A DIATOM-BASED RECONSTRUCTION OF DRAFT INTENSITY, DURATION, AND FREQUENCY FROM MOON LAKE, NORTH-DAKOTA - A SUB-DECADAL RECORD OF THE LAST 2300 YEARS

Diatom assemblages preserved in sediment cores from closed-basin lakes can provide high-resolution records of past hydrologic and climatic c onditions, including long-term patterns in the intensity, duration, an d frequency of droughts. At Moon Lake, a closed-basin lake in eastern North Dakota, a comparison of diatom-inferred salinity and the precipi tation-based Bhalme-Mooley Drought Index (BMDI) over the last 100 year s was highly significant, suggesting that the diatom record contains a sensitive archive of past climatic conditions. A sub-decadal record o f inferred salinity for the past 2300 years indicates that extreme dro ughts of greater intensity than those during the 1930s 'Dust Bowl' wer e more frequent prior to A.D. 1200. This high frequency of extreme dro ughts persisted for centuries and was most pronounced from A.D. 200-37 0, A.D. 700-850 and A.D. 1000-1200. A pronounced shift to generally we tter conditions with less severe droughts of shorter duration occured at A.D. 1200. This abrupt change coincided with the end of the 'Mediev al Warm Period' (A.D. 1000-1200) and the onset of the 'Little Ice Age' (A.D.1300-1850).