Annual layers

To infer accumulation rate history from an ice core, one needs to measure the thickness of annual layers (either directly if annual layers are resolvable, or by differentiating the depth-age scale determined by other means) and then correct for the thinning of these layers caused by the ice flow (the vertical strain Fig. 18-4). Estimates of vertical strain can be very uncertain for the deep part of an ice core. But vertical strain will not change rapidly with depth. Thus, if annual layers are resolvable one can learn relative accumulation rate changes across climate transitions with great confidence. 

Patterson s friend Edward D. Goldberg at the Scripps Institution of Oceanography in La Jolla, California, had tipped him off that one of the best records of the world s climate is embedded in thin layers of glacial ice at high altitudes or near the poles. Snow, dust, and fog deposit chemicals from the atmosphere onto the ice, where they remain undisturbed for thousands of years. As Patterson quickly realized, Only the quiescent ice sheets in the arid, perpetually frozen polar regions of the Earth provide annual layers of precipitation that are undisturbed by percolation and mechanical mixing, that are relatively free of dusts and salts, and also are thin enough to be accessible even when centuries old. ... 

A logical explanation for the global nature of these correlations is that they are all related to variations of the sun, which cause variations in the temperature of the sea surface, thus causing variations in the isotopic composition of water vapor which distills off the sea and is stored as wood in trees and also forms the annual layers of the ice cap. The variations of the sun are furthermore related to the flux of solar neutrons in the earth s atmosphere and so cause small variations in the carbon-14 content of the bristle cones. During times of a quiet sun the average carbon-14 production is about 25 percent larger than when solar activity is high [43]. 

Glaciochemical horizons are intervals of core with substantially higher or lower than average concentrations of certain chemical constituents. If a historical event of known age can be correlated with the event horizon in the core, the assigned age of that interval can be used to confirm the depth-age relationship which has been determined from seasonal variations or other dating methods. In addition, in deep ice where annual layers are too thin to count seasons reliably and dating is only possible by model calculations [15,30], these horizons provide check points for calculated ages. 

In summary, a number of chemical constituents in polar snow and ice have seasonal concentration variations that make them suitable for dating ice cores by counting annual layers. In... 

The climate has the ability to shift into radically different states according to ice cores extracted from Greenland s massive ice sheet in the early 1990s. These rods of ice are up to three kilometers long and provide a set of climate records for the past 110,000 years. They allow the investigation of annual layers in the ice cores which are dated using a variety of methods. The composition of the ice provides the temperature at which it formed. 

Fig. 4 Depth-age relationship of the ice cores from Fiescherhom glacier [12] and Colle Gnifetti [13, 14], Besides annual layer counting and radiocarbon ( C) dating, two types of time markers were used Saharan dust events (labeled by the year only) and volcanic eruptions (labeled by year and name of volcano). Depth is given in water equivalent. This is the amount of water contained in the ice core which is calculated using fim and ice density, respectively, both increasing with depth...

The oxygen-isotope ratio ( 0/ 0) found in the calcareous tests has been used as an indicator of the amount of water locked up in glaciers, which reflects the state of the global climate. Another source of continuous records of oxygen-isotope variation, as well as dust content and acidity, is found in the long cores drilled in polar ice caps in which annual layers can be counted and seasonal variation can be established [65]. 

The measurements on Fairbanks Barbados corals spurred efforts to find other means of extending the calibration curves back in time. Several tacks were taken. One obvious strategy was to count annual layers (varves) in lake and marine sediments. Smiver, the hero of the calibration effort, had adopted this approach way back in the... 

Whereas interval dating can be carried out by counting annual layers (e.g. Baldini et al., 2002), radiometric techniques are needed to provide the... 

McMillan, E., Fairchild, I.J., Frisia, S. Borsato, A. (2005) Calcite-aragonite trace element behaviour in annually layered speleothems evidence of drought in the Western Mediterranean 1200 years ago. Journal of Quaternary Science 20, 423 33. 

Railsback, L.B., Brook, G.A., Chen, J., Kalin, R. Fleischer, C.J. (1994) Environmental controls on the petrology of a late Holocene speleothem from Botswana with annual layers of aragonite and calcite. Journal of Sedimentary Research A64, 147-155. 

Fig. 7.5 Section of a Greenland ice core with visible annual layers...

NSIDC (National Snow and Ice Data Center) (1997) The Greenland Ice Cores CD-ROM. Available from the National Snow and Ice Data Center, University of Colorado at Bonlder, and the World Data Center-A for Paleoclimatology, National Geophysical Data Center, Boulder Colorado, http //www.ngdc.noaa.gov/paleo/icecore/greenland/summit/index.html Nye IF (1963) Correction factor for accnmnlation measnred by the thickness of annual layers in an ice sheet. J Glaciol 4 785-788... 

The acid concentration in ice core layers as a function of depth as determined from electrical measurements. The magnitudes of some volcanic eruptions in the Northern Hemisphere have been estimated from the acidity of annual layers in ice cores taken in Greenland. This methodology is sometimes referred to as acidity signal or acidity record. 

Annually laminated sediments Sediments with annual layers (usually couplets, i. e., varves) produced by the seasonal delivery of minerogenic material, by seasonal precipitation and/or biological activity. Formed in deep lakes with oxygen-deficient bottom waters and little or no bioturbation one yearly package is often called a varve. 

Chronology 1) A data series or framework referenced with respect to time (i.e., a time series). For lake sediment records, often the chronology describes the sediment depth-age relationship. 2) Time scale, which may be obtained through radiometric dating (e.g., C, U/Th) or counting of annual layers. 

This technique of dating ice cores by counting annual layers, identified either visually or isotopically (or both), has been widely used to develop climate histories for the ice sheets in East and West Antarctica. However, the method is limited to about 1,000 years by the progressive thinning of annual layers with depth in the ice sheet. 

Fig. 17.29 The thickness of the annual layers of ice in the Vostok core (Fig. 17.28) decreases with increasing depth in the upper part of the core and approaches a constant value below a depth of about 1,600 m. This property causes the age of ice in a long core to vary non-linearly with depth (Derived from Figure 1 of Lorius et al. 1985)...

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