<i>δ</i><sup>13</sup>C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

Campos, Marília C.; Chiessi, Cristiano M.; Voigt, Ines; Piola, Alberto R.; Kuhnert, Henning; Mulitza, Stefan

doi:https://doi.org/10.5194/cp-13-345-2017

Articles | Volume 13, issue 4

https://doi.org/10.5194/cp-13-345-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-13-345-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 4

Research article

|

18 Apr 2017

Research article |

| 18 Apr 2017

δ¹³C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

Marília C. Campos, Cristiano M. Chiessi, Ines Voigt, Alberto R. Piola, Henning Kuhnert, and Stefan Mulitza

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Final revised paper (published on 18 Apr 2017)
Supplement to the final revised paper
Preprint (discussion started on 20 Jun 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'Please consider different mechanism', Andreas Schmittner, 20 Jul 2016
- AC1: 'Final author comment to Dr. Schmittner (short comment)', Marília de Carvalho Campos, 14 Oct 2016
RC1: 'Review of Campos et al. 2016', Anonymous Referee #1, 24 Jul 2016
- AC2: 'Final author comment to Referee #1', Marília de Carvalho Campos, 14 Oct 2016
RC2: 'Review of Campos et al 2016, submitted to Climate of the Past', Anonymous Referee #2, 25 Aug 2016
- AC3: 'Final author comment to Referee #2', Marília de Carvalho Campos, 14 Oct 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (31 Oct 2016) by Zhengtang Guo

AR by Marília de Carvalho Campos on behalf of the Authors (12 Dec 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Dec 2016) by Zhengtang Guo

RR by Anonymous Referee #1 (04 Jan 2017)

Suggestions for revision or reasons for rejection

Review of Campos et al.,

Campos et al., significantly improved the manuscript from the first draft and I appreciate the effort put into the answer to previous comments. Please find below a few comments that should be taken into account before publication.

1) The new figure 5 is very nice, and the similarity between d13C GeoB6212-1 and atm. pCO2 is striking. This also raises the issue of the interpretation given by the authors: D13C decreases in phase with pCO2 decrease, contrarily to the hypothesis given.

Please note that some 3-dimensional numerical experiments of an AMOC shut-down have been performed with prognostic oceanic and atmospheric d13C. For example, in Menviel et al., 2015, you can see that both the Bern3D and LOVECLIM suggest that NADW shutdown leads to a surface d13C decrease on the Brazilian margin even without enhanced Southern Ocean upwelling. An AMOC shutdown without enhanced Southern Ocean upwelling leads to a small pCO2 decrease (Menviel et al., 2014).

2) Abstract and P11, L20-22: With the currently proposed hypothesis (AMOC shutdown leads to enhanced SO upwelling, which decreases SO d13C, releases low d13C into the atmosphere and induce surface d13C decrease on the Brazilian margin), I don’t see how the d13C records presented here add evidence to the fact that atmospheric CO2 increase during HS2 and HS3 could have originated from the ocean. Terrestrial carbon release would also increase pCO2, decrease d13CO2 and thus oceanic d13C.

3) In several part of the paper, the authors suggest that similar mechanisms/teleconnctions were at play during all Heinrich stadials (HS2, HS3 often compared to HS1). I understand the rationale, but this is not necessarily true, particularly in the case of HS1, which occurred during the deglaciation … For example, they assume that d13CO2 decreased during HS2 and HS3. It is quite possible, but not yet firmly shown. The very nice d13CO2 record from Eggleston et al., 2016, unfortunately cannot give that information. They note on p14: “Note that while Heinrich event 2 may also have left a small imprint in our 𝛿13C(atm) record, an unambiguous signal of the weaker Heinrich event 3 cannot be discerned. However, the low resolution in the 𝛿13C(atm) record at that time precludes a final conclusion.”

4) P7, L.26-28: Bauska et al., 2016 present a high-resolution high-quality atmospheric d13CO2 of the last deglaciation, but the modeling experiments performed are extremely idealized and performed with a simple box model. Please note that the impact of changes in southern hemispheric westerlies and/or changes in Antarctic Bottom Water formation on oceanic and atmospheric d13C has been explored using 3-dimensional ocean models including OGCM (e.g. Tschumi et al., 2011; Menviel et al., 2015).

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RR by Anonymous Referee #2 (15 Jan 2017)

Suggestions for revision or reasons for rejection

Second review of "Glacial d13C decreases in the western South Atlantic forced by millennial changes in Southern Ocean ventilation" by Campos et al.

Overall the authors have addressed most of the major issues raised by the reviewers and the manuscript is close to ready for publication. However, there remain several points that still require attention:

Page 6, Line 12: Air sea gas exchange very likely played a role in creating the surface ocean d13C anomalies. For example, a large fraction of the modeled decrease in d13C in subtropical gyre locations is due to exchange with a d13C depleted atmosphere (see Figure 6A in Schmittner and Lund, 2015).

Page 2, Line 10: While the surface ocean d13C anomalies require a global-scale driver, perhaps via the Southern Ocean, the high resolution intermediate depth records compiled by Hertzberg et al. (2016) show that d13C increased during HS1, which is inconsistent with transport of light carbon by AAIW.

Page 2, Line 20: Note that temperature change is not required... an isotopically light surface ocean will cause the atmosphere to be lighter (assuming constant gas exchange rates), with the temperature or wind-driven gas exchange effects acting to modify the isotopic offset between the surface ocean and atmosphere.

Page 4, Line 6: A minor detail, but air-sea exchange influences d13C of DIC, which then may be incorporated into the foraminiferal shell, with modification by vital effects.

Page 4, Line 27: What is the interpretation of the sand lenses? Are they turbidites? If so, is there any evidence of anomalous stable isotope results or sedimentation rates above and below the sand layers?

Page 5, Line 3-7: Did the authors use mixed planktonic or individual species for the radiocarbon dates? If mixed, were they surface mixed layer or thermocline bugs, or both? What is the basis for assuming the 400 year surface water reservoir age? Is this because surface mixed layer bugs were used for 14C analysis? What is the assumed reservoir age error for the age calibration?

Page 6, Line 16: There are two clear negative d13C anomalies during HS2 but the pattern is murkier prior to HS2. While there is a small anomaly at the end of HS3, there is a much larger negative excursion between HS3 and HS2 that doesn't fit the assumed pattern of low d13C during Heinrich stadial events. The authors need to acknowledge this inconsistency and briefly discuss what may explain the large negative anomaly between HS3 and HS2.

Page 7, Line 20: While it is true that the d13C of CDW should decrease with weakening of NADW, there are several additional factors that will influence the d13C tracer field of the ocean interior, related to changes in both preformed and remineralized d13C. For example, the model results of Schmittner and Lund (2015) suggest that remineralization plays the primary role in setting d13C in below 500m water depth in the South Atlantic, Indian, and Pacific Oceans following shutdown of the AMOC (see Figures 5 and 6, Schmittner and Lund, 2015).

Page 7, Line 24: Please qualify that the large Southern Ocean upwelling response occurs only in models with low resolution. Higher resolution models show only minor upwelling sensitivity to large changes in prescribed wind stress in the Southern Ocean, mainly due to eddy compensation (e.g. Farneti and Delworth, Journal of Physical Oceanography, v. 40, 2010).

Page 8, Line 7: This is somewhat selective presentation of evidence... as mentioned above, four separate intermediate depth sites show higher d13C during HS1, which is inconsistent with transport of isotopically light carbon via mode and intermediate waters (Hertzberg et al., 2016).

Page 9, Line 20, Line 23: The d13C of DIC in the surface ocean IS affected by air-sea gas exchange and this would have also been the case during HS events, the question is to what extent. While it is true that the surface ocean rarely reaches equilibrium with the atmosphere, this does not mean the surface ocean doesn't respond to atmospheric variation, especially places like subtropical gyres where there is greater time for equilibration with the atmosphere.

Page 10, Line 3: Please include best estimate of how much temperature may have affected surface ocean d13C (assuming full equilibration with atmosphere, so it would be a maximum effect).

Page 10, Line 9: The relationship between sedimentation rate and d13C is inconsistent. While low d13C at the beginning of HS2 corresponds to high sedimentation rate, the even more negative d13C anomaly in mid-HS2 occurs during a relative peak in sedimentation. Also, there is no clear changes in sed. rate during HS3, which should be the case is southward ITCZ migration and greater riverine input of terrigenous material were the main control on sediment accumulation at the core site. To make a convincing case would require multiple cores to constrain sediment discharge from the Plata River basin. Any individual core could represent accumulation related to bottom currents or dynamic sediment accumulation along the margin.

While it is helpful that the authors now include d18O data in the supplemental information, there is no mention of the records here in the discussion. Interestingly, it looks like there is a long-term minimum in d18O at the beginning of HS2, which may be consistent with greater fresh water input to the core site.

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ED: Publish subject to minor revisions (review by Editor) (29 Jan 2017) by Zhengtang Guo

AR by Marília de Carvalho Campos on behalf of the Authors (22 Feb 2017) Author's response Manuscript

ED: Publish as is (16 Mar 2017) by Zhengtang Guo

AR by Marília de Carvalho Campos on behalf of the Authors (22 Mar 2017)

Short summary

Our new planktonic foraminiferal stable carbon isotopic data from the western South Atlantic show major decreases during abrupt climate change events of the last glacial. These anomalies are likely related to periods of a sluggish Atlantic meridional overturning circulation and increase (decrease) in atmospheric CO₂ (stable carbon isotopic ratios). We hypothesize that strengthening of Southern Ocean deep-water ventilation and weakening of the biological pump are responsible for these decreases.

δ13C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

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δ¹³C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2