Lindsay McHenry's Research
Tephrostratigraphy of Tanzanian Archaeological Sites:
I use the composition of volcanic ash to correlate between archaeological sites in East Africa. Individual volcanic eruptions produce and deposit volcanic ash layers of distinct compositions that can often be uniquely identified over broad areas. Where these ashes are preserved between stratigraphic layers of archaeological interest, we can create stratigraphic frameworks for important evolutionary and cultural changes throughout a region.
I have applied this technique to the Plio-Pleistocene volcanic ashes of Bed I, the oldest and thickest bed at Olduvai Gorge, Tanzania. Specimens of Homo habilis and Paranthropus boisei, along with abundant stone tools, faunal remains, and cut-marked bones, have been found at sites throughout the Olduvai basin. These specimens are found between the layers of volcanic ash produced by the nearby Ngorongoro Volcanic Highlands. Unfortunately, the volcanic ash layers are poorly preserved because of the saline-alkaline lake and groundwater conditions at the time of (and since) deposition. At most sites the volcanic glass, the phase most often used for "fingerprinting" volcanic ashes, is altered to clay and zeolite and unusable. I have instead focused on the phenocrysts (volcanic minerals) for characterization and correlation. Using phenocryst composition, I have uniquely identified and correlated the major Bed I volcanic ashes throughout the basin, creating a high-resolution stratigraphic framework for Bed I. This framework has been used to place the new finds (including a Homo habilis maxilla, OH65) from the lesser-known western part of Olduvai into context with the better-known eastern sites.
I am currently working to expand both the regional and temporal scope of this project. I have collected volcanic ash samples from Laetoli, an older (Late Pliocene) archaeological site to the southwest of Olduvai, from Peninj, a Pleistocene site to the north, and from the younger Beds (Bed II and up) at Olduvai. My goal is to establish a regional stratigraphic framework based on volcanic ash compositions for Plio-Pleistocene (and Holocene) Tanzania that can be used for regional correlation between sites of paleoanthropological, palaeoclimatological, and palaeoecological interest.
As a part of this study I also investigate the geochemistry and mineralogy of the source volcanoes to help determine the volcanic history of the region. I have established direct correlations between Olduvai Bed I tephra and the nearby Olmoti and Ngorongoro volcanoes. I have also visited the currently active volcano Oldoinyo Lengai, collecting samples of fresh volcanic material at different stages of its current cycle of activity.
Mars Analogue Research:
I am also interested in Mars geology, and finding terrestrial analogues for the minerals, sediments, and geochemistry of the Martian surface. Much of the Martian surface, as documented by the MER Spirit and Opportunity rovers and recent and current orbiters, likely consists of altered volcanic material. These deposits are also enriched in sulfate minerals, likely the products of dirty evaporites during a wetter episode of Mars’ past or possibly sulfur-rich volcanic fumaroles. I am interested in studying places on Earth where sulfate minerals are formed during the alteration of volcanic materials, under both volcanic and aqueous conditions.
I have started a new project examining the precipitation of sulfate minerals in basaltic caves at Craters of the Moon National Monument in Idaho. Craters of the Moon basalts are particularly high in iron, and are thus more similar to Mars basalt compositions than are other commonly used analogues.
I am also studying the altered volcanic ashes of Olduvai Gorge, some of which contain the sulfate mineral jarosite. Jarosite is an abundant sulfate mineral on Mars and is used as an indicator of predominantly acidic conditions, as it is only stable at low pH. At Olduvai it is found directly associated with high-pH zeolite minerals, which suggests that jarosite might not always indicate widespread acidic conditions.