Also … thanks for putting together and riding herd on this wonderful blog.

w.

Re 8, 9:

Is this the source of the 1.6C constant in the calibration equation of Dekens but not in yours?

the plankton maximum are about 2 deg C cooler than the surface Warm Pool.

If dissolution occurs at depth then surface temps wouldn’t be such a factor.

glacial climates had lower dissolution rates

Comment by David Stockwell “¢’‚¬? 2 October 2006 @ 10:37 am

#8. 1.6 deg C – That makes a lot of sense although they didn’t mention it. I’ll try it that way and update the note.

Comment by Steve McIntyre “¢’‚¬? 2 October 2006 @ 10:44 am

This is not the case. The 1.6° constant is to adjust for a difference between the Atlantic and the Pacific. Dekens says:

The resulting equation is:

Mg/Ca = 0.38 exp (0.09*(SST -0.61*core depth km))

for G. ruber, Atlantic and

Mg/Ca = 0.38 exp (0.09*(SST -0.61*core depth km) – 1.6 C ))

for G. ruber, Pacific …

However, this is related to another problem. Steve M., you say,

Equation (8) follows logically from the literature, but the Dekens et al 2002 equation only appears possible by dropping a bracket or some other error in the simple algebra.

Actually, Dekens has done something quite clever. He has done a multiple regression of the form

where Y is SST [Levitus and Boyer, 1994], X1 is ln(Mg/Ca), X2 is water depth of sediment (km), and X3 is 0 if the core is in the Atlantic or 1 if the core is in the Pacific. Regressing this gives coefficients that match the data best. Investigation showed that the b3X1X2 term was very small, so he was left (for the Pacific) with

This resolves to the formula he gave. It appears that the similarity between the 0.61 and the 0.58 was coincidental.

However, there’s more to the story. I examined the Figure 5 you showed above:

http://www.climateaudit.org/wp-images/lea.ht24.jpg

I noticed that there’s a whole raft of species in there, whereas Lea only used G. ruber (white). So I went back to the Anand study referred to in Figure 5. Anand breaks it down into spinous species, which includes G. Ruber, and non-spinous species. His formula for the spinous species is

Mg/Ca = 0.58 e (0.075 * SST)

Note the difference between that and the formula for all species, with coefficients 0.38 and 0.9 respectively.

I took the G. ruber (white) data alone, and regressed the best match for that. Here are the results:

Note that the Lea and the Dekens results are very similar. This may be why, despite the advance, Lea continued to use his formula. However, neither one is a very good match to the G. ruber data. I show the best match. I then took the best match, and regressed it on the Dekens formula to find the correct depth adjustment. Using that formula gives the following results on the Lea data.

Note that the Dekens and Lea data are quite close. The Best Match results reflect the slightly different slope due to the improved match with the G. ruber data alone, and thus have a slightly wider spread. In this regard, it is worth noting that Lea commented that his results did not show as wide a spread as expected, saying

SSTs for glacial MIS 2 and MIS 6 determined
from OJP Hole 806B Mg/Ca data are
;3°C colder than modern values, compared to
the 6°C cooling estimate determined from Sr/
Ca of corals from the western tropical Pacific
and dated to the Younger Dryas interval and
termination II (6, 38).

Finally, a conundrum. I looked at the sedimentation rate at the OJP806 site. Here’s Lea’s figures:

Does that make sense to you? Look at the area about 250,000 years ago. Is it reasonable that the sedimentation rate would stay absolutely stable (within the limits of resolution) for tens of thousands of years, and then increase threefold in only 800 years? I don’t think so … but what do I know?

w.

from
http://www.jcdeelman.demon.nl/dolomite/bookprospectus.html

Well, Calcium carbonate has a solubility in cold water of about .0015 while Magnesium carbonate has a solubility of .17 or roughly 100 times as great. This is partly reflected in their relative concentrations in seawater on a molar basic where were magnesium is 6 times as common. But that still means magnesium would be more likely to dissolve than calcium, other things being equal. Though I notice there’s one form of Magnesium carbonate which isn’t nearly as soluble as the plain hydrate is. Still, as time passed I’d expect the ratio of magnesium to calcium to decline.

But as to the kinetics of the situation, I’m not positive. I don’t seem to recall the thermodynamic equations which govern such things making a distinction between different compounds. But there are lots of imponderables unless you look at a particular situation. There might be a different rate-limiting step which would favor one compound’s dissolving over another. And the crystalline form could also be important. And are the compounds co-mingled or do they form separate crystals?

2. BTW, without getting into religion, y’all should adopt the motto supposedly enshrined in a plaque at NASA’s Johnson Space Center: “In God we trust. All else bring data.”

The control on dissolution appears to be the pCO2 value as discussed in Dekens. But they don’t know this column and depth is a “proxy” for pCO2.

w..