Archives for the Month of April 2006 on Ramblings of a Geologist
Why Lake Erie is special
Lake Erie is by far the shallowest Great Lake: only 15 meters deep in the western basin, 20 meters in the central basin, and 64 in the eastern basin, as compared to the others, which are hundreds of meters deep. This means that Lake Erie is much more sensitive to changes -- like the littlest sibling who over-reacts to everything.
I think Lake Erie's shallow basins and resulting sensitivity are the reason that its sedimentary record (ie, mud) is different than those from other lakes in the region. We see whiting deposits (lots of fine-grained calcite) in Lake Erie and not Ontario, Huron, or the Finger Lakes because climate changes that happened in the region had a much stronger impact on Lake Erie than the others. Certain conditions are necessary for calcite precipitation (also called "whiting events" or "whitings"), and Erie was simply pushed over the edge, while the other lakes were more stable.
So where is this edge? How do we define it? And what pushed Lake Erie over this threshhold, inducing calcite precipitation? There are two possible interpretations.
Warmer temperatures (jet stream shift?)increased water temperature, thus increasing CaCO3 saturation (ie, the point where water just can't hold any more dissolved ions).
The outlet sill eroded, lowering lake level, which increased Ca++ weathering from the shoreline, which increased CaCO3 saturation.
I like the climate explanation better, because I think temperature is the primary control on whitings, and lake level drops can't account for the degree of warming that is indicated by the other sediment properties. I also like the warm climate explanation because it is consistent with the increase in primary productivity (ie, more little green floaty organisms living, photosynthesizing, and dying) indicated by our stable isotopes. And finally (the icing on the cake), there have been several studies linking cyanobacteria abundance and whitings -- more cyanobacteria means more whitings (as described in previous entry). Altogether this seems to be a stronger interpretation, with lots of proxy data fitting together like puzzle pieces.
Of course I could be wrong. Before I officially root for this interpretation in my final paper (and in the article that we will hopefully publish!) I'm going to try to see if it's possible to quantitatively analyze how much a temperature change would have impacted cyanobacteria, and calcite solubility. In science, a self-consistent interpretation is good, but a self-consistent interpretation with numbers backing it up is better ...
Regardless of what caused these whiting events, the fact remains: Lake Erie contains an extensive carbonate record that is absent in all the other lakes in the region. Since carbonate materials are extremely valuable for reconstructing past climates, I think these sediments will be key to understanding environmental changes in the Great Lakes/New York area.
Great Lakes Climate ... getting warmer?
I think my Lake Erie mud cores have recorded a warm/dry climate event that peaked about 3,000 years ago (see earlier entries for details). NOW I'm trying to figure out how this might influence or fit into the bigger regional picture. Was Lake Erie warm while other lakes were cool? That seems odd, but that's what data from some other research papers suggests. Here are some of my thoughts on the relationship/similarities/differences between Lake Erie and Lake Ontario.
McFadden's Ontario paper shows that carbonate precipitation drops and then essentially stops in the Neoglacial (the period between ~5200 to 250 years ago), and they interpret it to indicate cool/dry climate conditions. They also see a dramatic increase in diatoms (little floaty organisms made of silica), and a dramatic (fivefold) decrease in sedimentation rates (mud building up on the lake floor) during this period.
I'm not sure their data is actually documenting a cool period ... a couple ideas:
1) I think biology could actually be really important here. A study on whitings in Fayetteville Green Lake, NY (Thompson 1997) shows a really strong correlation between abundance of cyanobacteria and whiting events. Cyanobacteria blooms, coupled with warm conditions, seems to be initiating/intensifying calcite precipitation in Fayetteville Green lake more than any other factor. The paper talks about competition between cyanobacteria and diatoms -- says that diatoms need lots of nutrients but cyanobacteria (because of their shape/size, ability to absorb) can out-compete diatoms in oligotrophic conditions (because Green Lake is oligotrophic, cyanobacteria dominate and there are lots of whitings). In Lake Ontario, Mullins and Halfman said there was more wind, upwelling, and thus more nutrients in the Neoglacial -- so maybe diatoms out-competed the cyanobacteria there, and thus made it much more difficult for whitings to occur? No carbonate precipitation is associated with diatoms.
2) Maybe there were whitings, but the carbonate dissolved? Maybe oxygen levels were higher in the lake and there was more dissolution in the sediment Today Ontario mixes almost continuously from late fall to early spring (says McFadden). Is there more mixing in Ontario than in Erie? It's much deeper than Erie, so that doesn't seem quite right. Also magnetic susceptibility drops low and stays low during the Hypsithermal in Ontario -- since magnetic susceptibility is correlated with carbonate in Erie's central and eastern basins, could low mag. sus. could be evidence that there weren't whitings in Ontario?
3) Or, maybe McFadden is right, and it is a cool/dry period. What would make Ontario cool and Erie warm at the same time? Air masses/jet stream shifts?
Too many possibilities ... don't know if I can investigate them all! I almost wish I had more than 3 weeks of school left ... Almost.
Why should we care about lake mud? Part II
The Great Lakes hold 20% of the Earth’s surface fresh water, and are important natural and economic resources for the US and Canada. During the past several thousand years they have been strongly influenced by climate change and the evolving glacial landscape of the Great Lakes region. Lake Erie, the shallowest, has been very sensitive to environmental changes during its Holocene evolution and also to human influences during the modern era.
Lake level changes impact erosion rates; temperature shifts affect productivity and water chemistry; precipitation changes influence inflow from rivers and the Upper Great Lakes. Understanding these relationships and predicting future trends is important to maintaining these crucial natural resources. Also, understanding Lake Erie’s past climate is essential to predicting how the Great Lakes region will respond to both natural and human-induced climate change in the future.
My senior thesis project investigates Lake Erie’s history by analyzing sediments deposited in the lake’s eastern basin during the Holocene (to about 3,500 years ago). Since changes in the physical, biological, and chemical proxies found in these lake sediments can be influenced by a variety of factors, clear identification of the primary factor or factors acting at the time of deposition is not always possible. However, good interpretations can be made based on a critical analysis of the combined data. In these Lake Erie sediments, we use a variety of proxies, looking at relationships between them to better understand the lake’s paleo-depositional environment.
How do we know when proxy changes happen? (Or, how do we date mud?)
Cores can be correlated by matching magnetic susceptibility peaks that appear in sediment across the central and eastern basins. Radiocarbon dates from above the magnetic susceptibility shift are out of stratigraphic order, indicating contamination or sediment re-working. An approximate age for the shift in our Station 23 sediment was estimated from 2900 14C yrs BP from immediately above and below the shift.
CONCLUSIONS (so far)
Multi-proxy data from a Lake Erie sediment core indicate a warm climate event, peaking at about 2900 14C years BP, followed by a period of greater climate variability.
Lake Erie’s climate record differs from New York lake records, potentially indicating high regional variability.
Understanding the response of Lake Erie to climate change is crucial to predicting and preparing for future changes. Because it has such a shallow basin, Lake Erie’s water levels are particularly sensitive to climate. As we continue to see shifts in regional and global temperatures, and as human impact on the Great Lakes increases, we need to prepare for major environmental consequences.
Lake level fluctuations will impact coastal wetlands, commercial shipping, pleasure boating, and beach erosion. Temperature and water chemistry changes will impact primary productivity, fisheries, and invasive organisms. Further high-resolution paleo-climate work needs to be done in order to address these concerns.