Loehle Proxy MD95-2011

One of Loehle’s proxies – the one that ends in the 16th century – has occasioned some interest and I took some time to investigate it. It is an alkenone SST estimate from ocean sediment core MD95-2011 from the Norwegian Sea (66 58.19N, 07 38.36E and 1048 m). The series ends in the 16th century because it was a piston core and the top of it was lost. However box core JM97-948/2A was drilled i the same location and provides a high resolution series right to the 1990s.

Both cores are in high-accumulation zones and have been closely sampled – a sample spacing of 0.5 cm in core JM97-948/2A and 1.0 cm in core MD95-2011 – yielding resolution of about 3.4 years and 8.4 years, respectively. Both have been discussed in a number of scientific reports and a number of relevant data sets have been archived at http://www.pangaea.de, a repository for much ocean sediment data.

The archive includes dO18 values for both N pachy dex and N pachy sin for both MD95 and JM97; the combined data being shown in the plot below, with a slight difference in color denoting the change in core. Do these measurements indicate that dO18 values in the Norwegian Sea are at unprecedented levels? You tell me.

md95_211.gif
Figure 1. dO18 for Md95 and JM97. In the archive, the dates for the JM97 series are incorrectly reported. I asked CA reader Richart T about this and he was able to confirm that there was an error. I’ve plotted the data by re-calculating the dates in the JM97 dO18 series to match the date model. JM97 data from http://doi.pangaea.de/10.1594/PANGAEA.510799?format=textfile and MD95 data from http://doi.pangaea.de/10.1594/PANGAEA.510782?format=textfile. Dating from http://doi.pangaea.de/10.1594/PANGAEA.510806?format=textfile. X-axis in kyrs.

While dO18 is sometimes used as a temperature proxy (e.g. Lonnie Thompson’s ice core), it has not been used as one here. SST was estimated in two different ways for this core – (1) by an analog based on foram distributions; (2) alkenone SST. The SST estimates from foram distribution are archived for both MD95 and Jm97 and are plotted below; for alkenone SST, only the MD95 values are archived. I’ve been informed that JM97 values have been calculated but they have not been archived or reported, apparently because of inconsistency problems. Here is a plot:

md95_212.gif
Figure 2. SST estimates. Black – from foram distributions; red dots – alkenone SST from MD95. SST estimate and abundance of forams for MD95 from http://doi.pangaea.de/10.1594/PANGAEA.510781?format=html and for JM97 from http://doi.pangaea.de/10.1594/PANGAEA.510801?format=html. Alkenone SST for MD95 from loc=”http://doi.pangaea.de/10.1594/PANGAEA.438810?format=textfile.

Some References:
Calvo E., Grimalt J. O. and Jansen, E. (2002) High resolution UK37 sea surface temperature reconstruction in the Norwegian Sea during the Holocene. Quaternary Science Reviews, 21, 1385-1394. https://bora.uib.no/bitstream/1956/385/2/2001PA000654.pdf

Andersson, C. Bjørg Risebrobakken,2 Eystein Jansen,1,2,3 and Svein Olaf Dahl4 2003. Late Holocene surface ocean conditions of the Norwegian Sea (Vøring Plateau), Paleooceanography.

Risebrobakken, B; Jansen, E; Andersson, C et al. (2003): High-resolution study of Holocene paleoclimatic and paleoceanographic changes in the Nordic Seas, Paleoceanography


7 Comments

  1. Gary
    Posted Nov 28, 2007 at 12:33 PM | Permalink

    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.

  2. Posted Nov 28, 2007 at 7:09 PM | Permalink

    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?

  3. rafa
    Posted Nov 29, 2007 at 10:59 AM | Permalink

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

  4. richardT
    Posted Nov 29, 2007 at 12:46 PM | Permalink

    #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.

  5. Jimmy
    Posted Nov 29, 2007 at 7:35 PM | Permalink

    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.

  6. rafa
    Posted Nov 30, 2007 at 2:57 AM | Permalink

    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.

  7. Posted Dec 11, 2007 at 11:27 AM | Permalink

    Steve wrote,

    dO18 is sometimes used as a temperature proxy (e.g. Lonnie Thompson’s 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?

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