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