We analyze both new and previously published paleomagnetic records of secular variation (PSV) fro... more We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand. The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 90357170 cal. BP (calibrated years before 1950, ca 7950-8250 14 C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.
Seventeen box cores from Lake Superior were analyzed for total organic carbon (TOC), porosity, an... more Seventeen box cores from Lake Superior were analyzed for total organic carbon (TOC), porosity, and 210Pb activity. The data were used to determine relationships among bulk sedimentation rates, TOC sedimentation rates, and TOC degradation rates with depth in the core. Results are compared with predictive equations for TOC sedimentation rates and degradation rates in the oceans and show some striking similarities between the behavior of TOC in Lake Superior and in hemipelagic and pelagic marine environments.
Proceedings of the National Academy of Sciences, 2007
Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 an... more 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 resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m de...
International Continental Scientific Drilling Program (ICDP) drilled a complete succession of the... more International Continental Scientific Drilling Program (ICDP) drilled a complete succession of the lacustrine sediment sequence deposited during the last ~500,000 years in Lake Van, Eastern Anatolia (Turkey). Based on a detailed seismic site survey, two sites at a water depth of up to 360 m were drilled in summer 2010, and cores were retrieved from sub-lake-floor depths of 140 m (Northern Basin) and 220 m (Ahlat Ridge). To obtain a complete sedimentary section, the two sites were multiple-cored in order to investigate the paleoclimate history of a sensitive semi-arid region between the Black, Caspian, and Mediterranean seas. Further scientific goals of the PALEOVAN project are the reconstruction of earthquake activity, as well as the temporal, spatial, and compositional evolution of volcanism as reflected in the deposition of tephra layers. The sediments host organic matter from different sources and hence composition, which will be unravelled using biomarkers. Pathways for migration of continental and mantle-derived noble gases will be analyzed in pore waters. Preliminary 40 Ar/ 39 Ar single crystal dating of tephra layers and pollen analyses suggest that the Ahlat Ridge record encompasses more than half a million years of paleoclimate and volcanic/geodynamic history, providing the longest continental record in the entire Near East to date.
The frequent occurrence of adaptive radiations on oceanic islands and in lakes is often attribute... more The frequent occurrence of adaptive radiations on oceanic islands and in lakes is often attributed to ecological opportunity resulting from release from competition where arrival order among lineages predicts which lineage radiates. This occurs when the lineage that arrives first expands its niche breadth and diversifies into a set of ecological specialists with associated monopolization of the resources. Later-arriving species do not experience ecological opportunity and do not radiate. While theoretical support and evidence from microbial experiments for priority effects are strong, empirical evidence in nature is difficult to obtain. Lake Victoria (LV) is home to an exceptional adaptive radiation of haplochromine cichlid fishes, where 20 trophic guilds and several hundred species emerged in just 15 000 years, the age of the modern lake that was preceded by a complete desiccation lasting several thousand years. However, while about 50 other lineages of teleost fish also have estab...
Proceedings of the National Academy of Sciences, 2012
Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximatel... more 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 mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO 2 and CH 4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes exp...
The role that climate and environmental history may have played in influencing human evolution ha... more 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 where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered. We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012-2014 HSPDP coring campaign.
Across the glacial-interglacial cycles of the late Pleistocene (∼700 k.y.), temperature variabili... more Across the glacial-interglacial cycles of the late Pleistocene (∼700 k.y.), temperature variability at low latitudes is often considered to have been negligible compared to changes in precipitation. However, a paucity of quantified temperature records makes this difficult to reliably assess. In this study, we used the Bayesian method CREST (Climate REconstruction SofTware) to produce a 790,000 yr quantified temperature reconstruction from a marine pollen record from southeast Africa. The results reveal a strong similarity between temperature variability in subtropical Africa and global ice volume and CO2 concentrations, indicating that temperature in the region was not controlled by local insolation, but followed global trends at these time scales, with an amplitude of ∼4 °C between glacial minima and interglacial maxima. The data also enabled us to make an assessment of the impact of temperature change on pollen diversity, with results showing there is no link between glacial-age t...
Tephrochronology is a widely applied method recognized for its exceptional precision in geologic ... more 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 Greenland ice core records. Recent findings of a microscopic tephra accumulation, or cryptotephra, from Mazama in Newfoundland indicated that this horizon should also be found in Lake Superior sediments. We present findings that confirm the presence of 23 Mazama ash in two sediment cores from the Lake Superior basin, which indicates its 24 likely presence in the rest of the Laurentian Great Lakes and in deposits throughout much 25 of eastern North America and beyond. The ubiquity of this stratigraphic horizon should 26 be applicable to a higher resolution evaluation of climatological, ecological, and 27 archaeological events during the early-to mid-Holocene thermal maximum throughout 28 much of North America.
The sediment record from Lake Victoria is an important archive of regional environmental and clim... more 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, including controls on the Nile River flow. As well as important palaeoenvironmental proxies, the lake contains a unique record of explosive volcanism from the central Kenyan Rift, in the form of fine-grained volcanic ash (tephra) layers, interpreted as airfall deposits. In the V95-1P core, collected from the central northern basin of the lake, tephra layers vary in concentration from 10s to 10s of 1000s of glass shards per gram of sediment. None of the tephra are visible to the naked eye, and have only been revealed through careful laboratory processing. Compositional analyses of tephra glass shards has allowed the tephra layers to be correlated t...
Proceedings of the National Academy of Sciences of the United States of America, Jan 10, 2017
The sediment record of Lake Kivu reveals a complex volcanogenic and climatic Holocene history. In... more The sediment record of Lake Kivu reveals a complex volcanogenic and climatic Holocene history. Investigation of the inorganic carbonate record dates the onset of carbonate deposition in the mid-Holocene in Kivu's deep northern and eastern basins and identifies conditions enabling deposition. The magnitude and timing of carbonate-rich sedimentation is not so much controlled by climate but, instead, linked strongly to hydrothermal activity in the basin. Sublacustrine springs supply the vast majority of the calcium and carbonate ions required for supersaturation with respect to aragonite. This major hydrothermal activity that permanently stratifies Lake Kivu today was initiated ∼3,100 y before present (3.1 ka), when carbonate-rich sediments first appeared in the Holocene record. Aragonite is the dominant CaCO3 mineral present in the lake deposits. Both δ(13)C and δ(18)O of the aragonite are enriched above the expected kinetic fractionation of meteoric waters, suggesting a volcanoge...
Six ca. 9 m long piston cores and 16 multi-cores up to ca. 50 cm long were recovered from the Nor... more Six ca. 9 m long piston cores and 16 multi-cores up to ca. 50 cm long were recovered from the North Basin of Lake Malawi, East Africa. Five of these cores were selected for palaeoenvironmental studies. Pi ve stratigraphic units are recognised, which together with volcanic ash beds and magnetic susceptibility profiles, were used to correlate the cores. A geochronology for the cores has been established by means of 210Pb assays of the uppermost part of the record, radiocarbon dating and varve counts. Radiocarbon age determinations on bulk organic matter spanning the last-24,000 years are complicated by the presence of variable quantities of reworked organic debris and a calibration correction of 450 years has been provisionally assumed. Three facies: laminites (varves), homogenites, and unlaminated clays were recognised and used to interpret the depositional environment. The varved sections provide the first high-resolution varve chronology from the southern tropics. The uppermost varved section provides an outstanding basis for elucidating Lake Malawi's response to historically documented climatic, volcanic and seismic events in the region. Beds of homogeneous clay or 369
New molecular proxies of temperature and hydrology are helping to constrain tropical climate chan... more New molecular proxies of temperature and hydrology are helping to constrain tropical climate change and elucidate possible forcing mechanisms during the Holocene. Here, we examine a w14,000 year record of climate variability from Lake Victoria, East Africa, the world's second largest freshwater lake by surface area. We determined variations in local hydroclimate using compound specific dD of terrestrial leaf waxes, and compared these results to a new record of temperature utilizing the TEX 86 paleotemperature proxy, based on aquatic Thaumarchaeotal membrane lipids. In order to assess the impact of changing climate on the terrestrial environment, we generated a record of compound specific d 13 C from terrestrial leaf waxes, a proxy for ecosystem-level C 3 /C 4 plant abundances, and compared the results to previously published pollen-inferred regional vegetation shifts. We observe a general coherence between temperature and rainfall, with a warm, wet interval peaking w10e9 ka and subsequent gradual cooling and drying over the remainder of the Holocene. These results, particularly those of rainfall, are in general agreement with other tropical African climate records, indicating a somewhat consistent view of climate over a wide region of tropical East Africa. The d 13 C record from Lake Victoria leaf waxes does not appear to reflect changes in regional climate or vegetation. However, palynological analyses document an abrupt shift from a Poaceae (grasses)-dominated ecosystem during the cooler, arid late Pleistocene to a Moraceae-dominated (trees/shrubs) landscape during the warm, wet early Holocene. We theorize that these proxies are reflecting vegetation in different locations around Lake Victoria. Our results suggest a predominantly insolation-forced climate, with warm, wet conditions peaking at the maximum interhemispheric seasonal insolation contrast, likely intensifying monsoonal precipitation, while maximum aridity coincides with the rainy season insolation and the interhemispheric contrast gradient minima. We interpret a shift in conditions at the Younger Dryas to indicate a limited switch in insolationdominated control on climate of the Lake Victoria region, to remote teleconnections with the coupled Atlantic and Pacific climate system.
We present the first synchronously coupled transient simulation of the evolution of the northern ... more 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 records. Local precipitation, however, shows a much more gradual decline with time, implying a lack of strong positive vegetation feedback on annual rainfall during the collapse. The vegetation change in northern Africa is driven by local precipitation decline and strong precipitation variability. In contrast, the change of precipitation is dominated by internal climate variability and a gradual monsoonal climate response to orbital forcing. In addition, some minor vegetation changes are also simulated in different regions across northern Africa The model also simulated a gradual annual mean surface cooling in the subtropical North Atlantic towards the latest Holocene, as well as a reduced seasonal cycle of SST. The SST response is caused largely by the insolation forcing, while the annual mean cooling is also reinforced by the increased coastal upwelling near the east boundary. The increased upwelling results from a southward retreat of the North Africa monsoon system, and, in turn, an increased northeasterly trade wind. The simulated changes of SST and upwelling are also largely consistent with marine proxy records, albeit with a weaker magnitude in the model. The mismatch between the collapse of vegetation and gradual transition of rainfall suggests that the vegetation collapse is not caused by a strong positive vegetation feedback. Instead, it is suggested that the Mid-Holocene collapse of North African vegetation is caused mainly by a nonlinear response of the vegetation to a precipitation threshold in the presence of strong climate variability. The implication to the modeling and observations is also discussed.
Proceedings of the National Academy of Sciences, 2013
The most explosive volcanic event of the Quaternary was the eruption of Mt. Toba, Sumatra, 75,000... more 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 aerosols ejected during the eruption of the Youngest Toba Tuff (YTT). These events coincided roughly with a hypothesized human genetic bottleneck, when the number of our species in Africa may have been reduced to near extinction. Some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption. Others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors, and furthermore, that modern humans may have already expanded beyond Africa by this time. We report an observation of the YTT in Africa, recovered as a ...
Uploads
Papers by Thomas Johnson