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Abrupt pre-Bølling–Allerød warming and circulation changes in the deep ocean

Abstract

Several large and rapid changes in atmospheric temperature and the partial pressure of carbon dioxide in the atmosphere1—probably linked to changes in deep ocean circulation2—occurred during the last deglaciation. The abrupt temperature rise in the Northern Hemisphere and the restart of the Atlantic meridional overturning circulation at the start of the Bølling–Allerød interstadial, 14,700 years ago, are among the most dramatic deglacial events3, but their underlying physical causes are not known. Here we show that the release of heat from warm waters in the deep North Atlantic Ocean probably triggered the Bølling–Allerød warming and reinvigoration of the Atlantic meridional overturning circulation. Our results are based on coupled radiocarbon and uranium-series dates, along with clumped isotope temperature estimates, from water column profiles of fossil deep-sea corals in a limited area of the western North Atlantic. We find that during Heinrich stadial 1 (the cool period immediately before the Bølling–Allerød interstadial), the deep ocean was about three degrees Celsius warmer than shallower waters above. This reversal of the ocean’s usual thermal stratification pre-dates the Bølling–Allerød warming and must have been associated with increased salinity at depth to preserve the static stability of the water column. The depleted radiocarbon content of the warm and salty water mass implies a long-term disconnect from rapid surface exchanges, and, although uncertainties remain, is most consistent with a Southern Ocean source. The Heinrich stadial 1 ocean profile is distinct from the modern water column, that for the Last Glacial Maximum and that for the Younger Dryas, suggesting that the patterns we observe are a unique feature of the deglacial climate system. Our observations indicate that the deep ocean influenced dramatic Northern Hemisphere warming by storing heat at depth that preconditioned the system for a subsequent abrupt overturning event during the Bølling–Allerød interstadial.

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Figure 1: Ocean profiles of Δ14C and temperature during three different time intervals.
Figure 2: Temperature versus calendar age based on clumped isotopes in deep-sea corals across the deglaciation.
Figure 3: Δ14C–temperature cross-plot.
Figure 4: Benthic sections of δ13C during late HS1 and the Bølling–Allerød.

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Acknowledgements

We thank J. McManus and M. Miller for discussions. We also thank the captain and crew of the RV Atlantis cruise AT7-35 and the WHOI Deep Submergence Alvin and ABE groups.

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Authors

Contributions

N.T. and J.F.A. designed the study. N.T. collected the 14C and Δ47 data and compiled the benthic δ13C sections. J.R.S. facilitated and oversaw the 14C measurements and J.M.E. facilitated and oversaw the Δ47 measurements. A.V.S. collected the U-series data. J.F.A. facilitated and oversaw the U-series measurements. N.T. and J.F.A. wrote the first draft of the manuscript. All authors contributed to the interpretation and preparation of the final manuscript.

Corresponding author

Correspondence to Nivedita Thiagarajan.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Sample collection sites.

Sites of sample collection in the North Atlantic.

Extended Data Figure 2 Δ47 cleaning study.

Two different modern corals were selected to determine which cleaning method did not bias Δ47 temperatures. A live-when-collected coral with a growth temperature of 3.9 °C (a) and a coral with an Fe–Mn crust and a 14C age matching that of the modern water column from 5 °C waters in the Southern Ocean (b) were selected for the cleaning study. We found that physical cleaning with a Dremel tool gave accurate Δ47–temperature reconstructions. Error bars are 1 s.e.m.

Extended Data Figure 3 Box model and equations.

This schematic describes the box model and equations used to calculate the effect of geothermal heat on ocean temperatures. Cp is the heat capacity of sea water, V is the volume fraction of the northern or southern box, Θ is the temperature, ρ is the density of sea water, q is the overturning rate of the northern or southern box, A is the area of the ocean, kv is the vertical mixing coefficient and Fgh is the geothermal heat flux (0.1 W m−2).

Extended Data Figure 4 Steady-state temperature change with ocean overturning and mixing.

The steady-state solutions for the box model in Extended Data 3. a, If there is a well-flushed southern cell (10 Sv of overturning), the bottom water is only warmed by about 0.8 °C relative to the restoring temperature in the atmosphere. However, for a very slow overturning (1 Sv) this temperature increase can reach over 8 °C, well above the temperature change seen at our site. b, Increasing the difference between the restoring temperature in the south relative to the north can increase the warming by as much as 11 °C (Supplementary Fig. 4b).

Extended Data Figure 5 Transient-state temperature change with ocean overturning and mixing.

The transient-state solution for the box model in Extended Data 3. The vertical diffusivity was kept constant and the Southern Hemisphere restoring temperature at 277 K. We find that there is a rapid warming within the first few thousand years (denoted by the black box).

Extended Data Figure 6 Comparison with other Δ14C records during the deglaciation.

A comparison of the Δ14C of deep-sea corals (previously published work11,71 and current study) and foraminifera72 from the Northwest Atlantic (a) with Δ14C of the tropical Atlantic62 (b), Iceland53 (c), the Mediterranean54,55 (d), the Iberian margin57 (e) and the Southern Ocean58,59,60,61 (f). The three corals from our study showing a warm and Δ14C-depleted signature at 15 kyr ago are circled in red. Although the tropical Atlantic and Mediterranean are both warm, they have too-enriched Δ14C values to explain the warm, Δ14C-depleted water seen at our site at the mid-15-kyr event. Iceland has extremely Δ14C-depleted waters, but these are thought to form during brine formation, which would not generate warm waters. The Iberian margin also has Δ14C values, but it is bathed by cooler waters and does not show the abrupt mid-15-kyr warming (Extended Data 7). At the mid-15-kyr event, UCDW, LCDW, AABW, the corals at our site and the Icelandic records all converge to 50‰. We believe it is much more likely that some vertical convection is causing Southern Ocean waters all to have similar Δ14C values, and that some southern-sourced waters are influencing the Δ14C of our site as well as perhaps waters near Iceland (as previously suggested63). Note the different axes on f. (Points in c and d that are above the atmospheric value are connected with a dashed line instead of a solid line.) Uncertainties are 2σ error ellipses except for the Iberian margin and Mediterranean records, which are 1σ.

Extended Data Figure 7 Comparison with Iberian margin Δ14C and temperature.

a, A comparison of the Mg/Ca–temperature and Δ14C record from the Iberian margin57 with the record from our site. The Iberian margin shows a warming at the beginning of the Bølling–Allerød but not an abrupt mid-15-kyr warming. Asterisks indicate corals which have either a high δ234Ui or a Δ14C above the atmospheric value. In both cases, this open-system behaviour changes the Δ14C values but does not change calendar ages much on this plot. b, Δ14C measurements at the Iberian margin also show that the water bathing the Iberian margin is distinct from the warm and Δ14C-depleted water at our site. Error bars are 1 s.e.m.

Extended Data Figure 8 Compilation of Atlantic benthic δ13C records.

a, Benthic sections of δ13C from well-dated, high-resolution cores39,40,41,42,43,53 in the North Atlantic as well as from the GEOSECS database. Black dots indicate the latitudes and depths of cores used to make the sections. The time intervals compiled are as follows: Holocene (0–10 kyr ago; a), Younger Dryas (11.7–13 kyr ago; b), Bølling–Allerød (13–14.5 kyr ago; c), late HS1 (14.5–15.7 kyr ago; d), early HS1 (15.7–18 kyr ago; e), Last Glacial Maximum (19–22 kyr ago; f). At the LGM (f), cold, salty and δ13C-depleted water from the south lay below cold, fresher and δ13C-enriched water. During HS1 (d, e), intermediate waters changed more than deeper waters. By the Bølling–Allerød (c), the δ13C distribution seen today had been established. The Younger Dryas (b), while thought to be a return to Heinrich-like water masses, is structured differently than the late-HS1 section and was a progression towards the Holocene water column configuration (a).

Supplementary information

Supplementary Data

This file comprises 2 sheets: The first has the raw data for samples and standards run during all sessions; the second has the pertinent information for the heated gases run during each session and was used for sample correction. (XLSX 57 kb)

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Thiagarajan, N., Subhas, A., Southon, J. et al. Abrupt pre-Bølling–Allerød warming and circulation changes in the deep ocean. Nature 511, 75–78 (2014). https://doi.org/10.1038/nature13472

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