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

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.

Previous studies indicate that Krithe shell Mg/Ca ratios are predominantly controlled by water temperature at the time of shell growth and that postdepositional diagenetic factors such as dissolution, have negligible impact on original shell Mg/Ca ratios 35,36,37,38,11 . Carbonate ion effects, which are thought to influence deep-sea benthic foraminifera Mg/Ca ratios 37,38 , do not appear to influence Krithe Mg/Ca, especially in Arctic Ocean sediments 11 . Finally, although dissolution can affect ostracode shells, dissolution has negligible impact on ostracode Mg/Ca ratios 38 , but we nevertheless assess dissolution using a visual inspection index (VPI) that quantifies preservation state using VPI values of 1 [clear, translucent, pristine] to 7 [partially dissolved, chalky]. In our reconstruction we use well-preserved specimens (VPI -1-5) that additionally show no signs of secondary calcite.
The Mg/Ca-temperature relationship for the main Arctic species of Krithe, K. hunti, is based on Arctic-Nordic Sea coretop material from 50 sites (50 to 3500m water depth, temperatures from -1.6° to 1°C) and is expressed in the equation BWT (ºC) = (0.438 x Mg/CaKrithe) -5.14 (r 2 = 0.5). This calibration has a 1σ prediction error of ±1.0°C and a temperature sensitivity is ~2.3 mmol mol -1 °C -1 6,11 . This sensitivity is nearly double that of Krithe species from the North Atlantic (temperature range of 2° to 14°C) 37  Another possibility is that there was at certain times higher seawater Mg in the Arctic Ocean due to land-sea transport of dolomite, although it is difficult to imagine how this would affect bottom water dissolved ion chemistry, ostracode shell secretion, and the chemistry of molting fluids. This scenario is implausible for numerous reasons, including: (1) Ostracode Mg/Ca peaks in the record (TME's), including pre-and post-peak lower Mg/Ca values, occur stratigraphically outside dolomitic IRD layers, which are used as stratigraphic markers in the Arctic Ocean; (2) Dolomite is less soluble than calcite; thus water corrosive enough to dissolve dolomite and release Mg would likely dissolve all calcitic ostracode shells leaving sediment barren of ostracode shells; (3) Dissolution of dolomite would release Mg and Ca in 1:1 proportion thereby decreasing the seawater Mg/Ca ratio (~5:1), which in turn should actually lead to lower Mg/Ca ratios in ostracode shells, not higher.

Ostracode ecology and indicator species
Two indicator ostracode groups are used to reconstruct Arctic Ocean sea ice and productivity. The species Acetabulastoma arcticum has been used as a sea-ice proxy because it is a parasitic species living in the epipelagic, sea-ice dwelling amphipod Gammarus and its shells are commonly preserved in Arctic sediments 42 . The benthic genus Polycope, an opportunistic group that signifies high local surface ocean productivity 41 , often reaches 80% of total assemblages in late Quaternary sediments 10 . Prior study of A. arcticum and Polycope from many box and multicores shows large variability related to surface sea ice and productivity throughout most of the central Arctic 10 . These two ostracode taxa were studied in core HLY0503-6 from the Mendeleev Ridge indicating a sharp increase in both sea-ice and local productivity proxies between 400 and 350 ka 15, 32 . The near absence of Polycope spp. and associated species in pre-MBE sediments likely reflects its migration into the Arctic Ocean only when appropriate habitat, with local and pulsed surface productivity providing surface-to-bottom food for opportunistic genus. In addition, the sustained lack of summer sea ice and competition with a more diverse benthic ostracode fauna were likely factors influencing the pre-MBE benthic assemblages 32 .
A major environmental change near the MBE is also indicated by benthic foraminiferal assemblages 13 .

Revisionist views on Arctic Ice Cover during Glacials
There is growing evidence that the Arctic Ocean was at least partially covered with thick ice shelves up to 1-km thick and thicker-than-modern sea-ice cover during recent glacial periods. Evidence comes from geophysical and sediment records from the Hovgaard Ridge-Arctic Ocean 44 50,51,7 . Age estimates vary but include the last few major glacial maxima including MIS 2, 4, and 6. Laurentide Ice Sheet ice streams also exhibited complex behavior during periods of deglaciation 52 , which may have contributed to large ice discharges during Heinrich Events.

Continuous Mid-Brunhes Records
Previously published continuous records across the MBE include deep-sea temperature 53  Caption. Figure S1. Age-depth plots for Arctic cores used in paleothermomtry.
An age model for each core was developed using tiepoints for segments of the core from reference # 9 (