Are isotopes a reliable indicator of paleoclimate change?

[Andre]

Many of the old threads here are about ice cores, isotopes, CO2 etc and the problems I see with those. There might be interest in a simple wrap up, what those problems are and rather than linking to old threads I think it might be an idea to use fresh words in an attempt to explain it.

It may be known that the roots of the climate change hypotheses are in the research of paleo climate. See this elaborate work http://www.aip.org/history/climate/index.html

http://www.aip.org/history/climate/rapid.htm

The 1980s and 1990s brought proof (chiefly from studies of ancient ice) that the global climate could indeed shift, radically and catastrophically, within a century — perhaps even within a decade.

I intend to substantiate that the interpretation of indirect paleothermometers, especially isotopes to come to that conclusion is too simple.

 

[Mk]

Alright Andre, we're listening. I finally read that AIP link you often recommend. I read the "Especially" link, not the entire thing, it's pretty big.

How do you pronounce "loess?" What are some favorite places to find loess for testing?
An example was George Kukla's study of snail shells and pollen in layers of loess (wind-blown dust) in Czechoslovakia — another study that was designed to investigate gradual shifts, but in which a close look at the data revealed unexpectedly abrupt transitions.

 

[Andre]

Sorry Mk, for the delay, there were other priorities but we'll continue now.

Have been trying to find the best format, so let's try to start of with showing one of the problems:

Many publications like http://www.agu.org/pubs/crossref/1997/97GL02658.shtml focus on the synchronisation problem of the warming following the last glacial termination between 20,000 and 10,000 years ago as shown here

It appears that Antarctica (Vostok dD) came out of the ice age first, followed much later by Greenland (GISP-II d18O) and then both had clear dips at different periods.

Given that land warms faster than sea, this adds to the enigma that the predominant oceanic southern hemisphere was much earlier than the predominantly land northern hemispere to warm up.

Moreover, since Antarctica is still under a clear ice sheet, why do we think that the ice age ended there?

Such enigma's would require at least the confirmation of warming - cooling in the time frames as indicated in the graphs with more and different paleo temperature indicators, wouldn't you think?

To be continued


BTW Loess sounds like thus

 

[Andre]

So checking other records, as I suggested, was one of the first things done as Spencer Weart elaborates upon:

http://www.aip.org/history/climate/cycles.htm

Reliable Dates and Temperatures (1955-1971)
The tool that would unlock the secret was constructed in the 1950s, although it took scientists a decade to make full use of it. This tool was radiocarbon dating. It could tell with surprising precision the age of features like a glacial moraine. ..

..For example, dating of lake deposits in the Western United States showed surprisingly regular cycles of drought and flood — which seemed to match the 21,000-year cycle predicted by Milankovitch. But other carbon-14 dates seemed altogether out of step with the Milankovitch timetable.

But there was a problem as the last sentence showed. The reliability of the carbon dating. In those days the method hinged on a constant ratio of radiocarbon (14C) in the atmosphere but nothing proved to be more variable than that. Nowadays by comparing counted annual layers (tree rings, coral rings, stratified lake sediments) with their carbon dates we have a robust calibration table showing a difference of a few thousand years in this range of 10 – 20 Ky. Hence those early carbon dates, confirming warm and cold, did not do that all. But science moved on and sort of forgot about these problems.

So let’s focus on that first deviation period between 17.5 and 14.5 thousand years ago, known as Periglacial or Oldest Dryas when the northern hemisphere seemed to remain as cold as ever while the Antarctic clearly seem to warm and let’s check some recent studies covering that period, in which I converted the carbon dates (Ka BP) to calendar dates (Ka Cal BP) with the INTCAL04 table

Stone et al (1998) obtain Cosmogenic 36Cl ages for two samples from ice-scoured basalt outcrops of 17.6 ± 1.4 and 17.4 ± 1.3 cal. ka BP, at the The Storr, Isle of Skye, Scotland and provide minimal dates for the onset of ice-sheet thinning

A radiocarbon date of 13 870 ± 150 BP (c. 17.0–16.2 cal. ***ka BP) is maximal for ice-sheet deglaciation at Loch Ashik in eastern Skye (Walker et al., 1988; Walker and Lowe, 1990), Ballentyne et al 1998)

Vescovi et al 2007 reconstruct the vegetational history of the southern side of the Alps shows that Alpine deglaciation must have started before 18,000–17,500 cal yr BP south of the Alps

Clark (2003) examines evidence from alpine glacial deposits in the American Cordillera and observes glacial retreats In the Sierra Nevada, between 17,000 and ~15,000 14C yr BP(~20,100-18,500 cal yr B.P. and in the North Cascades by ~17,000 36Cl yr BP; in southern Idaho at 13ka BP (15,3 ka Cal BP).

Sandgren et al (1999) observe that sedimentation of Lake Kullatorpssjön in South Sweden started 14,660 14C years BP, recalibrated to 17,820 Cal BP years, denoting the time of deglaciation.

Andrews (2000) investigates the NE margin of the Laurentide Ice sheet in Canada and observes initial deglaciation at 14.5 ka or 17.5 ka Cal BP.

Kovanen and Easterbrook (2001) report rapid deglaciation in the North Cascades in Washington between 14 500 and 12 500 14C yr B.P (17.5 – 14.7 ka Cal BP)

Ager (2003) analyses the late Quaternary vegetation and climate history of the central Bering land bridge from St. Michael Island, western Alaska and infers a clear warming between 15 and 13 ka BP (ca 18.5 ka – 15.3 Ka Cal BP ).

Glover (2004) find carbon dates of 16500-15000 14C years BP for basin forming associated with glacial retreat, which calibrates to 19,700 – 18.,500 Cal years BP. Clastic to biologic sedimentation transition happened at 13,370 14C years, which calibrates to 15,900 cal years BP.

Hill et al 2006 find also indication early deglacial warmth 2 ka before the formal termination and remark that those findings "are consistent with a growing number of records from around the globe that exhibit pre-Bølling warming prior to Termination IA, and extends the record of such processes to the northern Pacific

Hubberten et al 2004 reconstruct Summer climate changes Laptev Sea area based on a fossil-insect record in the Mamontovy Khayata section, Bykovsky Peninsula (fig 6) and find a substantial summer climate improvement in two steps around 15 ka 14C yeasrs BP and 14 Ka 14C years BP, which calibrated to ~18,5 – 16,7 Ka Cal years BP

Shakun et al 2007 (Jemen) A gradual increase in moisture thereafter was interrupted by an abrupt drying event at 16.4 ka, perhaps related to Heinrich event 1.

Jacobi et al 2007 reconstruct precipitation variations in Northern Brazil for the last 20 ka deduced from biotic δD values, An abrupt change from arid towards much wetter conditions occurred from 17.3 to 16.8 k and coincides with a change from savannah to rainforest taxa. isotope data show only a small rise in aridity during Younger Dryas event (13–11.5 ka).

(quote from a draft article which merely sums up the essence of the studies)

References should be in this list

Denton et al compile the these problems:

GH Denton, WS Broecker, RB Alley, 2006; The mystery interval 17.5 to 14.5 kyrs ago, Past Global Changes (Pages) Volume 14 No 2 August 2006, pp14-17

Abstract
The time period between the beginning of Heinrich event #1 (H-1) and the onset of the Bølling/Allerød rivals the Younger Dryas in importance to our understanding of how the planet responds to abrupt mode switches. This interval also constitutes the onset of the most recent termination, arguably the most fundamental climate shift of the last 100-kyr glacial cycle. As some of the responses during this time appear to be mutually contradictory, we term it the “Mystery Interval”.

Obviously the biggest contradiction is that the isotopes of the Greenland ice cores suggested cold whereas the melting ice sheets and the changes in flora suggested warm in the Northern Hemisphere.
That should have triggered the curiousity, but it hardly did.

 

[Andre]

So following all those contradictions and there are plenty more, there are several more elements to be covered:

The second isotope - temperature contradiction period between roughly 12,670 +/- 25 and 11,560 +/-25 calendar year, known as the Younger Dryas. This intensely studied period is sort of a can of worms with much more complex features.

Alternately we could accept that the many other records on the Northern Hemisphere appear to show the same warming as we see on the Southern hemisphere in the period 17.5 - 14.5 thousand years, which would lead to the more satisfactory contention for now, that the Earth indeed warmed up at both hemispheres simultaneously. This would imply that the isotopes of Greenland are not about temperature.

In that case it would be opportune to investigate what the isotopes are telling us instead and review the hydrological cycle and it's impact on isotope fractination.

Lastly, if this is all correct what would a clearer view of these processes mean for the original substantiation of the AGW theory?

So what next? Any suggestions?