Well-constrained reconstruction of glacier activity during the Holocene epoch is difficult for a number of reasons, principally:
- (palaeo)glaciers are often located in harsh, high-altitude environments, which often lack sufficient productivity for radiocarbon dating;
- until recently, surface exposure dating techniques have lacked the precision to resolve the age of young landform formation (although see this study and this study);
- in the northern hemisphere at least, glaciers were at their maximum Holocene position relatively recently during the ‘Little Ice Age’ approx. 200-300 years ago, thereby removing any geomorphological record of previous glacier positions upvalley.
For these reasons alternative, indirect proxies of past glacier activity have often been sought. In a recent issue of Quaternary Science Reviews, Canadian-based authors Maurer et al. present a record of Holocene glacier fluctuation in British Columbia, Canada, using a sedimentary record from small mountain lake.
The reason this study caught my attention was the rather advantageous geomorphological setting, which allows the story of past glaciation to be told. The lake is located in a catchment which is currently unglaciated, but during times of more extensive glaciation the tongue of the nearby valley glacier extended into the lake catchment, providing inflow via glacial meltwater. When a glacier is present in the lake catchment the lake receives much coarser grained sediment than normal. Recognising this, Maurer et al. describe the past fluctuation of this glacier through the interpretation of a series of 3 lake sediment cores. They informally term the lake ‘On-Off lake’, presumably to reflect the numerous switches between glacial and non-glacial inputs. Radiocarbon dating of organic layers in the sediment cores and of tree remains in the glacier forefield provided the temporal constraint for identified glacier fluctuations.
It was found that during the first half of the Holocene, from 10000-5000yrs before present, the local glacier was not extensive enough to provide input to On-Off lake. Tree trunks showing evidence of having been sheared by an advancing glacier were dated to approximatey 5000yrs old, representing the first evidence for local glacier advance during the Holocene. Between approximately 2750 years ago and the present day, the glacier front advanced and retreated over the On-Off lake drainage divide several times, as indicated by the switches between glacial and non-glacial sediments in the lake cores. Maurer et al. use historical photographs to supplement their geological record of Holocene glacier behaviour to the present day.
This study corroborates other records of northern hemisphere glacier activity, which show reduced glacier extent similar to / less than that of the present day during the early Holocene, followed by glacier expansion in the latter half of the epoch, culminating in the Little Ice Age maxima 200-300years ago. Recent studies in the southern hemisphere appear to show the opposite behaviour, with glaciers at their maximum Holocene positions in the middle of the epoch (approximately 6000years ago) and have undergone overall retreat ever since. These studies (link and link) suggest that this asynchronous behaviour between hemispheres is connected and caused by the changing position of the intertropical convergence zone (ITCZ). This climatic boundary shifts northwards and southwards over various timescales, predominantly in accordance with changes in the Earth’s orbit of the sun. During the Holocene the ITCZ has slowly shifted southwards, increasing the proximity of southern hemisphere glaciers, such as those in New Zealand’s Southern Alps, to warm, tropical airflows. At the same time, this tropical influence is therefore reduced in the northern hemisphere mid-high latitudes. Glaciers are highly sensitive to changes in atmospheric temperature, therefore it is suggested that these changes in ITCZ position and the respective control on regional air temperature are responsible for the differences in Holocene glacier behaviour between the hemispheres.
These findings are interesting as they suggest that the controls on glacier extent can switch between regionally-dominant (such as that shown above for the Holocene) or globally-dominant (such as during the last glacial maximum approximately 210000years ago, when glaciers around the world advanced synchronously). As this study (link) points out, since the industrial revolution c.1850 AD, the majority of the worlds glaciers have been retreating in synchrony, likely due to rising temperatures in response to increased global greenhouse gas levels. Is this the case? If so, was global greenhouse gas concentration controlling the synchronous glacier behaviour during the last glacial maximum? And, what causes the switches between regional/global dominant controls on glacier behaviour? Further addition of well-dated, palaeoglacier and palaeoclimate records from around the globe will help to test hypotheses that aim to resolve these questions.