Retrieval of the two short cores from Kepler Lake, Alaska. Cores were taken using a plastic tube corer fit with a piston.
Two short cores (85 cm and 101 cm long) were taken from Kepler Lake, a groundwater-fed lake in south-central Alaska, with the goal of reconstructing climate and environmental changes in recent centuries for the region. Previous studies claimed a dominant relationship between δ18O (a ratio of the heavy and light isotopes of oxygen) and local atmospheric circulation patterns, rather than the conventional temperature interpretation (positive shifts reflect warmer temperatures, and negative colder).
The cores were analyzed for stable isotopes of carbon and oxygen from marl (calcium carbonate rich) sediment. This analysis served to help interpret changes in moisture balance (the amount of precipitation versus evaporation) as well as changing patterns of atmospheric circulation. During periods of evaporation, the lighter 16O isotope preferentially enters the vapor phase and leaves the system, leaving the lake water relatively enriched in 18O, causing a positive shift in δ18O values. Age constraints were provided by 210Pb analysis of sediment samples and AMS 14C dating of terrestrial macrofossils, such as fragments of leaves, twigs, and other debris from the vegetation surrounding the lake. Utilizing these two dating methods we are able to assign tight age control for the post 1950-period during which radiocarbon dating is not useful (due to bomb testing) and for the previous millennium, which is outside of the limitations of 210Pb dating (approximately 100 years before present).
Based on the results of our dating analysis, an 800-year record was established for the two cores. The most positive values of δ18O occurred during the Little Ice Age (LIA) period, contrary to the conventional temperature interpretation. The LIA is well established as a period of colder temperatures in the region, with studies showing reduced tree ring width and glacial advance; however moisture balance is still relatively unconstrained. Our record from Kepler Lake supports the hypothesis of a changing source of moisture input to the region and through comparison with other regional records suggests spatial complexity to changes in moisture during the last millennium.
More details on this study are forthcoming in a paper which is currently in review with the Journal of Paleoliminology.
Gonyo, A., Yu, Z., Bebout, G. (In Review) An An 800-Year Multiple-Proxy Record of Atmospheric Circulation and Climate Change from Kepler Lake, South-Central Alaska. Journal of Paleoliminology.
Overall, there is a lot of good information within this post. I think it would be easier to understand for all audiences if the information was organized a bit differently and included a few more details to explain the underlying concepts and terminology.
ReplyDeleteThough the very first sentence does state the goal of the study as reconstructing climate history, the explanation and details regarding what the author will be exploring more particularly are not evident until the end of the entry. It might be helpful to state these intentions earlier on so that the reader has a better idea of what to look for in the study. Instead, the author jumps right into the claims of previous research without really explaining the implications or concepts behind those claims. What does the claim of a dominant relationship between δ18O imply? What is a conventional temperature interpretation and what does it imply? Positive shifts in “what” reflect warmer temperatures, while negative shifts reflect colder temperatures? These are all questions that the author does begin to address in the summary, but not until later. These details and explanations might be better placed at the beginning of the summary to ensure the reader comprehends the issue being addressed.
Also, some of the details in the second paragraph leave the reader with unanswered questions. The explanation relating oxygen isotopes involvement in the evaporation process is helpful. The points regarding Pb and C dating could be simplified though as they do not seem to be as immediately relevant to the research summary. Finally, a little more detail regarding the Little Ice Age may be helpful, particularly some mention of the relative or absolute time frame of the LIA with respect to the 800 year record. The connection between colder temperatures and tree rings could be elaborated as well, especially for those beyond this field. Climate history is a very interesting topic, so a few extra details would really help to emphasize the different aspects for the audience.
This post did a good job at explaining the terms and practices used, such as explaining Charophytes and what δ18 O tells us. I think that some more information on the little ice age could have been used to help spark interest in the article. I also think that the results could have been discussed more, especially since it contradicts the conventional temperature interpretation.
ReplyDeleteThis is a detailed description of an interesting study. The explanation of oxygen isotope systematics with respect to evaporation and climate change is comprehensible and provides the reader the background, which is required to understand the report. But I'm wondering if it is really necessary to mention O,C and Pb isotopes at all in order to report on the fundamental aims and conclusions of the study. Telling the story in a simpler and more general way would prevent providing too many details.
ReplyDeleteThe pictures give good impressions of the work environment and the study object.
I found this post to be very interesting and easy to read. My only suggestion is to add a cartoon showing the relevant carbon and oxygen isotopic fractionation pathways.
ReplyDeleteI was a little confused about the dating methods you used. If I understood correctly, you used the two methods to cover the limitations of each. So where one was not applicable, the other could be used? I think you could be a bit more clear about that. That's really interesting though. I would like to know more about that, though it may be outside the realms of this post. Also, how old the cores were is a little unclear. You said it was an 800 year record, but when exactly the record is for is not stated outright, unless one is familiar with the LIA.
ReplyDeleteInteresting post!
This is an interesting study and post. The pictures are well chosen. I was a little confused about how the oxygen isotope data were interpreted, however. Perhaps I missed something, but how did you interpret the positive shift (i.e., "warmer" signal, normally) for the LIA? Are you arguing that the meteoric water source to the lake was isotopically heavier than normal (causing a positive excursion in the O-isotopic comp. for the lake), thus reflecting some change in the isotopic composition of moisture in the atmosphere (due to shifting circulation patterns?)? This may need some clarifying.
ReplyDeleteI think the above posts hit most of the major points, so I will only highlight what I think is new: it may bear explaining what a core is, and how it is taken.
ReplyDeleteThis is interesting research and a good post. I would have liked to read a little more on the previous studies you mentioned, to give your research a bit more context. Also, stating your findings near the beginning of the post would be a good idea. The little ice age is definitely one of the most interesting parts of this post, so be sure to discuss it in more detail.
ReplyDeleteThis is an interesting post. I'd like to know what time resolution for your sampling.
ReplyDeletehow is coupled with the atmospheric circulation change?
I thought there should be more information on how variations of oxygen isotopic ratios is interpreted in terms of paleotempeture.
A simple Distillation model would be helpful to see how the precipitation Vs Evaporate influence the 180/16O of the lake, hence the ratios of carbonate.
Similar to a lot of the previous commenters, I am interested in hearing more about the systematics of oxygen isotope records for northern lakes, though I can see how that information might add significant complexity to the post.
ReplyDeleteI think climate change is a good topic for a post that will have good appeal to a general audience. This post is very jargon heavy, which will probably confuse people. If possible, I think it may help to shorten the methods section. The pictures are really nice, but the caption on the 2nd image is a bit too complicated. Also, I really like your closing sentences, maybe add similar ones in your introduction.
ReplyDelete