Biases in radiocarbon dating of organic fractions in sediments from meromictic and seasonally hypoxic lakes

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Ojala , A , Saarnisto , M , Jungner , H , Snowball , I & Muscheler , R 2019 , ' Biases in radiocarbon dating of organic fractions in sediments from meromictic and seasonally hypoxic lakes ' , Bulletin of the Geological Society of Finland , vol. 91 , no. 2 , pp. 221-235 . https://doi.org/10.17741/bgsf/91.2.004

Title: Biases in radiocarbon dating of organic fractions in sediments from meromictic and seasonally hypoxic lakes
Author: Ojala, Antti; Saarnisto, Matti; Jungner, Högne; Snowball, Ian; Muscheler, Raimund
Contributor: University of Helsinki, Natural Sciences Unit
Date: 2019
Language: eng
Number of pages: 15
Belongs to series: Bulletin of the Geological Society of Finland
ISSN: 0367-5211
URI: http://hdl.handle.net/10138/308675
Abstract: We present here radiocarbon dating results from two boreal lakes in Finland, which are permanently (meromictic) or seasonally stratified and contain continuous sequences of annually laminated sediments that started to form in the early Holocene. The radiocarbon dating results of different organic components were compared with the varve-based sediment chronologies. The deviation between the Lake Valkiajarvi varve chronology (8400 varve years 2-3% error estimate) and 33 C-14 dates taken from insoluble and soluble organic phases vary inconsistently throughout the Holocene. In extreme cases mean calibrated radiocarbon dates with 95.4% confidence levels (2 sigma) are -2350 and +2040 years offset when compared with the varve chronology. On average, the radiocarbon dates are offset by ca. +550 years. The deviation between the Lake Nautajarvi varve chronology (9898 varve years +/- 1% error estimate) and 26 C-14 dates analyzed with conventional and AMS methods indicates that radiocarbon dates are systematically older by 500-1300 years (about 900 years on average). This significant offset mean that radiocarbon dates obtained from organic bulk sediment of meromictic and seasonally hypoxic lakes must be cautiously interpreted because of the reservoir effect and carbon cycling at the sediment-water interface. Direct evidence was obtained from the dating of soluble fraction and insoluble organic matter from near bottom water in the monimolimnion of Lake Valkiajarvi, which yielded C-14 ages of 560 +/- 80 BP and 2070 +/- 140 BP, respectively. Our study reinforces previous results that age-depth models based on bulk sediment radiocarbon dates obtained on sediments of stratified lakes are of limited value for accurate dating of changes in land use and especially the commence of agriculture.
Subject: 1171 Geosciences
lake sediment
dating
radiocarbon
varves
anoxia
Finland
ANNUALLY LAMINATED SEDIMENTS
VARVE CHRONOLOGY
PEAT SAMPLES
S FINLAND
C-14
PALEOECOLOGY
CULTIVATION
AHVENAINEN
RESOLUTION
HISTORY
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