the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jan 2025
Southern Hemisphere tree-rings as proxies to reconstruct Southern Ocean upwelling
Abstract. The Southern Ocean plays a key role in regulating global climate and acting as a carbon sink. This region, defined as south of 35° S, is accountable for 40 % of all oceanic anthropogenic CO2 uptake, and 75 % of ocean heat uptake between 1861 and 2005. However, the strength of the Southern Ocean sink (air-sea CO2 flux) is variable – weakening in the 1990s and strengthening again in the 2000s. Typical methods of constraining the flux must grapple with two opposing forces: outgassing of natural CO2 and uptake of anthropogenic CO2. Reconstructions of atmospheric radiocarbon (Δ14CO2) from Southern Hemisphere tree-rings may be a viable method of observing the one-way outgassing flux of natural CO2, driven by Southern Ocean upwelling. Here we present more than 400 tree-ring ∆14C measurements from 13 sites in Chile and New Zealand from the 1980s to the present. These measurements dramatically expand the dataset of Southern Hemisphere atmospheric Δ14CO2 records. We use these records to analyse latitudinal gradients in reconstructed atmospheric Δ14CO2 across the Southern Ocean. Tree-rings from New Zealand’s Campbell Island (52.5S, 169.1E) show Δ14CO2 was on average 3.3±3.5 ‰ lower than atmospheric background, driving a latitudinal gradient among New Zealand sites between 41.1° S and 52.5° S, whereas samples from similar latitudes in Chile do not exhibit such a strong gradient. We demonstrate that the gradient is driven by the combination of CO2 outgassing from the Antarctic Southern Zone (ASZ) and atmospheric transport to the sampling sites.
- Preprint
(1865 KB) - Metadata XML
-
Supplement
(1275 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Review', Anonymous Referee #1, 27 Jan 2025
This paper presents new 14C data from tree rings in New Zealand from the last 45 years or so. I believe the data are worth publishing, but I have a few concern on the present state of the draft, which might need improvements:
1. In my view the new data should be set in a better context of existing 14C data of the southern hemisphere (SH) of the last 100 years or so. While the authors cite Hua et al 2021 (which should be cited as published in 2022, https://doi.org/10.1017/RDC.2021.95), which split (following earlier papers) the 14C data 1950-2019 into 5 zones (2 in the SH), I am missing how the new data would agree/disagree with SH zone 1-2 (which is where the positions of the trees from which the new 14C data are measured are situated). Furthermore, Levin et al. (2022, https://doi.org/10.1017/RDC.2021.102) also showed 14C data for different latitudes. Here, especially Neumeyer Station in Antarctica and Macquary Island in the Southern Ocean are of interest. So, at best, the new data from Lewis et al are presented in the wider context of the data presented in these two papers. In this context it might be necessary to recalculate the Southern Hemisphere atmospheric background of ∆14CO2, from which the anomalies of individual sites are calculated.
2. For the back-trajectories (Figure 4) the Southern Ozean is split in different zones according to the positions of the Antarctic Polar Front and other fronts as published in Orsi et al 1995. My understanding of postitions of fronts, widely based on Freeman and Lovenduski (2016, doi:10.5194/essd-8-191-2016) is that the defintion of these fronts might be different when based on newer studies. Thus, it might be necessary to revise the calculations of back-trajectories and if and how this might help to distinguish which water masses might influence via sea-air gas exchange the measured 14C data. This might also influence Figure 6.
3. The overall motivation of the study, that these new 14C data might be used as proxies for Southern Ocean upwelling, which is already mentioned in the title, might need more support. My concern is based on the fact that not only the upwelling water mass in the Southern Ocean is depleted in 14C (old), but also the anthropogenic CO2 flux which operates in the the opposite direction (atmosphere->ocean), since this is mainly based on 14C-free fossil fuel emissions. If you look at the atmospheric D14CO2 time series in Hua et al 2022 you see that the southern most data for the last decades have actually higher values than the northern ones. I believe this is understood by the fact that the 14C-free CO2 emission (14C Suess Effect)being mainly in the NH, but I cannot remember a citation to this idea. Here, it is now argued that the small scale differences between some sides in the SH can be used to pin down the upwelling flux of old waters. I think it is necessary to discuss that what is concluded here on smaller scale (upwelling of old 14C-depleted waters reduces atm D14CO2 in some places around the Southern Ocean) is opposite (and maybe counterintuitive) to the more global picture (14C Suess effect larger in N than in S).
Minors:
- Line 40: Citation to „Peter Landschutzer, 2015“ is weird (first name?) and missing in the reference list.
- The DOI to 10.5281/zenodo.14532802 (Data Availability) does not work.Citation: https://doi.org/10.5194/egusphere-2024-4107-RC1 -
RC2: 'Comment on egusphere-2024-4107', Anonymous Referee #2, 27 Jan 2025
This study lays the groundwork for the use of Southern Hemisphere tree-rings for studying changes in Southern Ocean upwelling and air-sea CO2 flux. The authors demonstrate that trees from Chile and New Zealand from the 1980s to present accurately record atmospheric 14C (with a few exceptions) with a latitudinal gradient observed for New Zealand sites. Using HYSPLIT software they model back-trajectories for the air masses reaching the site and show that the New Zealand gradient is driven by Southern Ocean outgassing and atmospheric transport. The methods used are robust and the manuscript is generally well-written. Due to the importance of the Southern Ocean in global climate, this study will be critical for future work on constraining temporal changes in upwelling and air-sea gas exchange.
Minor comments:
Line 30: Insert space after (Talley, 2013)
Line 33: Possibly insert ‘content’ after ‘carbon’ just to be clear that this doesn’t refer to 14C (although probably fine).
Table 1: ‘The final column references numbered locations on Figure 2a/b maps.’ Note: The final column with location number is missing.
Line 163: ‘using a FFT filter cut-off of 667’ What are the units?
Line 168: ‘Data will be included in Supplementary Material’ Is this yet to be done or will it be at 10.5281/zenodo.14532802?
Figure 2: Perhaps change the label ‘CCGCRV Trend Reference’ to ‘CCGCRV SHB Trend’ for clarity
Line 209: ‘The data was filtered for existing radiocarbon measurements ('G2c14')’ Please insert ‘ocean’ in front of radiocarbon.
Line 213: ‘on bottle latitude/longitudes’ Some readers may not be familiar with the use of bottles for ocean water collection- perhaps replace with 'sample collection latitude/longitudes’
Line 309: ‘(Prend et al., 2022) discovered a deep maximum mixed layer depth surrounding Campbell Island and westward in the ASZ (Prend 2022 Fig. 1c).’ The sentence should start with ‘Prend et al. (2022) discovered’ and presumably end with (Prend et al. 2022, Fig. 1c)
Line 313: ‘Low ΔΔ14CO2 at Campbell Island may be linked air originating’. Insert ‘to’ after ‘linked.
Line 326: ‘local effects such as nearby obduction from deep mixed layer’ Doesn’t obduction refer to crustal movements?
Line 329: ‘This suggests that the influence of non-local fossil fuel and biospheric signals are small in the Southern Hemisphere.’ Does this apply to the entire Southern Hemisphere?
The citations need to be carefully checked as many in the text are missing from the reference section or incomplete. I’ve found the following problems with citations but may have missed some:
Line 49: ‘Levin et al.)’ Is this Levin et al. 2010?
Line 110: Zondervan et al., 2016. This is given as 2015 in references
Line 200: Warner 2018 is not in references. Should this be Millissa & Warner 2018?
Line 490: ‘Norris, M. W. (2015). Reconstruction of historic fossil CO2 emissions using radiocarbon measurements from tree rings’ This reference is incomplete.
Citations not in reference section:
Line 52: Turnbull et al. 2009
Line 58: Rend et al. 2022
Line 108: Baisden et al. 2013
Line 109 Turnbull et al. 2015
Line 110: Stuiver & Polach 1977
Line 161: Thoning et al. 1989
Line 182: Suess 1955
Citation: https://doi.org/10.5194/egusphere-2024-4107-RC2 -
RC3: 'Comment on egusphere-2024-4107', Anonymous Referee #3, 08 Feb 2025
This study aims to reconstruct atmospheric Δ14C variations near the Southern Ocean to infer past changes in Southern Ocean upwelling rates. Given the crucial role of Southern Ocean overturning circulation in the climate system, this is a highly relevant and important objective. The authors present a latitudinal Δ14C gradient derived from tree-ring records in Chile and New Zealand, spanning the past several decades. By applying backtracking approaches, they distinguish contributions from different Southern Ocean regions and assess the reliability of this proxy. This approach appears promising, and ground-truthing studies are essential to validate proxy-based reconstructions. As such, the study deserves publication, though some clarifications are necessary before acceptance.
The authors report a meridional Δ14C gradient in the New Zealand records, with increasing 14C depletion toward the south, which they attribute to the upwelling of 14C-depleted waters from the Southern Ocean. In contrast, no significant gradient is observed in the Chilean records. They suggest this discrepancy arises because tree-ring records from Chile are less influenced by the degassing of polar surface waters. Although that it is supported by atmospheric back-trajectory modelling, a more thorough description on the systematics of the Southern Ocean 14C-CO2 exchange is somehow lacking in the text. This exchange is complicated and may explain some of the difference between New Zealand and Chile Δ14C.
The error bars in Fig. 3 are large, raising questions about the significance of the New Zealand trend. Please specify in both the figure caption and the main text what these error bars represent and how they were calculated (e.g., 1 or 2 standard deviations). Reporting the p-value would also help assess the statistical significance of this trend. Additionally, it would be useful to display individual observations—perhaps using smaller, empty, or slightly shaded symbols—to allow readers to better evaluate the robustness of the trends.
The Cape Grim and Baring Head records are considered as representative of the Southern Hemisphere record and serve as the baseline for the Southern Hemisphere atmospheric background. It would be interesting to provide a more detailed discussion on their representativeness by comparing them with other global and Southern Hemisphere records, especially that the authors emphasize several times that that their study significantly expands Δ14C records for the Southern Hemisphere. Additionally, since Cape Grim and Baring Head are influenced by Southern Ocean air masses, could lower-latitude Southern Hemisphere records provide a more representative baseline?
Citation: https://doi.org/10.5194/egusphere-2024-4107-RC3
Model code and software
Code used in this manuscript Christian Lewis https://github.com/christianlewis091/science_projects/tree/main/soar_tree_rings/scripts_EGU_submission
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 196 | 45 | 8 | 249 | 22 | 5 | 5 |
- HTML: 196
- PDF: 45
- XML: 8
- Total: 249
- Supplement: 22
- BibTeX: 5
- EndNote: 5
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1