CLIMATIC AND ENVIRONMENTAL FACTORS GOVERNING ALKENONE DISTRIBUTIONS AND UNSATURATION INDICES IN LACUSTRINE SEDIMENTS

Sedimentary alkenones reflect survival of intact biosynthetic products derived from a few haptophyte species defined as three phylotypes, Groups I, II, and III, that are associated with freshwater environments, saline or alkaline settings, and marine systems, respectively [1]. Subclades of Group II haptophytes have also been proposed [2], while both the timing of divergence of phylotypes I, II, and III are poorly defined [1,2,3]. Complexities in the origins of lacustrine alkenones parallel the proliferation of reports affirming their prevalence in diverse lake environments and expanding their geographical range, predominantly in the northern hemisphere [4]. The capability of alkenone unsaturation indices to record lake temperatures has also been proven, albeit typically restricted to localized settings [5]. However, alkenone distributions can also reflect temporal changes in contributions from Group I versus Group II phylotypes associated with fluctuations in salinity [6]. They may also derive from a Group II subclade favored during warmer seasons that doesn’t produce alkatetraenones [7]. Thus, alkenone compositions in lacustrine sediments can record fluctuations in haptophyte populations associated with changes in salinity or alkalinity rather than temperature [8], which may govern stratigraphic variations in unsaturation indices for lakes experiencing climatic shifts from warm/dry to cold/wet conditions [9]. A global compilation of alkenones in lake sediments [4] reveals expansive ranges for unsaturation indices in saline/alkaline systems from arid regions, likely reflecting variations in strains of alkenone-producing haptophytes in response to hydrological changes associated with alternating wetter/drier cycles driven by climate.

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