Readers have inquired recently about how tropical speleothems are calibrated to temperature. Judd Partin, Kim Cobb (both of Georgia Tech) and associates wrote an excellent article last year (Cobb et al, EPSL 2007) on detailed O18 observations near a Borneo speleothem about which they had published. They introduced the article with the following observation that will resonate with most CA readers:
Detailed on-site analyses of the relationship between large-scale climate and local rainfall d18O are critical to accurate climatic interpretations of many terrestrial paleoclimate reconstructions based on ice core, tree ring, or stalagmite d18O, but few such studies exist.
They report a pronounced variation in northern Borneo dO18 values as follows (see graphic below):
Rainfall d18O values range from −11.5‰ to −2.5‰, with a mean of − 6.7 ± 2.8‰. A ~ 6‰ seasonal cycle is visible, with lighter values (− 10‰) occurring from August to October and heavier values (− 4‰) occurring from December to March.
Fig. 4. Interannual variability of northern Borneo precipitation (gridpoint centered at 3.75°N, 113.75°E,CMAPdata) (Xie and Arkin, 1997) plotted with the Southern Oscillation Index (http://www.cpc.noaa.gov/data/indices/). Both timeseries have been filtered with a 2–7 yr bandpass filter
They observe that these substantial variations cannot be explained by local temperature variations since:
Temperatures lie between 26 and 27°C year-round, as recorded by on-site temperature loggers.
Indeed, they note that this viewpoint is also held by other students of tropical speleothems:
In the tropics, stalagmite d18O records are largely interpreted as rainfall d18O reconstructions, with a minor role for relatively small temperature changes that occur in the tropics.
However, this seemingly plausible explanation cannot be transposed directly to northern Borneo as precipitation amount is not nearly as variable as the dO18 fluctuations. Cobb et al:
The ~ 6‰ seasonal cycle in rainfall d18O cannot be ascribed to seasonality of precipitation amount, which has a weak semi-annual nature, with relative rainfall maxima occurring in late boreal spring and from September to December (Fig. 2). In fact, a low correlation between Mulu rainfall d18O and Mulu precipitation (R = 0.05) suggests a limited role for a local “amount effect” stemming from fractionation in a local convective event. Likewise, the observed seasonal variability cannot be explained by variations in source water d18O, which are less than 1‰ across the Warm Pool (Brown et al., 2006).
An interesting conundrum. Strong annual variations in dO18, but not explainable either by temperature or precipitation amount.
The authors conclude that the strong variations are caused by changes in seasonal wind direction, with low dO18 coming when the wind direction leads to a water source further from northern Borneo and thus greater rainout.
Rather, we hypothesize that the observed rainfall d18O seasonality is caused by the increased rainout, and hence greater isotopic fractionation, that occurs during late boreal summer, and vice versa during late boreal winter. In late boreal summer, the ITCZ has reached its northernmost position, and mean southeasterly winds carry moisture from the Java Sea to Gunung Mulu, leading to significant rainout as moisture is carried long distances over the mountainous interior (Fig. 3). The remaining water vapor would be significantly depleted in d18O via both the “degree of rainout” and orographic fractionation mechanisms. Conversely, during late boreal winter, when the ITCZ lies south of Borneo, northeasterly winds carry moisture from the Sulu and South China Seas to Gunung Mulu (Fig. 3). The heavier rainfall d18O values measured during late boreal winter are consistent with the relatively short moisture pathway during this time of year.
All of which is fairly sensible.
People who like Hubert Lamb (of Little Ice Age and Medieval Warm Period fame) can hardly cavil at someone using wind direction as a proxy. So readers should not immediately start piling on to the obvious point that these dO18 records merely indicate wind direction, as Hubert Lamb placed great importance on prevailing wind directions as indicator of centennial climate fluctuations.
Equally, however, climate scientists who want to use such information in Mannomatics and the like need to remind themselves that the best information that they are getting from such records is information on wind direction. If you had detailed measurements of wind direction in northern Borneo (or south China or Yemen), how helpful would that be in determining global temperature?
Cobb, K. M., J. F. Adkins, J. W. Partin, and B. Clark. 2007. Regional-scale climate influences on temporal variations of rainwater and cave dripwater oxygen isotopes in northern Borneo. Earth and Planetary Science Letters 263, no. 3-4: 207-220. http://shadow.eas.gatech.edu/~kcobb/cobb07.pdf