Franck Bassinot

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.

Radiocarbon Reservoir Ages In The Mediterranean Sea And Black Sea

We measured apparent marine radiocarbon ages for the Mediterranean Sea, Black Sea, and Red Sea by accel- erator mass spectrometry radiocarbon analyses of 26 modern, pre-bomb mollusk shells collected living between AD 1837 and 1950. The marine reservoir (R(t)) ages were estimated at some 390 ± 85 yr BP, 415 ± 90 yr BP and 440 ± 40 yr BP, respec- tively. R(t) ages in the Mediterranean Sea and Black Sea are comparable to those for the North Atlantic Ocean (<65° N), in accordance with the modern oceanic circulation pattern. The DR values of about 35 ± 70 yr and 75 ± 60 yr in the Mediterra- nean area show that the global box-diffusion carbon model, used to calculate R(t) ages, reproduces the measured marine 14 C R(t) ages in these oceanic areas. Nevertheless, high values of standard deviations, larger than measurement uncertainties are obtained and express decadal R(t) changes. Such large standard deviations are indeed related to a decrease of the apparent marine ages of some 220 yr from 1900 AD to 1930 AD in both the Mediterranean Sea and the western North Atlantic Ocean.

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

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Mg/Ca and Sr/Ca ratios in planktonic foraminifera: Proxies for upper water column temperature reconstruction

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.

Asymmetrical saw-tooth pattern of the geomagnetic field intensity from equatorial sediments in the Pacific and Indian Oceans

] Reliable temperature estimates from both surface and subsurface ocean waters are needed to reconstructpast upper water column temperature gradients and past oceanic heat content. This work examines therelationships between trace element ratios in fossil shells and seawater temperature for surface-dwellingforaminifera species, Globigerinoides ruber (white) and Globigerina bulloides, and deep-dwelling species,Globorotalia inflata, Globorotalia truncatulinoides (dextral and sinistral) and Pulleniatina obliquiloculata.Mg/Ca and Sr/Ca ratios in shells picked in 29 modern core tops from the North Atlantic Ocean are calibratedusing calculated isotopic temperatures. Mg/Ca ratios on G. ruber and G. bulloides agree with published dataand relationships. For deep-dwelling species, Mg/Ca calibration follows the equation Mg/Ca = 0.78 (±0.04) exp (0.051 (±0.003) T) with a significant correlation coefficient of R

An unusual mid-Pleistocene monsoon period over Africa and Asia

Abstract A record of relative paleointensity from marine sediments in the equatorial Indian Ocean spanning the last 4 Ma completes a previous dataset from Ocean Drilling Program (ODP) Leg 138 sites in the equatorial Pacific [1]. The timescale was defined with a precision better than 20 kyr with different and independent methods. Most of the features present in the record from the equatorial Pacific are confirmed: the overall saw-tooth pattern is observed across every field reversal and the short-term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites. The global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo. A synthetic curve of field intensity changes during the last 4 Ma is proposed as a first paleointensity timescale for future stratigraphic studies.

Low-latitude hydrological cycle and rapid climate changes during the last deglaciation

During the Quaternary period, organic-rich black layers called sapropels were intermittently deposited in the deep eastern Mediterranean Sea, following high flood periods of the Nile River. During the past 250 kyr, timing of sapropel formation coincides with astronomically driven maximum summer insolation in the northern tropics. The insolation variations — described by a monsoon index — modulate the intensity of the African monsoon feeding the Nile flood. Here, we report the observation of a thick sapropel in eastern Mediterranean sediments that conspicuously deviates from the usual pattern. The sapropel, dated at 528–525 kyr by astronomical tuning of the stratigraphic oxygen-isotopic record, is anomalous because the tropical summer insolation, while at a peak at this time, was much lower than during the deposition of the more recent sapropels. The Mediterranean climate was cold and dry, at least at sea level. At the same time, in the equatorial Indian Ocean there was an extreme event of low surface-water salinity caused by heavy monsoonal fluvial discharge. The simultaneity, within dating uncertainties, of unusually heavy monsoon rainfall over Africa and Asia while summer insolation (and the monsoon index) was relatively low indicates a large, regional-scale monsoon anomaly that cannot be explained in terms of current understanding of astronomical forcing.

Surface and subsurface seawater temperature reconstruction using Mg/Ca microanalysis of planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer, and Pulleniatina obliquiloculata

Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Nino–Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.

Relative paleointensity across the last geomagnetic reversal from sediments of the Atlantic, Indian and Pacific oceans

Laser-ablation inductively coupled plasma-mass spectrometry microanalyses of Mg/Ca across individual final chambers of three planktonic foraminifera species, Globigerinoides ruber, G. sacculifer, and Pulleniatina obliquiloculata, reveal significant interspecies differences in test Mg concentrations. Whereas these three species have similar Mg/Ca values at low sea surface temperatures (similar to 22 degrees C), they diverge markedly at high sea surface temperatures (similar to 29 degrees C). Explanations for these differences in species Mg/Ca values based on detailed comparison of species intratest Mg/Ca distributions suggest that compositional variability within tests cannot account for the observed deviation of species Mg/Ca values in warm-water equatorial regions. Multiple regression modeling and delta O-18 analysis of Globigerinoides sacculifer tests indicate that interspecies differences in Mg/Ca values result from different depth habitats. The average Mg/Ca values of G. ruber final chambers reflect the temperature of the surface mixed layer (0-25 m), whereas those of G. sacculifer and Pulleniatina obliquiloculata correlate best with subsurface temperatures at 50-75 m and 100-125 m water depths, respectively. Mg/Ca calibration to the temperatures at these depths reveals a similar temperature control on Mg test composition in all species. Combining our results with Mg/Ca values from published culturing experiments, we derive a generalized equation for the effect of temperature and seawater salinity on foraminiferal Mg/Ca. We also show that the Mg/Ca composition of specific calcite layers within foraminiferal tests, including the low-Mg/Ca layers of Globigerinoides ruber and G. sacculifer and the cortex layer of Pulleniatina obliquiloculata, correlates with seawater temperature and can be used as an additional proxy for seawater temperature.

Duration and dynamics of the best orbital analogue to the present interglacial

Paleointensity records from marine sediments at three locations in the Atlantic, Indian and Pacific oceans show coherent and reproducible signals across the Brunhes-Matuyama reversal. The low-field susceptibility signals are predominantly anticorrelated between the Indo-Pacific and the Atlantic sites while the normalization of the natural remanent magnetization by any rock magnetic parameter yield identical results. We deduce that in these cases climatic components do not induce first order effects in the determination of relative paleointensity. The results establish the worldwide character of the triangular pattern displayed by the decay and recovery phases of the field intensity variations across the Brunhes-Matuyama reversal.