Holocene paleoshorelines, water levels and submerged prehistoric site potential of Rice Lake (Ontario, Canada)

摘要

Rice Lake, located in the eastern Great Lakes of North America, has a high density of prehistoric (11-0.5 ka BP) terrestrial archaeological sites. It has been speculated that a large number of sites are submerged on the lakebed, as lake levels have risen >9 m since the arrival of Early Paleoindian peoples (～11 ka BP). In order to better understand the submerged landscape and its archaeological potential, a detailed bathymetric survey and sediment-coring program was conducted across a 30-km~2 area of northeastern Rice Lake. Changes in Holocene water levels and shoreline positions were reconstructed by integrating core data with a digital elevation and bathymetric model (DEBM) that accounted for differential isostatic uplift and basin sedimentation. The DEBM was used to generate a series of maps showing changes in the lake paleobathmetry and paleogeography and areas of prehistoric archaeological potential. Isostatic uplift of the eastern basin outlet (>30 m) had a dramatic influence on water levels and shoreline positions since the inception of Rice Lake (～12 ka BP). During the Early Paleoindian occupation phase (～11-10.5 ka BP), water levels were at a maximum lowstand (10 m bpl) and much of the present lakebed was an exposed lake plain with extensive wetlands. At the time of the Late Paleoindian/Early Archaic occupation (9.5-8.7 ka bp; 11,150-9560 cal. BP), the lake was about half its modern extent and during the second lowstand phase (<6000 cal. BP) water levels dropped to >4 m bpl and the lake became hydrologically closed. After 4000 cal. BP, water levels recovered and the lake approached its modern extents. An archaeological potential map based on the reconstructed paleoshorelines identified four areas with archaeological potential: these include drowned river mouths, submerged wetlands and an area of uplifted Early Holocene lakebed in northeast Rice Lake. The results illustrate that isostatically driven changes in lake paleobathymetry can be substantial and must be accounted for when attempting to reconstruct paleoshoreline positions. The approach can be applied more broadly to other basins within the Great Lakes and elsewhere to assess prehistoric site archaeological potential.