The transport of moisture in the tropics is a critical process for the global energy budget and on geologic timescales, has markedly influenced continental landscapes, migratory pathways, and ...biological evolution. Here we present a continuous, first-of-its-kind 1.3-My record of continental hydroclimate and lake-level variability derived from drill core data from Lake Malawi, East Africa (9–15° S). Over the Quaternary, we observe dramatic shifts in effective moisture, resulting in large-scale changes in one of the world’s largest lakes and most diverse freshwater ecosystems. Results show evidence for 24 lake level drops of more than 200 m during the Late Quaternary, including 15 lowstands when water levels were more than 400 m lower than modern. A dramatic shift is observed at the Mid-Pleistocene Transition (MPT), consistent with far-field climate forcing, which separates vastly different hydroclimate regimes before and after ∼800,000 years ago. Before 800 ka, lake levels were lower, indicating a climate drier than today, and water levels changed frequently. Following the MPT high-amplitude lake level variations dominate the record. From 800 to 100 ka, a deep, often overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of extreme drought. Periods of high lake level are observed during times of high eccentricity. The extreme hydroclimate variability exerted a profound influence on the Lake Malawi endemic cichlid fish species flock; the geographically extensive habitat reconfiguration provided novel ecological opportunities, enabling new populations to differentiate rapidly to distinct species.
Sediment records from the northern basin of Lake Malawi provide a means of evaluating the lake basin's response to climate change over the past 75
ky, notably to increased precipitation at the ...terminations of droughts. Transitions from drier to wetter conditions provide an opportunity to evaluate the system's response to climate shifts. Upon termination of drought episodes at 62 and 72
ka, enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters. These processes appear to have left their mark in the sedimentary record, bringing about a spike of deposition of organic matter (probably of terrestrial origin) at times of increased rainfall. This was immediately followed by a period of deposition of chemically-weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation. After this altered material was removed (perhaps a thousand years after the transition to wetter conditions), fresher material, richer in soluble elements including nutrients, was exposed to chemical weathering, leading to substantial diatom blooms. The lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system. However, biological cycling of silica, including formation and dissolution of phytoliths, may have played a role in mobilization of the silica necessary for the diatom productivity.
An array of lake sediment proxies including paleobotanic, geochemical, and historical records has been used to determine former environments of Bugbee Pond, a small, mesotrophic pond in northeastern ...Minnesota. Much research has been produced on the history of climate and vegetation change of the region, yet we have little information on the impact of human settlement. This well-dated, high resolution, multi-proxy record is important for its length and concentration on the historic period. The lake itself became established by ~7000 years ago. Pollen evidence suggests a transition between the regional Prairie Period to the Great Lakes mixed conifer – hardwood forest was established in the region at this time. XRF data suggest dry basin accumulation early in the record after ~7000 cal yr BP, but lake levels substantially increased by ~5600 cal yr BP, during a regionwide climatic transition to more humid conditions. Birch and boreal conifers increased after about 3800 cal yr BP; further increases in boreal conifers occurred by ~2000 cal yr BP. Anthropogenic vegetation changes during the Historic period, beginning in the late 19th century, is well represented by forest clearance of white pine (Pinus strobus), followed by increases in early successional species and an increased sediment accumulation rate due to land clearance. Establishment of farming communities locally are shown by occurrence of corn (Zea mays) and oat (Avena sativa) pollen, and pasturing and grazing are documented by Rumex, Fabaceae and Poaceae pollen, as well as coprophilous fungi, such as Sordaria. The increase and subsequent decline in Pb and S concentrations in the uppermost sediments are mirrored by historically documented, nearby industrial activities.
Forcing mechanisms of tropical climate in continental areas remain poorly understood, due in large part to a lack of continuous, long-term, high-fidelity records. Sediment core T97-52V from Lake ...Tanganyika provides new insight into the timing and mechanisms behind East African climate change over the past 90+
kyr. This record is particularly important, because, other than a recently recovered scientific drill core from Lake Malawi, there are no other continuous, well-dated records from East Africa prior to 60
ka. The high resolution age model presented here provides a large degree of age certainty for the past 45+
kyr, and our suite of proxies allows a thorough examination of Lake Tanganyika's dynamics. From core stratigraphy and chemical analyses, we present evidence of a lake level drop greater than 400
m sometime prior to ∼
90
ka, much greater than that inferred for the LGM, suggesting a period of intense aridity sometime around 100
ka. Additionally, core T97-52
V preserves evidence of worm burrows detected by X-radiographic imagery as indicated by burrow-shaped deposits of iron oxide, indicating a shallow lake at the time of deposition of that material. Intermittently high lake levels between ∼
78
ka and ∼
72
ka developed at the same time as a weakened Asian monsoon and a pluvial phase in Northeast Brazil, suggesting a global reorganization of climate, possibly forced by a reduction in orbital eccentricity. Over the past 60
ka this core preserves the same events recorded in a core collected ∼
100
km away in the southern basin of Lake Tanganyika, including an unexplained increase in biogenic silica at ∼
37
ka, suggesting that this vast lake is responding coherently across both major bathymetric basins to regional and global climate forcing.
Major components of lacustrine sediments, such as carbonates, organic matter, and biogenic silica, provide significant paleoenvironmental information about lake systems. Fourier transform infrared ...spectroscopy (FTIR) and scanning X-ray fluorescence (XRF) techniques are fast, cost effective, efficient methods to determine the relative abundances of these components. We investigate the potential of these techniques using sediments from two large lakes, Lake Malawi in Africa and Lake Qinghai in China. Our results show statistically significant correlations of conventionally measured concentrations of carbonate (%CaCO₃), total organic carbon (%TOC), and biogenic silica (%BSi), with absorbance in the corresponding FTIR spectral regions and with XRF elemental ratios including calcium:titanium (Ca/Ti), incoherent:coherent X-ray scatter intensities (Inc/Coh), and silicon:titanium (Si/Ti), respectively. The correlation coefficients (R) range from 0.66 to 0.96 for comparisons of FTIR results and conventional measurements, and from 0.70 to 0.90 for XRF results and conventional measurements. Both FTIR and XRF techniques exhibit great potential for rapid assessment of inorganic and organic contents of lacustrine sediments. However, the relationship between XRF-ratios or FTIR-absorbances and abundances of corresponding sedimentary components can vary with sediment source and lithology.
Annually resolved sedimentological records (including annual varves) can be used to develop precise chronologies for key climatic and tectonic events. Varved records, however, are most common in high ...latitude lakes, resulting in a spatial bias with respect to annually resolved records in tropical regions. Here we report on the sedimentology of two sediment cores from Lake Izabal, eastern Guatemala, that contain a well-preserved thinly laminated section spanning ca. 2200 years of the mid-Holocene. We integrate radiocarbon age-depth modeling, sedimentological observations, laminae counting, µX-ray fluorescence scanning, and multivariate statistical analyses to constrain the nature and chronology of the laminations. Our sedimentological and geochemical results suggest that the alternating clastic (dark) and biogenic (light) laminae couplets were deposited annually. Dark laminae are characterized by an abundance of detrital grains, organic detritus, total organic carbon, and terrigenic elements, and most likely formed during times of increased discharge during the rainy season. In contrast, light laminae are characterized by a decrease in detrital grains and total organic carbon, and an increase in biogenic silica constituents, and were likely deposited at times of increased lake productivity during the dry season. We compare a floating varve chronology that spans ca. 2200 years with three radiocarbon-based age-depth models. Consistency between the varve chronology and one of the models partially supports the annual character of the laminated section in Lake Izabal. This laminated section, one of the first annually resolved sedimentological records from Central America, can help explore mid-Holocene hydroclimate variability and regional tectonic processes in this understudied region.
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 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 almost equal to70 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.
The climate of tropical Africa transitioned from an interval of pronounced, orbitally-paced megadroughts to more humid and stable conditions approximately 70,000
years ago (Scholz et al., 2007). The ...regional atmospheric circulation patterns that accompanied these climatic changes, however, are unclear due to a paucity of continental paleoclimate records from tropical Africa extending into the last interglacial. We present a new 140-kyr record of the deuterium/hydrogen isotopic ratio of terrestrial leaf waxes (δD
wax) from drill cores from Lake Malawi, southeast Africa, that spans this important climatic transition. δD
wax shifts from highly variable and relatively D-depleted to more stable and D-enriched around 56
ka, contemporary with the onset of more humid conditions in the region. Moisture source and transport history dominate the δD
wax signal at Lake Malawi, with local rainfall amount playing a secondary role for much of the paleorecord. Analysis of modern moisture sources for Lake Malawi suggests that D-depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier, D-depleted air mass currently located in central southern Africa to the Lake Malawi catchment. This D-depleted air mass is associated with the descending limb of the Hadley cell, which implies significant changes in the Hadley circulation during the megadroughts and related changes in the position of the Intertropical Convergence Zone over Africa. These findings demonstrate the ability of δD
wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance, and elucidate potential mechanisms for pronounced hydrological change in southeast Africa during the late Pleistocene.
► We analyze the δD of leaf wax in sediments from Lake Malawi, Africa, from 0 to 140
ka. ► Leaf wax δD at Lake Malawi generally tracks moisture source and transport history. ► Waxes deposited during the African megadroughts 140–70
ka are relatively D-depleted. ► Modified Hadley circulation expanded D-depleted southern air masses at this time.
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