MILLENNIAL-SCALE CYCLES OF COASTAL DUNE FORMATION DURING THE LATE HOLOCENE, LAKE MICHIGAN

Published OSL (n=107) and ¹⁴C (n=123) assays from the northern and eastern shores of Lake Michigan indicate that coastal dunes were constructed during six episodes that were identified through a Probability Density Distribution (PDD) of the OSL ages. PDD peaks mark times when dunes were more active. PDD lows represent intervals of dune stability. OSL PDD peaks are cyclical on millennial scale (5.5, 4.3, 3.3, 2, 1, and 0.3 ka). The ¹⁴C ages were collected from paleosols and archaeological sites stratified within dunes and mark intervals of dune stability. A PDD of ¹⁴C ages was also created. OSL PDD peaks mark times of relative dune stability and should be inversely associated with lows in ¹⁴C PDD if the coastal dune system is generally regionally consistent. Comparing both PDDs shows that ¹⁴C PDD peaks only occur after peaks (or within lows) of the OSL PDD, which indicates Lake Michigan coastal dunes are regionally consistent.

The OSL and ¹⁴C PDDs were also compared to reconstructed middle and late Holocene lake-level hydrographs and to a composite 7000-year-long continous record of El Niño events from coastal South America. These data provide clues about what drives the millennial-scaled cycles of coastal dunes along Lake Michigan. Three of the dune-building events are associated with significant rising lake level (transgressive) events (5.5, 3.3, and 2.3 ka) while others occurred during regressive (4.3 and 0.5 ka) or uncertain lake level events. Although likely important, the specific connection between water level change and coastal dune building is more complex than just “transgressive or regressive.”

The link between El Niño events and dune activity is clearer. Comparison of OSL and ¹⁴C PDD with the El Niño record shows that dunes grew during intervals when El Nino events were uncommon (i.e., <5 events/century). If these correlate with more La Nina events, then increased storminess played a role in dune formation. Conversely, intervals of dune stability and soil formation occur during intervals when El Niño events are more common (i.e., >10-15 events/century) and have greater oscillation. Collectively, these data suggest that dunes are built or stabilized when a complex set of climate, water-level, and sand-supply factors intersect within the coastal zones to create the proper conditions to construct dunes.