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Atlantic inflow into the southern Nordic Seas at the onset of the LGM promotes open-ocean conditions and Fennoscandian Ice Sheet growth
The Atlantic water inflow into the Nordic Seas has proven difficult to reconstruct for the Last Glacial Maximum (LGM). At that time, the Fennoscandian Ice Sheet grew potentially to its maximum extent. Sea-ice free conditions in the eastern Nordic Seas have been proposed as an essential moisture source contributing to this build-up. It has been hypothesized that the inflow of warm and saline Atlantic surface waters was important for maintaining these seasonally sea-ice free conditions in the Nordic Seas at that time. However, the difference between a perennially frozen ocean and a seasonally open ocean on ice sheet build-up remains unquantified. Here we use, tephra-constrained surface ventilation ages from a network of marine sediment cores and model experiments, to show that Atlantic inflow to the southern Nordic Seas likely occurred predominately via the Iceland-Faroe Atlantic inflow pathway helping to maintain seasonal open waters at the onset of the LGM. Using a numerical snow model, we further demonstrate that such open-ocean conditions may have been a factor contributing to the Fennoscandian Ice Sheet growth with a ~150% increase in surface mass balance over Norwegian coastal areas, compared to sea-ice covered conditions.
Nordic Seas, Irminger Sea, marine radiocarbon dates, Tephra, marine reservoir age
70.12195N, 50.976765S, 10.079068E, -40.020688W
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).
Adolphi, F., Ramsey, C.B., Erhardt, T., Edwards, R.L., Cheng, H., Turney, C.S.M., Cooper, A., Svensson, A., Rasmussen, S.O., Fischer, H., Muscheler, R., 2018. Connecting the Greenland ice-core and U∕Th timescales via cosmogenic radionuclides: testing the synchroneity of Dansgaard–Oeschger events. Climate of the Past 14, 1755-1781.
Greco, M., Jonkers, L., Kretschmer, K., Bijma, J., Kucera, M., 2019. Depth habitat of the planktonic foraminifera Neogloboquadrina pachyderma in the northern high latitudes explained by sea-ice and chlorophyll concentrations. Biogeosciences 16, 3425.
Hajdas, I., Bonani, G., Herrgesell Zimmerman, S., Mendelson, M., Hemming, S., 2004. 14C Ages of Ostracodes from Pleistocene Lake Sediments of the Western Great Basin, Usa—Results of Progressive Acid Leaching. Radiocarbon 46, 189-200.
Reimer, P.J., Austin, W.E.N., Bard, E., Bayliss, A., Blackwell, P.G., Ramsey, C.B., Butzin, M., Cheng, H., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kromer, B., Manning, S.W., Muscheler, R., Palmer, J.G., Pearson, C., van der Plicht, J., Reimer, R.W., Richards, D.A., Scott, E.M., Southon, J.R., Turney, C.S.M., Wacker, L., Adolphi, F., Buntgen, U., Capano, M., Fahrni, S.M., Fogtmann-Schulz, A., Friedrich, R., Kohler, P., Kudsk, S., Miyake, F., Olsen, J., Reinig, F., Sakamoto, M., Sookdeo, A., Talamo, S., 2020. The Intcal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 Cal k BP). Radiocarbon 62, 725-757.
Simstich, J., Sarnthein, M., Erlenkeuser, H., 2003. Paired δ18O signals of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba show thermal stratification structure in Nordic Seas. Marine Micropaleontology 48, 107-125.
Wacker, L., Fülöp, R.H., Hajdas, I., Molnár, M., Rethemeyer, J., 2013. A novel approach to process carbonate samples for radiocarbon measurements with helium carrier gas. Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms 294, 214-217.
Wastegård, S., Rasmussen, T.L., Kuijpers, A., Nielsen, T., van Weering, T.C.E., 2006. Composition and origin of ash zones from Marine Isotope Stages 3 and 2 in the North Atlantic. Quaternary Science Reviews 25, 2409-2419.
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