Iceberg Discharge Research Articles

Modelling abrupt glacial North Atlantic freshening: Rates of change and their implications for Heinrich events

The abrupt delivery of large amounts of freshwater to the North Atlantic in the form of water or icebergs has been thought to lead to significant climate change, including abrupt slowing of the Atlantic Ocean meridional overturning circulation. In this paper we examine intermediate complexity coupled modelling evidence to estimate the rates of change, and recovery, in oceanic climate that would be expected for such events occurring during glacial times from likely sources around the North Atlantic and Arctic periphery. We show that rates of climate change are slower for events with a European or Arctic origin. Palaeoceanographic data are presented to consider, through the model results, the origin and likely strength of major ice-rafting, or Heinrich, events during the last glacial period. We suggest that Heinrich events H1–H3 are likely to have had a significant contribution from an Arctic source as well as Hudson Strait, leading to the observed climate change. In the case of H1 and H2, we hypothesise that this secondary input is from a Laurentide Arctic source, but the dominant iceberg release for H3 is hypothesised to derive from the northern Fennoscandian Ice Sheet, rather than Hudson Strait. Earlier Heinrich events are suggested to be predominantly Hudson Strait in origin, with H6 having the lowest climate impact, and hence iceberg flux, but H4 having a climate signal of geographically variable length. We hypothesise that this is linked to a combination of climate-affecting events occurring around the globe at this time, and not just of Laurentide origin.

Research Spotlight: Columbia Glacier's changing calving behavior

In 2007, the terminus of Columbia Glacier, a tidewater glacier in Alaska, became ungrounded and began floating. Columbia is now the only known floating Alaskan tidewater glacier. A new study analyzed the glacier's changing calving (iceberg release) behavior during the transition from grounded to floating.

Millennial‐scale iceberg surges after intensification of Northern Hemisphere glaciation

Iceberg discharges from continental ice sheets are widely believed to have exerted a great influence on global climate, but an iceberg discharge regime in early glacial periods after intensification of Northern Hemisphere glaciation (NHG) remains largely unclear. Here we present high‐resolution rock magnetic records during the period from 2.1 to 2.75 Ma after intensification of NHG, reconstructed from the subpolar North Atlantic. Although the establishment of the middle Pliocene chronology of North Atlantic sediments is often a serious problem, we overcame it based on findings concerning the properties of magnetic susceptibility and natural gamma radiation. Reconstructed rock magnetic records indicate that millennial‐scale iceberg surges were dominant features in the early glacial periods. Additionally, the millennial‐scale iceberg surges occurred within glacial stages during intervals when ratios of global oxygen isotope stack from benthic foraminifera (LR04 δ18O stack) surpassed approximately 3.5‰. These are comparable to the climatic and environmental changes in Pleistocene glacial periods as represented by last glacial Dansgaard‐Oeschger cycles, suggesting that continental ice sheets have oscillated and calved icebergs in a similar manner since intensification of NHG.

Submarine landforms and shallow acoustic stratigraphy of a 400 km-long fjord-shelf-slope transect, Kangerlussuaq margin, East Greenland

Kangerlussuaq Fjord is a relatively uniform, steep-walled basin, whose floor has an almost smooth surface. Debris is supplied mainly from icebergs from the fast-flowing Kangerlussuaq Glacier. Sedimentation after iceberg release from multi-year sea ice is mainly by rain-out of fine-grained englacial debris. Streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into Kangerlussuaq Trough at the Last Glacial Maximum (LGM). Bedrock channels and crescentic overdeepenings indicate warm-based ice and free water beneath parts of the former ice sheet. Cross-cutting iceberg scour marks, which characterise outer Kangerlussuaq shelf, were produced not only during deglaciation, but also occasionally through the Holocene by deep-keeled icebergs from further north in East Greenland. The outward-convex contours of the shelf edge and slope beyond Kangerlussuaq Trough, and debris flows on the slope, suggest a glacier-influenced high-latitude fan. The distribution of streamlined subglacial landforms demonstrates that the Greenland Ice Sheet extended throughout Kangerlussuaq Fjord and reached at least 200 km across the shelf in Kangerlussuaq Trough at the LGM. Streamlined landform orientation indicates ice flow from the interior of Greenland down the axis of Kangerlussuaq Trough. There is little evidence for discrete sedimentary depocentres in the trough, implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation.

Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea

Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483.Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS 14C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 14C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 14C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 14C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 14C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.

Iceberg discharge to the Chukchi shelf during the Younger Dryas

The extent of glaciation in northwestern Alaska, the source of sediment supply to the Chukchi shelf and slope, and the movement of sea ice and icebergs across the shelf during the last glacial maximum (LGM) remain poorly constrained. Here we present geophysical and geological data from the outer Chukchi margin that reveal a regionally extensive, heavily ice-scoured surface ∼ 5–8 m below the modern seafloor. Radiocarbon dating of this discrete event yields age estimates between 10,600 and 11,900 14C yr BP, indicating the discharge event occurred during the Younger Dryas. Based on mineralogy of the ice-rafted debris, the icebergs appear to be sourced from the northwestern Alaskan margin, which places important constraints on the ice extent in northern Alaska during the LGM as well as existing circulation models for the region.

Changes in North Atlantic Deep Water strength and bottom water masses during Marine Isotope Stage 3 (45–35 ka BP)

The strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present high-resolution Nd isotope compositions of past seawater derived from authigenic Fe–Mn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481 m water depth). The data cover the period from 45 to 35 ka BP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, ∼40–39 ka BP). The Nd isotope record suggests that there was no northern-source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35 ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500 m at this location. The ɛ Nd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 ( ɛ Nd = −11.3) and most radiogenic immediately preceding IS 9 ( ɛ Nd = −9.8). More radiogenic compositions were also recorded during H4 (−10.2 ≤ ɛ Nd ≤ −9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3 ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South.

Cross‐cutting moraines reveal evidence for North Atlantic influence on glaciers in the tropical Andes

AbstractSurface exposure dating of boulders on an exceptionally well‐preserved sequence of moraines in the Peruvian Andes reveals the most detailed record of glaciation heretofore recognised in the region. The high degree of moraine preservation resulted from dramatic changes in the flow path of piedmont palaeoglaciers at the southern end of the Cordillera Blanca (10° 00′ S, 77° 16′ W), which, in turn, generated a series of cross‐cutting moraines. Sixty 10Be surface exposure ages indicate at least four episodes of palaeoglacier stabilisation (>65, ca. 65, ca. 32 and ca. 18–15 ka) and several minor advances or stillstands on the western side of the Nevado Jeulla Rajo massif. The absence of ages close to the global Last Glacial Maximum (ca. 21 ka) suggests that if an advance culminated at that time any resulting moraines were subsequently overridden. The timing of expanded ice cover in the central Peruvian Andes correlates broadly with the timing of massive iceberg discharge (Heinrich) events in the North Atlantic Ocean, suggesting a possible causal connection between southward migration of the Intertropical Convergence Zone during Heinrich events and a resultant increase in precipitation in the tropical Andes. Copyright © 2010 John Wiley & Sons, Ltd.

Rapid submarine melting of the calving faces of West Greenland glaciers

Widespread glacier acceleration has been observed in Greenland in the past few years. Oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three West Greenland glaciers, indicating that submarine melting has a profound influence on grounding-line stability. Widespread glacier acceleration has been observed in Greenland in the past few years1,2,3,4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5,6,7. These glaciers thin both at the surface, from warm air temperatures, and along their submerged faces in contact with warm ocean waters8. Little is known about the rates of submarine melting9,10,11 and how they may affect glacier dynamics. Here we present measurements of ocean currents, temperature and salinity near the calving fronts of the Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry and geographical positions. We calculate water-mass and heat budgets that reveal summer submarine melt rates ranging from 0.7±0.2 to 3.9±0.8 m d−1. These rates of submarine melting are two orders of magnitude larger than surface melt rates, but comparable to rates of iceberg discharge. We conclude that ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs, and suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics.

Extreme iceberg generation exposed

In the North Atlantic region, six massive iceberg discharge events marked the last glacial period. A numerical model now links these events to ocean temperatures and ice-shelf conditions.

Links between ocean temperature and iceberg discharge during Heinrich events

The last glacial period was punctuated by several periods of massive iceberg discharge from the Northern Hemisphere ice sheets. Numerical simulations indicate that these discharge events are linked to an interplay between ice-sheet accumulation, marine ice-shelf stability and periodically oscillating surface ocean temperature. Palaeoclimate records have revealed the presence of millennial-scale climate oscillations throughout the last glacial period1. Six periods of extreme cooling in the Northern Hemisphere—known as Heinrich events—were marked by an enhanced discharge of icebergs into the North Atlantic Ocean2,3, increasing the deposition of ice-rafted debris2. Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses4,5,6, but recent observations7,8 suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions. Here we use a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resultant changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf. We also find that the irregular occurrence of Heinrich events seen in the palaeoclimate records can be simulated by periodic ocean forcing combined with varying accumulation rates of the ice sheet. Our model simulations support a link between millennial-scale ocean temperature variability and Heinrich events during the last glacial period.

Evidence for iceberg armadas from East Antarctica in the Southern Ocean during the late Miocene and early Pliocene

Abstract Sediments from Ocean Drilling Program Site 1165 in the Indian Ocean sector of the Southern Ocean (off Prydz Bay) contain a series of layers that are rich in ice-rafted debris (IRD). Here we present evidence that IRD-rich layers at Site 1165 at 7, 4.8, and 3.5 Ma record short-lived, massive discharges of icebergs from Wilkes Land and Adelie Land, more than 1500 kilometers to the east of the depositional site. This distant source of icebergs is clearly defined by the presence of IRD hornblende grains with 40Ar/39Ar ages of 1200–1100 Ma and 1550–1500 Ma, ages that are not found on the East Antarctic continent in locations closer to Site 1165. This observation requires enormous amounts of detritus-carrying drifting icebergs, most likely in the form of large icebergs. These events probably reflect destabilization, surge, and break-up of ice streams on the Wilkes Land and Adelie Land margins of the East Antarctic Ice Sheet, in the vicinity of the low-lying Aurora and Wilkes Basins. They occurred under warming conditions, but each coast seems to have produced ice-rafting events independently, at different times. The data presented here constitute the first evidence of far-traveled icebergs from specific source areas around the East Antarctic perimeter. Launch of these icebergs may have happened during quite dramatic events, perhaps analogous to “Heinrich Events” in the North Atlantic.

The nature of millennial-scale climate variability during the past two glacial periods

Periodic iceberg discharges during the last glacial period led to a slowdown of the Atlantic meridional overturning circulation. Sediment records from the Portuguese margin show that similar events punctuated the penultimate glacial period as well, although their duration and broader climatic impacts were modified by different background climate conditions. During the last glacial period, iceberg discharges into the North Atlantic disrupted the meridional overturning circulation, leading to cooling in the Northern Hemisphere and warming in Antarctica1,2. This asymmetric response can be explained by a bipolar see-saw mechanism3,4,5, whereby changes in the strength of the meridional overturning circulation lead to changes in the interhemispheric heat transport. It is unclear, however, to what extent the response of the overturning circulation is a function of freshwater flux and boundary climate conditions4. Here we use foraminiferal isotope and pollen records from the Portuguese margin to reconstruct surface- and deep-water hydrography and atmospheric changes during the last and penultimate glacial periods. When we compare our records with temperature reconstructions from Antarctica6, we find that the bipolar see-saw was a characteristic feature of both glacial periods. However, the comparison also underlines the dependence of the bipolar see-saw on background climate and magnitude of iceberg discharge. It also suggests that an intensified hydrological cycle may lead to a weaker overturning circulation with a smaller disruption threshold and extended North Atlantic stadial durations.

Basal melting and freezing under the Amery Ice Shelf, East Antarctica

AbstractThe basal melting and freezing rates under the Amery Ice Shelf, East Antarctica, are evaluated, and their spatial distributions mapped. Ice velocity, surface elevation and accumulation rate datasets are employed in the analysis, along with a column-averaged ice density model. Our analysis shows that the total area of basal melting is 34 700 km2, with a total annual melt of 62.5 ± 9.3 Gt and an average melting rate of 1.8 ± 0.3 m a−1. Basal freezing mainly occurs in the northwestern part of the ice shelf, over a total area of 26 100 km2 and with a maximum freezing rate of 2.4 ± 0.4 m a−1. The total marine ice that accretes to the ice-shelf base is estimated to be 16.2 ± 2.4 Gt a−1. Using a redefined grounding line and geometry of the Amery Ice Shelf, we estimate the net melt over the ice-shelf base is about 46.4 ± 6.9 G ta−1, which is higher than previous modeling and oceanographic estimates. Net basal melting accounts for about half of the total ice-shelf mass loss, with the rest being from iceberg discharge. Our basal melting and freezing distribution map provides a scientific basis for quantitative analysis of ice–ocean interaction at the ice-shelf–ocean interface.

Deglacial diatom productivity and surface ocean properties over the Bermuda Rise, northeast Sargasso Sea

Diatom assemblages document surface hydrographic changes over the Bermuda Rise. Between 19.2 and 14.5 ka, subtropical diatom species and Chaetoceros resting spores dominate the flora, as in North Atlantic productive regions today. From 16.9 to 14.6 ka, brackish and fresh water diatoms are common and their contribution is generally coupled with total diatom abundance. This same interval also contains rare grains of ice‐rafted debris. Coupling between those proxies suggests that successive discharge of icebergs might have stimulated productivity during Heinrich event 1 (H1). Iceberg migration to the subtropics likely created an isolated environment involving turbulent mixing, upwelled water, and nutrient‐rich meltwater, supporting diatom productivity in an otherwise oligotrophic setting. In addition, the occurrence of mode water eddies likely brought silica‐rich waters of Southern Ocean origin to the euphotic zone. The persistence of lower‐salinity surface water beyond the last ice rafting suggests continued injection of fresh water by cold‐core rings and advection around the subtropical gyre. These results indicate that opal productivity may have biased estimates of meridional overturning based on 231Pa/230Th ratios in Bermuda Rise sediments during H1.

Late Quaternary ice sheet, lake and sea history of southwest Scandinavia – a synthesis

Based on a large number of new boreholes in northern Denmark, and on the existing data, a revised event‐stratigraphy is presented for southwestern Scandinavia. Five significant Late Saalian to Late Weichselian glacial events, each separated by periods of interglacial or interstadial marine or glaciolacustrine conditions, are identified in northern Denmark. The first glacial event is attributed to the Late Saalianc. 160–140 kyr BP, when the Warthe Ice Sheet advanced from easterly and southeasterly directions through the Baltic depression into Germany and Denmark. This Baltic ice extended as far as northern Denmark, where it probably merged with the Norwegian Channel Ice Stream (NCIS) and contributed to a large discharge of icebergs into the Norwegian Sea. Following the break up, marine conditions were established that persisted from the Late Saalian until the end of the Early Weichselian. The next glaciation occurredc. 65–60 kyr BP, when the Sundsøre ice advanced from the north into Denmark and the North Sea, where the Scandinavian and British Ice Sheets merged. During the subsequent deglaciation, large ice‐dammed lakes formed before the ice disintegrated in the Norwegian Channel, and marine conditions were re‐established. The following Ristinge advance from the Baltic, initiatedc. 55 kyr BP, also reached northern Denmark, where it probably merged with the NCIS. The deglaciation,c. 50 kyr BP, was followed by a long period of marine arctic conditions. Around 30 kyr BP, the Scandinavian Ice Sheet expanded from the north into the Norwegian Channel, where it dammed the Kattegat ice lake. Shortly after,c. 29 kyr BP, the Kattegat advance began, and once again the Scandinavian and British Ice Sheets merged in the North Sea. The subsequent retreat to the Norwegian Channel led to the formation of Ribjerg ice lake, which persisted from 27 to 23 kyr BP. The expansion of the last ice sheet startedc. 23 kyr BP, when the main advance occurred from north–northeasterly directions into Denmark. An ice‐dammed lake was formed during deglaciation, while the NCIS was still active. During a re‐advance and subsequent retreatc. 19 kyr BP, a number of tunnel‐valley systems were formed in association with ice‐marginal positions. The NCIS finally began to break up in the Norwegian Sea 18.8 kyr BP, and the YoungerYoldiaSea inundated northern Denmark around 18 kyr BP. The extensive amount of new and existing data applied to this synthesis has provided a better understanding of the timing and dynamics of the Scandinavian Ice Sheet (SIS) during the lastc. 160 kyr. Furthermore, our model contributes to the understanding of the timing of the occasional release of large quantities of meltwater from the southwestern part of the SIS that are likely to enter the North Atlantic and possibly affect the thermohaline circulation.

Sediment sources of northern Québec and Labrador glacial deposits and the northeastern sector of the Laurentide Ice Sheet during ice-rafting events of the last glacial cycle

Provenance studies of anomalously high-flux layers of ice-rafted detritus (IRD) in North Atlantic sediments of the last glacial cycle show evidence for massive iceberg discharges coming from the Hudson Strait region of the Laurentide Ice Sheet (LIS). Although these so-called Heinrich events (H events) are commonly thought to be associated with abrupt drawdown of the LIS interior, uncertainties remain regarding the sector(s) of this multi-domed ice sheet that conveyed ice through Hudson Strait. In Northern Québec and Labrador (NQL), large-scale patterns of glacial lineations indicate massive ice flows towards Ungava Bay and Hudson Strait that could reflect the participation of the Labrador–Québec ice dome in H events. Here we evaluate this hypothesis by constraining the source of NQL glacial deposits, which provide an estimate of the provenance characteristics of IRD originating from this sector. Specifically, we use 40Ar/ 39Ar ages of detrital hornblende grains in 25 till samples distributed along a latitudinal transect (lat. 58°) extending east and west of Ungava Bay. The data show that tills located west and southwest of the Ungava Bay region are largely dominated by hornblende grains with Archean ages (>2.6 Ga), while tills located east of Ungava Bay are characterized by grains with early Paleoproterozoic ages (2.0–1.8 Ga), although most samples contain a few Archean-age grains. IRD derived from the NQL region should thus be characterized by a large proportion of Archean-age detrital grains, which contrasts significantly with the predominant Paleoproterozoic 40Ar/ 39Ar ages (1.8–1.6 Ga) typically reported for the dominant age population of hornblende grains in H layers. Comparisons with IRD through the last glacial cycle from a western North Atlantic core off Newfoundland do not show evidence for any prominent ice-rafted event with the provenance characteristics of NQL glacial deposits, thereby suggesting that significant ice-calving event(s) from the Labrador–Québec sector may have been limited throughout that interval. Although these results tend to point towards a relative stability of this ice dome during H events, our study also indicates that further provenance work is required on IRD proximal to the Hudson Strait mouth in order to constrain with a greater confidence the sector(s) of the LIS that fed ice into Hudson Strait during H events. Alternatively, these results and other paleogeographic considerations tend to support models suggesting that part of the Ungava Bay glacial lineations could be associated with a Late-Glacial ice flow across Hudson Strait.

A role for icebergs in the 8.2 ka climate event

We investigate the potential role of icebergs in the 8.2 ka climate event, using a coupled climate model equipped with an iceberg component. First, we evaluate the effect of a large iceberg discharge originating from the decaying Laurentide ice sheet on ocean circulation, compared to a release of an identical volume of freshwater alone. Our results show that, on top of the freshwater effect, a large iceberg discharge facilitates sea-ice growth as a result of lower sea-surface temperatures induced by latent heat of melting. This causes an 8% increased sea-ice cover, 5% stronger reduction in North Atlantic Deep Water production and 1°C lower temperature in Greenland. Second, we use the model to investigate the effect of a hypothetical two-stage lake drainage, which is suggested by several investigators to have triggered the 8.2 ka climate event. To account for the final collapse of the ice-dam holding the Laurentide Lakes we accompany the secondary freshwater pulse in one scenario with a fast 5-year iceberg discharge and in a second scenario with a slow 100-year iceberg discharge. Our experiments show that a two-stage lake drainage accompanied by the collapsing ice-dam could explain the anomalies observed around the 8.2 ka climate event in various climate records. In addition, they advocate a potential role for icebergs in the 8.2 ka climate event and illustrate the importance of latent heat of melting in the simulation of climate events that involve icebergs. Our two-stage lake drainage experiments provide a framework in the discussion of two-stage lake drainage and ice sheet collapse.

The sensitivity of the climate response to the magnitude and location of freshwater forcing: last glacial maximum experiments

Proxy records indicate that the locations and magnitudes of freshwater forcing to the Atlantic Ocean basin as iceberg discharges into the high-latitude North Atlantic, Laurentide meltwater input to the Gulf of Mexico, or meltwater diversion to the North Atlantic via the St. Lawrence River and other eastern outlets may have influenced the North Atlantic thermohaline circulation and global climate. We have performed Last Glacial Maximum (LGM) simulations with the NCAR Community Climate System Model (CCSM3) in which the magnitude of the freshwater forcing has been varied from 0.1 to 1 Sv and inserted either into the subpolar North Atlantic Ocean or the Gulf of Mexico. In these glacial freshening experiments, the less dense freshwater provides a lid on the ocean water below, suppressing ocean convection and interaction with the atmosphere above and reducing the Atlantic Meridional Overturning Circulation (AMOC). This is the case whether the freshwater is added directly to the area of convection south of Greenland or transported there by the subtropical and subpolar gyres when added to the Gulf of Mexico. The AMOC reduction is less for the smaller freshwater forcings, but is not linear with the size of the freshwater perturbation. The recovery of the AMOC from a “slow” state is ∼200 years for the 0.1 Sv experiment and ∼500 years for the 1 Sv experiment. For glacial climates, with large Northern Hemisphere ice sheets and reduced greenhouse gases, the cold subpolar North Atlantic is primed to respond rapidly and dramatically to freshwater that is either directly dumped into this region or after being advected from the Gulf of Mexico. Greenland temperatures cool by 6–8 °C in all the experiments, with little sensitivity to the magnitude, location or duration of the freshwater forcing, but exhibiting large seasonality. Sea ice is important for explaining the responses. The Northern Hemisphere high latitudes are slow to recover. Antarctica and the Southern Ocean show a bipolar response, with warming and reduced sea ice. This warming continues after the cessation of the freshwater forcing and shows a dependence on the duration of the freshwater forcing. Equatorward of the expanded sea ice, the simulated temperature and salinity anomalies are sensitive to the amount of colder and fresher waters that are advected out of the subpolar North Atlantic. In the tropical Atlantic, the recovery of the Intertropical Convergence Zone (ITCZ) from its more southerly position during the freshwater forcing is much more rapid than the recovery of the AMOC, and is more related to the recovery of low-latitude surface temperatures than Greenland temperature or sea ice. These results have implications for using proxy records as indirect measures of the AMOC.

Position of the Polar Front along the western Iberian margin during key cold episodes of the last 45 ka

This paper documents the migration of the Polar Front (PF) over the Iberian margin during some of the cold climatic extremes of the last 45 ka. It is based on a compilation of robust and coherent paleohydrological proxies obtained from eleven cores distributed between 36 and 42°N. Planktonic δ18O (Globigerina bulloides), ice‐rafted detritus concentrations, and the relative abundance of the polar foraminifera Neogloboquadrina pachyderma sinistral were used to track the PF position. These three data sets, compared from core to core, show a consistent evolution of the sea surface paleohydrology along the Iberian margin over the last 45 ka. We focused on five time slices representative of cold periods under distinct paleoenvironmental forcings: the 8.2 ka event and the Younger Dryas (two recent cold events occurring within high values of summer insolation), Heinrich events 1 and 4 (reflecting major episodes of massive iceberg discharges into the North Atlantic), and the Last Glacial Maximum (typifying the highest ice volume accumulated in the Northern Hemisphere). For each event, we generated schematic maps mirroring past sea surface hydrological conditions. The maps revealed that the Polar Front presence along the Iberian margin was restricted to Heinrich events. The sea surface conditions during the Last Glacial Maximum were close to those at present day, except for the northern sites which briefly experienced subarctic conditions.