|The authors have made some improvements and removed some unsupported statements in the revision. However, the abstract and conclusions are unchanged from the original version. These both include the misleading and unsupported statements about the Southern Ocean influence on the interhemispheric gradient noted in the first review, and they both need to be revised to remove these and to focus on the new results reported here.|
The paper has a strange organization that mixes results (for comparison and validation of measurements) into the Methods section, and the Results and Discussion section is dominated by discussion (most of the data has been presented before and there is no new analysis of the causes of observed cycles, trend and gradient). The authors should revise the organization to have a Results section which is clearly showing new results from this analysis including the comparison and validation of measurements. The Results section should clearly report revisions to the Wellington record that were made since Currie 2009 and new observations of the BHD D14C trend over 2005-14 (an interesting new result that is not currently included in the paper), in addition to the analysis of seasonal cycles and estimates of interhemispheric gradients with other stations already included. Then the Results and Discussion section should be renamed Discussion, and shortened.
Section 3.7: I still find this analysis and figure to not be very useful. Why don’t the authors use the footprints to calculate fossil fuel CO2 from local combustion to make this point more clearly? In any case, please note here that the model simulations are only used to detect local influences of fossil fuel combustion and potential seasonality
Section 4.2: This section is too long and it does not give a clear impression of the drivers of the seasonal cycle. Following the recommendation above for reorganization, the seasonal cycle results should be separated from the discussion, which can be shorted to a couple of paragraphs. Ocean exchange needs to be mentioned as an influence. The authors do not mention the analysis of Levin 2010 on seasonality at CGO, who found stratosphere-troposphere exchange and oceanic exchange were the main contributors. Also, Randerson 2002 Fig 6 for the 1980s. The authors should be careful not to over-interpret the data in this section. Are the authors stating that the biosphere is the primary contributor to the seasonal cycle at BHD in the 1980s-90s when they say “This is consistent with a change in sign of the terrestrial biosphere contribution as the bomb 14C pulse began to return to the atmosphere from the biosphere (Randerson et al., 2002)”? Why would the biosphere contribution be strongest in winter?
L471 Also shown in Graven et al. 2012, La Jolla trend
L578-580 It’s strange to introduce Cape Grim here, when it has already been referred to several times and compared to BHD data in the previous section. This comparison with other sites should be introduced in the Methods rather than midway through the Results and Discussion. Did the authors consider laboratory offsets between the three laboratories?
L593-601 This is not a new or robust estimate of the exchange time and needs to be deleted. Many studies have noted the delay of about a year between the peak D14C in the two hemispheres, e.g. Nydal 1966, with Lal and Rama 1966 providing a more rigorous estimate than offered here.
L633 Levin et al. 2010 also attribute the change to the (Southern) Ocean
L636 Where is it shown that the interhemispheric gradient develops rapidly? From Levin 2010, Fig 6i there appears to be a clear trend over 1985-2005, albeit with interannual variation. No such figure is included here.
L638-651 should be deleted and replaced with one sentence such as “The interhemispheric gradient may also be influenced by the apparent reorganization of Southern Ocean carbon exchange in the early 2000s (Landschützer et al., 2015), which is postulated to be associated with changes in upwelling of deep water (DeVries et al., 2017) to which atmospheric Δ14CO2 is highly sensitive (Rodgers et al., 2011; Graven et al., 2012b).”
For “aging”, it may be better to say “dating”
The authors argue for retaining the data with large scatter over 1995-2005, but these data flagged “T” and the smoothed record over this period need to at least include larger uncertainties. The two smoothed records provided “BHD” and “BHDCGO” differ by up to 10 per mil, but the reported uncertainties are about 2 per mil. Shouldn’t the RMSE of 5 per mil over this period be incorporated in the uncertainty of the smoothed record of BHDCGO? Can more information be provided on the “additional error term, determined from the long-term repeatability of secondary standard materials”? The Turnbull 2016 reference appears to report only recent secondary standard measurements since 2011.
The excel spreadsheet refers to WLG in the header text on several sheets.
It seems unnecessary to repeat so much text from the paper in the supplement.
Supplement does not define “trend” and “fit” reported in the spreadsheet. “Trend” suggests a rate of change rather than deseasonalized data.