The role of Arctic Ocean freshwater during the past 200 ky
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As part of the hydrologic cycle, the freshwater system plays a pivotal role for the Arctic Ocean. It maintains the strong stratification in the upper waters and fosters the formation of sea ice on the circum-Arctic shelves from where the ice is being exported toward Fram Strait and into the Nordic Seas. Recent projections of climate change under the greenhouse effect predict severe changes for the hydrologic cycle in the Arctic. This manuscript reviews the current knowledge of past changes in freshwater fluxes to and from the Arctic Ocean and their possible impact on ocean circulation and climate outside the Arctic during the past 200,000 years. It becomes evident that abrupt and large-volume discharges into the Arctic Ocean during times of major climate transitions were capable of disturbing the global ocean circulation and triggering further climate change, e.g., at the onset of the Younger Dryas cold event. During sea-level rise in the Holocene, a connection between the increasing areas available for sea ice formation, the position of the ice margin in the ice export area (the Fram Strait) and the deepwater convection in the Greenland Sea is suggested. Further work is needed to investigate the effects of other catastrophic freshwater discharges from previously ice-dammed lakes in northern Eurasia during the Weichselian and Saalian glaciations. Events like the 8.2 ka and the Younger Dryas, which were associated with flooding and routing of glacial meltwaters and had a significant effect on climate, could serve as a template to better validate the impact of similar occurrences in the past. To date, the actual influence of the earlier events on ocean circulation and climate remains elusive.
KeywordsArctic Ocean Freshwater Climate Quaternary Sea ice
Rapid increases in freshwater input to the North Atlantic Ocean are recognized as a potential threat to the global climate system because they may cause a drastic reduction in the Atlantic meridional overturning circulation (AMOC) which is responsible for the heat transport to higher latitudes . A wealth of paleoclimatic studies has shown that this has happened a number of times in the geological past. Probably the most prominent examples were the so-called Heinrich events [19, 37], millennial-scale cooling events in the last glacial recorded around the North Atlantic, triggered by iceberg and freshwater discharges mostly from the Laurentian ice sheet in eastern Canada. Similar disturbances of the climate system could also be proven for pre-Weichselian times (e.g., [3, 59]), but the sources of the icebergs may have varied through time. During the last deglaciation and in the early Holocene, two rapid cooling events occurred in the North Atlantic realm. The Younger Dryas and 8.2 ka (8.200 ky before present; all ages are given as calendar years) events were ascribed to the release of large amounts of freshwater from ice-dammed lakes Agassiz and Ojibway in North America to the North Atlantic (e.g., [4, 21]). In the last decade, however, alternative hypotheses have been presented concerning the trigger of the Younger Dryas event. The most widely accepted one involves a routing of freshwater from Lake Agassiz and the Keewatin ice dome through the valley of Mackenzie River and into the Arctic Ocean [57, 82]. Considering the fact that there have been multiple large-scale glaciations on the circum-Arctic continents in the past, the role of Arctic freshwater events in rapid climate change has received rather little attention yet. In this manuscript, we address this issue on longer geological timescales and review available marine geological records to investigate the causes and possible consequences of freshwater events in the Arctic Ocean for the oceanic and climate system.
Freshwater in the modern Arctic Ocean
The last centennial in the Arctic has seen a rapid retreat of glacier ice on the continents  as well as ongoing freshwater accumulations in the Arctic Ocean from increasing river runoff [65, 66] and changes in the oceanic freshwater pathways . The release of a freshwater accumulation from the interior Arctic Ocean led to the “Great Salinity Anomaly” observed in the northern North Atlantic in the 1970s [16, 26]. A recent study indicated a drastic reduction (~10 %) in AMOC strength as a consequence of this freshwater export from the Arctic . A number of numerical modeling studies (e.g., [29, 41, 42, 94]) have investigated the response to present and future freshwater fluxes from the Greenland ice sheet under ongoing global warming. They unanimously concluded on a future weakening of the AMOC which will lead to a reduced northward heat transport that may, at least around the northern North Atlantic, compensate to a certain degree for the overall warming as a result of the greenhouse effect.
Detection of freshwater events in Arctic history
To define periods or short-term events in the geological history, when large amounts of freshwater were discharged to the Arctic Ocean requires evidence in the geological record which may point directly or indirectly to extremely low salinities in the surface waters. Several analytical approaches can lead to such evidence. High concentrations of freshwater diatoms and aquatic palynomorphs versus marine species in sediments from Siberian shelves have been shown to reflect the proximity of a terrestrial freshwater source [68, 69]. Hydrogen isotopes (δD) of short-chain biogenic compounds (n-alkane n-C17) were used to infer information on salinity changes in the Arctic Ocean at ~55 Ma, during the so-called Paleocene/Eocene Thermal Maximum . Findings of large amounts of the free-floating fern Azolla together with abundant freshwater organic and siliceous microfossils in Arctic sediments from ~50 Ma indicate an episodic freshening of Arctic surface waters .
The almost linear correlation between salinity and the oxygen isotope composition of seawater is slightly disturbed in Arctic near-surface waters by the formation and melting of sea ice . Nevertheless, the isotopic composition of living planktic foraminifers quite well reflects that of the ambient seawater [5, 63, 92]. Accordingly, the pattern of decreasing near-surface salinities from the northern Barents Sea margin to the Arctic Ocean interior is documented in oxygen isotope values (δ18O) of planktic foraminifers from sediment surface samples . A recent study by Xiao et al.  confirmed this result and the earlier hypothesis that higher δ18O values in these foraminifers reflect a deeper habitat of these organisms at the shelf break.
Although the modern Arctic Ocean stores huge amounts of freshwater, conditions during freshwater events in the Late Quaternary were probably significantly different. Evidence comes from the carbon isotope values (δ13C) of living planktic foraminifers from the freshwater-rich near-surface layer and specimens from interior Arctic sediment surface samples which are all unusually high (up to 1.5 ‰; [63, 78, 92, 97]). This correlates with the high δ13C values of dissolved inorganic carbon (DIC) in the uppermost water column of the Arctic Ocean which are in contrast to the low δ13CDIC values of Arctic river water (cf. Bauch et al. this issue). Nutrient availability and δ13CDIC values are anticorrelated in the Canada Basin, while in the Eurasian Basin, both show little variability (Bauch et al. this issue). Most likely, in the latter area there is little alteration of the carbon isotope budget by bioproduction under the sea ice, along the drift path of the freshwater from the Siberian shelves toward the Fram Strait [78, 97]. The high values found in living and Late Quaternary planktic foraminifers can thus be explained by an input of low-13C atmospheric CO2 that is balanced by the fixation of 12C through bioproduction.
Events triggered by rapid outbursts of freshwater from a well-defined source, however, produce a different signal in the paired planktic isotope records from the Arctic and sub-Arctic than found in modern surface sediment samples. While low δ18O values of the foraminifers, as can be expected, reflect the light isotopic composition of the additional freshwater, planktic δ13C values are typically low (e.g., [14, 45, 74, 77, 80]). This is interpreted to result from an increased stratification which blocked convection and trapped the nutrients so that the planktic foraminifers favored a (less ventilated) habitat underneath the freshwater layer. The strong stratification allowed metabolic CO2 from residual bacterial respiration (with low δ13C values) to accumulate in the water and decrease the δ13C of the dissolved inorganic carbon (DIC) of the water column. Accordingly, freshwater events can be identified in planktic isotope records by a combination of both low oxygen and carbon isotope values (e.g., [74, 77, 80]). In cases of extreme or very proximal freshwater discharges, salinities in the upper water layers may have dropped below the tolerance limit of planktic foraminifers (i.e., S < 28, estimated by correlation with the limits of related species; cf. ), with the consequence that no specimens are deposited in the respective sediments below (e.g., ).
Late Quaternary freshwater variability and events in the Arctic
Holocene and Eemian interglacials
Arctic Ocean environmental conditions during the last interglacial (Eemian) are far less well understood than in the Holocene. There is evidence for a cooler Atlantic Water inflow to and a stronger freshwater export from the Arctic Ocean in the early Eemian (e.g., [8, 13, 89]). The latter likely resulted from melting of the huge Saalian ice sheets on northern Eurasia which were much larger than later during Weichselian times (cf. Svendsen et al. ). Details of the interdependency of Arctic freshwater export, sea ice formation and the convective activity in the northern North Atlantic during the entire Eemian, however, remain elusive—mostly due to the lack of well-dated higher-resolution Eemian records from the Arctic Ocean.
In the last decade, evidence has been accumulating that the trigger for the most abrupt and severe climatic event during the last deglaciation originated in the Arctic. Besides latest hypotheses which also involved considerations of extraterrestrial impacts and volcanic eruptions, etc. (e.g., [33, 54, 58]), earlier explanations for the YD event (12.8–11.6 ka) focused on a deviation of freshwater discharge from the glacial Lake Agassiz south of the Laurentian ice sheet in North America (e.g., [21, 22, 23]). It was suggested that an abrupt change from a southward drainage (to the Mississippi) to a westward discharge through the St. Lawrence River valley at ~12.9 ka established a freshwater lid on the northern North Atlantic that reduced or shut down vertical convection and significantly weakened the AMOC. Several authors, however, have claimed a lack of field evidence for large-volume freshwater fluxes in the proposed through-flow regions west of the St. Lawrence River (e.g., [49, 86]). Combining results from modeling and field studies, an alternative theory involves a northward routing of freshwater discharge through the Mackenzie valley, entering the Arctic Ocean at 135°W in the Beaufort Sea [57, 82, 83]. Recent results from high-resolution numerical ocean modeling are in strong support of this hypothesis. Condron and Winsor  show that only minor amounts of the freshwater from a Lake Agassiz outburst toward the St. Lawrence valley could ever reach the areas of present ocean overturning in the Greenland and Labrador seas, while a discharge toward the Arctic Ocean and export through Fram Strait should result in a significant freshening in these areas. Marine evidence for a large-scale freshwater event in the Arctic Ocean and in waters off East Greenland is still scarce, probably because of effects such as low sedimentation rates and enhanced bioturbational mixing. A few records, however, have captured some signs. Planktic isotope data series from the Laptev Sea continental margin, the western Fram Strait and the Greenland Sea show a peak of low values centered at 13 ka [9, 79, 84] and are consistent with a reconstruction of enhanced sea ice formation off the Laptev Sea . Radiogenic and IRD data from the central Lomonosov Ridge suggest a significantly increased export of sea ice from Arctic North America, close to the area where the freshwater may have entered the Arctic Ocean . These data corroborate the results from terrestrial fieldwork and modeling that argue for a paradigm shift regarding the likely origin of the freshwater as the possible trigger for the YD cold event. Apparently, this major cooling, which interrupted the warming trend after the last glacial and which was recorded in a wealth of marine and terrestrial paleoclimatic data series all across the northern hemisphere, was caused by an excess freshening of Arctic Ocean surface waters and the export of these waters (also in the form of sea ice) to the major areas of oceanic deep convection.
The cause of a younger and significantly weaker cooling event than the YD may, at least in part, also be attributed to the Arctic Ocean. A core from the northern Alaskan continental margin (142°W) showed a distinct freshwater spike at ~11.5 ka in the isotopic records . That one correlates well in time with another massive outburst of meltwater from glacial Lake Agassiz which went down the Mackenzie River valley and into the Arctic Ocean [34, 86]. This low-salinity event may have contributed to the freshening of surface waters in the Nordic Seas which, together with the discharge from the Baltic Ice Lake, triggered the so-called Preboreal Oscillation , a brief (max. 200 year) but distinct cooling event recognized in many records from areas around the Nordic Seas.
Weichselian and Saalian glaciations
Conclusions and outstanding research questions
Freshwater discharge to the Arctic Ocean is an essential player in the modern ocean and climate system due to its critical role in the maintenance of near-surface salinity contrasts and the sea ice cover. The analysis of past changes in the Arctic freshwater system and their consequences is a valuable tool to better understand and predict the impact of ongoing climate changes on the Arctic environmental system and its future.
In the Holocene, sea-level rise played an important role in the freshwater system. Flooding of the vast Eurasian shelves strongly increased the areas available for seasonal sea ice formation. Enhanced ice production after the Holocene Thermal Maximum likely resulted in more freshwater export to the Nordic Seas through Fram Strait. This may have increased the oceanic contrasts between the regions leading to intensified deepwater convection in the Greenland Sea.
The last deglaciation witnessed a diachronous decay of the circum-Arctic ice sheets and the discharge of meltwater from various sources to the Arctic Ocean. Evidence from terrestrial field data, marine cores and numerical modeling is accumulating and supports the hypothesis that the Younger Dryas cold event was triggered by an outflow of freshwater from glacial Lake Agassiz and melting of the Keewatin ice dome toward the Arctic Ocean and further through the Fram Strait to the areas of deep ocean convection.
During the Weichselian and Saalian glaciations, enormous amounts of freshwater were stored in ice-dammed lakes south of the northern Eurasian ice sheets. Catastrophic drainage events were recorded inside the Arctic Ocean, but apparently the vertical convection in the northern North Atlantic was less responsive to this excess freshwater than during the later phase of the last deglaciation.
A number of open issues need the attention of the Arctic research community to improve our understanding of the interplay between Arctic freshwater, sea ice and the meridional overturning circulation in the past. While evidence is accumulating that points to an Arctic trigger for the Younger Dryas cold event, there is only indirect or ambiguous evidence from Arctic Ocean records of a strong freshwater flow to the basin. Well-dated marine records of higher-resolution, preferably from areas next to the potential outlets in the Mackenzie delta, are needed to better tie modeling and terrestrial fieldwork results to a freshwater event in the Arctic Ocean.
Considering the size of the ice-dammed lakes which repeatedly developed south of the northern Eurasian ice sheets during Weichselian and Saalian glaciations, the possible influence of the discharge of the trapped water toward the Arctic and potentially to the areas of deepwater renewal needs more refined studies. These should involve coupled ocean modeling and detailed paleoceanographic studies to trace the fate of the freshwater in the northern North Atlantic.
The Eemian, although of high interest as a potential analog for the Arctic under enhanced global warming, remains one of the least understood time periods in the Late Quaternary Arctic history. While there is evidence from the Nordic Seas for a strong Arctic freshwater discharge during the early Eemian, virtually no information exists in the published literature on the oceanic conditions in the potential source areas of this freshwater on the northern Eurasian shelves during the early Eemian. The interplay of sea-level rise and sea ice formation on these shelves is also of interest because it may have been critical for ice conditions in the Eemian Arctic—another open issue which needs further attention.
This manuscript builds on research done in various projects funded by the German Science Foundation (DFG), the German Federal Ministry of Research and Education (BMBF), and the European Union. Of particular importance was the generous support by the Academy of Sciences, Humanities and Literature Mainz through the Akademienprogramm in 2003–2015. The authors express their sincere thanks to these organizations. Thoughtful comments by two anonymous reviewers were very much appreciated and helped to improve the manuscript.
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