History of thermohaline circulation

[Mk]

How could we possibly chart its history? The Greenpeace website cites ice core data as backing them up (what? no references? ), but then we would have to have some gas isotope ratio that is used to pin down some quality.

 

[Andre]

Could you please be more specific? Anyway there are dozens of publications about changing THC's with the spikes of the Greenland Ice cores at the last termination.

 

[Mk]

I am asking how it is done, not possible ways it is done.

 

[Andre]

Ok,
Well obviously no-one in the past has been recording it so we need indications of indications and hints which are found in sediment cores of the ocean floors. Anything what can be measured is measured, biota remains, macro fossils of foraminifera, alkenone contents of the muds, ratios of elements, and isotopes, isotopes of everything you can think of.

Alleged tell tale signs of changes in the thermohaline current is changes in geographic spread of a thousands species of foraminifera,

Furthermore, a fast flow evens out the temperature gradient with lattitude, as the equatorial heat is more quickly transported to the poles. So rates of changes in sea surface temperatures can be used to measure this speed. Sea surface temperatures are decuded from isotope ratios, element ratios (Mg Ca Sr) and alkenone ratios.

So thermohaline current flows are basically deductions of deductions. It may be clear that this is very tricky, since sometimes other events may cause the same type of remaining evidence.

 

[Sinimod]

Pa/Th ratios in ocean bottom sediments

The ratio of protactinium 231 to thorium 230 in sediments recovered from the ocean floor in the Atlantic provide indication of when thermohaline circulation was strong, weak, and not operating. Read this article for a start on this subject:

http://www.whoi.edu/mr/pr.do?id=8839

 

[Andre]

Thanks for the link S,

There are problems though,

They found that the coldest interval occurred when the overturning circulation collapsed following the discharge of icebergs into the North Atlantic 17,500 years ago. This regional climatic extreme began suddenly and lasted for two thousand years.

Curiously enough that cold periglacial surge is just 'busted' by Schaefer et al Science 2006, who observed that both hemispheres came out of the ice age together some 17ka BP, disdaining the Greenland ice core isotopes that insist that it happened some 14,700 years ago at the onset of the Bolling "interstadial".

See also: http://www.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=2273&posts=20&start=1

Incidently one of the links that you send me today (thanks again) confirms that too:

http://www.geol.lu.se/PERSONAL/DNH/images/SAN1999.pdf

The lowermost radiocarbon date, immediately above the till, indicates that Lake Kullatorpssjo¨n was deglaciated before 14 000 14C yr BP, which corresponds to more than 17 200 cal. yr BP and provides further evidence that the Kullen Peninsula belongs to the earliest deglaciated area in
Sweden.

No, many of the earliest? many other proxies show the same. Actually this widespread early warming is a much better debunker for Greenland and other NH isotopes for being warming instead of precipitation change.

My guess is that the protactinium - thorium ratio variation may have more than one explanation (as usual).

 

[billiards]

It's a very tough science, in fact they still don't know all that much because they can only work with things like ice cores and sea floor sediment which can be very difficult to interpret. There's some good stuff going on with the "palaeo people" they can look at a shell and tell you if it was deep water, they can tell you about ocean acidity and all sorts of things. The problem is that its based on statistical analysis, if they find some anomaly it might mean something as drastic as an ice age or it could just be an erroneous spike.

Working out what the ice sheets were doing has major implications for what the ocean might have been doing, they are interlinked and should really be understood together. Also where the continents were, where the major basins were are all important. Palaeo current data can tell us some stuff about which direction the currents were going, these are sedimentary features that show orientation like ripples and flute casts, again you need lots of them and have to be aware of over interpreting.