this issue of CO₂ origins.

But they might have it all back to front and be leaving out ocean chemistry and biology. In fact, I’m convinced they are.

The Keeling Curve—CO₂’s climb from 280 to 420 ppm—carries their blame. But what if the ocean’s fizzing more than they think? Their rock-solid evidence could be mostly myth.

I’ve been digging into this with Ivan Kennedy, my second guest for the webinar series ‘Towards a New Theory of Climate Resilience’. That was back in February and I’m still to process the audio from this discussion.

Instead, my focus has been on writing technical papers. Ivan and I are working through a hypothesis that could perhaps flip the climate script.

Engelbeen claims fossil fuels’ isotopic fingerprint—light ¹²C (isotope C12) dragging the air’s ¹³C-to-¹²C ratio from -6.5‰ (per mille)* to -8.5‰ since 1850—is proof of coal and oil’s guilt. Ocean CO₂, averaging 0‰ from deep waters, should nudge it up—not down. Case closed.

Except. That ¹²C/¹³C tale’s shakier than they admit. What if the ocean’s surface, warmed by the sun, fizzes CO₂ richer in ¹²C than the deep oceans 0‰?

Calcification—limestone forming in seawater—might churn out CO₂ at -10‰ or lower, diluting that delta 13 signal just like fossil fuels. It’s not the deep ocean I’m on about—it’s the top 65 meters, the mixed layer, where sunlight and warmth cause biological action. So much action that it has built the biosphere’s great carbonate deposits, even the White Cliffs of Dover.

Ivan and I talked some of this over—Great Barrier Reef, North Pacific—during our webinar (soon my first podcast—thanks for waiting!). Calcification’s no sleepy trick; it’s a biological buzzsaw—corals, algae, phytoplankton like coccolithophores churning limestone. In summer blooms, they might pump out tonnes of CO₂, light on ¹³C. Our Thermal Acid Calcification (TAC) hypothesis says nature’s pitching in more than you might think.

Ponder this next time you sip a soda: could the ocean be bubbling up a CO₂ twist?

TAC’s perhaps a second plank in my New Theory of Climate Resilience. Subscribe for irregular updates, and to know about next webinars.

This is Part 2 of How Climate Works. Part 1 was with Bill Kininmonth. I never properly processed the audio from Part 1, and I accepted the AI summary of our meeting click here.

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When we say deep ocean carbon is 0‰ (per mille), we’re talking about its carbon isotope ratio, specifically the δ¹³C value. This is a measure of how much carbon-13 (¹³C) is present relative to carbon-12 (¹²C), compared to a standard reference.

In this case, 0‰ doesn’t mean there’s no carbon-13 in the deep ocean—it means the ratio of ¹³C to ¹²C in deep ocean dissolved inorganic carbon (DIC) is about the same as the standard reference, which is usually the Vienna Pee Dee Belemnite (VPDB). A δ¹³C of 0‰ indicates no enrichment or depletion of ¹³C relative to that standard.

Now, why is deep ocean carbon around 0‰? It’s because the deep ocean is a massive, well-mixed reservoir of carbon that’s been cycled through various processes over long timescales. Surface ocean carbon starts with a δ¹³C of about +1 to +2‰ due to photosynthesis, where phytoplankton preferentially take up ¹²C, leaving the surface water slightly enriched in ¹³C. But as organic matter sinks and decays, it releases carbon back into the deep ocean. This process, along with the mixing of water masses, balances out the isotopic signature. The deep ocean ends up with a δ¹³C close to 0‰ because it reflects a long-term average of all these inputs—biological, physical, and chemical—without much net fractionation.

In terms of carbon-13, this means the deep ocean has a pretty stable and “neutral” amount of ¹³C compared to the global carbon cycle. It’s not heavily skewed like surface waters or organic matter (which can be -20‰ or lower due to that photosynthetic preference for ¹²C). So, a δ¹³C of 0‰ tells us the deep ocean is kind of a baseline, a big pool where carbon isotopes have settled into equilibrium over thousands of years.

Addendum by Anthony: I was skeptical (gasp!) of Marohasy’s claims, so I decided to run it through Grok, but purposely asked the question in reverse of her premise, which I thought might prompt better results.. Grok does quite well with scientific questions. It seems to agree.

Marohasy states:

Except. That ¹²C/¹³C tale’s shakier than they admit. What if the ocean’s surface, warmed by the sun, fizzes CO₂ richer in ¹²C than the deep oceans 0‰?

The Grok response to this question is below:

Grok question:
On the ocean surface, does carbon dioxide with carbon 13 isotope come out of solution and into the atmosphere faster than carbon dioxide containing carbon 12 isotope?

Grok response:

Let’s dive into this intriguing question about carbon dioxide isotopologues—specifically, whether CO₂ with carbon-13 (¹³CO₂) comes out of solution and enters the atmosphere faster than CO₂ with carbon-12 (¹²CO₂) at the ocean surface. The process we’re examining is the outgassing of CO₂ from seawater into the air, and the key is to determine if the isotopic difference between ¹³C and ¹²C influences this rate.

The exchange of CO₂ between the ocean and atmosphere involves several steps: dissolved CO₂ must diffuse from the ocean’s bulk to the surface, cross the air-sea interface, and enter the gas phase. This process can be affected by both physical and chemical factors, including diffusion rates and reaction kinetics. Since ¹³C is heavier than ¹²C (atomic mass ~13 vs. ~12), we need to consider the kinetic isotope effect (KIE), where the mass difference can alter the speed of these processes.

First, let’s look at diffusion. In water, dissolved CO₂ moves toward the surface based on concentration gradients. According to Graham’s law of diffusion, the rate of diffusion is inversely proportional to the square root of the molecule’s mass. For ¹²CO₂ (molecular mass ~44) and ¹³CO₂ (molecular mass ~45), the difference is small, but ¹²CO₂ diffuses slightly faster because it’s lighter. The ratio of diffusion rates is roughly √(45/44) ≈ 1.011, meaning ¹²CO₂ moves about 1.1% faster than ¹³CO₂. This suggests that, all else being equal, ¹²CO₂ would reach the surface and outgas more quickly.

Next, there’s the chemistry. In seawater, CO₂ exists in equilibrium with carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻). The conversion of dissolved CO₂ to these species (and back) involves hydration/dehydration reactions, which can also exhibit a kinetic isotope effect. Because ¹³C forms slightly stronger bonds (due to its higher mass and lower vibrational frequency), reactions involving ¹³CO₂ tend to be slower. Experimental studies confirm that the hydration of ¹²CO₂ to bicarbonate is faster than for ¹³CO₂, meaning ¹²CO₂ is more readily available to outgas as it spends less time locked in these dissolved forms.

At the air-sea interface itself, the transfer of CO₂ into the atmosphere is driven by the partial pressure difference and turbulence, but isotopic fractionation can still occur. Research on gas exchange, including models and measurements of carbon isotope ratios (e.g., δ¹³C), shows that ¹²CO₂ is preferentially released into the atmosphere. This is partly because its faster diffusion and reaction kinetics give it an edge in escaping the ocean. The overall fractionation during outgassing enriches the atmosphere in ¹²C relative to ¹³C, leaving the ocean slightly enriched in ¹³C, as detailed in studies like those from NOAA’s Global Monitoring Laboratory.
So, does ¹³CO₂ come out of solution faster than ¹²CO₂? No, the opposite is true. Due to its heavier mass, ¹³CO₂ diffuses more slowly, reacts more sluggishly in seawater, and crosses into the atmosphere at a slightly lower rate than ¹²CO₂. The difference is subtle—on the order of a few percent—but consistent across physical and chemical processes. Therefore, carbon dioxide with carbon-12 outgasses into the atmosphere faster than carbon dioxide with carbon-13 at the ocean surface.

References

1. Kinetic Isotope Effect – LibreTexts Chemistry
2. Graham’s Law of Diffusion – Britannica
3. Kinetic Isotope Effects in CO₂ Hydration – PNAS
4. Carbon Isotope Measurements – NOAA Global Monitoring Laboratory