Regional and local changes inferred from lacustrine organic matter deposited between the Late Glacial and mid-Holocene in the Skaliska Basin (north-eastern Poland)

Abstract

The article presents the results of a combination of isotopic (δ15N and δ13C), geochemical (TOC, TN, C/N) and palaeobotanical investigations. They were carried out on organic matter (OM) that had been deposited from the Younger Dryas to the Subboreal chronozone in palaolake sediments (north-eastern Poland). OM from the late Younger Dryas consists only a small amount of TOC and TN, which together with the lack of aquatic macrophyte macrofossils confirm low primary production in the then reservoir. The higher values of C/N ratio (18–27) and δ13C (ca. −26‰) suggest that OM was mainly terrestrial in origin. During the Preboreal chronozone (11,450–10,150 cal. BP), there was a probable deepening of the reservoir through the melting of dead ice. This might have been intensified by an increase in precipitation and a fall in evapotranspiration during the ‘Preboreal oscillation’. From the early Preboreal chronozone (ca. 11,345 cal. BP), a mixed source of OM is indicated by C/N ratio (from 8 to 13). At the same time significant increase in TOC and TN probably registered a rise in primary production in the palaeolake. From the older part of the Boreal chronozone (ca. 9865 cal. BP) algae productivity increased (the C/N ratio dropped to ca. 8 and was then constant). At the beginning of the Atlantic chronozone (ca. 8650 cal. BP) a probable increase in the water level caused the partial relocation of OM from the reed swamp belt and/or the adjacent alder carr into deeper sections of the palaeolake (an increase in TOC and TN). In the late Atlantic chronozone (ca. 6965–6650 cal. BP), TOC and TN decreased while δ13C had high values, which probably reflected a more distinct stratification of the water column with anoxic bottom waters. Next, in the late Atlantic and early Subboreal chronozone (ca. 6650–5205 cal. BP) the amount of OM in palaeolake sediments was still rising. The δ13C values dropped, suggesting an intensification of processes of water mixing. These processes were simultaneous with the constant shallowing of the palaeolake and the disappearance of habitats suitable for submerged macrophytes. The topmost part of the profile is characteristic of a strong decomposition of peat, causing a release of carbon dioxide and therefore the depletion of the carbon pool in the peat (a drop in the C/N ratio). The concurrent upward trend of the δ15N curve is probably the effect of a greater anthropogenic impact on the palaeolake/mire and its catchment and/or more recent human interference in the topmost peat layers.