the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Late Eocene to early Oligocene productivity events in the proto-Southern Ocean as drivers of global cooling and Antarctica glaciation
Gabrielle Rodrigues de Faria
David Lazarus
Johan Renaudie
Jessica Stammeier
Volkan Özen
Ulrich Struck
Abstract. The Eocene-Oligocene transition (ca 40–33 Ma) marks a transformation from an ice-free to an ice-house climate mode that is well recorded by oxygen stable isotopes and sea surface temperature proxies. Opening of the Southern Ocean gateways and decline in atmospheric carbon dioxide have been hypothesised as possible triggers of the major climate shift during the Cenozoic. However, identifying the driving mechanisms remains controversial and depends on a better understanding of how the different environmental changes correlate. In this study, we investigate the spatio-temporal variation in export productivity using biogenic Ba (bio-Ba) from different Ocean Drilling Program (ODP) Sites in the Southern Ocean, focusing on possible mechanisms that controlled them as well as the correlation of export productivity changes to changes in the global carbon cycle. We document two significant SO region high export productivity late-Eocene events (ca. 37 and 33.5 Ma) correlated to pronounced changes in global atmospheric pCO2. We propose that paleoceanographic changes that followed Southern Ocean gateway openings, along with more variable increases in circulation driven by episodic expansion and decline of the Antarctic ice sheet, drove enhanced SO export production in the late Eocene through basal Oligocene. These factors may have driven the episodic reduction of atmospheric carbon dioxide and contributed to Antarctic glaciation during the Eocene-Oligocene transition.
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Gabrielle Rodrigues de Faria et al.
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RC1: 'Comment on egusphere-2023-1276', Peter Bijl, 21 Sep 2023
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The authors have generated biogenic barium and stable carbon and oxygen isotope records at three drill sites in the Southern Ocean spanning the late Eocene to early Oligocene. By intricately comparing the high-resolution datasets, with age models that were constructed in previous work, the authors recognized 2 episodes of increased export productivity, in the lower-Priabonian and around the Eocene-Oligocene transition. The authors correlate these to episodes of cooling and decline in atmospheric CO2, and conclude that the records express phases of enhanced carbon drawdown by the Southern Ocean, storage of that carbon and through that explains the CO2 decline. The stratigraphic correlation of the sites seems robust, although since the work on the age models was done elsewhere, there is very little that the authors provide in this paper to verify that. It would help the reader to have a (supplementary) figure showing those age models. I am not an expert on the methods used by the data seems of high quality, generating using standard methodologies and are of high enough resolution to detect the trends. The paper is overall well written (occasionally the wrong use of the word “if” confused me) and structured. My main concern is on the interpretation step from export productivity (which in my view has been clearly demonstrated) to CO2 storage and through that 1 million-year CO2 decline.
In conclusion, I value the records, and find the consistency between the records definitely fascinating, but I am not convinced of the evidence put forward by the authors that link export productivity in the Southern Ocean upwelling zone to atmospheric CO2 decline without carbon cycle box modelling exercises that marries all observations. I will substantiate this further below.
From export productivity to atmospheric CO2 decline
I have some serious concerns about the way the authors interpret their local export productivity records to ocean carbon drawdown and atmospheric CO2 decline. Indeed, during the Pleistocene, the ocean plays a large role in the glacial-interglacial variability of atmospheric CO2 concentrations through ocean carbon storage, but on longer time scales, atmospheric and ocean carbon reservoirs are in equilibrium (which explains the long-term Pleistocene CO2 stability). Regions of upwelling bring the excess deep-ocean carbon back into the atmosphere.
In fact the argumentation by the authors demonstrates the complexity: they invoke an increase of upwelling to explain the stimulation of export productivity, but omit to confess that upwelling areas are vast ventilators of deep-ocean carbon into the atmosphere. Invoking increased upwelling to explain the extra carbon export complicates the interpretation of the pCO2 decline. In the modern Southern Ocean it is not the upwelling areas with high primary productivity that constitute the carbon sink: it is the subantarctic zone, where surface waters low in DIC flow northwards and adsorb atmospheric CO2, after which they sink in intermediate water formation. The zone of maximum export productivity is in an upwelling zone and therefore a zone of CO2 flux out of the ocean into the atmosphere (see, e.g, the work by (Egleston et al., 2010; Sabine et al., 2004)).
Coming back to the timescale issue, (Sluijs et al., 2013) has demonstrated the complexity there. It showed that on timescales of 0.5-1 million years it is really difficult to induce carbon cycle perturbations, but at least it is doubtful whether just export productivity to the deep sea can do the trick. According to Sluijs et al., 2013, carbon deposition on the continental shelves play an important role, but in the late Eocene and across the EOT, ice sheet formation creates progressively less flooded continental shelve area. In any case, it seems like OC burial records are needed to seriously affect atmospheric CO2 decline on these time scales. The thought experiment (around line 400) that combines assumptions from the modern Southern Ocean that we know are not realistic for the Eocene (deep ocean ventilation, ocean flow strength) is grossly inadequate and doesn’t do justice to the knowledge we have on late Eocene ocean conditions. It is actually quite crucial that the authors demonstrate excess carbon burial to relate their export productivity trends to carbon removal from the exogenic carbon pool.
The authors suggest that knowledge on the carbon cycle is limited to get full understanding (lines 385/386. But this bypasses a slur of knowledge obtained from carbon cycle box modelling such as LOSCAR (Zeebe, 2011) as was also used in Sluijs et al., 2013. This box model takes into account the full carbon cycle, upwelling as well as ocean carbon uptake, alkalinity and CCD trends in all ocean basins, and includes stable isotope tracers to simulate the effects on deep-sea carbon isotopes. I would claim to say that without such a modelling exercise, the translation from one element of the carbon cycle (in this case export productivity) to atmospheric CO2 is impossible to make.
The importance of bathymetry
The location of the sites is not trivial, and I feel the authors must pay a little bit more attention to the local bathymetry at these sites. Bathymetry, however high, plays a huge role in steering local ocean flow, even up to the surface. For the Late Eocene, this was demonstrated in the high-resolution ocean model simulation by (Nooteboom et al., 2022). As such, the site record first and foremost local changes in oceanography, and depending on the bathymetric anomalies around, they are representative of what happens on a larger area. Many modern bathymetric highs have upwelling associated to them, because the high pushes deep ocean water upwards. Many ocean regions with bathymetric obstructions also are sensitive to small climate or oceanographic changes, as ocean fronts are unable to flow over them, so have to go around. Therefore, in most cases, close to bathymetric obstructions, oceanographic changes are strongly amplified compared to the forcing or the latitudinal average oceanographic changes. I just note that all three sites come from bathymetric highs, but are in this study used in a large extrapolation exercise to calculate whole-Southern Ocean carbon storage. I think that, given what we know of the amplification effect of local ocean change by bathymetry, this important point must be addressed in the paper, and particularly at an extrapolation exercise.
Smaller points of attention
Abstract: I categorically disagree with the way two consecutive theories for the trigger for the EOT “climate shifts” (I would write ice sheet formation, but ok) are presented as equal. They are not, and have never been. The gateway opening theory as primary trigger for AA glaciation from the ‘70s has been thoroughly refuted in the ‘90s, ‘00s and ‘10s by meticulous dissection of its argumentation with evidence from modern physics (Sloan and Rea, 1995), numerical modelling (Huber et al., 2004; Huber and Nof, 2006) and a lot of microfossil data e.g. (Bijl et al., 2011; Houben et al., 2019). At the same time that this hypothesis was refuted, the evidence for the role of CO2 decline in explaining AA glaciation appeared (Deconto and Pollard, 2003a, b; Deconto et al., 2008). So these two hypotheses were never really competing, they were merely consecutive. Later studies have asked the question: “if gateway opening wasn’t the primary trigger of AA glaciation, then what was their secondary role?” (Sauermilch et al., 2021), in line with the thinking since the ’00s that CO2 was the primary force. But that was new to compete with CO2 having been the primary trigger. Compelling new evidence in support for the gateways providing the primary trigger for AA glaciation has not been presented since then. So, the driving mechanisms are not controversial, but surely we do need a better understanding about how different environmental parameters relate to each other (line 15). The more important question surrounding environmental changes around EOT revolve around forcings versus response, particularly in the Southern Ocean: given that CO2 forced climate (and also ocean) cooling, and Antarctic glaciation, which in turn could have triggered further ocean changes, which part of the oceanographic changes observed in records is forced by the CO2 decline and which part is the consequence of the continental-scale ice sheet buildup, perhaps though atmospheric feedbacks (Houben et al., 2019).
Lines 27/28: These papers do not really demonstrate that. Perhaps the study that really tries to quantify the inportance of Cenozoic climate boundaries is (Westerhold et al., 2020)
Lines 55/56: What is mostly unclear is that this threshold is model-dependent and strongly dependent on boundary conditions (Gasson et al., 2014)
Lines 61/62: A lot of more recent literature suggests it is not - well, it depends on how you define an ACC. It triggered possibly some kind of circumpolar flow, but the onset of modern-like strength was strongly stalled uptil about 10 million years ago (e.g., (Evangelinos et al., 2020; Evangelinos et al., 2022)
Line 75: Not the development of all fronts: There always was a subtropical front as the boundary between the subtropical and subpolar gyres (see all modelling studies). The authors picture it in their Figure 5 quite clearly.
Lines 126/127: the direction of flow of the ACC and in part strength is driven by westerly winds, but its exact flowpath is governed by bathymetry (not unimportant given that the sites you present data from are onto those main obstructions)
Lines 153/154: very gradual. For most of the time, Australia still blocked the ideal flow path of the ACC (Hill et al., 2013)
Lines 161/162: Although this is already a long list, I feel that some crucial papers are missing here, mostly because they represent a leap forward in the approach. All model simulations here attempt to simulate equilibrium ocean circulation in fully coupled climate models, but these simulations come at the expense of spatial resolution. That this seriously impacts results of ocean flow has been demonstrated by Nooteboom et al., 2022 who simulates late Eocene ocean flow in a high-resolution simulation, that resolved Eddie flow. Secondly, as the authors rightfully say before, tectonic changes are really important, and Sauermilch et al., 2021 demonstrated the oceanographic effect in several steps across the EOT.
Lines 290/291: Very old data, some of which are now deemed unreliable (Pearson and Palmer). Please refer to the paleoCO2 compilation for the most recent work.
Lines 375/376: An important study that precedes the current study is that of Houben et al., 2019, that documented carefully the surface oceanographic conditions preceding the onset of AA glaciation in the Southern Ocean. This paper concluded a spinup of the ocean flow as evidenced by widespread glauconite formation in the Southern Ocean, as well as microplankton evidence.
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Citation: https://doi.org/10.5194/egusphere-2023-1276-RC1
Gabrielle Rodrigues de Faria et al.
Data sets
Late Eocene to Early Oligocene Oxygen and Carbon Isotope Records and Biogenic Barium Accumulation Rates in Maud Rise, Kerguelen Plateau and Agulhas Ridge. Gabrielle R. Faria, David B. Lazarus, Johan Renaudie, Jessica Stammeier, Volkan Özen, and Ulrich Struck https://doi.pangaea.de/10.1594/PANGAEA.959619
Gabrielle Rodrigues de Faria et al.
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