Why are the seas salty? - CrowdScience podcast, BBC World Service

Namaskar. Namaskar. Deepika.

Nice to meet you. I'm Chhavi Sachdev and you're listening to CrowdScience from the BBC World Service. Deepika is a salt farmer here on the outskirts of Bhavnagar, a town in the state of Gujarat on the West coast of India.

We’re standing in the blazing heat amidst five different sized fields of salt water. This is her salt farm and it covers 10 acres. Can you tell me what are your processes?

She wakes up at about 3:30/4 in the morning, early in the morning. Okay, so they aim to finish by nine o'clock because it's too hot after nine to be outside and they feel really hot and they get heat stroke and faint so they start early. She's telling me that it takes her at least  four hours, sometimes four and a half hours, to rake the salt in these pans.

So how long does the  whole process take to turn sea water into salt? Ten days to just to crystallise it and then the water is pumped out and  then the next one takes 10 days. They have their own tractor that they use to take the harvest but it doesn't work that well.

She has says they have to push it and push it and make it go somehow. As you can hear salt farming is hard hot work. It takes Deepika's family - her husband, brother in law, sister in law and her - eight hours of raking through the hot, salty ankle-deep water each day to keep the salt crystals from clumping while the water evaporates until all that's left is salt to be scooped  up and trucked away, eventually making its way to your dining table.

Salt holds an important place here in India's history, breaking British laws on salt production was the first act of  civil disobedience led by Gandhi that would eventually lead India to independence. Today India produces one third of the world's salt and nearly 75% of that comes from here in Gujarat. So why am I here?

Well because we've received a lot of salty science questions in the CrowdScience  inbox and we're tackling a couple of them in this episode. We'll come back to salt farming and getting salt out of the water later in the programme. But listener Julie's question is about how salt  got into the sea.

Hi my name is Julie and I'm from Auckland, New Zealand. My question is: 'Why is the sea salty? ' Given that the rivers that flow into them are fresh water.

I don't don't think rain is salty. Lakes are fresh and some lakes are as deep as some seas, so why are the seas salty? That is such a great question and do you live near the sea?

Auckland is a narrow isthmus, so to get to the sea in one direction is probably five minutes driving which goes straight to the Waitemata Harbour and if I drive in the other direction it's about 10 minutes to get to the Manukau Harbour,  which goes to the Tasman sea. Amazing. So I also live by the sea.

I'm in Mumbai, we've got salty water quite a ways around us because we're a peninsula. I have never wondered that question  so I'm very happy to dig into it and find out. Sifting for Julie's answer took us to a lake nowhere near the ocean.

Julie mentioned lakes are fresh not salty but that's not always true. We are at the southern boundary of the Great Salt Lake which is in Northern Utah, right outside of  Salt Lake City. It's nestled between many different mountain ranges and it is the saltiest lake in  the Western Hemisphere.

This lake in America is five times saltier than the ocean and it's a microcosm of how salt gets into the sea, so it may hold some answers for us. Our guide here is a PhD candidate in geology and geophysics at the University of Utah. My name is Mark Radwin.

I look at the landscape evolution of the Great Salt Lake and the Bonneville Salt Flats, thinking about how it's been changing and how it might continue to change in the future. We wanted to see some of the salt this lake holds. Mark's with CrowdScience producer Sam looking for it the oldfashioned way, with our eyes and noses.

The lovely smells. What is that smell, it's kind of. .

. ? Sulfury things.

Mirabilite being a sulfur mineral or sulfur-based mineral, it can cause that smell once it gets oxidised by natural conditions. It kind of makes that funny rotten egg smell. Mirabilite.

One of the many types of salt we're trying to spot here on the lake's shoreline and with a bit of  luck Mark and Sam happen upon something. It's a big pile of multiple different salts. And this is all coming out of a spring, so some water's coming up through a hole in the ground .

Looks like this is a longstanding mound that has slowly been evaporating out and you get these  interesting stacks of all these different salts. There's a there's at least three different salts here in this mound that we're standing on. So we've got some like crystal-looking little structures here.

I don't know maybe the size of your thumb or something. What are these? Is this salt This is salt this is not typical table salt though it's a salt we call mirabilite and this is a common salt that forms in the Great Salt Lake area.

It's made out of sodium just like table salt  but instead of having sodium chloride it's actually a sodium sulfate. And it  makes these transparent crystals that you can find in these little mounds and they actually  can like melt in your hands they're really cool. Salt as you've heard isn't just the sodium  chloride that we sprinkle on our food.

That's the kind that makes up most of the salt in the ocean, 90% in fact. But salts can be made up of various different elements and this lake contains a lot  of them. Most lakes are freshwater, not salty at all, so what's going on here?

This lake is in a  closed basin so it can't flow out to the ocean, doesn't flow out anywhere else and that's  really important for why it gets so salty because the only way water can escape is through evaporation. So most of the large accumulations of salt in the world were once some inland sea,  some large body of water, that had to evaporate The Great Salt Lake is so salty because unlike rivers or freshwater lakes where water is constantly flushing out to the oceans, any salt that gets in here is stuck. Water can escape by evaporation  but the salt is left behind.

What listener Julie wants to know though is how does all this salt  get into the water in the first place? Nearby rocks. As the rivers and as the rain is  in contact with all these rocks and as that rain and river water comes into the lake basin  it dissolves elements from the rocks and those are what get put into the water.

The landscape isn't  extra salty or anything like that so you'll find this all over the the world where independent  of the rock type around it you can find salty lakes. So this is an example of a really ancient salt. This is from New Mexico, Permian salt, so 225 million years ago.

Brenda Bowen is a professor of geology and geophysics at the University of Utah and Mark's PhD adviser. In her office she's  showing us various salts she's collected over the years. Here's one that is actually from a salt  lake in East Africa in the East African Rift and this is a really cool salt.

It's actually called Magadiite, it forms in Lake Magadi. It's really beautiful with, you know, all these stripes.  It kind of looks like toothpaste oozing up and this was actually forming along fractures in the  lake bottom that the minerals that can grow from the brine.

I have another sample over here, this  one's crinkly. Yeah so that is a piece of halite crust from the Bonneville salt flats. So actually  this grew as a crust on the surface like this and the crystals are growing down from the uppermost surface into the brine through time and so then we reached down and plucked it up and so you're looking at it upside down here and can see those beautiful cubic crystals.

This piece of salt is from the nearby Bonneville Salt Flats, once part of the same giant lake, Lake Bonneville, that covered a huge area including what's now the Great Salt Lake. Earlier Mark told us that salt  gets into the water from rocks and that's also how it gets into the ocean or any body of water. We asked Brenda to explain the chemistry in this process.

Salt forms and accumulates on our planet  because of the water cycle and water that falls as rain and snow and as it travels through  the atmosphere it picks up a little bit of carbon dioxide from the atmosphere and becomes slightly acidic, so that then when it falls on rocks and the surface of the planet you end  up breaking down the rocks and get weathering and erosion. The main salt in seawater and salty lakes is sodium chloride, what we know as regular table salt. There are other minerals besides this, which are released from rocks during erosion but not as many of them find their way into bodies  of water.

So you have waters that are flowing over the surface of the Earth and rocks are made up of things like sodium and calcium, magnesium, potassium and then chlorides and sulfates and other ions. A lot of those materials get used up in other biogeochemical cycles as  nutrients for plants or animals or they form new types of rocks like clays or other types of  minerals. But sodium and chloride stick around the longest.

They're the ones that don't get used up  in some of those other biological processes. While sodium chloride might be the most abundant at the end of the eroded rock's journey, some other salts do make it into the sea and one of those common salts actually gave oceanographers a crucial insight into the saltiness of the sea. People had realised after about 100 years of studying oceanography that the balance of the salts didn't work, that we had all this water flowing into the oceans, it was running off the continents, rivers  were carrying salts through estuaries and into the oceans and we didn't know where they were all  going and we didn't know why the Earth wasn't just getting saltier and saltier and so there had to be something that had to be taking some of these elements back out.

This is Chris German, a senior  scientist at the Woods Hole Oceanographic Institution in Massachusetts. Oceans maintain  a salinity level of about 3. 5%.

But one thing mystified scientists: If the salts that wash out  of the rocks are continually flowing into the sea, why didn't the oceans keep getting saltier? Just  like the Great Salt Lake. That's where one of the less common salts offered an important clue.

One of the salts that is particularly out of balance is magnesium sulfate or Epsom salts. There's way too much magnesium being washed into the sea that it could be accounted for with anything else we know about things like sort of biology or in the salt flats. So we knew there had to be some other mechanism that was taking magnesium out of seawater.

Meanwhile in the 1970s oceanographer John Edmund had observed some water bubbling up from the seafloor that was just a bit warmer than ordinary seawater. It also had a slight magnesium deficiency. And then he published something that seemed like an outrageous interpretation of the data.

Based on the slightly warm waters coming  out of the seafloor and the amount of magnesium they lacked Edmund did some calculations  and came up with a prediction. He said I can predict that there must be fluids out there somewhere at the bottom of Earth's oceans that are somewhere around 350°C or hotter. So a lot  of people were just like you must be crazy you can't possibly imagine there is such a thing  and two years later they discovered them.

It was a different group, working in a different part of the world's ocean and found things and they measured them and they were actually 362°C, so if anything he slightly underestimated. The things they found were hydrothermal vents,  which are exactly what they sound like - gashes in the seabed spewing out extremely hot water  and minerals from below the Earth's crust with great pressure, like a fire hose. Chris now leads  expeditions to find the many and varied types of hydrothermal vents that exist out there.

As I've continued to explore not only have I kept finding new hydrothermal vents around the world but I keep getting surprised and humbled by the fact that there's an even wider diversity of  hydrothermal sites than even I anticipated. Even as recently as last summer we were finding new kinds of hydrothermal vents that nobody had ever seen before. Black smokers, white smokers and snowblowers are some of the names given to hydrothermal vents because the minerals they eject usually form  large billowing chimneys above them.

The main difference between the types of vents  Chris is talking about is their chemistry - what they're spewing out and what they're taking in. But what remains the same is how they work. It seems to be that the water basically it percolates downwards and it gets warmer and warmer.

So beneath the sea floor, the deeper you go the hotter the  rock becomes and so water can percolate down over large areas very slowly into the rock, so  it can take maybe a couple of years for water to actually reach the point where it's at its  hottest and it's buoyant but then once it gets to that point it only seems to take about an hour to come back out. And where does salt fit in here? Is salt one of the compounds that's coming out?

So the sea water that goes down is about a 3. 5% sodium chloride solution and one thing  we have noticed is that the total amount of salt can actually vary in hydrothermal vents when they come back out. Sometimes it comes out as more salty and sometimes it comes out as less salty and what that allows us to deduce is that there's actually a different thing that happens, which is called  phase separation, which is kind of complex but basically what it allows a very concentrated  salty brine to condense from the water.

That's probably the explanation is why sometimes we see hydrothermal vents that can have fluids that are up to twice the salt content of the original  sea water. But if these vents are releasing extra briny plumes, why don't the world's oceans get even saltier? It's a young science and still something of a mystery but the answer may have to do with hydrothermal circulation.

The movement of water through the deep ocean crust. If you have this underwater high pressure boiling effect called phase separation that maybe you're sequestering a lot of the salt in the rocks down beneath the seafloor and maybe more of the hydrothermal vents  are fresher than seawater rather than salty than seawater, so the net balance is that it's actually  taking salts out of the ocean and storing them in the rocks beneath the seafloor. I see so it's like a little bit of a give and take.

Yes it's a yin and yang thing that the continents generate the salts and wash them into the sea, the ocean crust is something that helps take some of that  salt back out again and around the cycle goes again. You're listening to CrowdScience from the BBC World Service. I'm Chhavi Sachdev and today we're answering your questions about salt.

So far we've tackled Julie's question about why the sea is salty.  We learned how the salt gets into the water and how it's regulated thanks to rain, rock  erosion and ocean vents. Back in Bhavnagar, one of India's salt manufacturing centres, it's  nine am and already 36°C, that's 96.

8°F. I'm with salt farmer Deepika again who tells me this work has been getting  hotter and harder in recent years. There's a lot of difficulties.

It's hotter in the last five- six years, okay. Has the rain changed? So she says there is unseasonable rain.

Salt gets spoiled, work  stops, okay, oh that is a shame. I travelled down the road to meet Prasan Khemka who owns Chandan Salt Works, a 710 acre industrial salt operation, which produces a thousand  times more salt than Deepika rakes in salt crystals. Instead of harvesting edible salt like Deepika does his operation  produces salt for industrial purposes.

We majorly supply to all the chlor-alkali industries in India. These industries they manufacture caustic soda and soda ash. And where does caustic soda and soda ash end up in our listeners lives?

Soda ash is used as a raw material to manufacture washing powder and detergent. Caustic soda, it is used in many things like the textile industry they use caustic. There are more than 400 uses of caustic.

Like Deepika's small farm, this industrial salt works is also facing business losses as weather patterns shift. This year I could just harvest two crops not third crop because now rainy season is near and it's humid and maybe by 15 June the monsoon may come so I cannot take the risk of opening the new pens  and harvesting because if there is rain the salt will dilute. It's clear to see the huge impact that diluted salt has on these salt farmers.

An increasing problem as the climate warms. And another of our listeners - Will - is wondering whether the seas themselves will get diluted due to climate  change. Actually I live pretty much the furthest place from the sea in England.

It's right in the middle of the country. And what is your question for CrowdScience? Well I'm worried because the melting ice caps is clearly going to reduce the salinity, is that going to have any effect  on the weather or the wildlife or anything like that?

I have no idea if the ice caps are going to  put in a tiny bit of fresh water or a lot of fresh water. Climate change in the Arctic and elsewhere  is adding more fresh water to the oceans in the form of giant walls of melting glaciers carving  off into the ocean. So what does this mean for the world's oceans?

Is Will right in thinking all that melting ice will dilute its saltiness? The oceans have an average salinity of 35g per kg of water and so salinity varies between around about 32g up to around about 38g per kg. And  how much is 32g?

So is that a teaspoon? Is that five teaspoons? Yeah I've never thought about ocean salt in  teaspoons, it's a very good question.

I really don't know I might have to go downstairs and measure some salt and see how much teaspoons you know 35g would be. We checked - it's about 5 teaspoons per kg of water. My name is Paul Durack, I'm an oceanographer by training actually.

An Australian, as you can probably hear in my accent and I've been based in the US researching climate change in the ocean since 2011. And where in the US are you now, you're on the Pacific? That's right, so I'm on the west coast in California based at the back of San Francisco actually.

So Lawrence Livermore National Laboratory. All right so you're clearly in a good place to talk to us about the world's oceans, literally. So for our listeners we are trying to find out about the oceans becoming fresher.

I mean the oceans are pretty big. They're very big. Is the amount of fresh water that's melting into them enough to have an impact on the overall salinity of the oceans?

Our observing network for the ocean  is not as comprehensive as we have over land so we don't have a good enough observing system to answer the question of how much is the global salinity changing, but what we do have is regional information, which then gives us this insight such as you know freshening Southern Ocean for  example and that environment is actually pretty sensitive to fresh water flowing into the ocean  if it doesn't get mixed by winds. Scientists have found out that the Southern Ocean, which surrounds Antarctica, is getting fresher - evidence for listener Will's hunch that ice melt will  make the oceans less salty. But evaporation and precipitation also play a part, a big bigger one  in fact when it comes to the global picture.

The patterns of evaporation and precipitation at the surface of the ocean actually then sets up what the salinity looks like underneath it. So there's  a very tight relationship between regions of the ocean that have more evaporation than rainfall  and they're generally salty and then there's other parts of the ocean that have more rainfall  than evaporation and they're fresher and then what happens is that the ocean circulation responds to the places that are salty and the places that are fresh and tries to balance all of the gradients so that the ocean is much more even. Climate change is melting ice on a worrying scale but it's also having a dramatic effect on evaporation and precipitation, throwing off the global water cycle.

As a rule of thumb we consider for every degree of warming has about a 7% increase in the amount of evaporation precip. For every degree Celsius that we increase the temperature of the atmosphere, that atmosphere can hold 7% more water and so that then leads to an enhancement of the water cycle. And by enhancement Paul means the water cycle is running on overdrive.

The warmer air has a much larger capacity to hold water vapour, which means more evaporation, more precipitation and a knock-on impact on ocean salinity. Paul tells me this is already playing out. For example on both sides of the Indian peninsula in the Bay of Bengal and the Arabian Sea.

The Bay of Bengal actually has quite a fair amount of runoff from land, ice melting etc. that flows into the Bay of Bengal in addition to  what's happening just with the evaporation and precipitation fluxes over the ocean itself. And the Arabian Sea is in an evaporative regime and then eventually that will rain out not only over  say the Bay of Bengal but also over some of the land areas and then through runoff that will find  its way back into the Bay of Bengal as well.

And so consequently what's happening is as the world  is warming up and the water cycle is amplifying the contrasts that are naturally there in the  background are actually amplifying and this is leading then also to salinity changes. The Arabian Sea getting saltier and the Bay of Bengal getting fresher. As Paul said, the Bay of Bengal on India's East Coast is getting fresher while the Arabian Sea on the west coast is getting saltier.

Well the salt farms I visited in Bhavnagar are on the west coast where salt levels in the sea are rising. You think this would help salt farmers like Deepika but as Paul pointed out the warmer atmosphere here is not just evaporating more water out of the Arabian Sea, it's also dumping it down on this  region when it's not expected. I asked Prasan how things have changed in recent years.

Since the last five or seven years I can say the monsoon pattern has changed. So this has badly affected the salt industry because we have almost reduced our production all over Gujarat you can say more than 30%. More than 30%?

Yeah more than 30%. It's hard. Yeah it's a hard truth.

If there is normal rain it is good for salt industry, if there is extensive rain it is bad for salt industry  and if there is less rain that is also good for salt industry. But extensive rain, extended  monsoon, will always hamper the industry. And now as a salt manufacturer we have to change  ourselves so that we can get more production in this climate change.

A shifting monsoon season and unpredictable rainfall clearly impact annual salt harvests. The smallscale salt farmer I met,  Deepika, told me that she's lost about 40% of her salt crop in recent years and that's largely due  to the turbocharged water cycle we've been hearing about - increased humidity and unseasonal rains. For our last stop today in Bhavnagar I visit one place that's trying to help salt farmers navigate these  uncharted conditions.

Presently it is low tide so that is why we cannot see the sea water. It comes up to here? Yeah it comes up to here and now we come forward for the pumping station.

All across the vista I can see hillocks of this salt, more salt work factories, again more salt and if I just  turn in a 360° loop salt everywhere. And around me are dried up salt pans. This is all part of the  experimental salt farm, 180 acres, that belong to the Central Salt and Marine Chemical Research Institute.

And I'm here with DrBhoomi Andharia, who's going to tell me about what their plans are to  help smallscale farmers as well as big industrial salt farmers with climate change. Earlier rain was there. The monsoon season was fixed.

June to October is monsoon season but now what happens that rain you cannot predict. Cyclones are also frequent, so this salt production is entirely in the open pans.  So you cannot protect it with the roofs, it is not feasible, so it get diluted completely.

Second, silt will come in the brine, so it will affect the quality of the brine, so they will not get good market value because its whiteness get affected nobody will eat that type of dusty salt. Bhoomi and her colleagues use this experimental salt farm to test new salt harvesting methods that are more climate resistant. One of these innovations is to guide the flow of brine between the salt  basins especially when they start to flood.

They use gravity and directional gates, as well as solar-powered pumps. So starting from this pumping, this is energy efficient pumping system. Then this brine will be transferred through gates, so this is the mechanism.

Further, the most important aspect is to discharge the rain water and flooding. So through this in the monsoon season generally salt production gets stopped, but here we have designed in such a manner we will continue the  salt production in the monsoon. Even these innovations won't protect against all the water coming in the wake of climate change and melting ice.

Actually sea level is rising every year. All the saltworks are near to the coastal regions, so once the sea level will rise it will be predicted that  majority of the salt work some partial area will be submerged completely due to the sea level rise. So it is going to affect a lot to the salt industries and major salt manufacturers they are already  worried about that.

Are they doing anything to mitigate this? Are you advising anything? For sea level rise we cannot do anything because once it will be rise no production will work but for  flood level rise we are working on that and we are trying to implement that engineering  and technologies with the scientific design in major salt works by raising the embankment  according to the flood mappings.

Climate change is inescapable. Even more than the melting ice caps and glaciers, the unseasonal rain and evaporation patterns are affecting salinity in our water  bodies and ultimately the salt we get from the oceans. Scientists like Paul, Chris, Mark and Brenda are trying to understand how things work so scientists like Bhoomi can help salt farmers  adapt to the changing climate.

I know this has certainly given me something to think about when I add salt to my chips or go for a dip in the sea. Now over to our listeners for the credits. .

. That's it for this edition of CrowdScience. Today's questions were from me Julie in New Zealand.

And me, Will in the UK. If you have a science question, salty or sweet, that you'd like made into a programme by the CrowdScience team, email crowdscience@bbc. co.

uk This episode was produced by Sam Baker and  presented by Chhavi Sachdev. Thanks for listening.