Underwater beach?!

So today’s post will be short — I’m off shift now and would really like to get some sleep before the lifeboat drill at 10:30.  Today was the first main day of coring after…many…unsuccessful attempts at getting started during the night shift.  But at least actually having something to do makes the time go faster, even if that something is pretty much just moving water from one container to another and baking little bits of sediment to run on the gas chromatograph (see yesterday’s post for more on that).

Partway through today’s core, between some nice layers of good seafloor mud — you know the kind you could charge people to use at a fancy spa — we hit between 20 and 50 m of sand (depending on how far the piston corer advanced each time).  And this wasn’t just like slightly coarser grains but still muddy.  It was the kind of thing no one would ever pay money to have smeared on their face.  We’re talking 60-grit here, like something straight off a beach.  But wait, I don’t remember ever learning about underwater beaches…would they have underwater palm trees too?  Apparently some of the sedimentology folks have found bits of terrestrial organic matter, so maybe the underwater beach explains it all?!

Actually no, the trick is that this sand didn’t come from nearby.  In fact it didn’t even come from just a little far away.  It’s from the Himalayas (probably)!  The neat thing about the oceans is that seawater is pretty dense (a bit like yours truly?) at around 1025 kg/m^3, while air is about 1000 times less dense (around 1 kg/m^3).  What this means is that the density difference between seawater and sand (density around say 2500 kg/m^3) is much less than between air and sand.  So if you get a landslide in the Himalayas themselves, the sand and debris and everything will just settle out of the air pretty quickly and won’t go very far since the speed at which a solid sinks through a liquid is (at least ideally) proportional to the difference between their densities (see the Stokes settling “law”).  And furthermore, water is substantially more viscous than air, meaning it resists much more or is “stickier” (think of maple syrup as an example of something more viscous than water).

Now think about a similar “land”slide underwater.  These things — called turbidity flows — are really neat because the sand and debris can stay suspended in the water for thousands of kilometers.  Basically the turbulence in the water is so great that the sediment the water is carrying never really has enough time to sink before it gets swept back up into the flow (remember the density difference is lower and the viscosity is higher than in an above-water landslide).

What we encountered today was the result of an absolutely huge turbidity flow that brought coarse sand all the way from the Bay of Bengal, the only major river emptying into this part of the ocean, to where we are, a distance of around 2500 km!  And these flows can go even farther — layers from them have been found near the Sunda Strait, another ~1500 km from here!

While an underwater beach would be fun, what we’ve found might be even neater.  Core after core we brought up today was full of sand that washed off a mountain range that’s about as far from us as St. Louis is from San Francisco (I’m guessing a bit here).  And to get from there to here, it flowed along the bottom of the ocean with nothing more than its initial “push”.  I’d say that’s pretty cool!

That’s all for now from here — until we cross paths again,

Brian (who might end up dreaming about transferring water from one container to another)

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