Françoise Yiou

Solar irradiance during the last 1200 years based on cosmogenic nuclides

Based on a quantitative study of the common fluctuations of 14C and 10Be production rates, we have derived a time series of the solar magnetic variability over the last 1200 years. This record is converted into irradiance variations by linear scaling based on previous studies of sun-like stars and of the sun’s behavior over the last few centuries. The new solar irradiance record exhibits low values during the well-known solar minima centered at about 1900, 1810 (Dalton) and 1690 ad(Maunder). Further back in time, a rather long period between 1450 and 1750 ad is characterized by low irradiance values. A shorter period is centered at about 1200 ad, with irradiance slightly higher or similar to present day values. It is tempting to correlate these periods with the so-called “little ice age” and “medieval warm period” respectively An accurate quantification of the climatic impact of this new irradiance record requires the use of coupled atmosphere−ocean general circulation models (GCMs). Nevertheless, our record is already compatible with a global cooling of about 0.5-1°C during the “little ice age”, and with a general cooling trend during the past millenium followed by global warming during the 20th century (Mann et al., 1999).

Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, Puerto Rico

We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of ≈ 43 m Ma−1, consistent with mass balance results.

Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be and 26Al

Concentrations of 10Be (t12 = 1.5 × 106 y) and 26A1 (t12 = 0.72 × 106 y) have been determined by accelerator mass spectrometry (AMS) in a suite of quartz samples taken from sandstone boulders in several moraines in Arena Valley, a dry valley adjacent to the Taylor Glacier in the Quatermain Mountains, Southern Victoria Land, East Antarctica. These isotopes are produced in surficial quartz by cosmic ray spallation of O and Si. The concentrations in these samples ranged from 6.1 × 105 to 3.0 × 107 at g−1 for 10Be and from 9.4 × 106 to 1.2 × 108 at g−1 for 26A1, depending upon the extent of exposure at the surface. Analyses of 10Be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons; the value of 142+96−41 g cm−2 is consistent with other published values. Production rates of 17+6−4 at g−1 y−1 for 10Be and 113+54−16 at g−1 y−1 for 26A1 at 1300 m and 87°S and a 26Al: 10Be production ratio of 6.5+1.3−1.3 were calculated from the data. These values correspond to sea-level production rates at high geomagnetic latitude of 6.4 at g−1 y−1 and 41.7 at g−1 y−1 for 10Be and 26A1, respectively, consistent with determinations based on ~ 11 Ky glacially polished surfaces in the Sierra Nevada in California. These production rates imply exposure ages for the various moraines ranging from 50 Ky to 2.5 My, in accordance with other geological evidence. The 10Be and 26Al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3He. Examination of the 3He concentrations in conjunction with the present data set yields an estimate of 3He production of 230+8540 at g−1 y−1, corresponding to 85+31−15 at g−1 y−1 at sea level. This is lower than previously reported values of 240 ± 60 at g−1 y−1 and ~190 at g−1 y−1 (scaled to sea-level and high geomagnetic latitude) based, respectively, on volcanic flows less than 2000 years old at low latitude, and samples collected from 14,400-year-old volcanic flows. This discrepancy is likely due to diffusive losses which may become significant (up to ~50%) for surface exposure ages on the order of 2.0 My. Nevertheless, the general correlations among the three isotopes suggest that 3He does not suffer gross losses through diffusion.

Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14C and 10Be records

For about the last 30 years it has been recognized that the high frequency component of the tree rings 14C/12C record is dominated by the modulation of the cosmic ray flux by the solar wind. In particular, it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric 14C/12C. During the last millennium other periods of high 14C/12C values were observed but their solar origin is still debatable. In the present work we compare these fluctuations with an independent record of cosmogenic 10Be measured in ice from the South Pole to check the solar origin of the observed 14C/12C variations. In order to compare quantitatively the results obtained on 10Be and 14C, it is necessary to take into account the different behaviour of these two cosmogenic isotopes, and especially the damping effect of the carbon cycle in the case of 14C. As an input to a 12-box numerical model we used the relative fluctuations of the 10Be concentrations record measured in South Pole ice and converted it into a synthetic 14C record. We took into account the fact that 10Be modulation is enhanced in polar regions due to the orientation of the geomagnetic field. As expected, the fluctuations of the modelled 14C record are much smaller (a factor of 20) than those observed for the raw 10Be record. In addition, the variations are smoother and shifted in time by a few decades. The 10Be-based 14C variations closely resemble the 14C measurements obtained on tree rings (R = 0.81). In particular, it is easy to identify periods of maximal 14C/12C which correspond to solar activity minima centred at about 1060, 1320 (Wolf), 1500 (Sporer), 1690 (Maunder) and 1820 (Dalton) yr A.D. Cross-correlation calculations suggest that there is no significant lag between the 10Be-based 14C and the tree-ring 14C records. Our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium.

Beryllium 10 in the Greenland Ice core Project ice core at Summit, Greenland

Concentrations of the cosmogenic isotope 10Be have been measured in more than 1350 samples from the Greenland Ice Core Project (GRIP) ice core drilled at Summit, Greenland. Although a dust-associated component of 10Be retained by 0.45 μm filters in some of the samples complicates the interpretations, the results confirm that the first-order origin of 10Be concentration variations is changes in precipitation rate associated with different climate regimes. This effect is seen not only between glacial and interglacial periods, but also during the shorter “Dansgaard-Oeschger” interstadials. By contrast, the 10Be data do not support the interpretation of rapidly varying accumulation (i.e., climate) during the last interglacial. They can, however, be used to help place limits on the origin of the ice in these events. After taking into account variable snow accumulation effects, variations in the 10Be flux are observed, probably caused by solar and geomagnetic modulation, but possibly also by primary cosmic ray variations. The most dramatic is a 10Be peak ∼40,000 years ago, similar to that found in the Vostok ice core, thus permitting a very precise correlation between climate records from Arctic and Antarctic ice cores. The 36Cl/10Be ratio (considering either “total” or only ice-associated 10Be) shows significant variability over the whole core depth, thus confirming the difficulty in using this parameter for “dating” ice cores.

Quaternary Climate Change and the Formation of River Terraces across Growing Anticlines on the North Flank of the Tien Shan, China

Nested stream terraces, warped upward over actively growing anticlines along the north flank of the Tien Shan in western China, appear to record alternating phases of valley widening and incision. Differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines. We infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr. The crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm/a. The maximum heights of 25 and 35 ( ± 10) m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr ( ± 10 kyr). Hence, they suggest that flood plains, which were abandoned to form the terraces, developed adjacent to active stream beds during the last glacial period, when climates were relatively cold and dry. We presume that they were incised during deglacial periods when discharges and stream power increased. Apparent durations of exposure, obtained from

Presence of the Solar de Vries Cycle (∼205 years) during the Last Ice Age

^{10}Be

Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10Be and 14C records

in quartz cobbles lying on the surface of the lower terrace from one anticline, concur with abandonment and deep (~150 m) incision of the flood plain during the last global deglaciation (ca. 20 to 13 kyr B.P.). A minimum carbon-14 date of 33.9 kyr B.P. from deposits on the lowest terrace sequence from the other anticline, however, implies that such abandonment and incision of this flood plain occurred before the most recent global glacial maximum, about 20 kyr B.P. We infer that incision of this second anticlines floodplain began during an earlier deglacial epoch within the last glacial period (between about 70 and 20 kyr, and perhaps near 35 kyr B.P.).