Journal of Paleolimnology 17: 23–31, 1997. 23 c 1997 Kluwer Academic Publishers. Printed in Belgium. A Holocene paleosalinity diatom record from southwestern Saskatchewan, Canada: Harris Lake revisited Susan E. Wilson 1 , John P. Smol 1 & David J. Sauchyn 2 1 Paleoecological Environmental Assessment and Research Laboratory (PEARL), Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6 2 Department of Geography, The University of Regina, Regina, Saskatchewan, Canada S4S OA2 Received 29 August 1995; accepted 3 January 1996 Key words: diatoms, salinity, climatic change, Holocene, Cypress Hills, Saskatchewan Abstract Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom- salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh ( 0.5 g l 1 ) throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake’s history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well. Introduction Lakes in the southern Interior Plains of western Canada remain largely unexplored with respect to the paleoen- vironmental and paleoclimatic information contained in their stratigraphic sequences. This is partially due to the scarcity of suitable study lakes and to problems involved in retrieving cores from lakes with cement- ed or desiccated horizons in the sediment sequence (Vance et al., 1992). Of those lakes studied, few contain continuous lake sediment records that span the entire This is the third in a series of papers published on Paleolimnol- ogy in the Great Plains of North America. The papers were select- ed from oral and poster presentations made at the Theme Session: ‘Modern & Ancient Lake Environments of the Great Plains’ held at the Geological Society of America meeting in Lincoln, Nebraska, April 27–28, 1995. These papers were collected by D. C. Gos- selin and W. M. Last. Additional manuscripts submitted as part of this Theme Session will appear in future issues of Journal of Paleolimnology. Holocene. As such, this important agricultural region – one of the bread baskets of the world – remains one of the least understood regions of Canada with respect to Holocene environmental history (Ritchie, 1985). Harris Lake, in the Cypress Hills of southwest- ern Saskatchewan, contains one of the few con- tinuous sediment records from the southern Interi- or Plains that spans most of the Holocene. Previous studies on the pollen (Sauchyn & Sauchyn, 1991; Sauchyn, 1990), sedimentology and mineralogy (Last & Sauchyn, 1993), and ostracods (Porter, 1993) of Harris Lake suggest that the region was affected by a climatic change around 5000 years BP when presum- ably drier and warmer early to mid-Holocene condi- tions gave way to a cooler, moister climate. Sauchyn & Sauchyn (1991) propose a date of 7700–5000 years BP for the altithermal based on pollen and organic matter data, while Porter (1993) suggests the interval