Omnipresent authigenic calcite overgrowth on microfossils distorts Arctic radiocarbon chronology and isotope stratigraphy

The majority of paleoceanographic proxies uses calcareous microfossils as source. In the Arctic Ocean these microfossils are often altered by authigenic calcite overgrowth, this has an significant impact on the reliability of such proxies
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Recently we, a group of senior scientists, each specialized in a specific field, have published a study on the impact of authigenic calcite on radiocarbon, carbon- and oxygen isotope measurements on foraminiferal shells from sediment cores of the central Arctic Ocean in Communications Earth and Environment. This study started in 2018 when I (Jutta Wollenburg) (Fig. 1) was selecting pristine Neogloboquadrina pachyderma specimens from a sediment core collected in 2015 on the Yermak Plateau, for radiocarbon dating in the AWI Micadas laboratory.

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Out of pure curiosity, I also picked translucent-white specimens with adherent authigenic overgrowth for parallel dating. Although for both sample sets only translucent white specimens were picked, I noticed significant age-offsets. At this point micropaleontologist/geologist Jens Matthiessen, as well as Gesine Mollenhauer and Hendrik Grotheer from the Micadas radiocarbon laboratory, and myself decided to precisely document the occurrence of authigenic calcite in sediments of the central Arctic Ocean, where foraminiferal shells are more severely affected by authigenic calcite precipitates than on the Yermak Plateau or Barents Sea continental slope, which are close to the ocean margin. We opted

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for core PS72/413-3 from the Mendeleev Ridge, because of its excellent calcite and aragonite microfossil preservation. This enabled 1 cm-step wise radiocarbon measurements on translucent-white shells with and without overgrowth. Where enough shells of respective preservation states could be isolated, additionally carbon and oxygen isotope measurements were performed. Finally, we applied the same strategy to an old sediment core (PS2185-6) used in multiple publications to compare our data with published isotope and radiocarbon records.

Radiocarbon measurements are considered the most reliable dating method for sediments of the last 45000 years. Therefore, this method is widely used, not only by paleontologists and micropaleontologists with a comprehensive understanding of species taxonomy, ecology and shell structure. Planktonic foraminifera shells are the preferred proxy source for surface water reconstructions and radiocarbon dating. However, as their calcareous shells consist of a puzzle of tiny hyaline mesocrystals, intermediate organic layers and have the highest porosity amongst all foraminifera, they are extremely vulnerable to diagenetic changes. Both dissolution and overgrowth by authigenic calcite/aragonite leads to an incorporation of exogenous components into the biogenic shell. As we showed, this changes not only the 14C content but potentially all isotope and trace metal contents of the biogenic shell. With deep-water temperatures of -0.9°C and ventilation provided mainly by highly-oxygenated saline brines released during seasonal sea-ice formation, the deep central Arctic Ocean belongs today to the most extreme oceanographic areas, and even more so when deep-water ventilation diminished during glacial periods. Large down-core fluctuations in the planktonic vs. benthic foraminifera ratios, and high fragmentation of white instead of translucent-white planktonic foraminifera shells report from times of lowered bottom-/porewater pH. Even some of the barren intervals amounting to about 1/3 of a common central Arctic Ocean sediment core may reflect times with calcite-aggressive pH. However, today the deep Arctic Ocean has no carbonate compensation depth (CCD), and as curious as it may sound the published study reports on the omnipresent occurrence of authigenic calcite as discrete sedimentary crystals and, more importantly, by encrusting the majority of foraminifera shells. As has been recorded elsewhere, authigenic calcite overgrowth is often accompanied by a slight brownish discoloration of planktonic foraminiferal shells, and although dominant in arctic sediments (Fig. 2), for measurements such specimens should be avoided. In our study we show that it is important to document the diagenetic preservation status of the specimens selected for proxy measurements in each sample, and that it requires a trained person and good microscope to identify suitable specimens. However, our comparative downcore measurements are more than a plea for a cautious selection of specimens for measurements, it puts our basic understanding of paleoceanographic changes in the Arctic Ocean over the last 50.000 years at stake. We show that almost all pre-Holocene N. pachyderma specimens are significantly altered by authigenic overgrowth, which leads to age-offsets of >+10.000 years for shells with overgrowth, a fact that was obviously not noticed in previous paleoceanographic studies of the Arctic Ocean. As consequence sedimentation rates, which have been considered close to zero during MIS2, are in our cores much higher, reporting different late glacial sedimentation processes, younger and potentially changed oceanographic events. For example: Freshwater events as indicated by low d18O N. pachyderma records, need to be re-interpreted as we found extremely depleted d18O values in some of the measured authigenic crystals, and furthermore, according to our radiocarbon dating, they should have occurred at a much younger time. Diagenetic alterations of calcareous foraminifera and other microfossils are not restricted to the Arctic Ocean, thus, potential errors in radiocarbon ages and other biogenic calcite-based proxies elsewhere in the world’s ocean cannot be excluded until investigators document the preservation status of the specimens measured.

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