Subannual record of climate change in Lake Malawi

Abstract:

East Africa is vulnerable to future climate change. However, there is a poor understating of recent climate changes and especially the long-term impact of climatic cyclicities on rainfall. In this study we use scanning XRF to understand climate changes over the last 2000 years from Lake Malawi the southern most of Africa’s Great Lakes. The record starts about 850 AD to the 1998 AD when the core was taken. We scanned the varved upper most part of the Lake Malawi record. Using ultra fine scanning we were able to have an average of 5 data points per varve. In this study we use Ti/Ca to reflect rainfall and Si/Ti to represent diatom deposition (upwelling strength) during the dry season. The records show dry conditions during the ‘little Ice age’ and wetter conditions during the ‘Medieval warm period’ with wetter conditions continuing with a couple of major dry periods throughout the record. There are also a full set of cycles associated with ENSO and Solar forcing throughout the record. Interestingly while the ENSO cycles where found before at Lake Challa the record looks completely different. Whereas they found a range of cycles within the 2-5 year band we found only a significant 3.4-year cycle in the Ti/Ca record suggesting a different impact of the ENSO cycle on different parts of Africa. Also we found evidence for the 11 year cycle in the Si/Ti only suggesting that this cycle had more of an impact on the dry period. The reason that the record in Malawi is different then the rest of east Africa is the influence of the Mascarene High (MH) located in the southern Indian Ocean. It is known that a strong MH can prevent the ITCZ from migrating as far south but long records of this system are none existent and its role in future climate change is debated. Therefore our record suggests that change the strength of the MH lead to different climate conditions in southern East Africa and have major impactions for understanding the impact of future climate change in the region. On this note there is some indication that conditions were changing in the last 150 years. This could be a result of human impacts on the record leading to weakened seasonality and drier conditions. This fits both previous work done on the lake and short-term studies of the MH. Therefore our record helps fill in a major gap in our understating of East African climate.