Phytoliths Used to Investigate the Effects of the Indonesian Mount Toba Super-Eruption (~75 kyr) in East Africa: A Subdecadal Record from Lake Malawi

Abstract:

The recent discovery of cryptotephra visually and chemically matched to the Youngest Toba Tuff (YTT, 75.0 ± 0.9 kyr) in Lake Malawi drill core sediments has spurred renewed interest in this period of time in East Africa. The YTT is the most recent and largest of the four Mount Toba eruptions, and is the only super-eruption to have taken place during the Quaternary. The timing of the YTT approximately coincides with a hypothesized human genetic bottleneck. Several climate models have proposed an episode of global cooling following the YTT; however, the magnitude and duration of the cooling is much debated, ranging from just a few degrees of cooling to a state of volcanic winter. Cored sediments from Lake Malawi provide an excellent record of local variability in the lake’s watershed that may be linked to specific climatic events. To investigate the possible effects of the YTT in East Africa, we continuously sampled Lake Malawi drill core 2A-10H-2 at 2-4 mm (~6 yr) intervals above and below the first occurrence of the YTT. Poaceae phytoliths were grouped into plant functional types (C₃, C₄, xerophytic, mesophytic, arboreal, etc.), revealing mostly subtle changes in terrestrial vegetation over the ~400 yr time period examined. Abrupt increases in concentration values for phytoliths derived from riverine Podostemaceae plants appear to signal increased discharge from rivers draining the surrounding uplands. Perhaps most significant is the increasing trend in burned phytoliths and decreasing trend in tree phytoliths post-YTT. Although there appears to be a very weak cooling signal synchronous with the YTT, the most abrupt terrestrial vegetation changes appear to be better correlated with the deposition of a slightly older cryptotephra horizon derived from the local Rungwe Volcanic Province. A potential complication with this record is the existence of a turbidite pre-YTT that encompasses the Rungwe horizon.