Glacier Moraines - Dating and Mapping Past Glacial Fluctuations
Although the changing environment can sometime seem distant, especially when talking about glaciers by which many people are affected indirectly (IPCC, 2019), massive changes in glacier extent are increasingly well documented. But how significant are these changes?
Reconstructing the Past
In order to examine the significance of glaciers' disappearance it is crucial to look at their past evolution and fluctuations in order to obtain a ''well-constrained temporal perspective'' (Thompson, 2017).
Map of Africa indicating present-day glaciers as well as past glacial activity
Understanding how glaciers formed in past periods enables a comparison to the present, as can be seen in the map above which shows the location of glacial activity in Africa during the Last Glacial period. The period following the Last Glacial period, known as the Holocene, is especially interesting since it experienced periods of stark contrast in glacial extents. Its relatively warm stable climate, in comparison to greater glacial-interglacial changes (Wanner, 2008), also included some significant climatic oscillations - dramatically altering the functioning of Earth systems, including the cryosphere (Mayewski, 2004).
Moraines as an Indicator of Past Environments
Moraine refers to the material deposited by a moving glacier and the resulting accumulation of debris.
A terminal moraine, also called end moraine, forms at the edge of the glacier, marking its maximum advance. The debris, transported and pushed by the ice, accumulates at the bottom.
Medial moraines form at the glacier surface when two tributary glaciers meet. The debris is contained within the ice, exposed at the surface. This type is rarely preserved once the ice disappears.
Lateral moraines form along the side of the glacier. The debris can either be deposited on the surface of the glacier through frost weathering of the rock or from tributary streams entering the valley.
Dating and and mapping the extent of moraines deposited by glaciers is a process often used to produce a record of both the magnitude and timing of past climate change in the tropics (Shanahan, 2000). Terminal moraines, for example, indicate the greatest extent of glacier advances. They are therefore used to reconstruct the previous size of now shrunken or even disappeared glaciers.
The dating of moraines has been quite difficult and inaccurate for a while due to limited dating methodologies. The majority of dating used to be done through radiocarbon and only provided data on the maximum and minimum ages of organic-rich deposits on moraines. The introduction and improvement of terrestrial cosmogenic nuclide (TCN) dating, however, has allowed researchers to date moraines directly, consequently gaining insights into the nature of glacier oscillations during the Holocene (Solomina, 2015).
Tracing Changes of East African Glaciers
Tropical East African glaciers contain direct glacial-geomorphologic evidence of past, more extensive glaciers (Shanahan, 2000). The age and extent of moraines present on these glaciers can provide valuable information of both magnitude and timing of past climate change.
Studies using cosmogenic 36Cl to look at moraines on Mt. Kenya and Kilimanjaro have been able to reconstruct the timing and magnitude of their glaciations. Their widespread advances were found to be in synchrony with the Last Glacial Maximum (LGM), the most recent period with the maximum global ice volume (Mark, 2008). All three glacier sites contain significant LGM moraines which serve as evidence of this extensive glaciation (Osmaston, 2004).
Significant variability has been also reported during the current Holocene epoch. Its onset is characterised by a rapid retreat of glaciers as the climate warms, which is marked in the Ruwenzori range where rapid glacial recession was identified during this early Holocene period (~11.7-8.2 ka) (Jackson, 2020). Although the Little Ice Age period (AD 1300-1850) is often associated with glacial advances, studies show that the maximum tropical glacier extent on East Africa's Mt Kenya reached during the Holocene has been around AD 5700 instead. These mid-Holocene tropical glacier advances have been generally attributed to the weakening of monsoon rains and low temperature (Karlén, 1999). Even though the glacial expansion during the LIA period was not as extensive on Mt Kenya as earlier on, paleoenvironmental reconstructions of the Ruwenzori indicate that the deposits mark the greatest expansions during the Late Holocene (after ~4.2 ka) which suggests that the LIA did influence the range significantly (Jackson, 2020).
What is the Significance of these Findings?
Although the Holocene is a period that has been widely studied in terms of its climatic variability, little is known about past glacial fluctuations of the tropical regions (Jackson, 2020). It is evident that Holocene glacier oscillations in East Africa, and the variability within the region, remain poorly understood (Solomina, 2015). Understanding how glaciers responded, especially during warmer periods in the Holocene, is key for predicting and adapting to any future changes, and therefore requires more attention (Jackson, 2020).
Moreover, the unusually high glacier behaviour in the last few decades in most regions in comparison to past Holocene glacier changes, highlights the escalating rate of their retreat as a result of anthropogenic forcing; especially as it is happening during a period in which orbital forcing is favourable for glacier expansion (Solomina, 2015). These glacial recessions reflect the urgency of addressing growing human interference, signifying the potential beginning of a new epoch - The Anthropocene.
Comments
Post a Comment