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I randomly had a question pop into my head: "Why are oceans salty, but rivers are not?"

This article seems to provide a plausible explanation: http://oceanservice.noaa.gov/facts/riversnotsalty.html

It basically says:

  1. Rain keeps putting fresh water into the rivers, which is why they aren't "salty".
  2. Rivers, as they flow, pick up sediment from the earth and push that into the oceans. It's more or less this sediment that makes the oceans taste "salty".

That leads me to my question: Is the ocean really "rising", or is the floor just getting "thicker/higher". The article states that millions of tons of sediment is being deposited on the ocean floor every year. Most people say/believe that melting ice is causing the ocean level to rise. I'm not trying to counter that, but couldn't all that sediment be a significant factor as well? i.e. Maybe the ocean level isn't "rising" as much as we believe, but, rather, the water is getting displaced by all that sediment?

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    $\begingroup$ Three things: sediment is subducted back into the mantle, so only a limited amount accumulates before it's gone. Also, the additional mass "presses" the crust down into the mantle, aka isostasy. Third, the time scales are completely different. Ice melting happens (nowadays) in decades. Sedimentation is a much longer process, thousands of years and longer. $\endgroup$ – Gimelist Oct 6 '16 at 18:47
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In a geologically static world you might have a point. Erosion would wear down the land surface, depositing the sediment in the ocean - where 'Archimedes principle' would apply, and seas level would rise by displacement. There are two main reasons why this doesn't account for the rate of sea level rise that we are seeing: plate tectonics and observed rates.

The Earth is a tremendously dynamic environment in which plate tectonics drive an approximate balance between crustal destruction by erosion and subduction, as against freshly generated land at 'constructive margins'. This 'wobbly balance' has been going for the past 3+ billion years, and is likely to continue for several billion years more. The balance of rise and fall in sea level averages close to zero but has positive and negative anomalies over time. In all cases, fluctuations are slow - in the order of millions of years (excluding the much larger short-term fluctuations caused by ice ages). So for all practical purposes the base-line is 'zero change', upon which is superimposed the effects of sea level rise as we exit the last ice age, i.e. the last 12000 years and continuing. On top of that is the recent effect of human-induced climate change. Let us consider these rates compared to the hypothetical rate of erosion and sea level rise in a static world.

During the 20th century, sea level rise was estimated from tide-gauges, which raises the problem of 'are we measuring rising sea or sinking land?' For the last decade or so we have been able to measure absolute mean sea levels by satellite. That is, independent of land levels. Now we can re-evaluate old data, and come up with 'real' average rates of rising sea level. They turn out to be about 2.5 +/- 1.5 mm/year for most of the last century. Now they have accelerated to more than twice that amount. See, for example:

https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level

This is the global average, and yes, there are all manner of positive and negative anomalies. Now consider sediment displacement. Measuring the global sediment flux from rivers to oceans is extraordinarily difficult, for reasons which would take several pages to explain. But of the diverse range of estimates the consensus seems to be between 20 and 60 Gtonnes per year for the world's major rivers. Leaning towards the upper bound, and adding in a factor for the minor rivers, and assuming an average sediment density of about 2.5, we arrive at about 40 billion cubic metres of sediment entering the oceans each year. There are about 360 million square kilometres of ocean, so on average, the annual increase in ocean sediment is about 1.1 mm. In reality, this is an upper bound with the a more realistic estimate being about 0.6 mm. So, even without taking oceanic subduction into account, the observed sea level is at least an order of magnitude more than can be accounted for by sediment displacement. Or to answer your question, no, the ocean floor isn't rising at any rate that we can measure. Conversely, sea level is rising at a rate that is consistent with thermal expansion of the ocean surface, combined with nett global ice melt.

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  • $\begingroup$ How it is that "2.5 +/- 1.5 mm/year" is "at least an order of magnitude more" than 1.1 mm/year? Is this a special "climate algebra"? Also, the ocean floor is made of tectonic plates that are known to move. Why your explanations do not account for a possibility that the ocean floor just flattens, and continents simply sink down a little bit? $\endgroup$ – Ale..chenski Nov 19 '16 at 3:14

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