Nathalie Fagel

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

Late Quaternary clay mineral record in Central Lake Baikal (Academician Ridge, Siberia)

Abstract We investigated the mineralogical composition of two cores recovered on the Academician Ridge (Central Lake Baikal, Siberia). Sedimentological features show that the cores are unaffected by turbidity currents. However, hemipelagic deposition is not continuous, but intermittently disturbed by syn- or post-sediment reworking (e.g., bioturbation, slumps, faulting). Such modes of deposition are consistent with the complex uplift history of the ridge. Bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial/interglacial stages, and diluted by diatom-rich intervals related to warmer interglacial stages. The core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve SPECMAP. The ∼8-m cores partly recover the last four interglacial/glacial cycles, i.e., since oxygen isotope stage 8. We test the use of clay minerals as a proxy for paleoclimatic reconstruction. The clays are more weathered during the diatom-rich intervals in agreement with warmer climate conditions. However, the mean clay composition does not change significantly through glacial/interglacial stages. This observation implies that, in the Academician Ridge sediments, a simple smectite/illite ratio (S/I) does not alone provide a reliable indicator of climatic variation. It reflects the complex clay assemblages, especially the smectite group, delivered to Central Lake Baikal. Smectites include primarily illite–smectite mixed layers, made of a mixture of montmorillonite and beidellite. According to their behavior after cation saturation, the illite–smectite mixed layers are primarily transformed smectites, with some neoformed smectites intermittently observed. In addition, Al-smectites occur in minor proportions. We conclude that the S/I ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites.

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.

Sm–Nd isotope systematics in deep-sea sediments: clay-size versus coarser fractions

Sm-Nd studies were carried out on Last Glacial and Holocene sediments from the Labrador Sea. Three grain-size fractions were analyzed for each sediment: the clay -size fraction (<2 µm), the fine cohesive silts (2- 10 µm) and the coarse silts (10-63 µm). In most cases, Sm-Nd s ignatures are different from one grain-size to anot her. In addition, different Sm-Nd signatures are derived fr om the clay-size fractions compared to the cohesive silt fraction, indicating that the cohesive fraction is heterogeneous and that distinct origins in terms of source-areas are implied. Silt fractions record the influence of supplies from the North American Precambrian Shield, whereas clay-size fractions (<2 µm) are more sensitive to s edimentary input from the Eastern Basins, in relati on with deep currents. Coupling Sm-Nd studies on silt and c lay fractions may be a powerful tool to unravel the evolution of deep circulations versus detrital supplies.

SmNd signature of modern and late Quaternary sediments from the northwest North Atlantic: Implications for deep current changes since the Last Glacial Maximum

Abstract Nd isotopes and SmNd concentrations have been investigated in the clay size fraction of surface and late Quaternary sediments of the northern North Atlantic. Four sources of Nd have been identified in the surface sediments: (1) a radiogenic Nd typical of the Iceland volcanism; (2) a “Pan-African” Nd issued from eastern Greenland; (3) a “Variscan” Nd derived from the Arctic Ocean; (4) an old unradiogenic Nd related to the North American Shield. In the Iceland and Irminger basins, the major sources of Nd are the Variscan Nd derived from the Arctic Ocean and the Iceland Nd, except in the western Irminger Basin where the Pan-African Nd originating from eastern Greenland is dominant and labels sedimentary supplies linked to the Denmark Strait Overflow Water. Nd derived from Icelandic volcanism is never clearly dominant, even on the Reykjanes Ridge, and the Nd derived from the American Shield becomes dominant only in the Labrador Sea. Sediments lying in the pathway of the Western Boundary Undercurrent have Nd isotopic compositions which are shifted towards more radiogenic values indicating an input of material from the eastern basins, carried by this deep current. Late Quaternary sediments were cored from the southern Greenland Rise (core 90-013-013) and eastern Labrador Rise (Orphan Knoll; core 91-045-094). In core 90-013-013, ϵNd values during the Last Glacial Maximum are as low as −20, but subsequently rose to −9 in the modern sediments, indicating that the sedimentary material during the Last Glacial Maximum was dominated by unradiogenic Nd from the North American Shield. This may be related to a lesser outflow of the Western Boundary Undercurrent. In core 91-045-094, located along the Labrador Current trajectory, a dominant supply of North American Shield Nd is recorded, perhaps originating at least partially from the Hudson Strait. The SmNd data are compatible with smectite vs. illite studies at the same sites which suggest a continuous but more modest Western Boundary Undercurrent supply during Last Glacial Maximum.

Anthropogenic impacts in North Poland over the last 1300 years - a record of Pb, Zn, Cu, Ni and S in an ombrotrophic peat bog.

Lead pollution history over Northern Poland was reconstructed for the last ca. 1300 years using the elemental and Pb isotope geochemistry of a dated Polish peat bog. The data show that Polish Pb-Zn ores and coal were the main sources of Pb, other heavy metals and S over Northern Poland up until the industrial revolution. After review of the potential mobility of each element, most of the historical interpretation was based on Pb and Pb isotopes, the other chemical elements (Zn, Cu, Ni, S) being considered secondary indicators of pollution. During the last century, leaded gasoline also contributed to anthropogenic Pb pollution over Poland. Coal and Pb-Zn ores, however, remained important sources of pollution in Eastern European countries during the last 50 years, as demonstrated by a high (206)Pb/(207)Pb ratio (1.153) relative to that of Western Europe (ca. 1.10). The Pb data for the last century were also in good agreement with modelled Pb inventories over Poland and the Baltic region.

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.

CLAY SUPPLIES IN THE CENTRAL INDIAN BASIN SINCE THE LATE MIOCENE - CLIMATIC OR TECTONIC CONTROL

Abstract Mineralogical (X-ray diffraction, differential thermal analysis), geochemical [microprobe, inductively coupled plasma (ICP)-atomic emission spectrometry, ICP-mass spectrometry] and Sr-Nd isotopic analyses have been carried out on the clay size fraction of Late Miocene to Pleistocene sediments from the Central Indian Basin. The samples were taken from five giant cores recovered between 1° and 10°S on a transect along 80°E. The clay assemblages are homogeneous and characterized by an alternation of illite- and smectite-rich levels. Most of the clays are detrital and were derived from a unique source: the weathering of the Indo-Gangetic Plain supplied most of the eroded material. Temporal clay mineralogical fluctuations in the depositional basin reflect environmental changes in the provenance. On the basis of spectral analyses of a mineralogical parameter (peak height ratios), the fluctuating smectite-illite clay sedimentation is controlled by periodic Late Miocene climatic changes. During the Late Pliocene, an irregular, probably tectonic, control appeared.

Smectite composition as a tracer of deep circulation: the case of the Northern North Atlantic

Abstract The link between smectite composition in sediments from the northern North Atlantic and Labrador Sea, and deep circulation is being further investigated through detailed studies of the X-ray pattern of smectites and cation saturations. This allows clear distinction of dominant terrigenous sources associated to the main components of the modern Western Boundary Undercurrent. Time variations of smectite characteristics in two piston cores from the inlet and outlet of the Western Boundary Undercurrent gyre in the Labrador Sea indicate: (1) a more southern circulation of North East Atlantic Deep Water during the Late Glacial; (2) a step by step transition to the modern pattern of deep circulation during the Late Glacial/Holocene transition, with intensification of North East Atlantic Deep Water and Davis Strait Overflow; (3) an expansion of Davis Strait Overflow and Labrador Sea Water circulation in relation to ice surges and deposition of detrital layers; (4) an intensified circulation of North East Atlantic Deep Water during the Younger Dryas; and (5) a very recent increased influence of Denmark Strait Overflow Water beginning between 4.4 and

Clay mineral signature of the NW Atlantic Boundary Undercurrent

Abstract Surface sediments were sampled along 2 transects, across the Iceland and Irminger basins and in the Labrador Sea. Clay mineral assemblages (deduced from X-ray analyses of the carbonate-free