I’m spending a little extra time examining the “new” Mann 2008 MWP non-dendro “proxies”. There are 4 series from varved lake sediments in Punta Laguna, Mexico: two dO18 and two C13 series, each for a different gastropod species.
Original data is archived at WDCP here. I compared this data to Mann’s versions and noticed some puzzling differences. The coronatus data (both C13 and O18) contains only one value in the 20th century. Although the Mann version is consistent with interpolation in the pre-modern period, Mann’s modern values contain values that do not exist in the official archive. Where on earth did Mann’s 20th century values come from?
Mann reports flashy correlations for a couple of these series (382, 383) with NA values reported for the other 2 series – I’m not sure why. For 382, Mann’s SI shows correlations for 1850-1995 (0.40), 1896-1995 (0.60) and 1850-1949 (0.52) and for 383, he shows correlations for 1850-1995 (0.63), 1896-1995 (0.8693) and 1850-1949 (0.7467). In the latter series, the underlying data set has reported values for 1893 and 1993 and none in between, so it would be interesting to know how the calibration was done.
The underlying publication only discusses the dO18 series – NOT the dC13 series, and does not interpret them as a temperature proxy. They observe that the O18 variability is much greater than can be accounted for by temperature and hypothesize that changing O18 values correspond to changing circulation patterns, moving zones north and south. This is a theme that is picked up in other studies – I’ve mentioned this in connection with Newton et al 2006 on Pacific Warm Pool proxies, who discusses this.
The original publication clearly marks a MWP, though it ends in the 950-1100 period that is often highlighted by other data. I’m not convinced that a lot of weight can be placed on the author’s dates.
The sediments are dated by 5 dates from terrestrial wood (as there is a large reservoir effect in the gastropod radiocarbon dates) with only 2 dates from the last millennium. Dates are interpolated assuming that each date is exact i.e. varying sedimentation rates. If a uniform rate were assumed (and there are only 5 dates), this would reduce the estimated age of sediments in this period by about 90 years or so and would delay the estimated date of the end of the MWP in this region. (Craig Loehle has always emphasized these sorts of dating issues as a potential source of incoherence.)
It’s hard to develop an objective way of handling such things, as efforts to wiggle match can easily cause one to find what one “Wants” to see. For the present, I merely urge people not to place undue weight on the dates of these squiggles as the dating uncertainties could easily be a century or so.
Curtis, J.H., D.A. Hodell, and M. Brenner, 1996, Climate Variability on the Yucatan Peninsula (Mexico) during the last 3500 years, and implications for Maya cultural evolution, Quaternary Research 46: 37-47.