The records synthesized here confirm that lakes across mid-latitude North America have fluctuated on a wide range of time scales despite their locations in humid or snow-dominated regions. Their fluctuations indicate multi-millennial to centennial variations in the moisture gradient across semi-arid to humid mid-latitude North America during the Holocene when external climate forcing was modest and monotonic. At face value, they imply three major geographic patterns of hydrologic change during the Holocene (Fig. 3).
First, effective moisture increased over much of the region since the last glacial period. Late-Pleistocene climates may have been arid across much of the region as most lakes in both the eastern and western clusters were low from 15-11 ka (Fig. 1-2). In addition to effects related to the ice sheets, the change may relate to long-term warming driven by greenhouse gas forcing, as well as winter insolation anomalies (Liu et al. 2014; Bova et al. 2021), which could have increased the water vapor available to storms, particularly in winter. Any increase in water vapor was likely then amplified by insolation-driven changes in the tropics-to-polar thermal gradient that favored increased cyclogenesis and therefore production of precipitation from water vapor over the course of the Holocene (Routson et al. 2019).
Second, the east-west gradient has not been stable, but varied in strength on multi-millennial time scales (Fig. 2-3). A rise in the level of western lakes marks the beginning of the Holocene and represents a reduced gradient before 8 ka compared to today (Fig. 2). Other mid-continental moisture records also provide evidence of high effective moisture in the early Holocene (prior to the mid-Holocene arid period there), such as represented by dust deposition at Elk Lake, Minnesota (Dean 1997)(Fig. 5), diatom records from sites such as Moon Lake, South Dakota (Laird et al. 1996), and numerous fossil pollen records (Bartlein et al. 1984; Bartlein and Whitlock 1993; Grimm 2001; Nelson and Hu 2008; Williams et al. 2010; Grimm et al. 2011).
Rocky Mountain lake levels likely rose at 11 ka in response to changes in moisture delivery caused by the acceleration of Atlantic Meridional Overturning Circulation at the end of the Younger Dryas (McManus et al. 2004; Zhang and Delworth 2005), but the decline in western water-levels by ca. 8 ka coincides with the diminished influence of the Laurentide ice sheet (Fig. 5). Prior to 8 ka, the presence of a large, intact Laurentide Ice Sheet and its associated glacial anti-cyclone reduced the influence of the sub-tropical high and advection of moisture in eastern North America (Shuman et al. 2002), while enhancing moisture delivery to mid-latitude central and western North America (Williams et al. 2010; Oster et al. 2015; Lora et al. 2017; Morrill et al. 2018). The effects would have favored the weak early-Holocene gradient, but after 8 ka, eastern lakes rose, western lakes fell, and the east-west gradient appears to have become steeper than today (Fig. 2).
Subsequent weakening of the gradient re-developed from 5.5-4.5 before it returned to a near modern difference (Fig. 2-4). The weak gradient (wet west-dry east) coincided on millennial time scales with cool mid-continent summers whereas warm continental summers correlated with a strong gradient (dry west-wet east) at other times in the Holocene (Fig. 5). The relationship could relate to reinforcing changes between precipitation, soil moisture, and the resulting effects of latent heating on air temperatures (Oglesby and Erickson 1989; Notaro and Zarrin 2011), but potentially also arose from the overarching atmospheric and ocean circulation changes (O’Brien et al. 1995; Oppo et al. 2003), which could have produced phases of mid-Holocene cooling in central North America (Muschitiello et al. 2015) in conjunction with regional moistening (Kelly et al. 2018).
Third, the latitudinal distribution of moisture along the Rocky Mountains also varied on multi-millennial timescales (Fig. 2-3). Enhanced aridity in the mid-Holocene affected Wyoming from ca. 9-5.5 ka when it extended from the Great Basin to the northern Great Plains (Mock and Brunelle-Daines 1999; Hermann et al. 2018; Liefert and Shuman 2020). Fossil pollen indicate an eastward expansion of prairie in Minnesota (Nelson and Hu 2008) and aeolian datasets indicate increased dune and loess activity in the northern Great Plains (Dean 1997; Miao et al. 2007b)(Fig. 5). Northern Rocky Mountain forest composition during this time also shifted in favor of xeric taxa (Whitlock 1993) and western forest-steppe ecotones shifted as areas of grasslands expanded (Macdonald 1989; Alt et al. 2018).
In Colorado, maximum aridity occurred later from 8-2 ka (Fig. 2-3). The north-south difference in the timing of the Rocky Mountain aridity could represent long-term shifts in winter precipitation patterns similar to those associated with interannual to multidecadal variations over the Pacific Ocean (Wise 2010; Pederson et al. 2011). However, the difference between PC2 and PC3 (Fig. 3) indicates that the western lakes were influenced by multiple dynamics. Both regions experienced at least a modest reduction in the aridity at ca. 5.5-4.5 ka, although the wet millennial period appears to have been drier than today in the south (e.g., Upper Big Creek and Emerald lakes).
The increase in western water levels at ca. 5.5 ka may relate to large-scale hemispheric phenomena (Fig. 6-7), connecting African and North American changes in a manner consistent with simulations (Muschitiello et al. 2015; Kelly et al. 2018; Sun et al. 2019). The change coincided with an apparent shift in European and North American mean temperatures and in sea-surface temperatures in the western North Atlantic (Fig. 6)(Kim et al. 2007; Sachs 2007; Marsicek et al. 2018). The change also correlates with other evidence of a non-linear transition in the mid-Holocene over portions of the North Atlantic from Iceland (Larsen et al. 2012).
“Green Sahara” simulations show that the synoptic consequences of a rapid end to the African humid period in the mid-Holocene could have propagated northward to the Arctic via the weakened northern mid-latitude westerlies (Muschitiello et al. 2015; Kelly et al. 2018)(Fig. 7). As anticipated by simulations (Hopcroft and Valdes 2019), dust deposition downwind of the Great Plains declined rapidly as it increased off the coast of west Africa (Fig. 6)(Bradbury and Dean 1993; Dean 1997; deMenocal et al. 2000a; McGee et al. 2013). The changes in the dust loading may be a critical feedback (Pausata et al. 2016; Messori et al. 2019) when foraminifera assemblages in the upwelling region off west Africa also shifted abruptly, consistent with a change in the African monsoon (deMenocal et al. 2000b), which would have provided the thermodynamic mechanism for displacing atmospheric circulation over the Atlantic sub-tropics and enhancing moisture delivery to North America (Kelly et al. 2018).
Consistent with such dynamics, the belt of sub-tropical moisture over west Africa shifted southward at ca. 5.5 ka, rapidly increasing effective moisture in a step-wise reversal of precession-forced drying trends in Ghana (Shanahan et al. 2015) while rapidly accelerating them elsewhere in north Africa (deMenocal and Tierney 2012; Tierney and deMenocal 2013). Abrupt cooling in the Atlantic may have favored the southward shift (Fig. 6)(Kim et al. 2007; Sachs 2007), although it may have coincided with a net warming of adjacent continents (Fig. 6)(Marsicek et al. 2018). Even though the African changes have important spatial differences (Claussen et al. 2017), the rapid increase in effective precipitation at Lake Bosumtwi, Ghana from 5.8-5.2 ka (Shanahan et al. 2015) is synchronous with the rapid drying in the Sahel indicated by increasing dust deposition off west Africa at 5.6-5.1 ka in ODP core 658c (Fig. 8)(deMenocal et al. 2000a). Drying in portions of the Sahel (but not synchronously across the Sahara; Kropelin et al. 2008) parallels modern precipitation correlations associated with shifts in Atlantic pressure fields that increase in precipitation elsewhere, including the Rocky Mountains (Fig. 7A)(Landsea and Gray 1992; Zhang and Delworth 2006; Wang et al. 2012).
The Rocky Mountains have some of the highest correlations with Sahelian precipitation outside of Africa, especially in the northern hemisphere summer months (Fig. 7A). Reduced precipitation in the Sahel tends to coincide with increased African surface pressures and increased 500 mb geopotential heights (Fig. 7B) just as expected in response to a reduction in the African monsoon in response to precessional forcing (Claussen et al. 2017; Kelly et al. 2018). Easterly (dust laden) winds from the Sahel are then enhanced, while mid-latitude westerly zonal velocities decline between central North America and western Europe (Fig. 7C). Specific humidity increases over the Americas (Fig. 7D) as water vapor is advected northward into (Fig. 7F) and not eastward away from (Fig. 7C) the North American mid-continent where uplift produces precipitation (Fig. 7E). The changes in surface heating in Africa, thus, propagate to North America through dynamics linked to the North Atlantic Subtropical High, which have consequences for the east-west moisture gradient and have analogs in mid-Holocene simulations (Kelly et al. 2018).
If the change at 5.5 ka represents a step-shift in Holocene climates that connected Africa, the North Atlantic, and North America (Fig. 6)(deMenocal et al. 2000a; Marsicek et al. 2018), it also marks the beginning of the anomalous millennium when the east-west moisture gradient was reduced (Fig. 3-4). The period from 5.5-4.5 ka appears as a prominent millennial anomaly in many regions of mid-latitude North America (Fig. 5)(Shuman, submitted) and more broadly in the northern hemisphere (Magny and Haas 2004; Mayewski et al. 2004; Marsicek et al. 2018; Helama et al. 2021). At the time, inland lakes (Blanding, Davis) in the northeast were low, but coastal lakes (Deep, New Long) rose (Fig. 1)(Newby et al. 2014; Shuman and Burrell 2017). In the Great Plains, loess activity declined at 5.5-5.2 ka, remained low for >300 years, and then increased after 4.9-4.5 ka (Fig. 6)(Dean 1997). A prominent Ab soil horizon amid the Holocene Bignell Loess in the western Great Plains dates to the same wet millennial period (Miao et al. 2007a) when prairie pollen records mark a millennial decline in Ambrosia (ragweed) pollen (Grimm 2001) and diatoms indicate a millennial freshening of Moon Lake, North Dakota (Laird et al. 1996). Further west, submerged tree stumps in Lake Tahoe, California, document aridity and low water from 5.75-4.44 ka (Lindstrom 1990; Benson et al. 2002). Similar mid-Holocene millennial anomalies exist in Africa (Thompson et al. 2002; Berke et al. 2012).
Potentially, as indicated by combinations of precessional trends and the millennial east-west changes (Fig. 4), the apparent step change at 5.5 ka could represent the interference of long-term Holocene trends and a prominent millennial-scale variation (Fig. 8). In some areas, such as in the northern Rocky Mountains (Fig. 4B) and the Sahel as recorded by ODP658c (Fig. 8A-B), the two changes constructively interfere to accelerate long-term trends, but in other areas, such as Elk Lake, Minnesota (Fig. 5) and Lake Bosumtwi, Ghana (Fig. 8A), the two patterns may have negatively interfered to produce a millennial oscillation (Fig. 5). The outcome depends upon the relative magnitudes and signs of the trends and millennial fluctuation, such that negative interference could appear as a delayed abrupt shift like recorded at ca. 4.5 ka in northeastern lakes (Fig. 4A), in eastern North American speleothems (Hardt et al. 2010), or potentially in southern hemisphere regions with opposing insolation trends (Cruz et al. 2009). The timing can also depend on interactions with other patterns of variability such as reconstructed over the northwest Atlantic and observed in the northeastern coastal lakes (Fig. 1)(Shuman, submitted; Shuman et al. 2019).
Whether the two phenomena (step change and millennial oscillation) are linked is important because a step-shift spanning from Africa to North America could relate to threshold effects related to surface-atmosphere or dust feedbacks (Claussen et al. 1999; Pausata et al. 2016), but such feedbacks are unlikely to have produced (although they could amplify) a millennial-scale climatic fluctuation. Anomalous conditions from ca. 5.5-4.5 ka could relate to either external volcanic or solar forcing (Hernández et al. 2020; Helama et al. 2021) or internal variability, such as in the North Atlantic (Oppo et al. 2003; Thornalley et al. 2009). The interference with orbital and ice volume trends differs, however, from that observed during abrupt Pleistocene events associated with North Atlantic overturning such as the Younger Dryas (YD) because the east-west gradient in North America decreased after the YD and increased after the anomalous millennium from 5.5-4.5 ka (Fig. 8C). In Africa, Ghana and the Sahel appear to have changed in concert during the YD, but in opposition at 5.5 ka (Fig. 8A).