the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Aug 2025
Decoding the North Atlantic Ocean Circulation Breakthrough in the Aptian–Albian Transition
Abstract. The Aptian–Albian interval was marked by significant climatic changes driven by intense volcanism, monsoonal activity, and shifts in ocean circulation, which influenced sedimentary expression of oceanic anoxic events (OAEs) and Cretaceous oceanic red beds (CORBs). The formation of CORBs was primarily influenced by oxygen flux, sea-level changes, and atmospheric dust, with thermohaline circulation playing a key role in deep-water oxygenation. This study combines magneto-cyclostratigraphic analyses from Ocean Drilling Program (ODP) Site 1049 to assess the temporal synchrony of CORB-related events between the Tethys and North Atlantic. The results provide new insights into CORB formation and paleoclimatic conditions during the Aptian–Albian interval. The onset of long-term Aptian CORBs is linked to global cooling and intensified thermohaline circulation, while Albian CORBs exhibit shorter, cyclic deposition influenced by orbital forcing. Orbital tuning of short geomagnetic reversals at ODP Site 1049 reveals that the M-2r reversal occurred at 110.76 Ma with a timespan of 150 kyr, and the reversed-polarity subchron "3" was between 111.45 and 111.53 Ma, which represent important tie points for geochronological models of Aptian–Albian interval.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3832', Helmut Weissert, 26 Sep 2025
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RC2: 'Comment on egusphere-2025-3832', Anonymous Referee #2, 28 Sep 2025
The study’s contribution lies in establishing a highly precise chronostratigraphic framework for the Aptian–Albian section at ODP Site 1049. This precise age control rigorously constrains the timing of CORBs, thereby providing a solid foundation for subsequent regional correlation and mechanistic studies. However, the discussion regarding the CORB formation mechanism could be significantly enhanced by providing greater depth and incorporating more independent evidence, which currently represents the main area for improvement in this manuscript.
Main Concerns:
The authors propose that "long-term Aptian CORBs were driven by sustained thermohaline circulation and global cooling (Cold Snap)." To enhance the persuasiveness of this mechanistic conclusion, the authors are encouraged to consider the following points:
- To fully verify the proposed "enhanced thermohaline circulation" and "global cooling" mechanisms, it would be highly beneficial to incorporate or discuss critical geochemical and paleoclimatic evidence.
- Given the clear cyclical characteristics of the Aptian CORBs, a more detailed investigation into the coupling relationship between short-term orbital control and the proposed long-term thermohaline circulation drive would strengthen the analysis.
- The assertion that the erosional event between ∼114.5 and 111.9 Ma may have caused a global hiatus needs careful re-evaluation. If a global scale is proposed, the manuscript should clearly address: a) the reasons for any inconsistency in the record (unconformity presence/absence) across correlated sections; and b) supporting evidence from other global sections.
Specific Text and Figure Issues
Lines 103-106: To strengthen the paleoclimate argument, the discussion regarding the use of clay mineral assemblages and benthic foraminifera species could benefit from including necessary detail and specific literature support on how these proxies reflect the inferred climate changes.
Lines 137-143: The evidence provided to overrule previous interpretations of the Aptian magnetic reversal as a diagenetic artifact appears insufficient. Relying on a single stained sample retaining initial magnetization is not fully convincing; incorporating more direct paleomagnetic evidence would be valuable for conclusively excluding diagenetic influence.
Lines 148-149: Clarification is needed regarding the statement citing Ryan et al. (1978) concerning seven short reversals. This reference is currently vague and potentially confusing in both the text and figures. It would be helpful if the authors explicitly named these reversals and clearly indicated their positions in the section.
Figure 2d: The reference to "Holes" in the Figure 2d legend appears inconsistent with the main text description.
Figure 3: The contrast between the vertical axes of the two boreholes (depth vs. age) in Figure 3 hinders direct data comparison. It is highly recommended that the authors unify the vertical axis unit (e.g., convert both to age) or provide the depth-to-age conversion clearly in the figure/caption. Additionally, the text should include a necessary background description for the PLG core.
Line 223: Please verify the figure citation in line 223: "Figures 3c and 3d" appears to be an error and should likely be corrected to "Figures 5c and 5d."
Lines 256-257: The description of CORB B, particularly the assertion that its duration is "consistent with Milankovitch cycles," is currently too brief. To support this claim, the authors should provide a more detailed description of the eight main reddish-brown levels and relevant data.
Citation: https://doi.org/10.5194/egusphere-2025-3832-RC2
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- 1
Ramos et al. present a cyclostratigraphic dataset based on earlier published magnetic susceptibility (MS) records from ODP Site 1049 (Blake Nose, North Atlantic). In the methods chapter, the authors reinterpret the published Aptian magnetostratigraphy of ODP Site 1049 containing several short reversals.. The authors consider these reversals as initial and this provides them with a higher resolution of Aptian stratigraphy. Aim of the their stratigraphic study is a more precise dating of CORB (red bed) intervals and, in consequence, a new (?) paleoceanographic interpretation regarding the formation of CORBs.
The manuscript is clearly structured, with data shown in several graphical displays, some of the figures are discussed below. The interpretation/paleoceanography section remains, in part, rather general and not veryinformative (see below). In the following paragraphs I add comments which may help improving the manuscript.
Introduction
Citation style: e.g. LIP’s and climate: the authors cite two papers from 2024, this is fine, however, the authors need to show with a selection of citations that this is a topic which has been investigated over the last 40 years. I recommend including early literature, as these authors offered key hypotheses that later research expanded upon (Arthur; Larson etc). The same comment is valid for the summary on CORB research.
Line 57 add role of cooling or Aptian Ice Ages as forcing factors (see e.g. Weissert, 1989, Surveys in Geophysics including early relavant literature).
Geological setting
Line 93 Carbonate Compensation Depth CCD, I prefer Calcite Compensation Depth sensu Bramlette 1961 (in contrast to Aragonite Compensation Depth, ACD)
Line 94 -97 Composition of sediment: you write correctly that the sediment is a clayey calcareous nannofossil chalk and claystone, but you also write that the sediment is composed of quartz, limestone clasts, dolomite etc. Please clarify.
Line 105 please cite earlier literature on this topic which has been investigated over the last 40 years (see works by Premoli Silva etc, or, a younger article, Giorgioni et al, 2017 among others).
Figure 3 please define PLG when you use the term for the first time. I recommend to add a graph of the PLG core plotted against depth in meters (you may add this in figure 2 ?).
Figure 3 Chemostratigraphy: Most of OAE1b is missing at ODP Site 1049. Published C-isotope stratigraphy supports this observation. According to your documentation the long gap (red dots) begins at 114.5 Ma. If I look at the C-isotope stratigraphic curve below the gap, it shows a trend from more positive values less positive values (143-145m) at the ODP Site (amplitude 1permil) but a much smaller fluctuation at the PLG core (amplitude < 0.5 permil). Please comment on this discrepancy (?) in the pre-gap isotope curve (why not correlate the decreasing trend with the trend between Jacob and Kilian?). And, please use the same scale for the plot of your C-isotope data in your figure 3 (much more expanded C-isotope scale at PLG!).
Fig 3 and Fig 6: ODP core is plotted against meters, PLG core against age. Please use meters in both records. In Fig 3 CORB A ends at level143 m (younger than 114.6 Ma). In Fig 6 you end CORB A near 114. 6 Ma, if I am correct (it is difficult to switch from meters to age in the two sections).> I recommend to also mark the end-Aptian gap in fig 6, this makes reading of the figure easier. (“A” ends at the unconformity, if I am correct).
Line 257 Please provide a more detailed description of the eight reddish levels, including the thickness of the beds, the characteristics of the sediments between the reddish beds, carbonate content, and other relevant details. Additionally, clarify the duration of these “levels”, shown in your figure 6, which is presumed to be measured in thousands of years (not in Ma as indicated in “duration” in your fig. 6).
Origin of CORBs
This discussion is rather long and it summarizes, in part, quite well-established interpretations of the link between red sediments and cold snap(s) > cite relevant literature. CO2 reductions and Aptian “Ice Age” (see for early literature e.g. Weissert and Lini, 1991 and earlier literature therein).
325 Your discussion on deep-water oxygenation remains rather general: “sustained and effective thermohaline circulation”, where were sources of deep-water formation, during greenhouse times and during cold snaps, was evaporation a possible way to form dense water e.g. on Arabian platform as suggested by Nd-Isotope data for the Late Cretaceous etc. See also early discussions on Cretaceous paleoceanography at times of no major polar ice caps in several papers by William (Bill) Hay. There are also several studies on Albian cyclostratigraphy and paleoceanography available in the literature (e.g. Giorgioni and others).
336 Here you list several processes which may or may not have influenced deep water circulation in the Cretaceous. Please consider the time frame for these processes, storms and cyclones , for example, had an impact which is most probably not recorded in oxygenation state of Cretaceous pelagic sediments, and so on. > Please shorten or discuss more accurately.
Unconformity
Unconformities remain a significant topic in Cretaceous paleoceanography. The correlation of gaps between ocean basins will require further studies. At Blake Nose, the gap spans the Late Aptian, while other notable pelagic gaps, such as at the Cismon Site in northern Italy (Tethys), began earlier and ended later. And, look also at discussion of Albian to Turonian Ocean circulation and deep-water currents in the Tethys, including discussion of red beds in Gambacorta et al., 2016.
The 356 and 366 prominent gaps in shelf successions likely indicate stronger currents during greenhouse climate periods (see current patterns during OAE2, Wohlwend et al. 2015). Both orbital changes and significant mid-Cretaceous climate fluctuations appear to have greatly influenced deep-water and shelf current strength. I suggest discussing additional factors that may affect erosive currents and referencing literature that highlights the complexity of gap stratigraphy beyond what is presented in this study.