Tuesday, March 27, 2012

2911.txt

date: Mon, 18 Sep 2006 11:20:46 +0100
from: Tim Osborn <t.osbornatXYZxyz.ac.uk>
subject: review of Wu 2006
to: Keith Briffa <k.briffaatXYZxyz.ac.uk>

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----------------------
Review of manuscript by Wu, Yu, Zeng and Wang
"Possible solar forcing of 400-year wet-dry climate cycles in
northwestern China"

This paper presents results from a new lake sediment record in NW
China, covering approximately the last 1500 years. This is an
interesting area in which to develop new records spanning this time
period and the authors should be encouraged in their endeavours to do
so. Despite this, I do not recommend that the present manuscript
should be published, for two main reasons.

First, the age-model cannot be relied upon with confidence. For most
of the length of the record the age-model appears to be defined by
the untested assumption of constant sedimentation rate between the
surface and one single calibrated radiocarbon date. Not only is this
assumption not tested by using further dates (except in the top 8cm
of a neighbouring core, compared with the 150cm of the main core) but
the changes in sediment composition which are interpreted as major
changes in river inflow would surely cast doubt upon this
assumption? The age-model is, of course, important anyway, but
becomes even more so because of the comparisons made later in the
manuscript with other records, including interpreting those
comparisons as evidence for a response to changes in solar forcing.

Second, the comparisons made between different indicator time series
within the new core, and also with records from elsewhere in the
region or the world, are not done with sufficient quantitative rigour
to justify the conclusions that are made.

The doubtful age-model should not limit the use of quantitative
statistical methods (at least simple correlation coefficients) for
the comparison between the different indicators, because the multiple
proxies were presumably derived from the same depths of the same core
and therefore age-model errors will affect all these proxy series in
the same way. These comparisons should, therefore, be more
quantitative. For example it is stated that the oxygen and carbon
isotopes of the bulk carbonate have "a tendency to covary": this
statement is not clearly supported by a visual comparison, so state
the correlation between these series. The del18O does not, for
example, appear to show the "large negative excursions" in 750 or
1050, and neither isotope shows a negative excursion at the stated
1650 date. There are other cases too that the authors should
re-visit using quantitiative statistics to support the conclusions
that they draw from their data.

The doubtful age-model does invalidate the comparisons made with
other records and the radiocarbon record and so these should be
removed completely from the manuscript (e.g. all of page 11, first
half of page 12, points 3 and 4 in the conclusions and figures 4 and
5. Even if the age model was good enough to justify such
comparisons, the comparisons are not made rigorously. For example,
the lake record wet intervals "appear to correlate" with snow
accumulation rate and snow oxygen isotopes -- do they correlate or
don't they? State the correlation coefficient. Visually, it looks
as if they may correlate with the rate, but not at all with the
isotopes! There is a "close correlation" between the lake organic
matter del13C record and snow accumulation -- if true, state the
correlation coefficient. Visually this statement looks
false. Another problem is that the MWP is not well-defined and it is
not possible therefore to have confidence in the stated onset and
termination dates (for which region? dates are probably different in
different places!) nor their coincidence with wet intervals in the
poorly-dated lake record.
----------------------


Dr Timothy J Osborn, Academic Fellow
Climatic Research Unit
School of Environmental Sciences, University of East Anglia
Norwich NR4 7TJ, UK

e-mail: t.osbornatXYZxyz.ac.uk
phone: +44 1603 592089
fax: +44 1603 507784
web: http://www.cru.uea.ac.uk/~timo/
sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm

**Norwich -- City for Science:
**Hosting the BA Festival 2-9 September 2006

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