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Latest Pliocene Northern Hemisphere Glaciation Amplified by Intensified Atlantic Meridional Overturning Circulation

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Latest Pliocene Northern Hemisphere Glaciation Amplified by Intensified Atlantic Meridional Overturning Circulation ARTICLE https://doi.org/10.1038/s43247-020-00023-4 OPEN Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation ✉ Tatsuya Hayashi 1 , Toshiro Yamanaka 2, Yuki Hikasa3, Masahiko Sato4, Yoshihiro Kuwahara1 & Masao Ohno1 1234567890():,; The global climate has been dominated by glacial–interglacial variations since the intensifi- cation of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic mer- idional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning cir- culation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly increased after 2.7 million years ago and was actively maintained even in early stages of individual glacials, in contrast with late stages when it drastically decreased because of iceberg melting. Probably, the active Nordic Seas overturning during the early stages of glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations. 1 Division of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan. 2 School of Marine Resources and Environment, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Tokyo 108-8477, Japan. 3 Department of Earth Sciences, Graduate School of Natural Science and Technology, Okayama University, 1-1, Naka 3-chome, Tsushima, Okayama 700-8530, Japan. 4 Department of Earth ✉ and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan. email: [email protected] COMMUNICATIONS EARTH & ENVIRONMENT | (2020) 1:25 | https://doi.org/10.1038/s43247-020-00023-4 | www.nature.com/commsenv 1 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-00023-4 orthern Hemisphere glaciation gradually developed after 70° N its onset 3.6 million years ago (Ma)1 and further intensi- N 2 fi fied after 2.7 Ma . The latter intensi cation of Northern Hemisphere glaciation (iNHG) was an irreversible climatic dete- rioration3, and the 41-kyr component of glacial–interglacial var- 65° iations was amplified2. The primary cause of iNHG has been DSOW inger debated, with two leading hypotheses: severe cooling induced by rm C – Sea I u 4 6 60° Labrador r r decreased atmospheric CO2 concentration , and increased rain Water e ODP 982 n t and snow fall over Greenland and Europe attributed to the closure t n 7,8 IODP e of the Central American Seaway (CAS) . Maintaining the 41-kyr r 55° Arctic Front U1314 r fi u glacial cycles requires additional oceanic ampli er mechanisms, C 3 ic ISOW t such as the Atlantic meridional overturning circulation (AMOC) n 50° a 9 tl and storage of carbon dioxide . The meridional overturning cir- A h culation is driven by deep waters produced by sinking of cold, rt o salty, dense waters at high latitudes in both hemispheres, and in 45° N this process, Atlantic surface currents transport heat and moisture northward10. Although the AMOC played a dominant role in 40° IODP U1313 (DSDP 607) – – 0 recent glacial interglacial and stadial interstadial climate condi- 0 35° tions11,12, studies based on benthic foraminiferal carbon isotopes 13 13,14 NADW (δ C) , a major proxy of deep-water formation, suggested that 30° North Atlantic Deep Water (NADW), the deep component of the 60°W 50° 40° 30° 20° 10° 0° 10°E AMOC, weakened after iNHG. Fig. 1 Oceanographic setting of the subpolar North Atlantic. Major ocean The weakened NADW and AMOC after iNHG are an enigma currents and locations of this study (IODP Site U1314, 56°21.9′N, 27°53.3′ because the density of surface water in the high-latitude North W, water depth 2820 m) and additional deep-sea drilling sites (IODP Site fi Atlantic may have been increased during iNHG by a signi cant U1313, Deep Sea Drilling Project [DSDP] Site 607, and Ocean Drilling 15 decrease in sea surface temperature (SST) and by removal of a Program [ODP] Site 982) discussed in the text are shown. Brown arrows large amount of moisture by the development of continental ice denote interglacial movement of the Arctic Front30. sheets16. In fact, general circulation models4,5 have predicted that fi the AMOC intensi ed during iNHG, whether the atmospheric 25,26 fl 4–6 7,8 by entraining surrounding water . Afterward, the over ows CO2 or CAS closure hypothesis was adopted. Because form the Deep Western Boundary Current around Cape Farewell benthic foraminiferal δ13C values are restricted by uncertainties fi – and eventually become lower NADW around the Labrador Basin. associated with biological modi cation, air sea gas exchange, and – 26 17–20 In addition, the less dense Labrador Sea Water (4 6Sv ) forms remineralisation effects , it is necessary to examine the upper NADW mainly above 2 km depth. Because ISOW, activities of the deep water and AMOC based on other proxies. – including the entrained downslope water, is the major source of In this paper, we present glacial interglacial variability of the lower NADW, its record from Site U1314 provides valuable Nordic Seas overturning circulation, northern sector of the ‐ information on the behavior of the lower limb of the AMOC AMOC, during iNHG. Our multi proxy reconstructions of during iNHG. surface-water and bottom-water environments indicate that the fi The analyses performed were (i) decomposition of isothermal Nordic Seas overturning intensi ed after the Marine Isotope remanent magnetization (IRM) of sediments as an ISOW activity Stage (MIS) G4/G3 deglaciation. More importantly, our high- proxy, (ii) measurement of coarse fractions (>150 μm) of ice resolution proxy data reveal that the overturning remained rela- rafted debris (IRD) and mineral amounts in sediments as a proxy tively strong even in early stages of individual glacials after iNHG. of arriving and melted icebergs, (iii) alkenone biomarkers as We propose that the active AMOC during early glacial stages fi proxies of SST and productivity, and (iv) abundances of diatoms facilitated the ef cient growth of continental ice sheets by miti- as proxies of surface water productivity and NAC activity (see gating heat and utilizing moisture supplied by the North Atlantic “ ” fi Methods section). IRM decomposition is an effective method to Current (NAC), resulting in the ampli cation of Northern reconstruct ISOW activity; the high-coercivity component (HCC) Hemisphere glaciation. of IRM reflects the amount of basaltic sediments from Iceland transported by the ISOW flow, whereas the low-coercivity component (LCC) is attributed to acidic sediments of southern Results 27 – origin . An alternative explanation for the IRM-HCC, that the Reconstruction of ice sheet AMOC interactions. We performed increased import of Icelandic basalts is a result of freshly multi-proxy analyses of sediments recovered from the southern weathered and eroded material in the aftermath of shrinking ice Gardar Drift, subpolar North Atlantic (Integrated Ocean Drilling sheets, is unlikely because glacial HCC greatly increased with the Project [IODP] Site U1314) (Fig. 1) (see “Methods” section). This – development of ice sheets covering regolith after iNHG, 2.7 Ma site is well suited to reconstruct ice sheet ocean interaction, (Fig. 2; see below discussion). Our ISOW and iceberg records including the AMOC, because of its location under the strong 21,22 seem different from interpretations by previous studies (e.g., influence of icebergs , the Irminger Current (north-western 28 – fl ref. ) because of the much higher resolution of our records. For branch of the NAC), and the Iceland Scotland Over ow Water example, a peak in our iceberg record represented by one analysis (ISOW). Currently, two kinds of cold, dense waters pass south- – fl point (a composite of a few hundred years) (Fig. 2)is ward across the Greenland Scotland Ridge as over ows: the undetectable in low-resolution IRD records on millennial time ISOW to the east and Denmark Strait Overflow Water (DSOW) fl scales. Our high-resolution records succeed in reconstructing ice to the west (Fig. 1). The individual over ows are roughly 3 sheet–ocean interactions that were previously undetectable. Sverdrups (1 Sverdrup= 106 m3 s−1)23, and volume transport by both overflows corresponds to about one-third of total NADW production24. After passing the Greenland–Scotland Ridge, the ISOW and icebergs. A new IRM record confirms the results of a overflows descend sills and further double their volume transport previous study on low-resolution IRM data27 that the HCC 2 COMMUNICATIONS EARTH & ENVIRONMENT | (2020) 1:25 | https://doi.org/10.1038/s43247-020-00023-4 | www.nature.com/commsenv COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-00023-4 ARTICLE 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 g 0MIS 100abc def h a 4,000 8,000 AMOC intensification IRD (grains/g) 0 12,000 b 100 15 75 30 50 Quartz (wt%) 45 25 1.5 IRM-HCC (%) c 0 1.0 0.5 C (‰ PDB) 13 3.0 δ d 0 G3 G7 G9 G11 3.4 103 G1 G8 O stack G13 (IODP Site U1313) MIS 101 -0.5 18 G12 Benthic δ 99 3.8 102 G5 7 (‰ PDB) G4 G10 e 100 104 G2 G6 6 LR04 4.2 5 4 3 19 2 BWT (ºC) 18 f 1 17 (DSDP Site 607) 0 16 15 14 g SST (ºC) 13 20 (ODP Site 982) 12 11 17 10 80 14 60 11 SST (ºC) 40 h 8 (IODP Site U1314) 20 (µg/TOCg) i 40 0 30 Alkenone concentration 20 100 10 j Calcite (wt%) 75 All species 0 Thalassionema bacillare 50 25 (valves/µg) k 0 30 (%) Abundance of diatoms 20 10 Relative l abundance of 0 T.
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