Simulating ice core <sup>10</sup>Be on the glacial–interglacial timescale

Abstract

Abstract. 10Be ice core measurements are an important tool for paleoclimate research, e.g., allowing for the reconstruction of past solar activity or changes in the geomagnetic dipole field. However, especially on multi-millennial timescales, the share of production and climate-induced variations of respective 10Be ice core records is still up for debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow-pit measurements are used for model calibration and validation. Being specifically configured for 10Be in polar ice, this tool thus allows for a straightforward investigation of production- and non-production-related modulation of this nuclide. We find that the polar 10Be ice concentration does not immediately record the globally mixed cosmogenic production signal. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input, we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable of reproducing the largest portion of the observed 10Be changes. However, model–measurement differences exhibit multi-millennial trends (differences up to 87% in case of normalized to the Holocene records) which call for closer investigation. Focusing on the (12–37) b2k (before the year AD 2000) period, mean model–measurement differences of 30% cannot be attributed to production changes. However, unconsidered climate-induced changes could likely explain the model–measurement mismatch. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) b2k model–measurement differences.

Synchronizing the Greenland ice core and radiocarbon timescales over the Holocene – Bayesian wiggle-matching of cosmogenic radionuclide records

“…This has led to the proposition of a so-called "polar bias" in ice core 10 Be records, stating that if polar 10 Be records were dominated by 10 Be produced at high latitudes, the anisotropy of the geomagnetic shielding would lead to an enhanced solar-and an attenuated geomagnetic modulation signal in polar 10 Be records. There is contradicting evidence from data and modelling studies to whether this is the case (Field et al, 2006;Bard et al, 1997;Pedro et al, 2012;Muscheler and Heikkilä, 2011;Heikkilä et al, 2009;Elsässer et al, 2015).…”

“…In reality, both modes of deposition contribute to the accumulation of 10 Be on the ice sheet. Today, wet deposition processes dominate over dry deposition which accounts for about one third or less of the deposited 10 Be in Greenland (Heikkilä et al, 2011;Elsässer et al, 2015). However, this dry / wet deposition ratio has likely been variable over time (Alley et al, 1995).…”

Reconstruction of the 236U input function for the Northeast Atlantic Ocean: Implications for 129I/236U and 236U/238U‐based tracer ages

“… Simulated atmospheric deposition of 236 U using the two dimensional atmospheric box model GRACE [ Elsässer et al ., ; Levin et al ., ]. As an example, the depositional flux of 236 U in the latitudinal band 50°N‐60°N is shown (red curve).…”

“…The atmospheric input of 236 U into the surface ocean is calculated with the two dimensional (laterally averaged) atmospheric multibox model GRACE [ Levin et al ., ]. To simulate the depositional flux of 236 U from the atmosphere the most recent version of GRACE that includes the simulation of aerosol‐borne radionuclides is used [ Elsässer et al ., ]. The model was calibrated with measured data of 137 Cs, 90 Sr, 210 Pb, SF 6 , 14 C, and 7 Be, and therefore seems appropriate for the simulation of the depositional flux of bomb produced and aerosol‐borne 236 U.…”

“…The horizontal breakdown of the model is 30° in the free troposphere and stratospheric layers which represents the three main atmospheric circulation cells (Hadley, Ferrel and Polar cell). The resolution of the boundary layer is enlarged to 10° per box [ Elsässer et al ., ]. Since major atmospheric processes show variations on the latitudinal scale, the 2‐D setup is a reasonable first‐order approach for the simulation of 236 U deposition.…”