On the causes of Arctic sea ice in the warm Early Pliocene

Scattered and indirect evidence suggests that sea ice occurred as far south as the Iceland Sea during the Early Pliocene, when the global climate was warmer than present. However, conclusive evidence as well as potential mechanisms governing sea ice occurrence outside the Arctic Ocean during a time with elevated greenhouse gas concentrations are still elusive. Here we present a suite of organic biomarkers and palynological records from the Iceland Sea and Yermak Plateau. We show that sea ice appeared as early as ~4.5 Ma in the Iceland Sea. The sea ice either occurred seasonally or was transported southward with the East Greenland Current. The Yermak Plateau mostly remained free of sea ice and was influenced dominantly by Atlantic water. From ~4.0 Ma, occurrence of extended sea ice conditions at both the Yermak Plateau and Iceland Sea document a substantial expansion of sea ice in the Arctic. The expansion occurred contemporaneous with increased northward heat and moisture transport in the North Atlantic region, which likely led to a fresher Arctic Ocean that favors sea ice formation. This extensive sea ice cover along the pathway of the East Greenland Current gradually isolated Greenland from warmer Atlantic water in the Late Pliocene, providing a positive feedback for ice sheet expansion in Greenland.

Scientific RepoRts | (2019) 9:989 | https://doi.org/10.1038/s41598-018-37047-y formation. Crucial evidence supporting freshening of the surface waters in the Russian Arctic is still lacking, but paleoclimatic records from the North Atlantic 23 and Norwegian Sea 24 indicate increased heat transport and the first seasonal sea ice in the marginal Arctic Ocean (Yermak Plateau) around 4.0 Ma 11 . Given the current trajectory towards a globally warmer climate and amplified Arctic climate change, it is crucial to document the occurrence of Arctic sea ice during warmer-than-present conditions and understand the underlying mechanisms. Despite not being a perfect analogue for the future climate of our planet 25 , the Early Pliocene does provide the opportunity to study the mechanisms governing sea ice presence in the Iceland Sea in a world characterized by high CO 2 levels 17 and global temperatures 26 . Here, we present the first Early Pliocene (~4.9-3.5 Ma) sea ice reconstructions based on the sea ice proxy IP 25 , sterols and palynology from the Iceland Sea Ocean Drilling Program (ODP) Site 907 and Yermak Plateau ODP Hole 911A (Fig. 1) to determine and understand the underlying causes of (seasonal) sea ice presence in the Early Pliocene Arctic.

Dominantly Ice Free Conditions in the earliest pliocene Nordic seas
Our reconstructions reveal sea ice-free conditions and relatively high marine productivity in the Iceland Sea between 5.0 and 4.6 Ma, evidenced by the absence of IP 25 and relatively high concentrations of the open water biomarker brassicasterol (Fig. 2B). Dinoflagellate cyst concentrations are high (~3,000 to ~12,000 cysts/g; Fig. 2A), and the assemblage has clear Atlantic water characteristics 14 , suggesting a Nordic Seas circulation different from today 13,14,27 . Ice-free conditions are supported by reconstructed summer SSTs ~5 °C higher than modern (Fig. 2D) 28,29 and high phytoplankton productivity related to the presence of Atlantic water masses in the Iceland Sea. The low-resolution Yermak Plateau biomarker record suggests occasional sea ice edge conditions at 4.9 and 4.6 Ma ( Supplementary Fig. 1B). High IP 25 and low brassicasterol concentrations indicate seasonal sea ice occurrence at ODP Hole 911A (Fig. 2D), while the absence of IP 25 and high brassicasterol concentrations at the neighboring ODP Hole 910C indicate sea ice-free conditions and enhanced primary productivity at this time 11 . Yet, a clear, uniform interpretation of the low-resolution biomarker data from these sites, located very close to each other, is difficult to make. The difference in biomarker signature at the two sites could be consistent with a highly variable sea ice margin, comparable to modern conditions in this region ( Supplementary Fig. 2) 30 . High brassicasterol concentrations at ODP Hole 910C suggests substantial primary productivity, which is often elevated close to marginal ice zones 10,31 . However, SSTs remain relatively high and indicate an Atlantic rather than polar water influence 11 . Alternatively, as seasonal sea ice was already present in the Arctic Ocean since the Late Miocene 10 , sea ice could have also been exported from the Arctic Ocean towards the Yermak Plateau until it encountered the warmer Atlantic waters of the West Spitsbergen Current. With a sub-aerially exposed Barents Sea 20 , the heat advection through the West Spitsbergen Current towards the Yermak Plateau is increased 32,33 (Fig. 3A), thereby inhibiting a long-term sea ice cover but allowing occasional local sea ice formation or sea ice export from the Arctic. It is, however, very likely that the different paleoceanographic interpretations for ODP holes 910C and 911A are best explained by the different ages of the investigated samples at both sites and that sea ice appeared in the region only sporadically.  14 , which has been related to the emergence of a modern-like EGC 14 and the replacement of Atlantic water with cooler, fresher arctic-sourced water. When sea ice occurred at 4.5 Ma, reconstructed summer SSTs of ~10 °C remained well above modern values (Fig. 2D), suggesting either a strong seasonal contrast between warm summers and winters with sea ice, or that transported sea ice melted in the Iceland Sea. While our data unequivocally demonstrates sea ice presence in the Iceland Sea at 4.5 Ma, we cannot conclude whether sea ice formed locally in the Iceland Sea, was exported from the Arctic to the Iceland Sea with the EGC or was a combination of both. In ODP Hole 910C, a significant drop in brassicasterol concentrations (to values below 2 ng/g sediment) and the absence of IP 25 ( Fig. 2C) could be indicative for a permanent sea ice cover at 4.5 Ma ( Supplementary Fig. 1B), but the Yermak Plateau most likely still remained sea ice free and dominated by Atlantic water 11 . Between 4.4 and 4.1 Ma, samples for biomarker analyses were not available at ODP Site 907 due to a heavily sampled sediment core. However, earlier studies show a decrease in summer SSTs from 10 to 6 °C 29 around 4.4-4.3 Ma and a fragmentary dinoflagellate cyst record with low concentrations to even barren samples 13,14 ( Fig. 2A,D). This suggests a strengthened influence of cool, Arctic waters via the EGC in the Iceland Sea about 100-200 ka later than the first sea ice occurrence at 4.5 Ma. Although this lag is not fully understood, it may be reflecting a gradually intensifying EGC that leads to long-term, cool conditions in the Iceland Sea. The Yermak Plateau generally remained sea ice free 11 , but the occurrence of IP 25 in a single sample (~4.4 Ma) in ODP Hole 911A reflects that conditions were sporadically suitable for sea ice to be present (Fig. 2C).
Thus, the sea ice reconstructions for the Iceland Sea and the Yermak Plateau around 4.5 Ma correspond favorably to the reorganization of Nordic Seas surface circulation. This paleoceanographic change most likely occurred as a consequence of changed flow direction through the Bering Strait 14 allowing cool and low salinity Pacific water to enter the Arctic Ocean 15,34-36 . While today, Arctic-Atlantic surface water exchange occurs through both the Fram Strait and the CAA 37 , the latter was closed during the Early Pliocene 19 . Such setting favors the role of the EGC as the only pathway for cool, fresher water and possibly also as sea ice exporter from the Arctic already in the Early Pliocene (Fig. 3B).

Warming in the Norwegian sea promotes Arctic sea Ice Formation
An extended sea ice cover (high IP 25 and low brassicasterol) alternated with ice edge conditions (high IP 25 and high brassicasterol) in the Iceland Sea between 4.0 and 3.7 Ma (Fig. 2B; Supplementary Fig. 1A). Dinoflagellate cyst samples are either barren or show very low concentrations during this interval indicating limited productivity ( Fig. 2A) 13,14 and hence possibly harsh conditions due to sea ice presence and/orlow SSTs. Indeed, Iceland Sea summer SSTs decreased further from ~8 to 3 °C 29 within this interval (Fig. 2D), yet summers largely remained sea ice-free because alkenone production still occurred. At Yermak Plateau ODP Hole 910C, seasonal sea ice appeared shortly after 4.0 Ma 11 , while at ODP Hole 911A, a single sample around 3.8 Ma with high IP 25 and low brassicasterol concentrations suggests an extended sea ice cover. One sample in ODP Hole 911A could indicate a permanent sea ice cover around 3.5 Ma (Fig. 2C, Supplementary Fig. 1B), but taking the data from ODP Hole 910C into account, a seasonally sea ice covered Yermak Plateau seems more likely. Around 4.0 Ma, rapid ocean surface warming in the Norwegian Sea (~7 °C in 40 ka; Fig. 2D) 24 , cooler SSTs in the Iceland Sea 29 and a sea ice cover stretching from the Arctic Ocean to the Iceland Sea all occurred at the same time and established the characteristic zonal surface water gradient of the Nordic Seas. Together, these observations support two major parts of the theory proposed by ref. 21 , namely the northward heat (and associated moisture) transport in the North Atlantic and the enhanced formation of sea ice in the Arctic. Essential for enhanced sea ice formation is a freshening of the Arctic Ocean. It was proposed that the moisture source for this freshening originates from increased northward heat and moisture transport in the North Atlantic 21 . This is evident in the reconstructed high SSTs in the eastern North Atlantic 23 and Norwegian Sea 24 around 4.0 Ma (Fig. 2D). There is currently no corroborating data that atmospheric moisture was consequently transported via the westerlies from the Atlantic region to the Eurasian continent where it fed northward-draining rivers, which ultimately freshened the Arctic Ocean to promote sea ice formation. However, our new data do provide evidence that shortly after the rapid temperature increase in the eastern North Atlantic and Norwegian Sea, a major expansion of sea ice cover occurred in the Arctic and extended to the Fram Strait and the Iceland Sea (Figs 2B, 2C and 3C). Further testing of this theory will require gathering data that documents the moisture transport to Siberia, the freshening of the Arctic Ocean via Siberian rivers and consequent sea ice formation.

Influence of Sea Ice Expansion on Pliocene Arctic Climate
Despite the relatively high Early Pliocene atmospheric CO 2 concentrations of 380-400 ppm 17,18 , the changes in North Atlantic and Arctic paleoceanography caused sea ice to occur as far south as the Iceland Sea at 4.5 Ma. This development was likely controlled by the emergence of a modern-like EGC, which was established as a consequence of a surface water flow reversal across the Bering Strait 14 . The changed paleoceanography was crucial to allow the import of fresher, cooler Arctic water and sea ice into the Iceland Sea. Whether sea ice was exported directly from the Arctic, or the fresher, cooler water favored local sea ice formation in the Iceland Sea yet remains ambiguous. Nevertheless, the effects of a changed EGC and appearance of sea ice along its pathway are also recognized from the dinoflagellate cyst turnover in the Iceland Sea 14 and the onset of biosiliceous sedimentation in the Labrador Sea 15 at 4.5 Ma. Around that time, small IRD amounts are recorded at ODP Site 907, indicating that Greenland did have ice caps or small ice sheets that could produce icebergs and ice rafted detritus (IRD) 38 , albeit in volumes considerably lower than in the Late Pliocene and Quaternary. Tectonic uplift made elevated plateaus available in Greenland during the Early Pliocene 39 where glaciers and ice caps could nucleate and eventually expand into a large, IRD producing ice sheet. Such large ice sheet started to deliver considerable IRD into the North Atlantic during the Late Pliocene 40 , when the zonal gradient in the Nordic Seas was already established. In fact, the cooler water and more substantial sea ice presence in the Iceland Sea after 4.0 Ma and in the Late Pliocene 16 , may have contributed to the gradual expansion of continental ice in Greenland. A more substantial and long-term sea ice presence along the East Greenland coast acts to thermally isolate Greenland from relatively warmer Atlantic waters 41 and reduces heat advection 42 , as well as providing a sea ice-albedo feedback and inhibiting ocean-atmosphere heat exchange. These combined effects, together with the tectonic uplift of the circum-Arctic land masses 43 all provide positive feedbacks for expansion of the Greenland Ice Seet (GIS).
In the modern context of increasing atmospheric greenhouse gas concentrations and rapidly declining Arctic sea ice, our study provides fundamental insights into consequences of a (seasonally) sea ice-free Arctic by demonstrating that limited sea ice presence in the Arctic Ocean and Iceland Sea together with a high Early Pliocene atmospheric CO 2 concentration (380-400 ppm 17,18 ) correspond to a strongly reduced GIS. The occasional presence of sea ice in the Early Pliocene Iceland Sea and Yermak Plateau was insufficient to provide an insolating buffer between the warm surface waters in the Nordic Seas and the GIS before ~4.0 Ma. Together with the high atmospheric Pliocene greenhouse gas concentrations, this setting likely inhibited major glaciation in Greenland. It is only when a more stable ice edge developed along the coast of East Greenland in the Late Pliocene 16 and CO 2 concentrations gradually decreased 17,18 that the GIS could expand to reach the coast line 40,44 . As such, sea ice along the east Greenland coast acts as a positive feedback for sustaining and expanding the GIS. Our data do not allow disentangling the relative effects of Arctic sea ice extent and greenhouse gas concentrations on the GIS, which should be addressed in future modeling studies.

Methods
Age model. We studied the interval between 85.63-73.42 meter composite depth (mcd) of ODP Site 907. The age model is based on paleomagnetostratigraphy 45 . Ages for the three uppermost studied samples were calculated using the astronomically tuned IRD record from ref. 46 . All paleomagnetic reversals were updated to the most recent Astronomically Tuned Neogene Timescale 2012 47 ( Table 1). The alkenone SST reconstructions 29 and the IRD record 38 from ODP Site 907 (Fig. 2D,E) were placed on this age model to ensure a direct comparison. We adopted the age model reported in ref. 11 for ODP Hole 910C and ref. 48 for ODP Hole 911A.

Data Availability
All data generated or analyzed within this study are available at doi.pangaea.de/10.1594/PANGAEA.896652.  Table 1. Early Pliocene tie points used for our age model. Depth (mbsf = meters below sea floor, mcd = meters composite depth) for polarity chron boundaries from ODP Hole 907A and the corresponding age from ref. 45 . We updated these ages to the Astronomically Tuned Neogene Timescale 2012 47 .