The 10Be/9Be ratio is acknowledged as an effective tool for establishing the stratigraphic position of paleomagnetic excursions. Still, our data suggest that, in particular depositional settings, the interplay between climate, sedimentation and oceanography may jeopardize a realistic depiction of the natural 10Be/9Be record

Boschi, Vanessa

Capraro, Luca

Ferretti, Patrizia

Fornaciari, Eliana

Kong, Xianghui

Macrì, Patrizia

Scarponi, Daniele

Xian, Feng

Zhou, Weijian

Archivio Ricerca Ca'Foscari

The 10Be record as a proxy of paleomagnetic reversals and excursions: A Mediterranean perspective

The 10Be/9Be ratio is acknowledged as an effective tool for establishing the stratigraphic position of paleomagnetic excur-sions. Still, our data suggest that, in particular depositional settings, the interplay between climate, sedimentation and oceanography may jeopardize a realistic depiction of the natural 10Be/9Be record

Archivio istituzionale della ricerca - Alma Mater Studiorum Università di Bologna

Name not available

     Available online http://amq.aiqua.it ISSN (online): 2279-7335    Alpine and Mediterranean Quaternary, Vol. 31 (Quaternary: Past, Present, Future - AIQUA Conference, Florence, 13-14/06/2018), 173 - 175 THE 10BE RECORD AS A PROXY OF PALEOMAGNETIC REVERSALS AND EXCURSIONS: A MEDITERRANEAN PERSPECTIVE  Luca Capraro 1, Patrizia Ferretti 2, Patrizia Macrì 3, Daniele Scarponi 4, Eliana Fornaciari 1, Feng Xian 5, Weijian Zhou 5, Xianghui Kong 5, Vanessa Boschi 6  1 Dipartimento di Geoscienze, University of Padova, Padova, Italy 2 CNR-IDPA, Mestre (Venezia), Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy 4 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, University of Bologna, Bologna, Italy 5 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Xi’an, China Corresponding author: L. Capraro <luca.capraro@unipd.it>   ABSTRACT: The 10Be/9Be ratio is acknowledged as an effective tool for establishing the stratigraphic position of paleomagnetic excur-sions. Still, our data suggest that, in particular depositional settings, the interplay between climate, sedimentation and oceanography may jeopardize a realistic depiction of the natural 10Be/9Be record.  KEYWORDS: Lower/middle Pleistocene, central Mediterranean, stratigraphy, paleomagnetism, 10Be/9Be 1. INTRODUCTION  The concentration of 10Be atoms adsorbed on sedi-ment particles is measured for tracking major geomag-netic reversals and events, whenever magnetic proper-ties are poor and/or conventional palaeomagnetic analy-ses yield ambiguous results. This approach relies on both the assumptions that 1) the production rate of 10Be in the atmosphere mainly depends on the strength of the Earth’s magnetic field, and 2) the 10Be generated in the atmosphere is rapidly conveyed to the Earth’s sur-face and locked into sediments. However, the study of marine successions demonstrates that 10Be record and palaeomagnetic events are mostly asynchronous (Valet et al., 2014). In addition, the period of low intensity of the Earth magnetic field associated to geomagnetic reversals, during which an overproduction of cos-mogenic 10Be is believed to occur, lasts significantly longer than the polarity switch itself (Raisbeck et al., 2006). Accordingly, there is no independent verification that a given 10Be peak marks the exact stratigraphic position of the associated polarity switch, as it may have been generated at any time during the period of inten-sity low. To our knowledge, the only record reporting on a perfect synchronicity between the 10Be and palaeo-magnetic signals at the M–B reversal is that of Valet et al. (2014), reconstructed in the Equatorial Indian Ocean from a deep–sea sediment core virtually void of terri-genous influxes. In contrast, stratigraphic successions affected by a significant terrigenous input show that the 10Be peak lags significantly behind the M–B boundary (e.g., Suganuma et al., 2010). According to the interpre-tation given by Suganuma et al. (2010), the 10Be peak would point to the correct stratigraphic position (and age) of the reversal, while the geomagnetic signal would be “frozen” at a lower stratigraphic position, well below the sediment surface, in response to lock–in processes. Mechanisms that control the flow of cosmogenic 10Be particles to the Earth surface and, more signifi-cantly, their transfer to the seafloor are still ambiguous. We may reasonably assume that the deposition of cos-mogenic 10Be in ice caps and loess deposits results from the instantaneous “freezing” of the atmospheric signal, although 10Be deposition is known to respond to numer-ous environmental and climatic factors, such as the re-gional precipitation rates and regimes (e.g., McHargue & Damon, 1991; Raisbeck et al., 2006). When dealing with marine terrigenous successions, further levels of uncer-tainty should be added in order to account for the much more complex arrangement of the marine sedimentary system (e.g., Brown et al., 1987, 1988; Yiou et al., 1988; Simon et al., 2017).  2. MATERIAL AND METHODS  We reconstructed a continuous 10Be/9Be record straddling the M-B reversal for the Valle di Manche (VdM) section (Calabria, Southern Italy) via the analysis of 68 sedimentary samples collected with an average resolution of ca. 9 cm, which results in a ca. 0.35 kyr resolution according to our age model (Capraro et al., 2017). Samples were prepared at the University of Xi’an, China, according to the procedures reported by Zhou et al. (2007), with minor modifications accounting for the difference between loess and marine sediments. BeO measurements were performed using the 3-MV accelera-tor mass spectrometer (AMS) at the Xi’an AMS Center, IEECAS. Test chemistry blank ratios are very small, in the order of 1 ×10-14.  https://doi.org/10.26382/AIQUA.2018.AIQUAconference       3. RESULTS  Our results compare very well with the high–resolution record of 10Be and 9Be reconstructed for the coeval Montalbano Jonico (MJ) section (Basilicata, Southern Italy; Simon et al., 2016), where a conven-tional palaeomagnetic record of the M-B reversal can-not be achieved (Sagnotti et al., 2010). The 9Be and 10Be concentrations measured at VdM are ca. four times those found at MJ. The excess of 9Be probably depends on the different primary sources of the terri-genous fraction, these being crystalline rocks from the Sila massif at VdM and young sediments from the uplift-ing Apennines at MJ. In addition, the estimated sedi-ment accumulation rates are significantly higher at MJ, suggesting that, at VdM, the influx of 9Be-free material was smaller. Probably, the lower sedimentation rates at VdM also account for the higher background concentra-tions of 10Be. At VdM, a prominent 9Be spike is centered in corre-spondence to the “Pitagora ash” (Capraro et al., 2017), similarly to what documented at MJ for the V3 and V4 tephra (Simon et al., 2016). Most likely, emplacement of the “Pitagora ash” provided a massive injection into the water column of highly soluble 9Be, which is very abun-dant in mantle sources (e.g., Baroni et al., 2011).   4. DISCUSSION AND CONCLUSIONS  Our record does not provide a complete documen-tation across the MIS 19–MIS 18 transition, where 10Be concentration attains a relevant peak that correlates almost perfectly to that recognized at MJ at ca. 775 ka, which Simon et al. (2016) interpret as the geochemical signature of the M-B reversal. This interpretation is at odds with that accomplished at VdM, where the palaeo-magnetic record provides unquestionable evidence that the M-B reversal occurs in the midst of full MIS 19 (ca. 787 ka according to our age model; Macrì et al., 2018).  At VdM, 10Be concentrations peak ca. 3.5 m above the M-B reversal, i.e. ca. 12 kyr later than the geomag-netic event. This delay is grossly in agreement with that calculated by for many open-ocean records straddling the M–B reversal, where the effects of lock–in proc-esses are invoked (Suganuma et al., 2010). 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Macrì, Patrizia

Archivio istituzionale della ricerca - Università degli Studi di Venezia Ca' Foscari

https://iris.unive.it/bitstream/10278/3710893/1/Capraro_AMQ2018.pdf

The 10Be record as a proxy of paleomagnetic reversals and excursions: A Mediterranean perspective

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