Yves Lancelot

The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal

Abstract Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. [1] from ODP site 677 in the equatorial Pacific differs significantly from previous ones [e.g., 2–5], yielding estimated ages for the last Earth magnetic reversals that are 5–7% older than the K Ar values [6–8] but are in good agreement with recent Ar Ar dating [9–11]. These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in δ18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal [12]. Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the specmap composite curves [4,13], and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. [1]. The precession peaks found in the δ18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our δ18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 ± 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. [1] and recent radiochronological Ar Ar datings on lavas [9–11]. We developed a new low-latitude, Upper Pleistocene δ18O reference record by stacking and tuning the δ18O records from core MD900963 and site 677 to orbital forcing functions.

Marine Isotope Substage 5e and the Eemian Interglacial

The subdivision of Marine Isotope Stage 5 (MIS5) into five substages is robustly applicable to benthic oxygen isotope records from almost all areas of the ocean. The association of the Eemian Interglacial (in the sense that this concept is utilized by palynologists) with MIS 5e is widely agreed on the basis of a diversity of evidence. Here we present the first direct evidence regarding the relationships between the boundaries of these two stratigraphic entities. The base of the Eemian as recognized here is significantly younger than the base of MIS 5, and indeed falls within the isotopic ‘‘plateau’’ of MIS 5e during which global sea-level is thought to have been a few meters higher than at present. The termination of the Eemian Interglacial in Portugal as recognized in the palynological record in this core off southern Portugal is well within MIS 5d. D 2002 Elsevier Science B.V. All rights reserved.

Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: A 1500‐kyr record of carbonate dissolution

Appendix Table Al Is available with entire article onmicrofiche. Order from the American Geophysical Union, 2000Florida Avenue, N.W., Washington, D.C. 20009. DocumentP94-001;

Paleoceanographic reconstructions from planktonic foraminifera off the Iberian Margin: Temperature, salinity, and Heinrich events

2.50. Payment must accompany order. We examined coarse fraction contents of pelagic carbonates deposited between 2000-and 3700-m water depth in the tropical Indian Ocean using Ocean Drilling Program (ODP) sites 722 (Owen Ridge, Arabian Sea) and 758 (Ninetyeast Ridge, eastern equatorial Indian Ocean), and four giant piston cores collected by the French R/V Marion Dufresne during the SEYMAMA expedition. Over the last 1500 kyr, coarse fraction records display high-amplitude oscillations with an irregular wavelength on the order of ∼500 kyr. These oscillations can be correlated throughout the entire equatorial Indian Ocean, from the Seychelles area eastward to the Ninetyeast Ridge, and into the Arabian Sea. Changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation. The well-known “mid-Bruhes dissolution cycle”represents the last part of this irregular long-term dissolution oscillation. The origin of this long-term oscillation is still poorly understood. Our observations suggest that it is not a true cycle (it has an irregular wavelength) and we propose that it may result from long-term changes in Ca++flux to the ocean. Sites 722 and 758 δ18O records provide a high-resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr. Site 722 (2030 m) lies above the present and late Pleistocene lysocline. The lysocline shoaled to the position of site 758 (2925 m) only during the interglacial intervals that occurred between about 300 and 500 ka (Peterson and Prell, 1985a). Despite these supralysoclinal positions of the two sites, short-term changes in coarse fraction contents are correctable from one site to another and probably result from regional (or global) dissolution pulses. By stacking the normalized coarse fraction records from sites 722 and 758, we constructed a Composite Coarse Fraction Index (CCFI) curve in which most of the local signals cancelled out. The last 800 kyr of this curve appear to compare extremely well with the Composite Dissolution Index curve from core V34-53 (Ninetyeast Ridge), which unambiguously records past variations of carbonate dissolution in the equatorial Indian Ocean (Peterson and Prell, 1985a). In the late Pleistocene the CCFI variations are mainly associated with glacial-interglacial changes. They show strong 100 and 41 kyr periodicities but no clear precession-related periodicities. As proposed earlier by Peterson and Prell (1985a), the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation. We cannot totally reject the possibility, however, that low temporal resolution and/or bioturbation degrade somehow the precessional signal at ODP sites 722 and 758. In contrast, spectral density of dissolution cycles in the giant (53 m long) piston core MD900963 (Maldives area) displays clear maxima centered on the precession frequencies (23 and 19 kyr−1) as well as on the kyr−1 frequency but shows little power at the 100- kyr−1 frequency. These high-frequency changes most probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon.

The Seismic Stratigraphy and Sedimentary History of the East Mariana and Pigafetta Basins of the Western Pacific

A quantitative analysis of planktonic foraminifera in a core from the Iberian Margin allows a reconstruction of the evolution of oceanographic parameters during the last glacial cycle with a resolution of ∼1000 years. A principal component analysis performed on 19 species allows the identification of 11 intervals characterized by increased abundances of the subpolar species. The youngest six of these intervals are correlated with the last 6 Heinrich events (HEs). The five cold events older than stage 4 are dated at 81, 90, 110, 129, and 140 ka, respectively. Paleotemperatures reconstructed using the modern analog technique indicate 4°C decreases during all even-numbered isotopic stages and stage 3. During the HEs, temperature decreases reach ∼10°C and seawater δ18O anomalies reach ∼1‰. Temperature and salinity reconstructions indicate that the environment of the Iberian Margin has been under the combined influence of global factors such as the migration of the polar front and iceberg discharge and of regional factors such as the precipitation/evaporation regime on both oceanic and continental area.

Sur la mise en place des depots grossiers du plateau continental dans la partie nord du golfe de Gascogne

The successful completion of Leg 129, resulting in the first and only holes to penetrate igneous basement (not necessarily layer 2) in the East Mariana and Pigafetta basins, now allows the calibration of our regional multichannel seismic site surveys and the extrapolation of drilling results throughout these oldest Pacific basins. Our study indicates that mid-Cretaceous flows/sills overlie Jurassic/Lower Cretaceous sediments and oceanic crust throughout the East Mariana Basin and the southeast Pigafetta Basin. Jurassic-age oceanic crust and overlying Upper Jurassic-Lower Cretaceous sediments unquestionably exist at Site 801 and extend semicontinuously between Sites 801 and 800. Turbidite sequences of varying thicknesses and ages are ubiquitous features of both basins. Cretaceous turbidite sequences were derived from Magellan and Marcus-Wake seamounts and Seamount chains of Aptian age or younger. The seamounts/atolls of the Caroline Ridge more than 300 km to the south of Site 802 were the source of extensive Miocene-age volcanogenic turbidites which are restricted to the south central East Mariana Basin, while the carbonate caps that developed on Ita Mai Tai Guyot and other edifices of the Magellan chain were the source for the redeposited shallow-water carbonate sequences recovered along the eastern margin of the East Mariana Basin. The Ogasawara Fracture Zone, Magellan Seamounts, and associated flexural moat separate the Pigafetta Basin from the East Mariana Basin and influence the source and distribution of redeposited material. A distinct basinwide reflection that is correlated with the shallowest chert/porcellanite/clay and chert/chalk sequences in the Pigafetta Basin and East Mariana Basin, respectively, is a time-transgressive horizon (Eocene-Campanian) resulting from the passage of the Pacific Plate beneath the equatorial zone of high productivity.

Reconstructing sea surface temperature and salinity using δ18O and alkenone records

Relic deposits of cold periods of Quaternary Flandrian transgression, covered with fine sediment, France