Brown's Lake Bog and the 8200 yrBP Event (Lutz et al., 2007)

A unique bog managed by The Nature Conservancy

Also the subject of a senior IS project by Clint Bailey

 

Abstract
Many Northern Hemisphere paleoclimatic records, including ice cores, speleothems, lake sediments, ocean cores and glacier chronologies, indicate an abrupt cooling event about 8200 cal yr BP. A new well-dated series of sediment cores taken from Brown’s Lake, a kettle in Northeast Ohio, shows two closely spaced intervals of loess deposition during this time period. The source of loess is uncertain; however, it is likely a sensitive site because of an abandoned drainage and former glacial lake basin located to the north of the stagnate ice topography that gave rise to the kettle lake. Strong visual stratigraphy, loss on ignition data, and sediment grain size analyses dated with 3 AMS radiocarbon dates place the two intervals of loess deposition between 8950 and 8005 cal yr BP. The possibility of a two-phase abrupt climate change at this time is a finding that has been suggested in other research. This record adds detail to the spatial extent and timing as well as possible structure of the 8.2 ka abrupt climate change event in Ohio.

Brian Lutz and Tom Lowell (left) coring along the shore of Browns Lake. On the right is a SEM photograph of a fly that was recovered from one to the Browns Lake cores. Note that the fly has wings in tact and was likely not transported or reworked.

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Figure 1. (A) Location of Brown’s Lake Bog (BLB) and some of the other sites reporting evidence of the 8.2 ka event in North America: a) White Mountains (Kurek et al., 2004), b) North Pond (Shuman et al., 2002), c) Crawford Lake (Yu and Eicher, 1998), d) Deep Lake (Hu et al., 1999). (B) Bathymetric map in meters of Brown’s Lake Basin and core sites (numbers indicate core site locations).


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Figure 2. Photographs of core lengths containing the 8.2 ka horizons. [All 3 cores are the same scale; the loess layers get smaller from core 1 to core 3 due to the sediment dynamics of the lake basin where each core was retrieved.] Solid lines to the left of the cores illustrate the extent of the loess layers. Letters to the left of the cores indicate where material was collected for AMS dating (See Table 1). LOI and grain size data from core 3 is scaled to the same units as the core 3 image and 0cm is the midpoint between the two loess layers.

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Figure 3. Comparison of late Wisconsin loess (bold solid line) to samples taken from the silt layers in core 3 from the Brown’s Lake site (cm measurements in legend correlate to Figure 2 and indicate from which silt layer each sample was taken). As illustrated, the characteristics of the sediment in the silt layers in the Brown’s Lake record are nearly identical to the loess standard.

Discussion and Conclusions
The recognition of the 8.2 ka event at Brown’s Lake adds to the growing recognition of the widespread impact of this event (Alley and Ágústsdóttir, 2005). The double loess layer suggests two discrete events of loess mobilization during this interval. Hu et al. (1999) summarized the dating and oxygen-isotope composition of sedimentary carbonate in cores from Deep Lake in Minnesota and found evidence for an abrupt cooling event between 8900 and 8300 cal yr BP. The carbonate composition did not, however, indicate an abrupt change during the 8200 interval. The only change in their core around 8200 cal yr BP was an increase in varve thickness due to loess deposition, which suggests the possibility that there were two closely-spaced, yet independent abrupt changes about this time.

Hu et al. (1999) suggested that the first event was characterized by decreased precipitation temperatures, increased polar air outbreaks, and snowfall accounting for a greater amount of winter precipitation, whereas the second event resulted from sustained windy conditions depositing aeolian dust. At Brown’s Lake both events appear to be characterized by similar conditions—sustained periods of loess deposition. Moreover, because there is some disagreement when the two events occurred in each of our cores, it is possible that the Brown’s Lake and Deep Lake events are not the same.
Baldini et al. (2002), in a study of a high-resolution speleothem record in western Ireland, have also identified the 8.2 ka cooling as being composed of two discrete events.. Two shifts in strontium and phosphorus concentrations centered on 8330±80 cal yr BP are consistent with the Brown’s Lake record. However, Baldini et al. report that the cooling events occurred on a multi-decadal scale whereas the Brown’s Lake data indicates that the events may have taken place over a longer time period of approximately 320±40 years.

More support for the double event is gained from high resolution, well-dated sediment cores retrieved in the North Atlantic off of the eastern coast of Canada. In these cores, Keigwin et al. (2005) identified a clear change in the species composition and ?18O of planktonic foraminifera species sensitive to changes in sea surface temperatures. Their record also clearly indicates that the cooling was a two-stage event and it had an onset and duration very similar to the Brown’s Lake record.
All of these well-dated records, including our findings from Brown’s Lake, are increasing our knowledge of both the spatial extent and characteristics of the 8.2 ka event. Although much work remains to fully characterize this cooling, we anticipate that future investigations are likely to further support this two-phase attribute given the strength of the signal as it has been identified at these sites throughout the Northern Hemisphere. We also hope that this work calls attention to the sediment records from kettle lakes in North America, similar to Brown’s Lake, where changes resulting from the drainage of the glacial lakes thought to trigger this event may be recorded in high resolution.

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