Some basic geochemistry of carbonates, carbonic acid and carbon dioxide
This is part of the of How To…series… on carbonate rocks
Sedimentary carbonate petrology is concerned first and foremost with the precipitation and dissolution of mineral phases; principally calcite, aragonite and dolomite. Both processes involve chemical reactions and the two primary requirements for these reactions are:
thermodynamics – there needs to be sufficient energy to drive the reactions, and
an excess or deficit of dissolved mass that, for the minerals of interest, includes Ca2+(aq), Mg2+(aq), and CO32-(aq) (aq = aqueous)
All sedimentary carbonate reactions at the surface or during sediment burial take place in water: fresh water, sea water, or concentrated brines. Furthermore, the chemical composition of these fluids can evolve, for example during burial (sea water to brine), or uplift and exposure (sea water to fresh meteoric water). Such changes are commonly manifested as cement stratigraphy (e.g. high-Mg calcite overlain by low-Mg calcite), or where clasts, matrix and early cements are replaced by new generations of calcite or dolomite.
We are also interested in carbonate reactions because of the present trend towards climate change and the possibility that ocean chemistry may change; ocean acidification tops the list here. To get beyond the hyperbole, to determine whether this is a real possibility or not, we need to understand some basic chemistry. Some introductory concepts are outlined herein.
To delve deeper into the complexities of the carbonate system, have a read of the contributions cited below. Continue reading →
Photosynthesis, a process that had its beginnings about 2.5 billion years ago, has an awesome responsibility; it keeps us breathing. It is a metabolic process in plants that uses the energy from sunlight to drive chemical reactions; reactions that produce amino acids, proteins, sugars and other compounds that create the architecture of plants. The process takes atmospheric CO2, converts the carbon plus other nutrients to organic compounds, then expels the left-over oxygen. Plants help regulate the composition of the atmosphere – they are our other set of lungs.
It has been shown experimentally that photosynthesis increases in many kinds of plants (some more than others), as the supply of atmospheric CO2 also increases. On a global scale, this is referred to as greening of the earth, where both regional studies, and more recently satellite data show an overall increase in plant growth, and an increase in growing seasons. In Europe and North America, the seasonal leaf-out (or bud-break) for the period 1950 to the 1990s was 2-4 weeks earlier than pre-1950. Continue reading →
Read any scientific paper or blog on climate and you’re bound to come across the phrase radiative forcing. Radiative forcing is central to all climate science. Radiation from the sun heats our atmosphere and earth surface. Some of this radiation is reflected back to space. If there is a balance between incoming and outgoing radiation then average global atmospheric temperatures neither increase or decrease. However, if the balance is perturbed, climate will warm or cool. Radiative forcing causes climate imbalances. Thus, volcanic aerosols tend to cool things off, decreasing albedo will tend to warm them. Continue reading →
Of the two certainties in life, volcanoes offer the most excitement (death and taxes are basically the same thing). They are magnificent while asleep; a primeval ruggedness that stirs the imagination. We paint them, we eulogise them. And when they awaken, we run for cover. Whether in a state of dormancy or high agitation, they leave an impression on our inner and outer landscapes.
All active volcanoes emit gas; pre-, during and post-eruption. On average, 96% of volcanic gases are water vapour, the remaining components being CO2, SO2 (most common), plus a little helium, nitrogen, carbon monoxide, hydrogen sulphide, and a few halides. Volcano-derived carbon dioxide is frequently cited as a culprit for increasing atmospheric CO2 concentrations in Continue reading →
It seems that global catastrophes and the ensuing mass extinction of all manner of life-forms, asteroid impacts and Dinosaurs immediately come to mind, were made for Popular Science. Even Hollywood is in on the act. Perhaps it’s because, in the telling, they appeal to some innate sense of nihilism, a bit like the existential threats that politicians trot out from time to time.
A recent scientific paper by Steven Stanley published by the US National Academy of Sciences, provides some good news on this score; past estimates of life forms snuffed out by such global events, have been exaggerated. Stanley’s reassessment accounts for the fact that extinctions are taking place all the time, in the background, and that these individual, long-term biotic events need to be subtracted from the total species loss resulting from some catastrophe. Continue reading →
Venus and Mars. Popular mythologies aside, neither planet has a particularly hospitable temperament. Both are rocky planets, like earth, and both have atmospheres of carbon dioxide (about 95%) and nitrogen (about 3%). Surface temperatures on Venus hover around 450oC; those on Mars about -53oC. Venus is not only a tad warm, its surface atmospheric pressure is 92 times that on earth; that on Mars about 6 one thousandths of earths comfort zone. Venus’ inclemency is the result of run-away Greenhouse processes; Mars’ is due to virtually no Greenhouse effects. Earth resides in that ‘Goldilocks zone’, being neither too hot nor too cold. Lucky us!
An energy budget
The sun heats the earth’s surface, atmosphere and oceans; earth’s internal heat contributes very little to this process. For the climate to be stable over a reasonable length of time (decades, centuries) there must be a balance between incoming heat from the sun and outgoing heat lost to space; this heat is lost via reflection, convection and conduction. Perturbations in this balance result in the surface either heating up or cooling down. We know that these long-term imbalances do occur because there have been glaciations and intervening periods of more clement surface conditions. Major climatic variations like these are mainly the result of (predictable) periodic perturbations in earth’s orbit and rotation, namely the Milankovitch cycles.
The total energy that earth receives from the sun is called the total radiance. It is primarily in the form of visible and infrared light, with lesser proportions of short wave-length UV, gamma and x-ray frequencies, and longer wavelength frequencies like microwaves. Partitioning of the incoming light is illustrated in the cartoon below. About 30% is reflected directly back to space by clouds, aerosols and the earth’s surface, especially ice sheets (referred to as the albedo); 70% is absorbed by the atmosphere, and both the solid and liquid earth.
However, to maintain an energy balance, some of this absorbed light energy must be converted and re-radiated back to space. We witness this re-radiation in our everyday lives. The many hues of red, blue, green, and yellow in my backyard are visible because a part of the light spectrum is reflected. If the entire spectrum is reflected, we see white; the red flowers are reflecting only light in the red part of the spectrum – the remaining light energy is absorbed. The absorbed energy is absolutely necessary for biological growth. If no light or heat energy is reflected, we see nothing – black.
The greenhouse blanket
The Martian surface is frigid because most of the light-heat energy that gets to the planet’s surface is reflected and re-radiated back to space – Mars has no blanket. Our own atmosphere is made up of 78% nitrogen, 21% oxygen, and very small amounts of carbon dioxide, methane, and water vapour. Nitrogen and oxygen are almost completely transparent to sunlight in the visible and infrared part of the spectrum – they do little to help warm or cool the planet. That task has been appointed to the greenhouse gasses; water vapour (most abundant), carbon dioxide, methane, nitrous oxide, and more recently certain industrial hydrocarbons that have chlorine and fluorine in their molecular structure. Despite their very low concentrations in the atmosphere (water vapour is most abundant, CO2 is 0.04%, methane is even less), they are solely responsible for maintaining the kind of climatic conditions we have become accustomed to. All except the chlorofluorocarbons occur naturally.
How does this greenhouse process work? As an example, a carbon dioxide molecule consists of a carbon atom bonded to two oxygen atoms. Bonding is loose enough such that infrared light energy will cause the atomic bonds to vibrate; in doing so the molecule absorbs heat. All the greenhouse gasses operate on the same principle. However, they also react to heat energy at different frequencies in the infrared part of the spectrum – this is illustrated below, comparing water and CO2. Water vapour absorbs energy over certain frequencies across the light spectrum, but importantly there is a ‘window’ between about 8-15 micrometres (part of the infrared frequency range) where it does not absorb heat; heat can escape through this frequency window.
This creates for us a wonderfully fortuitous balance between some infrared heat being absorbed by water vapour and other parts of the heat energy spectrum that can escape to space. Part of theenergy balance is also moderated by CO2 that absorbs heat in parts of the infrared spectrum that water does not. Nitrous oxide and methane also play an important role in maintaining the balance between heat that is absorbed, and heat that is redirected to space. It is worth reiterating that, except for water, the concentrations of these greenhouse gasses are very low and that even minor changes to these amounts will result in some degree of warming or cooling of the atmosphere.
An additional factor in the greenhouse effect is the amount of aerosol and extremely fine particulate matter in the atmosphere. Volcanic eruptions contribute some of these although addition to the atmosphere is sporadic. The potential cooling effect from violent eruptions is well documented (Krakatoa 1883, Pinatubo 1991). Soot from industrial burning and clearing of forests is also present and may influence atmospheric heating.
Although oxygen has no role in greenhouse maintenance, it does interact with certain ultraviolet light frequencies to produce ozone (O3) in the upper atmosphere; this happy circumstance means that most of the harmful UV energy is filtered out by ozone before it reaches the surface.
Postnote
There is an enduring image of Earth rising above the moon’s horizon, taken during the first manned lunar mission. Small, cloud swirled, seemingly fragile. Our atmosphere looks thin. When you look at this image, and then consider some of the details of how the atmosphere works, the balancing acts among all the gas components, the partitioning of heat and mass from air, earth and oceans, you realize how precarious the conditions conducive to our well-being really are. I’m not sure who said it first, but it really is all we’ve got.
CO2 has a bad rep. We can’t do without it (GOOD – it’s part of the photosynthetic process), but it looks like we’re upsetting the balance between having too little and producing too much (BAD). I take some of the blame for this: I drive a car (out of necessity), run a small boat (that I really enjoy), use a gas stove (the best cooking device ever), use a couple of lawn/orchard mowers (also necessary to keep the weeds at bay in our organic kiwifruit orchard), and take trips to Canada and beyond (which is life-affirming). I guess we all have our crosses to bear (INDIFFERENT), but I do take solace in the knowledge that my carbon footprint is more than offset by the biomass on my organic orchard.
Cycle: ( noun) A series of events that are regularly repeated in the same order (Oxford Dictionary)
How Milankovitch cycles effect Earth’s climate
Natural cycles are all around us; tides, seasons, sun spots, birthdays, El Niño. In geology we can identify cycles at many different scales, from the really grand to the wafer-thin (deference to Monty Python), from those that span eons, to cycles that repeat every few seconds. Perhaps the grandest of earth cycles are those that last 100-300 million years and involve the formation and destruction of tectonic plates. On a more human time scale there is the seemingly never-ending train of waves rushing to meet you on your favourite beach. Continue reading →
One of my geology field seasons in the Canadian Arctic worked out of a base-camp on Axel Heiberg Island (west of and snuggled against Ellesmere Island). It was the spring thaw and all rivers and streams were muddy. Our only source of clean water turned out to be a small melt-water pond atop an iceberg in Strand Fiord, a few hundred metres offshore. The helicopter would make daily trips with a 45-gallon drum to collect the water. The ice and its water were crystal clear and probably a few thousand years old. It was a treat. Perhaps the only thing missing was the occasional Scotch or G&T. Continue reading →
Carbon dioxide is a significant by-product of oil and natural gas production at the well-head, hydrocarbon and coal combustion (especially in power generation) and several manufacturing industries (e.g. cement). Carbon Capture and Storage (CCS) involves technology that captures CO2 produced by these industrial processes and stores it underground. In doing so, CCS technology attempts to prevent the CO2 from being released into the atmosphere. From the point of view of potential climate change it seems like a sensible thing to do. However, CCS does have its detractors who argue primarily that either it doesn’t matter how much CO2 enters the atmosphere, or that the costs far outweigh the benefits. Indeed, the cost of CCS programs is high. Regardless, the science of CCS is fascinating. Continue reading →
