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Andre
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Glacier retreat and dating problems.We are (still) interested in the correlation between the ice core *paleo-temperatures* and the retreat of the glaciers. This should be quite familiar:
So it is rather widespread believed that the Last Glacial Maximum glaciers really retreated when the *temperature* jumped allegedly 10 degrees or so at the beginning of the Bolling Allerod around 14500 years ago.
So for correlating glacier datings we need accurate dates but we know that there were some complications with the most widely used dating method, carbon dating, which is solved nowadays with accurate calibration tables like INTCAL04. However these correction had to be develloped and became available only slowly in the 1990ies. So how about older and even newer carbon dating. Are the correct calibrations carried out and what if not?
Let's compare two papers on glacier activity during the last glacial maximum as recorded in accumulation of "glacial flour" in lake records.
| Quote: | Bischoff, J.L. K. Cummings 2001, Wisconsin Glaciation of the Sierra Nevada (79,000–15,000 yr B.P.) as Recorded by Rock Flour in Sediments of Owens Lake, California Quaternary Research 55, 14–24 (2001) doi:10.1006/qres.2000.2183,
Chemical analyses of the clay-sized fractions of 564 continuous sediment samples (200-yr resolution) from composite core OL90/92 allow quantification of an abundance of glacial rock flour. Rock flour produced during glacier advances is represented by clay-sized plagioclase, K-feldspar, and biotite in homogeneous internal composition. The abundance of rock flour is deemed proportional to the intensity of glacier advances. Age control for the composite section is provided by combining previously published radiocarbon dates on organics, U/Th dates on ostracode shells, and U/Th dates on saline minerals from nearby Searles Lake correlated to OL92 by pollen.
The rock flour record displays three levels of variability: (1) a dominant one of about 20,000 yr related to summer insolation and precipitation; (2) an intermediate one of 3000–5000 yr, perhaps related to North Atlantic Heinrich events; and (3) a minor one of 1000–2000 yr, perhaps related to North Atlantic thermohalinedriven air-temperature variation. |
| Quote: | Rosenbaum J.G., R.L. Reynolds 2004 Record of Late Pleistocene glaciation and deglaciation in the southern Cascade Range. II. Flux of glacial flour in a sediment core from Upper Klamath Lake, Oregon Journal of Paleolimnology 31: 235–252, 2004. # 2004 Kluwer Academic Publishers
Abstract
During the late Wisconsin, glacial flour from alpine glaciers along the east side of the Cascade Range in southern Oregon was deposited in Upper Klamath Lake. Quantitative interpretation of magnetic properties and grain-size data of cored sediments from Caledonia Marsh on the west side of the lake provides a continuous record of the flux of glacial flour spanning the last ~37 000 calendar years. For modelling purposes, the lake sediments from the 13-m core were divided into three sedimentary components defined from magnetic, geochemical, petrographic, and grain-size data. T
he components are (1) strongly magnetic, glacial flour made up of extremely fine-grained, fresh volcanic rock particles, (2) less magnetic lithic material made up of coarser, weathered volcanic detritus, and (3) non-magnetic biogenic material (largely biogenic silica). Quantitative interpretation is possible because there has been no significant postdepositional destruction or formation of magnetic minerals, nor alteration affecting grain-size distributions.
Major steps involved in the interpretation include: (1) computation of biogenic and lithic components; (2) determination of magnetic properties and grain-size distributions of the non-glacial and glacial flour end-members; (3) computation of the contents of weathered and glacial flour components for each sample; (4) development of an age model based on the mass accumulation of the non-glacial lithic component; and (5) use of the age model and glacial flour contents to compute the flux of glacial flour.
Comparison of the glacial flour record from Upper Klamath Lake to mapped glacial features suggests a nearly linear relation between flux of glacial flour and the extent of nearby glaciers. At ~22 ka, following an extended period during which glaciers of limited size waxed and waned, late Wisconsin (Waban) glaciers began to grow, reaching their maximum extent at ~19 ka. Glaciers remained near their maximum extent for ~1000 years. During this period, lake sediments were made up of ~80% glacial flour. The content of glacial flour decreased as the glaciers receded, and reached undetectable levels by 14 ka. |
Let's compare in the next posts how these dates were generated.
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