Tephra analysis (FMAZ II-1 tephra marker) on a network of ten marine sediment cores in the northern North Atlantic is presented as well as radiocarbon dates (14C) by Accelerator Mass Spectrometry (AMS) of the same layers. We use published data for the Tephra horizon´s identified in the cores together with eight already published 14C AMS dates. In two out of the ten cores we newly 14C AMS date. Here, we add radiocarbon dates to the existing ones from the FMAZ II-1 tephra layer from two key sites in the high-latitude North Atlantic Ocean: offshore southeast Greenland (GS16-204-18CC) and the southern Norwegian Sea (MD99-2284). The purpose of radiocarbon dating the volcanic ash layers in the marine sediments is to derive marine 14C reservoir ages (MRAs) from a spatial network of ten marine sediment cores for investigating past changes in the North Atlantic surface ocean circulation.
For the near-surface MRA reconstruction, approximately 1.5 mg of planktic foraminifera specimens (150-500 µm) in pristine condition were picked from the same depth level as the identified FMAZ II-1 tephra marker. The samples were then radiocarbon-dated using AMS 14C measurement procedures at ETH Zürich, Switzerland. There, the samples were processed using a newly developed method (Wacker et al., 2013) involving direct CO2 measurements of ~ 0.5 mg using an AMS facility equipped with a gas ion source. In addition, we performed leaching experiments on the sample surface material using HCl 0.02 M, following procedures in ref. (Hajdas et al., 2004). The AMS 14C dates from all sites were measured on the near-surface planktonic species Neogloboquadrina pachyderma (N. pachyderma) (calcification depth ∼30-200 m (Greco et al., 2019; Simstich et al., 2003)) permitting reconstruction of near-surface water mass properties.
We calculated the near-surface MRAs (in 14C years) as the difference between the measured planktonic (N. pachyderma) 14C age and the IntCal20 atmospheric 14C calibration curve (Reimer et al., 2020). The uncertainty of the reconstructed MRAs is calculated by combining the uncertainty (summation in quadrature method) related to (1) 14C analytical error, (2) GICC05 ice core age model error for FMAZ II-1 age, (3) IntCal20 calibration curve, and (4) transfer-function related to IntCal20 and GICC05 age offset (Adolphi et al., 2018). For two of the marine sediment cores, the AMS 14C date was not extracted from the exact same core depth as the FMAZ II-1 tephra layer. For sediment core LINK04, the spacing between the AMS 14C date and the position of the FMAZ II-1 tephra layer is 12 cm. Therefore, the AMS 14C date was adjusted using the correction originally provided by Wastegård et al. (2006).
References:
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