From CO2 Science, another peer reviewed paper with a paleoclimatology reconstruction based on cores containing plankton shells, show that both the Medieval Warm Period (MWP) and the Little Ice Age (LIA) can be seen in Indonesia. In the past, critics have said these events to be “regional” implying they occurred only around Europe, due to lack of historical records in other regions of the world.
Since the Oxygen18 isotope dating method seems well proven, it would seem this study has a good basis for its claims. Even RC’s Gavin Schmidt likes it.
Co2 Science writes:
From the authors’ Figure 2b, adapted below, we calculate that the Medieval Warm Period was about 0.4°C warmer than the Current Warm Period.
The Makassar Strait (Wikipedia)
Foraminifera samples.
Description
Oppo et al. derived a continuous sea surface temperature (SST) reconstruction from the Indo-Pacific Warm Pool (IPWP), which they describe as “the largest reservoir of warm surface water on the earth and the main source of heat for the global atmosphere.” This history — which was based on δ18O and Mg/Ca data obtained from samples of the planktonic foraminifera Globigerinoides ruber found in two gravity cores, a nearby multi-core (all at 3°53′S, 119°27′E), and a piston core (at 5°12′S, 117°29′E) that were recovered from the Makassar Strait on the Sulawesi margin — spans the past two millennia and, as they describe it, “overlaps the instrumental record, enabling both a direct comparison of proxy data to the instrumental record and an evaluation of past changes in the context of twentieth century trends.” Reconstructed SSTs were, in their words, “warmest from AD 1000 to AD 1250 and during short periods of first millennium.”
Reference
Oppo, D.W., Rosenthal, Y. and Linsley, B.K. 2009. 2,000-year-long temperature and hydrology reconstructions from the Indo-Pacific warm pool. Nature 460: 1113-1116.
.gif)
.gif)
Unlike plate tectonic theory – which brought together disparate material into a unified and explainable whole – global warming has required the discrediting of the Medieval Warm Period, the Urban Heat Island effect, and solar fluctuation in order to make its case. My scepticism has risen in direct proportion to the amount of past climate evidence that the AGWers have demanded be discarded.
Excellent account of the LIA and the superimposed (coincidental?) impact of Icelandic eruptions given by Prof Dawson in “So foul and Fair a Day”.
http://europe.theoildrum.com/node/6387
Some convincing evidence for natural climate cycles comes form Scott Stine’s description of Owen’s Lake (California) being dry 800 years ago. It filled up with water during the LIA, a nice environment for those escaping Hell in Europe to settle.
I think its safe to say anyone who does not have an axe to grind knows that the current warming is not unprecedented.
Also proofs of the same from south america:
http://www.springerlink.com/content/m11m129238u61484/
euan mearns (12:16:26) :
The same case of a dry lake being filled durin the Maunder Minimum:
To the last two peaks of large inflow of meltwater, radiocarbon dates corrected to sidereal ages, are AD 1280/1420 and AD 1443/1656. These ages agree with two cold episodes clearly recorded in dendrological studies from the Patagonian Andes and were correlated to the Little Ice Age. Thus, older Holocene episodes of large inflow of water to the basin were correlated with the Neoglacial Advances defined by Mercer (1976) for the Andes.
http://www.springerlink.com/content/m11m129238u61484/
(cited above)
I read that way of thinking a lot, but It doesn’t make sense under the scientific point of view. If you claim that The MWP and the LIA were regional, you have to prove it, otherwise it shouldn’t be a scientific argument.
The obvious question now-in these “interesting times”- is: Are solar minimums (minima) ,low solar magnetic field epochs, related to volcanic eruptions-lowering temperatures-/big earthquakes ?
@John Egan: I wouldn’t bring plate tectonics into this. It’s a very whacky idea that gained prominence because the older theory couldn’t explain the mechanism. Plate tectonics has changed explanations countless times on its own mechanics and will change again. If you look at the geometry, PT is physically impossible for long distances. In fact, PT fell out of favour with the people responsible for bringing it to prominence. Also, subduction doesn’t exist on other planets. And it doesn’t exist on Earth either even though plenty of “scientists” will try to claim otherwise.
Oxygen 18
http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/
A quick explanation
Peer-reviewed to boot!
(Not that it’s important anymore – at least not for the IPCC),
FoxNews covers the IPCC’s F-graded report.
http://www.foxnews.com/scitech/2010/04/19/united-nations-climate-global-warming-ipcc/
Concerning MWP, it was consensus that it existed – until the Hockey-Stick Bros. came around and tried to airbrush it out.
This is yet another arrow through the heart of the bogus AGW tales.
Make that 501
http://www.populartechnology.net/2009/10/peer-reviewed-papers-supporting.html
blacks whitewash (12:17:48) :
I think its safe to say anyone who does not have an axe to grind knows that the current warming is not unprecedented.
—-
Indeed. Ever since this all started being about axe-grinding agendas I have relied on this little summary as a guide. Yes, from Nature, but back when it wasn’t compromised by politics, and also in a paper not directly dealing with climate change per se.
I must ask this extremely well informed group commenting here if this still rings true or whether new research has somehow changed this:
“The Greenland (Arctic) and Vostok (Antarctic) ice cores are particularly informative, offering fine temporal resolution and continuity. This has revealed surprising oscillations of climate on a millennial scale within the main 100-kyr cycle. The Greenland Ice Core Project (GRIP) identifies some 24 interstadials through the last ice age with average temperature rising rapidly by ~7 C over just decades. Further ice and sediment cores from around the world are demonstrating the global scale of these major climatic events.”
From: Hewitt, G. 2000. The genetic legacy of the Quarternary ice ages. NATURE, Vol. 405, 22 June 2000 (www.nature.com)
Again: “with average temperature rising rapidly by ~7 C over just decades.”
So did the IPCC gang rebore the ice and find something different?
Do you think we could get a key or legend describing the different coloured lines in the graph??
Woah! Lets not get too excited… these are “proxies” aren’t they?!? Or are there “good proxies” and “bad proxies”???
Yep, it’s definitely getting tougher for those deniers of MWP and LIA. btw, C3 put up a purrr-ty version of same chart, which was on their site earlier in the day.
http://www.c3headlines.com/2010/04/new-climate-data-reveals-modern-warming-to-be-only-lukewarm-versus-medieval-roman-periods.html
MrX (12:34:53) :
“I wouldn’t bring plate tectonics into this. It’s a very whacky idea …”
Oh yeah? Got a better theory?
With all the proof, this is our real problem according to Joe Bastardi for his latest blog post:
“It does show the folly of man thinking he has all these grand plans and is in control. But in the battle, and with the latest NOAA release upping the ante even more than man is causing all this
( it’s open season in spite of all we have seen, the ice cap returning, the leveling of temps, Climate Gate… my prediction this Congress would be right back at it with the Cap and Trade after they got health care, though not meteorological, is spot on. see this: http://www.alertnet.org/thenews/newsdesk/N19159977.htm )
we find that nothing will stop the zealots. They even go so far as to blame global warming for volcanoes. There are no limits, and a willing public, many of them under 40 and knowing no better than what they have been told the last 20 years, is not willing to dig in and look”.
http://www.accuweather.com/ukie/bastardi-europe-blog.asp?partner=accuweather
blacks whitewash (12:17:48) :
I think its safe to say anyone who does not have an axe to grind knows that the current warming is not unprecedented.
Hey there, climategatestuff aka blackwhitewash! Where you been?
Nah, the Mann-made disinformation campaign has run a long time, gone very deep, and is still spreading wide. It’s going to take awhile to get the truth out to the innocently deceived, of which our school systems are currently programming even more as we speak. Then we have the assorted types of the brainwashed to deal with, from the forcefully indoctrinated (“They kept sending me to Save-the-Earth seminars!”) to the willfully-following (“Now I understand why everyone deserves to die.”)
Yup, we got a job ahead of us.
I’ve never understood how anyone could believe that when it was warm enough to support farming in Greenland over the course of two or three centuries, this was just a local condition.
Or maybe no one really believes it. If they did, they’d be queueing up around the block with fancy models to explain an effect like that.
Urederra (12:29:54) :
“… If you claim that The MWP and the LIA were regional, you have to prove it, otherwise it shouldn’t be a scientific argument.”
Actually, this amounts to a classical falsifiable hypothesis. The accumulating evidence of correlated warm episodes in other parts of the world falsifies the regional limitation.
“Oxygen18 isotope dating method”
Novel! I prefer to use radiocarbon for dating.
[sarc]
Since this wasn’t reviewed by the relevant people it is irrelevant …
[/sarc]
MrX (12:34:53) :
@John Egan: I wouldn’t bring plate tectonics into this. It’s a very whacky idea that gained prominence because the older theory couldn’t explain the mechanism. Plate tectonics has changed explanations countless times on its own mechanics and will change again. If you look at the geometry, PT is physically impossible for long distances. In fact, PT fell out of favour with the people responsible for bringing it to prominence. Also, subduction doesn’t exist on other planets. And it doesn’t exist on Earth either even though plenty of “scientists” will try to claim otherwise.
Well. There you go. Everything I was taught in school, out the window in one fell swoop.
Thanks, MrX, for enlightening us. I guess it’s time to start all over from square 1. In the meantime, how about providing some proof of your hypothesis? It should be an interesting read.
Are there any climate changes that cannot be explained by a simple cyclical oscillation of all the air circulation systems latitudinally ?
@Duster
I know. What I mean is that it cannot be said that MWP and LIA are regional phenomena without proving it. And somehow the proponents of the AGW theory haven’t done much to prove it, other than saying “nope, that’s regional”
“MrX (12:34:53) :
… I wouldn’t bring plate tectonics into this. It’s a very whacky idea that gained prominence because the older theory couldn’t explain the mechanism. Plate tectonics has changed explanations countless times on its own mechanics and will change again. If you look at the geometry, PT is physically impossible for long distances. …”
PT is at present the only viable model going for large scale geological phenomena. It is the only one that DOES handle the geometry. The geosynclinal theory never did. There are no other explanations that integrate the motions of large scale faults like the San Andreas, volcanic arcs, their geographic correlation with extremely deep trenches, space/depth patterns in seisimicity along continental margins and particularly along subduction zones, triple junctions and a number of other phenomena such as magnetic striping, into one relatively neat mechanical model. Most objections are focused on crustal rigidity and don’t want to accept empirical and laboratory data of plasticity at depth; there’s a propensity to argue that rocks are brittle and can’t really deform. A theory can remain viable as long as it continues to examine itself, which is why the changing ideas about the mechanisms PT are a good thing.
The Indonesian study isn’t the only one showing that the MWP extended well beyond Europe. Would you believe Mono County, California?
Late Holocene forest dynamics, volcanism, and climate change at Whitewing Mountain and San Joaquin Ridge, Mono County, Sierra Nevada, California, USA
Constance I. Millar, John C. King, Robert D. Westfall, Harry A. Alden, Diane L. Delaney
Abstract
Deadwood tree stems scattered above treeline on tephra-covered slopes of Whitewing Mtn (3051 m) and San Joaquin Ridge (3122 m) show evidence of being killed in an eruption from adjacent Glass Creek Vent, Inyo Craters. Using tree-ring methods, we dated deadwood to AD 815– 1350 and infer from death dates that the eruption occurred in late summer AD 1350. Based on wood anatomy, we identified deadwood species as Pinus albicaulis, P. monticola, P. lambertiana, P. contorta, P. jeffreyi, and Tsuga mertensiana. Only P. albicaulis grows at these elevations currently; P. lambertiana is not locally native. Using contemporary distributions of the species, we modeled paleoclimate during the time of sympatry to be significantly warmer (+3.2°C annual minimum temperature) and slightly drier (−24 mm annual precipitation) than present, resembling values projected for California in the next 70–100 yr.
http://www.fs.fed.us/psw/publications/millar/psw_2006_millar027.pdf
The Medieval Warm Period happened, no if or buts.
To deny the occurrence is to deny reality, geologists and archaeologists have never denied the existence of the MWP.
Some ’scientists’ who because of it’s inconvenient warming attempted to efface fact from their own fiction. Time and again, more and more confirmation of the MWP comes to light and each fact just simply reinforces the obvious; that the cover up and fudged computer algorithms (Mann) cannot deny the truth and that their flawed hypothesis is further blown out of the water.
Now that is denial for you.
“Bonehead (13:13:20) :
Woah! Lets not get too excited… these are “proxies” aren’t they?!? Or are there “good proxies” and “bad proxies”???”
There are many proxies untouched by M. Mann, so the answer to your question is yes.
To be moderately nit-picky, I suggest you reorder a couple sentences to:
“Since the Oxygen18 isotope dating method seems well proven –even RC’s Gavin Schmidt likes it– it would seem this study has a good basis for its claims.”
Otherwise Gavin’s “it” is ambiguous, and could be seen to imply his approval of the study (which so far as shown, he hasn’t commented on it) rather than the underlying methodology.
My own opinion re MWP is that someday we’ll clean up the dating accuracy issues of separate proxies in different regions, and then the MWP will just snap back into view quite distinctly.
I’d really like to see someone undertake, as a peer-reviewed article, the assumption that the dating uncertainties of the proxies is the real problem here, and center the dating of all of them based on the best understanding of when the MWP actually happened.
Otherwise, the dating uncertainty problems of the various proxies “flatten out the sine wave” inherently.
And in related news:
http://www.prnewswire.com/news-releases/climategate-figure-threatens-lawsuit-over-satirical-youtube-video-hide-the-decline—no-cap-and-trade-coalition-says-bring-it-on-91591369.html
A warm anomaly in Indonesian waters might suggest La Nina like conditions (as would profound drought in the western USA). La Nina are usually linked to cool global anomalies. To demonstrate a globally warm MWP, you need, as a minimum, to find proxies from places expected to be cool in La Nina and show that they are average or warm.
The curve is similar to the data from Newton et al (2007) “Indo-Pacific Warm Pool SST Reconstruction (1004 to 1840)”:
http://i34.tinypic.com/2wg7hxy.jpg
Except the Newton curve has a blip in 1463:
http://i35.tinypic.com/11rb3ae.jpg
Those are from my post on SST (paleoclimatological) reconstructions:
http://bobtisdale.blogspot.com/2008/07/sst-reconstructions.html
Or are there “good proxies” and “bad proxies”???
Surely.
Stephen Wilde (13:40:39) : You asked, “Are there any climate changes that cannot be explained by a simple cyclical oscillation of all the air circulation systems latitudinally ?”
ENSO, AMO. The apparent 80- to 100-year cycle in the Southern Ocean:
http://i39.tinypic.com/m7wf7t.jpg
“MrX (12:34:53) :
… I wouldn’t bring plate tectonics into this. It’s a very whacky idea that gained prominence because the older theory couldn’t explain the mechanism. Plate tectonics has changed explanations countless times on its own mechanics and will change again. If you look at the geometry, PT is physically impossible for long distances. …”
Sorry buddy your in a 1950’s time warp. This was the argument in the 50’s and early 60’s when I went to university geology classes. Plate tectonics is proven beyond on all reasonable doubt since then by all the evidence.
MrX (12:34:53) :
“I wouldn’t bring plate tectonics into this. It’s a very whacky idea …”
————
Reply: I agree with Jimmy Haigh’s request for a better theory, MrX. Plate tectonics might be a very whacky (sic) idea, but I’m finding too much evidence to just toss it out unless you’ve got something much more credible than the theory we’re currently using.
Any references? We’re open-minded around here and you won’t get yelled at if what you’ve got is any good.
For those interested here are some more information and papers showing MWP being a global phenomenon.
http://www.cfa.harvard.edu/news/archive/pr0310.html
http://www.co2science.org/data/mwp/mwpp.php
http://uanews.org/node/30720
http://joannenova.com.au/2009/12/fraudulent-hockey-sticks-and-hidden-data/
http://translate.google.com/translate?hl=de&sl=de&tl=en&u=http%3A%2F%2Fwww.science-skeptical.de%2Fblog%2Fbeispiellose-erwarmung-oder-beispiellose-datenmanipulation%2F001195%2F
Bonehead (13:13:20)
There is a huge and compelling literature on the calculation of past sea surface temperatures. This includes thousands of sudies from thousands of marine drill cores and a variety of methods for estimating past water temperatures. The Mg/Ca ratio in planktonic foraminifera shells is one of these methods. It has been cross correlated with several other methods of temperature estimation from foraminifera. It has been checked in the lab by culturing the wee beasties at different temperatures. It has been checked in the field by sampling mordern waters of varying depth, latitude, longitude, temperature, clarity etc. It is also backed by some seriously detailed geochemistry. The effect has been studied extensively in foraminifera. It has also been studied extensively in coral.
The ratio of Mg to Ca in calcite which is secreted during shell production is dependant to a large extent on the temperature of the environment that the organism inhabits. There are some slight variations in the curve for this ratio between different foraminiferal species. This is why the authors of this study concentrated on a single species.
The method has been shown to work in samples from many sites going back hundreds of thousands of years. Now it is being applied with scalpal-like precision to very young sediments. In the future we can expect many more results rather like the those referenced in this article.
If anybody cared to read the paper, they would soon realise that the temperature reconstruction is based on Mg/Ca ratios, not d18O. The latter is used to reconstruct the hydrography.
It’s not a question of good and bad proxies it’s a question of cherry-picking. See Briffa and Mann. Even without proxies historical accounts tell us it was global.
Richard Telford (14:18:05) :
A warm anomaly in Indonesian waters might suggest La Nina like conditions (as would profound drought in the western USA). La Nina are usually linked to cool global anomalies. To demonstrate a globally warm MWP, you need, as a minimum, to find proxies from places expected to be cool in La Nina and show that they are average or warm.
________________________________
How about farming in Greenland?
It’s also worth remembering that during the overall cooling shown by the graph, hundreds of ice-core CO2 measurements show atmospheric concentration rising from less than 280 ppm to more than 310 ppm. Obviously, CO2 causes Global Cooling.
MrX “Also, subduction doesn’t exist on other planets. ”
And the sample size is, what, one other planet?
Two?
For what it’s worth, computer modeling indicates that rocky planets somewhat larger than Earth will have subduction, and hence plate techtonics, even without oceans. For Earth, the oceans are necessary, which explains why Venus shows no signs of it. Of course, what we know of Venus’s surface is limited to radar surveys and short lived probes. Another requirement is a molten mantle.
Mars, in its distant and very wet past, before its core cooled off, may have had subduction and plate techtonics, the debate is still open.
The problem with the MVP and the LIA is that they falsify the current generation of climate models. None of the modelers have the foggiest idea of what caused either, nor how to code them in. However, all of the models are easily tuned to match the hockey stick even with wildly different forcings and feedbacks, just vary the tuning parameters until you get a match. They just can’t predict the MWP or LIA from any currently know scientific principles. In order to validate the models they are left with only two options, either figure out how to include the MWP and LIA into the models, or remove the MWP and LIA from history, which they have been trying to do for the last 20 years or so.
Neither the MWP nor the LIA will be accepted by the ‘True Believers’ in the climate science community until they can figure out how to model them.
Richard Telford (14:18:05),
Here is an interactive map showing the Medieval Warm Period around the globe: click
The MWP was world wide. Which shows that the current climate is normal, and nothing unusual is happening. Furthermore, the Greek optimum prior to both the MWP and the SUV was even warmer.
“MrX (12:34:53) :
Also, subduction doesn’t exist on other planets.”
There is no part of the surface of Venus older than 600 million years.
“DG: ………… The biggest surprise of Magellan was that the surface seems like it’s all the same age. That’s what I’m calling the second great transition. Something changed on Venus 600 or 700 million years ago to make the surface all the same age.
If you use the word catastrophic it rubs some people the wrong way, but something dramatic happened on Venus which wiped out almost all signs of an older surface. The planet got re-paved, basically, 600 or 700 million years ago.
AM: Did some huge impact melt the surface? Or was it the last gasp of volcanic activity?
DG: Clearly, whatever this second great transition was, it involved massive amounts of volcanism. You can see these flows that appear to be flood basalts all over, covering 80 percent of the planet. The remarkable thing is that they seem to be all the same age. The crater density is relatively uniform and random around the planet. So the planet seems to have been flooded with basaltic lavas in a geologically short period of time, simultaneously around the planet.”
http://www.astrobio.net/interview/1137/venus-hothouse-planet
I suspect that is why Venus has such a dense atmosphere and is so damned hot.
Oppo archives her data too. http://www.ncdc.noaa.gov/paleo/pubs/oppo2009/oppo2009.html
Of most importance is the accuracy of the downcore dating. Unfortunately the full-article is behind the Nature paywall so this means a trip to the library.
What we need are well-dated series sampled from the globe that show the MWP happened AT THE SAME TIME around the globe. Otherwise, it can’t be said that the GLOBAL temperature was hotter at that time than before and after. After all, we all know the perils of averaging different proxies together!
Richard,
That’s an amazing map. Thanks for the link!
One problem with calling the MWP “local” is that it is fairly impossible to create a weather map that can loop a warm jet stream up that far north, and then have it come back south over the rest of the northern hemisphere.
This past winter’s record-setting AO created a blocking pattern which could warm Greenland, but not northern Europe, which (as we all know) was very cold.
To have the “local” MWP, which Alarmists suggest existed, you need both Greenland and Europe warm, but Asia cool. The jet would have to be amazingly lop-sided, like a beret stylishly tilted to one side of a head. Basically a huge trough would have to lock itself in place, and not budge for a hundred years, even to thaw the Vikings fields in Greenland.
It is great fun to play around with the Gulf Stream, and to try to create fantastic weather maps. The problem is that most maps that I have created are incredibly unstable. They wouldn’t last a month, let alone a century.
I am really interested to see the map Alarmists use, which makes the MWP manage to be “local.”
Too bad Richard won’t benefit from the map. But others will, and it was posted for them ☺
Caleb (17:49:39),
Right about the MWP being global. It seems there would be a violation of the 2nd Law if one part of the globe stayed significantly warmer than the rest of the planet, for hundreds of years.
The jet stream acts to even out temperatures along the same general latitudes. But the alarmist crowd doesn’t care about thermodynamics, they want to get rid of the MWP completely: click
But that wouldn’t work even if they could pull it off, because 2,000 years before the MWP there was the Minoan climate optimum, which was much warmer than the MWP: click
The current climate is just fine. Could be a little more pleasantly warm. But history tells us that’s controlled by factors beyond the ability of humans – or CO2 – to influence.
… personally I will take any continuous line of proxy data which isn’t stupified (by combining it with other sources or data gathered by different methodologies) … rather than present temperature data sets. The temperature data sets are so compromised by bad methodology I personally couldn’t trust any of it. Unless we say ’station a’ which hasn’t had any instrumentation changed and has been calibrated and maintained the same way for ‘x years’ shows this trend over this period of time. ’station a’ however, tells me nothing about ’station b’ or the aggregate of stations and the aggregate is meaningless.
Great map! Thanks!
/Mr Lynn
MrX (12:34:53) : “I wouldn’t bring plate tectonics into this. It’s a very whacky idea that gained prominence because the older theory couldn’t explain the mechanism. Plate tectonics has changed explanations countless times on its own mechanics and will change again. If you look at the geometry, PT is physically impossible for long distances.”
All of the geologists that I know subscribe to standard plate tectonics theory. All of the geophysicists that I know feel that the continents are fairly fixed in place. I can’t say that I know enough (in numbers) of geologists and geophysicists to say that it is a fair representation of the two fields. There are definitely some problems with the mechanism of plate tectonics, but you could say that about the theories of how the solar system formed, also. And in science, you don’t have to have a “better” theory before criticizing someone else’s theory. Sometimes we just really don’t know why things are the way they are.
The real importance of this study is that it is reconstructing ocean temps rather Northern Hemisphere atmospheric temps. If we calculate the total accumulated joules during the MWP in the oceans vs the atmosphere one will not make a significant digit of the other. Why are the climate scientists not trying to find the missing heat in the MWP? Well it looks like its not missing.
@Madman (13:03:14) : “Do you think we could get a key or legend describing the different coloured lines in the graph??”
Here is the entire paper. (You need a sub to get the Nature copy, but I found this.)
https://darchive.mblwhoilibrary.org/bitstream/handle/1912/3188/ppnature08233_with_fig%26supple.pdf?sequence=1
@Duster (13:46:57) :
Good response to a silly comment.
@Janice (18:47:23) :
“…All of the geologists that I know subscribe to standard plate tectonics theory. All of the geophysicists that I know feel that the continents are fairly fixed in place….”
The great battles over the validity of plate tectonics were basically settled in the ’70s. No other theory explains so much about the features of the earth. But the earth doesn’t expose all its secrets, and it takes a long time for most geologic processes to work, so in some cases we don’t have good explanations as to why things happen.
As to the continents being fixed in place, I’m not sure what you’re saying. The continents are part of crustal plates that move about, albeit at rates of a few centimeters a year, top speed. However, the continents are essentially immune, due to their low density, from being subducted. Subduction is a function of density, and old, cold, dense oceanic crust (essentially basaltic lavas and ultramafic intrusives) finally sinks into the upper mantle material, dragging with it pieces of sedimentary prisms which form at the trenches that demarcate subduction zones.
Density differences have a great deal to do with many geologic phenomena, and with some oceanic and atmospheric phenomena, too.