| 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.
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.