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Climate of the Past An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Apr 2019

Research article | 16 Apr 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Climate of the Past (CP).

Modal shift in North Atlantic seasonality during the last deglaciation

Geert-Jan A. Brummer1,2, Brett Metcalfe2,3, Wouter Feldmeijer2,a, Maarten A. Prins2, Jasmijn van 't Hoff2,b, and Gerald M. Ganssen2 Geert-Jan A. Brummer et al.
  • 1NIOZ Royal Netherlands Institute for Sea Research, Department of Ocean Systems, 1790 AB, Den Burg, and Utrecht University, The Netherlands
  • 2Earth and Climate Cluster, Department of Earth Sciences, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands
  • 3Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France
  • anow at: Nebest B.V., Marconiweg 2, 4131 PD, Vianen, The Netherlands
  • bnow at: Institute of Geology and Mineralogy, University of Cologne, Zuelpicher Str. 49a, 50674 Cologne, Germany

Abstract. Change-over from a glacial to an interglacial climate is considered as transitional between two stable modes. Palaeoceanographic reconstructions using the polar foraminifera Neogloboquadrina pachyderma highlight the retreat of the polar front during the last deglaciation in terms of both its decreasing abundance and stable oxygen isotope values (δ18O) in sediment cores. While conventional isotope analysis of pooled N. pachyderma shells show a warming trend concurrent with the retreating ice, new single shell measurements reveal that this trend is composed of two isotopically different populations that are morphologically indistinguishable. Using modern time-series as analogues for interpreting down-core data, glacial productivity in the mid North Atlantic appears limited to a single maximum in late summer, followed by the melting of drifting icebergs and winter sea ice. Despite collapsing ice sheets and global warming during the deglaciation, a second warm population of N. pachyderma appears in a bimodal seasonal succession separated by the subpolar G. bulloides. This represents a shift in the timing of the main plankton bloom from late to early summer in a deglacial intermediate mode that persisted for ca. 10,000 years until the last deglaciation ended. When seawater temperatures exceeded the threshold values, first the cold (glacial) then the warm (deglacial) population of N. pachyderma disappeared, whilst G. bulloides with a greater tolerance to higher temperatures persisted throughout the Holocene to the present day in the mid-latitude North Atlantic. Single specimen δ18O of polar N. pachyderma reveal a steeper rate of ocean warming during the last deglaciation than appears from conventional pooled δ18O average values.

Geert-Jan A. Brummer et al.
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Geert-Jan A. Brummer et al.
Geert-Jan A. Brummer et al.
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Short summary
Here mid ocean seasonality is resolved through time, using differences in the oxygen isotope composition between individual shells of the commonly used (sub)polar planktonic foraminifera species in ocean-climate reconstruction N. pachyderma and G. bulloides. Single specimen isotope measurements during the deglacial period revealed a surprising bimodality of the polar N. pachyderma not seen in concurrent specimens of subpolar G. bulloides, the cause of which was investigated.
Here mid ocean seasonality is resolved through time, using differences in the oxygen isotope...