Digital Varves

Varves of the Month for 8/1/2010 - 8/31/2010

Varves from Glastonbury, Connecticut

Scale bar in cm.

Click on image to download original image file

This month's varves are relatively thick red varves from a 152-ft core taken in the parking lot of the Putnam Bridge Center in Glastonbury, Connecticut near the Glastonbury/East Hartford town line. The varves are from a depth of 132 ft. and the split core has been partially dried to make the summer and winter layers more easily discernable. The varves on this image have been matched to the New England Varve Chronology and numbers on the image (2944-2952) are New England varve years. A plot of the outcrop core from which the image was taken and adjacent overlapping cores is also shown below. The varves on the image were deposited at about ~17,900 yr ago in an environment 18 km south of the receding ice front when it was in East Windsor and Windsor Locks, Connecticut.

The varves shown here are typical of varves transitional from ice-proximal to distal with winter and summer layers having about the same thickness. There is a positive correlation between winter and summer layer thickness suggesting that: 1) winter layer clay (dark layers) was introduced to the lake during the summer and 2) time was not a factor in determining the thickness of winter layers. All of the summer layers have many laminations in them that represent melting or storm events during the summer and in some cases possibly diurnal cycles. Some of the varves, especially the thicker ones show thin early melt season layers but all the varves are dominated by their main melt season layers. The early melt season layer is sometimes expressed as a thin red or brown horizon at the bottom of each varve (see for example 2945, 2947, 2948, 2950 and 2951). In some cases this layer is nothing more than a red pigment in a silty layer, possibly the result of clay resuspended by the first melt season currents. Late melt season layers are not discernable except in varve 2945 (the thickest varve on the image) that also has a summer layer that becomes finer upward and shows a rhythmic, possibly diurnal, pattern in its upper part. There appears to be a relatively sharp change from summer to winter layers without the gradation often seen in thicker varves, except in varve 2947 where there is a gradational summer to winter transition. Winter layers in the varves show a very faint gradation in color from olive gray at the bottom to gray with a reddish brown tint near the top (see also last month's varves). This faint color change appears to be gradational and probably reflects grain size gradation (decreasing upward) in the winter layer with iron oxide pigments settling later in the winter than gray clay, which may have started to flocculate immediately after the melt season.

Plot of the New England Varve Chronology for varve years 2900-3000. Shown are Antevs (1928) original "normal curves" for Connecticut (black, CON29-32) and the Hudson Valley (HUD29-32) with matches of 5-ft core sections collected at the Glastonbury drill site including GL2#29-26 (red) and GL1#28-25 (blue). The image of varves above shows varves 2944-2952 in core GL2#27. Click on image to download original file (~3MB)

The varves also show a change from a relatively thick varve interval (2944-2948) to a thinner varve interval (2949-2952). On running average plots of varve thickness these thick and thin cycles are distinct and show sub-century scale changes in varve thickness in response to subtle changes in climate or weather patterns. Relatively warm periods are times of more meltwater production and deposition of thicker varves while cooler intervals were times of less meltwater production and thinner varve deposition. It is tempting to interpret varve thickness simply as a function of temperature, however, there are two factors that control the thickness of varves. Not only is mean annual temperature important but also the length of the melt season. Both of these factors contribute to the number of degree-days in the melt season

Past Varves of the Month...