Dominique Blamart

Precise dating of Dansgaard-Oeschger climate oscillations in western Europe from stalagmite data

The signature of Dansgaard–Oeschger events—millennial-scale abrupt climate oscillations during the last glacial period—is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of 14C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard–Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 ± 0.6 and 67.4 ± 0.9 kyr ago.

RAPID CLIMATIC VARIATIONS DURING MARINE ISOTOPIC STAGE 3 : MAGNETIC ANALYSIS OF SEDIMENTS FROM NORDIC SEAS AND NORTH ATLANTIC

Abstract The bulk magnetic parameters of seven deep-sea cores distributed from the Nordic Seas (67°N) to the North Atlantic as far south as the Bermuda Rise (33°N) exhibit short-term variations which correlate with rapid climatic changes during marine isotopic stage 3 (MIS3). The magnetic mineralogy is uniformly dominated by well sorted low Ti-content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals. Because the magnetic minerals predominantly originate from one common source area (the Nordic basaltic province), these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition. These results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep-sea circulation. The latter was active/reduced during the interstadials/stadials and Heinrich events transporting the magnetic particles from the Norwegian Sea into the North Atlantic ocean along a path similar to the present path of the NADW. It is tentatively suggested that the Faeroe-Shetland Channel and the Denmark Strait were the only two active paths for the overflow water during MIS3. The presence of magnetic oscillations in the Bermuda Rise core in phase with those from the North Atlantic indicates that the activity of the southern Newfoundland Basin gyre was linked to that of the NADW during MIS3.

Eastern North Atlantic deep-sea corals: tracing upper intermediate water Δ14C during the Holocene

Abstract Paired 230Th/U and 14C dating were performed on deep-sea corals (Lophelia pertusa and Madrepora oculata) from the northeastern North Atlantic at ∼730 m bsl to investigate past changes of the thermohaline circulation. These were estimated using the Δ14C value of the upper intermediate waters, based on the 14C ages of the top and base of each coral, where possible, and the 230Th/U dating. The reliability of these estimates was checked by dating two very young corals of the species L. pertusa. One of these corals, collected alive in 1999 AD, gave a 230Th/U age of 1995±4 AD after correction for non-radiogenic 230Th. Another coral, the top of which dated to 1969±6 AD, recorded the atmospheric 14C/12C increase due to the nuclear tests in the early 1960s. The calculated Δ14C values from these two corals agree with those measured at GEOSECS Station 23 in 1972–1973 [Ostlund et al., Earth Planet. Sci. Lett. 23 (1974) 69–86] and 1991–1992 [Nydal and Gisfelos, Radiocarbon 38 (1996) 389–406]. This, together with the 100% aragonite content and the δ234U and 230Th/232Th values of all the dated corals, indicates that none of the corals behaved as open systems with respect to their U-series nuclides and that they closely represent the water mass properties in which they lived. The pre-anthropogenic Δ14C value of the North Atlantic intermediate waters was estimated at −69±4‰. The reservoir age varies from ∼400 years to ∼600 years, and this variation is due to atmospheric 14C/12C changes. A reservoir age of 610±80 years, close to the pre-anthropogenic value, was determined from one coral dated at 10 430±120 cal yr BP, when the global sea level was approximately at −35 m [Bard et al., Nature 382 (1996) 241–244]. This suggests a modern-like pattern of the oceanic circulation prevailed in the Northeast Atlantic Ocean at this time although the deglaciation was not completely achieved.

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon

Abstract The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and 14C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.

Evolution of weathering patterns in the Indo‐Burman Ranges over the last 280 kyr: Effects of sediment provenance on 87Sr/86Sr ratios tracer

[1] A high-resolution study of mineralogy and major element geochemistry combined with Sr and Nd isotopes has been conducted on high sedimentation rate cores collected off the Irrawaddy River mouth in the Andaman Sea and the Bay of Bengal to reconstruct the erosional and weathering history of the Irrawaddy River basin. In both cores, eNd(0) values imply that both glacial and interglacial sediments share a common crustal source: the Irrawaddy River. Strong glacial/interglacial cycles are recorded by Sr/Sr: interglacial periods yield values between 0.713 and 0.717, whereas glacial periods show higher values between 0.717 and 0.719. Variations of the pedogenic clays (smectite and kaolinite) to primary mineral (feldspar, quartz, illite, and chlorite) ratios show strong precessional cycles, suggesting a control by past changes in the summer monsoon intensity. Each increase in pedogenic clays content is also associated with a net loss of labile elements (Na, K, and Ca) from the detrital minerals under chemical weathering. Wet periods of summer monsoon reinforcement correspond to an increase in weathering of the Irrawaddy plain soils and a decrease of Sr/Sr ratio. Plotting Sr/Sr versus Rb/Sr gives a pseudoisochrons interpreted as a mixing line representing the strength of chemical weathering. During glacial stages, enhanced physical erosion induced by glacier scour and frost action in the highland of the Irrawaddy River basins produced high volumes of unaltered, Rb-rich minerals. The low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered Rb-rich minerals with high radiogenic Sr composition from the high relief of the Indo-Burman Ranges and the Tibetan plateau to the Indian Ocean.

Northeastern Atlantic cold-water coral reefs and climate

U-series age patterns obtained on reef framework-forming cold-water corals collected over a nearly 6000-km-long continental margin sector, extending from off Mauritania (17 degrees N; northwest Africa) to the southwestern Barents Sea (70 degrees N; northeastern Europe), reveal strong climate influences on the geographical distribution and sustained development of these ecosystems. Over the past three glacial-interglacial cycles, framework-forming cold-water corals (Lophelia pertusa and Madrepora oculata) seem to have predominantly populated reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. Above 50 degrees N corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from similar to 50 degrees N to similar to 70 degrees N. We propose that north-south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity related to changes of the polar front.

A discontinuous climatic record from 187 to 74 ka from a speleothem of the Clamouse Cave (south of France)

This study presents a continental paleoclimatic record in the south of France,based on N 18 O, N 13 C and the 234 U/ 238 U activity ratio,dated by the 230 Th/ 234 U disequilibrium method with thermal ionization mass spectrometry. A stalagmite (Cla4) from the Clamouse Cave offers a discontinuous stable isotopic record between 189 ka and 74 ka which covers marine isotope stages (MIS) 7,6.4,5.5,5.3 and 5.1. The growth phases of the Cla4 stalagmite correspond to high sea level stages,except during MIS 6.4 (169^162 ka). All the growth phases of Cla4 correspond to humid periods,corresponding to the sapropel events observed in the eastern Mediterranean basin. Thus,the influence of a strong hydrological activity in the eastern Mediterranean basin during these periods prevailed as far west as the south of France. Because the karstic system studied strongly buffers the isotopic composition of water,isotopic variations of the calcite deposited in the cave represent mainly global and large-scale environmental variations when isotopic equilibrium conditions prevailed for calcite crystallization. Sub-stage 5.3,the end of 5.5 and MIS 7 were colder by about 4^6‡C (calculated temperature) compared to present-day temperature while the growth phases of substages 5.1 and the beginning of 5.5 reflect environmental conditions close to present ones. The N 18 O and N 13 C values of cave deposits of the sub-stage 6.4 are: (1) strongly marked by kinetic fractionation processes such as evaporation due to moisture deficit within the cave atmosphere during the first step of this growth phase and (2) related to higher humidity due to a second period of enhanced rainfall during the second step of growth. This study shows that even if calcite has not been deposited at isotopic equilibrium,its isotopic composition can give insights into the environmental conditions at the time of deposition. > 2002 Elsevier Science B.V. All rights reserved.

Paleotemperatures from deep-sea corals: scale effects

Like other biogenic carbonate that can be dated, aragonite skeleton of deep-sea corals is a potential archive of oceanographic changes over time. Stable isotope analysis is commonly used in paleoceanographic reconstruction of past seawater temperatures, however, offset from isotopic equilibrium as well as recent observations about isotope distribution with the micro-structure of deep-water corals implies non direct paleoclimate reconstructions. Here we test the influence of the sampling scale on oceanographic interpretations.

Fossil water in large stalagmite voids as a tool for paleoprecipitation stable isotope composition reconstitution and paleotemperature calculation

The deuterium content of macroscopic fluid inclusions, several millimetres large, observed on two stalagmite polished sections has been measured with high accuracy (<F0.5x). Although several recent studies have already focused on the stable isotope content of microscopic fluid inclusions, this is the first time that such measurements have been made on large inclusions by direct injection of the water in the spectrometer, eliminating technical problems due to the extraction of water from microscopic inclusions (crushing and heating). The first results from four fluid inclusions of two stalagmites of South Western and Southern France demonstrate that: (1) for the same stalagmite and the same age, the deuterium content is similar; (2) the fluid inclusion deuterium content difference between the sites is in agreement with the present day dripping water; (3) calculated paleotemperatures at 100 ka are in the range of those derived from pollen, marine and ice core records. Despite the fact that macroscopic fluid inclusions have rarely been observed, it appears that they can give precious data for paleoclimatologists on the stable isotope content (d 18 O, dD and deuterium excess d) of the past cave dripping water, which is a proxy of the rainfall water, and that it can be accurately dated by the TIMS U/Th method on the host calcite up to 500 ka. D 2002 Elsevier Science B.V. All rights reserved.

C and O isotopes in a deep-sea coral (Lophelia pertusa) related to skeletal microstructure

Lophelia pertusa is a deep-sea scleractinian coral (azooxanthellate) found on the continental margins of the major world oceans. Built of aragonite it can be precisely dated and measured for stable isotope composition (C–O) to reconstruct past oceanic conditions. However, the relation between stable isotope and skeleton microstructures, i.e. centres of calcification and surrounding fibres, is crucial for understanding the isotopic patterns. Values for δ18O and δ13C in Lophelia pertusa were determined at a micrometer scale using an ion microprobe (SIMS - Secondary Ion Mass Spectrometry). In this coral species, centres of calcification are large (50 µm) and arranged in lines. The centres of calcification have a restricted range of variation in δ18O (−2.8 ± 0.3 ‰ (V-PDB)), and a larger range in δ13C (14.3 to 10.9 ‰ (V-PDB)). Surrounding skeletal fibres exhibit large isotopic variation both for C and O (up to 12 ‰) and δ13C and δ18O are positively correlated. The C and O isotopic composition of the centres of calcification deviate from this linear trend at the lightest δ18O values of the surrounding fibres. The fine-scaled variation of δ18O is probably the result of two processes: (1) isotopic equilibrium calcification with at least 1 pH unit variation in the calcification fluid and (2) kinetic fractionation. The apparent δ13C disequilibrium in Lophelia pertusa may be the result of mixing between depleted δ13C metabolic CO2 (respiration) and DIC coming directly from seawater. This study underlines the close relationship between microstructure and stable isotopes in corals. This relationship must also be taken into consideration for major elements like Mg and trace elements (U-Sr-Ba) increasing the reliability of the geochemical tools used in paleoceanography.