Using the N/C ratio to correct bulk radiocarbon ages from lake sediments: Insights from Chilean Patagonia

Bertrand, Sébastien; Araneda, Alberto; Vargas, Patricia; Jana, Patricia; Fagel, Nathalie; Urrutia, Roberto

doi:10.1016/j.quageo.2012.06.003

Cited by 37 publications

(33 citation statements)

References 26 publications

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“…This systematic difference between ages of tephra determined from lake compared to bog deposits is statistically significant. Bertrand et al (2012) demonstrated that bulk radiocarbon AMS age dates from lake sediments were 300 to 1,100 years older than true ages in some northern Patagonian lakes due to variable inputs of terrestrial organic carbon from the Holocene soils that cover the lake watersheds. However, our finding of nearly identical radiocarbon ages for H1 from multiple small, closed-basin lakes located at different elevations, in watersheds having contrasting slopes and degrees of soil development, argues against site-specific processes of soil erosion and redeposition for the striking radiocarbon synchrony among lakes.…”

Section: H1 Eruption Agementioning

confidence: 99%

Holocene tephrochronology around Cochrane (~47° S), southern Chile

Stern

Moreno²,

Henríquez³

et al. 2015

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ABSTRACT. Two Holocene tephras encountered in outcrops, cores and trenches in bogs, and lake cores in the area around Cochrane, southern Chile, are identified (based on their age, tephra glass color and morphology, mineralogy, and both bulk and glass chemistry) as H1 derived from Hudson volcano, and MEN1 derived from Mentolat volcano. New AMS radiocarbon ages indicate systematic differences between those determined in lake cores (MEN1=7,689 and H1=8,440 cal yrs BP) and surface deposits (MEN1=7,471 and H1=7,891 cal yrs BP), with the lake cores being somewhat older. H1 tephra layers range from 8 to 18 cm thick, suggesting that both the area of the 10 cm isopach and the volume of this eruption were larger than previously suggested, but not greatly, and that the direction of maximum dispersion was more to the south. MEN1 tephra layers range from 1-4 cm in thickness, indicating that this was probably a reasonably large (>5 km 3 ) eruption. Some of the lake cores also contain thin layers (<2 cm) of late Holocene H2 tephra and the recent H3 (1991 AD) tephra, both derived from the Hudson volcano. No tephra evidence has been observed for any late Pleistocene tephra, nor for the existence of the supposed Arenales volcano, proposed to be located west of Cochrane. Keywords: Tephra, Tephrochronology, Volcanism, Hudson, Mentolat, Andes, Chile. RESUMEN. Tefrocronología holocénica cerca de Cochrane (~47° S), Chile Austral. Dos tefras holocenas reconocidas en af loramientos, testigos y trincheras en pantanos y lagos de la zona de Cochrane, Chile, han sido identificadas (sobre la base de su edad, color y morfología de las partículas vítreas, mineralogía y composición química de la roca total y del vidrio) como H1 y MEN1, provenientes de los volcanes Hudson y Mentolat, respectivamente. Nuevas edades radiocarbónicas AMS asociadas a estas tefras revelan diferencias sistemáticas entre los testigos lacustres (MEN1=7,689 y H1=8,440 años cal. AP) y los depósitos en superficie (MEN1=7,471 y H1=7,891 años cal. AP), en que las edades en los testigos lacustres son algo más antiguas. Los horizontes de la tefra H1 varían entre 8 y 18 cm de espesor, lo que indicaría que tanto el área de la isópaca de 10 cm como el volumen de esta erupción fue levemente mayor a lo previamente reconocido, y que la dirección de dispersión fue más hacia el sur. Los niveles de la tefra MEN1 varían entre 1 y 4 cm de espesor, lo que sugiere que esta fue una erupción relativamente grande (>5km 3 ). Algunos testigos lacustres también incluyen niveles más finos (<2 cm) de otra tefra (H2) del Holoceno tardío y de la tefra (H3) asociada a la erupción del año 1991, ambas derivadas del volcán Hudson. No se encontraron evidencias de tefras pleistocénicas tardías ni tampoco de la presencia del volcán Arenales, supuestamente ubicado al oeste de Cochrane y cuya existencia ha sido puesta en duda.

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Section: H1 Eruption Agementioning

confidence: 99%

Holocene tephrochronology around Cochrane (~47° S), southern Chile

Stern

Moreno²,

Henríquez³

et al. 2015

andgeo

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“…Radiocarbon dating was done on a parallel core collected from the lake, and depths of both cores were correlated using magnetic susceptibility and organic content profiles. The dating, organic content measurement, and charcoal analysis are described in detail elsewhere (Bertrand et al 2012;Araneda et al 2013). The macrocharcoal particles ([125 lm) in sediment subsamples were disaggregated in hot (70°C) 10 % KOH solution for 20 min, sieved through a 125-lm mesh, and the retained particles were counted under a stereomicroscope as detailed by Araneda et al 2013.…”

Section: Study Sitementioning

confidence: 99%

Remobilization of trace elements by forest fire in Patagonia, Chile

et al. 2015

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Temporal changes in the amounts of trace elements (As, Co, Cu, Mn, Ni, Pb, and Zn) and their correlations with temporal changes in charcoal abundance in age-dated sediments collected from Lake Thompson in Patagonia, Chile, attest to the substantial pyrogenic remobilization of contaminants that occurred in Patagonia during the mid-1900s. This remobilization was concurrent with the extensive slash and burn period in the region during that period. The changes in concentrations of Co, Cu, and Ni in relation to charcoal abundance in the lacustrine sediments over time were small compared to those of As, Mn, Pb, and Zn. However, the relatively low enrichment factors of all those trace elements, normalized to Fe, indicate that they were predominantly derived from local, natural sources impacted by fires rather than industrial sources. The primarily local source of Pb in the sediments was corroborated by the temporal consistency of its isotopic ratios ( 206 Pb/ 207 Pb: 208 Pb/ 206 Pb), which were similar to previously reported values for natural lead in Central and Southern Chile. However, the pyrogenic remobilization of both natural and industrial trace elements by forest fires in Chile and elsewhere is expected to rise as a consequence of climate change, which is projected to increase both the frequency and intensity of forest fires on a global scale.

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“…The old-carbon offsets are the age differences between bulk organic matter and leaf samples. Although it is well known that using 14 C ages of bulk organic matter overestimates the sedimentation age (Bjorck et al, 1998;Wang et al, 2002;Xu and Zheng, 2003;Fallu et al, 2004;Bertrand et al, 2012;Hou et al, 2012;Yamane et al, 2014), studies discussing the cause of down core variability in old-carbon offsets are scarce (Blaauw et al, 2011;Gaglioti et al, 2014). Here, we propose that 14 C offsets are causally linked to surface processes and soil dynamics in the catchment, driven by hydroclimate variability.…”

Section: Introductionmentioning

confidence: 60%