arising from J.E. Wollenburg et al. Communications Earth & Environment https://doi.org/10.1038/s43247-023-00802-9 (2023)

Wollenburg et al.1 convincingly demonstrate that foraminiferal tests obtained from Arctic Ocean sediment cores are often affected by authigenic calcite overgrowth, potentially biasing isotopic and elemental ratios, and resulting in radiocarbon ages that are thousands, or even tens of thousands, of years too old. The phenomenon is particularly severe in sediments older than the Holocene, and may provide an alternative explanation for the apparent hiatus that is widespread in the central Arctic Ocean during the last glacial maximum. To avoid this problem, the authors recommend careful screening under the microscope in combination with SEM analysis to ensure that only pristine foraminifers, free of overgrowth, are used.

However, although the authors recognize that some foraminifers have been displaced through bioturbation, they overlook two important aspects of deep-reaching burrows. First, the piping down of young foraminifer tests from the sediment surface offers an alternative explanation for the young radiocarbon ages of relatively unaffected foraminifers (PI_1 and PI_2) observed in the sediment below the Holocene interval. Second, pristine tests piped down into deeper, older sediment would make an alluring source of material for any paleoceanographer. Accordingly, if these piped-down pristine foraminifera were to be used, radiocarbon ages, isotopes, and elemental ratios would then rather reflect conditions at the (paleo) sea floor than those of the sampled stratigraphic position.

How common are these kinds of deep-reaching burrows, and how deep can surface material be piped down into underlying sediment? Several studies have addressed sediment shifts caused by deep-reaching bioturbation2,3. Detailed studies on the trace fossil Zoophycos in late Quaternary sediment have demonstrated that this trace fossil occurs in about a third of the cores taken from water depths deeper than 1000 m, and that the material inside the burrow is several thousand years younger than surrounding sediment4,5. Zoophycos is believed to represent a cache behavior, where the producing organism collects food on the sea floor and stores the collected material in distinct galleries called spreiten as much as one meter below the sea floor6,7.

Consequently, an alternative hypothesis for the large age offset between foraminifera free from authigenic overgrowth (PI_1 and PI_2) and foraminifera with overgrowth (PI_3), is that the PI_1 and PI_2 foraminifera used for dating largely consist of Holocene foraminifer tests piped down into late glacial sediment during the time of burrow construction in the Holocene.

In fact, when the radiocarbon ages from PS72/413-3 are overlaid onto the core photograph (Fig. 1), it becomes obvious that the interval where the largest differences in radiocarbon ages are observed corresponds to an interval of abundant Zoophycos spreiten. The best way to assess bioturbation and trace fossils in unlithified core material is through radiographs (X-ray images), but unfortunately, the radiographs of PS72/413-3 only extend to about 14 cm core depth. However, radiographs of core PS72/413-5 from the same location (~1.3 km apart)8, and the radiographs of core PS8125-69, used for the analysis in Figure 6 of Wollenburg et al.1, clearly show intense bioturbation and abundant occurrences of the trace fossil Zoophycos (Fig. 2). Detailed observations of the radiographs show that Zoophycos spreiten are particularly common in the interval 17-24 cm in PS72/413-5 (Fig. 3) and from 12 to 23 cm in PS2185-6. These intervals correspond to the depths where large radiocarbon age offsets were observed between foraminifera with and without overgrowth. Moreover, the interval with abundant Zoophycos spreiten also corresponds to the depths where young, presumably displaced1, pristine foraminifera were found, further suggesting that these foraminifera were brought down by the Zoophycos producer. It is important to note that the foraminifera in the Zoophycos spreiten were collected on the sea floor by the trace-maker and emplaced in deeper sediments corresponding to MIS 2 and MIS 3. Moreover, the authors attribute an age shift of up to 10 ky solely to authigenic calcite precipitation. This corresponds to about two radiocarbon half-lives and, therefore, would require that the weight of the foraminiferal tests was more than quadrupled by the addition of radiocarbon-dead calcite.

Fig. 1: Downcore distribution of radiocarbon ages and Zoophycos spreiten.
figure 1

a Radiocarbon ages of PI_1 (pristine = red), PI_2 (visually free of overgrowth = orange), and PI_3 (with overgrowth = blue) of giant box core PS72/413-3 plotted on top of the core photo for comparison with the distribution of Zoophycos (Z) and other trace fossils. Arrows indicate that radiocarbon ages are older than 39 ky. Tentative stratigraphy based on sedimentology, color, bioturbation and radiocarbon ages. b Schematic illustration of the construction of the Zoophycos trace fossil. Detritus (e.g., foraminifera) is collected on the sediment surface and stored deep in the sediment cf.7.

Fig. 2: Radiographs showing the distribution of Zoophycos spreiten.
figure 2

Radiographs showing the uppermost 35 cm of gravity cores PS72/413-5 (a) and PS2185-6 (b) with the positions of Zoophycos spreiten marked. Abundant Zoophycos occurrences are focused to a depth interval from slightly below 10 cm down to roughly 25 cm, corresponding to the typical burrowing depth of 10–20 cm by the causative organism, although considerably deeper reaching Zoophycos have been observed2,7. Radiographs8,9 obtained from the open access library12 at www.pangaea.de.

Fig. 3: Close-up of Zoophycos spreiten.
figure 3

a Radiograph showing a close-up of the interval 20-25 cm in core PS72/413-5. The abundant Zoophycos spreiten in this interval would make it virtually impossible to sample unaffected foraminifera. b Same interval but with Zoophycos spreiten traced out for better clarity. Radiographs13 obtained from the open access library12 at www.pangaea.de.

Thus, before the presence of a widespread hiatus during MIS 2 in the Arctic Ocean can be dismissed, detailed studies on the possible influence of deep-reaching trace fossils must be conducted. Because of the large heterogeneity of bioturbation in marine sediments10, the most straightforward way to test the two hypotheses would be to sample foraminifera directly from the sediment slabs used for the production of the radiographs. In this way, the radiocarbon ages of foraminifera of ambient sediment and Zoophycos spreiten can be directly compared11.

The piping down of younger material from higher stratigraphic levels also poses a general problem for paleoceanographic studies in the Arctic Ocean. Because of the ubiquitous occurrence of authigenic calcite, the few foraminifera without visible overgrowth would be preferentially selected when tests are picked out under the microscope. As a result, we would expect a strong bias towards foraminifera actually originating from a stratigraphically much higher position.

Because of the usually low color contrast to ambient sediment, Zoophycos spreiten, and other trace fossils are often overlooked in core descriptions. The most efficient way to assess the extent of bioturbation and trace fossils in marine sediments is through radiographs. We therefore strongly recommend that radiographs are routinely made from all sediment cores used for paleoceanographic purposes and that the radiographs are evaluated for the presence of deep-reaching burrows such as Zoophycos.

To summarize, authigenic calcite causes large age offsets in Arctic Ocean chronostratigraphies based on radiocarbon dating. In order to assess the true size of these offsets, deep-reaching bioturbation must be taken into consideration.