The problems of the isotope paleo thermometer

Andre

In some threads around here I have hinted to problems with the use of 'water' isotope ratio's (δ¹⁸O, δ²H aka δD) as thermometer of past temperatures. This could become a dense thread, so I'll give the conclusion first:

The isotope ratio's in ice cores and other records of meteoric water are mainly proxies for absolute humidity at the water source.

That's a bummer since the isotopes are used a prominent tool to reconstruct paleo climate. Also it took me years to find this out, while the actual principle is so simple.

No this is not a new theory, it's just testing an existing theory, looking how paleo climatology and hydrography compare to basic meteorology. And actually combining some threads in the first page of this forum also gives this result.

The theorie for using isotopes as temperature proxies is wrapped up in Jouzel et al 1997.

Actually we could also keep it quite simple. The most important sentence in Jouzel et al is (page 46,481):

Finally, we should keep in mind that isotope changes record cloud temperature [Cuffey et al., 1995]...

Cuffy et al 1995 state (page 455):

...many factors in addition to local environmental temperature affect isotopic composition. Thes include changes in sea-surface composition and temperature (10), changes in atmospheric circulation (11), changes in cloud temperature, which may be different from changes in surface temperature (12), changes in the seasonality of precipitation (13),....

For the moment I don't think we have to dig deeper, because I have not found any isotope -temperature study that elaborates on cloud temperature. But we have a thread here about that.

To be continued soon

Andre

Sorry if I cliffhangered that a bit but I was thinking of this recent thread

Andre

Meanwhile, here is a comprehensive textbook about the isotopes in the water cycle. Well worth reading if you have a few hours to spare.

So how about the clouds?

3.3.1 IN-CLOUD PROCESSES
The formation of precipitation comes about as a result of the lifting of an air mass (dynamically or orographically). Due to adiabatic expansion, the air mass then cools until the dew point is reached....

...The isotopic ratio of the precipitation condensing from atmospheric water vapour is: RL = L/VRV where L/V is determined by the condensation temperature or rather the temperature at the cloud base.

But that temperature is the dew point

The plot is tightening. Well I'm just saying that to encourage you all to crush this stuff, because the outcome is important.

to be continued.

Andre

So there we have it:

At a given temperature but independent of barometric pressure, the dew point is a consequence of the absolute humidity, the mass of water per unit volume of air.

and

Due to adiabatic expansion, the air mass then cools until the dew point is reached. (forming clouds - A)

So what is causing....

changes in cloud temperature, which may be different from changes in surface temperature.

That's the absolute humity that fixes condensation and hence cloud temperature.

So if we look at Jouzel et al (1997) fig 4

We are looking at the comparison of snow accummulation and isotope-reconstructed temperatures of the GISP-2 ice core in Greenland. It reflects a most interesting timeframe from right to left the (cold or warm?) mystery interval until 14.5 thousand years, the (warm?) Bolling Allerod until 12,7 thousand years, the (cold?) Younger Dryas until 11.6 thousand years and finally the (warm?) Holocene.

But with such an extreme tight correlation, shouldn't the warning flags be out? Aren't we looking at twice the same? Snow accumulation being a function of absolute humidity and Isotope-cloud temperature being a function of absolute humidity?

In such cases it may help to consult other data

Andre

Hey, I'm challenging a major fundamental idea in paleo-climatology here, discussion is allowed.

Andre

Quote by Andre

In such cases it may help to consult other data

Anomalously mild Younger Dryas summer conditions in southern Greenland

The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14,400 and 10, 500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12 800–11 550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Fohn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling
experiments.

So, if you'd accept that the Greenland Ice core graph just reflects absolute humidity both by direct precipitation and isotope dew point reconstructions, the Younger Dryas was just very dry/arid and cloudless skies in combination with an approaching solar summer insolation maximum caused mild temperatures, while during the wet Bolling Allerod which preceded it, it was also cool due to the more prevailing cloud cover. So all of sudden there is no anomaly at all and there is no need to model problems away.

Andre

Another highly detailed record of the same period can be found in the pollen counts of the sediments of the Meerfelder maar volcanic pond/lake in central Germany (Lücke and Brauer 2004)

The abstract:

Abstract

The response of a lacustrine ecosystem to climatic changes from 13,500 to 10,800 BP was studied in a varve dated sediment profile of Lake Meerfelder Maar, western Germany. Bulk biogeochemical parameters, stable carbon and nitrogen isotopes of sedimentary organic matter and micro-facies analysis are used for a detailed investigation of the lake's development from the Allerød interstadial (AL) to the Younger Dryas stadial (YD) and into the early Holocene (Preboreal). Varve micro-facies reveal rapid changes in composition and seasonal structure of the depositional environment mainly in temporal accordance with changes of terrestrial biozones. The respective responses of bulk proxy parameters (∼bi-decadal resolution) indicate different sensitivities to diverse climatic changes. A prominent transition took place within two decades at the AL/YD boundary (12,690–12,670 BP). Increased flux of nutrients released from redeposited littoral sediments and shorter YD summer seasons led to an acceleration and concentration of lacustrine primary production with reduced discrimination against 13C. High lacustrine primary production was further favored by relatively warm YD summer temperatures. At 12,240 varve years BP, from 1 year to the next, a regular deposition of detrital layers in spring set in which is related to local hydrologic threshold processes amplifying the response to increased snowmelt discharge. This distinct change in the middle of the YD is clearly detected by varve micro-facies but not equally recorded in organic bulk proxy data. Especially, carbon isotope ratios remain constant and indicate a negligible effect of this process on the productivity of lacustrine algae. The YD/Preboreal transition (11,645–11,585 BP) is marked by a characteristic change in micro-facies. Throughout this transition, stable carbon isotope ratios strongly decline while the accumulation of opal and organic matter (OM) remained constant. This is seen as an increasing importance of spring and autumn for gross primary production at the onset of the Holocene.

This is the pollen diagram they reconstructed:

As trees and grasses usually use wind for pollination, their pollen is most abundant. We see a distict shift in both total pollen count and tree/grass ratio throughout the Younger Dryas. Apparantly the trees reduced during the Younger Dryas, while the grassed (gramineae) held up better. It should be noted that both facts could be explained by aridity. Steppes are generally dry, forests are usually wet. And under arid conditions the pollen production may reduce, (ask the hay fever sufferers)

Especially interesting are the water plant/algae fern swamp pollen Potamogeton, Botrychium and Pediastrum suggesting swampy conditions, or a low stand of the lake, consistent with aridity while it could not really have been arctic.

But the relative abundance of meadow flower species like Helianthenum is interesting. Since these are not polinated by the wind, these pollen are naturally rare. These species are still common there today, moreover there are no clear arctic pollen around like the Dryas octopetala, famous name giver for the period.

So it looks like these species are a good reason for the authors to talk about warm summers during the younger dryas, but also more arid as suggested by pollen shift and abundance.

How about the winters? Not a lot to say about that except that the Greater Burnet (Sanguisorba Officinalis) is limited to winter hardiness zone 4 and Potamogeton to zone 5. Cold yes, maybe, but not too extreme.

So it looks like these pollen reflect clear changes in moist-arid conditions but an extreme temperature swing like this cannot be considered supported here, imo.

Andre

This thread could go on for a lot more information, we could go for evidence of cold events in the allegdly warm Bolling Allerod and the onset of the Holocene, the Preboreal. We could also dig deeper in the isotope physics, and see if we can find more evidence in Meteoric Water Lines and Deuterium Excess

And after all that we could ask questions about the consequences of all this for the reconstruction of the geologic past.

But is it useful?

Andre

I drafted a little study about all this. I don't think I should post an open link to it yet. But anybody who wants to have look at it, is welcome, Just drop me a pm.

Andre

The study is work in progress, draft number 0.2.

I found some pretty convincing support recently, for instance this:

https://gsa.confex.com/gsa/2012AM/fi...act_207396.htm

Suggesting that the timing may be right to discuss things.

Meanwhile this is the abstract:

Abstract

In the precipitation cycle the dew point at which the first condensation takes place is the main determining factor for the isotope ratios. Together with the Rayleigh process it controls the actual decline of the isotope ratios (δ18O and δD) in the precipitation during the path of the moist air mass from the source to the ice sheets at the poles. The dew point however is a direct function of the absolute or specific humidity at the moisture source. Therefore isotope reconstructions like for instance in Greenland ice cores should logically be reconstructions of the humidity, not the temperature. This explains previously considered anomalies, like warm Younger Dryas Summers and glacial readvances in the USA and Scandinavia during the the Allerød/Older Dryas oscilations, preceding the transition to the Younger Dryas by a substantial margin. This suggest that it was not the Younger Dryas that was cold but the preceding period, which is at odds with the isotope paleo thermometer of the Greenland Ice Cores.

Just to make sure:

Quote by Andre

No this is not a new theory, it's just testing an existing theory, looking how paleo climatology and hydrography compare to basic meteorology.

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