North Atlantic Ice-rafted Debris Research Articles

Late Pleistocene 100-kyr glacial cycles paced by precession forcing of summer insolation

Climate variability over the past 800,000 years has long been described as being dominated by ~100-kyr glacial cycles, and researchers have debated whether these glacial cycles were driven by Earth’s orbital cycles of eccentricity, obliquity and precession. Some recent studies have suggested that these ~100-kyr glacial cycles are best characterized as groupings of two or three 41-kyr obliquity cycles; however, age uncertainties have made it difficult to distinguish whether the dramatic changes in ice-sheet size were more associated with 41-kyr obliquity or ~23-kyr precession cycles. We compare the impacts of obliquity and precession on glacial cycles using improved age estimates to analyse orbital phases during the onset of glacial terminations. Terminations are dated using a 640-kyr multiproxy stack of eight North Atlantic benthic δ18O records with well-constrained probabilistic age estimates derived from correlating North Atlantic ice-rafted debris to instances of abrupt Asian monsoon variability in high-resolution 230Th-dated speleothems. Rayleigh’s R statistics for the precession and obliquity phases of terminations demonstrate that, although both have statistically significant effects, the precession phase is more predictive of termination onset, particularly for the largest events. Thus, we conclude that Late Pleistocene ice sheets were sensitive to the precession forcing of Northern Hemisphere summer insolation intensity.

Atlantic ITCZ variability during the Holocene based on high-resolution speleothem isotope records from northern Venezuela

Few high-resolution paleoclimate proxy records exist in the region located under the direct influence of the Intertropical Convergence Zone (ITCZ) in South America and most of them were retrieved from the Cariaco Basin off the coast of Venezuela. Here we present new ẟ18O and ẟ13C records of Venezuelan speleothems collected in caves adjacent to Cariaco, covering the mid- and late-Holocene. We document previously undetected secular-to multidecadal-scale climate variability in the core region of the ITCZ, which is compared to other high-resolution records from the North Atlantic, Caribbean and tropical South America. Over the mid-Holocene our record exhibits broad swings between periods of reduced (8.3–8.0, 6.5–5.0, 4.1–3.6 ka BP) and increased (8.5, 8.0–6.5, 4.9–4.2 ka BP) rainfall. In particular, between 5.5 and 5.0 ka BP, increases in polar and subpolar North Atlantic ice rafted debris and a reduction in North Atlantic Deep Water (NADW) formation might have contributed to the southward displacement of the North Atlantic Subtropical High (NASH) and the ITCZ, which led to severe dry conditions in north central Venezuela and an enhancement of the South American Monsoon System (SAMS). During the late-Holocene, contrary to data from Cariaco reported in previous studies, our results point to drier conditions during the Medieval Climate Anomaly (MCA, 900–1100 CE), which were further amplified during positive Atlantic Multidecadal Oscillation (AMO) phases. Wet conditions, however, prevailed during the first part of the Little Ice Age (LIA, 1400–1500 CE). No speleothem deposition occurred during the main LIA period, which might be related to drier conditions in response to a southward displacement of the ITCZ that led to major moisture convergence over the SASM domain. Our new records from Venezuela provide a reliable proxy for ITCZ behavior over the Atlantic – South America domain and document past dynamics in relation to other climate systems (NASH and SAMS), while providing new evidence of ITCZ – North Atlantic teleconnections during the Holocene.

Holocene changes in biomass burning in the boreal Northern Hemisphere, reconstructed from anhydrosugar fluxes in an Arctic sediment profile

The rapid warming of Arctic is causing increased fire activities in the boreal Northern Hemisphere (NH), leading to unprecedent changes in the global carbon cycling, human health and ecosystems. Understanding the interaction between fire and climate in this far north region is crucial for predicting future changes of wildfires. However, fire records over geological time scales are still scarce in the high latitudes of NH to provide comprehensive pictures of the fire history in this region. Here, we used the flux of levoglucosan (Lev) and its isomers in a sediment profile YN from Svalbard, high Arctic, as proxies for the changes in biomass burning from ∼9–2 kyr BP (thousand years before present). Backward trajectories and comparison with charcoal syntheses from various regions confirmed that the Lev transport to the profile site is sourced from the fire activities in the boreal NH, especially in northern Europe and northern Siberia. The Lev flux exhibited a slight overall decreasing trend at ∼3 %/kyr (p = 0.09) over the study period, as well as centennial maxima at ∼9, 8–7, 6, 5, and 4–3 kyr BP (p = 0.06). On sub-orbital scales, the long-term decrease in fire activities corresponded to trends of summer temperature in the extratropics of the NH (p = 0.01, r = 0.42), reflecting their regulation of fuel availability and flammability. On centennial to sub-millennial time scales, high levels of biomass burning were associated with periods of increased North Atlantic ice-rafted debris (p = 0.02, r = 0.38), which were indicative of cold and dry conditions over most of the source regions, reflecting the impacts of dryness on fuel flammability. The results suggested that enhanced Arctic amplification on centennial time scales may reduce biomass burning in most of the boreal NH, although fires in some mid-latitude regions may be facilitated.

Quantification of temperature and precipitation changes in northern China during the “5000-year” Chinese History

High-quality paleoclimate reconstructions can provide crucial climate context to test the hypothesis of climatic impact on historical culture changes. Here we report high-resolution and quantitative temperature and precipitation records covering the entire “5000-year” Chinese history in northern China. Our temperature record shows a slight decrease before 1800 cal yr BP and a ∼4 °C rapid cooling afterwards, superimposed with four major ∼2–3 °C centennial-scale cold events, potentially corresponding to the widespread North Atlantic ice-rafted debris Bond events. While precipitation record shows high value before 3000 cal yr BP, and a gradual decrease of ∼250 mm with two distinct ∼100 mm centennial-scale dry intervals after 1100 cal yr BP. Our data not only provide a more complete climate background for Chinese dynasties, but show the coincidence in the timing between abrupt cold and/or dry events and large-scale social unrests and southern migration of nomads. This finding reveals climate fluctuations, in particular variations in temperature, played an important role in affecting Chinese historical cultural changes.

East Asian monsoon changes early in the last deglaciation and insights into the interpretation of oxygen isotope changes in the Chinese stalagmite record

Stalagmite oxygen isotope (δ18O) records have enhanced our understanding of the history of the East Asian monsoon. However, abrupt changes in the monsoon are not constrained well enough to address certain issues and there are still unknowns in the interpretation of cave δ18O records. Here we present a new high-resolution stalagmite record from Shima Cave, central China. Anchored with 24 230Th/U dates, our sample grew from 19.7 to 17.8 ka and from 16.3 to 13.3 ka, covering much of the early portion of the last deglaciation with a temporal resolution of 7 years. Using this record and other available Asian δ18O records, we test model predictions for shifts in δ18O in Asian caves and further investigate monsoon variations on centennial to decadal timescales during the early portion of the last termination. We conclude the following regarding the interpretation of δ18O in Chinese caves. Two mechanisms affect δ18O: changes in the fraction of monsoon rainfall in annual totals (the Wang-Cheng mechanism) and changes in the amount of rainout between tropical sources and cave sites (the Yuan mechanism). The former is caused by changes in the seasonal migration of the sub-tropical jet and likely has a smaller effect on cave δ18O than the latter. The latter involves changes in rainout from both the Pacific and the Indian Ocean sources. Precisely how that change in rainout is partitioned between sources and cave sites is not fully understood; however, it is clear that some of the change takes place in China; i.e. it is not restricted to upstream sites. Rough calculations for our site suggest that mean annual rainfall may have been two-thirds of modern values at the time of the highest δ18O values during Heinrich Stadial 1 (HS1) (16.1 ka) and one fifth again higher than modern values during the Bølling (14.3 ka). Within HS1, consistent with observations from other localities, Chinese cave records exhibit a twofold structure. Within the phases, we observe a truly remarkable relationship between the North Atlantic ice rafted debris record and the Hulu-Shima Cave record, where events ranging from multi-centennial to sub-decadal timescale can be related across Eurasia. The origin of this two-phased structure may relate to different sources and extent of ice-rafted debris in the North Atlantic as well as the extent of sea ice and the Atlantic Meridional Overturning Circulation.

Western Mediterranean hydro-climatic consequences of Holocene ice-rafted debris (Bond) events

Abstract. Gerard C. Bond established a Holocene series of North Atlantic ice-rafted debris events based on quartz and haematite-stained grains recovered from subpolar North Atlantic marine cores. These so-called “Bond events” document nine large-scale and multi-centennial North Atlantic cooling phases that might be linked to a reduced thermohaline circulation. Regardless of the high prominence of the Holocene North Atlantic ice-rafted debris record, there are critical scientific comments on the study: the Holocene Bond curve has not yet been replicated in other marine archives of the North Atlantic and there exist only very few palaeoclimatic studies that indicate all individual Bond events in their own record. Therefore, evidence of consistent hydro-climatic teleconnections between the subpolar North Atlantic and distant regions is not clear. In this context, the Western Mediterranean region presents key hydro-climatic sites for the reconstruction of a teleconnection with the subpolar North Atlantic. In particular, variability in Western Mediterranean winter precipitation might be the result of atmosphere–ocean coupled processes in the outer-tropical North Atlantic realm. Based on an improved Holocene δ18O record from Lake Sidi Ali (Middle Atlas, Morocco), we correlate Western Mediterranean precipitation anomalies with North Atlantic Bond events to identify a probable teleconnection between Western Mediterranean winter rains and subpolar North Atlantic cooling phases. Our data show a noticeable similarity between Western Mediterranean winter rain minima and Bond events during the Early Holocene and an opposite pattern during the Late Holocene. There is evidence of an enduring hydro-climatic change in the overall Atlantic atmosphere–ocean system and the response to external forcing during the Middle Holocene. Regarding a potential climatic anomaly around 4.2 ka (Bond event 3) in the Western Mediterranean, a centennial-scale winter rain maximum is generally in-phase with the overall pattern of alternating “wet and cool” and “dry and warm” intervals during the last 5000 years.

Early- to mid-Holocene paleohydrology in northeast Mediterranean: The detrital record of Aliakmon River in Loudias Lake, Greece

One of the prominent features of northeast Mediterranean (NEM) Holocene climate are recurrent phases of cold and aridity; their impacts on the hydrological cycle remain at large unknown, as few existing paleohydrological records are either restricted to lake-level fluctuations or focus near the ‘8.2 kyr BP’ event. Here, we present the detrital record of Aliakmon River in Lake Loudias between 9500 and 3000 cal. BP. Magnetic susceptibility (MS) exhibits high correlation with mean grain size ( r = 0.7) of silt-sized fractions and is used as a proxy of the distal clastic input of Aliakmon River, whereas organic matter (OM), carbonate content (CaCO3), water content (WC), and clay concentrations decipher sedimentological and biological processes in Lake Loudias. Periods of high hydrological activity were interrupted by short intervals of low river discharge at c. 9400, 8600, 8200, 7500, 7000, 6200, 5300, and 4500 cal. BP and during a multi-century event centered at 3500 cal. BP, in agreement with marine and terrestrial paleoclimatic reconstructions from NEM. With exception of the wet period between c. 8.6 and 7.9 cal. BP, periods of increased hydrological activity are synchronous to contraction of Pinus forests and increased sea surface temperatures and silt transport in the Aegean Sea. The long-term (~580 years) variability of MS is in-phase with southeast Europe pollen-inferred annual temperature variations and with North Atlantic Ice Rafted Debris (IRD) events.

Coherency of late Holocene European speleothem δ18O records linked to North Atlantic Ocean circulation

Speleothem δ18O records provide valuable information about past continental environmental and climatic conditions, although their interpretation is often not straightforward. Here we evaluate a compilation of late Holocene speleothem δ18O records using a Monte Carlo based Principal Component Analysis (MC-PCA) method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each δ18O record. The MC-PCA approach permits not only the identification of temporally coherent changes in speleothem δ18O; it also facilitates their graphical depiction and evaluation of their spatial coherency. The MC-PCA method was applied to 11 Holocene speleothem δ18O records that span most of the European continent (apart from the circum-Mediterranean region). We observe a common (shared) mode of speleothem δ18O variability that suggests millennial-scale coherency and cyclicity during the last 4.5 ka. These changes are likely caused by variability in atmospheric circulation akin to that associated with the North Atlantic Oscillation, reflecting meridionally shifted westerlies. We argue that these common large-scale variations in European speleothem δ18O records are in phase with changes in the North Atlantic Ocean circulation indicated by the vigour of the Iceland Scotland Overflow Water (ISOW), the strength of the subpolar gyre (SPG) and an ocean stacked North Atlantic ice rafted debris (IRD) index. Based on a recent modelling study, we conclude that these changes in the North Atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies. However, the mechanisms that ultimately force the westerlies remain unclear.

Climate signatures through Marine Isotope Stage 19 in the Montalbano Jonico section (Southern Italy): A land–sea perspective

A multi-proxy record based on calcareous plankton, dinocysts, pollen, mineralogy and grain size has been acquired from the “Ideale” section, the portion of Montalbano Jonico succession (Southern Italy) that could host the GSSP for the Middle Pleistocene. The direct correlation between marine and terrestrial proxies provides a valuable indication of atmospheric and oceanic connections. The record shows a distinct climate variability well expressed by the vegetational changes, sediment input into the basin and sea surface–water modifications. Biotic proxies indicate that, during the study interval, periods of prevailing subpolar/transitional surface–water conditions were concurrent with steppe and halophytic vegetation. Erosional processes on the hillslopes and terrigenous supply, during these periods, were likely favored due to reduced arboreal vegetation, as attested to by sediment composition and pollen assemblages. Temperate/subtropical surface water conditions were concomitant with prevalent expansions of thermophilous arboreal plants and moister conditions on land, which likely promoted chemical weathering leading to an enhanced supply of kaolinite and smectite in the basin. A relationship between mineral content and the nature of the outcropping units in the source area, as well as sea-level fluctuations, has also been considered. A well-defined period of polar–subpolar water incursion, ranging from 793.2ka to 788.1ka, matches the major expansion of steppic vegetation on land and the increase of terrigenous supply into the basin. It likely represents the local response to North Atlantic ice-rafted debris deposition. Short-lived warming/cooling episodes punctuate the latest part of MIS 20 and are tentatively correlated with analogue climate phases predating MIS 1 inception. Full interglacial conditions of MIS 19c, lasting 10.6kyr, started at 784.3ka and were characterized by warm, oligotrophic and stratified surface–waters coupled with forest expansion, suggesting prevalent warm and humid conditions on land. A potential sapropel-like interval is identified within MIS 19c and correlated to i-cycle 74. The MIS 19c/19b transition is marked, at 773.2ka, by the reestablishment of millennial-scale variability, similarly to North Atlantic marine and ice-core records. The results improve the paleoclimate framework of a Middle Pleistocene reference section and add further insight regarding climate development across MIS 19 and on potential similarities with its closest Holocene analogue.

Drought modulated by North Atlantic sea surface temperatures for the last 3,000 years along the northwestern Gulf of Mexico

In 2012 the most severe United States drought since the 1930's occurred, highlighting the need for a better understanding of the climate factors driving droughts. Spatial-temporal analysis of United States precipitation data from 1900 to 1999 indicates that the Atlantic Multidecadal Oscillation (AMO) primarily modulates drought frequency. Tree rings provide the highest resolution and most spatially distributed drought records beyond secular time-series; however, as most tree-ring records only extend to ca. 1000 years before present (BP), a new broadly distributed, quantitative, multi-millennial, proxy of past drought is needed to determine if the AMO modulated drought across North America through the late Holocene. In this study, we develop a new quantitative drought proxy from a transfer function between X-Ray Fluorescence (XRF) derived elemental data from a Texas playa and a tree-ring drought record. Using this transfer function, a 954-year tree-ring drought record was extended to ca. 3000 BP. Changes in the extended drought record correspond with timing of the Roman Climate Optimum, Medieval Warm Period, Little Ice Age, and changes in the AMO as recorded in a proxy record derived from North Atlantic ice-rafted debris. These results indicate that lacustrine-derived XRF element data can be used as a quantitative tool to reconstruct past drought records, and suggest that AMO modulated drought in southern Texas for the last 3000 years. Additional studies using XRF-derived element data as a drought proxy are needed to determine the utility of this proxy in non-playa lacustrine systems.

The Upper Pleistocene deglaciation on the Apennines (Peninsular Italy)

The synthesis of the published data and new researches allow to establish a more reliable chronology of the phases of deglaciation that occurred after the Apennine last glacial maximum. The chronological framework was assured by radiocarbon dating and by the presence of four geochemically and chronologically characterized tephra layers and a quartz-rich loess, that were found on different mountain massifs, mainly in Central Italy. After the Campo Imperatore Stade (local LGM) dated 28-27 ka cal BP, a first retreat took place during the Campo Felice Arid Spell, followed by a glaciers advance (Le Capannelle Stade) that started between 26 and 25 ka cal BP. During the readvance the glaciers reached a length about 90% of the local LGM glacier. The readvance was followed by a retreat broken by at least two readvances (Piano Pietranzoni and Lago Pietranzoni readvances) and then by a very fast withdrawal at the beginning of the Fornaca Interstade, dated ca. 22-21 ka cal BP. The Fontari Stade glacier’s advance began ca. 18 ka cal BP and was followed by a retreat broken by at least three small readvances (Fontari 2; Fontari 3; M. Aquila 1). The M. Aquila 1 readvance ended ca. 15-14 ka cal BP. The Venaquaro Interstade, dated 14-13 ka cal BP, was followed by the M. Aquila Stade. This stade was the last Late Pleistocene small glacial expansion and can be correlated to the Younger Dryas. The variations in the atmospheric circulation in the Mediterranean area likely played a role during the first deglaciation phases (between 27-26 and 21-22 ka cal BP). During the period between 19-18 and 12 ka cal BP the glacial fluctuations were linked, very likely, to the climatic impact of the North Atlantic ice rafted debris events. It is possible, therefore, that during the Fornaca Interstade (between 22-21 and 19-18 ka cal BP), there has been such a change in atmospheric circulation and the Central Mediterranean area became more prone to be influenced by the effects of the North Atlantic D/O and IRD events.

Vegetation and climate across the Early–Middle Pleistocene transition at Montalbano Jonico, southern Italy

Pollen analysis of the Montalbano Jonico marine succession (MJS), southern Italy, provides a continuous vegetational record between 858 ka and 745.13 ka which includes Marine Isotope Stage (MIS) 19, now considered the closest orbital analogue to the Holocene. A comparison between paleoclimate proxies and global signals of past climate change allows the pollen record to be interpreted in terms of major environmental and climate modifications through MIS 21–18 at the orbital- and sub-orbital-scale. This is also central to understanding migration patterns of the genus Homo. Interglacials are expressed by the dominance of a mesophilic vegetation typical of a (warm) temperate and relatively humid climate. Deciduous Quercus dominated fully temperate arboreal forest already devoid of subtropical taxa. Wooded steppes to steppes expanded during glacials when cold and dry conditions prevailed. At MJS the main expansion of temperate forest, dominated by broad-leaved trees, correlates to MIS 19.3, whereas the expansion of steppe vegetation, during drier conditions, marks MIS 19.2 directly above the volcaniclastic layer V4, dated at 773.9 ± 1.3 ka. More intensively arid conditions developed during MIS 20 and MIS 18; the Pollen Temperature Index reaches its minimum in uppermost MIS 20 in agreement with a significant cold and arid climate phase of wide significance in the North Hemisphere, as documented by prominent peaks in North Atlantic ice rafted debris (IRD) and Mediterranean aeolian dust records. On the other hand, Artemisia, a significant component of steppes, shows its maximum expansion during earlier phases of MIS 18. Millennial to sub-millenial-scale climate variability is evidenced by two abrupt short-term pollen events within MIS 19.3, which show a dominant increase of the cosmopolitan herbaceous component at around 783.54 ka and 774.84 ka, respectively. The vegetational and inferred climate changes for MIS 19 offer two possibilities: (1) to investigate the interglacial climate variability and discuss the linkage between climate and changes in geomagnetic field intensity close to the Matuyama–Brunhes paleomagnetic boundary (MBB), and (2) to help evaluate the role of the MBB in defining the Middle Pleistocene global boundary stratotype section and point (GSSP). With respect to point 2, a comparison of data at MJS with the pollen record at the nearby and coeval Valle di Manche section is relevant and evidences the complex response of vegetation to climate events possibly under the effects of both astronomical and local factors.

Paleoenvironmental and climatostratigraphic insights for Marine Isotope Stage 19 (Pleistocene) at the Montalbano Jonico succession, South Italy

Quantitative analyses on pollen and ostracods were performed on the Montalbano Jonico succession (MJS) through Marine Isotope Stages (MIS) 18–21 in order to acquire details of paleoenvironmental changes and climate pattern during MIS 19, which includes the Matuyama–Brunhes paleomagnetic boundary (MBB). While the MJS does not record the MBB, which is the main criterion for defining the Early–Middle Pleistocene boundary, it is a candidate Global Boundary Stratotype Section and Point (GSSP) for the Ionian Stage and Middle Pleistocene Subseries because of its excellent exposure, continuous deposition, and astronomically tuned record between MIS 37 and MIS 16. The new 40Ar/39Ar age of 773.9 ± 1.3ka for volcaniclastic layer V4 allows the age-model for the succession to be refined. Ostracod assemblages show significant paleodepth fluctuations at the glacial–interglacial scale, in agreement with the pollen distality index (Pinus vs. Caryophyllaceae, Amaranthaceae and Ephedra) which also suggests higher values during MIS 19. Pollen analysis provides supplementary paleoclimate data that are valuable to reconstruct temperature variation during the identified phases. A major expansion of steppic vegetation at 790ka in the uppermost MIS 20 correlates with a significant cold and arid Northern Hemisphere climate phase, as documented by a concurrent peak both in North Atlantic ice rafted debris and Mediterranean dust records. Climate conditions recorded in MIS 19.3 were similar to those of the present interglacial, whereas a dry climate phase marks MIS 19.2 at 771.84ka, slightly above the V4 layer. Millennial scale climate variability occurred during MIS 19 as evidenced by two abrupt short-term moderately dry pollen events at 783.5ka and 774.8ka that could be related to a period of reduced geomagnetic field intensity through the MBB transition. Additional environmental/chronological events accompanied this stratigraphic interval: i) the volcaniclastic layers V3 and V4, radiometrically dated at 801.2 ± 19.5ka and 773.9 ± 1.3ka, respectively; ii) the co-occurrence of maximum flooding and thermal maximum in agreement with the previously recorded co-occurrence of the Neopycnodonte community and tropical–subtropical mesopelagic teleostean Bonapartia pedaliota, close to the increase of Quercus; and iii) the maximum depth (MD) between 777.3 and 773.25ka as recorded by outer shelf/upper slope macroinvertebrate fauna, close to the highest value of pollen distality index in MIS 19. Results enhance the understanding of orbital-suborbital paleoenvironmental change during the mid-Pleistocene, and particularly of the climate pattern during MIS 19 and across the MBB.

Iceberg-rafted tephra as a potential tool for the reconstruction of ice-sheet processes and ocean surface circulation in the glacial North Atlantic

AbstractIce-rafted tephra deposits, of Marine Isotope Stage 6 (MIS 6) age, from Site U 1304 on the Gardar Drift, North Atlantic were examined for their shard size distribution and major element composition. The heterogeneous composition, large shard sizes and association with ice-rafted debris (IRD) indicate that these late MIS 6 deposits were transported by iceberg-rafting from Iceland to Site U 1304. Comparison of individual shard geochemistry with the geochemistry of Holocene volcanic systems from Iceland allows the identification of different potential volcanic source regions. This detailed geochemical analysis, when combined with Icelandic Ice Sheet (IIS) flow models for the Last Glacial Maximum (LGM), suggests that the IIS had calving margins to both the north and south during the late MIS 6 and that icebergs could have been transported to the Site U 1304 by following surface ocean circulation patterns similar to those that prevailed during the LGM. We demonstrate that the descriptive concept of Icelandic glass in the characterization of tephra components within North Atlantic IRD can be significantly improved through quantitative characterization and that such data hold the potential to help constrain surface ocean circulation models, while also potentially yielding new information about the IIS during earlier glacial periods.Supplementary material:Statistical tests, major element concentrations of analysed shards, primary and secondary standards are available at http://www.geolsoc.org.uk/SUP18716

Provenance of North Atlantic ice-rafted debris during the last deglaciation—A new application of U-Pb rutile and zircon geochronology

Understanding the provenance of ice-rafted debris (IRD) provides a means to link the behavior of individual ice sheets to proxy records of climate change. Here we present a new approach to determining IRD provenance using U-Pb geochronology of detrital minerals rutile and zircon. We characterize potential source regions from Scotland using detrital rutile from modern fluvial systems, and demonstrate that their unimodal rutile U-Pb ages reflect the timing of the last amphibolite facies metamorphism of the source rocks, imparting a distinctive source signature. Contrasts between these spectra and the bimodal IRD (ca. 470 Ma and ca. 1800–2000 Ma) rutile age signatures rule out Scotland as the sole source and suggest a Laurentian contribution; IRD zircon ages further support this view. U-Pb mineral dating has the potential to provide new insight on IRD provenance, because it allows linkage between IRD and individual source terranes based on their differing magmatic and tectonothermal histories. The occurrence of Laurentian-sourced IRD proximal to Scotland demonstrates widespread and rapid dispersal of debris across the subpolar North Atlantic during the Older Dryas cold oscillation, and implicates the Atlantic meridional overturning circulation as a control. This highlights the sensitivity of some IRD records to rapid climate change during the last deglaciation and supports the interpretation of Heinrich events as time-parallel marker horizons.

Abrupt climate changes of Holocene

This paper is a review of studies of abrupt climate changes (ACCs) during the Holocene published during the past ten years. North Atlantic cold events are indicators of ACCs. As indicated by North Atlantic ice-rafted debris (IRD), there were nine confirmed cold events during the Holocene, occurring at 11.1 kyr, 10.3 kyr, 9.4 kyr, 8.1 kyr, 5.9 kyr, 4.2 kyr, 2.8 kyr, 1.4 kyr, and 0.4 kyr respectively according to most representative results from Bond et al. (1997). However, the identification of chronology has been made with some uncertainties. Considerable climatic proxy data have shown that, during the cold events, substantial climate abnormalities have occurred widely across the globe, particularly in the areas surrounding the North Atlantic. These abnormalities were in the form of high-latitude cold in the both hemispheres, expansion of the Westerlies to low latitudes, drought in the monsoon regions, recession of summer monsoons, and intensification of the winter monsoons. Studies have indicated that the four ACCs occurring in the early Holocene may be related to freshwater pulses from ice melting in the northern part of the North Atlantic, and the other five ACCs that occurred during the middle and late Holocene may be related to the decreased solar activity.

British Ice Sheet dynamics inferred from North Atlantic ice-rafted debris records spanning the last 175 000 years

British Ice Sheet dynamics inferred from North Atlantic ice-rafted debris records spanning the last 175 000 years

Centennial- to millennial-scale climate shifts in continental interior Asia repeated between warm–dry and cool–wet conditions during the last three interglacial states: evidence from uranium and biogenic silica in the sediment of Lake Baikal, southeast Siberia

We observed timescale-dependent changes in the correlations between biogenic silica (bioSi) and uranium concentrations of bulk sediment from the Buguldeika Saddle, Lake Baikal. There was a positive correlation for the glacial–interglacial (100 kyr) scales over the 180 kyr, with inverse correlations for centennial- to-millennial scales during the last three interglacial states (MIS 5, 3, and 1). The distinction of these correlations between the time scales suggests that the sedimentation processes of bioSi and U differ from each other. The Baikal bioSi concentration record is generally regarded as a paleotemperature proxy, reflecting the diatom production of the lake. On the other hand, we concluded that the temporal variation of the U concentration in the sediment reflected the weathering intensity in the south of Lake Baikal watershed associated with changes in the rainfall/moisture levels.Based on the paleoclimate proxies from Lake Baikal, climates in continental interior Asia are identified as having dry and wet conditions during the glacial and interglacial periods, respectively. This accounts for the rhythms in growth and retreat of ice sheets in Eurasia on glacial–interglacial scales. In the interglacial periods (MIS 5, 3, and 1), the continental interior is, contrary to the glacial–interglacial climate changes, characterized by alternating warm–dry and cool–wet climates with periods in tens of thousands years. In particular, such shifts in the climate stand out prominently at the beginning of MIS 5d. The detrended U record for the last 5.2 kyr shows wet events at 4.3–3.7, 3.2–2.3, 1.8–1.2, and 0.8–0.3 kyr. These events coincide with the timing of the North Atlantic ice-rafted debris events, which were possibly related to solar activity. All this evidence suggests that climate in the continental interior followed significantly different fluctuation patterns in the glacial–interglacial and shorter time scales.

Phase relationships of North Atlantic ice-rafted debris and surface-deep climate proxies during the last glacial period

We report stable isotope, core scanning XRF, and ice-rafted detritus (IRD) data in glacial-aged sediments from piston Core KN166-14-JPC-13 (hereafter referred to as JPC-13) retrieved at 53.1°N 33.5°W on the southern Gardar Drift, North Atlantic. A chronology was established by correlating millennial-scale features in the benthic δ 18O record to Portuguese Margin Core MD95-2042. Once the alignment of benthic δ 18O of JPC-13 to MD95-2042 is fixed, the relative timing of proxy variables is used to determine the phasing of changes in IRD and surface-deep hydrography. Each peak in North Atlantic IRD coincided with a decrease in benthic δ 18O that, in turn, has been linked to warming in Antarctica (Events A1–A7). The IRD pulses are followed shortly thereafter by abrupt decreases in planktonic δ 18O, indicating warming and increased heat transport to the subpolar North Atlantic associated with Greenland interstadials (GIS) 8, 12,14, and 16–17. Grain-size and elemental ( K/Ti) variations indicate that Iceland–Scotland Overflow Water (ISOW) was stronger during these long, warm interstadial periods of Marine Isotope Stage (MIS) 3. The results are consistent with interhemispheric phase relationships inferred from Iberian Margin sediment cores and with methane synchronization of Greenland and Antarctic ice cores.

North Atlantic region atmospheric circulation dynamics inferred from a late-Holocene lacustrine carbonate isotope record, northern Swedish Lapland

The first high-resolution record of climate variation based on the oxygen and carbon isotope composition of authigenic carbonate for northern Scandinavia is presented. Modern lake-water isotope data indicate that controls on its oxygen and hydrogen (δ18Ow and δDw) composition are unlikely to be evaporation or temperature, and its variations must therefore reflect changes in, or at the source of, precipitation. Substantial and persistent changes of the isotopic composition of the precipitation are required to change the mean annual isotope composition of lake surface water. For this reason we argue that the recorded changes were significant and that the recurrence of such changes would greatly affect future regional climate conditions in the North Atlantic region. Oxygen isotope (δ18O) minima occurring at ∼ 200, 500, 1300, 1600 and at 2900 cal. yr BP all coincide with major peaks in North Atlantic ice rafted debris deposition. We suggest that the depletion events in δ18O cycles recorded in several lakes in northern Swedish Lapland are caused by the same climatic shifts as those noted in the North Atlantic marine records. This is because changes of atmospheric circulation pattern and the lower ocean and atmospheric temperatures associated with the IRD events help to explain why 18O depletion of precipitation occurred during these events. Our findings indicate that the recorded changes in North Atlantic ice drift and surface hydrography are coupled to changes in atmospheric circulation.