Thomas C. Johnson

University of Minnesota Duluth | UMD · Large Lakes Observatory

Seismic-reflection surveys of the Isle Royale sub-basin, central Lake Superior, reveal two large end moraines and associated glacial sediments deposited during the last cycle of the Laurentide Ice Sheet in the basin. The Isle Royale moraines directly overlie bedrock and are cored with dense, acoustically massive till intercalated down-ice with acou...

The sediment record from Lake Victoria is an important archive of regional environmental and climatic conditions, reaching back more than 15,000 cal. years before present (15 ka BP). As the largest lake by area in East Africa, its evolution is key to understanding regional palaeohydrological change during the late Pleistocene and Holocene, includin...

Tephrochronology is a widely applied method recognized for its exceptional precision in geologic dating and stratigraphic correlation. Tephra from the ~7.6 kyr B.P. Mount Mazama caldera-forming (”climactic”) eruption have been widely identified and applied as stratigraphic isochrons sediments of northwestern North America, as well as in the Greenla...

Tephrochronology is a widely applied method recognized for its exceptional precision in geologic dating and stratigraphic correlation. Tephra from the ca. 7.6 ka Mount Mazama caldera-forming (climactic) eruption has been widely identified and applied as stratigraphic isochron sediments of northwestern North America, as well as in the Greenland ice...

Anthropogenic climate change has the potential to alter many facets of Earth's freshwater resources, especially lacustrine ecosystems. The effects of anthropogenic changes in Lake Superior, which is Earth's largest freshwater lake by area, are not well documented (spatially or temporally) and predicted future states in response to climate change va...

Supplementary Tables, Supplementary Discussion, Supplementary Figures, and Supplementary References

Significance Lake Kivu is unusually stratified as a result of active volcanism nearby, with warmer, more saline water holding high concentrations of methane and CO 2 , underlying cooler fresh water. We find that the history of carbonate sedimentation in Lake Kivu reflects the timing and duration of past hydrothermal input to the lake. The stable is...

African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial-interglacial change intensified worldwide. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year...

The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to wh...

Tephrochronology is a widely applied method recognized for its exceptional precision in geologic dating and stratigraphic correlation. Tephra from the ~7.6 kyr BP Mount Mazama “climactic eruption”—the caldera collapse that created Crater Lake in Oregon—have been widely identified and applied as stratigraphic isochrons in terrestrial, oceanic, and l...

https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/69854

Significance Lake Malawi is one of the world’s oldest and deepest lakes, with >1,000 species of endemic cichlid fish; its water bottom anoxia prevents bioturbation of deep-water sediments, which preserve exceptional paleoclimate signals. The Lake Malawi Drilling Project recovered the first continuous 1.3-My record of past climates of the African in...

The sediments of western Lake Superior hold a record of environmental changes that have accompanied the settlement and urbanization of the surrounding watershed. Organic carbon concentrations are low (1.5%) with little variation in stable isotope composition (− 26.5 ± 0.5‰) prior to 1900. Organic carbon and nitrogen concentrations begin to rise aft...

Alan Robock (Eos, 94(35), 305–306, doi:10.1002/2013EO350001) nicely summarizes past volcanic eruptions and climate, but we wish to correct his assessment of our recent discovery of the Youngest Toba ash in the sediments of Lake Malawi in East Africa [Lane et al., 2013].

New biomarker analyses from Lake Albert, East Africa spanning ~ 15-9 ka show the most extreme, abrupt, multi-stage climate and environmental shifts during the last deglacial transition of anywhere in Africa. Records of hydroclimate expressed in compound specific δD values from terrestrial leaf waxes and a TEX86 paleotemperature record support multi...

Lake Nicaragua, the largest lake in Central America, is a promising site for paleolimnological study of past climate change, tectonic and volcanic activity, and pre-Columbian agriculture in the region. It is near the northern limit of the Intertropical Convergence Zone (ITCZ), which brings the rainy season to the tropics, so effects of decreasing p...

The most explosive volcanic event of the Quaternary was the eruption of Mt. Toba, Sumatra, 75,000 y ago, which produced voluminous ash deposits found across much of the Indian Ocean, Indian Peninsula, and South China Sea. A major climatic downturn observed within the Greenland ice cores has been attributed to the cooling effects of the ash and aero...

Testing hypotheses linking environmental change and human evolution requires detailed climatic, tectonic and hydrographic records for the areas of interest. In eastern Africa, stratigraphically continuous lake beds often occur near important fossil hominin and artifact sites, providing an opportunity to reconstruct paleoenvironmental conditions at...

An 8 km2 area representative of deep offshore basins in Lake Superior was surveyed with multi-beam sonar and a high-frequency seismic-reflection system to create a high-resolution bathymetric map of the lake floor morphology, which is dominated by ring-shaped depressions attributed to the dewatering of glacial-lacustrine clays. Ten multi-cores were...

The termination of the African Humid Period (AHP) about 5 thousand years ago (ka) was the most dramatic climate shift in northern and equatorial Africa since the end of the Pleistocene. Based on TEX86 paleotemperature data from Lake Turkana, Kenya, we show that a temperature shift of 2–4 °C occurred over the two millennia spanning the end of the AH...

No abstract available. doi:10.2204/iodp.sd.14.08.2012

Affected by the shifting Intertropical Convergence Zone, Lake Malawi holds an outstanding limnic sediment archive storing palaeo-environmental information of a climatically highly responsive landscape in the East African Rift Valley. Reliable chronological data are essential to interpret the lake-bottom sediments and optically stimulated luminescen...

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth's climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global m...

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 re...

Currently, the source of organic matter to surface sediments of Lake Malawi (East Africa) is unclear; studies of offshore north-basin cores (363 m to 403 m water depth) have produced conflicting results regarding the proportion of aquatic versus terrestrial organic carbon (OC) contained in these sediments. To address this question, ten multi-cores...

New molecular proxy records of temperature and rainfall from lacustrine sedimentary archives are helping to constrain the tropical response to global climate change and elucidate possible forcing mechanisms during the latest Pleistocene and Holocene in East Africa. It is often noted that tropical ecosystems are more sensitive to perturbations in ra...

We have applied the TEX86 paleothermometer to produce a surface water temperature record for Lake Malawi spanning the past 700 years. Over much of the record temperature fluctuates from ~ 24–27 °C with a mean of ~ 25 °C ; however, there has been a substantial increase in temperature of ~ 2.0 °C during the past ~ 100 years. The TEX86 temperature rec...

We present a TEX86-derived surface water temperature record for Lake Malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past ∼ 74 kyr with millennial-scale resolution. Average temperature during Marine Isotope Stage (MIS) 5A was 26.5 °C, with a range from 25.7 to 27.3 °C,...

The recovery of detailed and continuous paleoclimate records from the interior of the African continent has long been of interest for understanding climate dynamics of the tropics, and also for constraining the environmental backdrop to the evolution and spread of early Homo sapiens. In 2005 an international team of scientists collected a series of...

We report a 150,000 year record of the abundance of biogenic silica (BSi) in drill cores from the northern and central basins of Lake Malawi. The periods of highest diatom burial at both sites occurred around 65–69 and 51–60 thousand years ago (ka) after the termination of prolonged, intense drought in the region. These peaks are believed to reflec...

Tropical heating is the driving force behind global atmospheric circulation and thus is an essential component to our understanding of climate dynamics through time. We present a record of temperature and aridity from Lake Malawi, the largest and southern-most of the large East African Rift lakes, which extends from 9 to 14 °S. The temperature reco...

Proxy records from Africa identify large and abrupt changes in the hydrological cycle during the last deglaciation. Explaining the complex spatial and temporal variations in African hydrology faces several challenges. We use transient simulations of the climate evolution from the Last Glacial Maximum (21,000 years ago) to the early Holocene (10,000...

Recent organic geochemical advances have facilitated the comparison between continental temperature change and hydrologic variability. TEX86, a proxy based on the lipids of aquatic Crenarchaeota that show a positive correlation with growth temperature, was used to reconstruct surface water temperatures from Lake Victoria, East Africa during the lat...

Situated in the volcanic highlands of the East African Rift Valley's western branch, Lake Kivu contains one of the most unusual and fascinating aquatic ecosystems on the planet. Bottom waters in the 480-meter-deep lake are warmer and saltier than its surface waters. The concentrations of dissolved carbon dioxide and methane are so high in the deep...

This paper discusses the potential offered by both Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery in estimating lake surface temperature. The findings from regression analysis show that MODIS/Terra Land Surface Temperature data (MOD11A1) and Ocean Color MODIS/Terra Sea Su...

This paper describes results of research to develop algorithms for estimating chlorophyll-a concentrations in Lake Malawi from MODIS satellite imagery. A procedure relating chlorophyll concentration in Lake Malawi to the atmospherically corrected reflectance ratio of MODIS/AQUA (Ocean Color) bands 9 and 12 (i.e., R443/R551) was established using th...

The application of new organic geochemical proxies for temperature and precipitation to sediment cores from Africa's deep lakes, combined with recent paleoceanographic investigations of continental climate recorded in major river deltas, has enormously improved our view of Africa's hydrologic and thermal evolution since the Last Glacial Maximum (LG...

We present a new record of the deuterium/hydrogen ratio of terrestrial leaf waxes from Lake Malawi from 0-75 kyr BP. The D/H ratio of leaf waxes has proven to be a robust indicator of terrestrial hydrological conditions through time in East Africa, and application of this new tool to drill cores from Lake Malawi provides the longest D/H record avai...

We obtained a paleotemperature record from Lake Malawi (10 - 14 S lat) that spans the past 150,000 years, based on analyses for TEX86 in the drill core sites in the central and north basins of the lake. This is the longest continuous record of temperature thus far recovered from the African continent. The temperature records for the past 25,000 yea...

In 2005 the Lake Malawi Drilling Project recovered ~623 m of continuous drill core from seven holes at two sites, including a core to 380 m subbottom in the deep Central Basin (592 m water depth). One of the oldest lakes in the world, Lake Malawi is ~580 km long, spans 5 degrees of latitude, and its sedimentary record integrates signals from across...

Biomarkers of aquatic algae and compound-specific carbon isotopes are used to examine past variations in algal community composition and primary productivity in Lake Malawi. We note major changes at the Pleistocene-Holocene boundary. From the Last Glacial Maximum until ∼11.8 calendar years before present (kyr B.P.), Lake Malawi was characterized by...

Accurate reconstructions of past hydrological variability are essential for understanding the climate history of the tropics. In tropical Africa, the relative proportion of vegetation utilizing the C3 vs. C4 photosynthetic pathway is mainly controlled by precipitation and thus past hydrological changes can be inferred from the vegetation record. In...

Between 10,500 and 9000 cal yr BP, δ18O values of benthic ostracodes within glaciolacustrine varves from Lake Superior range from − 18 to − 22‰ PDB. In contrast, coeval ostracode and bivalve records from the Lake Huron and Lake Michigan basins are characterized by extreme δ18O variations, ranging from values that reflect a source that is primarily...

SummaryA comprehensive geophysical survey of Lake Malawi, located in the southern portion of the East African rift system (EAR), has provided new insights to modern geological processes in the synrift lacustrine environment. In particular, depositional processes have been identified by detailed interpretation of over 12000 km of 28 kHz echosounder...

Recent investigations by the International Decade for the East African Lakes (IDEAL) have significantly advanced our knowledge on the history of Lake Victoria. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400 000 years for the lake basin. Three major desiccation events ar...

Lake Turkana is a large, closed-basin lake in the northern Kenyan rift that occasionally overflowed first to the Indian Ocean and then, after about 1.3 million years ago, into the Nile drainage basin. The lake lies in a broad, arid depression surrounded by late Cenozoic fluvial, lacustrine, and volcanic sequences. The climate of the Turkana basin i...

There are few high-resolution climate records available for the tropics; this situation is particularly acute in Africa. We present a sub-annual-resolution record of East African climate for the past ~1200 years, based analyses of varved sections of three replicate cores taken as part of the Lake Malawi Scientific Drilling Project. The varves consi...

We present preliminary results from a study using the TEX86 temperature proxy from sediments of East African Rift Lakes (including Lakes Turkana, Albert, and Malawi) to reconstruct the thermal history of tropical Africa for the last ~ 20,000 years at a subcentennial to multicentennial resolution. The TEX86 proxy, based on tetraether membrane lipids...

Lingering debate among evolutionary biologists over whether or not Lake Victoria dried out during the late Pleistocene focuses on perceived conflicts between biological and geological evidence for the age of its endemic species. This article reviews and updates the geophysical and paleoecological evidence for lake-wide desiccation and describes the...

We analyzed a varved sequence of sediment from a 350-m depth in the north basin of Lake Malawi, East Africa, for the size distribution of the sortable silt fraction (10-64 micrometers). Mean size of the sortable silt $\overline {{\rm{(SS}}} )$ varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of te...

1] The tropics play a major role in global climate dynamics, and are vulnerable to future climate change. We present a record of East African climate since 55 ka, preserved in Lake Malawi sediments, that indicates rapid shifts between discrete climate modes related to abrupt warming (D-O) events observed in Greenland. Although the timing of the Mal...

Lake Malawi extends from 9-14 degrees S within the East African Rift Valley, and at 700 m deep, contains more than 20 percent of the surface water on the African continent. In 2005 the Lake Malawi Scientific Drilling Project drilled 7 holes at two sites in the lake, recovering a continuous sediment record that samples much of the Quaternary. Detail...

As part of the Lake Malawi Drilling Project we have undertaken a 1 cm-scale resolution (ca. 20 yr) XRF scanning study of core from site 2A in the Northern Basin of the lake. We have focused on certain parameters, including Fe, Zr:Ti, and Si:Ti; these may be interpreted to represent changing input of terrigenous sediments (delivered to the lake by r...

We used scanning X-ray fluorescence to investigate fluctuations in calcium abundance in the upper 200 m of cores MAL05-1B and 1C recovered by the Lake Malawi Drilling Project from a central lake site. Conspicuous variations in calcium concentrations reflect the abundance of endogenic calcite, which accumulates during periods of relatively dry condi...

We present a record of tropical paleotemperature from one of the few continental sites available to date, Lake Malawi. Using the TEX86 molecular paleotemperature proxy, we have extended the previous Lake Malawi temperature record from 23,000 to 75,000 years. The record, taken from hole 2a Site MAL05-2A drilled during the Lake Malawi Drilling Projec...

The large lakes of the East African Rift Valley provide a magnificent array of sedimentary basins that are actively recording the modern climate dynamics of tropical East Africa. The basins are perhaps 10 million or more years old (Cohen et al. 1993), and their deepest reaches have archived continuous records of past climate change that are unparal...

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...

The tropics play a major role in global climate dynamics, and are vulnerable to future climate change. We present a record of East African climate since 55 ka, preserved in Lake Malawi sediments, that indicates rapid shifts between discrete climate modes related to abrupt warming (D-O) events observed in Greenland. Although the timing of the Malawi...

Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416–16421]. This re...

We present a synthesis of available estimates of primary production, organic carbon burial, and lake-atmosphere carbon dioxide exchange data for large lakes of the world. All three fluxes showed significant relationships with latitude and related climate variables, with lower production, higher evasion of carbon dioxide, and higher burial efficienc...

Plant leaf wax carbon isotopes provide a record of C3 versus C4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and...

We present the first synchronously coupled transient simulation of the evolution of the northern Africa climate-ecosystem for the last 6500 years in a global general circulation ocean–atmosphere–terrestrial ecosystem model. The model simulated the major abrupt vegetation collapse in the southern Sahara at about 5 ka, consistent with the proxy recor...

High-resolution analyses of the Mg concentration in authigenic calcite in five cores from Lake Edward provide a water balance history of central equatorial Africa spanning the past 1400 yr. A high ratio of Mg to Ca (%Mg) indicates strong droughts in central Africa during the Little Ice Age (A.D. 1400 1750), in contrast to records from Lake Naivasha...

Previous lack of independent lacustrine temperature proxies led to shortcomings in our understanding of continental paleoclimate records. Recent advances in a molecular biomarker paleotemperature proxy, known as TEX86, have led to new insights about tropical paleoclimate. Powers et al. (2005) reported the temperature history of Lake Malawi, East Af...

In order to fully understand abrupt climate events, both high and low-latitude climatic processes must be considered. Here, we examine the carbon isotopic composition of plant leaf wax biomarkers in order to examine changes in C3 versus C4 vegetation, a sensitive indicator of aridity, from the southeastern African tropics during the past 23,000 yea...

Climate proxy data and general circulation models indicate that tropical regions experience broad, zonal changes in precipitation driven by shifts in the mean position of the Intertropical Convergence Zone (ITCZ) during millennial-scale coolings and warmings in the North Atlantic. These ITCZ movements affected the climate of tropical Africa, where...

We present the first synchronously coupled transient simulation of the evolution of northern Africa climate- ecosystem for the last 6500 years in a global general circulation ocean-atmosphere-terrestrial ecosystem model. The model successfully simulated the major abrupt vegetation collapse in the southern Sahara at about 5ka, consistent with the pr...

As part of the Lake Malawi Drilling Project we have undertaken a 1 cm-scale resolution XRF scanning study of core from site 2A in the Northern Basin of the lake (ca. 10 South latitude). This 38-m core provides an essentially continuous record of regional climate over the past 70,000 years, so our XRF analyses correspond to an average temporal resol...

ABSTRACTA high-resolution side-scan sonar survey of the lake bed off the Keweenaw Peninsula, Lake Superior, demonstrates that bottom currents are affecting lake bed morphology at depths up to 240 m. Numerous lineations which run parallel to the shore appear to be sand ribbons. A field of sedimentary furrows which occurs in one area demonstrates the...

Introduction Lake Malawi, located in the southern part of the East African Rift Valley, is one of the world’s largest, deepest (700 m) and oldest (>7 Ma) lakes and is renowned for its biodiversity, especially its unique ecosystem including hundreds of species of fi sh and invertebrates found nowhere else on Earth. Geologists and paleolimnologists h...