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Posts tagged with "geochemical cycles"

Cambrian explosion and Ocean Chemistry

Thursday, 21. May 2009, 17:34:31

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About 530 million years ago the ocean chemistry changed dramatically with a jump in the concentration of sulphate in the oceans. This was more or less simultaneous with the seemingly rapid appearance of a wide variety of multiple-celled organisms, an event known as the Cambrian Explosion. The chicken or egg question is, whether one of these two events caused the other.

According to a paper in the 18 May 2009 issue of the scientific journal Proceedings of the National Academy of Sciences (PNAS) by Canfield, from Odense University, Denmark, and Farquhar, from Maryland University, US, the cause of the rise of sulphates in the oceans can be directly linked to the emergence of worms. They attribute the rise in sulphate to the onset of bioturbidity on the ocean floor — the burrowing, sluicing, pumping and mixing caused by masses of worms, clams, crustaceans and other animals that began to appear around this time in Earth's history.

Before the worms came about, sulphate — arriving in seas in the run-off from rivers — would largely be turned into hydrogen sulphide by bacteria living in the ocean floor. The sulphide would then be converted to pyrite (FeS2), which, once buried, removes the sulphate from the system. Once bioturbation turned on, however, oxygen in the deep ocean could mix more freely with the sediments, allowing bacteria and other processes to recycle pyrite and turn it back to sulphate. This excess sulphate would have reached a saturation point, giving rise to the formation of gypsum deposits — a mineral that, along with sulphate levels, also happened to rise in the rock record around this time. Gypsum is a calcium sulphate, CaSO4 (hydrated with water).

Here just a few words about sulphur and oxidation. I do not want to get into too many details about oxidation and reduction. More details can be found by googling redox or start with the Wikipedia article on redox.

An oxidation process (a.o. through the action of microorganisms) in the water could look like this:

H2S (hydrogen sulphide) → S (sulphur) → SO2 (sulphur dioxide) → SO3 (sulphur trioxide) → SO42- (sulphate ion)
The oxidation goes from left to right - a reaction in the opposite direction i.e. from right to left would be reduction. So to the left we have products related with hypoxia and anaerobic conditions and to the right oxygen rich and aerobic environments.

The hydrogen sulphide may react with metal ions, like iron, in the water to form insoluble sulphides. Pyrite (FeS2) is such a sulphide. Sulphate ions, on the other hand may react with metals to form sulphate, such as gypsum, that is calcium sulphate (CaSO4). I intend to come back to these reactions and the geochemical sulphur cycle in a later post. Just this final remark about oxidation. The term oxidation was originally applied to reactions in which substances combined with oxygen, and reduction was defined as the removal of oxygen from an oxygen-containing compound. The meaning of the terms have however broadened significantly. You will see from my example above that no oxygen is added from step 1 to step 2, but oxygen (O2) continuously added in the following steps.

And now two more images before I close. One of pyrite, this time with another crystal form than the cubes in the pyrite image above. And an image of gypsum, not in its pure form, but as desert rose.





• http://www.pnas.org/content/106/20/8123.abstract?etoc
• http://www3.interscience.wiley.com/journal/121682990/abstract
• http://www.nature.com/news/2009/090518/full/news.2009.485.html?s=news_rss
• http://news.softpedia.com/news/Ocean-Chemistry-Changed-530-Million-Years-Ago-111927.shtml



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A Fishy Carbon Cycle

Wednesday, 21. January 2009, 10:05:39

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The oceans play an important role (together with land plants) in taking up carbon dioxide (CO2) that has been emitted into the atmosphere. Understanding these uptake mechanisms is important in forecasting the rise of atmospheric CO2 because even though plants and bodies of water now absorb surplus greenhouse gas, they could become new trouble spots.

The following simplified carbon cycle box model is meant to illustrate the carbon emissions of human origin and the fact that vegetation and oceans for the time being serve as sinks for surplus carbon.


I hope I have got the sums right, but they are only meant to be indicative.

A paper published in the journal Science of 16 January 2009 highlights how little in fact is known about some aspects of the marine carbon cycle, which is undergoing rapid change as a result of global CO2 emissions.

Production of calcium carbonate in the ocean is generally attributed to plankton. It is less widely known that all marine bony fish (teleosts) produce and excrete carbonate. Bony fish includes 90 percent of marine fish species. “Gut rocks”, i.e carbonate precipitates, are excreted by bony fish via the guts (intestine) as a by-product of their osmosis regulation. the “gut rocks” are produced whether or not fish are feeding

The blood of fish has a higher water concentration than the surrounding sea water. As sea water passes over the gill membranes, water diffuse out of the blood into the sea water by osmosis. To replace the water they constantly lose by osmosis, they must continuously drink calcium- and magnesium-rich sea-water. Drinking sea water brings a large quantity of salt into the blood and this has to be removed.

The “gut rocks” are excreted at high rates. Based on estimates of global fish biomass the authors suggest that marine fish contribute 3 to 15% of total oceanic carbonate production, which means that fish are a major but previously unrecognized source of oceanic carbonate and contribute substantially to the marine inorganic carbon cycle.

The fish poop carbonate is soluble and dissolves in the upper sea water. Less soluble carbonates, produced by plankton, are more likely to sink further and become locked up in sediments.

The increase in carbon dioxide in the atmosphere not only drives global warming, but also raises the amount of CO2 dissolved in ocean water, tending to make it more acid, potentially a threat to sea life. The uptake of carbon dioxide into the carbonate of fish “gut rocks” (and into planktonic shells) may to a certain degree neutralise this acidification.

Calcium carbonate (CaCO3) is alkaline. Alkaline is the opposite of acid. Solutions with a pH below 7.0 is considered acid, while solutions with a pH above 7.0 is considered alkaline.

I wonder if the fish “gut rocks” - in spite of the high solubility - have any implications for carbonate sedimentation?

References:
Contribution of Fish to the Marine Inorganic Carbon Cycle
by Wilson et al.
in
Science 16 January 2009:
Vol. 323. no. 5912, pp. 359 - 362
DOI: 10.1126/science.1157972

• http://www.sciencemag.org/cgi/content/short/323/5912/343
• http://www.sciencemag.org/cgi/content/abstract/323/5912/359
• http://fe13.story.media.ac4.yahoo.com/news/us/story/ap/20090115/ap_on_sc/sci_fish_poop
• http://www.eurekalert.org/pub_releases/2009-01/uomr-fms011609.php
• http://www.sciencedaily.com/releases/2009/01/090115164607.htm

See also http://www.seeddaily.com/reports/First_Ever_Estimate_Of_Worldwide_Fish_Biomass_And_Impact_On_Climate_Change_999.html on First-Ever Estimate Of Worldwide Fish Biomass And Impact On Climate Change


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More Mercury Misery - from Bulbs to Teeth

Monday, 19. January 2009, 10:22:12

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After a couple of days in good geologists company with evening dinners and visits to a limestone cave and a sandstone quarry, or was it the other way round, I am now back to normal - and time enough to read some mercury news from Scandinavia. News that reached my eyes after I had written the post on Mercury Pollution and the Mercury Geochemical Cycle .

First the bad news.

Energy saving light aka compact fluorescent lamp (CFL) or compact fluorescent light or less commonly as a compact fluorescent tube (CFT) is quite efficiently saving energy all right. These bulbs have however created a new serious problem - the bulbs contain small amounts of mercury.

For energy saving reasons many countries are now banning the use of “traditional” incandescent light bulbs (high-energy bulbs) in favour of more energy-efficient lighting like the low-energy CFL bulbs. Wikipedia has a list of the countries concerned. In December 2008 the member states of the EU agreed to a phasing out of incandescent light bulbs by 2012.

The amounts of mercury vapour inside the glass tubing of CFLs average 4.0 mg per bulb (according to EU rules they are allowed to contain up to 0.5 mg), and it is a concern for landfills and waste incinerators where the mercury from lamps is released and contributes to air and water pollution. I have to add at this stage that in areas powered by coal, CFLs end up saving on mercury emissions versus incandescent bulbs, because coal releases a lot of mercury as it is burned - and I may of course also add that the light from incandescent bulbs may cause (skin) cancer. Nothing is as simple as it looks like.

Energy saving bulbs will break before they get to the landfill. They'll break in containers, or they'll break in a dumpster or they'll break in the trucks. In many regions it is not allowed to put CFL bulbs in the trash. They should be sorted out and collected for recycling.

Sorted waste collection is the rule in Denmark but nevertheless an estimated 2 million CFL bulbs per annum end up in incineration plants where the mercury is emitted to the atmosphere. With only about 5.5 million inhabitants I consider that a lot, or rather much too much. In Sweden about 8-10 kg mercury from CFL bulbs are thrown into the garbage bin each year.

And now the good news.

In Sweden the government has banned all use of mercury (as from 1 June 2009). The ban prohibits products containing the heavy metal from being brought to market in Sweden. The prohibition also means mercury can no longer be used in manufacturing or dentistry. The Swedish dentists may no longer fill it into Swedish teeth as dental amalgam. Dental amalgam is an alloy of a number of metals, mainly silver, tin and mercury. The decision makes Sweden, along with Norway, the country with the most stringent restrictions on mercury. Mercury waste, of which there is roughly 1,400 tons, will be shipped to Germany, the government has decided (poor Germans!).

As Sweden, like Denmark, is a member of the EU, I am not quite sure what is going to happen with energy saving bulbs in Sweden? Although EU has banned use of mercury in electric and electronic products, low-energy bulbs are not included in this ban! I know, however (of course), that Sweden wants to see them banned.

In Swedish:
• http://www.svd.se/nyheter/inrikes/artikel_2316491.svd
• http://www.sr.se/cgi-bin/isidorpub/PrinterFriendlyArticle.asp?Artikel=2564374&ProgramID=83




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Mercury Pollution and the Mercury Geochemical Cycle

Wednesday, 14. January 2009, 19:05:23

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Of the geochemical cycles I mentioned here the mercury cycle is the one with the smallest fluxes (Mercury is an extremely rare element in the Earth's crust, having an average crustal abundance by mass of only 0.08 parts per million). Nevertheless it is well worth watching carefully because of the toxicity of mercury. All mercury spills are potentially very dangerous.

Mercury (Hg), also known as quicksilver, is the only metal that is liquid at normal temperatures. The production of mercury has luckily declined since the early 1970s. This is partly due to increased recycling and partly to the concern about environmental pollution. Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, and other scientific apparatus, but concerns about the toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in favour of alcohol-filled, digital, or thermistor-based instruments.

Small-scale gold mining is the second-worst source of mercury pollution in the world, after the burning of fossil fuels. Gold can be extracted by amalgamation. The principle of amalgamation is to extract gold from the pulverized ore by mercury. The ore dust is rubbed with mercury which amalgamates with the gold. The amalgam thus produced is an alloy of mercury and gold. Gold is then separated from the mercury either by filtering it through leather or by distillation. The process is forbidden in most countries, but ...

Mercury exists in two different forms, organic and inorganic. In water the most prevalent form of mercury is the organic form. Most fish have trace amounts of mercury. The level of mercury found in a fish is related to the level of mercury in its environment and its place in the food chain. Mercury tends to accumulate in the food chain, so large predatory fish species tend to have higher levels than non-predatory fish or species at lower levels in the food chain. Eating fish contaminated with mercury can cause serious health problems, especially for children and pregnant women, so the general advice in some parts of the world is not to eat fish every day.

Natural sources such as volcanoes are responsible for approximately half of atmospheric mercury emissions.

The human-generated half can be divided into the following estimated percentages:
* 65% from combustion, of which coal-fired power plants are the largest source
* 11% from gold production.
* (34% a lot of other different sources).

As I said mercury pollution has already spurred public health officials to advise eating less fish, but it could become a more pressing concern in a warmer world according to a paper that appears in a recent issue of the journal Oecologia.

Carbon dioxide, the main greenhouse gas, has increased nearly 40 percent since the industrial revolution and is expected to continue climbing unless power plant and other emissions are restricted or curtailed. Carbon dioxide-enriched soil may contain much more mercury, because such soil has greater capacity to trap and hold on to mercury.

While I was writing the first paragraphs of this post, Andrew Alden published an interesting post on mercury (12 January 2009) titled And Now, Conflict Mercury at geology.about.com.


• http://www.sciencedaily.com/releases/2009/01/090107134635.htm
• http://geology.about.com/b/2009/01/12/and-now-conflict-mercury.htm
• http://www.msnbc.msn.com/id/28596948/
• http://geology.about.com/od/mercury/a/Hgmercury.htm




PS of 15 January 2009:
The Swedish Government has forbidden all use of mercury - also in dental fillings - as from 1 June 2009.

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Ocean Nutrient Cycling

Monday, 12. January 2009, 18:44:56

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Most of the nutrient requirements for life are available to excess in sea-water, but concentration of nitrogen, phosphorus, and silicon can be very low at times and in certain areas. They are most heavily used in the photic zone (that is the sunlit zone), where their availability can limit production, and they can be almost totally depleted in surface waters. The photic zone is the upper part of the ocean reached by sufficient sunlight to allow photosynthesis, and thereby plant growth.

Nitrogen, phosphorous, and silicon are called biolimiting nutrients since they limit the amount of life in the ocean. If we have more of these nutrients we will likely have more life, if we have less nutrients, then we will have less life (or biomass). Nitrogen, phosphorous, and silicon are exhausted first in the surface waters since each is essential to the growth of phytoplankton, the microscopic algae that float with the ocean currents.

Their concentration-depth profiles usually looks a bit like the following diagram:
The profiles often reach maxima at about 1 km depth - silicon somewhat deeper than nitrogen and phosphorus. The profiles decrease a bit in concentration below the maxima, partly due to slow upward mixing from deeper waters.

In the cold waters of the Southern Ocean (surrounding Antactica) iron is biolimiting, and it has for some years been suggested that fertilising it with iron could slow global warming by enhanced phytoplankton photosynthesis that would pull large amounts of carbon dioxide from the atmosphere, helping to counteract the buildup of this greenhouse gas. A German research ship laden with 20 tons of iron sulphate has just whipped up a storm of protest as it sails towards the Antarctic, where it intends to dump the iron sulphate into the ocean. See http://www.nature.com/news/2009/090109/full/news.2009.13.html .

More about geochemical cycles in general in my post yesterday.

PS of 14 January 2009:
The German science ministry has suspended the planned Indo–German ocean fertilization experiment in the Southern Ocean, and asked the German research institute behind it to commission an independent assessment of the study's environmental safety. See http://www.nature.com/news/2009/090114/full/news.2009.26.html




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Nutrient Cycles or Geochemical Cycles

Sunday, 11. January 2009, 19:21:18

It was my intention to write something today about nitrogen fixation in oceans and ocean nutrient cycling. As many of my readers are not earth scientists, and cycles in general are fundamental in many earth science disciplines, I shall however first write a few basic paragraphs on cycles in general. I am talking about cycles like the carbon cycle, the nitrogen cycle, the sulphur cycle and the cycles of other individual chemical elements like magnesium, cadmium, lead, mercury, zinc, cadmium, phosphorous ...

Such cycles are known as geochemical cycles, or biogeochemical cycles, or nutrient cycles. Each geochemical cycle is a model that describe the flow of a chemical element between land mass, ocean and atmosphere (and plants and animals). They are often shown as box model. Here is a general model of a geochemical cycle.



You may also see hydrosphere or water instead of ocean, and lithosphere instead of land, and air instead of atmosphere. And you can have many more boxes to give more details. The chemicals are sometimes held for long periods of time in one place. Such a place is called a reservoir. We may also talk about source, especially where the reservoir in decreasing in size, and sink, especially where the reservoir is increasing in size.

The pathways in the model above will be precipitation (rain, hail, snow), dust and other aerosols like sea spray from atmosphere to land and to ocean. Dust and degassing from land to atmosphere. Sea spray and gas evolution from ocean to atmosphere. Dissolved and suspended material transported by rivers to the ocean. Sedimentation from ocean to sediments. Uplift of sediments to land.

The vast majority of material near the Earth’s surface stays in the crust, the ocean, and the atmosphere. There appears to be little loss either to the mantle or to outer space. The material is continuously recycled. We talk about flux. The flux is the amount of material passing via a particular transport pathway in a fixed period of time - e.g. 7.4 gigatons per year. (A gigaton is 1 billion tons or 1,000,000 tons.

Geochemical cycles can also be illustrated by more descriptive diagrams, like the one I used in my post on the Oceanic Nitrogen Cycle for the nitrogen cycle:



The cow represents animals as a reservoir. This diagram is lacking in detail, and things like water and volcanoes (degassing) are missing.

Here is a more worked out model of the carbon cycle from UNEP - indeed with fluxes (called exchanges in the model) measured in gigatons.



For larger image - click here

The majority of carbon is found in rocks, either as carbonate, usually associated with calcium in limestones (as CaCO3), or as dispersed organic carbon, in sedimentary rocks, particularly shales. About three quarters of the total carbon in the outer regions of the Earth (the regions represented in geochemical cycle models) is contained in carbonates containing inorganic carbon and one quarter in rocks containing dispersed organic compounds. The combined carbon content of all other carbon reservoirs (atmosphere, plants and animals, soil, fossil fuels, dissolved compounds) comes to less than 1% of the total.

I hope that will do for an introduction.






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