John A. Peck

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins

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 lakes water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene 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 ≈70 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.

Atlantic Forcing of Persistent Drought in West Africa

Although persistent drought in West Africa is well documented from the instrumental record and has been primarily attributed to changing Atlantic sea surface temperatures, little is known about the length, severity, and origin of drought before the 20th century. We combined geomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, to reconstruct natural variability in the African monsoon over the past three millennia. We find that intervals of severe drought lasting for periods ranging from decades to centuries are characteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts.

Mid to late Holocene climate evolution of the Lake Telmen Basin, North Central Mongolia, based on palynological data

Abstract Palynological and sedimentological data from Lake Telmen, in north-central Mongolia, permit qualitative reconstruction of relative changes in moisture balance throughout the mid to late Holocene. The climate of the Atlantic period (7500–4500 yr ago) was relatively arid, indicating that Lake Telmen lay beyond the region of enhanced precipitation delivered by the expanded Asian monsoon. Maximum humidity is recorded between ∼4500 and 1600 cal yr B.P., during the Subboreal (4500–2500 yr ago) and early Subatlantic (2500 yr–present) periods. Additional humid intervals during the Medieval Warm Epoch (∼1000–1300 A.D. or 950–650 ago) and the Little Ice Age (1500– 1900 A.D. or 450–50 yr B.P.) demonstrate the lack of long-term correlation between temperature and moisture availability in this region. A brief aridification centered around 1410 cal yr B.P. encompasses a decade of cold temperatures and summer frost between A.D. 536 and 545 (1414–1405 yr B.P.) inferred from records of Mongolian tree-ring widths. These data suggest that steppe vegetation of the Lake Telmen region is sensitive to centennial- and decadal-scale climatic perturbations.

Mid to Late Holocene climate change in north central Mongolia as recorded in the sediments of Lake Telmen

Abstract A record of climatic change has been reconstructed for the past 7110 calibrated calendar (cal) years for Lake Telmen, Mongolia, based upon sedimentologic and geomorphic evidence from the lake–watershed system. Lake Telmen is a saline (presently 4 g l−1) closed-basin lake and is therefore sensitive to changes in effective moisture balance. Between 7110 and 6260 cal yr ago, conditions in the Lake Telmen region were hyperarid and a small saline (approximately 20 g l−1) lake was present. Increased effective moisture balance but still arid conditions prevailed between 6260 and 4390 cal yr ago. Since 4390 cal yr ago, generally more humid conditions prevailed in the Lake Telmen region. As the lake deepened, a hypoxic hypolimnion became established below a well-developed summer thermocline resulting in varved sediment accumulation since 4390 cal yr ago. Between about 2710 and 1260 cal yr ago, a greater than present-day effective moisture balance is recorded by a series of four lake highstand terraces. Sedimentologic profiles from lacustrine cores suggest an increase in effective moisture during the past approximately 680 cal yr.

Muted climate variations in continental Siberia during the mid-Pleistocene epoch

The large difference in carbon and oxygen isotope data from the marine record between marine oxygen isotope stage 12 (MIS 12) and MIS 11, spanning the interval between about 480 and 380 kyr ago, has been interpreted as a transition between an extremely cold glacial period and an unusually warm interglacial period, with consequences for global ice volume, sea level and the global carbon cycle. The extent of the change is intriguing, because orbital forcing is predicted to have been relatively weak at that time. Here we analyse a continuous sediment record from Lake Baikal, Siberia, which reveals a virtually continuous interglacial diatom assemblage, a stable littoral benthic diatom assemblage and lithogenic sediments with ‘interglacial’ characteristics for the period from MIS 15a to MIS 11 (from about 580 to 380 kyr ago). From these data, we infer significantly weaker climate contrasts between MIS 12 and 11 than during more recent glacial–interglacial transitions in the late Pleistocene epoch (about 130 to 10 kyr ago). For the period from MIS 15a to MIS 11, we also infer an apparent lack of extensive mountain glaciation.

Transparent Large-Strain Thermoplastic Polyurethane Magnetoactive Nanocomposites

Organically modified superparamagnetic MnFe(2)O(4)/thermoplastic polyurethane elastomer (TPU) nanocomposites (0.1-8 wt %) were prepared by solvent mixing followed by solution casting. Linear aliphatic alkyl chain modification of spherical MnFe(2)O(4) provided compatibility with the TPU containing a butanediol extended polyester polyol-MDI. All MnFe(2)O(4)/TPU nanocomposite films were superparamagnetic and their saturation magnetization, σ(s), increased with increasing MnFe(2)O(4) content. All nanocomposite films exhibited large deformations (>10 mm) under a magneto-static field. This is the first report of large actuation of magnetic nanoparticle nanocomposites at low-loading levels of 0.1 wt % (0.025 vol %). The maximum actuation deformation of the MnFe(2)O(4)/TPU nanocomposite films increased exponentially with increasing nanoparticle concentration. An empirical correlation between the maximum displacement, saturation magnetization, and magnetic nanoparticle loading is proposed. The cyclic deformation actuation of a 6 wt % surface modified MnFe(2)O(4)/TPU, in a low magnetic field 151 < B(y) < 303 Oe, exhibited excellent reproducibility and controllability. MnFe(2)O(4)/TPU nanocomposite films (0.1-2 wt %) were transparent and semitransparent over the wavelengths from 350 to 700 nm.

Use of Paleomagnetism in Studies of Lake Sediments

SV and magnetostratigraphy studies are well established for dating and correlation of lake sediments. SV and radiocarbon studies of suitable, rapidly deposited (>30 cm/ka) Holocene lake sediments can be used to provide dating and correlation with centennial resolution for regional (≤5×103 km) paleoenvironmental studies.

Magnetic record of Lake Baikal sediments: chronological and paleoclimatic implication for the last 6.7 Myr

Magnetic remanence vectors for 1472 samples taken from a 601 m core through Lake Baikal sediments are reported along with a complete magnetic susceptibility profile obtained from a pass-through system. Matching the stable remanence directions to the standard geomagnetic polarity time scale (GPTS) provides a robust chronology from the present back to V6.7 Ma and yields a remarkably constant sediment accumulation rate of 3.9 cm/kyr. For earlier times ^ represented by depths s 270 m ^ correlation to the GPTS is more problematic. Susceptibility fluctuations reflect climatic changes that can be matched to the marine oxygen isotope pattern for the last 6.7 Myr. Spectral analysis of the resulting susceptibility time series then indicates that, for the most part, the Milankovitch obliquity signal dominates. However, when the temporal evolution of the frequency content is investigated by analyzing sequences of time windows, a complex picture emerges in which eccentricity and precession power appear during some intervals. Furthermore, there is persistent evidence for significant power in a ‘non-Milankovitch’ band between 28 and 35 kyr. A 2003 Elsevier Science B.V. All rights reserved.

Iron transformations induced by an acid-tolerant Desulfosporosinus species.

ABSTRACT The mineralogical transformations of Fe phases induced by an acid-tolerant, Fe(III)- and sulfate-reducing bacterium, Desulfosporosinus sp. strain GBSRB4.2 were evaluated under geochemical conditions associated with acid mine drainage-impacted systems (i.e., low pH and high Fe concentrations). X-ray powder diffractometry coupled with magnetic analysis by first-order reversal curve diagrams were used to evaluate mineral phases produced by GBSRB4.2 in media containing different ratios of Fe(II) and Fe(III). In medium containing Fe predominately in the +II oxidation state, ferrimagnetic, single-domain greigite (Fe3S4) was formed, but the addition of Fe(III) inhibited greigite formation. In media that contained abundant Fe(III) [as schwertmannite; Fe8O8(OH)6SO4 · nH2O], the activities of strain GBSRB4.2 enhanced the transformation of schwertmannite to goethite (α-FeOOH), due to the increased pH and Fe(II) concentrations that resulted from the activities of GBSRB4.2.

Short-term Sediment and Morphologic Response of the Middle Cuyahoga River to the Removal of the Munroe Falls Dam, Summit County, Ohio

ABSTRACT The removal of the 3.66-m-high Munroe Falls Dam from the Cuyahoga River in Summit County, Ohio, between August and October 2005, follows an accelerating trend to remove dams for the purpose of river restoration. The response of the middle reach of the Cuyahoga River to this dam removal resulted in sedimentologic, morphologic and hydrologic changes. These changes were monitored by measuring transects, discharge and velocity, and collecting bed sediment both pre-removal and post-removal. The overall change to the mean grain size of the channel-floor deposits was characterized by coarsening upstream, and fining downstream of the former dam. The greatest degree of coarsening occurred within 1,000 m upstream of the former dam site. Following dam removal, changes in channel morphology were characterized by approximately 1 m of bed aggradation downstream of the dam site. Upstream, the channel quickly incised to the pre-1817 (pre-dam) substrate within a month of dam removal. Once the pre-1817 substrate was reached, downcutting stopped, and channel-widening became the dominant morphologic response. Prior to dam removal, flow velocity within the impoundment limited sediment transport to suspended load in all but the largest flows of the year. Following removal, reduced cross-sectional area and greater slope, increased flow velocity by 4 to 15 times. Now the river erodes and transports sand-sized sediment as bedload even during the low-flow periods. Bedload discharge measurements indicate the largest sediment source is the former impoundment sediment.