The Earth Story

Why are oceans salty? Oceans have saltwater but rivers and most lakes do not. Why? Where did the salt come from?

Nacreous (‘mother of pearl’) cloud Nacreous clouds, also known as polar stratospheric clouds (PSCs), appear high in the atmosphere, some 15 to 30km (10 to 20 miles) above the Earth, generally in latitudes higher than 50°, particularly in the northern hemisphere. They form in the freezing temperatures of the lower stratosphere, often below -80°C (-112°F), and are usually a mixture of nitric acid and ice crystals, sourced from parcels of moist air that are forced up through the tropopause by the same orographic oscillations that are responsible for producing high lenticular wave clouds.

Salt mine of many hues When sea evaporate, often time and again as climate oscillates, very thick zones of salt can result, such as the Permian era (roughly 300-250 million years ago) Zechstein that underlies much of Europe. Near the type locality for the era near around the Russian city of Perm is an abandoned salt mine, where the evaporating waters were rich in potassium as well as sodium, and a mixture of evaporite minerals were deposited. These have created amazing swirls and psychedelic patterns om the tunnels, made of mixed halite (rock salt) and the potassium magnesium chloride mineral carnallite.

Wulfenite and mimetite

Lead ores produce some beautiful minerals, especially in the oxidised zones of primary deposits, where percolating groundwater and hydrothermal circulation have altered the primary dull sulphide minerals. Here are two of them together, the first a molybdate (see http://tinyurl.com/k95bcub andhttp://tinyurl.com/pn8w5cj), the second a chlorinated arsenate created from the oxidation of galena. Mimetite, part of the apatite group of minerals gets its name from its resemblance to pyromorphite (lead phosphate) and vanadinite (lead vanadide) and is usually yellow, more rarely green as in this beautiful specimen from the Ojuela mine in Mexico. It usually occurs as dusting on rock surfaces, or in botryoidal masses. The specimen measures 4.5 x 2.8 x 2.1 cm, the yellow crystals are wulfenite and the green ones mimetite.

Loz

Image credit: Spirifer Minerals

http://www.mindat.org/min-2714.html http://www.minerals.net/mineral/mimetite.aspx http://www.galleries.com/Mimetite

Why the sea is (not too) salt …

The Earth contains far less chlorine than it should. At least, far less than other representative samples of the early solar system do, and far less than the solar abundance, compared to other elements. If you plot the amount of each chemical element found on Earth against that found in primitive meteorites, or seen in the Sun, you will find a consistent trend. But halogens such as chlorine and bromine lie off the trend, and appear to be depleted on Earth compared to their neighbours on the periodic table.

Almost half of Earth's chlorine and bromine is found in the oceans, and in salt deposits from ancient oceans. These elements love to be in aqueous solution. But their total amount is still around ten times less than expected from measurements of meteorites, which otherwise represent well the stuff that Earth seems to have formed from. Put another way, something has removed around 90% of the chlorine that should be here, and if it hadn't the oceans would be ten times saltier. It's likely that such salty oceans in Earth's past would've prevented the development of complex life.

Sharp and Draper, researchers from Albuquerque and Houston, have recently moved our understanding of this conundrum a step forward. In a paper to be published in Earth and Planetary Sciences Letters they discuss the three generally-assumed explanations for the missing chlorine. By carrying out experiments on mixtures of silicate minerals and metallic iron at very high pressures and temperatures they show that the chlorine does not strongly favour the metal. From this they dismiss the idea that the chlorine has been trapped in Earth's core during the early stages of its core formation and metal-silicate differentiation. Another possibility is that chlorine and bromine are more volatile within the solar nebular, preventing their inclusion in planets and planetismals in the early solar system. Sharp and Draper, however, claim that the presence of the heavy halogen elements at higher abundances in meteorites indicates otherwise. Instead, they favour a model in which these halogens were "blown away" by meteorite impacts when Earth was still young, but after they had already become concentrated in the outer crust of the planet.

Without this impact-induced collisional erosion the oceans would be as salty as the Dead Sea. And the early oceans could well have remained the Dead Oceans. So, meteorite impacts may well have been responsible for mass extinctions over Earth history, but also have laid the foundations for evolution to have developed in the first place.

~SATR

Image: Meteor Crater, Arizona, by Mike Hendren (creative commons license)

Link: http://www.sciencedirect.com/science/article/pii/S0012821X13001192

Alternative explanations of "Why the sea is salt": http://etc.usf.edu/lit2go/134/stories-from-around-the-world/5302/why-the-sea-is-salt/

http://www.sacred-texts.com/etc/bnm/bnm40.htm

A new study suggests that exposure to chemical used to chlorinate drinking water and kill crop pests may increase the probability of developing a food allergy. The class of chemicals under scrutiny are the dichlorophenols. Dichlorophenols do not directly cause food allergies, but it is thought that by altering the population of microorganisms in the human body, it is sensitising the immune system’s reaction to food triggers. Most of us will know someone with a food allergy; with food allergies affecting between 6 and 8% of children in the US- a 20% increase in the last decade. If we look at previous generations however, the number decreases; with 1-3% of adults in the US with a food allergy. If we look further back in time again, food allergies were almost unheard of. Why is this? The whole mechanism behind this increase in susceptibility is accredited to what is known as the “The Hygiene Hypothesis”. This hypothesis essentially proposes that keeping our living environments too clean can actually backfire. It is suggested that a reduced exposure to bacteria and viruses can cause the immune system to react aggressively to other everyday allergens; like food.  Dichlorophenols are antimicrobial agents, they are added to water to inhibit and kill any microorganisms that may be present. They are also used in pesticides. This means that by drinking water where dichlorophenols have been used or ingesting pesticides residues; we are taking an antimicrobial into our bodies. It is suggested that this may then reduce the population of the natural microorganisms within our bodies and sensitise our immune systems. Using data collected by the U.S. National Health and Nutrition Examination Survey (NHANES) in 2005 and 2006, the researchers looked at concentrations of a variety of dichlorophenols in the urine of more than 2,200 people, ages 6 and older. They also looked at blood-test results indicating allergies to peanuts, eggs, milk or shrimp. People with the highest levels of dichlorophenols were 80% more likely to have food allergies compared to people with the lowest levels. The research has been published in the journal Annals of Allergy, Asthma and Immunology. Of course this is the early stages of research and nothing can be written in stone quite yet. It is however a new line of research worth pursuing. A good message to take away from this is; often we can be “too clean”. We need, particularly children, to be exposed to a certain level of microbiological activity. Although there are some nasty microorganisms out there, for the most part, they are beneficial and live symbiotically with us.  (I hope no-one sees this as scaremongering, not disinfecting our water supplies is far more dangerous than the implications of disinfection, and other disinfection methods are available.)  -Jean  More information: http://www.telegraph.co.uk/health/healthnews/9717546/Water-purifier-chemical-increases-food-allergy-risk.html http://www.sciencedaily.com/releases/2012/12/121203081621.htm Water treatment: http://www.excelwater.com/eng/b2c/about_8.php

In 2007, the Department of Water Protection in Los Angeles detected high levels of bromate in the Ivanhoe reservoir. Bromate forms when sunlight causes chlorine to oxidise bromide. Bromide is a natural constituent in water, particularly groundwater, while chlorine is generally added to water supplies to kill any bacteria, viruses and protozoa.  The reservoir facility, which is 102 years old, supplies more than 600,000 people with potable water in downtown and South LA. When the Department of Water Protection realised the problem with bromate, they began construction of a new underground reservoir, but while the new facility was being built they had to determine a way to keep the sunlight out of the water.  The method had to be quick and cost effective. The solution was “bird balls” which are made of polyethylene and cost only 40 cents each. 400,000 balls were dropped into the reservoir on June 2008, where they were to remain for the next four to five years until the new underground reservoir is completed. -Jean  Photograph by Gerd Ludwig For more info see: http://pubs.acs.org/cen/news/85/i52/8552notw4.html

OZONE DEPLETION AND THE ARCTIC Please see this previous post about the ozone hole over Antarctica and its effect on the countries bordering it (including increased skin cancer rates):http://on.fb.me/SMU6uC To recap somewhat, ozone is a gas made up of three oxygen atoms (O3), which occurs naturally in trace amounts in the stratosphere (the upper atmosphere) and protects life on Earth from the Sun’s ultraviolet (UV) radiation. The stratospheric ozone layer screens all of the most energetic, UV-c, radiation, and most of the UV-b radiation. Ozone only screens about half of the UV-a radiation. Stratospheric ozone is typically measured in Dobson Units (DU), the number of molecules needed to create a layer of pure ozone 0.01 millimetres thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere. Earth’s atmosphere has an average amount of ozone of 300 Dobson Units. Ozone depletion is caused by chemicals called chlorofluorocarbon compounds (CFCs) known as "freons" and bromofluorocarbon compounds known as Halons, which escape into the atmosphere from refrigeration and propellant devices and processes. These chemicals are so stable that they can persist for decades within the lower atmosphere, but in the stratosphere ultraviolet light breaks the chemical bond holding chlorine to the CFC molecule. This destruction of the ozone does not happen immediately; these roaming chlorine molecules become part of two chemicals that are so stable that scientists consider them to be long-term reservoirs for chlorine, under normal atmospheric conditions. The atmospheric conditions over Antarctica during winter are unusual however; there is an endlessly circling whirlpool of stratospheric winds called the polar vortex that isolates the air in the centre. As there is no sunlight in Antarctica over winter, the air in the vortex gets so cold that clouds form and chemical reactions take place which could not take place anywhere else. These reactions can occur only on the surface of polar stratospheric cloud particles, as their frozen crystals provide a surface for the chemical reactions that free chlorine atoms in the Antarctic stratosphere. The inactive chlorine chemicals are converted into more active forms like chlorine gas (Cl2). When sunlight returns to the South Pole, the UV light breaks the bond between the two chlorine atoms which releases free chlorine into the stratosphere. The free chlorine molecules are then responsible for a series of chemical reactions that destroy ozone molecules but return the free chlorine molecule unchanged and free to do more damage (a catalytic reaction). There is also a second catalytic reaction with chlorine that contributes a large fraction of ozone loss, which involves bromine. The ozone hole grows until temperatures warm enough that the polar vortex weakens, enabling air from the surrounding latitudes to mix with the air in the polar vortex. The ozone-destroying forms of chlorine are then dispersed, until the following spring. The Montreal Protocol on Substances that Deplete the Ozone Layer, which was signed in 1987, limited production of ozone-depleting substances. All non-essential products containing CFCs were banned; these included all aerosol products, pressurised dispensers and foam products. All CFC containing air conditioning and refrigeration appliances were also banned in 2001. Hydrofluorocarbons (HFCs) were used to replace chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) for many uses, for example solvents or refrigerating agents. This in turn was partly responsible for the increased concentration of HCFCs in the atmosphere. HFCs were used as they do not pose any harm to the ozone layer, having no chlorine. HCFCs and HFCs are now thought to contribute to anthropogenic climate change, as these compounds are capable of trapping enormous amounts of infrared radiations in the atmosphere. They are thought to be up to 10,000 times more potent greenhouse gases than carbon dioxide on a molecule-for-molecule basis. The Montreal Protocol currently calls for a complete phase-out of HCFCs by 2030, but does not place any restriction on HFCs.  In the first three months of 2011, a large hole appeared in the ozone layer above the Arctic. Every year the Arctic ozone layer had suffered some damage over the winter period but the effect was normally short-lived, as there are generally warmer stratospheric conditions over the Arctic than over the Antarctic. The Arctic polar vortex is also about 40 percent smaller than a typical Antarctic vortex, but is more mobile. In 2011, between 18 and 20 kilometres above the ground, over 80 per cent of the existing ozone was destroyed. The ozone loss in 2011 over the Arctic was twice the levels seen in 1996 and 2011, previously the highest records. The hole was similar in size to the holes seen over Antarctica in the 1980’s. Please read the previous post for more information on the Antarctic ozone hole (http://on.fb.me/SMU6uC). Though the Arctic ozone hole would have allowed in more UV radiation than before, it is unlikely this would have added much risk to the underlying population's risk of UV-related cancer.  Scientists are now examining why the hole over the Arctic grew so large. It is possible this occurred because the stratosphere remained cold for several months longer than usual. This cold air then allowed water vapour and nitric acid to condense into polar stratospheric clouds, which as explained above, allow chlorine to convert into chemically active forms. It is unknown why the stratosphere remained cold for so long, though climate change could be responsible. Global warming occurs only at the bottom of the atmosphere and warms the surface but cools the stratosphere. The Intergovernmental Panel on Climate Change (IPCC) concluded in 2007 that there has been global stratospheric cooling since 1979, but it is not yet clear whether this is a result of climate change.  Without the 1987 Montreal Protocol however, chlorine levels would be so high that an Arctic ozone hole would form every spring. The ozone-depleting chemicals that are already in the atmosphere mean that the ozone hole over Antarctica and possible future Arctic ozone loss will continue for decades. If an ozone-depletion area over the Arctic forms which is similar in size to the one over the South Pole, over 700+ million people, wildlife and plants could be exposed to dangerous UV ray levels. You can watch a video showing Arctic ozone loss 2010-2011 here:http://www.youtube.com/watch?v=aNNRjbBK1Ns. The maps used of ozone concentrations over the Arctic come from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite. The left image shows January 1 to March 23, 2010, and the right shows the same dates in 2011. March 2010 had relatively high ozone, while March 2011 has low levels. The image consists of maps of ozone concentrations over the Arctic from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite. The left image shows March 19, 2010, and the right shows the same date in 2011. March 2010 had relatively high ozone (the red colour denotes high DU), while March 2011 has low levels (blue and purple show low DU). -TEL Arctic ozone hole: http://www.nature.com/nature/journal/v478/n7370/full/nature10556.html; http://www.newscientist.com/article/dn20988-arctic-ozone-hole-breaks-all-records.html; http://www.theozonehole.com/arcticozone.htm; http://www.theozonehole.com/nasaarctic.htm; http://earthobservatory.nasa.gov/IOTD/view.php?id=49874 http://ozonewatch.gsfc.nasa.gov/facts/hole.html More on ozone layer protection: http://www.epa.gov/ozone/strathome.html You can read the Montreal Protocol here: http://ozone.unep.org/pdfs/Montreal-Protocol2000.pdf Image: Rob Simmon, with data courtesy of Ozone Hole Watch.