Heinrich events - why can't we understand them yet?

aspergers@40

There's been so much research on the iceberg armada events (Wikipedia Heinrich events) since the classic 1988 paper by Hermut Heinrich himself. There's a host of different data concerning the events, so why is still such a mystery? Here's an excellent introduction to the research carried out in slide show format Paleo Slide Set: Heinrich Events: Marine Record of Abrupt Climate Changes in the Late Pleistocene.

Here's a 2009 report which links the events with abrupt changes in Indonesian waters Evidence for Indonesian Throughflow slowdown during Heinrich events 3–5. What's going on?

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Xnn

Actually, we have a fairly good idea of what happened. In brief, a portion of the ice sheet covering North Eastern Canada near the Hudson strait broke up carrying a large amount of dirt and pebbles out into the nearby ocean. It was significant enough that it affected the sea level and global climate and was initiated by a rather subtle change. A tipping point was reached.

So, no suprise here; Ice sheets constrained by ice shelves are unstable. It doesn't take much and they will fall into to the sea. In fact, we have already recently observed several ice shelves breaking up surprisingly fast. The only differance is that Heinrich are much larger than what we have recently observed.

Now, as with most paleoclimatic data, the precise dating of events is inprecise and unreliable. So, it is never going to be difficult to point out a seeming contradiction in the data. This doesn't mean that we have no clue about what happened. However, it is important to understand these events very well as they provide potential clues to our future.

In particular, the West Antarctic Ice Sheet contains lots of ice constrained by enormous ice shelves. (Look up the size of the Ross Ice Shelf.) So, we know it is unstable and might collapse, thus generating a modern-day Heinrich like event in the Southern Hemisphere. Of course it won't the the same since it's on the opposite side of the World. However, the consequences of such a collapse would be dire: a rapid rise of several meters in sea levels around the world.

So, bottom line. It's not that we don't understand them as much as we need to understand them much better.

aspergers@40

I agree with this statement Xnn, but I'm not so convinced by your summations. It seems obvious that it is a huge global event, essentially a period of global warming that starts a sequence of events involving ice sheet discharge into the oceans. But what about the small clues:

1. Increased terrigenous runoff in Amazon fan
2. Increased grain size in wind-blown loess in China
3. Changes in relative Thorium-230 abundance, reflecting variations in ocean current velocity

It seems as though the world was generally much wetter, much windier with stronger ocean currents! How do you explain that?

Xnn

It's probably true relative to what it was before the event.

But again, be careful about Paleoclimate data as it is typically inprecise and unreliable.
Also, Heinrich events are from a particular ice shelve/sheet that broke up and reformed.
Response to a collapse of WAIS is unlikely to be exactly the same.

aspergers@40

Right, so what caused this then? An increase in insolation would do the job; more sunshine -> more evaporation -> more ppt + more wind. What are the other alternatives?

Skyhunter

Since Heinrich and Dansgaard/Oeschger events, and the associated temperature shifts, are localized events, The effect on global climate is not as extreme. Increased insolation would have more of a global impact. It can also be ruled out by beryllium-10 isotopic analysis.

The theory with the greatest acceptance is that the glaciers grow until they reach a critical mass, at which point they discharge the accumulated ice into the Atlantic where it is picked up by the currents. The ice melts in the North Atlantic depositing refrigerator size boulders on the sea bed. The discharge of fresh water causes a slowdown or stopping of the Atlantic current.

This theory explains the ice rafted debris, the cyclical nature of the events, the abrupt cooling during the event, the rapid warming following the event, and the fact that these events only seem to occur during glacial epochs.

Xnn

Right.

Strictly speaking Heinrich events involved the breakup of a particularly large ice shelf near the Hudson strait that forms during glacial periods. It lasted about 700 years. Since that ice shelf is long gone, strictly speaking it is not going happen again any time soon.

However, Antarctica has plenty of large ice shelves that are in the process of breaking up in a similar fashion right now. Just last week, an iceburg the size of Rhode Island broke free from the Wendall (Filchner-Ronne) shelf. Rhode Island is about 1% of the size of that ice shelf.

Ice burgs from earlier breakups have been sighted as far away as New Zealand (8000 miles).
More study will be needed to determine if this is the harbinger of a modern day "Heinrich" event or not. However, simple math shows that the Wendall (Filchner-Ronne) is breaking up at a rate comparable to such an event.

aspergers@40

Let's start at the beginning. The Heinrich events and D-O events ARE part of a global climate change, and are NOT just local (re-read the OP w.r.t to the Indonesian paper). Wikipedia:

Xnn

Here's a quote from the paper:

So, basically what happens is that a slight amount of warming from a Dansgaard type event triggers the breakup of the large ice shelf near the Hudson Strait. This in tern allows all the glaciers behind it to flow into the sea at an accelerated rate. The result (over about 700 years) is a large influx of icebergs and fresh water into the North Atlantic that cools the Northern Hemisphere which in tern disrupts the thermohaline circulation on a global scale.

aspergers@40

No, I don't agree on this point. What kind of temperature increase are you talking about, and what is the supposed mechanism? It's more intuitive to assume a relative amount of warming would lead to the same relative amount of cooling isn't it? Look at thumbnail #3 in the OP for more info.

Andre

Note that the relationship between Heinrich events and Dansgaard Oesgcher events is not that unambiguous:



source

Skyhunter

Any change in the THC would have global impacts on climate. But the impact would be amplified for the North Atlantic, as is evidenced in the GISP cores.

Andre

But it gets more complex when the Dansgaard Oeschger events also show up very distincively in the pacific as oxic-anoxic oscilations:

http://www.nature.com/nature/journal.../379243a0.html

aspergers@40

Quite right Andre. The evidence quite clearly shows that the Antarctic also experienced D-O events, but the significance was a lot less; not even being noticeable the first time the cores were analysed. The situation is very different in the Greenland ice cores, showing a bigger effect. It seems reasonable to suggest a global warming of around a few degrees, but a local amplification mechanism has resulted in the Arctic polar region to become around 5 degrees warmer. What do you think?

aspergers@40

What are you proposing as the initial mechanism for the change in the THC though, and why with such a regular pulse?

Wikipedia: Dansgaard-Oeschger events:

Also, rather interestingly, it quietly states:

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Skyhunter

I am not proposing anything, just advancing the leading theory.

If I were to speculate based on the evidence I've seen, I would say that because the growth of the ice sheets is fairly constant, varies from year to year, but over the centuries it balances out. Once the sheets reach a critical mass, an as yet poorly understood flux in the solar cycle triggers a massive discharge of fresh water into the North Atlantic. This pulse of fresh water disrupts the thermohaline circulation, causing it to slow down, tropical waters get warmer because the heat is no longer being carried to higher latitudes. When the THC resumes, there is a rapid rise in temperature in the North Atlantic as the warmer than usual tropical waters are transported north..

This is what I suspect is the cause of the D-O events.

When the edge of the sheet collapses entirely during a D-O event,we see the ice rafted debris associated with Heinrich events.

But that is just my amateur musings.

aspergers@40

Okay, so we agree on the general gist of things i.e. binge-purge model. The speculated additional local Arctic polar warming could be due to the loss of summer sea-ice imo. The loss of albedo would result in less sunlight being reflected back into space. This then would be the reason for heavy northern hemisphere iceberg discharge compared to light southern hemisphere iceberg discharge.

Quite rightly, the problem now seems to boil down to what causes D-O warming events. As you mentioned "an as yet poorly understood flux in the solar cycle triggers.." I was wondering whether you were refering to an increase in the radiation output from the Sun?