Lake Tuborg, Ellesmere Island Research site & overview of results Paleoclimatic Reconstruction from High Resolution Arctic Lake Sediments

Lake Tuborg

Lake Tuborg formed when an ice advance from the Agassiz Ice Cap sealed off the inner part of Greely Fiord. The resulting lake still contains seawater at depth, trapped at the time of this advance (Hattersley-Smith and Serson, 1964). Dates on this water (¹⁴C) indicate isolation occurred ~3,000 years ago (Bowman and Long, 1968), though the reliability of this early date is unknown. Today, the lake contains 50-60m of freshwater in the mixolimnion, overlying anoxic marine water in the monimolimnion. Sediments settling to the lake floor are not disturbed by burrowing organisms, and form varved couplets.

The thickness of clastic varves in Lake Tuborg is mainly controlled by summer temperature. We have demonstrated this from both hydrological studies on site, and also from empirical studies relating longer term meteorological data and ice core melt records from the Agassiz Ice Cap to varve thickness. Meteorological observations near the lake and at ~800m above the lake, indicate that temperature controls snowmelt, river discharge and sediment flux to the lake. However, internal characteristics of the Tuborg watershed complicate the relationship, and may introduce non-linearities. In 1995 for example, ponding of saturated snow on the Agassiz Ice Cap delayed runoff until a critical threshold was reached; a very large slushflow event then led to discharge and suspended sediment concentration increasing suddenly by two orders of magnitude. We do not know the recurrence interval of such catastrophic events, but we plan to establish the necessary conditions for them by examining the 48 year long upper air record from Eureka, thereby placing our limited set of observations in the context of the last few decades, including the unusually warm 1950s.

The sedimentary record from Lake Tuborg, and the melt record from Agassiz Ice Cap ice cores (80°49'N; 72°54'W, only 60km from Lake Tuborg), both show that dramatic changes in summer temperatures have occurred over the last century. Sediment deposition in Lake Tuborg (as evidenced by varve thickness) closely parallels the Agassiz melt record of the last 300 years. Both records show how profoundly different the 20th century has been from earlier centuries. It is also interesting to note that both records also document the abrupt shift to cooler summers after 1963, as noted in some of our earlier work (Bradley and England, 1978a,b). Summer temperatures in the last 30 years have been closer to late 19th century levels than the unusually warm period in the mid-20th century. Both systems are controlled by summer temperature; Bradley (1975) showed that mass balance in the region was mainly a function of freezing level heights in July. We can also show that Eureka air temperatures at 900mb (above the near-surface inversion) and Tuborg varve thickness vary in parallel. It therefore seems clear that summer temperatures affect snowmelt and ice cap ablation (or, at higher elevations, the production of superimposed ice from surface melting) and the resulting runoff leads to increased sediment flux and (consequently) thicker varves in Lake Tuborg. It therefore seems probable that we can extend the record of summer temperatures from Lake Tuborg by recovering longer cores. Furthermore, by comparing the Tuborg varve thickness record with the Agassiz melt series, we can improve our confidence in the chronology of the sedimentary record. Together with independent dating methods (210Pb, luminescence, AMS 14C) we plan to build up a chronologically reliable time series of sediment deposition from this site. We will then use this site to prepare a detailed secular paleomagnetic record for the Canadian High Arctic which can be used as a chronological template for dating other records which are poor in organic carbon and thus poorly constrained by conventional 14C dating. This may then enable other high resolution paleoclimatic studies to be carried out in lakes where varved sediments are not found, or where the laminated facies are discontinuous (cf. Lamoureux and Bradley, 1996).

Publications

1. Bowman, T.E. and A. Long, 1968. 14C age of saline waters in Lake Tuborg Arctic, 21, 172-180.
   
2. Bradley, R.S., 1975: Equilibrium line altitudes, mass balance and freezing level heights in the Canadian High Arctic. Journal of Glaciology, 14, 267-274.
   
3. Bradley, R.S. and England, J., 1978a. Volcanic dust: influence on glacier mass balance at high latitudes. Nature, 271, 736-738.
   
4. Bradley, R.S. and England, J., 1978b. Recent climatic fluctuations of the Canadian High Arctic and their significance for glaciology. Arctic and Alpine Research, 10, 715-731.
   
5. Hattersley-Smith, G. and Serson, H., 1964, Stratified water of a glacial lake in northern Ellesmere Island. Arctic, v. 17, 108.
   
6. Lamoureux, S.F. and R.S. Bradley, 1996. A late Holocene varved sediment record of environmental change from northern Ellesmere Island, Canada. J. Paleolimnology, 16, 239-255.

1. Braun, C., 1997. Streamflow and sediment transport prediction in two High Arctic watersheds, Nunavut, Canada. MS thesis, Geosciences, University of Massachusetts, Amherst, 167 pp.
   
2. Braun, C. and Hardy, D., 1995. Climatic and lacustrine controls of varve formation, Canadian High Arctic: Paleotimes, 3, 10-11.
   
3. Braun, C., Hardy, D., and Bradley, R.S., 1996. Streamflow and Sediment Transport Prediction, Lake Tuborg, Ellesmere Island, Nunavut, Canada. Transactions, American Geophysical Union, v. 77, no. 46.
   
4. Smith, S., 1997. A record of environmental change derived from varved lakes along the margin of the Agassiz Ice Cap. MS thesis, Geosciences, University of Massachusetts, Amherst.
   

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