Class Notes

• page 2
• page 3
• page 5
• page 6 - Volcanoes
• page 7 - Sources of Paleoclimatic Data
• page 9
• page 10
• page 12 - Tree rings
• page 13 - 18O/16O ratios in Greenland Ice
• page 14
• page 15 - Laurentide Ice Sheet
• page 16 - Land/Sea Ice
• page 17 - Ice Sheets
• page 18
• page 19 - Earth today

Ice Core Data

• Cores drilled in ice caps or glaciers hold a record of past climate in their layers of compressed snow.

• Trapped bubbles and dust grains within the ice record the composition of the atmosphere at the time of deposition.

• Isotopes and dust content are then used to reconstruct climate histories.

Tree Ring Data

• Trees record environmental conditions in their annual growth rings, and can therefore be used as a proxy for past climates.

• Data include ring width and wood density measurements, stand averages known as site chronologies, and reconstructions of past temperature and/or precipitation derived from tree-ring data.

Paleovegetation Data

• Pollen grains trapped in the sediments of lakes and bogs indicate the assemblage of plants growing at the site at the time of deposition.

• The plant community is, in turn, an indicator of the prevailing climate conditions.

Paleoceanographic Data

• A number of climate sensitive parameters are recorded in ocean sediments, including plankton abundances, isotope ratios, and carbonate content.

Methods of Dating Ice Cores

• Ice cores provide continuous information on key properties of paleoclimate including local temperature and precipitation rate, humidity, and wind speed.

• Ice cores also record changes in atmospheric composition. They can be used to measure trace gas concentrations, chemical impurities of terrestrial and marine origin, other trace compounds or isotopes, cosmogenic isotopes, extraterrestrial material, and aerosols of volcanic and anthropogenic origin.

• Counting of Annual Layers
  • The basis of this method lies with looking for items that vary with the seasons in a consistent manner.

  • Of these are items that depend on the temperature (colder in the winter and warmer in the summer) and solar irradience (less irradience in winter and more in summer).

  • Once such markers of seasonal variations are found, they can be used to find the number of years that the ice-core accumulated over.

  • This process is analagous to the counting of tree rings.

  • A major disadvantage of these types of dating is that they are extremely time consuming.

• Temperature Dependent
  • Of the temperature dependent markers the most important is the ratio of ¹⁸O to ¹⁶O.

  • The water molecules composed of H2(¹⁸O) evaporate less rapidly and condense more readily then water molecules composed of H2(¹⁶O).

  • Thus, water evaporating from the ocean it starts off H2(¹⁸O) poor.

  • As the water vapor travels towards the poles it becomes increasingly poorer in H2(¹⁸O) since the heavier molecules tend to precipitate out first.

  • This depletion is a temperature dependent process so in winter the precipitation is more enriched in H2(¹⁶O) than is the case in the summer.

  • Thus, each annual layer starts ¹⁸O rich, becomes ¹⁸O poor, and ends up ¹⁸O rich.

  • This process also depends on the relative temperatures of different years, which allows comparison with paleoclimatic data.

  • For similar reasons the ratio of deuterium to hydrogen acts the same way.

  • The major disadvantage of this dating method is that isotopes tend to diffuse as time proceeds.

• Previously Measured Ice-Cores
  • In this method one compares certain inclusions in a ice-core whose age has been determined with a seperate method to similar inclusions in an ice-core of a still undetermined age.

  • These inclusions are typically ash from volcanic eruptions and acidic layers.

  • The major disadvantage of this method is that one must have a previously age-dated ice core to start with.

• Volcanic Eruptions
  • After the eruption of volcanoes, the volcanic ash and chemicals are washed out of the atmosphere by precipitation.

  • These eruptions leave a distinct marker within the snow which washed the atmosphere.

  • We can then use recorded volcanic eruptions to calibrate the age of the ice-core.

  • Since volcanic ash is a common atmospheric constituent after an eruption, this is a nice signature to use in comparing calibrated time data and an ice-core of undetermined age.

  • Another signature of volcanism is acidity.

  • The major diasadvantage of this method is that one must previously know the date of the eruption which is usually not the case.

  • Furthermore the alkaline precipitants of the ice ages limits this measure to approximately 8000 BC.

• Oceanic Cores
  • In this method one compares certain inclusions in dated ocean cores with related inclusions found in the ice-core of a still undetermined age.

  • Examples of such inclusions are a decrease (or increase) in temperature over a period of years that can be determined from flora and fauna found in the oceanic core and a decrease (increase) in the 18O enrichment over this same period of years.

  • Another example is volcanic ash.

  • The major disadvantages of this method are that one must compare different signatures of climatic change that correspond to the same event and that one is not certain of the lag times (if any) between oceanic reactions and glacial reactions to the same climatic changes.

• Ph Balances
  • One unique marker of periods of glaciation is that precipitation during the ice ages are markedly alkaline.

  • This is due to the fact that the ice ages tied up a large quantity of the available water thus exposing a larger portion of the continental shelves.

  • From these shelves huge clouds of alkaline dusts (primarily CaCO₃) were blown across the landscape.

  • The major disadvantage of this method is that it gives only very approximate age ranges (i.e. this ice was laid down during the ice age).

  • Furthermore, the lag time between the onset of glaciation and increased alkalinity are uncertain.

• Paleoclimatic Comparisons
  • In this method, one compares long range climatic changes (e.g. ice ages and interglacial warmings) with markers (such as the ¹⁸O/¹⁶O ratios) found within the ice-cores.

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• The Vostok Ice-Core

• Antarctica Map
  • The history of the world's climate is recorded in the layers of sediment that accumulated over thousands of years in ice and rock. Paleoclimatologists are studying sediment encapsulated in deep Antarctic ice to answer a few perplexing questions about the conditions that prevailed during the ice ages.

  • Ice cores from Vostok, Antarctica, were the first to cover a full glacial-interglacial cycle.

  • The Vostok Ice-Core was collected in East Antarctica by the Russian Antarctic expedition.

  • The Vostok Ice-Core is 2,083 meters long and was collected in two portions: 1) 0 - 950 m in 1970-1974, 2) 950 - 2083 m in 1982-1983.

  • The total depth of the ice sheet from which the core was collected is approximately 3,700 meters.

  • The ice core was sliced into 1.5-2.0 meter segments.

  • A discontinuous series sampled every 25 meters and a continuous series from 1,406 to 2,803 meters were then sent in solid form to Grenoble, France for further analysis.

  • The samples were crushed and then melted with the gases given off collected and saved for further analysis.

  • The melt water was tested for chemical composition and then electrolysised.

  • The methods used in the determination of the ages include:
    • ¹⁸O/¹⁶O isotopic analysis

    • comparison with other ice cores

    • comparison with deep sea cores

    • 10Be/9Be isotopic analysis

    • deuterium/hydrogen isotopic analysis

    • CO2 correspondances between dated ice-cores and CO2 correspondances with dated oceanic cores

  • The results determined from these various samples were consistent between the continuous and discontinuous slices within the sections that overlapped.

  • They were also consistent with Greenland ice-cores, other Antarctic ice-cores, dated volcanic records, deep sea cores, and paleoclimatic evidence.

  • The analysis show definate evidence of the the last two ice ages.

  • Using the methods listed above the bottom of the ice-core was laid down 160,000 +- 15,000 years ago.

  • Air initially enclosed in Vostok ice provides our only record of variations in the atmospheric concentrations of CO₂ and CH₄ over a complete glacial-interglacial cycle. For both greenhouse gases, concentrations are higher during interglacial periods than during full glacial periods. Since preindustrial times, levels of CO₂ and CH₄ have increased sharply. A close correlation between these gas concentrations and the Vostok isotopic temperature has been confirmed by extending the record over part of the previous cycle. However, at the end of the last interglacial, the CO₂ decrease significantly lags Antarctic cooling, while CO₂ and Antarctic temperatures increase during the warmings of the glacial-interglacial transitions. Interestingly, at least during certain deglaciation periods, the trace gas increase precedes the onset of most melting of the northern ice sheets by several thousand years.

  • From a climatic viewpoint, CO₂ and CH₄ have played an important role. Together with the growth and decay of the Northern Hemisphere ice sheets, these greenhouse gases have amplified the initial orbital forcing, and they account for about half of the glacial-interglacial climate changes. This supports the idea that significant greenhouse warming will occur in the next century.