Enhanced Arctic Amplification Began at the Mid-Brunhes Event ~400,000 years ago

T M Cronin,G S Dwyer,L H Deninno,L Gemery,J Rodriguez-Lazaro,E K Caverly,J Farmer

doi:10.1038/s41598-017-13821-2

T M Cronin, G S Dwyer + Show 5 more

Open Access

https://doi.org/10.1038/s41598-017-13821-2

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Abstract

Arctic Ocean temperatures influence ecosystems, sea ice, species diversity, biogeochemical cycling, seafloor methane stability, deep-sea circulation, and CO2 cycling. Today’s Arctic Ocean and surrounding regions are undergoing climatic changes often attributed to “Arctic amplification” – that is, amplified warming in Arctic regions due to sea-ice loss and other processes, relative to global mean temperature. However, the long-term evolution of Arctic amplification is poorly constrained due to lack of continuous sediment proxy records of Arctic Ocean temperature, sea ice cover and circulation. Here we present reconstructions of Arctic Ocean intermediate depth water (AIW) temperatures and sea-ice cover spanning the last ~ 1.5 million years (Ma) of orbitally-paced glacial/interglacial cycles (GIC). Using Mg/Ca paleothermometry of the ostracode Krithe and sea-ice planktic and benthic indicator species, we suggest that the Mid-Brunhes Event (MBE), a major climate transition ~ 400–350 ka, involved fundamental changes in AIW temperature and sea-ice variability. Enhanced Arctic amplification at the MBE suggests a major climate threshold was reached at ~ 400 ka involving Atlantic Meridional Overturning Circulation (AMOC), inflowing warm Atlantic Layer water, ice sheet, sea-ice and ice-shelf feedbacks, and sensitivity to higher post-MBE interglacial CO2 concentrations.

Highlights

Today’s central Arctic Ocean is characterized by a shallow halocline that separates the Polar Surface Layer and its perennial sea-ice from the warm Atlantic Water (200–500 m), which flows into central basins from the North Atlantic via the Fram Strait and Barents Sea
Trends in ostracode magnesium/calcium ratios (Mg/Ca) ratios and bottom intermediate water temperature (BWT) shown in Fig. 2a reveal a major transition beginning slowly, near 430 ka and increasing in amplitude after 350 ka, when Mg/Ca ratios rose from a pre-Mid-Brunhes Event (MBE) level of 11–12 mmol/mol to post-MBE levels averaging around 14 mmol/mol
Converting the ostracode Mg/Ca ratios to temperature[11] (Extended Data), the MBE shift equates to a rise in bottom temperatures in the Arctic Ocean intermediate depth water (AIW) layers from near 0 °C to as much as 3–5 °C

Summary

OPEN Enhanced Arctic Amplification

~400,000 years ago Received: 16 March 2017 Accepted: 2 October 2017 Published: xx xx xxxx. To understand the changes in ocean circulation during the MBE, we reconstructed Arctic Ocean temperatures from benthic ostracode magnesium/calcium ratios (Mg/Ca), a tool for Arctic marine paleothermometry (Extended Data), and sea ice history from abundance patterns of ostracode species from 7 sediment cores from the western and central Arctic Ocean (Fig. 1). Both reconstructions mainly record interglacial and interstadial intervals, which are reflected in the abundances of calcareous microfossils[9]. Data on these tiepoints and the estimated core depths for Marine Isotope Stratigraphy stages for each core are given in the Extended Data, which includes Supplementary Figure S1 and Supplementary Table 1

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