Enhanced surface melting of the Fennoscandian Ice Sheet during periods of North Atlantic cooling

Abstract

12 Heinrich Events (HEs) are dramatic episodes of marine-terminating ice discharge and 13 sediment rafting during periods of cold North Atlantic climate. However, the causal chain of 14 events leading to their occurrence is unresolved. Here, we demonstrate that enhanced surface 15 melting of land-terminating margins of the southern Fennoscandian Ice Sheet (FIS) is a recurring 16 feature of Heinrich Stadials (HSs), the cold periods during which HEs occur. We use neodymium 17 isotopes to show that the Channel River transported detrital sediments from the interior of 18 eastern Europe to the Bay of Biscay in the Northeast Atlantic Ocean ca. 158 to 154 ka. Based on 19 similar evidence from the last glacial period, we infer that this interval corresponds to the 20 melting and retreat of the southern FIS margin despite contemporaneous cooling in the North 21 Atlantic and central Europe. The FIS melting episode occurred just prior to a HE, consistent with 22 findings from the more recent HSs 1, 2, and 3. Based on this evidence, we clarify a sequence of 23 events that precedes HEs. Precursor melting of North Atlantic-adjacent ice sheets induces an 24 initial Atlantic meridional overturning circulation (AMOC) slowdown. Atmospheric changes 25 during the resulting HS cause summertime warming in northern Europe that drives enhanced FIS 26 melting. Subsequent meltwater discharge to the North Atlantic further weakens the AMOC and 27 warms the intermediate water masses that contribute to HEs. 28 29 INTRODUCTION 30 The widespread melting of North (N.) Atlantic-adjacent ice sheets during periods of 31 exceptionally cold polar climate is a paradoxical feature of recent glacial periods (e.g., Barker et 32 al., 2015; Toucanne et al., 2015). Heinrich Events, in which armadas of icebergs discharge from 33 marine-terminating ice margins into the N. Atlantic, punctuate the termination of cold Heinrich 34 Stadials. HSs likely are caused by ocean surface cooling in response to freshwater-induced 35 disruptions of the AMOC (e.g., Clark et al., 2007; Ivanovic et al., 2018). HEs then are triggered 36 by the melting of marine-terminating grounded ice by the poleward transport of subsurface heat 37 (700-1100 m depth, Alvarez-Solas et al., 2013) from low latitudes in response to further 38 weakening of the AMOC (Shaffer et al., 2004; Marcott et al., 2011; Alvarez-Solas et al., 2013). 39 However, the continental sources of freshwater that induce this AMOC destabilization during 40 HSs remain debated. 41 During the last glacial period, HEs were preceded by the melting of terrestrial42 terminating FIS margins. These FIS melting episodes, focused in the continental interior of 43 Europe, lasted from the onset of HSs until the resulting HE as revealed by detailed study of HS1 44 (~18-15 ka), HS2 (~26-23 ka), and HS3 (~31-29 ka) (Toucanne et al., 2015). Here, we document 45 sedimentary and geochemical evidence of terrestrial-terminating FIS margin melting during a 46 period of extensive N. Atlantic cooling ca. 158-152 ka. Our results demonstrate that FIS melting 47 during HSs precedes and contributes to the AMOC disruption that leads to HEs during both the 48 last and penultimate glacial periods. Enhanced surface melting of the FIS prior to HEs is 49 consistent with summertime warming in Europe during stadials (Schenk et al., 2018; Bromley et 50 al., 2018). 51 Terminal moraines show that the British-Irish Ice Sheet (BIIS) and FIS coalesced in the 52 North Sea ca. 160 ka during the Drenthe Stage of Marine Isotope Stage (MIS) 6 (Gibbard et al., 53 1988). When the BIIS and FIS coalesced, rivers of Britain, France, and the North European Plain 54 (NEP; Fig. 1) integrated as tributaries of the Channel River, the sea level lowstand precursor of 55 the modern English Channel (Fig. 1; Busschers et al., 2008). The Channel River drainage basin 56 extended across much of northern Europe and drained large quantities of meltwater to the Bay of 57 Biscay (e.g., Zaragosi et al., 2001; Toucanne et al., 2009, 2015). Sediments deposited off the 58 Channel River mouth therefore record the timing and nature of ice sheet melting. 59 60 METHODS 61 FIS melting in the continental interior of Europe during MIS 6 is supported by the Nd 62 isotopic composition of detrital sediments from Bay of Biscay core MD03-2692. This core is 63 located in front of the former Channel River and records sedimentary discharge with high fidelity 64 (Fig. 1; 46°49.72′ N, 9°30.97′ W, 4064 m; Eynaud et al., 2007). The Nd isotopic compositions of 65 detrital sediments from the Channel River fingerprint their geographic origin within Europe 66 (Toucanne et al., 2015). Following Toucanne et al. (2015), we determine that anomalously non67 radiogenic Nd isotope signatures in the core sediments correspond to periods of southern FIS 68 margin melting and retreat. To reconcile the Nd isotope signatures of the MD03-2692 sediments 69 with their continental sources, we acquired Saalian (MIS 6-10) glacigenic sediments deposited 70 by the Baltic Ice Stream in Denmark and Poland (Ehlers et al., 2011) (Fig. 1). 71 Nd isotope ratios were measured for the fine-fractions (<63 μm) of both the MD03-2692 72 core sediments (n=55; Table S1) and glacigenic sediments from the NEP (n=17; Table S2). We 73 focus on the <63 μm fraction because the meltwaters from ice margins predominantly transport 74 the clay and silt fractions of continental detritus (Brown and Kennett, 1998; Boswell et al., 75 2018). All samples were prepared per Bayon et al. (2002) prior to isolation of the Nd by ion76 exchange chromatography. Nd isotope measurements were performed on a Thermo Scientific 77 Neptune MC-ICP-MS at the Pôle Spectrométrie Océan, France, using a sample-standard 78 bracketing method. Procedural Nd blanks were negligible compared to the amount of Nd in the 79 studied samples. We estimate the 2σ uncertainty of our measurements to be ±0.3 ε-units based on 80 replicate analyses of the JNdi-1 standard solution (143Nd/144Nd = 0.512115 ± 0.000009, 2σ, 81 n=31). We report 143Nd/144Nd ratios in εNd notation, [(Nd/Nd)sample/(Nd/Nd)CHUR − 1] 82 × 104, using the (Nd/Nd)CHUR value of 0.512638 (Jacobsen and Wasserburg, 1980). 83 The MIS 6 chronology for MD03-2692 (Table S3) is constructed by tuning the 84 abundances of the polar planktic foraminifera N. pachyderma (s.s. sinistral) in the core to those 85 from the ODP 983 core (Barker et al., 2015) that has been recently synchronized to the synthetic 86 Greenland ‘Speleo-Age’, a U-Th based chronology (Barker et al., 2011). The dominance of N. 87 pachyderma in the sediments corresponds to periods of intense cooling, and we presume that the 88 onset of these cold periods, interpreted to represent the southward migration of the polar front, is 89 concurrent across the N. Atlantic (Barker et al., 2015). From this initial chronology, we observe 90 that the high-resolution Ca/Fe ratios of MD03-2692 sediments, reflecting climatically-driven 91 biogenic carbonate fluxes, are closely aligned with the synthetic Greenland temperatures 92 (GLT_syn) of Barker et al. (2011). This coupling of Ca/Fe ratios and GLT_syn allows us to fine93 tune the final age model (e.g., Hodell et al., 2013) (Table S3; Fig. S1). 94 95 LINKING BALTIC SEDIMENT TO SOUTHERN FIS MARGIN RETREAT 96 Throughout most of MIS 6, the εNd values of the core sediments vary between -10.8 and 97 -12.0 (Fig. 2F). These values are consistent with downstream Channel River sources (e.g., 98 Ireland, Great Britain, and France), including the BIIS (Toucanne et al., 2015). As inferred from 99 the radiogenic Nd signatures and two-fold increase in mass accumulation rate (MAR) of detrital 100 sediments at the core site (Fig. 2F, G), enhanced melting of the BIIS began ca. 160 ka. However, 101 the εNd of the core sediments from 158 to 154 ka reached values of -14.0 (Fig. 2F), revealing 102 that the dominant portion of Channel River sediments were sourced from the eastern NEP (-14.4) 103 by 156 ka. This Baltic sediment provenance demonstrates that the southern margin of the FIS 104 was melting, retreating, and dispatching large quantities of sediment to the Channel River (Fig. 105 2F, G). Benthic foraminifera record an ~12 m sea level equivalent (SLE) reduction in the size of 106 global ice sheets ca. 159 to 156 ka (Fig. 2A; Waelbroeck et al., 2002). This ice volume decrease 107 is synchronous with a substantial retreat of the southern FIS margin from the Drenthe maximum 108 to a spatial extent even more restricted than the subsequent Warthe limits (Fig. 1; Toucanne et 109 al., 2009). Considering the size of the FIS ca. 160 ka (~60 m SLE, Lambeck et al., 2006), a large 110 volume of FIS meltwater was discharged through the Channel River (Fig. 2F). The 111 corresponding increase in Channel River flow led to greater volumes of anchor ice (from 112 wintertime freezing of the river bed) that were transported to the Bay of Biscay during the spring 113 thaw (Toucanne et al., 2009). In total, the melting episode resulted in a 2.5 m section of 114 seasonally laminated IRD termed ‘Channel River IRD’ and massive muds in the deep Bay of 115 Biscay (Fig. 2E, G). This accumulation is 1.5 times greater than at Termination I (ca. 18-17 ka, 116 Zaragosi et al., 2001). 117 To verify that the ca. 156 ka event reflects FIS margin melting, we draw on evidence 118 from the last glacial period. Terrestrial-based paleogeographical reconstructions of the FIS 119 (Hughes et al., 2016) reveal that the southern FIS margins retreated in phase with Channel River 120 discharge events identified during HS1, HS2, and HS3 (Fig. 3). Based on the similarity of 121 sedimentary and geochemical evidence, we infer that the terrestrial-terminating FIS margin was 122 melting and retreating from ~158 to 154 ka (Fig. 2A, F, G). 123 124 ICE SHEET MELTING AND AMOC SLOWDOWN DURING STADIALS 125 FIS melting in the continental interior ca. 158-154 ka occurs during a period of cooling 126 that extends