Greetings, I was willing to reconstruct paleo-temperature of Last Glacial Maximum (LGM) from the oxygen isotope ( δ18O) or Mg/Ca ratio. Therefore. I obtained data from the Pangaea site and arranged them in 3 columns depth, time (Kyrs BP), parameters. The δ18O data were calculated based on G.ruber and benthic, planktonic foraminifera. While the Mg/Ca ratio was extracted from H. elegans.
Now, I am wondering whether I could use any equation(s) that would take the previously mentioned parameters into variables and reconstruct paleo-temperature. I studied some literature where I found the following equation published by McCrea (1950), which was subsequently revised by Epstein et al. (1953):
T(°C) = 16.5− 4.3(δ18Occ − δ18Osw) + 0.14(δ18Occ−δ18Osw)
where δ18Occ is the measured value in calcium carbonate and δ18Osw is the isotope ratio of the water from which it is precipitated. The slope of this relationship means that a 0.23‰ increase in δ18Occ corresponds to a difference of about 1°C (Paul N. Pearson, 2012). I understand that I would put the Isotope ratio's column in the δ18Occ variable, but what should I do about the δ18Osw variable ? As I am planning to derive the paleo-temperature of the LGM, I was wondering that if there's any standard δ18Osw value of that time or do I need separate dataset for that too?
Regarding the ratio of the water, there was something such as conversion of VSMOW/SMOW to VPDB/PDB scale, I would be grateful if someone clarified these with their expertise. Although I do believe my data is already converted to PDB when I downloaded from Pangaea. It also needs to be mentioned that these dataset are from cores in Bay of Bengal. Finally, I welcome any suggestion, advice or tips to reconstruct temperature from the data I gathered. If my approach is wrong, it would be a great help to me if you could me point me out to the right directions or the right equations. I am attaching some screenshot here as well. Thanks in advance.
You can find all information needed in this paper, all the main equations used and how to chose.
Dear Dr. Silvia Spezzaferri ,
Thank you so much for referring me that article. I went through the paper, especially through method section. I must say that the equations, variables are well arranged as well as easier to understand in Supplementary table-1; especially for undergraduate students like me, who is totally new to this field. I would like to list some of my understandings and I'd be grateful if you could tell me if my ideas are correct.
T(°C) = 14.32− 4.28(-3.13 − (-0.4539)) + 0.07(-3.13−(-0.4539))2 = 26.2
Do you think my calculation is correct (-3.13 is the δ18Occ measured value from the core at 1.73 m depth)?
2. Also do I need to take δ18Osw for different depths or values from the surface is applicable to entire column ? (in the screenshot)
3. Finally, in order to calculate SST directly from Mg/Ca ratio, I noticed that the following equation was used widely (Dekens et al., 2002):
Mg/Ca (mmol mol1) = 0.38 exp [0.09 * SST (°C)]
I am assuming that simply putting the corresponding value of the Mg/Ca from the core would give me the SST after I am done solving the equation. Would it be correct ?
I am thankful for your response. I would also be obliged if you could take a few moments to clarify my confusion and queries.
Thank you.
Md. Ashif Imam Khan.
Dear friend，I am also interested in this topic and method as I'm facing a similar study. As you mentioned, that equation is applicable for calculating past SST using sediment δ18Occ values, actually, depending on a prerequisite of assumption: δ18Osw of the surface water in your study site is constant with time.
However, is there any variation on it? I guess impossible. But if its variations during LGM to now are negligible, okay, your calculation is correct.
PS: In our group, one manuscript about using δ18Occ values of lake sediments to calculated paleo temperature was rejected in last year. Because you did not know δ18Osw of temperary lake water.
——take δ18Osw for different depths or values from the surface is applicable to entire column ? (maybe that of seawater varied slightly? than lake water)
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