Disentangling magnetic and environmental signatures of sedimentary 10Be/9Be records

Reconstructions of the global production rate of the cosmogenic isotope Be-10 from sedimentary records of authigenic Be-10/Be-9 ratios have been successfully used to obtain independent estimates of geomagnetic dipole moment variations caused by field excursions or reversals. In this study, we assess the reliability of Be-10/Be-9 as a proxy for the cosmogenic Be-10 production rate by evaluating two potential biasing sources represented by sediment composition and climatic modulation. For this purpose, we compare five high-resolution Be-10/Be-9 records of the Matuyama-Brunhes (M-B) field reversal from sediment cores of the Indian, West Pacific, and North Atlantic oceans. Significant increase of Be-10/Be-9 ratios at similar to 774 ka is explained in terms of the dominant control of geomagnetic modulation during the M-B reversal. Results do not support the existence of a direct proportionality between measured sedimentary Be-10/Be-9 ratio and cosmogenic Be-10 production rate, as shown by Be-10/Be-9 records that offset relative to each other during and outside the M-B reversal. Residual differences between offset-corrected rescaled records do not appear to be related to an incomplete correction of variable sediment scavenging efficiencies by Be-9 normalization. Instead, these differences can be explained by a common climatic modulation model, assuming a linear relation between Be-10/Be-9 and the global Be-10 production rate with site- and time-dependent additive and multiplicative coefficients. These coefficients are linear functions of a single global climate proxy identified with the benthic delta O-18 record. Additive coefficients are almost constant in time and can represent up to similar to 60% of the average Be-10/Be-9 value during periods of stable field polarity. Multiplicative coefficients are also site-specific, with mean values representing the bulk scavenging efficiency of the site, and variations about this mean expressing a multiplicative climatic modulation of the Be-10 production rate. The amplitude of this modulation amounts to 10-15% of the maximum variations recorded during the M-B reversal and is sufficiently large to mask minor variations of the dipole moment during stable polarity periods. Reconstructions of the geomagnetic dipole intensity can benefit from the information about climatic modulation effects gained with our modelling approach. Best suited sites for magnetic field reconstructions should be characterized by minimal Be-recycling contributions from ancient Be-10 reservoirs and minimal climatic modulation, as far as it can be determined from relative comparisons with other records. These conditions are most likely encountered in open basins at sites (1) with < 2.8 km water depth, (2) > 200 km offshore, and (3) located underneath a large current system extending over regions with minimum terrigenous inputs. Scaling all records with respect to a chosen reference enables to produce Be-10/Be-9 stacks with reduced noise and short-term local environmental effects. Differences between stacks obtained in this manner highlight global climatic effects that need to be considered when generating calibrated reconstructions of the geomagnetic dipole moment. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

The global production rate of the cosmogenic isotope Be-10 has been reconstructed from sediment records to estimate geomagnetic dipole moment variations, with findings showing no direct proportionality between sedimentary Be-10/Be-9 ratios and cosmogenic Be-10 production rate. Climatic modulation and geomagnetic control during field reversals are important factors affecting Be-10/Be-9 ratios.