Middle Holocene dry climate caused by change in atmospheric circulation patterns: Evidence from lake levels and stable isotopes

Paleoclimatic studies with multiple proxy data have increasingly improved our understanding of Holocene climate. A dry and warm climate in the early and middle Holocene has been documented by numerous studies in eastern North America (T. Webb et al., 1983, 1993; COHMAP Members, 1988; Baker et al., 1992; R. Webb et al., 1993). Recent studies show that the classic dry and warm prairie period in midwestern North America was time transgressive (Baker et al., 1992; Wright, 1992); it occurred between ca. 8 and 5 ka in Minnesota (McAndrews, 1966; Webb et al., 1983) and from 5.5 to 3 ka in eastern Iowa, southern Wisconsin (Winkler et al., 1986; Baker et al., 1992), and southern Michigan (Manny et al., 1978). In southern Ontario, several sites show that lake levels were low during the late middle Holocene (ca. 6–2 ka), as interpreted from depositional hiatuses, low sedimentation rates, lithologic changes, and diatom species distribution (see Yu, 1995). The low lake levels reflect decreased effective moisture caused by a dry and warm climate (Yu and McAndrews, 1994; Yu, 1995). In contrast, stable isotopes from fossil wood and marl sediment indicate a dry and warm early middle Holocene climate at 7.4–5.8 ka and a very moist climate at 5.8–1.5 ka (Edwards and Fritz, 1988). Therefore, there is a significant discrepancy in timing for the middle Holocene dry period in southern Ontario, as interpreted from lake levels and stable isotope data. Here we present multiple proxy evidence for low lake levels from five cores at Crawford Lake, southern Ontario.1 The combined records of interpreted lake levels and carbonate isotopic data are linked to moisture regime and atmospheric circulation. We show that (1) low lake level was caused by decreased effective moisture, (2) the