Abstract
The well characterized oxygen-isotopic fractionation during cellulose biosynthesis has been utilised by numerous studies of stable isotopes in fine-grained aquatic cellulose. We measured the δ13Ccellulose and δ18Ocellulose values of bulk cellulose and moss fragments from an ∼11.4ka-long core obtained from a shallow, productive, spring-fed, hardwater lake, Äntu Sinijärv, Estonia (59˚3.8′N; 26˚14.5′E; 94.6 m a.s.l.; maximum depth 7.3 m), in order to reconstruct regional Holocene climate and lake-basin evolution. Isotopically, the modern waterbody is a well-behaved, open, hydrological system with negligible evaporative effects. Cellulose-isotope records were compared with down-core measurements of loss-on-ignition (LOI), carbonate and mineral contents, total organic carbon (TOC), total nitrogen (TN), C/N ratio, δ13CTOC, biomarker indices (Palg and Paq), published palaeoecological data and a δ18Ocarbonate record from the same palaeolake. Green microalgae, freshwater macroalgae (Chara) and aquatic bryophytes were important sources of sedimentary cellulose during different phases in the environmental history of the lake. Although a strong palaeoclimatic imprint can be detected in the δ18Ocellulose record from Äntu Sinijärv, notably the Preboreal oscillation, the 8.2ka event and an unnamed cold oscillation ∼3.25ka BP, the isotopic signal of these events may have been amplified by increases in 18O-depleted spring snowmelt. In contrast, δ13Ccellulose was tightly coupled to the Holocene evolution of terrestrial ecosystems and soils by significant inputs of biogenic carbon from the catchment and sublacustrine springs. During the early Holocene, ∼11 – 9ka BP, the δ18Ocellulose and δ18Ocarbonate records diverge markedly, which can be attributed to “no-analogue” seasonal, climatic, hydrological and isotopic conditions resulting from orbital forcing and residual ice-sheet impacts.
Highlights
This study has demonstrated that with a soundly based chronology, major abrupt palaeoclimatic events can be identified within a century or so of their accepted dates of occurrence, their isotopic magnitude and estimated palaeotemperature implications may be exaggerated in this sequence due to past departures from observed seasonal, hydrological and isotopic behaviour resulting from orbital forcing and changing climatic-boundary conditions
With significant effort, this study has identified past changes in major cellulose sources with the aid of C/N ratios and biomarker indices calculated from GC-MS analyses of the solvent extracts remaining after cellulose preparation, together with several types of palaeoecological data that are available for earlier cores from the same lake
Shallow hardwater lakes have significant advantages for cellulose-isotope analysis, including biodiverse ecosystems that produce abundant cellulose, which can be analysed in parallel with lacustrine carbonates in order to achieve a more multidimensional perspective on past limnological changes
Summary
We present stable-isotope records from the organic fraction of lacustrine marls from Lake A€ntu Sinija€rv, Estonia, in order to investigate regional climatic and lake-basin evolution through the course of the Holocene. Use of both cellulose d18O and d13C in lake-. F.A. Street-Perrott et al / Quaternary Science Reviews 193 (2018) 68e83 reconstruction relies on the uniformitarian assumptions that: 1) A€ntu Sinija€rv was always a well-mixed, open, groundwater-fed lake with a short residence time and minimal evaporation; and 2) present-day isotopic relationships are applicable throughout the Holocene. The contrasting isotopic records from these two materials raise important questions about the stability of isotopic relationships on millennial to multi-millennial time scales
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