Effects of diagenesis on benthic and planktic foraminiferal radiocarbon ages in the Eurasian Basin (Arctic Ocean)

Jutta.Wollenburg [ at ] awi.de


Radiocarbon (14C) ages obtained from foraminifera calcite are used for paleoseawater reconstruction and carbon cycle dynamics for the past 30,000 years. Up to now, the effects of diagenesis on this proxy were poorly understood. The aim of this master thesis is to investigate if systematic differences in radiocarbon ages in pristine and diagenetic overprinted benthic and planktic arctic foraminifera exist. The foraminifera shells were obtained from the sediment core PS92/93-2 close to the Yermak Plateau in the Eurasian Basin. Optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis were used to check for diagenetic alteration. Authigenic calcite precipitation on planktic arctic foraminifera (N. pachyderma sin.) were observed in four intervals (between 15–230 cm bsf) and three types of overgrowth are defined for classification. Radiocarbon measurements are done on untreated pristine shells and shells with overgrowth. The results of this study demonstrate that secondary calcite overgrowth, precipitated on foraminifera shells, causes systematic younger 14C-ages. Leaching experiments are done on planktic foraminifera shells which show tall overgrowth crystals. With leaching of the outer shell and the measurement of the radiocarbon content of both the leachate and the leached sample it is possible to identify the effect of the overgrowth. The main outcome of the leaching experiment is that the leachates are composed of the 14C signal of the shell, 14C-enriched overgrowth, and adsorbed atmospheric CO2. The source of the overgrowth needs to be determined by further research. The stable isotopic composition of pristine foraminifera and foraminifera with overgrowth reveal no significant offset. It is likely that the calcite precipitated from 14C-enriched pore water.

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Haettig, K. (2020): Effects of diagenesis on benthic and planktic foraminiferal radiocarbon ages in the Eurasian Basin (Arctic Ocean) , Master thesis, Fachbereich Geowissenschaften Universität Bremen, Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung.

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