Climate change: Warmer... but why? | Upsetting the balance | Mammals on the move
Connecting climate and disease | How do sea shells record temperature change?
Taking Earth's temperature

 

Unlike areas that have been inhabited for long periods – where people have measured temperatures every day for hundreds of years – there are few instrumental records of temperatures on and around the Antarctic continent that extend back to the beginning of the industrial revolution. That's a problem for scientists who want to know how the area has been affected by global warming. Shells from the Antarctic can help them reconstruct climate.

 

Getting data from a shell

 

 

Professor Kacey Lohmann of the U-M Department of Geological Sciences has performed detailed chemical analyses on growth bands in the shells of many Antarctic scallops and other species of shelled marine invertebrates. In doing so, he has been able to piece together Antarctica's temperature history with the data that is stored in the shells.

 

  In the photo above, Professor Lohmann uses a computerized milling system to take samples for analysis. Only very small samples are needed.
Life below freezing


Adamussium colbecki is a species of scallop, a shelled marine animal. It lives in waters off the coast of Antarctica. This is very cold water, with a temperature of 28° F. That’s four degrees below freezing. Because marine waters are salty, they can maintain temperatures below the freezing point without turning to ice.

 


The specimen to the left was found near McMurdo Bay by biologist Paul Berkmann of the University of California at Santa Barbara. Professor Berkmann dove under the ice where he collected this scallop, measured and tagged it, and released it during the 1984-85 field season. In January of 1997 it was found again, and collected for analysis.



How can a shell record temperature?

When air temperatures on Antarctica rise, the continent's glaciers melt and retreat, and the meltwater runs off into coastal waters, dramatically changing the water chemistry. Each growth band in the shell reflects the chemical signature of the water – and, indirectly, the continent's temperature – at the time the band formed. Because Adamussium colbecki is a type of scallop that can live 100 years or longer, shells collected today may contain records of the Antarctic environment going back to the beginning of the century. Professor Lohmann reads these records by analyzing ratios of different isotopes (alternate forms) of oxygen in the growth bands.




This close-up photo shows the sampling path
milled across the surface of the shell, going down
from the top edge on the left side.

 

 

Each sample is placed in a small container called a boat, and placed into a holder called a convoy.



 


The sample powders from the shells are put into a machine called a mass spectrometer, pictured to the right. This machine dissolves the samples and analyzes them to determine ratios of oxygen isotopes in the growth bands.

 

 

 



The graph illustrates the similarity in the records of temperature variation measured from the scallop shell isotope data, and recorded air temperatures from Orcadas Station in the South Orkney Islands near Antarctica. This station was first inhabited in 1904 and provides the longest instrumental temperature record for the Antarctic region, spanning from 1904 to 2003. The similarity of variation in the shell isotope record and the measured air temperatures shows the usefulness of the shell as an archive for reconstructing past temperature changes where no air temperature records exist. Blue areas denote times when the climate was cooler, in contrast to yellow areas, which denote the warm times.


Evidence of global warming

Professor Lohmann uses the oxygen isotope data to calculate what the air temperature was when a particular growth band was produced. Because of the high resolution of the computer milling, individual samples can represent intervals of time as small as two weeks or less. This results in a set of data giving the variability and range of local temperature over the year. Like everywhere else in the world, Antarctica shows evidence of warming during the past 50 years.

Ensuring scientific accuracy

Professor Lohmann's research is focused on trying to get accurate, detailed data of past climates. Atmospheric scientists can then use these data in the climate models they construct to predict what our actions will do to the global climate in the future. Says Professor Lohmann: "I'm trying to ensure that their computer models provide accurate predictions." One of Professor Lohmann's current projects is to accurately calibrate measurements of how much Antarctic ice is actually melting as global temperatures rise.

 

Along with analyzing shells from Antarctica’s recent past, Professor Lohmann is working on reconstructing the very long-term climate history of the continent. He analyzes data gathered from fossil clam shells found off the coast of Antarctica on Seymour Island.
Because fossilized clam shells still contain the actual material that made up the shell of the live animal, Professor Lohmann is able to analyze them in the same way as shells from recently living scallops.

 

This fossil clam, called Cucullaea raea, was found on Seymour Island, and is 40 million years old. It has been cut in half to allow thin slices to be taken from it for chemical analysis. (Click image for higher resolution)
This thin section of fossil clam shell was embedded in resin and glued to glass for support so it could be milled to get samples for analysis. From this and other data, we know that when this fossil clam was alive, Antarctica was much warmer than it is now—there were palm trees and crocodiles living there. (Click image for higher resolution)

 

 


Climate change: Warmer... but why? | Upsetting the balance | Mammals on the move
Connecting climate and disease | How do sea shells record temperature change?
Taking Earth's temperature