|Raitzsch et al. present us a relatively high resolution record of atmospheric CO2 from a Southern Ocean site (ODP Site 1092) for the middle Miocene climate transition. The manuscript, both data processing and interpretation, has been substantially improved, and it will be a great addition to the scientific toolkit on climate-carbon-ice sheet interactions during the Neogene and beyond. |
I only have a few additional comments and suggestions, for which indicated line numbers refer to the marked version of the manuscript.
1.1 Sampling strategy: please include the size fraction selected.
Line 165: Please consider rephrasing as (or similar to): “By contrast, for T. trilobus d11B the T. sacculifer calibration is applied, with four available equations ( …..). The calibrations refer to different size fractions of T. sac and different analytical techniques…”
T. sacculifer d11B calibrations are dependent on the size fraction of the foraminifera shells (e.g. see summary in Foster et al. 2012). Comparing individual records with different calibrations without considering the size fraction of the foraminifera used might be misleading.
Lines 209-212: Mg/Ca is pH insensitive for T. sac (Gray and Evans 2019). Therefore the pH correction should not be applied to T. trilobus, but it is not clear if it is.
Lines 261-262. The CO2 record has large uncertainties on the scale of ~ 50-170 ppm. How confidently could we interpret variations on the scale of 50-100 ppm? The ~100ppm step change at 13.82 Ma is probably meaningful as it displays some consistency with the following CO2 estimates and the general cyclicity of the record, but the 50ppm higher frequency variations remain a question. Relative CO2 changes must carry a smaller uncertainty, and an estimate of this would be very useful to demonstrate that such variations are indeed meaningful and interpretable.
Lines 355 and below:
At the risk of “wiggle matching”, comparing the B/Ca record of Sosdian et al., 2020 with this CO2 record one might see some striking similarities. There is a long term decline in CO2, just as the B/Ca record displays a long term increase (and thus decline in surface water DIC). Within the overlapping period, there are two clear ~ 400ky larger cycles, each of which include a CM. The cycle that includes CM5b is characterised by both lower B/Ca (higher DIC) and higher CO2, thus both records appear consistent. The cycle that includes CM6 is intriguing because of the structure within the B/Ca record of Sosdian et al., 2020 in comparison to the d11B and B/Ca in Badger et al., 2013 and this CO2 record. If we ignore the point at ~13.8 Ma of the record in Badger et al., 2013, and focus on the period transitioning into CM6, B/Ca and d11B/CO2 follow the expected covariation so as higher d11B (higher pH and lower CO2) occurs when B/Ca is higher (lower DIC). But then, the expected minima in B/Ca at CM6, when compared to this CO2 record here, and the correlation of records within CM5b, collapses at the peak of the d13C positive excursion but recovers shortly after. Of course, the disagreement could be driven by issues on either of the two proxies, e.g. non-DIC effects on B/Ca, or the assumptions regarding the d11B-CO2 proxy, or regional effects on either of these two records, or a more global scale change. It would be great if both the similarities and the disagreements can be explored, and in that context identify what could make CM6 different to CM5b.
Line 422 and below:
Clearly CM6 is characterized by a more pronounced d13C change compared to all the other CM events. However, this is not something observed in this CO2 record. If anything, comparing the maximum and minimum one might say CM5b CO2 is more pronounced than CM6, since the maximum is ~ equivalent but the minimum of CM5b CO2 is clearly higher, and the amplitude of change is either equivalent or larger for CM5b than CM6. This comparison gets further complicated by the argument of enhanced deep water formation during CM6 (which is plausible), which would likely place Site 1092 into a frontal system in the Southern Ocean, recording higher [CO2] than atmospheric CO2 (and possibly also contributing to the divergence in the records of B/Ca and CO2). What would drive the difference in amplitude of change between the d13C and CO2 records for CM6 in comparison to the other CM?