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| 29 Dec 2025
Pleistocene Benthic Foraminifer Bioevents in the Central Arctic Ocean: stratigraphic and paleoceanographic implications
Abstract. Benthic foraminifers show distinct temporal and spatial distribution patterns in the Central Arctic Ocean (CAO) demonstrating their potential to provide robust age constraints and to address paleoceanographic change in the Pleistocene. Several benthic foraminifer bioevents have been previously reported from the upper and middle Pleistocene that are here critically evaluated by studying three sediment cores from the Mendeleev and Lomonosov ridges and analysing published data sets. Based on this data bioevents are defined by using absolute abundances of species in the >63 µm grain size fraction, whereas relative abundances are considered not reliable because taphonomic processes such as disintegration and/or dissolution overprint the original assemblage composition. Bioevents are calibrated to lithological horizons and then linked to Quaternary subseries and marine isotope stages based on available independent stratigraphic data.
Three calcareous bioevents can be defined in the Brunhes Chron (Middle Pleistocene): (1) the highest common occurrence of Bolivina arctica (~MIS 9) at the top of lithological unit L in brown bed B 7, (2) the lowest common occurrence of Oridorsalis umbonatus at the base of brown bed ?B 4 (~MIS 7), and (3) the acme of Bulimina aculeata (~MIS 7) in brown bed ?B 4 in water depths of less than ~ 2000 m. The lowest common occurrence of Oridorsalis umbonatus is coeval with the base of the acme of Bulimina aculeata at shallow sites (<2000 m). The proposed correlation to marine isotope stages should be considered provisional and subject to modifications as additional age tie-points become available. So far numerical ages for these bioevents are too imprecise due to the limited number of biostratigraphic and radiometric ages.
Further benthic foraminifer bioevents may be useful for stratigraphic correlation on a regional to supra-regional scale but require evaluation of previous taxonomic identifications and additional sediment core studies. The extinct agglutinated species Haplophragmoides obscurus disappeared on Lomonosov Ridge in the Middle Pleistocene but the complex taxonomy and the few data on the occurrence in arctic sediment cores currently prohibits the application as biostratigraphic marker. The assemblage turnover from agglutinated to calcareous benthic foraminifera occurred close to the first downcore change of normal to reverse magnetic polarity and might be a synchronous event in the eastern Arctic Ocean in middle Pleistocene sediments older than MIS 11 indicating a possible relation to the mid-Brunhes event. This fundamental change in assemblage composition is time-transgressive because it probably occurred in the Amerasian Basin in the Early Pleistocene. However, there is sedimentological evidence for a significant gap in the sedimentary sequences on Lomonosov Ridge at the stratigraphic level of the assemblage turnover. Since stratigraphic tie-points are not available for the sequences below this event, it remains speculative if the ages are closer to each other in both basins.
In the Late Pleistocene the identification of bioevents is hampered by sporadic occurrences of benthic foraminifera, and the disputable chronostratigraphy due to possible hiati and/or condensed sections in MIS 2 to MIS 5 sediments. The identification of MIS 5 is a controversial issue, and it might be missing in some cores from Lomonosov Ridge, possibly due to extensive carbonate dissolution, while certain brown layers in the Amerasian Basin are potential candidates for this interglacial. The acme of Siphotextularia rolshauseni that was previously described as stratigraphic marker for MIS 2 sediments in the Norwegian-Greenland Sea can only be used in the Fram Strait area and at the upper continental slope of the northern Barents Sea. Pullenia bulloides, frequently used to identify MIS 5a in polar to subpolar sediments, is only sporadically present in Pleistocene sediments from the CAO and is not confined to a specific stratigraphic interval. Since this species shows variable abundances in cores from water depths less than 2000 m in the Fram Strait area and at the northern Barents Sea continental margin in the Pleistocene, it is not anticipated that it is a stratigraphically useful species.
The bioevents in the CAO are caused by a complex interplay of various biological processes. Apart from B. arctica and H. obscurus that likely evolved in the Arctic Ocean, the species characterizing these bioevents such as B. aculeata and O. umbonatus must have invaded the Arctic Ocean from subpolar latitudes. Since an unrestricted exchange of water masses with subpolar latitudes is only facilitated through Fram Strait, these intermediate to deep-water species had to be transported as juvenile specimens (propagules) by Atlantic Water to CAO sites during time periods favourable for their propagation. The possible time span of a vital transport, and thus the maximum reachable location for settlement within the Arctic Ocean, depends on the species, the vitality of a respective specimen, the local environmental conditions, and the strength of Atlantic water advection. The environmental conditions, in particular the availability of food, play then a major role for the successful colonization at a particular site, not only for the invading species but also the species endemic to the CAO (H. obscurus, B. arctica). These sites must face a high (H. obscurus, B. arctica, O. umbonatus), or significantly higher particulate organic carbon export to the sea floor than today (B. aculeata). Such environmental conditions must have occurred basin-wide to trigger the synchronous and coincident changes in assemblage compositions. Moreover, external forcing may have triggered environmental change. The onset of a massive discharge of detrital dolomite-rich ice-rafted debris might have caused the abrupt collapse of a Bolivina arctica dominated fauna and almost disappearance of Haplophragmoides obscurus. The most conspicuous change in the environment is expressed in the turnover from predominance of agglutinated to calcareous foraminifer which was probably caused by a fundamental change in food supply and its quality. However, the formation of bioevents cannot be attributed alone to biological processes. Due to selective dissolution of thin-shelled epifaunal taxa, assemblages are enriched in robust epifaunal and/or infaunal calcareous species, or may consist only of a agglutinated taphocoenosis.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6290', Anonymous Referee #1, 29 Jan 2026
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AC1: 'Preliminary response to anonymous reviewer #1', Jutta Wollenburg, 23 Feb 2026
In this post we focus on the general comments by the anonymous reviewer whereas the rather detailed specific comments will be addressed in the formal response to this review.
The anonymous reviewer has raised some interesting viewpoints of the Arctic Ocean biostratigraphy and we fully agree that it is not an easy task to apply benthic foraminifer bioevents. However, we do not agree that the community should be convinced not to apply any bioevent for stratigraphic correlation in the Arctic Ocean. Below we will address the major points individually.
At the beginning of his general comments the reviewer criticizes that we did not propose a novel way forward nor did we make a strong assertion that researchers currently using the bioevents should stop applying these methods. The reviewer also raises methodological concerns in the application of the bioevents and critizes that many of the bioevents used in the manuscript rely on common occurrences as ‘biomarkers’ rather than first and last appearances typically viewed as necessary in biostratigraphy: The reviewer is correct in saying that the applied methods are not novel but previous research has shown that this bioevent approach is useful for stratigraphic interpretations in the Pleistocene. Since there is little evolutionary turnover in Pleistocene benthic foraminifers (see below) an alternative approach has been developed to provide biostratigraphic information for sediment cores from northern subpolar to polar latitudes. In deep-sea sediment cores from the Norwegian Sea, Streeter et al. (Streeter et al., 1982) observed that the calcareous benthic foraminifer Pullenia bulloides shows a distinct maximum in relative abundance at the transition of marine isotope stage 4 to 5. Subsequent studies by Haake and Pflaumann (Haake and Pflaumann, 1989) and Haake et al. (Haake et al., 1990) revealed consistent P. bulloides absolute abundance maxima (specimens per ccm) in the Norwegian-Greenland Sea in MIS 5a. Thereafter, Fronval and Jansen (Fronval and Jansen, 1996), Knies et al. (Knies et al., 1998), Wollenburg et al. (Wollenburg et al., 2001), Bauch et al. (Bauch et al., 2000) and many others used P. bulloides absolute abundance maxima as stratigraphic tool to constrain MIS 5a. Similarily, the agglutinated benthic foraminifer Siphotextularia rolshauseni depicts a consistent maximum in absolute abundance in cores from the Norwegian-Greenland Sea in early MIS 2 that may be used for correlation of sediment cores (e.g., Nees and Struck, (Nees and Struck, 1994), Bauch et al., (Bauch and Bauch, 2001); Wollenburg et al., (Wollenburg et al., 2001). Like the bioevents described in our manuscript, these events were attributed to periods of more intense Atlantic water advection. These studies show that certain benthic foraminifers show distinct coeval distribution patterns/abundance maxima that can be calibrated to specific marine isotope stages in defined oceanic areas. Therefore, we do not agree that this established stratigraphic approach in Pleistocene benthic foraminifer work should not be applied in the Arctic Ocean.
Since an Arctic Ocean chronostratigraphy cannot be established with a single method and in particular traditional planktic microfossil and stable oxygen isotope stratigraphy is only useful for specific time intervals, we reviewed published benthic foraminifer data to evaluate the potential of abundance patterns for stratigraphic correlation. In the central Arctic Ocean, distinct stratigraphic occurrences and abundance maxima of benthic foraminifer species have been recognized for quite some time (e.g. Herman (Herman, 1974); O´Neill (O'neill, 1981); Ishman et al. (Ishman et al., 1996); Cronin et al. (Cronin et al., 2014), e.g. a distinct abundance maximum of Bulimina aculeata is restricted to a narrow stratigraphic interval in the Pleistocene e.g.(Jakobsson et al., 2001; Polyak et al., 2004). Bolivina arctica that occurs in low abundancies in Pleistocene sediments from the central Arctic Ocean, shows a temporal spatial pronounced highest common occurrence terminated by a distinct decrease at a distinct stratigraphic level e.g. (Herman, 1974; Scott et al., 1989). Therefore, Backman et al. (Backman et al., 2004) and Polyak et al. (2004) used relative abundance maxima and range bottoms of certain taxa for correlating sediment cores across the Arctic Ocean.
In our manuscript the comparison of absolute abundances (specimens/g) with relative abundances (% of a species in an assemblage) revealed that absolute abundances are confined to narrower stratigraphic intervals than relative abundances, because bioturbation in these low sedimentation settings (Löwemark and Singh, 2024) may cause misleading occurrences of species in glacial intervals with low benthic foraminifer contents. Dislocation will result in high relative abundances but low absolute abundances of taxa in these glacial sediments. This point is clearly addressed in Figure 2 of the manuscript which compares relative and absolute abundances of Stetsonia horvathi and demonstrates this advantage of absolute abundances over relative abundances. As a high absolute abundance of a species in interglacial sediments increases the likelihood of dislocation by bioturbation into underlying glacial sediments, the relative abundance maximum of e.g. B. arctica is therefore, often shifted into glacial sediments that reflect adverse environmental conditions (Fig. 3).
Reply to the reviewers request to work with first and last appearances when applying stratigraphy: In an evolutionary sense within the time interval we are looking at, there are no first or last appearances of deep-sea benthic foraminifera species in the Arctic Ocean (except for the disappearance of Haplophragmoides obscurus). Studies suggest a drastic change in Arctic water mass structure, ice cover, and phytoplankton composition during the Mid-Brunhes Transition (MBT)~ 430–350 ka (Kender et al., 2019). With the increase in the amplitude of glacial cycles expressed after the MBT, an expansion of sea-ice extent and thickness and an intense Atlantic Water advection is presumed (Polyak et al., 2013). The phytoplankton is affected as well (see Fig. 5), thus, different phytodetritus for the benthic community became available. Very obviously the extinction of H. obscurus is linked to these environmental change, and B. arctica which has been recorded for at least the last 1.5 Mio. yrs diminished after the MBT. To us it is also logical that the B. aculeata and O. umbonatus bioevents are a reflection of the intensified Atlantic Water advection in one of those interglacials after the MBT, when the modern Arctic deep-water benthic foraminiferal community hadn’t stabilized yet. Thus, in our cores from 1073 m (PS2185-5), 2351 m (PS72/340-5) and 2732 m (PS72/396-5) (Figure 1, Table 1), and the cited literature we observe a basin-wide coincident acme of Bulimina aculeata (at water depths <2000 m), the lowest common occurrence of Oridorsalis umbonatus, and the highest common occurrence (HCO) of Bolivina arctica that we assign to respective bioevents routed in fundamental paleoecological changes. This manuscript focuses on the application of bioevents as a stratigraphic basis. More comprehensive paleoecological statements would require a detailed evaluation of all species present and statistical analyses that include modern fauna e.g. (Wollenburg et al., 2001; Wollenburg et al., 2004).
The reviewer comments that „the relative success of relative abundances and absolute abundances in identifying events is not systematically evaluated”. In all figures depicting the stratigraphic distribution of species both absolute and relative abundances are shown. We did not further expand on this issue because we have included as an example Figure 2 with a comparison of relative and absolute abundances. But, if requested we may address this issue with each bioevent discussion (e.g. above for B. arctica) in the revised version of the manuscript. Generally, using absolute abundance data makes stratigraphic work in the Arctic Ocean easy because taxomomic knowledge of only stratigraphically important taxa is needed. In contrast, for a correct counting of relative abundances all species and specimens must be identified and a comparison between different labs does require similar taxonomic concepts for all species.
Different to many central Arctic Ocean foraminiferal studies we worked on the size fraction >63 µm, as Polyak et al. (2004) already showed that species such as Bolivina arctica are too small to be adequately recorded in the size fraction >150 µm. In our manuscript we define bioevents using absolute abundances (specimens per gram dry sediment) of the grain size fraction >63 µm from large sample volumes (76->100 g per sample) and samples that contain large specimen numbers. Such a strict definition of the data used to describe Arctic bioevents is at present not applied in routine stratigraphic work. Often only the larger grain-size spectrum >125 µm or >100 µm is considered in foraminifera analyses and interpretations are based on relative abundances or the presence of species at a specific stratigraphic level. Information on sample size, specimen numbers per sample, actual specimen counts, relative (%) and absolute (nos./g dry sediment) abundances of calcareous and agglutinated species of all species mentioned in the manuscript and of all cores are deposited in Pangaea and will be available for download after acceptance of the manuscript.
The reviewer states that common occurrence bioevents are prone to spatial differences in environment and preservation and are generally not seen as reliable and that the impact of changing sedimentation on the abundances being used to recognize bioevents is not addressed: We agree that common occurrences need to be reliable for stratigraphic analyses. Therefore, we reviewed publications dealing with benthic foraminifer analyses in the Central Arctic Ocean, and studied three new cores in order to discuss the potential of a number of species for establishing bioevents. As we have described in the manuscript, only three species are currently useful and e.g. both P. bulloides and S. rolshauseni, used in the Norwegian-Greenland Sea for stratigraphic purposes, do not show spatial and temporal coincident occurrence peaks in the Central Arctic Ocean. Moreover, we discuss in the appendix that e.g. Epistominella exigua is not a suitable biostratigraphic marker because of taxonomic uncertainties, and because high abundances of this species are rather a local phenomenon. A detailed taxonomy with ecological requirements, shell characteristics, and preservation potential of selected species complement our manuscript (Appendix A and B).
Each fossil foraminifera assemblage has lost a significant percentage when compared to the modern fauna. We have discussed and described, that loss for the Arctic Ocean in general and, moreover, for each of the bioevents, extensively the influence of calcite dissolution on the high relative dominance of the respective infaunal taxon. We added the Appendix B: ‘Ecology which comprises shell characteristics, and preservation potential’ of main calcareous taxa, including e.g. the mean shell thickness of the discussed taxa, and their habitat. These information are important for describing the subjective preservation potential shown in Fig. 12. Fig. 13 provides images on exemplified assemblages to illustrate the extent of diagenetic alteration. Our analyses reveal that diagenetic alteration mainly influences the relative abundance of the bioevent characterizing infaunal species (B. arctica, B. aculeata, O. umbonatus) (Fig. 13, Appendix B), especially due to the preferential dissolution of small-sized, thin-shelled epi- to shallow infaunal taxa like Stetsonia horvathi. As the bioevent is characterized by moderate infaunal taxa the interval of highest common occurrence of B. aculeata, B. arctica and O. umbonatus are least affected by dissolution.
As absolute ages cannot be assigned to each depth in the sediment cores, it is impossible to calculate reliable sedimentation rates. From radiocarbon measurements of PS2185-6 (Wollenburg et al., 2023) we know that the top 20 cm were deposited over roughly 30 14C ka which would result, ignoring likely different sedimentation rates between glacial and Holocene sediments, for this core section in a mean sedimentation rate of ~6 mm/14C ka. In the box corer taken at site PS72/396 radiocarbon ages of 18.6 14C ka at 6.5 cm sediment depth indicate a mean sedimentation of ~3 mm/14C ka for MIS2-Holocene. In both cores below these depths calcareous shells were significantly affected by authigenic overgrowth (Wollenburg et al., 2023) resulting in unreliable 14C -ages. Without a robust age model and reliable sedimentation rates, no one should calculate accumulation rates. Therefore, we are just working with the intervals of HCO of certain taxa in specimen-rich samples when identifying the respective bioevents in our cores (e.g. the HCO B. arctica in samples with a mean of 92774 benthic foraminifera per sample for core PS2185-6). All respective tables can be downloaded from Pangaea once the manuscript is published
The reviewer further states that the use of “absolute abundance” and the approach to count 300 specimens from samples splits of each sample for defining bioevents, would be heavily affected by changes in sedimentation rates and hiatuses: The approach to count 300 specimens from sample splits of each sample is the traditional approach in foraminiferal investigations when maximum diversity stability is of interest and to ensure that every foraminifera from a sample proportion was identified. Why this approach should be heavily affected by changes in sedimentation rates or hiatuses, is unclear to us as this is only a method to ensure a constant quality in counting and sample representation. Furthermore, X-rays, photos, linescans, xrf-scans etc. from the respective sediment cores of the sites investigated were used to search for potential hiatuses. We even opted for PS2185-6 as one of the cores because of the large data set available for this core. However, independent on the kind of analyses performed on central Arctic Ocean cores, even with utmost care small hiatus can never be ruled out, that is why we are plotting our results versus depth and show at which depths age fix points are found. Despite low sedimentation rates and eventual hiatus in the sediment cores, the positive aspect is that bioevents are strictly linked to brown layers and thus interglacial/interstadial conditions. The highly variable sedimentation rates of glacial gray or pink sediments are irrelevant to this study, except to show that in such sediments, the very low foraminiferal abundances can indeed lead to relative abundance peaks.
The reviewer assumes that in discrete samples where a particular number of specimens would be counted, the absolute abundance would be very much affected by the other taxa. He states, ‘If I would pick 300 specimens (as recommended on line 992) and there are no other species, I would get 300 of one species and thus a higher abundance than if I would if many other taxa were present. Similarly if the constraint is to pick 1 gram of sediment’: We disagree because if only e.g. B. arctica specimens would be found in one sample of core PS2185-6 (mean sample weight ~100 g), these 300 specimens would have a relative abundance of 100%, while their absolute abundance would be only 3 nos/g.dry weight (300 specimen divided by 100 g sediment). This is not comparable with an absolute abundance of ~300-1500 nos/g. dry weight that this species may reach in our cores. Again, the abundance of other taxa in a sample is irrelevant if you calculate absolute abundances.
The reviewer further states that the manuscript concedes that proposed correlations are preliminary and numerical ages are “too imprecise” (in abstract) and states that there is no robust independent chronostratigraphy available (Line 571): We have to accept that the Arctic Ocean chronostratigraphy still has a lower temporal resolution than the Pleistocene chronostratigraphy in most other oceans (e.g. O´Regan et al., in press). Due to the high freshwater input (e.g. Morris, 1988; Nørgaard-Pedersen et al., 1998) and diagenetic alterations (Wollenburg et al., 2023), stable isotope curves in the Arctic Ocean do not correspond to those in the global ocean (Lisiecki and Raymo, 2005) and comprise considerable gaps due to absence of calcareous foraminifers. Therefore, a stable oxygen isotope stratigraphy cannot be established and different methods must be applied to define stratigraphic tie points for Pleistocene sediments. In the time interval studied, AMS 14C ages (e.g., Wollenburg et al., 2023 and references therein), 231Pa and 230Thxs extinction ages (Hillaire-Marcel et al., 2017; Song et al., 2023) and calcareous nannofossil bioevents (Razmjooei et al., 2023) form the basis to calibrate benthic foraminifer bioevents to independent chronostratigraphic data. This allows to relate bioevents to certain time intervals that are represented by marine isotope stages, and not to exact numerical ages. That is why we stated that the correlation to marine isotope stages is provisional and the bioevents and the assigned ages should be tested in future studies. To be honest this is the only way to prove or disprove the validity of these bioevents and their age assignments. But based on our new data and the extensive literature data we are quite confident that our suggestions are not too far way off from reality.
The reviewer also stated that he didn’t understand why we included 2 deep-water cores in our study: Since previous benthic foraminifera research has mainly focused on benthic foraminifers from sediment cores located in relatively shallow water depths (500-<1900 m), we used the relatively well-studied core PS2185-6 from the shallow Lomonosov Ridge (1073 m water depth) as the reference core for previously studied shallow water sites. The Lomonosov Ridge represents a barrier to deep water exchange >1870 m between the Eurasian Basin and Amerasian Basin (Björk et al., 2007), which is why deep-water sediment cores must be considered for a reconstruction of paleo-deep water circulation/change within the Arctic Basins. Therefore, it was important to us to gather for the first time respective information on bioevents from two deep-water cores (2351 and 2723 m) from the Amerasian Basin. These new data confirmed that Bulimina aculeata is a stratigraphic marker at sites located above 2000 m water depth (e.g. Backman et al., 2004), whereas Bolivina arctica and Oridorsalis umbonatus can be used at water depths ranging from ~3000 to 560 m.
The reviewer states that the discussion then provides extensive review of ecological and environmental reasons for the abundance changes in different taxa that are often speculative and not well-rooted in the results provided in the study or connected to the biostratigraphic questions, particularly given the emphasis in other parts of the manuscript that the Arctic has complex spatial differences in environment: The reviewer did not give any specific arguments to explain his opinion, making it difficult to address these issues he obviously saw. We provided an extensive review on the general and species-specific environmental needs and preservation potential of foraminifera in the discussion and Appendix B. Furthermore, we illustrate in figures 12 and 13 taphonomic changes. We have explained that the B. aculeata bioevent had to follow an invasion of propagules by enhanced Atlantic Water advection during that respective interglacial, and required enough food availability at the respective coring sites to flourish, and the bioevent was terminated by the subsequent glacial conditions. There also was no subsequent successful invasion by this species in younger interglacials. As Oridorsalis umbonatus (as O. tener) was rarely reported below the O. umbonatus bioevent, we presume that the species also invaded the Central Arctic Ocean via the same intensified Atlantic Water advection as B. aculeata. At all sites <2000 m water depth we observe a coincident bloom of both taxa, however, food availability obviously was not sufficient to sustain a bloom of B. aculeata.
The reviewer further doubts that any of the bioevents are indeed robust, particularly given the conceded lack of radiometric ages and the strong impacts of ecologic and taphonomic processes:
The proposed bioevents are relatively robust because data from a number of cores have been analysed before the three bioevents have been defined. A number of other taxa were excluded because of taxonomic uncertainties and an inconsistent stratigraphic occurrence. The assignment to marine isotope stages acknowledges the uncertainties of the age models due to the complex chronostratigraphy of the Arctic Ocean. The bioevents are characterized by infaunal taxa, which indicates times with increased labile organic matter accumulation in the sediment to allow for such a habitat. For B. aculeata the accumulation of organic matter had to be high as this is a non-arctic species is more frequently found in upwelling regions. Infaunal taxa are also more likely to be preserved in the fossil record because their shells are not exposed to the sediment-water interface were lowered pH leads to calcite dissolution. These facts are described in the discussion and summarized in Appendix B.
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Citation: https://doi.org/10.5194/egusphere-2025-6290-AC1 -
RC2: 'Comment on egusphere-2025-6290', Anonymous Referee #2, 25 Feb 2026
Based on three sediment cores from various sites in the (central) Arctic Ocean, Wollenburg and Matthiesen present benthic foraminiferal assemblage data (at quite a high resolution), thereby re-evaluating previous benthic foraminifer bioevents. These are then tentatively linked to Marine Isotope Stages. In conclusion, they find that the acme of Bulimina aculeata, the lowest common occurrence of Oridorsalis umbonatus, and the highest common occurrence of Bolivina arctica are applicable as robust bioevents in the Middle Pleistocene of the central Arctic Ocean. Overall, the manuscript presents an important improvement in the (much needed) benthic foraminifer biostratigraphy of the central Arctic Ocean. Therefore, it will present a useful contribution to the field.
Nevertheless, I have a series of remarks which should be addressed may help improve the manuscript;
-Reference to letter-named beds in the abstract. As presented, it appears as is if the letter-named beds would be universal across the basin. However, as explained by the authors further in the manuscript, this naming stems from the Western Arctic Ocean (Amerasian Basin) and does not necessarily apply across the basin (e.g. Lomonosov Ridge) – this should be made clear. In general, I’m not entirely sure how useful it is to refer to use this naming in the abstract, as many readers might not be familiar, and even for readers who are familiar some of the naming is obscure, e.g. what does “?B 4” mean?
-Line 11-12: unclear what “calibrated” means here. Perhaps change to “derived from” or “correlated across”?
-Line 14: “Brunhes Chron” Throughout the manuscript the authors seem to confidently imply that the Brunhes chron can be identified. However, the magnetostratigraphy of the central Arctic Ocean is highly controversial. Quite worryingly, the authors substantiate the statement with a reference to a non peer-reviewed document (phd-thesis). Uncertainty regarding this must be addressed throughout the document.
-Line 32: What is the reasoning for the suggestion that hiati and condensed intervals would only be limited to ‘MIS2 to MIS5 sections’? If this is the case, why wouldn’t it affect previous MIS’s too?
-Line 34: “MIS5…might be missing due to carbonate dissolution”. This sentence needs rephrasing as it currently implies only calcareous deposition occurred during that time, which was subsequently dissolved away... I think what the authors are suggesting is that MIS5 is not missing but that it would suffer from dissolution of calcareous microfossils? As this was clearly not the case for the Holocene, what is the reasoning/mechanism that his would have occurred during MIS5(e)?
-Line 48: What is “the vitality of a respective specimen”?
-Line 55-57; In arctic environments turnover form calcareous- to agglutinated-dominated assemblages are common and often linked to corrosive bottom waters, wouldn’t this be a more likely explanation?
-Line 74-75; For net catches under perennial ice, please see Vermassen et al. (2025), who report 100% N. pachyderma under perennial ice (at sites further north than C&W). Also note that according to Carstens and Wefer, N. pachyderma is the only reproducing species under perennial ice, the rare other species being expatriates from further south. https://bg.copernicus.org/articles/22/2261/2025/
-Line 118: “false specimen numbers per sample weight” is an exaggerated statement; as long as authors clearly report whether calcareous/agglutinated are counted (and in which size fraction, etc.) and how relative abundances are calculated, the results will be reproducible, not ‘false’.
-Line 154: Freeze-drying is not ideal for the preservation of agglutinated species, see e.g. https://doi.org/10.1177/0971102320200205. Given how much the authors emphasize the importance of agglutinated species this method is somewhat surprising.
-Line 167: “extrapolated to 100% of the size fraction” Always good to provide the used formula here, too.
-Line 221-224: Again, if data are provided and reported properly, one can still compare or recalculate relative abundances, it is not difficult necessarily. Even when both calcareous and agglutinated assemblages are provided, the relative abundance is sometimes calculated relative to the respective assemblage anyway.
-Lines 225-228; This raises the question how “noticeably abundant” and “low numbers” are defined in this study? This is particularly important because, as the authors point out, abundance can range from high to very low numbers. This is rather fundamental to the study but not explained.
- Lines 256-257 “Whether agglutinated and less common calcareous foraminifera were included in relative abundance calculations is usually not stated.” I am quite surprised to read this and wonder if this is true, as this is standard information that is usually reported in assemblage studies.
-Lines 261-262: “Since this work is based primarily on absolute abundances, data from Scott et al. (1989) and Lazar and Polyak (2016) could be included.”. This leaves the reader wondering why it is or is not included.
-Lines 264-266:“lithostratigraphy of the sediment cores is briefly described because sediments in the Arctic Ocean are generally siliciclastic in composition” Is not entirely sensical, and in general I would suggest to omit these introductory lines.
-Lines 281-282: Please define 'slow sedimentation' in cm/yr (or MAR) as this has different meaning for different researchers.
-287-288: “Sediments in the brown layers are sometimes coarser at the southeastern Mendeleev Ridge (Figs. 3, 4),…” Coarser than what?
-Lines 536-539: On the basis of correlation rather than direct observation, it does appears that the first occurrence of E. huxleyi in core PS1285 is inferred to lie above the uppermost foraminifer maximum and below the first diamict. Although the global evolutionary first appearance of E. huxleyi occurs in MIS 8, evidence from the high-latitude North Atlantic suggests that its first persistent occurrence in polar/subpolar basins is younger, potentially not preceding MIS 5 (Gard and Backman, 1990; Henrich and Baumann, 1994; Razmjooei et al., 2023). The precise timing of initial Arctic colonisation nevertheless remains uncertain, and resolving this diachroneity will require high-resolution studies from sites in the north the Nordic Seas. Due to this uncertainty, it would appear the uppermost foraminifer maximum could plausibly still fall within MIS 5, but an older placement (MIS 6-9?) cannot be excluded with current constraints. But it can be considered likely that at least the lower foraminifer maxima predate MIS 6. This uncertainty regarding the uppermost foraminifer maximum should be mentioned further down the manuscript too.
-Lines 545-547: This needs to be revised, Razmjooei et al. (2023) did not suggest that P. lacunosa’s extinction was due to a warm interglacial, they argue that if P. lacunosa was not present in the Arctic Ocean during glacials (e.g. it does not invade during glacials), and went globally extinct in MIS12, then logically it’s the Last Occurrence/Highest Occurrence should be indicative of MIS13. They reason that the stratigraphic “last occurrence” observed in Arctic sediments may not represent the true extinction horizon, but rather the last interval in which P. lacunosa was able to colonize the Arctic (or be preserved there) before glacial suppression of production and/or enhanced dissolution.
-Line 582: The authors mention multiple times that the (well-known) turnover from agglutinates to calcareous assemblage would be time-transgressive, but it was unclear to me what the evidence (or reasoning) for it being time-trangressive is.
-Line 707: change “eventually” to “possibly”
-Lines 841-843 are unclear, please reformulate.
-Line 975 “associated main species” reads rather awkward and unclear, I think the authors mean Subdominant/ Associated, or perhaps Accessory, species.
-Conclusions: I think it would be quite useful if the authors could provide concrete recommendations of where in the Arctic (which basins/ridges, water depths) future work on benthic foram biostratigraphy could/should be focused.
-The taxonomy appears thorough and well documented.
-In general, a table or schematic that gives an overview comparing the previously identified bioevents with the new results would be useful.
-Overall, the manuscript would benefit from a thorough redaction and spell check in order to improve readability.
Citation: https://doi.org/10.5194/egusphere-2025-6290-RC2
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General Comments
The manuscript demonstrates the difficulties of benthic foraminiferal biostratigraphy in the Central Arctic Ocean using multiple bioevents. However, the manuscript does not seem to propose a novel way forward nor does it make a strong assertion that researchers currently using the bioevents should stop applying these methods. The introduction implies that benthic forams are underutilized in biostratigraphy (starting line 89) and leads the reader to think benthic forams will be shown to be useful by the study, but this outcome does not occur. Thus, I am not clear what the authors intend to contribute with this manuscript other than to say others have said that benthic forams do not work well for biostratigraphy in the Central Arctic and when they looked at three cores to evaluate some potential biomarkers, they found that those others were correct. Since there was no real methodological advance or significant new source of data applied to challenge the prior assertion that benthic foraminifera are not useful for Arctic biostratigraphy, I don’t believe the findings are significant enough to warrant publication.
I also have methodological concerns in the application of the bioevents. Many of the bioevents used in the manuscript rely on common occurrences as biomarkers rather than first and last appearances typically viewed as necessary in biostratigraphy. Common occurrence bioevents are prone to spatial differences in environment and preservation and are generally not seen as reliable. Given unique spatial distribution patterns for foraminifera are acknowledged even in the first line of the abstract and other places in the manuscript, I’m not clear how common occurrence bioevents are valid in this setting. Further, the authors highlight that they use “absolute abundance” for defining bioevents, and figures report # of individuals per gram of sediment, which are heavily affected by changes in sedimentation rates and hiatuses. These features of the Arctic record are frequently highlighted in the manuscript (ex. line 487) as hindering biostratigraphic correlation, but the impact of changing sedimentation on the abundances being used to recognize bioevents is not addressed. Given how bioevents are being recognized and defined in the manuscript, they do not seem an appropriate method for assessing chronology in the region from first principles and I’m not clear why the exercise was done.
Further, the manuscript concedes that proposed correlations are preliminary and numerical ages are “too imprecise” (in abstract) and states that there is no robust independent chronostratigraphy available (Line 571). With the lack of robust chronological data, the exercise of evaluating the usefulness of bioevents seems futile given there is no reliable chronology to compare to. The outcome of the manuscript seems to just solidify existing uncertainty albeit with methods that may be not be expected to alleviate that uncertainty.
The discussion then provides extensive review of ecological and environmental reasons for the abundance changes in different taxa that are often speculative and not well-rooted in the results provided in the study or connected to the biostratigraphic questions, particularly given the emphasis in other parts of the manuscript that the Arctic has complex spatial differences in environment.
The conclusions state that “a standardized methodology is applied to define robust bioevents” but it does not appear that any of the bioevents investigated are indeed robust, particularly given the conceded lack of radiometric ages and the strong impacts of ecologic and taphonomic processes. Conclusions further make recommendations on how to best do biostratigraphy as if the study demonstrated their methods were successfully, but I have difficulty seeing that success. Some assertions in the conclusions are not tested by the study. For example, the relative success of relative abundances and absolute abundances in identifying events is not systematically evaluated. Although much of the discussion reviewed ecological drivers of species patterns, those are not mentioned in the conclusions except to say they could account for the formation of the bioevents.
Some of my confusion may be due to the organization of the manuscript and below I point out some aspects of organization that made understanding and following of the arguments within difficult.
Although I did not look at the appendixes in detail, they are well illustrated and taxa are thoroughly described. A publication presenting that effort would be very valuable to others working in the region.
Specific Comments
Line 45: Does no water mass exchange happen on the Pacific side of the Arctic? It does not seem that interaction between the subpolar latitudes and the Arctic is only occurring through the Fram Strait based on most maps of high latitude currents.
Line 47: propagules of foraminifera are known to be viable for (at least) decades, so using “vital transport” to imply that transport must occur rapidly while the individuals are alive seems misleading.
Line 126-line 130: This discusses that bioevents were defined for 1500-1700 m, but focuses on two cores that are more than 2300 m water depth. It is not well explained why this is a “test of whether species are restricted to certain water depths,” or why the depth ranges of these taxa are not known. Is the test more about whether the bioevents can be recognized in deeper waters? The depth of the “reference core PS2185-6" is not given here.
Line 130: citations for “published data” are not given. Perhaps direct the reader to the table of sources?
Line 221: Here the assertion is made that absolute abundances are not affected by other taxa in a sample like relative abundance are. However, in discrete samples where a particular number of specimens is counted to, the absolute abundance is very much affected by the other taxa. If I pick 300 specimens (as recommended on line 992) and there are no other species, I would get 300 of one species and thus a higher abundance than I would if many other taxa were present. Similarly if the constraint is to pick 1 gram of sediment.
Line 223: If comparison of relative abundance data is “difficult” because agglutinated taxa are sometimes not included, why can’t the relative abundances simply be recalculated excluding the agglutinated taxa? By restricting the calculation to only calcareous taxa, this issue would be avoided.
Line 265: Pronounced lithological variability is mentioned, which could profoundly affect the density of foraminifera in ways that are uninformative to biostratigraphy or to ecological analyses. Line 355 reemphasizes this by point out that some lithologies do no have forams at all. Again on line 487 talks about variable accumulation and stratigraphic breaks, which will affect the densities for foraminifera obtained, and thus, create patterns in “absolute abundance.”
Section 3.2. Figures are referred to qualitatively and with subjective terms when quantitative, objective, comparisons would be more useful. Ex. “Bolivina arctica are rarely abundant to dominant” however, it is not clear the meaning of “rarely,” “abundant,” or “dominant.” Or “Benthic foraminifer assemblages are generally dominated by Stetsonia horvathi” does not appear to be true from the figures (perhaps this is because each panel has different y-axes, which makes comparison difficult) and without quantification, the sentence is hard to rely on. The generalization of patterns in calcareous taxa across the cores is also difficult because some of the statements seem to be true for one core and not others.
Line 519: In the discussion the term “foraminifer maximum” is introduced for the first time and it is unclear what this is referring to.
Section 4.2.1 of the discussion relies on the change between agglutinated-dominated foram assemblages and calcareous-dominated assemblages for correlation, but in the results the authors note that the distribution of agglutinated foraminifera is different in each core examined in the manuscript. The change over is only obvious in Figure 5, but it is claimed for two of the cores (line 592) even though only Figure 5 is the only stratigraphic figure referenced in the section. The majority of the section is simply reviewing past work that seems unaffected by the new data even though the claim (Line 580) is made that the new data have an effect. The support for the argument is not clear.
Line 928: Assertions about switch from r to k strategists in the Arctic are tenuous and not well supported by data. It appears to rely on only one taxon in one core and a different taxon in another core.
Some data that is used as supporting evidence of some claims is cited as unpublished ideas by one of the authors and relying on unpublished information does not give confidence in the interpretations. For example, in section 3.2, unpublished data (line 366) is mentioned and attributed to one of the authors rather than being presented in the current manuscript as results, but this data on the abundance of a planktonic could easily be provided. Later in the discussion (line 940) unpublished information about the ecology of a purported k-strategist (Pyrgo) is given as unpublished observations by one of the authors. This same taxon is further supported as being a k-strategist based on the lack of reports of food-triggered reproduction, but no citation is given so it is not clear it anyone even tested the relationship and lack of knowledge should not be used a supporting evidence.
Technical Comments
On organization
The abstract is very long and should be shortened by about half. Synthesizing the results rather than listing each in turn would also help the reader understand the main thesis of the manuscript, which is not currently evident.
Organization of the manuscript is at times confusing and some paragraphs are not logically linked to each other or structured with clear topical themes. For example, section 3.2 starts with the calcareous assemblage, then reports on agglutinated assemblage and then shifts back to calcareous taxa on line 397 and back to agglutinated on line 445. The paragraphs from line 393-448 are all about single taxon with no connections between the paragraphs or a clear narrative. It then switches back to assemblage-level results. Subheadings and topic sentences are needed in order to follow the ideas.
The current organization of the manuscript also puts information in unexpected places. For example:
Section 3.1 in the Results appears to be a review of prior work rather than presenting any new results. This should be moved above results into methods or a background section about the study site.
Section 3.2 is in the Results, but is primarily discussion and review, making it very difficult to focus on the new information.
Section 4.1 of the discussion does not seem to be connected to any results and instead is background on the chronology of the cores, which would be more appropriate before the results in a section on site background.
Section 4.3 also does not seem connected to any results and is background on the ecology of foraminifera and what controls their distribution in the Arctic. The only potential connection provided is to the shift from agglutinated to calcareous taxa.
Figure 1 needs a legend for the bathymetrical color scale.
Table 1 provides water depths, but some are negative and some are positive. Needs standardization.
Having all the time series for the cores plotted in different figures (Figures 3-5) on different pages also makes it hard to compare among the cores and see any common patterns necessary for evaluation biostratigraphy utility of the bioevents.
Line 424: “NP26 record” is confusing. There are two cores with this designation in Table 1 and the abbreviation is the same as used for nannoplankton biozones.
Figure 12. I am not clear on how this illustrates preservation potential. Where does the orange triangle come from? How is enrichment of robust taxa being illustrated? There are clearly samples where less robust taxa are present and robust taxa are not.
All figures with abundance data and relative abundance data are plotted on different scales making it very hard to compare across species in a single figure or across the figures. Axes should be standardized.
There are numerous typographical and formatting errors that need careful proof reading.