The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early latePleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after Ϸ70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.human origins ͉ Lake Malawi ͉ paleoclimate ͉ Pleistocene
We constrain, in detail, fluctuations of two former ice caps in NW Scotland with multibeam seabed surveys, geomorphological mapping and cosmogenic 10 Be isotope analyses. We map a continuous sequence of 40 recessional moraines stretching from ~10 km offshore to the Wester Ross mountains. Surface-exposure ages from boulders on moraine ridges in Assynt and the Summer Isles region show that substantial, dynamic, ice caps existed in NW Scotland between 13-14 ka BP. We interpret this as strong evidence that large active glaciers probably survived throughout the Lateglacial Interstadial, and that during the Older Dryas period (c.14 ka BP) ice caps in NW Scotland were thicker and considerably more extensive than in the subsequent Younger Dryas Stadial. By inference, we suggest that Lateglacial ice-cap oscillations in Scotland reflect the complex interplay between changing temperature and precipitation regimes during this climatically unstable period (~15-11 ka BP). words
The cosmogenic radionuclide •øBe has generated much interest because of its potential as a tracer in the environment and applications to geology, archaeology, glaciology, and oceanography. Nevertheless, for •øBe to be useful as a tool in the Earth sciences its geochemical cycle as outlined below needs to be understood more fully. Beryllium 10 (t•a = 1.5 x 106 years) is mainly produced in the atmosphere by spallation of oxygen and nitrogen induced by secondary neutrons formed by cosmic ray interactions with the atmosphere, but some is produced in situ on the surface of the Earth. Deposition of •øBe onto the surface of the Earth depends primarily on precipitation. Deposited •øBe is made up of several components, primarily •øBe •r•oduced in the stratosphere and in the troposphere and "Be recycled from dust and soil particles, and secondarily •øBe recycled from the ocean as hygroscopic nuclei and from cosmic dust. Even though paleoprecipitation dominated •øBe deposition at any one location in the past, cosmic ray flux and m•ajor changes in the Earth's magnetic field also influenced •øBe deposition. The •øBe deposited on land will either be fixed in soils or be carried away in overland flow through the fluvial system, or locked in ice. Most of the beryllium is transported in the sediment load and that which stays in solution shows a strong pH dependence and is highly mobile in organic-rich continental waters. Beryllium 10 from sediments and fiver water is quickly deposited in the nearshore sediment along the coastlines along with a small amount of •øBe that is released and dispersed to the deep sea. In the open sea, most of the beryllium is in solution and the rest resides on particulate matter, much of which is of biogenic origin. Beryllium 10 that is added to the sea may be scavenged by such particles, but as they settle out into deeper waters the organic matter may oxidize and calcareous organisms may slowly dissolve, releasing •øBe back into solution, though fecal pellets may carry much of the •øBe to the seafloor. Slow-growing manganese nodules absorb some •øBe directly from the surrounding water, but pelagic and slope sediments act as the ultimate sinks for •øBe as the residence time for •øBe in the sediments approaches that of the mean life of •øBe, 2.18 x 106 years. Nevertheless, a small portion of •øBe is subducted or accreted at the world's trenches, and it has been used as a tracer for the study of island-arc volcanism. Recently, the cosmogenic radionuclide •øBe has generated much interest because of its potential as a tracer in the environment and applications to geology, archaeology, glaciology, and oceanography. For the past several years, •øBe research has been conducted on relatively concentrated •øBe in meteorites, deep-sea sediments, manganese deposits, and soils. Current research on the production of •øBe within terrestrial matter requires added sensitivity from AMS techniques. The potential for Quaternary research may be substantial, but for •øBe to be used as a tool in the Earth sciences its geoch...
The cosmogenic isotope 10Be, total Be, and Al were measured in partly varved sediments from the upper 50 m of core 480, leg 64 (DSDP), Gulf of California. The concentration of 10Be from 1 to 50 kyr is in general agreement with estimates of the geomagnetic dipole moment obtained from archaeomagnetic and marine core research. 10Be anomalies were also found at 32 kyr and 43 kyr, contemporaneous with the Mono Lake and Laschamp excursions, respectively. The production of 10Be required to explain these anomalies is too high, particularly for the Mono Lake excursion, to be produced by a combination of decreased geomagnetic field and unprecedented long‐term solar activity. We conclude that the cause is a change in the galactic cosmic‐ray flux consistent with a supernova event. The coincidence with the two excursions remains a paradox.
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