Abstract
Former glaciolacustrine systems are an important archive of palaeoglaciological, palaeoenvironmental and palaeoclimatic change. The annually laminated (varved) sediments that, under certain conditions, accumulate in former glacial lakes, offer a rare opportunity to reconstruct such changes (e.g. glacier advance and retreat cycles, glacier ablation trends, permafrost melt, nival events) at annual or even sub-annual temporal resolution. Data of this kind are desirable for their ability to guide and test numerical model simulations of glacier dynamics and palaeoclimatic change that occur over rapid time intervals, with implications for predicting future glacier response to climatic change, or the effects of weather and climate events on lake sedimentation. The most valuable records preserved in glaciolacustrine systems are continuous varved sequences formed in the distal parts of glacial lakes, where microscale lamination structures can accumulate relatively undisturbed. Technological advances, in the last few decades, have enabled improved characterisation of glaciolacustrine varve microfacies and the precise measurement of varve thickness at the micrometre scale. However, unlike in cognate fields (e.g. soil science), protocols for the robust and consistent description and interpretation of glaciolacustrine varve sediments are lacking. To fill this gap, and to provide a resource for future studies of glaciolacustrine varved sediments, this paper reviews the processes of sedimentation in glacial lake basins, and presents the defining microfacies characteristics of glacial varves using a descriptive protocol that uses consistent examination of grain size, sorting, structure, nature of contacts, development of plasmic fabrics and features such as dropgrains and intraclasts within individual laminations. These lamination types are then combined into lamination sets, whose structures can be interpreted as glaciolacustrine varves. Within this framework, we define five principal assemblages of glaciolacustrine varve microfacies which, if clearly identified in palaeoglaciolacustrine settings, enable more detailed palaeoenvironmental interpretations to be made. Finally, we discuss the utility and complexities of reconstructing the evolution of former glacial lake systems using varve microfacies and thickness datasets.
Highlights
Annually-laminated lake sediments provide high temporal resolution records and precise chronologies of modern and palaeoenvironmental change (Ojala et al, 2012; Zolitschka et al, 2015)
Material can be transported into the middle of lake systems from icebergs and deposited as ice-rafted debris (IRD) (Thomas and Connell, 1985) or wind-blown particles that fall onto the lake surface whether it be open-water or a frozen surface (Lewis et al, 2002)
This study demonstrates that a pattern of decreasing varve thickness at the centennial scale can be identified by combining macro- and micro-scale analysis at Lago General Carrera/Buenos Aires (Bendle et al, 2017) and solely at the microscale at Llangorse, South Wales (Palmer et al, 2008a,b)
Summary
Annually-laminated lake sediments (varves) provide high temporal resolution records and precise chronologies of modern and palaeoenvironmental change (Ojala et al, 2012; Zolitschka et al, 2015). The paper is presented in three parts It synthesises and reviews glaciolacustrine sedimentology, covering sources of sediment, modes of transport, and seasonal sedimentation patterns in laminated lake sediments (with a specific focus on the delivery of sediment to distal locations) and summarises key studies that have presented microscopic analysis of extant and palaeo-glacial lake sediments. This provides context for the second part of the paper that establishes the reasoning behind a consistent descriptive framework for a microfacies system. We evaluate how this information can be used to refine palaeoclimatic and palaeoenvironmental reconstructions
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