the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2026
Evolution of deep-water circulation in the North-East Atlantic during the latest Miocene warming
Abstract. Understanding the possible responses of Atlantic Meridional Overturning Circulation (AMOC) to climate warming is one of the major challenges of modern oceanography. Today, the lower (deeper) southward flowing limb of AMOC consists of the North Atlantic deep water (NADW), which is predominantly formed by deep water convection in the ocean basins to the north and south of Iceland. The southward transit of deep water formed in the northerly basins (Greenland-Iceland-Norwegian Seas) is constrained by gateway geometry to two major flow pathways to the east and west of Iceland. To the south of Iceland extensive deep-sea sediment archives, in the form of contourite drifts, are deposited by these currents and have provided critical information about AMOC and NADW dynamics through the Pleistocene. Here we make use of recently recovered cores from one of these sediment drifts (Gardar Drift, IODP Expedition 395, Site U1564,), that records the deep Iceland-Scotland Overflow water (ISOW) dynamics back to the warm climates states of the late Miocene to Pliocene. By combining sedimentological and X-Ray fluorescence derived elemental proxy evidence, we reconstructed deep ocean current activity and carbonate preservation between 5.0–6.2 million years ago (Ma). The record supports the periodic presence of deep currents since the latest Miocene, as well as a distinct ISOW weakening that coincided with the global warming trend, and the severe restriction on Mediterranean Outflow Water, just before the Miocene-Pliocene boundary. Low carbonate preservation hints to the presence of corrosive water masses in the North Atlantic following the termination of the Messinian salinity crisis.
- Preprint
(1463 KB) - Metadata XML
-
Supplement
(399 KB) - BibTeX
- EndNote
Status: open (until 30 Sep 2026)
- RC1: 'Comment on egusphere-2026-3764', Anonymous Referee #1, 19 Aug 2026 reply
-
RC2: 'Comment on egusphere-2026-3764', Anonymous Referee #2, 08 Sep 2026
reply
Karatsolis and others present new elemental records from sediments of drift/contourite deposits in the North Atlantic Ocean, which likely recorded changes in strength of currents composing the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). Therefore, these records have the potential to shed light on deep water formation and AMOC dynamics during warmer-than-today climate states of the Late Miocene to Early Pliocene. Identification of intervals I, II and III seems robust from the presented datasets, however their paleoenvironmental interpretation should be supported by additional data. Below, I present suggestions that could improve paleoceanographic interpretations and the argumentation to support the presented age model, as well as general and minor comments.
Interpretation of XRF-derived elemental records:
- Possible changes in carbonate vs biosiliceous components should be assessed, or at least ruled out, with a version of the ln(Si/Ti) ratio, as previously applied in the eastern Equatorial Pacific Ocean.
- Since Ti counts can be influenced by provenance changes (i.e., basaltic grains from Iceland), authors should provide in the supplement evidence that the ln(Ca/Ti) ratio shows trends comparable to those of ratios using Ca over other terrigenous elements (e.g., Fe, K, Al).
- Authors should perform an overall assessment (it does not have to be a detailed micropaleontological assessment) of discrete samples for signs of dissolution of calcareous microfossils, and proportions of high productivity taxa (e.g., infaunal benthic foraminifera). This would allow to disentangle whether shifts in carbonate content were driven by preservation or productivity changes. The paleoenvironmental interpretation of interval II strongly depends on the assessment of micropaleontological samples to disentangle dilution-dissolution-productivity.
Age model:
- Can high energy depositional processes of contourite/drift deposits affect the identification of magnetochrons? This possibility should be discussed and ruled out for Site U1564.
- The revised age model for Site U1564 is based on a maximal tuning approach, with a high number of tie points. Authors should provide a comparison with a minimal tuned version of the age model (as supplementary material) to ensure that the maximal tuning is not adding artificial features to the record in the time domain.
General comments:
- The introduction focuses excessively on basic information on sedimentary archives, and fails to explore in depth the role of the AMOC in the Earth system. For instance, the role of AMOC in heat exchange between the two hemispheres, and the resulting impacts of hydroclimate changes, is not even mentioned. Therefore, I strongly suggest to rewrite this section, streamlining the importance of deep time reconstructions of deep-water dynamics in the North Atlantic to better understand late Cenozoic climate change.
- There is no statement in the Conclusions on whether overturning intensity was reduced during interval II.
Minor comments:
- What does (high/low) mean in Table 1? Is it the phase relationship between record and target? Please clearly state it.
- Please revise throughout the text the use of before/after (should only be used in the time domain) and above/below (should only be used in the depth domain).
Citation: https://doi.org/10.5194/egusphere-2026-3764-RC2 -
RC3: 'Comment on egusphere-2026-3764', Anonymous Referee #3, 25 Sep 2026
reply
General comments
Karatsolis et al. present a new high-resolution sedimentological and geochemical record from IODP Site U1564 on Gardar Drift spanning approximately 6.2-5.0 Ma. The study extends the record of this North Atlantic contourite system into the latest Miocene and earliest Pliocene, complementing the younger record from the same site presented by Sinnesael et al. (2025). The authors combine XRF-derived elemental ratios, a calibrated CaCO3 record, sedimentation and carbonate accumulation rates, and a newly constructed astronomical chronology to investigate changes in bottom-current activity across the late Miocene cooling–warming transition and the Miocene-Pliocene boundary.
I find the dataset valuable. High-resolution records of North Atlantic deep-current behaviour across this interval remain relatively scarce, and Site U1564 provides an important record from Gardar Drift within the modern depth range of ISOW-related deposition. The manuscript also makes a useful effort to integrate the Site U1564 observations with records from ODP Site 982 and with broader climatic and gateway changes in the North Atlantic. My main concern is not the importance of the record, but how far some of the interpretations extend beyond what is directly constrained by the proxies. In several places, changes in sediment accumulation and elemental composition at Site U1564 are interpreted in terms of changes in ISOW-related bottom-current activity and subsequently discussed in the context of deep-water formation and broader Atlantic overturning. These processes are related, but they are not equivalent, and the present dataset constrains them at different levels of confidence.
I think the manuscript would be stronger if it more clearly distinguished the observed sedimentological and geochemical changes at Site U1564 from their interpretation in terms of local bottom-current and depositional conditions, inferred changes in ISOW-related influence, and possible implications for broader North Atlantic overturning.
I therefore recommend major revision.
Major comments
1. Interpretation of bottom-current activity and ISOW
The interpretation of the U1564 record in terms of ISOW activity is central to the manuscript. The combination of sedimentation rates, Zr/Rb and Ti/K provides reasonable evidence for changes in bottom-current-related deposition, and the location of U1564 on Gardar Drift provides a basis for discussing these changes in relation to ISOW. However, I think the manuscript sometimes moves too directly from these sedimentary signals to changes in ISOW strength.
In particular, sedimentation rate within a contourite system is not a direct or necessarily linear measure of current strength. Accumulation can respond to current velocity, but also to sediment supply and focusing, winnowing, erosion, bypass and migration of the current core. Likewise, Zr/Rb provides information about hydrodynamic grain-size sorting, but does not directly quantify the volume transport of ISOW. The manuscript itself shows some of this complexity: during Interval II, for example, low Zr/Rb and Ti/K are interpreted as reflecting “reduced current strength and/or sediment supply,” whereas the subsequent Discussion more specifically attributes the change to reduced ISOW influence.
The authors later acknowledge that the apparent reduction in ISOW activity could partly reflect lateral migration of drift depocentres, short-lived changes in bottom-current pathways, or localised weakening of an ISOW segment. This is an important qualification, but it does not yet seem fully integrated into the preceding interpretation. I suggest carrying this distinction more consistently through the Discussion, Abstract and Conclusions: the proxies provide evidence for changing bottom-current/depositional conditions at Gardar Drift and can reasonably be interpreted in terms of changing ISOW-related influence, whereas changes in total ISOW transport or basin-scale overturning are less directly constrained.
McCave and Hall (2006; https://doi.org/10.1029/2006GC001284) would be a useful additional reference here. Their discussion is particularly relevant because current-sensitive sediment properties primarily constrain local depositional flow conditions and can also be affected by source and winnowing.
2. Astronomical chronology and temporal uncertainty
The new astronomical chronology is an important part of the study, but I think the distinction between construction of the tuning and independent evaluation of the resulting age model needs to be clearer.
The authors tune the CaCO3 record to ET-P and subsequently note the phase and amplitude agreement between the tuned CaCO3 series and ET-P as supporting the validity and robustness of the astronomical tuning. Because this correspondence is partly imposed by the tuning itself, I do not think it provides an independent test of the age model. The comparison with the independently tuned Site 982 benthic δ18O record is more informative in this respect. I suggest distinguishing more clearly between correspondence with ET-P as a result of the tuning and agreement with independent chronostratigraphic constraints and Site 982 as evaluation of the tuning.
Some additional discussion of uncertainty would also be useful. The authors report offsets of approximately 10-25 kyr between U1564 and Site 982 and describe these as potentially reflecting small tuning errors or minor hiatuses. These offsets become relevant when the timing of changes at U1564 is subsequently compared with relatively short-lived climatic and gateway changes.
Relatedly, the manuscript is transparent that the 5.55 Ma boundary is the midpoint of a gradual transition occurring approximately between 5.60 and 5.50 Ma. However, later discussion sometimes treats 5.55 Ma as a more precise onset age. I suggest retaining the transition range when comparing the U1564 change with the LMW, Mediterranean restriction and Bering Strait opening. A new age model is not necessary, but the uncertainty inherent in the existing chronology and in the definition of the interval boundaries should be reflected in these comparisons.
3. Carbonate preservation and the interpretation of Interval III
The manuscript appropriately recognises that variations in CaCO3 can reflect several processes, including terrigenous dilution, carbonate production and dissolution/preservation. My concern is therefore not that these alternatives have been overlooked, but that the Site 982 comparison is used to narrow them more strongly than the available evidence may allow.
For Interval II, the manuscript argues that similar average CaCO3 fluxes at U1564 and Site 982 support the absence of a major sustained change in productivity or selective preservation and therefore favour reduced terrigenous input at U1564. This is plausible, but the two sites differ substantially in water depth and depositional setting, and U1564 is a contourite drift where lateral sediment redistribution is important. I would therefore present reduced terrigenous dilution as the interpretation most consistent with the observations rather than as one that excludes changes in production or preservation.
During Interval III, the divergence in CaCO3 flux between the two sites is consistent with the authors’ interpretation of reduced carbonate preservation at the deeper site, but it does not uniquely demonstrate this mechanism, and identifying the responsible water mass is a further interpretative step. The manuscript already recognises the need for additional isotopic and micropalaeontological evidence to substantiate the proposed oceanographic changes. I therefore suggest keeping the interpretation of increased influence of corrosive southern-sourced water clearly as a hypothesis rather than a water-mass reconstruction demonstrated by the present proxies.
4. Regional versus basin-scale circulation
The contrasting records from Gardar and Eirik Drifts provide an opportunity to strengthen the regional interpretation. The manuscript already notes that evidence from Eirik Drift indicates a different history from that inferred at Gardar Drift and considers differentiated DSOW and ISOW pathways. It also acknowledges the possibility of changes in drift depocentres and bottom-current flow paths.
I think this contrast deserves greater weight in the overall interpretation. Rather than expecting the two drift systems necessarily to record a spatially uniform strengthening or weakening of North Atlantic overturning, their different behaviour may indicate changes in the routing or relative contribution of the major overflow pathways. This would also provide a useful framework for interpreting the U1564 record without requiring every change in Gardar Drift deposition to represent a proportional basin-wide change in overturning.
The existing comparison with longer-term North Atlantic records could also be developed in this context. The manuscript notes evidence for generally stronger northern-component water production and overturning over an interval extending from approximately 6 to 2.5 Ma, while recognising that those records do not resolve the higher-frequency variability observed at U1564. The relationship between these different spatial and temporal signals could therefore be discussed more directly. In particular, the authors could consider whether the U1564 record reflects a reduction in northern-source export or a more regional change in the expression or routing of ISOW at Gardar Drift. The present dataset does not appear sufficient to distinguish these possibilities, but acknowledging them would provide a more balanced interpretation of the regional significance of the U1564 record.
5. Climatic and gateway controls
The discussion of the LMW, Bering Strait and Mediterranean restriction provides useful context, and the manuscript generally uses appropriately conditional language when discussing possible mechanisms. However, I think the distinction between temporal correspondence and evidence that these changes drove the U1564 transition could be maintained more consistently.
For example, the manuscript notes that a key phase of Bering Strait opening at approximately 5.6-5.4 Ma was coeval with reduced bottom-current activity during Interval II. It similarly notes that the shutdown of Mediterranean outflow and subsequent isolation of the Mediterranean occurred close to the inferred onset of weaker ISOW. These temporal relationships are interesting, and modelling studies provide plausible mechanisms by which either gateway could influence North Atlantic circulation. They do not, however, establish which mechanism, if any, caused the transition recorded at U1564. This distinction is particularly important given that the Discussion section is currently titled “Tectonic drivers of change in deep water formation.” I suggest a more neutral framing such as “Potential controls on late Miocene deep-water circulation.” This would better match the level of evidence and would also accommodate the LMW itself, which is climatic rather than tectonic.
6. Abstract and Conclusions
Following the points above, I suggest revisiting the Abstract and Conclusions so that the level of confidence in the main statements more closely reflects what is directly constrained by the record. The evidence for a substantial change in the depositional and bottom-current regime at Gardar Drift is convincing. The interpretation of this change as reduced ISOW-related influence is reasonable given the location of the site and the combined proxy evidence. However, implications for total ISOW transport and basin-scale overturning are less directly constrained, while attribution to warming or particular gateway changes remains based primarily on temporal correspondence and physically plausible mechanisms.
Similarly, the Interval III carbonate record provides evidence consistent with changing preservation at the deeper site, but the specific attribution to corrosive southern-sourced water remains a hypothesis without an independent water-mass tracer. I suggest retaining these distinctions in the final synthesis rather than presenting all stages of the interpretation with the same degree of certainty.
I hope the above comments are helpful. Overall, I find this a valuable study that presents an important new record from Gardar Drift. I hope the manuscript will be considerably strengthened by addressing the points above, and I look forward to seeing a revised version.
Citation: https://doi.org/10.5194/egusphere-2026-3764-RC3
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 220 | 110 | 36 | 366 | 48 | 22 | 19 |
- HTML: 220
- PDF: 110
- XML: 36
- Total: 366
- Supplement: 48
- BibTeX: 22
- EndNote: 19
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Manuscript titled " Evolution of deep-water circulation in the North-East Atlantic during the latest Miocene warming" by Karatsolis et al., et al. [egusphere-2026-3764] present an original unpublished major element data (XRF-scan) and astronomical tunning for site U1564 (Gardar drift) between 5.0 to 6.2 Ma. A large number of studies have already shown that Gardar drift is a great site for paleaoceanographic studies aimed at reconstructing the AMOC and the study period, which includes the Miocene/Pliocene transition, is a key moment when major climatic, tectonic and global thermohaline circulation changes of maximum interest took place.
The data obtained is high resolution and looks fantastic. Astronomical tunning is base in magnetostratigraphy (only two tie points) and Carbonate content obtained from major element data (Ca/Ti ratio) and shipboard-measured CaCO3.
The greatest weakness of the presented study is that authors base all their work on element composition obtained by XRF-Scan (mainly Ca/Ti, and slightly Zr/Rb and Ti/K ratios). With these data, the authors have done a magnificent job but at the same it is a major limitation for the discussion and the conclusions that can be drawn. The study also is based on a previous paper at the same site by Sinnesael et al., 2025 that covers a different period, which gives greater robustness to the work presented
Despite all the issues presented, I believe that the conclusions obtained by the authors are in line with the nature of the data and the range of error of the age model. Three intervals are distinguished that affect the evolution of the AMOC and that are similar to those observed in other regions on a global scale, only for this reason I think it is worth publishing this study although we must wait a little longer to have more data (SST, oxygen isotopes in forams, a detailed study of carbonate, increase age model tie point etc…) to reach more robust conclusions, as the authors point out in their conclusions.
However, at this moment I can only recommend publication after minor/major revision because the whole interpretation about the presence of waters originating in the southern hemisphere that promote corrosion in carbonates seems to me too poorly supported. To add something more in this regard, having only XRF-Scan data available, the authors could explore the correlation between Sr and Ca along the control or the Ca/Sr ratio. In both cases it can give us an idea of what may be happening with carbonates. Plotting this elemental ratio could be a nice support for carbonate discussion.
It is also not clear whether the Ti/K ratio is associated with changes in Iceland-sourced terrigenous input (as proposed in previous studies) or with changes in the ratio of fine to coarse material associated with the activity of the currents that give rise to the contourite. The two ratios (Zr/Rb and Ti/K) have a very high similarity (Fig. 7) during the Interval I and II, and only show some differences during interval III. The authors could include a simple figure to indicate how an intense/weak current contributes more or less material from Iceland or if, on the contrary, both ratios depend on the current intensity and therefore on the concentration between fine (Rb and K) or coarse material (Ti and Zr).
In addition, in table 1 the authors assume that warm moments result in a higher carbonate content in the record. However, the Ca/Ti ratio in the Atlantic represents in most cases the dilution of carbonate by the detrital material (in stead of paleoproductivity, even more in a contourite drift setting) and taking into account the dramatic changes that occur during the period studied, I dare to predict that when the authors have paleo-SST or stable isotope data available, it is quite possible that the Ca/Ti peaks will change their relationship with orbital parameters along studied interval. This should not be a problem for the observations and conclusions reached and the authors themselves recognize a gap (error range) between 10 and 25 kyr. So, again, the problem is that Ca/Ti ratio is sensitive to everything (carbonate corrosion, detrital input, carbonate production, winnowing...) and it is impossible to know what is happening in the environment having this ratio alone, without further support from other proxies or observations.
It would also be helpful, but not mandatory, to perform Principal Component Analyses (PCA) in obtained elemental database at each interval, to see if Ca and Sr lose their correlation showing that something is changing in carbonates, or if we are moving from a more contouritic sedimentation to one dominated by more pelagic component (May be Bahr, A. et al., 2014 G3 approach could help to see how the different components of PC1 or PC2 could be associate with more or less pelagic/contourite influence).
Finally, in the discussion chapter dedicated to tectonics/thermohaline circulation, authors could also briefly talk about the changes in the thermohaline circulation forced by tectonics that is seen in other regions, like these affecting the AMOC associate with the closure of the Central American Seaway (Karas et al., 2017 Sci Rep.) or the Indian ocean (Antartic ice sheet expansion, Indonesian Troughflow (Tagliaro et al., 2022 Paleoceanography and Paleoclimatology)… with these planetary changes authors may explain why the relationship observed in the study area (AMOC) with CO2 is not so obvious. In fact, the discussion on CO2 is not very clear in the current version either.
Minor comments:
I hope these comments are helpful to the authors.