Wednesday, July 10, 2013

The poop on worm poop as a paleotemperature marker, corrected and updated

by Joan Savage

Why are researchers about paleoclimate so interested in earthworms, or actually in one kind of earthworm?

The common nightcrawler earthworm, Lumbricus terrestris, produces castings year round that include calcium carbonate granules. Not exciting you might think, and even with a vivid imagination,  no, no, earthworm farming is not the latest foolish idea for carbon dioxide sequestration.

Photo of Lumbricus terrestris from wikipedia commons

The calcium-carbonate-flecked worm poop was among those curiosities of nature known to few, and among the few were several scientists collaborating in the UK,  one group is Emma A.A. Versteegh, Stuart Black  Matthew G. Canti  Mark E. Hodson.
Their article Earthworm-produced calcite granules: a new terrestrial palaeothermometer? is fortunately Open Access. What they  produced is a newprocedure for tracking LOCAL temperature conditions over thousands of years in many areas of the planet. This method uses  δ18O values, as determined on individual calcite granules. As they say, "As the granules are abundant in modern soils, buried soils and archaeological contexts, and can be dated using U-Th disequilibria, the developed palaeotemperature relationship has enormous potential for application to Holocene and Pleistocene time intervals."
The correlation to  δ18O data by Kim & O'Neil was at p < 0.0005

In the world of science, particularly climate science, that's a rock star. 

The more recent of the two article's authors are Loredana Brinza, Paul D. Quinn, Paul F. Schofield,  J. Frederick W. Mosselmans, and Mark E. Hodson.  The Brinza et al article published earlier this week in Geochimica et Cosmochimica Acta, "Incorporation of strontium in earthworm-secreted calcium carbonate granules produced in strontium-amended and strontium-bearing soil," was treated like a rock star, possibly because it was confused with the previous article's bolder stance. The title and abstract modestly didn't hint at the implication of their work.
The full article is available for free for what you can read in five minutes! The full article is 17 pages including references. I raced to the conclusions which were both optimistic about the eventual use as a paleoproxy, yet they cautioned that more research is needed on soil conditions and temperature. What had been a variable in the study was the amount of strontium in the soil.
Basically, here's how the Sr/Ca works. The ratio of strontium to calcium in calcium carbonate formation is affected by temperature, but strontium90 presence has also been used to monitor nuclear fallout.  The Sr/Ca ratio has been tracked in coral reefs, over 10^5 years to within 0.5 degrees C accuracy when tested against oxygen isotope records.  That's over 100,000 years, count those zeroes.  So, the earthworms' casting granules with their distinct Sr/Ca ratios are miniature temperature records for the time that they pooped, but soils are not the watery solutions from which corals took their minerals.  

Keep both the bold Versteegh and the cautious Brinza articles in mind.  One's ready to roll and the other might prove useful, too.

Where has the nightcrawler been in the Holocene and Pleistocene? 

In the now-rare definitive work by John W. Reynolds, The Earthworms  (Lubricidae and Sparganophilidae) of Ontario,  Reynolds cites Gates (1972).  Lumbricus terrestris Linnaeus 1758 is a native of Palearctis, now known as Europe, Iceland, North America, South America, Siberia, South Africa, and Australasia.

Reynolds also comments on a belief that the worms had been introduced to North America from Europe, which he corrects to say the worms were native south of the last continental glaciation.  That does suggest that eager researchers may be limited to recently re-introduced post-Pleistocene worm castings in forests of Wisconsin, upper New York, Canada, and other areas that were glaciated up to about 11K years ago.

Palearctis leaves out North Africa and Antarctica. Australasia doesn't include mainland Asia, the subcontinent of India or the Middle East, either.

Even so,  what an opportunity to pinpoint temperature conditions at archaeological sites that have been dated by carbon isotopes, or in soils or geologic strata!   It's not just for coral and mussels anymore.

I am grateful to BBC news for correctly reporting that the Versteegh et al article was the breakthrough.

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