Dongge Cave is a very long speleothem in southeast China, which is held to provide evidence on changes in the Asian monsoon.
It was most recently considered in Bao Yang et al (QSR 2007) in juxtaposition with the Dasuopu ice core, Oman speleothem S3 and the RC2730 Arabian Sea core showing G bulloides percentage. (This latter core was an important contributor to the Moberg reconstruction and has been discussed on several occasions – G bulloides are evidence for cold water and are interpreted as evidence of increased upwelling of cold water, which in turn is considered to be evidence of wind speed, and thus monsoon activity.)
Kim Cobb has also just reported a new speleothem in Borneo, somewhat the same part of the world, which I’ll try to look at soon. I’ve not previously looked at the Dongge Cave speleothem. Its results have been properly archived at WDCP, much facilitating examination.
First, here is a published image of the dO18 values from Dongge cave (this can be determined to be a plot of stalagmite DA). This shows a strong decline in dO18 values following the Holocene Optimum, which was reversed around 1400, after which there is an increase in values reversing the previous trend.
Next here is a graphic of the Oman speleothem S3, referred to in the Yang et al article. (This information is not archived; I’ve written to Stephen Burns, an associate of Raymond Bradley, suggesting that he archive his interesting data.) In this case, you can see a very distinct Holocene Optimum, with a very rapid increase in values emerging out of the LGM, followed by a long decline following the Holocene Optimum, with recent discontinuity in the records.
The next graphic compares the Dongge DA speleothem shown above to the Dongge D4 speleothem, which goes back to the LGM and to another Arabian Sea G bulloides series (723A) which, unlike RC2730, goes back to the Holocene Optimum.
The longer Dongge speleothem (D4) shows the decrease in dO18 values as you go back to the LGM, which is very marked in this series as well as the Oman speleothem. However, the LGM transition to higher dO18 values is dated about 2000 years earlier for Dongge Cave than for Oman speleothem S3.
Is this a real difference in dates reflecting “regionalization”? Or is it an artefact of different dating? (and these speleothems are benchmarked with U/Th methods that can have pretty wide error bars?
I find it difficult to believe that there would be 2000 years difference in the dO18 regime changes and I dare say that the events are coeval, notwithstanding the author datings. (Obviously this can lead to the Crowley-esque perception of “regional” differences in timing, which may not be real.)
One last point about the Dongge Cave spelothem series – that is not obvious from the first plot of this series shown above.
Here is a blown up version of the series for the last millennium (arranged in this case with the most recent value on the right.)
In this case, the maximum values were reached in the 1930s, and there has been sharp decrease in the last half of the 20th century – another “Divergence Problem” if you will.
I’m not saying that this shows that temperatures have decreased; or that this somehow invalidates the perception that these speleothem dO18 values show changes from the LGM to the Holocene Optimum and back again. I don’t know what it means – except that it deserves to be talked about.
My attention was drawn to this discrepancy by Yang et al 2007, who said that this series and Dasuopu were “very similar” – the sort of qualitative arm-waving that is so typical of climate science. Given that Dongge Cave dO18 goes sharply down in the last half of the 20th century, I don’t notice anything “similar” at all in the 20th century portion, let alone “very similar”.
Obviously these time series show a very marked Holocene Optimum in both Oman (17N) and southeast China (25N), both of which give evidence on tropical conditions. They are interesting to juxtapose against statements from realclimate and NOAA, discussed previously here. NOAA stated:
Paleoclimatologists have long suspected that the “middle Holocene” or a period roughly from 7,000 to 5,000 years ago, was warmer than the present Image courtesy of Kerwin et al., 1999. Click here for larger viewing image. day. Terms like the Alti-thermal or Hypsi-thermal or Climatic Optimum have all been used to refer to this warm period that marked the middle of the current interglacial period. Today, however, we know that these terms are obsolete…
Obviously the speleothem data from Oman and Dongge shows that the Holocene Optimum was very marked in these areas and not just the NH extratropics.
If the Team wish to cavil that this information does not bear on annual temperatures, they must explain why, for example, the same type of data (e.g. percent G bulloides in the Arabian Sea, speleothem dO18) is permitted as a temperature proxy in Moberg et al 2005. All in all, the speleothem data certainly doesn’t support the claim that the term is “obsolete”.