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A recently publicized discovery of a Columbian Mammoth tusk located well of the coast of California in quite deep water made me wonder if geological conditions 100,000 years ago can explain how it got to the location it was found at?

Pilot Randy Prickett and scientist Steven Haddock, researchers with Monterey Bay Aquarium Research Institute (MBARI), discovered a Columbian mammoth tusk 185 miles offshore and 10,000 feet deep in the ocean in 2019, the institution said in a news release.

According to the Monterey Bay Aquarium Research Institute, the tusk portion is at least 100,000 years old:

The team believes it could be the oldest well-preserved mammoth tusk recovered from this region of North America. Dating of the tusk is being done by the UCSC Geochronology Lab led by Terrence Blackburn, associate professor of Earth and planetary sciences. Blackburn analyzed radioisotopes to show that the tusk is much more than 100,000 years old. “Our age estimate on the tusk is largely based on the natural radioactive decay of certain uranium and thorium isotopes imparted to the tusk from the ocean. If the tusk had been found on land, deciphering its history would not be as straightforward,” said Blackburn. Christopher Edwards and Patrick Drake will examine the oceanographic currents to better pinpoint where the tusk originally came from.

Another article I looked at mentioned they were exploring an underwater seamount formation with the ROV, so it looks like the approximate location might be about here: enter image description here

This looks well beyond any continental shelf or continuous undersea canyon type formations which might connect the location to the shoreline.

So, is there a geological explanation for how this mammoth tusk might have ended up at this location?

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    $\begingroup$ Several possible scenarios. Glacial ice could have retreated moving body parts out to sea. During the peak of the last Ice Age (~20,000 years ago), sea level was about 120 meters lower than today. So the coast was presumably several miles farther out $\endgroup$
    – LazyReader
    Nov 28 '21 at 5:20
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    $\begingroup$ I am much more intrigued with how the bleep you go about finding a 100,000 year-old mammoth tusk 185 miles offshore under two miles of ocean?!? Phrases like "needle in a haystack" and "incredibly lucky" seem weak by comparison. It absolutely beggars the imagination. $\endgroup$ Nov 28 '21 at 15:04
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    $\begingroup$ Definitely should have been an exciting moment for the ROV team. I've watched quite a bit of ROV streams on youtube, and there is always an expectation of 'what will show up next'. $\endgroup$
    – justCal
    Nov 28 '21 at 15:12
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  1. The mammoth probably died on land.
  2. Its remains got picked up by a glacier.
  3. The glacier carried the tusk down to the sea.
  4. Eventually, the ice containing the tusk broke off as an iceberg.
  5. The iceberg floated out to sea..
  6. When it got near to where the researchers found the tusk, the ice around the tusk melted and dropped it into the water, where it sank to the bottom.

When this happens to a rock a glacier picks up, it is called a "dropstone".

During glacial maxima, icebergs can survive much farther south than now. The presence of dropstones in what were supposedly equatorial sediments led to the theory of a Snowball Earth a billion or so years ago.

Based upon CO2 data, 100,000 years ago, the Earth was working its way into the most recent glacial maximum, called the Wisconsin glaciation in North America ( the likely source of this iceberg).

Glacial maxima graph

Source: Wikimedia Commons, Author Tom Ruen, CC BY-SA 3.0

Notice that the article did not call the tusk a "fossil". You can only call it that after a specific process of mineralization after being buried in sediment. They might very well have found original ivory from the mammoth.

But this is not necessary. There are two ways the tusk got into the glacier:

  • The carcass was buried in sediment and some or all of its skeleton, including the tusk, became fossilized. Later, the glacier picked up the rock containing the fossil.

  • The mammoth died on top of the glacier, the carcass was buried in snow and more glacier may have built itself on top. This is the most likely scenario if they found original ivory.

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    $\begingroup$ This would explain why they mentioned studying oceanographic currents in the MBARI release I cited. Can you coordinate this time frame with a glacial maxima you mention? $\endgroup$
    – justCal
    Nov 26 '21 at 23:55
  • $\begingroup$ Great answer so far, thanks for the update with the corresponding information on glaciation. Do you think we can further narrow down the possible source of this mammoth since the California current splits along the western North American coast, or is that one of those behaviors that may have been different due to ocean temperature / salinity changes in the past? $\endgroup$
    – justCal
    Nov 27 '21 at 17:01
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    $\begingroup$ @justCal Not only temperature and salinity, but there are geometric issues as well, sea level being well below what it is now. $\endgroup$
    – Spencer
    Nov 27 '21 at 18:16
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    $\begingroup$ there is also a small chance of the classic bloat and float. $\endgroup$
    – John
    Nov 28 '21 at 3:26
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    $\begingroup$ @John One of the scenarios I did consider was based on the something I read once about the Cascade coastal region and a past mega-tsunami. If this occurred in the distant past (100,000 years ago or so) could current patterns (then) account for a mammoth body caught in the flotsam to travel that far? $\endgroup$
    – justCal
    Nov 28 '21 at 15:26

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