Hello, my name is David Schultz. Welcome to Our Earth, Its Climate, History, and Processes. In this lecture, I want to ask the question, are the oceans in steady state? And by this, I mean the amount of water vapor in the Earth's climate system. So, in other words, is the volume of the oceans rem, been remaining fixed in time? And also the salinity, has the amount of salts in the ocean remained constant or changed over time? Well, we know that water is lost to the Earth system by subduction of the ocean plates. In the sediments on top of the down-going plate is carried water, and that water will induce the early melting of those sediments. And then that bubbles up to the surface here through volcanoes, where some of that water vapor is re-released back into the atmosphere. So, we know there's this cycling going on between water in the bottom of the ocean, and then turning around and being re-emitted to the atmosphere. We don't have much evidence for when, if a steady state was reached, that it, that it was reached. It was probably fairly early in the history of the Earth after the oceans had already been established. Absent any change in the amount of water on the Earth, since we don't have that evidence, we assume then that this is a prerequisite for any study of global changes in sea level. And we know that the sea level, or the depth of the oceans, has changed quite dramatically over relatively short geologic periods. We see sedimentary rock structures that show that in the past, there were much shallower and deeper sequences, and some of these sequences covered parts of the land that have, have, are now exposed. So, these continents would have been underwater. So, here's just two examples. In the UK here, we have Late Carboniferous rocks, a lot of coal-bearing rocks around Manchester here that indicated that there was this gradual rising of sea level flooding the UK at the time. And then an abrupt decrease, which then exposed this to the air. Also in North America during the Cretaceous, we know that there were periods of large seas, perhaps 2 to 300 meters deep across the central United States. Now, these changes in sea level can occur for one of two reasons. They could be global. In other words, the, the water is changing all over the globe for various reasons. As water warms, it expands. As then it cools, it contracts. And so, even if the amount of water in the oceans does not change, then you can see changes in the apparent volume of the ocean because of these thermal expansions. Also, if you have ice or fresh water that's on land, not in the ocean, that then melts, comes into the oceans, of course you can change the amount of water in the oceans and then raise sea level that way. Now, the land could, could sink or rise due to uplift. Of course, these are going to be local changes. We're not going to see that over the entire globe. So, this would be a result of isostatic adjustment. Let's say that there was a period of glaciation and the ice on the land cause the land to, to sink down into the the, cause the crust to sink down. And when this ice is removed during an interglacial period, then the continent would, would rebound. And we also have earthquakes that could shift the sea bed and locally change sea level as well. So, we have both global and local changes in sea level. And we can identify these changes due to changes in sedimentary layers, and these would indicate periods of what we call marine transgressions or marine regressions. Now, the basic principle here is that you have these coarse-grained clastic rocks, like sands, and these tended to be deposits, deposited in high-energy environments such as beaches. In comparison, there are fine-grained sentiments such as silts and muds. These tend to be deposited in calmer, offshore environments. And so, as you go from a beach scenario to an offshore scenario, then you can see one type of sedimentary progression and vice versa for the other way. So, we have a transgression when the sea level rises and moves onto higher ground. We have regressions when the, the sea level sinks and allows more land to become exposed. And so, here are those sedimentary sequences that we would see during a transgression. In this case here, as sea level rises, the sands move up towards to higher ground, and, and then the mudstones and limestones then follow behind the deeper sediment. Likewise, in a regression when sea level is sinking either globally or locally, we would get a pier, a, a sands being left, abandoned at higher terrain, and the sequence is in reverse. So, that's the water in the Earth's system. What about the salts? Where did the salts come from? Well, present ocean salinity average around the globe is about 3.5% by weight. Now, oceanographers measure this as 35 parts per 1,000. You may see that more commonly in the literature. Now, once land formed in the Hadean and the Archean, then there would have been weathering from the, the rain and the precipitation dissolving ions and transporting these to the oceans. And also outgassing from the mantle would have incorporated water-soluble ions such as chlorine and, and sulfate as well. So, you might reasonably assume then, because there are all these weathering and transports of salts to the ocean, that the ocean's salinity has been steadily increasing. But again, there may be local variations in salinity across the oceans as we've seen, but we don't have a lot of evidence that there have been abrupt changes in salinity globally. So again, we're back to talking about the balance of salts then. What if rivers bring salt into the ocean, then what removes salts? And I think it's important to remember this time that Joly calculated that the oceans would replenish their present salinity with, within 80 to 100 million years. So you can see that this is a much shorter time scale than the billions of years that the oceans have been around. So, this cycling then is actually relatively frequent, relatively active compared to the age of the oceans. So, how do we remove salts from the ocean? Well, it was originally thought that this would have been done by evaporites. And this is where the seas start to shallow, the salinity increases, and then evaporation would, would simply just leave these salt deposits, such as gypsum and anhydrite and the more familiar sodium chloride, which appears as the mineral halite. Now, this has been common at certain times in the geologic record, but it's never been sufficiently abundant to account for all the salt removal that you would need over geologic time. And for that, I want to go to this example here. Here, this is the Mediterranean Sea, and you can see it's a relatively enclosed basin fed by the rivers around the Mediterranean, and then just this small gap right here between the Straits of Gibraltar connecting the Mediterranean Sea to the Atlantic Ocean. Now, about 5.5 million years ago, the Strait of Gibraltar was closed due to changes in sea level and changes in sedimentation. And we see abundant gypsum and other evaporite deposits around the Mediterranean. This one here being from Tunisia on the south coast of the Mediterranean in Africa. So, here we are about this period around 5 and a half million years ago, where the Straits of Gibraltar have closed up,. And essentially, what's been left has just been this very shallow, salty basin that's been left of the Mediterranean, and these have left those evaporite deposits. Now, over time, these sediments got covered with soil or, or dust or whatever, and, and have been protected so that when the Straits of Gibraltar opened up again at 3.3 million years ago, then the salts would not have been reevapor re absorbed back into the water, redissolved into the water. So, if these evaporites then are only periodic and can't explain the removal of salts, what does? Well, we think that what goes on here at mid-ocean ridges is pretty important. The basalts that are erupted on the seafloor are cooled by circulation by, by encountering the relatively cold seawater. These causes cracks in the basalts, and then the salts, being corrosive and interacting with the minerals in the basalt, cause elements to be exchanged. And so, whereas water may lose some of the magnesium, the sulfate and perhaps some of the sodium and chloride, it gains others, a lot of silica, iron, manganese and, and calcium. And so, here's a schematic representation of that. Here's the mid-ocean ridge and the lava, the, the basaltic lava being ev, erupted here, but you can also see these cracks along the edge here where you get this exchange in the seawater. And if you ever seen these underwater images of these worms at these hot springs underwater, it's often near the mid-ocean ridge here where there's a lot of heat content and, and the exchange of minerals allowing these extremophiles to live in this environment. So, as the temperature is quite hot it dissolves these minerals, and when it interacts mixes with the cooler ocean water, then you get precipitate. You get iron oxides precipitating out. So you get the manganese oxides, iron oxides, and sulfides precipitating out, whereas other ions remain in the seawater. Now, this may a bel, be only a partial answer to the problem of exchange of salts, but we continue to look for these answers. So, to answer the question then, has salinity reached a steady state, again, there's the evidence that life evolved in the oceans 3 billion years ago, and of course, the descendants of that life are still in the oceans today. Some of us have come on shore, onto land. Most organisms, when you look at the salt content in their body, to have similar salt concentrations to seawater. And so it's likely that it's no surprise there that that similar salt concentration in our bodies is a result of you know, our ancestors evolving within that seawater of the same composition.
