Episodic struvite deposits in a Northern Great Plains flyway lake: indicators of mid-Holocene drought?

A series of 34 layers of struvite (hydrous Mg-ammonium-phosphate) were deposited in laminated sediments of Kettle Lake, North Dakota, in the Northern Great Plains, USA. Sedimentologic, mineralogic and nitrogen isotopic evidence suggest that struvite was deposited during nutrient-enriched eutrophication events within a Mg-rich lacustrine environment. The struvite layers are dispersed between 4650 and 8700 cal. yr BP, with particularly high frequency between 8068 and 8700 cal. yr BP. The youngest struvite layer dates to 2734 cal. yr BP. Key features of the struvite-forming events were (a) relatively low water stage and consequent elevated water salinity associated with the dry mid Holocene, (b) disappearance of most lakes and wetlands in the region, and (c) focused, but brief, visitations by large populations of migratory waterfowl. The lack of more frequent occurrence of struvite is ascribed to the rarity with which this combination of conditions was achieved, in combination with difficulty of preservation for this salt. 15N isotopes in struvite (mean 7.51‰) are heavy relative to 15N in sediment and also extremely uniform over time. The isotopes are interpreted to reflect a waterfowl waste source without extensive NH4 volatilizaton. The timing of the struvite events does not closely correspond to century-scale mid-Holocene drought cycles inferred from oscillating aragonite concentrations, although there is a weak preferred occurrence in the humid phase of these cycles. Thus the struvite events are ephemeral prairie features of this generally arid period but tend not to occur at precisely the most arid intervals.