LATEST PROJECT NEWS
First, the MPEP Team has been invited, both formally and informally, to present papers regarding its work to date at the Annual Meeting of the Northeastern Section of the Geological Society of America this coming March in Portland, Maine. These invitations are the result of the scientific interest our project is generating in the post-glacial geologic and paleolimnological communities of the northeastern U.S. and adjacent Canada during the past year. This is an important opportunity for us to present and discuss our current findings with others active in the same scientific fields, and we're excited to be a part of such a process. The abstracts for each presentation can be found by clicking on the following link icon:
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| Abstracts |
As you already know, we obtained a new basal carbon-14 date for the bottom of the Pond that shows when the influences of the last glacial ice left the basin (~14,000 calendar years ago; see earlier Post). Since that time, we've analyzed the bottom 6 core samples (6 more to go), and have established that while the temperature of the Pond's water varied somewhat from time to time, the organic content of the sediments increased fairly steadily after the Pond was de-iced up to about 9,660 calendar years ago. At that point, something unusual occurred here, and right now we don't know precisely what.
Near the top of core sample T-7, we found two +/- 1 inch-thick light tan to yellow colored bands about 2 inches apart within the otherwise dark greenish brown to black sediment. These were the first such variations we found up to that point in the otherwise consistent sedimentation shown in the cores, and we immediately determined that they must represent some significant change in the environment around the point in time. We initially thought, based on what we estimated the likely rate of sedimentation in the Pond basin to be, that these bands might be the local representation of an important global cooling event that occurred 8,200 calendar years ago. If it did, it would be a very significant finding.
Accordingly, we carefully sampled each band to check their organic content, the relative temperature of the water at the time (from the species of chironomids present in the band's sediment), and their age via a sample for carbon-14 (AMS) dating taken from the regular sediment in the area between the two bands. The date came out at ~ 9,660 calendar years - too old for the bands to be related to the 8,200-year cooling event, but the organic content and the relative temperature of the water both drop significantly within each band. Thus, the bands do represent two significant cooling events of perhaps 100-years each with a break of maybe 150-200 years in between. We've tried to specifically relate the date of these bands with other known climatic cooling events, but have so far been unsuccessful.
Microscopic observation of the contents of the sediment within the bands after all organic material has been removed show they are composed of a combination of millions of siliceous diatom remains intermixed with a generally light-tan to yellow, but in some places orangey and pinkish residue that survived the high heat of the organic removal process with its various colors intact. We don't yet know what this residue is composed of, but we've sent samples out for geochemical analysis. When we get the results and combine them with the pollen analyses that are to begin soon on all the cores, we should be able to determine what these unusual sediments indicate about climate changes in the Pond basin back about 9,600 years ago.
We, of course, will keep you posted here on the blog in the next few weeks. Meantime, if you have comments or questions, please contact us by commenting here on the blog (instruction for doing so on the homepage). So far, very few folks have chosen to comment, but please do. It's easy, and it will be great to hear what you think the bands may represent about our Pond "back in "The 96-Hundreds".
4 comments:
Where there any glacial lakes in the reagion that may have begun draining into GPPP and reduce it's water temp?
Dan Corindia
Hi Dan:
Not as far as is presently known. By the time the lighter colored bands were apparently deposited, the Connecticut Valley Lobe of the Laurentide Ice Sheet was far to the north of the BPPP region. The Bethlehem Moraine System, near where you and Chris worked, is almost certainly of Older Dryas age and thus significantly older than the bands, and it was being actively deposited there about the same time as the first organic material begen to be deposited in BPPP. So even at that earlier date, active ice lay better than 20miles to the north and well beyond the BPPP basin.
Anyway, we're still trying to establish the rest of the data in re: these lighter bands. Pollen analysis, which may really define the local climate for the period, will hopefully get underway in the next month or two. We'll keep everyone posted via the blog.
By the way, thanks for your comment! We wish more folks would get involved like you have. Also, let me know if you've any other ideas or possibilities to consider. With your experience with this same kind of project, I'd appreciate any ideas/comments.
Brian
Brian,
Rereading the post you said the layers are mostly siliceous diatoms, right? do we know if tey are algae diatoms or other? Is the 9.6K event corrolated in an of the other cores? If not are we looking in the wrong direction (cooling). Maybe it was a sustained algea bloom? Just brainstorming form the other side of the world!
Dan
Hello Brian and Dan (Dan, I'm Brian's neighbor who is doing the chironomid/temperature proxy part of the analysis),
Just a couple of contributions to the thinking about the "yellow bands". A first rather cursory look shows most of the diatom frustules to be the planktonic alga, Melosira, or something close to it. I gather that Melosira is a common cold-water bloomer, e.g., in the Great Lakes today. So the possibility does exist that these bands may have accumulated over a brief period (rather than a millimeter per year, in which case the 1" band would represent ca 250 yrs) and could represent a couple of dramatic surges in diatom productivity. At the moment, we have just the one date, between the two bands. Another date from just below or above the bands would help solve the timing issue. Also, Lisa Doner, who did the LOI analyses that produced the residue samples we looked at, has retained all the residue samples she has generated. She is going to send those to me so that we can compare the diatom composition and density from more "typical", organic-rich sediments near the bands to those from within the bands. That could be instructive as well. There are temperature drops associated with the two bands, although the temp continues to drop for a distance up-core from the top band while the organic content and appearance of the core returns to brownish gytja.
And so the mystery continues! We are all still brainstorming on this. What fun!
Lee
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