Sediment Core and Glacial Environment Reconstruction

Dr. Jostein Bakke and Dr. Øyvind Paasche Sediment Core and Glacial Environment Reconstruction

SpringerReference

Sediment Core and Glacial Environment Reconstruction Synonyms Glacier reconstructions

Definition Sediment core and glacial environment reconstructions describe the methods used to reconstruct past glacier activity based on sediments deposited in distal glacier-fed lakes (Figure 1). By quantifying physical properties of glacial and extra-glacial sediments deposited in catchments, and in downstream lakes and fjords, it is possible to isolate and identify past glacier activity - size and production rate - that subsequently can be used to reconstruct changing environmental shifts and trends. Moreover, detailed glacier reconstructions can also be used to assess denudation rates, chemical and physical weathering, as well specific glaciological changes.

Sediment Core and Glacial Environment Reconstruction, Figure 1 Shows a simplified representation of different organic and minerogenic components making up the bulk sediments typically found in records retrieved from distal glacier-fed lakes. It is commonly the physical properties of the sediments that are used for glacier reconstructions.

Introduction Alpine glaciers are often located in remote and high-altitude regions of the world, areas that only rarely are covered by instrumental records. Understanding the behavior of glaciers and linking their activity to climate changes can therefore be challenging, but when successful glacier reconstructions can shed light on past and present climate variability on both shorter and longer time scales. Few, if any, other proxies have responded in a more unambiguous way to global warming than glaciers, which now are receding in a hitherto unrecognized pattern. Robust glacier reconstructions can thus be an important source of knowledge for a better understanding of not only natural climate variability, but also change induced by anthropogenic emissions of greenhouse gases. Producing glacier records is not straightforward and is frequently based on a blend of different methods, which occasionally is difficult to reconcile. This can, for instance, be due to the method applied for estimating the equilibrium-line-altitude (ELA) or even the preferred physical parameter that is interpreted to reflect glacier activity. One major drawback with glacier reconstructions based solely on moraine chronologies - by far the most common - is that

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23 May 2012 16:17 1

Dr. Jostein Bakke and Dr. Øyvind Paasche Sediment Core and Glacial Environment Reconstruction

SpringerReference

due to selective preservation of moraine ridges such records do not exclude the possibility of multiple Holocene glacier advances. This problem is true regardless of whether cosmogenic isotopes or lichenometry have been used to date the moraines, or also radiocarbon dating of mega fossils buried in till or underneath the moraines themselves. To overcome this problem, Karlén (1976) initially suggested that glacial erosion and the associated production of rock-flour deposited in downstream lakes could provide a continuous record of glacial fluctuations, hence overcoming the problem of temporal incomplete reconstructions. Reading the glacial signal, as preserved in downstream lake sediments, now includes the application of various methods such as measuring the amount of minerogenic versus biologic matter (typically inferred from Loss-on-Ignition (LOI)), grain-size analysis (GSA), magnetic properties (MP), geochemical elements (GE), Rare-Earth Elements (REE), Bulk Sediment Density (BSD), but also other techniques (e.g., Bakke et al., 2009; Guyard et al., 2007; Leeman and Niessen, 1994; Leonard and Reasoner, 1999; Lie et al., 2004; Paasche et al., 2007). Paleorecords of natural climate variability based on glacier reconstructions are arguably more reliable if separate events, like those that happened to produce individual moraines, can be compared relative to each other. Being able to do so requires knowledge not only about the conditions during the deposition of a single moraine, but also the conditions both prior and after the advance. It is not until such knowledge is made available through continuous glacier reconstructions that changing climate conditions adequately can be evaluated in terms of glacier response. Being able to do so is the main advantage with using sediment cores as basis for glacial environment reconstruction. Here we offer a brief review on prospects and problems associated with the employment of such an approach and also how continuous glacier reconstructions can be used as proxies for past climates.

Methods review Changes in average sediment evacuation from alpine glaciers are mainly governed by glacier size and the mass turnover gradient, determining the deformation rate at any given time. The amount of solid precipitation (mainly winter accumulation) versus loss due to melting during the ablation-season (mainly summer temperature) determines the mass turnover gradient in either positive or negative direction. In this simplified world, a prevailing positive net balance will lead to higher sedimentation rates and vice versa, which in turn can be recorded in downstream lakes. To retrieve these glacial sediments it is necessary to collect sediment cores from the lake bottom. A range of coring equipment is now available, with different operating mechanisms and different levels of success in core recovery (see Table 1). A number of problems (often overlooked) are encountered during coring operations or during subsequent transport: (1) sediment disturbance/deformation due to coring, (2) cores not capturing all sedimentary units, (3) not enough cores retrieved, and (4) potential onsite pollution of the sediments in the cores. Sediment Core and Glacial Environment Reconstruction, Table 1 An overview of available coring devices, how they are operated, what maximum water coring depth they are suited for and what the maximum length of the retrieved core is expected to. The last column contains reference to some papers describing these devices

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Dr. Jostein Bakke and Dr. Øyvind Paasche Sediment Core and Glacial Environment Reconstruction

SpringerReference

Coring device

Operation

Operating water depth

Core References length (m)

Piston corers driven by rods (also modified with hammer or percussion)a

Hand operated and motorized