Comment #114 Darrell Kaufman @ 2023-09-09 10:30

Thank you for posting a preprint of your comment, and for the opportunity to clarify the role of amino acid geochronology in dating the Quaternary sedimentary sequence in the Arctic Ocean. Consistent with other evidence highlighted in your comment, amino acid geochronology also points to significantly older ages than those used in Vermassen et al.’s (2023) paper.

You state, “The amino-acid racemization rates of some foraminifera, previously used for age estimation in several papers (e.g., Kaufman et al., 2008), have been recently invalidated by West et al. (2023).” However, the purpose of the Kaufman et al. (2008) study was to develop an age equation for amino acid racemization in foraminifera from the Arctic Ocean sediment. To do this, the rate of racemization was calibrated using the assumption that the ages of the then-accepted chronology from the Lomonosov Ridge were correct: “The stratigraphy of the cores in this study was tied to the age model developed for the central Lomonosov Ridge… [which] enables the ages of each AAR sample to be estimated independently.” In other words, no ages were determined using amino acid geochronology. To my knowledge, the age equation developed by Kaufman et al. (2008) has never been used to estimate ages in other studies.

Instead, Kaufman et al. (2013) showed that the accepted chronology from the Lomonosov Ridge would require a rate of racemization that is significantly higher than expected when compared to measurements from multiple other independently dated marine cores from cold bottom water sites. This finding was then further supported by amino acid racemization in foraminifera from the Yermak Plateau in the eastern Arctic Ocean (West et al., 2019), and again from the Nordic Seas (West et al., 2023). Indeed, West et al. found, “The calibrated AAR ages suggest that intervals previously interpreted as substages in MIS 5 are instead separate interglacial periods extending back to MIS 9.”

Amino acid geochronology should not be discounted in future attempts to reconcile the currently disparate results from different dating techniques applied to Arctic Ocean sediments.

References
Vermassen, F., O’Regan, M., de Boer, A., Schenk, F., Razmjooei, M., West, G., Cronin, T., Jakobsson, M. & Coxall, H.K. A seasonally ice-free Arctic Ocean during the Last Interglacial. Nature Geosciences 16, 723–729 (2023). 10.1038/s41561-023- 01227-x

Kaufman, D. S., Polyak, L., Adler, R., Channell, J.E.T. & Xuan, C. Dating late Quaternary planktonic foraminifer Neogloboquadrina pachyderma from the Arctic Ocean using amino acid racemization. Paleoceanography 23, PA3224 (2008). 10.1029/2008PA001618

Kaufman, D. S., Cooper, K., Behl, R., Billups, K., Bright, J., Gardner, K., Hearty, P., Jokobbson, M., Mendes, I., O’Leary, M., Polyak, L., Rasmussen, T., Rosa, F. & Schmidt, M. Amino acid racemization in mono-specific foraminifera from Quaternary deep-sea sediments. Quaternary Geochronology 16, 50-61 (2013). 10.1016/j.quageo.2012.07.006

West, G., Kaufman, D. S., Muschitiello, F., Forwick, M., Matthiansen, J., Wollenburg, J. & O’Regan, M. Amino acid racemization in Quaternary foraminifera from the Yermak Plateau, Arctic Ocean. Geochronology 1, 53-67 (2019). 10.5194/gchron-1-53-2019

West, G., Kaufman, D.S., Jakobsson, M. & O'Regan, M. Amino acid racemization in Neogloboquadrina pachyderma and Cibicidoides wuellerstorfi from the Arctic Ocean and its implications for age models. Geochronology 5, 285–299 (2023). 10.5194/gchron-5-285-2023