PALEOHYDROLOGICAL SIGNIFICANCE OF STRUVITE OCCURRENCE IN MID-HOLOCENE SEDIMENTS OF KETTLE LAKE, WESTERN NORTH DAKOTA

Kettle Lake, North Dakota, is today a small (2.2 ha), shallow (10 m) low-salinity "flow-through" lake within an esker containing >19 meters of Holocene marl with either authigenic aragonite or calcite plus a detrital allogenic suite. The lake has been meromictic through much or all of the Holocene and the sediments are clearly laminated throughout the core. In places, summer layers of nearly pure aragonite can be discriminated from organic winter layers, but no varve chronology may be established due to slumping, laminar discontinuity, and other factors. The marl from 704-1660 cm sediment depth (2733-8735 ¹⁴C years B.P., based on charcoal dates) is punctuated by approximately 19 layers of clear tabular struvite (NH₄MgPO₄·6H₂O). The highest frequency of struvite layers (13) is between 1166-1353 cm, from 6036-7290 ¹⁴ C years B.P. X-ray diffraction and SEM analysis indicate the struvite is a relatively pure phase occuring only as laminae. ¹⁵N of the struvite (n=16) shows little variance (7.56 ^o/_oo ± 0.74). The mineral is interpreted to be fecal in origin with minimal ¹⁵N enrichment due to ammonium volatilization. The mean ¹⁵N value is similar to published means for NGP Chironomidae, Lepidopterae, and Odonata populations, as well as to guano deposits of seabird colonies. Struvite occurrence appears to vary in timing, covarying most frequently with peaks in authigenic aragonite concentration that suggest relatively humid intervals within short-term climatic cycles. Based on the ¹⁵N evidence, we interpret the struvite as avian guano, deposited intermittently during individual years in the mid-Holocene, particularly from 6000-7300 B.P. We believe it is a regional drought indicator that reflects waterfowl habitat constriction in a period when depressed lake and groundwater levels caused nearly all nearby lakes to be dry and/or deflated. That this salt would recur at Kettle Lake is thought to relate primarily to its greater depth than nearly all lakes in the region, but may also relate to persistent meromixis favoring preservation, low lake volume and salinity, and/or avian whimsy.