date: Thu, 21 Dec 2000 15:02:55 +0000 from: Giancarlo Bianchi subject: Re: deep water flow curve to: Keith Briffa Dear Keith, First things first. I've attached a text file containing the published data you requested vs. cal. years. A few general comments that you may find of use follow: a - you may have noticed from the data that although we observe a correlation between weaker near-bottom flow intensity of ISOW with cooler climates in recent historical times (and, by extension, in earlier parts of the Holocene), we do not record the lowest values during the Little Ice Age. So, assuming (most probably incorrectly) a simple linear correlation between deep flow speed and climate, the Little Ice Age event is rather unremarkable compared to earlier minima (e.g. ~2,800 yrs BP). b - with regards to the significance of the record with regards to global thermohaline circulation changes (thus also varying heat flux to NW Europe), it is rather hard to assess the extent to which the results reflect changing production rates of deep water in the Nordic Seas. As we mention in the penultimate paragraph of the paper, it could well be that the data reflect changes in the properties of the waters entrained beyond the sills the separate the Nordic Seas and the North Atlantic. The implication of this is that, for instance, if the properties of Labrador Sea Water (one of the important water masses entrained downstream of the sills) are the main modulator of the density structure, and thus flow speed, of ISOW, the origin of the signal we record may lie in the Labrador Sea. This is quite plausible in view of the evidence by Bob Dickson and others of dramatic changes in the depth of convection in the Labrador Sea over the past few decades. c - In contrast with our results, Keigwin finds a pronounced shift in benthic del13C in a core from the Laurentian Fan associated with the Little Ice Age. His excursion is also the most pronounced of the Holocene. However, when he compares his results with another deep ocean palaeonutrient proxy (Cd/Ca ratios) on the same samples, there is no change associated with the LIA. So, in summary the situation is still unclear. By the way, if you want to look at all these results, you can find them in a paper by Keigwin & Boyle that can be downloaded as a pdf file at: http://www.pnas.org/cgi/reprint/97/4/1343.pdf The paper is entitled "Detecting Holocene changes in thermohaline circulation" and provides a good, concise review of the current knowledge. d - finally, I think Keigwin's work has shown quite clearly that there are surface ocean changes in the past 1,000 yrs (mainly in the Sargasso Sea) and I believe he is on the right track when he suggests that the mechanisms involved in the North Atlantic Oscillation may be a viable model to use for longer-term climatic fluctuations in this region. In fact we also use this model as partial explanation for the apparent moderate degree of correlation between the grain size data from the Iceland Basin and the Sargasso Sea. Now, having gone through what I just wrote once more, I don't think I directly addressed the main topic of your paper but hopefully you'll find some useful information scattered here and there. Please contact me again should you need more help. By the way, I may leave early today but will be in tomorrow morning. Best wishes and happy festivities, Giancarlo Attachment Converted: "c:\eudora\attach\Size15K.txt" Department of Earth Sciences University of Cambridge Downing Street Cambridge CB2 3EQ U.K. Tel.: 01223 333400 Fax.: 01223 333450 http://www.esc.cam.ac.uk/