Comments on: Loehle Proxy MD95-2011

By: Hu McCulloch

Hu McCulloch — Tue, 11 Dec 2007 17:27:11 +0000

Steve wrote,

dO18 is sometimes used as a temperature proxy (e.g. Lonnie Thompsons ice core),

In Thompson’s 2003 paper (the infamous one whose Figure 7d, showing the MBH99 hockey stick
plus Jones 99 instrumental series, is the true source of AIT’s “Dr. Thompson’s
Thermometer”), and again in his 2006 PNAS paper, it is intimated that dO18 proxies temperature, but he does not actually calibrate any of his series to temp. Even his PNAS composite index is just a Z-mometer, not a true thermometer.
Has he actually calibrated any of his ice cores to temperature?

By: rafa

rafa — Fri, 30 Nov 2007 08:57:02 +0000

I do not have access to the full text of the Calvo et al. paper but they seem pretty confident on the method used, alkenones, showing a maximum 5.5-6.5 Kyrs BP. Any link to a free access to the full paper?. Thanks.

By: Jimmy

Jimmy — Fri, 30 Nov 2007 01:35:35 +0000

I am suprised by the lack of an early Holocene maximum in the d18O of the forams. No Holocene Optimum.

Although the alkenone SST does have a trend decreasing to present, there are no error bars on the points so it’s hard to tell if it’s outside of the error of the measurement.

By: richardT

richardT — Thu, 29 Nov 2007 18:46:38 +0000

#3
The alkenones are produced by planktonic algae that grow in summer in the photic zone. This uppermost part of the ocean warms strongly in summer.
In this area, most of the forams are living in sub-surface water, perhaps 100-150 m deep. This is below the seasonal thermocline, and so does not warm in summer. The temperature at this depth is set during overturn in winter.
As the two proxies are sensitive to the temperature at different times of year, they can show different trends if summer and winter temperatures don’t have the same trend.

By: rafa

rafa — Thu, 29 Nov 2007 16:59:29 +0000

Someone who knows can explain how foram. and alkenones yield opposite trends?, best

By: Hu McCulloch

Hu McCulloch — Thu, 29 Nov 2007 01:09:13 +0000

I gather the update hasn’t been peer-reviewed published, and so won’t meet Craig’s Published Or Perish criterion. Could the authors be encouraged to published the update, perhaps as a short note in the same journal, so that it would qualify in future studies?

By: Gary

Gary — Wed, 28 Nov 2007 18:33:57 +0000

IIRC, error bars on SST estimates from foram frequency distributions are about one degree C. I don’t know what they are for alkenone-based estimates. It’s curious that the trends in the 8 kyr overlap are opposite.