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Icebergs not the trigger for North Atlantic cold events

Abstract

Abrupt climate change is a ubiquitous feature of the Late Pleistocene epoch1. In particular, the sequence of Dansgaard–Oeschger events (repeated transitions between warm interstadial and cold stadial conditions), as recorded by ice cores in Greenland2, are thought to be linked to changes in the mode of overturning circulation in the Atlantic Ocean3. Moreover, the observed correspondence between North Atlantic cold events and increased iceberg calving and dispersal from ice sheets surrounding the North Atlantic4 has inspired many ocean and climate modelling studies that make use of freshwater forcing scenarios to simulate abrupt change across the North Atlantic region and beyond5,6,7. On the other hand, previous studies4,8 identified an apparent lag between North Atlantic cooling events and the appearance of ice-rafted debris over the last glacial cycle, leading to the hypothesis that iceberg discharge may be a consequence of stadial conditions rather than the cause4,9,10,11. Here we further establish this relationship and demonstrate a systematic delay between pronounced surface cooling and the arrival of ice-rafted debris at a site southwest of Iceland over the past four glacial cycles, implying that in general icebergs arrived too late to have triggered cooling. Instead we suggest that—on the basis of our comparisons of ice-rafted debris and polar planktonic foraminifera—abrupt transitions to stadial conditions should be considered as a nonlinear response to more gradual cooling across the North Atlantic. Although the freshwater derived from melting icebergs may provide a positive feedback for enhancing and or prolonging stadial conditions10,11, it does not trigger northern stadial events.

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Figure 1: Regional context of the study site.
Figure 2: Proxy records from ODP site 983.
Figure 3: Relative timing of temperature change versus ice rafting.
Figure 4: Gradual cooling precedes the transition to stadial conditions.

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Acknowledgements

We thank S. Edwards, L. Owen, F. Piggott, L. Skyrme and M. Theobald for assistance in the laboratory and J. McManus for discussions. This study was supported by a Philip Leverhulme Prize to S.B., the Comer Science and Education Foundation (GCCF3) and the UK Natural Environment Research Council (NERC) grants NE/L006405/1 and NE/J008133/1. Additional funding by ‘Helmholtz Climate Initiative REKLIM’ (Regional Climate Change), a joint research project of the Helmholtz Association of German research centres (HGF), is gratefully acknowledged (G.K.). L.J. was funded by the climate change consortium of Wales (http://www.C3Wales.org). This research used samples provided by the Integrated Ocean Drilling Program (IODP). We thank W. Hale for assistance in sampling and curation. All data and age models presented here are available in the Extended Data.

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Contributions

S.B. designed research and analysed datasets. J.C. processed samples and performed faunal counts with assistance from those mentioned in the Acknowledgements. X.G. performed ice core data analysis. S.B., L.J., X.G., G.K. and D.T. contributed to writing the paper.

Corresponding author

Correspondence to Stephen Barker.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 North Atlantic iceberg trajectories.

a, Map showing locations of core sites relevant to this study with simplified iceberg trajectories, after ref. 56. Sites where volcanics are reported to appear early within broader pulses of IRD are marked with a ‘V’. b, Core details and summary of IRD sources for each site4,20,21,22,57,58,59,60,61,62,63,64. In line with our observation of early cooling with respect to ice rafting at ODP site 983, early cooling has also been reported at sites SO82-58, DS97-2P61 and LO09-1861 within the Irminger Sea.

Extended Data Figure 2 IRD composition at ODP site 983.

Volcanics comprise 36% of the total IRD on average. Source data

Extended Data Figure 3 Threshold sensitivity analysis for the calculation of offsets between temperature and IRD.

a, b, Warming offsets are small (typically 0–50 yr) for a wide range of thresholds using either %NPS or IRD per gram as the primer (Methods) but the IQR (c, d) is larger for more or less sensitive thresholds. We define an optimal set of threshold values as those producing lower values for the IQR (delineated by the black square). e, Decreasing the threshold sensitivity (higher rates of change required to detect a transition) results in fewer paired transitions. Small white square is the threshold set that gives the highest number of paired transitions within optimal region and is used in Fig. 3 (dNPS/dt = ±14, dIRD/dt = ±170). f, h, Cooling offsets are all positive for the first, median and third quartiles.

Extended Data Figure 4 Age model development for ODP site 983 (0–150 kyr ago).

a, Splice (at orange circle) of NGRIP δ18O2 and a synthetic Greenland temperature record, GLT_syn48. b, %NPS. c, IRD per gram. d, Benthic δ13C (ref. 26). e, Per cent coarse fraction (yellow symbols are tuning points). f, Benthic δ18O from ODP site 983 (ref. 26) (blue curve) with LR04 stack (shifted by −0.5‰; orange curve) for comparison. g, Sedimentation rates implied by new age model. All records (except LR04 stack) are on the GICC05 age model53 back to 60 kyr ago and a modified version of the speleothem-tuned age model of ref. 48 (using the NALPS speleothems54 between 60 kyr ago and 108 kyr ago) for older ages52. The grey curve in be is the millennial-scale component of GLT_syn (GLT_syn_hi)48. Source data

Extended Data Figure 5 Age model development for ODP site 983 (100–250 kyr ago).

a, Synthetic Greenland temperature record, GLT_syn48. b, %NPS. c, IRD per gram. d, Benthic δ13C (ref. 26). e, Per cent coarse fraction (yellow symbols are tuning points). f, Benthic δ18O from ODP site 983 (ref. 26) (blue curve) with LR04 stack (shifted by −0.5‰; orange curve) for comparison. g, Sedimentation rates implied by new age model. All records (except the LR04 stack) are on the EDC3 age model49. The grey curve in be is the millennial-scale component of GLT_syn (GLT_syn_hi)48.

Extended Data Figure 6 Age model development for ODP site 983 (200–350 kyr ago).

a, Synthetic Greenland temperature record, GLT_syn48. b, %NPS. c, IRD per gram. d, Benthic δ13C (ref. 26).e, Per cent coarse fraction (yellow symbols are tuning points). f, Benthic δ18O from 98326 (blue curve) with LR04 stack (shifted by −0.5‰; orange curve) for comparison. g, Sedimentation rates implied by new age model. All records (except the LR04 stack) are on the EDC3 age model49. The grey curve in be is the millennial-scale component of GLT_syn (GLT_syn_hi)48.

Extended Data Figure 7 Age model development for ODP site 983 (300–450 kyr ago).

a, Synthetic Greenland temperature record, GLT_syn48. b, %NPS. c, IRD per gram. d, Benthic δ13C (ref. 26). e, Per cent coarse fraction (yellow symbols are tuning points). f, Benthic δ18O from ODP site 983 (ref. 26) (blue curve) with LR04 stack (shifted by −0.5‰; orange curve) for comparison. g, Sedimentation rates implied by new age model. All records (except the LR04 stack) are on the EDC3 age model49. The grey curve in be is the millennial-scale component of GLT_syn (GLT_syn_hi)48.

Extended Data Figure 8 Cooling and warming offsets calculated using the revised age model

Box and whisker plots show calculated offsets between temperature change (change in %NPS) and IRD at ODP site 983 using the LR04 age model (upper panel) and our revised EDC3 age model (lower panel). Boxes represent the interquartile range (IQR, 25%–75%) dissected by the median value. Whiskers are 1.5×IQR and extend to the last value included in this range. Positive values signify temperature change is earlier. Blue boxes represent cooling versus arrival of IRD; red/orange boxes represent warming versus IRD decrease. Dark blue/red boxes represent the start of a transition; light blue/orange boxes reflect the mid-point. n is the number of paired transitions detected. Source data

Extended Data Figure 9 Rate of cooling (according δ18O) versus duration of an interstadial in the NGRIP ice core.

a, Logarithmic scales (error bars represent the uncertainty in the calculated gradients). b, Linear scales. Interstadial durations were calculated using a thresholding approach on the first derivative of the smoothed δ18O record2 to identify their abrupt onsets and ends. 50 years were then subtracted from either end of each identified interval before calculating the gradients (using the raw, unsmoothed measurements) to avoid contamination from the sharp transitions. Shorter intervals have fewer data points and therefore greater scatter (leading to greater uncertainty in the calculated gradient). Two very short intervals (Dansgaard–Oeschger events 17 and 18) were omitted from the analysis. This analysis is an updated version of that made by Schulz28 on the GISP2 ice core. Source data

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Barker, S., Chen, J., Gong, X. et al. Icebergs not the trigger for North Atlantic cold events. Nature 520, 333–336 (2015). https://doi.org/10.1038/nature14330

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