Climatic forcing on varve sedimentation regimes through the Holocene from Lake Nautajrvi, Southern Finland.

During the Holocene, the European high latitudes experienced substantial alterations in temperature and precipitation patterns. These changes were driven by a complex combination of factors, including the impacts of orbital and solar forcing on insolation, the dynamics of ocean-atmospheric circulation regimes, and various related feedback mechanisms. Sedimentation regimes in Boreal lakes can be sensitive to these climate changes and thus provide a valuable archive to better understand climatic forcing processes. Limitations in sampling resolution, chronological precision, and proxy sensitivity however have traditionally made continuous proxy-based palaeoclimatic reconstructions difficult to achieve through the Holocene, particularly prior to the instrumental period (c. <1800 AD). Annually laminated lacustrine or varved sediments offer significant potential to improve our understanding of sub-millennial to sub-decadal climate forcing mechanisms in sensitive regions such as the northern high latitudes. Here we present XRF core scanning records of a new sedimentary sequence from the dimictic Lake Nautajrvi, Southern Finland, which is continuously varved for the last 10 ka. Statistical exploration of the XRF data clearly distinguishes elements associated with detrital material (Ti, K, Si), redox conditions (Fe, Mn) and organic content ((ln(inc/coh)). Consequently, we will show how these signals can be used as proxies for different seasonal sedimentation regimes in the lake, including winter catchment snow accumulation, lake stratification induced benthic redox, and biogenic productivity during the growing season. Spectral analysis reveals statistically significant and non-stationary decadal to multicentennial cyclicity in lake sedimentation processes through the Holocene, which align to the known Gleissberg and Suess solar cycles and periodic variability in North Atlantic atmospheric circulation regimes. Changes in these frequencies throughout the Holocene, demonstrates that different dominant external forcing mechanisms affected lake stratification and sedimentation regimes. The clearest shift occurs during the mid-Holocene, between 6 to 5 ka BP, when lake sedimentation switches from predominantly solar-cycle forced changes in stratification to decadal-centennial scale cycles in detrital influx and biogenic productivity. These shifts correspond to variability in North Atlantic circulation regimes <5 ka BP and align with trends observed in other records, indicating increased variability in the North Atlantic atmospheric circulation patterns during the mid-Holocene. Such findings highlight the valuable potential that XRF core scanning records from clastic-biogenic varved records have for tracing these non-stationary climatic changes in the high latitudes of northern Europe.