Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates

Identification of the sources of organic matter in sedimentary records prov ides important paleolimnologic information. As the types and abundances of plant life in and around lakes change, the composition and amount of organi c matter delivered to lake sediments changes. Despite the extensive early d iagenetic losses of organic matter in general and of some of its important biomarker compounds in particular, bulk identifiers of organic matter sourc es appear to undergo minimal alteration after sedimentation. Age-related ch anges in the elemental, isotopic, and petrographic compositions of bulk sed imentary organic matter therefore preserve evidence of past environmental c hanges. We review different bulk organic matter proxies of climate change in tropic al and temperate sedimentary records ranging in age from 10-500 ka. Times o f wetter climate result in enhanced algal productivity in lakes as a conseq uence of greater wash-in of soil nutrients, and these periods are recorded as elevated Rock-Eval hydrogen indices, lowered organic C/N ratios, less ne gative organic delta(13)C values, and increased organic carbon mass accumul ation rates. Lowering of lake water levels, which typically depresses algal productivity, can also cause an apparent increase in organic carbon mass a ccumulation rates through suspension of sediments from lake margins and red eposition in deeper basins. Alternations between C-3 and C-4 watershed plan ts accompany climate changes such as glacial/interglacial transitions and w et/dry cycles, and these changes in land-plant types are evident in delta(1 3)C values of organic matter in lake sediments. Changes in climate-driven h ydrologic balances of lakes are recorded in delta D values of sedimentary o rganic matter. Visual microscopic examination of organic matter detritus is particularly useful in identifying changes in bulk organic matter delivery to lake sediments and therefore is important as an indicator of climate ch anges.