marine diatoms provide biostratigraphic evidence for tsunami inundation
at Bradley Lake, a small freshwater lake on the south-central Oregon coast.
During the past 7,200 years, fine-grained lacustrine deposits in the deep
axis of the lake were disturbed 17 times by the erosion and emplacement
of coarse-grained gyttja and, in some cases, sand. By identifying diatoms
in closely spaced core samples, we determined that 13 of the 17 events
(termed ìdisturbance eventsî) record prehistoric tsunamis
in Bradley Lake. We consider the evidence strong for 11 events, based
on numbers and diversity of marine taxa: De1, De2, De4, De5, De6, De7,
De8, De11, De12, De13, and De17. The evidence is less compelling for an
additional 2 events (De9 and De10), although tsunami inundation is likely.
Finally, we identified 4 events (De3, De14, De15 and De16) in which there
were no marine diatoms to support tsunami inundation, although stratigraphic
data shows that the lake bottom was disturbed.
Freshwater diatoms dominate throughout the Bradley Lake record, showing that the lake has remained a freshwater habitat throughout its existence. However, anomalous occurrences of three species of brackish diatoms (Thalassiosira bramaputrae, Cyclotella meneghiniana, and Mastogloia smithii) may be evidence for short-lived periods of slightly elevated salinities in the lake following De16, De13, De12, De11, De9, De8, and De5. With the exception of De12, increased abundances of one or more of the brackish species is coincident with decreased numbers of freshwater diatoms. A temporary rise in salinity, as evidenced by short-lived increases in abundances of brackish species and decreases in abundances of freshwater species, is consistent with tsunami inundation into the lake.
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