A varve is composed of an annual sediment layer taken from a core. Like the annual tree ring layers (left), we are interested in the annual varve thickness (right). The Summer Layer: ◦ Lighter in color ◦ Sandy or silty in texture The Winter Layer: ◦ Darker in color ◦ Is often high in clay content and fine sediment A picture of a varve core (http://ase.tufts.edu/geology/varv es/default.asp) Tree ring (NOAA Paleoclimate) Dr. Ridge taking a core with students. (http://ase.tufts.edu/geology/v arves/default.a sp)
Varves can be used as a climate proxy due to the variability in their annual thickness. Thicker varves indicate warmer climate years. Thinner varves indicate colder climate years. Changes in varve thickness can be plotted against time to determine climate variability.
One varve year (summer and winter) is contained between the yellow arrows. The variability in varve thickness can be used as a climate proxy. A long term climate record can be assembled using changes in varve thicknesses in a core. A sample varve that is typical of the New England area (http://ase.tufts.edu/geology/varves/default.asp) Warm Year Cool Year
A warmer climate more meltwater from the glacier thicker varve layers. (More Sediment) A colder climate less meltwater from the glacier thinner varve layers. (Less Sediment) Water from the melting glacier carries sediment into a lake basin. In the picture to the right, we can observe a melting glacier (2003-2005).
Much of the Northeastern United States was covered by ice at one time. The red line indicates the furthest extent of the glaciers. Other lines indicate landforms where the glacier paused in its retreat. Map of NE deglaciation (http://ase.tufts.edu/geology/varves/default.asp) Bethlehem, PA
Glacial Period- A cool period in which glaciers advance. Interglacial Period- A warm period in which glaciers retreat.
Glacier http://earthobservatory.nasa.gov/IOTD/view.php?id=40962 Keep in mind that eventually the ice will retreat away from the lake during a warm period. This means that less sediment will be deposited into the lake.
Lake varves can only be found in regions that were affected by glaciers. The varves for this activity come from Dummerston, VT. Scientists study varves in this location because there used to be a large glacial lake where these lake varves could form.
Varves can be measured in section from bottom to top (right) Small sections of a varve chronology close to the surface can be cored. Cores can also be retrieved using large mechanized drilling system (pictures to the left) A student measuring a varve in section from bottom to top. (http://ase.tufts.edu/geology/varv es/default.asp) Once back in lab cores can be photographed and the thickness of each varve layer can be measured. Continuous Deep Drilling Truck Multiple drill bits that each contain a varve core A varve core packed in plastic tubing for transport to the lab and analysis
Each group will measure varve thicknesses within a piece of a core section. This section represents a part of a long term record (~300 years). Measurements will be recorded in a data table and then plotted on the appropriate graph paper. Graphed data will be aligned to form a paleoclimate reconstruction using lake varve measurements.
New England Varve Year Measurement (cm) 5685 2.71 5686 2.09 5687 3.00 5688 2.74 5689 1.55 5690 0.63 56911.57 5692 1.75 5693 3.13 5694 2.51 5695 0.93 5695 5685 Measurement Table 5690 1. Measure the width of each varve layer (cm) 2. Record the data in the measurement table to the right. 5693
3. Plot the points from the previous table onto the associated graph and connect the dots.
Keep in mind that glaciers advance and retreat. ◦ This greatly influences the amount of sediment deposited into a glacial lake. As glaciers retreat the amount of sediment being deposited into a glacial lake decreases. It is sometimes difficult to accurately interpret a varve record. ◦ Keep this in mind as you interpret this paleoclimate record.
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