Hydrology & Sediment Transfer


Paleoclimatic Reconstruction
from
High Resolution Arctic Lake Sediments

Hydrological field investigations

Patterns of suspended sediment transfer from watersheds in the Canadian High Arctic cannot be investigated using the same analytical tools available to lower latitude investigations. Within the region, there are simply no basins with data series longer than three years. As a result, little is known about how suspended sediment transfer varies spatially, or how transfer from a given basin varies in response to climatic variability. To ascertain the paleoclimatic signal of interannual thickness variations in a clastic sedimentary record, the climate signal represented by sediment transfer from the basin must be understood. In lieu of long data series, the 'calibration' technique typically applied to Arctic basins is to develop a simple statistical association between annual thickness and the nearest available climatic data. Unfortunately, without a clear understanding of the controlling processes, as only determined by on-site measurements, spurious associations may result.

To convert varve thickness into a quantitative estimate of climate with confidence (such as summer temperature), direct studies of the watershed are required. This kind of quantification is essential for verification of GCM paleoclimate reconstructions where climate parameters (not simply varve thicknesses) are needed. Just as with any paleoclimate proxy, calibration of varved records is essential. Unfortunately, the spatial array of climate data from the High Arctic is so sparse (only a few long-term weather stations) that we can not rely on the conventional approach which uses a spatial array of data to calibrate a temporal (down-core) record of variability. Rather, we must gain insight into key variables from in situ studies, involving one or more weather stations in each basin, plus streamflow measurement, and sampling of suspended sediment in transit. Our previous work has clearly demonstrated the value of this approach, particularly when measurements are made for several years (e.g. Hardy et al. 1996). We believe that the reliability of any paleoclimatic reconstruction based on lake sediments is greatly enhanced by being carried out in conjunction with field measurements of the important processes involved. Such measurements also provide key parameters for modeling studies seeking to understand, more generally, the relationship between precipitation, erosion and sediment flux in Arctic environments.

At the Sawtooth Range field site, a stream gaging station was established in May, 1998 at a stable cross-section of the primary stream entering the lake (C. Braun effort!). Manual discharge measurements were conducted here throughout the summer, along with electronic measurements of stream height (stage), as well as water temperature and electrical conductivity. The purpose was to provide a continuous record of discharge (calculated by stage-discharge relationships), water temperature, and a proxy measure of total dissolved solid load. In conjunction with the weather station measurements, these data will demonstrate hydrological responses to snowmelt. Combined with sediment sampling (see below), the continuous record of streamflow will be used to calculate sediment transfer into the lake. These measurements will also allow calculation of a continuous streamwater density time series; to better understand the dispersion and deposition of sediments within the lake.

Sampling and measurement of suspended sediment in transit to the lake was also done at the stream gaging station. The objective was to sample at sufficient frequency to allow interpolation between measured suspended sediment concentration (SSC) values. Arctic streams are characterized by a high amplitude diurnal cycle in SSC, as well as fluctuations in SSC that are nearly random, due to processes such as release of impounded meltwater. Therefore, adequately capturing SSC variability requires a flexible, yet intensive sampling program involving multiple samples each day. We currently sample suspended sediment using the USDH-48 depth-integrating manual sampler during daytime, and when discharge is not a danger to the observer. Collected samples are then filtered in the field (0.45 Ám retention), for laboratory analysis back at the university. For the 1999 field season we plan to employ an automatic sampling system, which obtains samples at regular intervals by pumping water and sediment into bottles. In addition to the collection of actual samples, we also operate a Optical Backscatterance (OBS) sensor, which provides a continuous record of SSC when the values are properly calibrated from suspended sediment samples.

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Climatology Lab | Department of Geosciences | University of Massachusetts

Last revised:  18 July 1998