BG-2019-507

Dear Zhiguo Rao and Goudong Jia and co-authors,

First of all, I would like to thank both reviewers for their reviews and you for your reply. I have read you manuscript with great pleasure, but agree with the comments of the reviewers. So please do address these in you next version of the manuscript. I do have a few other questions or suggestions. I totally agree that if the majority of your brGDGTs are produced within the lake, lake water temperature is probably more important than air temperature. That being said, there is still the option that soil derived brGDGTs crate part of the observed bias, right? In soils I would assume production is highest during the warm season and degreases or even completely stops in the cold season, when the soils are probably also frozen? What is the catchment of this lake and when does soil derived material end up in the lake? I could imagine soil brGDGTs with a warm season bias being stored in soils over winter ending up in the lake with the melting of snow and ice transporting soil derived material with meltwater to the lake ending up in the sediment. If something like this might happen, you would probably see such as soil signal in mainly the spring SPM and maybe only in certain areas of the lake. Did you sample in spring? So big question is the soil contribution always small and insignificant relative to the lake contribution? Or might there be a small, but significant, contribution to the sediment creating this warm bias? I totally agree with your lake temperatures and how they deviate from the air temperatures in winter, so a very valid explanation, but can you completely rule out the soil contribution based on the data set you have presented here?
That was one potential issue. What is reflected in you SPM, is there the potential for resuspension, for instance? I would expect activity and/or growth also to be reduced at 5 degrees C relative to the 14, 15, 20 degrees C in summer. Agreed potentially not zero, which would most likely be the case for the frozen soils. Your lake temperature lag behind the air temperature a little, the brGDGTs prably integrate a relatively long(er) time period, even in the spm, and therefore lag behind even more or average out a longer time period. Add to that the possibility of soil derived material which could be seasonally varying and possibly resuspension. Could it be that the relatively stable amount of brGDGTs in SPM reflects these process, slow growth and a “fossil” component leading to averaging out of the extremes and resulting in an apparent warm bias?
There one last remark, would it be possible that the population growing and active at 5 degrees C does something different than the one growing in summer in the lake? Something like this is happening in soils where the observed differences are related to population changes and not adaptations to changing conditions by the same population. If so, how would that interfere with your ideas?
Again, I think your idea is very valid, but I do think some of these other complicating mechanisms could be discussed. I assume you did not analyse a spring sample, or a sample from during or right after a major ice and snow melt. 14C age data of brGDGTs from different samples would also be very interesting, I think, but that is a completely different topic, never mind.

I am looking forward to your next version of the manuscript,

Marcel

Dear Zhiguo Rao and Goudong Jia and co-authors,

First of all, I would like to thank both reviewers for their reviews and you for your reply, also to my suggestions. Much appreciated. I think you manuscript can be published with technical corrections.

best regards,

Marcel