Cambrian explosion and Ocean Chemistry
Thursday, 21. May 2009, 17:34:31
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
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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