Christine S. Lane

Volcanic ash reveals time-transgressive abrupt climate change during the Younger Dryas

Knowledge of regional variations in response to abrupt climatic transitions is essential to understanding the climate system and anticipating future changes. Global climate models typically assume that major climatic changes occur synchronously over continental to hemispheric distances. The last major reorganization of the ocean-atmosphere system in the North Atlantic realm took place during the Younger Dryas (YD), an ∼1100 yr cold period at the end of the last glaciation. Within this region, several terrestrial records of the YD show at least two phases, an initial cold phase followed by a second phase of climatic amelioration related to a resumption of North Atlantic overturning. We show that the onset of climatic amelioration during the YD cold period was locally abrupt, but time-transgressive across Europe. Atmospheric proxy signals record the resumption of thermohaline circulation midway through the Younger Dryas, occurring 100 yr before deposition of ash from the Icelandic Vedde eruption in a German varve lake record, and 20 yr after the same isochron in western Norway, 1350 km farther north. Synchronization of two high-resolution continental records, using the Vedde Ash layer (12,140 ± 40 varve yr B.P.), allows us to trace the shifting of the polar front as a major control of regional climate amelioration during the YD in the North Atlantic realm. It is critical that future climate models are able to resolve such small spatial and chronological differences in order to properly encapsulate complex regional responses to global climate change.

Volcanic ash layers illuminate the resilience of Neanderthals and early modern humans to natural hazards

Marked changes in human dispersal and development during the Middle to Upper Paleolithic transition have been attributed to massive volcanic eruption and/or severe climatic deterioration. We test this concept using records of volcanic ash layers of the Campanian Ignimbrite eruption dated to ca. 40,000 y ago (40 ka B.P.). The distribution of the Campanian Ignimbrite has been enhanced by the discovery of cryptotephra deposits (volcanic ash layers that are not visible to the naked eye) in archaeological cave sequences. They enable us to synchronize archaeological and paleoclimatic records through the period of transition from Neanderthal to the earliest anatomically modern human populations in Europe. Our results confirm that the combined effects of a major volcanic eruption and severe climatic cooling failed to have lasting impacts on Neanderthals or early modern humans in Europe. We infer that modern humans proved a greater competitive threat to indigenous populations than natural disasters.

Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka

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 cryptotephra layer in Lake Malawi sediments, >7,000 km west of the source volcano. The YTT isochron provides an accurate and precise age estimate for the Lake Malawi paleoclimate record, which revises the chronology of past climatic events in East Africa. The YTT in Lake Malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change, implying that the eruption did not significantly impact the climate of East Africa and was not the cause of a human genetic bottleneck at that time.

Multi-proxy dating of Iceland’s major pre-settlement Katla eruption to 822–823 CE

U. Buntgen received funding from the Ministry of Education, Youth and Sports of CR within the National Sustainability Program I (NPU I; GN LO1415), and additional support was provided by NSF grant 0909541. We are particularly thankful to the NEEM project and its participants.

Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk, Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3 1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5 1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.

Reply to Roberts et al.: A subdecadal record of paleoclimate around the Youngest Toba Tuff in Lake Malawi

Roberts et al. (1) state that we found no evidence of climate change at multidecadal to millennial timescales following deposition of the Youngest Toba Tuff (YTT) in Lake Malawi (2). However, we examined smear slides at a 2-mm interval, corresponding to subdecadal resolution, and X-ray fluorescence scans run at 200-µm intervals correspond to subannual resolution. We observed no obvious change in sediment composition or Fe/Ti ratio, suggesting that no thermally driven overturn of the water column occurred following the Toba supereruption.

Humans thrived in South Africa through the Toba eruption about 74,000 years ago

Approximately 74 thousand years ago (ka), the Toba caldera erupted in Sumatra. Since the magnitude of this eruption was first established, its effects on climate, environment and humans have been debated. Here we describe the discovery of microscopic glass shards characteristic of the Youngest Toba Tuff—ashfall from the Toba eruption—in two archaeological sites on the south coast of South Africa, a region in which there is evidence for early human behavioural complexity. An independently derived dating model supports a date of approximately 74 ka for the sediments containing the Youngest Toba Tuff glass shards. By defining the input of shards at both sites, which are located nine kilometres apart, we are able to establish a close temporal correlation between them. Our high-resolution excavation and sampling technique enable exact comparisons between the input of Youngest Toba Tuff glass shards and the evidence for human occupation. Humans in this region thrived through the Toba event and the ensuing full glacial conditions, perhaps as a combined result of the uniquely rich resource base of the region and fully evolved modern human adaptation.

A cryptotephra record from the Lake Victoria sediment core record of Holocene palaeoenvironmental change

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 to previously unrecognized eruptions of Eburru volcano around 1.2 and 3.8 ka, and Olkaria volcano, prior to 15 ka. These volcanoes lie ~300 km east of the core site in the Kenyan Rift. Our results highlight the potential for developing cryptotephra analysis as a key tool in East African palaeolimnological research. Tephra layers offer opportunities for precise correlation of palaeoenvironmental sequences, as well as windows into the eruption frequency of regional volcanoes and the dispersal of volcanic ash.

Discovery of Mount Mazama cryptotephra in Lake Superior (North America): Implications and potential applications

This research was supported by a Regents Professor grant by the University of Minnesota to Johnson

Comment on “The Latest on Volcanic Eruptions and Climate”

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].