Ice-rafted Debris Events Research Articles

Impact of suborbital climate changes in the North Atlantic on ice sheet dynamics at the Mid‐Pleistocene Transition

Early and Mid‐Pleistocene climate, ocean hydrography and ice sheet dynamics have been reconstructed using a high‐resolution data set (planktonic and benthicδ18O time series, faunal‐based sea surface temperature (SST) reconstructions and ice‐rafted debris (IRD)) record from a high‐deposition‐rate sedimentary succession recovered at the Gardar Drift formation in the subpolar North Atlantic (Integrated Ocean Drilling Program Leg 306, Site U1314). Our sedimentary record spans from late in Marine Isotope Stage (MIS) 31 to MIS 19 (1069–779 ka). Different trends of the benthic and planktonic oxygen isotopes, SST and IRD records before and after MIS 25 (∼940 ka) evidence the large increase in Northern Hemisphere ice‐volume, linked to the cyclicity change from the 41‐kyr to the 100‐kyr that occurred during the Mid‐Pleistocene Transition (MPT). Beside longer glacial‐interglacial (G‐IG) variability, millennial‐scale fluctuations were a pervasive feature across our study. Negative excursions in the benthicδ18O time series observed at the times of IRD events may be related to glacio‐eustatic changes due to ice sheets retreats and/or to changes in deep hydrography. Time series analysis on surface water proxies (IRD, SST and planktonicδ18O) of the interval between MIS 31 to MIS 26 shows that the timing of these millennial‐scale climate changes are related to half‐precessional (10 kyr) components of the insolation forcing, which are interpreted as cross‐equatorial heat transport toward high latitudes during both equinox insolation maxima at the equator.

Aquatic ecosystem responses to Holocene climate change and biome development in boreal, central Asia

Boreal ecosystems are highly vulnerable to climate change, and severe ecological impacts in the near future are virtually certain to occur. We undertook a multiproxy study on an alpine lake (ESM-1) at the modern tree-line in boreal, southern Siberia. Steppe and tundra biomes were extensive in eastern Sayan landscapes during the early Holocene. Boreal forest quickly expanded by 9.1 ka BP, and dominated the landscape until c 0.7 ka BP, when the greatest period of compositional turnover occurred. At this time, alpine meadow landscape expanded and Picea obovata colonised new habitats along river valleys and lake shorelines, because of prevailing cool, moist conditions. During the early Holocene, chironomid assemblages were dominated by cold stenotherms. Diatoms for much of the Holocene were dominated by alkaliphilous, fragilarioid taxa, up until 0.2 ka BP, when epiphytic species expanded, indicative of increased habitat availability. C/N mass ratios ranged between 9.5 and 13.5 (11.1–15.8 C/N atomic ratios), indicative of algal communities dominating organic matter contributions to bottom sediments with small, persistent contributions from vascular plants. However, δ13C values increased steadily from −34.9‰ during the early Holocene (9.3 ka BP) to −24.8‰ by 0.6 ka BP. This large shift in magnitude may be due to a number of factors, including increasing within-lake productivity, increasing disequilibrium between the isotopic balance of the lake with the atmosphere as the lake became isotopically ‘mature’, and declining soil respiration linked to small, but distinct retreat in forest biomes. The influence of climatic variables on landscape vegetation was assessed using redundancy analysis (RDA), a linear, direct ordination technique. Changes in July insolation at 60 °N significantly explained over one-fifth of the variation in species composition, while changes in estimates of northern hemisphere temperature and ice-rafted debris events in the North Atlantic were also significant, but considerably less important. The potential importance of climate and biome development (tundra, steppe, cold deciduous forest and taiga) on different trophic levels (i.e. chironomid and diatom communities) in lake ESM-1 was also assessed using RDA. Climate predictors had a more significant influence on Holocene chironomid assemblages, especially July insolation at 60 °N, estimates of regional precipitation and estimates of northern hemisphere temperature, while only the development of the taiga biome had a significant impact on these primary consumers. Diatom communities also had a small, but significant influence on Holocene chironomid populations, perhaps linked to variation in faunal feeding strategies. In contrast, climatic and biome predictors explained similar amounts of variation in the Holocene diatom assemblage (approximately 20% each), while chironomids themselves as predictors explained just under 7% of diatom variation. Lake acidity was inferred using a diatom inference model. Results suggest that after deglaciation, the lake did not undergo a process of gradual acidification, most likely due to the presence of continuous permafrost and low levels of precipitation, preventing base cations and dissolved organic carbon entering the lake (except for the period between 1.7 and 0.7 ka BP). We conclude that lakes in continental, boreal regions undergo different models of lake ontogeny than oceanic boreal regions. Unlike other regions discussed, climate is a more important driver of ecosystem change than catchment changes. We also demonstrate that the start of the period coincident with the onset of the Little Ice Age resulted in important thresholds crossed in catchment vegetation and aquatic communities.

Evidence for climatic and oceanographic controls on terrigenous sediment supply to the Indian Ocean sector of the Southern Ocean over the past 63,000 years

Multiple proxy studies on sediment core SK 200/22a from the sub-Antarctic sector of the Indian Ocean revealed millennial-scale fluctuations in terrigenous input during the last 63,000 years. The marine isotope stages 1 (MIS 1) and MIS 3 are characterized by generally low concentrations of magnetic minerals, being dominated by fine-grained magnetite and titano-magnetite. Within the chronological constraints, periods of enhanced terrigenous input and calcite productivity over the last 63,000 years are nearly synchronous with the warming events recorded in Antarctic ice cores. An equatorward shift of the westerly wind system in association with a strengthening of the Antarctic Circumpolar Current (ACC) system may have promoted wind-induced shallow-water erosion around oceanic islands, leading to enhanced terrigenous input to the core site. Major ice-rafted debris events at 13–23, 25–30, 45–48 and 55–58 ka BP are asynchronous with δ18O and carbonate productivity records. This out-of-phase relation suggests that ice-sheet dynamics and ACC intensity were the primary factors influencing ice rafting and terrigenous input to the Indian sector of the Southern Ocean, with only limited support from sea-surface warming.

Temperature evolution from the δ 18O record of Hani peat, Northeast China, in the last 14000 years

From the last deglaciation to the Holocene, the Greenland Ice Core (GISP2) δ 18O records as well as the records of ice-rafted debris on the surface of the North Atlantic have revealed a succession of sudden cooling events on the centennial to millennial scales. However, the temperature proxy records are rarely studied systematically and directly to ensure that this air temperature cooling pattern simultaneously existed in the East Asian Region, in addition to the repeated pattern occurring in the Greater Atlantic Region. A peat cellulose δ 18O temperature proxy record proximately existing for 14000 years was picked up from the Hani peat in Jilin Province, China. It suggests by comparison that the sudden cooling events, such as the Older Dryas, Inter-Allerod, Younger Dryas, and nine ice-rafted debris events of the North Atlantic, are almost entirely reiterated in the temperature signals of Hani peat cellulose δ 18O. These cooling events show that the repeatedly occurring temperature cooling pattern not only appeared in the North Atlantic Region in the high latitudes, but also in the Northwest Pacific Region in the middle latitudes. The climate change events marking the start of the Holocene Epoch, the Holocene Megathermal, the “8.2 kyr” event, the “4.2 kyr” event, the Medieval Warm Period, and the Little Ice Age are further discussed. The sensitivity response of Hani peat cellulose δ 18O to the land surface temperature and the reason for the age accuracy of peat cellulose 14C are also discussed based on the characteristics of the peat bog environment.

8000 year marine record of climate variability and fjord dynamics from Southern Greenland

Holocene sediment records of glacio-marine environmental and climatic changes from Ikersuaq Fjord and Narsaq Sound in southern Greenland have been studied. The Ikersuaq Fjord record representing the last c. 500 years is dominated by turbidite sedimentation and very high sedimentations rates of up to 1 cm/yr. The adjacent Narsaq Sound site, on the other hand, represents a continuous and more slowly accumulating sedimentation record of the last c. 8000 years. The Narsaq record reveals changes in position and melting rates of glacier termini of the nearby Greenland Ice Sheet from the Holocene Thermal Optimum period terminating at about 5.0 ka to the Neoglaciation period during the late Holocene. From 8.0 ka to 4.8 ka, ice-rafted debris (IRD) influx progressively became reduced, supporting a landward retraction of glacier termini. Neoglaciation of the region appears to have been initiated at about 4.8 ka, with a culmination of IRD rain at about 4.6 ka, 3.6 ka, 2.2 ka, 1.0 ka, 0.7 ka, and 0.5 ka and onward. Some of these IRD events appear to be associated with enhanced melting and glacier instability during warming episodes. However, at the transition to the ‘Little Ice Age’ at about 1450 years AD, a marked increase in IRD influx took place. The bottom water of Narsaq Sound appears to have been dominated by modified Irminger Water (Atlantic) during most of the Holocene. Bottom current flow was quite sluggish before 3.2 ka, whereas the late Holocene period appears to have been characterised by three pronounced episodes of increased bottom current velocity and increased calcareous benthic foraminifera productivity. The long term climate pattern in the Narsaq Sound record follows climate changes consistent with patterns of East Greenland Current variability, which is partly related to dominant North Atlantic Oscillation forcing of wind and ice drift patterns.

Synchronous climate anomalies in the western North Pacific and North Atlantic regions during the last 14,000 years

A peat cellulose δ 18O record spanning around 14,000 years from the Hani peat mire in northeastern China reveals several abrupt temperature anomalies in the period from the last deglaciation through the Holocene. The timing of these anomalies coincides well with the notable cooling events recorded respectively using the GISP2 ice core and ice-rafted sediment of the North Atlantic Ocean, such as the Older Dryas, Inter-Allerød, Younger Dryas, and the nine ice-rafted debris events. The results demonstrate that this repeating pattern of abrupt temperature deterioration is not limited to the North Atlantic area at high latitude but also exists in the western North Pacific region at middle latitude. The synchronous temperature anomalies possibly are resulted from the joint effects of meltwater discharge into the North Atlantic Ocean and reduced solar activity. In the period from around 8600 to 8200 cal. yrs BP the Hani peat record shows a broad δ 18O peak that may reflect compound climate signals resulting from the two kinds of forcing factors: the temperature drop related to reduced solar activity at around 8600–8250 cal. yrs BP, and the temperature anomaly attributed to the meltwater effect at around 8220 ± 70 cal. yrs BP. This result may provide palaeo-temperature evidence for existence of the sharp “8.2 k” event in the western North Pacific region. In addition, our results have revealed that in the period from the last deglaciation through the Holocene the synchronous temperature anomalies before and after the “8.2 k” event seem to be related to meltwater outflow and reduced solar activity, respectively. It is important that the all temperature anomalies—whether because of reduced solar activity in the late Holocene or from meltwater discharge in the early Holocene—are accompanied by an abrupt decline in the Indian Ocean summer monsoon and abrupt strengthening of the East Asian summer monsoon. It is likely that reduced solar activity and meltwater outflow appear to modulate Earth system changes in the same direction. The influences could be compounded. Reduced solar activity and meltwater outburst both appear to act as triggers for occurrence of the El Niño phenomenon in the equatorial Pacific Ocean, which may result in broad teleconnections between the temperature anomaly in the Northern Hemisphere and abrupt variation of the Asian monsoon.

Vulnerability of the Cordilleran Ice Sheet to iceberg calving during late Quaternary rapid climate change events

We present the first high‐resolution record of iceberg calving on the continental slope of Vancouver Island, British Columbia (MD02‐2496, 48°58N, 127°02W, 1243 m water depth), through the last glacial from the Cordilleran Ice Sheet (CIS). These previously unknown ice‐rafted debris (IRD) events representing significant retreat of the western margin of the CIS out of marine waters show little correspondence with local climate change. High‐resolution radiocarbon dating indicates that the younger IRD events coincide with global radiocarbon age plateaus that allow direct correlation with distal climate records where the same plateaus have been identified. The coincidence of episodic shedding of IRD from the CIS with North Atlantic climate events (i.e., North Atlantic Deep Water shutdown, and Heinrich events) begs the question: What drove CIS destabilization? Forcing appears to be external to CIS dynamics perhaps resulting from either eustatic sea level rise or episodes of atmospheric warming over the North American continent.

The relationship of Heinrich events and their European precursors over the past 60 ka BP: a multi-proxy ice-rafted debris provenance study in the North East Atlantic

High resolution, multi-proxy records of ice-rafted debris (IRD) flux and provenance in the NE Atlantic detail the development, variability and decline of marine margins of the last glacial circum-North Atlantic ice sheets. Coupled lithological identification, Sr and Nd isotopic composition and 40Ar/ 39Ar ages of individual hornblende grains reduce ambiguity as to IRD potential source region, allowing clear differentiation between Laurentide (LIS), Icelandic and British (BIS) ice sheet sources (the Icelandic and BIS are collectively referred to as the NW European ice sheet, NWEIS). A step-wise increase in the flux of IRD to the core site at ∼26.5 ka BP documents BIS advance and glaciation of Ireland. Millennial-scale variability of the BIS at a ∼2 ka periodicity is inferred through clusters of pulsed IRD fluxes throughout the late glacial (26.5–10 ka BP). Combination of these European IRD events and the ∼7 ka periodicity of LIS instability is thought to account for quasi-synchronicity of the NWEIS and LIS IRD pulses at Heinrich event (H) 2 and H1, previously suggested to represent the possible involvement of the NWEIS in the initiation of H events. Furthermore, the lack of extensive NWEIS marine margin is inferred prior to H3 (31.5 ka BP), such that no ‘European precursor’ event is associated with either H5 or H4. This suggests that ‘precursor events’ were not directly implicated in the collapse of the LIS, and the persistent instabilities of the BIS that are clustered at a 2 ka periodicity are incompatible with the concept that both H events and their ‘precursors’ are independent responses to a common underlying trigger.

Inverse phase oscillations between the East Asian and Indian Ocean summer monsoons during the last 12 000 years and paleo-El Niño

The inverse phase variations between the East Asian and Indian Ocean summer monsoons on the interannual timescale result from the El Niño-Southern Oscillation activity in the tropical Pacific, which potentially provides a new way for studying paleo-ENSO. Here we reconstruct a 12 000-yr proxy record for the East Asian summer monsoon from δ 13C time series of the plant cellulose of the Hani peat bog in the northeastern China. The comparison of it with the peat proxy record of the Indian Ocean summer monsoon suggests that the El Niño conditions are coincident with strong East Asian summer monsoons, weak Indian summer monsoons, and drift ice events in the North Atlantic at both orbital and millennial time scales. The orbital-scale inverse phase relationships between the monsoons indicate the occurrence of a long-term ENSO-like pattern, confirming the sensitivity of the monsoon systems associated with ENSO to insolation forcing. The nine inverse phase variations on the millennial time scales may suggest the nine El Niño-like patterns superimposing the long-term ENSO-like pattern. The inverse phase variations between the monsoons also show close correspondence to the drift ice events at high northern latitudes. In every case when the abrupt ice-rafted debris events occurred in the North Atlantic, the inverse phase relationship established, and the El Niño-like pattern occurred in the tropical Pacific correspondingly. The discussions on the influence of ocean thermohaline circulation on these global linkages have been made.

Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene

There has been a number of investigations for the correlation between the Asia monsoon and the North Atlantic climate for the last glacial; however, little research has been done for the present interglacial, the Holocene. Here we present for the first time a high-resolution composite proxy record for the Indian Ocean summer monsoon spanning around 12 000 years based on the δ 13C time series of both a single plant species ( Carex mulieensis) remains cellulose and the total plant assemblage cellulose in the Hongyuan peat bog from the Tibet Plateau. The records show that the strength of the Indian Ocean summer monsoon had abrupt variations during the last 12 000 years. The weakest monsoon occurred in the Younger Dryas period. Following rapid strengthening from around 11 200 to 10 800 a BP the monsoon kept a generally strong level for around 5300 years. From around 5500 a BP onwards the monsoon strength tended to gradual decrease. In addition, there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales, which superimpose the general tendency of the monsoon variation. In every case when the ice-rafted debris events in the North Atlantic occurred, the summer monsoon strength decreased correspondingly. These evidences show that teleconnection between the Indian Ocean summer monsoon and the North Atlantic climate is present not only in the last glacial but also in the Holocene, which may be linked to abrupt reorganizations of the ocean thermohaline circulation, leading to redistribution of energy, changing temperature and moisture gradient over the southern subtropical Indian Ocean, and eventually controlling the variability of the Indian Ocean summer monsoon.

Reconstruction and palaeoclimatic interpretation of mid-Holocene vegetation and lake-level changes at Saint-Jorioz, Lake Annecy, French Pre-Alps

Pollen and sediment analyses were used to reconstruct vegetation and lake-level changes over the mid-Holocene period at Saint-Jorioz, Lake Annecy (northern French Pre-Alps). Episodes of forest clearings point to Neolithic cultural activities in the northern French Pre-Alps from c. 6500 cal. BP in agreement with other pollen and macrofossils records from eastern Switzerland and the Jura mountains. The lake-level record shows rises atc. 8300–8200, 6400, 5900 and after 5730 cal. BP. The rise at c. 8300–8200 cal. BP shows a tripartition. Lower water levels developed before 8300–8200 and 6665 cal. BP, at c. 6050 and 5730 cal. BP. Rises at c. 8300–8200, 6400, 5900 and after 5730 cal. BP can be related to the 8200-yr event and North Atlantic ice-rafted debris (IRD) events occurring at c. 6200, 5800 and 5500 cal. BP, respectively. Using a model based on pollen and lake-level data, the palaeoenvironmental changes reconstructed at Saint-Jorioz were translated into quantitative climate parameters. The results suggest that rises in lake level coincided with increasing annual precipitation, runoff and available moisture, and decreasing mean summer temperature and shortening of the growing season probably as a result of alternate southward/northward displacements of the Atlantic Westerly Jet. At Saint-Jorioz, the period around the 8200-yr event corresponded to a c. 2.5°C summer temperature cooling and a c. 130 mm P-E (precipitation-evaporation) increase in agreement with other European and marine palaeoclimate records; a c. 4°C summer-temperature cooling and a rise in annual precipitation by c. 175 mm are reconstructed at c. 5900 cal. BP and after 5730 cal. BP, i.e., close to the time of the North Atlantic IRD events dated at 5800 and 5500 cal. BP, and are to be tested by further investigations. A general trend toward temperature cooling and precipitation increase appears over the period documented by the sediment sequence of Saint-Jorioz possibly related to an orbitally induced reduction of summer insolation.

Late Quaternary ice-rafting events in the SW Pacific Ocean, off eastern New Zealand

New records of ice-rafted debris (IRD) in eight cores from Campbell Plateau reveal periodic incursions of icebergs over the last 200 kyr. Observations of modern iceberg trajectories and provenance studies show the icebergs originated mainly in Antarctica and traveled east and north to the plateau margins via the Antarctic Circumpolar Current. There, local currents and winds dispersed icebergs across the plateau and adjacent regions as far north as Chatham Rise at 43°S. The shallow rise crest, along-rise circulation and strong thermal gradient associated with the nearby Subtropical Front inhibited further northward transport. Although IRD concentrations are very low (≤26 grains/g), downcore profiles and a δ 18O chronology reveal distinct peaks during (i) the transition from marine isotope stages (MIS) 7 to 6, (ii) late interglacial MIS 5 and (iii) glacial MIS 2. In addition, smaller peaks occur in MIS 4 and 3. This generalised pattern is also evident off Antarctica, indicating that periodic destabilisation of ice shelves was the main driving force behind the Campbell Plateau IRD events. However, there are significant differences between Antarctic and New Zealand IRD records that probably reflect palaeoceanographic influences on iceberg dispersal. The MIS 5 IRD event is better represented off Antarctica, confirming that last interglacial changes in sea level and temperatures encouraged iceberg discharge and proximal melting. Conversely, the MIS 2 event is relatively better shown on Campbell Plateau, implying that last glacial temperatures inhibited proximal iceberg melting and, together with a more vigorous oceanic circulation, allowed more icebergs to reach the distant plateau. A tentative correlation with records from the Southeast Atlantic and Southwest Indian oceans suggests that the IRD events were felt over much of the Southern Ocean for at least the last 70 kyr.

Millennial and sub-millennial-scale variability in sediment colour from the Barra Fan, NW Scotland: implications for British ice sheet dynamics

Abstract Sediment colour, together with other proxy data, provides a novel, rapid and non-destructive tool in the investigation of glacier-influenced sedimentation on the Barra Fan, NW Scotland. Lightness ( L *) and reflectance (400–700 nm) measurements at this site provide a quantitative estimate of changes in calcium carbonate and clay content. Interstadials are carbonate-rich/clay-poor (higher L * and reflectivity), whereas stadials are carbonate-poor/clay-rich (lower L * and reflectivity). Detailed sedimentological investigations suggest that the last British Ice Sheet (BIS) extended to the outer continental shelfbreak shortly after 30 ka bp . This climatic response of the BIS to global cooling at the Marine Isotope Stage (MIS) 3 – 2 transition marks a significant increase in sediment delivery to the Barra Fan. Prior to 30 ka bp , strong Dansgaard/Oeschger (D/O) cyclicity dominates the record. After 30 ka bp , shorter periodicities prevailed as the BIS reached its maximum extent. Glacier dynamics plays a significant role in the delivery of ice-rafted debris (IRD) across this margin, highlighting the inherent difficulties of correlating millennial-scale IRD events when the IRD is derived from different ice sheets. An event stratigraphy based upon carbonate-rich interstadials provides a more robust means of amphi-Atlantic correlation during this interval.

Marine ice-rafted debris records constrain maximum extent of Saalian and Weichselian ice-sheets along the northern Eurasian margin

Ice-rafted debris (IRD) (>2 mm), input in eight sediment cores along the Eurasian continental margin (Arctic Ocean), have been studied over the last two glacial/interglacial cycles. Together with the revised chronologies and new micropaleontological data of two cores from the northern Barents Sea (PS2138) and northeastern Kara Sea (PS2741) spanning Marine Isotope Stages (MIS) 6 to 1, the IRD data give new insights into the glacial history of northern Eurasian ice-sheets over the last 150 ka. The chronologies of the cores are based on stable isotope records, AMS 14C datings, paleomagnetic and biostratigraphic data. Extensive episodes of northern Barents Sea ice-sheet growth, probably to the shelf edge, occurred during the late Weichselian (MIS 2) and the Saalian (MIS 6). Major IRD discharge at the MIS 4/3-transition hints to another severe glaciation, probably onto the outer shelf, during MIS 4. IRD-based instabilities of the marine-based ice margin along the northern Barents Sea between MIS 4 and 2 are similar in timing with North Atlantic Heinrich events and Nordic Seas IRD events, suggesting similar atmospheric cooling over a broad region or linkage of ice-sheet fluctuations through small sea-level events. In the relatively low-precipitation areas of eastern Eurasia, IRD peak values during Termination II and MIS 4/3-transition suggest a Kara Sea ice-sheet advance onto the outer shelf, probably to the shelf edge, during glacial MIS 6 and 4. This suggests that during the initial cooling following the interglacials MIS 5, and possibly MIS 7, the combined effect of sustained inflow of Atlantic water into the Arctic Ocean and penetration of moisture-bearing cyclones into easterly direction supported major ice build-up during Saalian (MIS 6) and Mid-Weichselian (MIS 4) glaciation. IRD peak values in MIS 5 indicate at least two advances of the Severnaya Semlya ice-sheet to the coast line during the Early Weichselian. In contrast, a distinct Kara Sea ice advance during the Late Weichselian (MIS 2) is not documented by the IRD records along the northeastern Kara Sea margin.

Millennial‐scale climate change and oceanic processes in the Late Pliocene and Early Pleistocene

We generated 200–500 year resolution records of oceanic processes in the North Atlantic (Ocean Drilling Program Site 983, 60°24′N, 23°38′W, 1983 meters water depth) for intervals in the latest Pliocene (1.86–1.93 Ma) and the earliest Pleistocene (1.75–1.83 Ma) in order to examine the linkages between millennial‐scale variations in the ocean and background glacial‐interglacial climate change. Within glacial intervals we find evidence for variations similar to those observed in the late Pleistocene. We find discrete ice‐rafted debris (IRD) events that reoccur every 2–5 kyr. These events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters. The timing of IRD events in the late Pliocene and early Pleistocene intervals is similar to that of Dansgaard‐Oeschger cycles, but we find no IRD events comparable in timing to late Pleistocene Heinrich events. Although interglacial intervals are much more stable, we do find evidence for low‐amplitude variations in deep water properties that reoccur every ∼2 kyr within interglacial intervals. The similarity between our late Pliocene—early Pleistocene records and late Pleistocene records implies that the mechanism driving millennial‐scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late Pleistocene.

Centurial‐millenial ice‐rafted debris pulses from ablating marine ice sheets

We use an ice‐sheet model to show that (i) margins of marine ice‐sheets can be expected to be frozen to the bed, except where ice‐streams discharge; (ii) 20–50km retreats induced by ablation rates of 2 m/yr provide sufficient debris flux through the grounding line to produce large sedimentation events. Such ablation would reduce ice‐shelf extent markedly, permitting debris to reach the calving front and be transported by icebergs leading to ice‐rafted debris (IRD) events. Ice shelf break‐up takes around a century (start of IRD pulse), while the creation of warm‐based conditions (end of IRD pulse) due to upwards motion of warm ice takes a few more centuries. Such IRD pulses are unlikely to explain Heinrich events, which are associated with relatively cold periods within glaciations. Surges are not necessary conditions for the production of large IRD events.

Is ocean thermohaline circulation linked to abrupt stadial/interstadial transitions?

Are the abrupt stadial/interstadial (S/IS) climate transitions observed in the Greenland ice cores also seen in marine climate records? Literature which shows that the S/IS events have a large footprint in ocean surface marine properties encompassing the entire Northern Hemisphere has been reviewed. Whether this influence extends to the deep circulation is more equivocal on most recent evidence. Several of the “Heinrich” ice rafting debris events are clearly associated with elimination of North Atlantic source waters from the deep Atlantic. But most cores studied do not resolve the other S/IS events, except for one new record from the Bermuda Rise which shows all of the major S/IS cycles in benthic foraminiferal. Deep-sea corals can tell us how rapid these events are in the deep sea.

Environmental changes since 13,000 cal. BP reflected in magnetic and sedimentological properties of sediments from Lake Holzmaar (Germany)

Late- and Postglacial sediments from a German maar lake (Holzmaar) in the West Eifel Volcanic Field were investigated with rockmagnetic and sedimentological methods. The concentration of magnetic minerals is very variable and zones exist, where magnetic properties are dominated by paramagnetic minerals. Remanence carrying (ferromagnetic) minerals are dominated by a low coercivity fraction, identified by its Curie temperature and by scanning electron microscopy as titanomagnetite with a grain size range between a few microns and ≈25 μm. An additional high coercivity phase, probably hematite, is present throughout the entire record in low concentrations. Rockmagnetic results indicate the possible postdepositional dissolution of (titano-) magnetite and the authigenic growth of greigite in one confined section. A positive correlation between initial magnetic susceptibility ( κ) and sediment accumulation rate and between κ and the non-arboreal pollen percentage is observed while κ and organic carbon content show a negative correlation. These coeval variations are due to a nearly exclusively detrital sedimentary input. The variations show a broad dependence on climatic conditions for times prior to significant human activities in the region and the impact of the human settlement history afterwards. Early Holocene climatic variations, indicating two cooling events, agree with ice rafted debris events of the North Atlantic.

Abrupt climatic changes induced by ice-rafting events in the eastern North Atlantic during the late Quaternary: stable isotope and X-ray mineralogical analyses

Stable oxygen- and carbon-isotope and X-ray mineralogical analyses are used to obtain information on ice-rafting (Heinrich) events in the eastern North Atlantic during the last glacial period. A sediment core collected from the continental slope off Portugal reveals that the X-ray mineralogy of some ice-rafted debris (IRD) layers is similar to that of the Heinrich layers H1, H2, and H4 in the North Atlantic, although the quantities of various detrital fragments are relatively low. The IRD layers are marked by a maximum abundance of planktonic foraminiferaNeogloboquadrina pachyderma (sin.), minima or lowered values of planktonic δ18O (Globigerina bulloides), and negative δ13C anomalies of benthic foraminiferaCibicidoides wuellerstorfi. Significant decreases in seasurface temperature and salinity are also characteristic. These data suggest that the icebergs passed along the western Iberian margin were of the same origin as those which led to the formation of Heinrich layers in the North Atlantic. In particular, a decrease in benthic δ13C of the IRD events implies a decreased ventilation of the North Atlantic that may have enhanced the advection of a12C-enriched intermediate water from the south. The absence of an IRD layer corresponding to the Heinrich layer H3 in the studied cores suggests that the reduction of convection in the North Atlantic prevented the iceberg drifting to the study area off Portugal during the period H3.