Claude Hillaire-Marcel

Absence of deep-water formation in the Labrador Sea during the last interglacial period

The two main constituent water masses of the deep North Atlantic Ocean—North Atlantic Deep Water at the bottom and Labrador Sea Water at an intermediate level—are currently formed in the Nordic seas and the Labrador Sea, respectively. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming. Here we use micropalaeontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2u2009°C warmer than present. Our results indicate that todays stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7,000 years ago, has no analogue throughout the last climate cycle.

Geomagnetic paleointensity and environmental record from Labrador Sea core MD95-2024: global marine sediment and ice core chronostratigraphy for the last 110 kyr

Piston core MD95-2024 from the Labrador Rise provides a continuous record of rapidly deposited detrital layers denoting Laurentide ice sheet (LIS) instability. The core also provides a high-resolution record of geomagnetic paleointensity, that is consistent with, but at higher temporal resolution than previous Labrador Sea records. Correlation to the Greenland Summit ice cores (GRIP/GISP2) is achieved by assuming that Labrador Sea detrital layers correspond to cold stadials in the ice cores. This allows a GISP2 official chronology to be placed on MD95-2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes ( 10 Be and 36 Cl) in Greenland ice cores. Synchronous millennial scale variability observed from the MD95-2024 paleointensity

Dynamics of North Atlantic deep water masses during the Holocene

High resolution flow speed reconstructions of two core sites located on Gardar Drift in the northeast Atlantic Basin and Orphan Knoll in the northwest Atlantic Basin reveal a long-term decrease in flow speed of Northeast Atlantic Deep Water (NEADW) after 6,500 years. Benthic foraminiferal oxygen isotopes of sites currently bathed in NEADW show a 0.2‰ depletion after 6,500 years, shortly after the start of the development of a carbon isotope gradient between NEADW and Norwegian Sea Deep Water. We consider these changes in near-bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the Holocene deep ocean circulation, and attribute the changes to a weakening of NEADW flow during the mid to late Holocene that allowed the shoaling of Lower Deep Water and deeper eastward advection of Labrador Sea Water into the northeast Atlantic Basin.

Nd and Pb isotope signatures of the clay-size fraction of Labrador Sea sediments during the Holocene: Implications for the inception of the modern deep circulation pattern

Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2–6.5 ka) is composed of material derived from the North American shield and from a ‘‘young’’ crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the MidAtlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4558 Oceanography: Physical: Sediment transport; 1040 Geochemistry: Isotopic composition/chemistry; 9325 Information Related to Geographic Region: Atlantic Ocean; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: clay-size fraction, sedimentary mixings, deep circulation, Nd and Pb isotopes, North Atlantic, Labrador Sea

Changes in the Western Boundary Undercurrent outflow since the Last Glacial Maximum, from smectite/illite ratios in deep Labrador Sea sediments

High-resolution mineralogical studies were performed on late glacial and deglacial sediments from two deep piston cores from the Labrador Sea, located at the inlet (SW Greenland Rise) and outlet (Labrador Rise) of the Western Boundary Undercurrent (WBUC) gyre. At the two sites, smectites transported from the eastern Iceland and Irminger basins by the WBUC are observed. Clay mineral changes are used as proxies for the paleointensity reconstruction of the WBUC. On the Greenland Rise, a clay mineral index (smectite/illite (S/I) ratio) is defined. A S/I ratio of ∼1 characterized the Last Glacial Maximum. It increased after ∼17 ka. and reached a maximum value of 4 during the early Holocene. The mineralogical changes are gradual and do not show any reversal during the Younger Dryas. This pattern, which is confirmed by first-order estimations of smectite and illite fluxes, suggests gradually increasing sedimentary fluxes and WBUC intensity since the Last Glacial Maximum. A peak in the velocity of the WBUC at ∼9 ka, as recorded by clay assemblages, is consistent with other regional studies based on pollen, foraminifera, or grain-size measurements. A massive dilution of smectites by illite and chlorite (S/I ≈ 3) occurs at ∼8.5 ka. It corresponds to a period of rapid sediment accumulation and reflects an intensified illite-rich detrital supply by meltwaters from the southern Greenland Ice Margin. On the Labrador Rise, the smectite content varies between 20 and 60% with no obvious trend through time. The mineralogical composition is strongly influenced by ice-rafted deposition and by the abundance of fast deposit units (cf. Heinrich layers in the North Atlantic) which contain abundant detrital carbonates spilled-over from the North-West Atlantic Mid-Ocean Channel. In such layers, smectites are present but are diluted by the addition of illites, chlorites, and kaolinites. This provides evidence for a discrete and continuous WBUC supply of fine particles from the Irminger and Iceland Basins as far as the southeastern part of the Labrador Basin. Early deglacial smectite-rich layers (up to 60%) are also observed at this site. They indicate an increase in the outflow of the WBUC at ∼13.5 ka. (Bolling-Allerod), as previously reported from grain size or foraminiferal assemblage studies.

Holocene fluctuations in Arctic sea-ice cover: dinocyst-based reconstructions for the eastern Chukchi SeaThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.GEOTOP Publication 2008-0023.

Cores from site HLY0501-05 on the Alaskan margin in the eastern Chukchi Sea were analyzed for their geochemical (organic carbon, δ13Corg, Corg/N, and CaCO3) and palynological (dinocyst, pollen, and spores) content to document oceanographic changes during the Holocene. The chronology of the cores was established from 210Pb dating of near-surface sediments and 14C dating of bivalve shells. The sediments span the last 9000xa0years, possibly more, but with a gap between the base of the trigger core and top of the piston core. Sedimentation rates are very high (∼156xa0cm/ka), allowing analyses with a decadal to centennial resolution. The data suggest a shift from a dominantly terrigenous to marine input from the early to late Holocene. Dinocyst assemblages are characterized by relatively high concentrations (600–7200xa0cysts/cm3) and high species diversity, allowing the use of the modern analogue technique for the reconstruction of sea-ice cover, summer temperature, and salinity. Results indicate a decrease in sea-i...

Sources of Labrador Sea sediments since the last glacial maximum inferred from Nd-Pb isotopes

Pb isotopes have been measured in the clay-size fraction of Late Glacial and Holocene deep-sea sediments recovered from two Labrador Sea piston cores that have been previously analyzed for Nd isotopes. The newly acquired Pb isotopic data allow us to better constrain the different source areas that supplied clay-size material during the last deglaciation, until 8.6 kyr (calendar ages). Nd-Pb data can be modeled mainly as a mixture between a Precambrian crust and Lower Paleozoic material originating from the denudation of the pan-African orogen. The old material originates mainly from the Archean, Lower Proterozoic, or both terranes of Greenland (and also probably corresponding terranes of Labrador), although minor input of other Precambrian material is recorded in some detrital carbonate-rich deglacial samples from Orphan Knoll. The Phanerozoic crustal end member consists of sediment material mainly originating from northwestern Europe. This source area is found to be the only significant source of young crustal material in early Holocene sediments from the Greenland Rise. No significant input from the mid-Atlantic volcanism is apparent. This study puts further constraints on the deep circulation pattern during the last deglaciation. It is concluded that at that time, European Phanerozoic material was carried from the Norwegian Sea through the Wyville Thompson Ridge into the Iceland Basin by the North East Atlantic Deep Water. No evidence for an overflow is found either south of the Iceland (Iceland-Scotland Ridge) or through the Denmark Strait.

Oxygen isotope compositions of sinistral Neogloboquadrina pachyderma tests in surface sediments: North Atlantic Ocean

Abstract Neogloboquadrina pachyderma (Left-coiled) is a planktonic foraminifer indicator species for polar waters. It is present in both the glacial and interglacial sediments of the high-latitude oceans, and therefore is frequently used as a signal-carrier for constructing late Pleistocene-Holocene planktonic δ 18 O records. In this study the δ 18 O compositions of N. pachyderma (L) tests from deep-sea surface sediments are compared with surface water δ 18 O values and summer sea surface temperatures (SST) in the North Atlantic Ocean, the Norwegian-Greenland Sea, and the Labrador Sea. We document that for a temperature range of 2–8°C, the planktonic δ 18 O values follow paleotemperature equations with an offset of less than 0.4%., and that above 8°C the planktonic δ 18 O compositions become almost independent of SST and remain nearly constant. This pattern suggests that N. pachyderma (L) precipitates its shell nearly at isotopic equilibrium with surface waters colder than 8°C, and that in warmer surface waters N. pachyderma (L) changes its habitat depth or its growth season to secrete its shell at an ambient temperature around 7–9°C.

Paleointensity‐assisted chronostratigraphy of detrital layers on the Eirik Drift (North Atlantic) since marine isotope stage 11

[1]xa0Four piston cores collected in 1999 and 2002 from the Eirik Drift (southern Labrador Sea, off SE Greenland) provide paleomagnetic, environmental magnetic, and oxygen isotope records back to marine isotope stage 11. Age models for the cores are based on a combination of planktonic oxygen isotope data, relative geomagnetic paleointensity proxies, and the identification of geomagnetic excursions (Laschamp and Iceland Basin). Environmental magnetic data delineate two distinct detrital signals, interpreted to reflect the behavior of the surrounding ice sheets (Greenland and Laurentide) to orbital- and millennial-scale climate forcing. Broad decimeter-scale intervals of increased magnetic concentration and grain size occur during the early part of interglacial marine isotopic stages (MIS) 1, 5, 7, 9, and 11. Discrete centimeter-scale layers, recognized by magnetic concentration and grain-size sensitive parameters, gamma ray attenuation (GRA) bulk density, and carbonate content, are observed in glacial and interglacial stages as well as during terminations. On the basis of glacial reconstructions on Greenland during the last termination, the broad decimeter-scale coarser-grained intervals can be attributed to detrital influx associated with the retreat of the terrestrial-based Greenland Ice Sheet in the early Holocene. A similar magnetic signal observed within interglacial MIS 5, 7, 9, and 11 indicates similar Greenland Ice Sheet behavior during these time intervals. Two types of centimeter-scale detrital layers are also recognized back to MIS 11. Detrital carbonate (DC) layers reflect predominately ice-rafted debris (IRD) deposition, while the low detrital carbonate (LDC) layers reflect mass movement as evidenced by sharp basal contacts, graded bedding, and traction structures, likely from the Greenland slope. Some detrital layers on Eirik Drift, particularly the DC layers, can be tentatively correlated to detrital layers observed in the central North Atlantic and to those documented on the southern side of the Labrador Sea at Orphan Knoll.

Reconstructing Sea Ice Conditions in the Arctic and Sub-Arctic Prior to Human Observations

Sea ice is a sensitive parameter characterized by a high variability in space and time that can be reconstructed from paleoclimatological archives. The most direct indication of past sea ice cover is found in marine sediments, which contain various tracers or proxies ofenvironments characterized by sea ice. They include sedimentary tracers of particles entrained and dispersed by sea ice, biogenic remains associated with production under/within sea ice or with ice-free conditions, in addition to geochemical and isotopic tracers of brine formation linked to sea ice growth. Reconstructing the extent of past sea ice is, however, difficult because proxies are only indirectly related to sea ice and require the use of transfer functions having inherent uncertainties. In particular, we have to assume a correspondence between sea ice cover values from modern observations and the sea ice proxies from surface sediment samples, which is a source of bias since the time intervals represented by modern observations (here 1954-2000) and surface sediments (10 0 ―10 3 years) are not equivalent. Moreover, suitable sedimentary sequences for reconstructing sea ice are rare, making the spatial resolution of reconstructions very patchy. Nevertheless, although fragmentary in time and space and despite uncertainties, available reconstructions reveal very large amplitude changes of sea ice in response to natural forcing during the recent geological past. For example, during the early Holocene, about 8000 years ago, data from dinocyst assemblages suggest reduced sea ice cover as compared to present in some subarctic basins (Labrador Sea, Baffin Bay, and Hudson Bay), whereas enhanced sea ice cover is reconstructed along the eastern Greenland margin and in the western Arctic, showing a pattern not unlike the dipole anomaly that was observed during the 20th century.