Seasonal formation of clastic-biogenic varves: the potential for palaeoenvironmental interpretations

Abstract

Clastic-biogenic varved lake sediments from Nautajärvi, Finland, covering the last ca. 10 000 years have previously been investigated with respect to varve chronology, laminae composition and thickness variations, palaeomagnetic secular variation, the pollen record and the diatom composition of selected sections. In this study, we provide detailed information on the seasonal sedimentation of different varve components and palaeolimnological indicators (diatoms, chrysophycaean cysts) using near-bottom sediment-trap monitoring. The monitoring results confirm previous interpretations of the formation of clastic-biogenic varves in Fennoscandian lakes. The results also indicate that seasonal sediment fluxes correspond with environmental changes. Higher snow storage in winter and the discharge intensity during the following spring snowmelt clearly increase the suspended sediment load, transportation and net accumulation of detrital mineral matter during spring and early summer. A prolonged autumn and subsequent freezing and thawing cycles in winter, conversely, result in an incremental but slow accumulation of assorted mineral matter and organic particles during winter, whereas the subsequent spring flooding and detrital sediment yield are diminished and thinner clastic laminae are formed. The accumulation rates of diatoms and chrysophyte cysts suggest that these algal groups are more dependent on seasonal processes (e.g. spring and autumnal overturn) than on rapid, short-lived environmental episodes such as the spring discharge peak.