High-resolution stalagmite stratigraphy supports the Late Holocene tephrochronology of southernmost Patagonia

Abstract

Volcanic ash layers are important markers for the chronostratigraphy of paleoclimate and paleoenvironmental archives at the southern tip of South America. However, this requires that tephras are well-dated. We report geochemical data from the MA1 stalagmite formed in a non-karst cave near Mt. Burney volcano in southernmost Patagonia (~53°S). High-resolution LA-ICP-MS analyses, SEM imagery, and NanoSIMS enable to identify volcanogenic signals during the last 4.5 kyrs BP from sub-annual trace element variations and tephra particles in distinct laminae. The new 230Th/U-chronology of MA1 provides precise dating of tephra from Mt. Burney (MB) and Aguilera (A) at 4216 +93/−193 yrs BP (MB2), 2291 ± 33 yrs BP (MB3), 853 +41/−60 yrs BP (MB4) and 2978 +91/−104 yrs BP (A1). This unique high-resolution record of MA1 holds potential to date further eruptions from Southern Andean volcanoes for the tephrochronology in this critical region, and potentially also large-volume explosive volcanism off South America.

Iron (hydr)oxide formation in Andosols under extreme climate conditions

“…However, Mn contents in primary and secondary minerals in basement rocks 20 , soils 20 (Table 1, Figs. S3 and S4) and cave stalagmites 26 from this area were low and rarely exceeded detection limits. Also based on previous findings, we suggest that the evolution of such redox interfaces in South Patagonian Andosols is caused by frequent water-table fluctuations 20,23 modulated by the strong and SWW-driven variations in rainfall intensity 27 .…”

“…In addition, our NanoSIMS measurements (Figs. 5 and S7) demonstrate the occurrence of residual MB 2 glass, represented by isolated areas high in 28 Si − and 16 O − secondary ion counts that are framed by irregularly shaped 27 Al 16 O − rims 26 . With a greater distance to the glass, 27 Al 16 O − merges gradually with 12 C − and 12 C 14 N − , while spots with elevated 28 Si − and 16 O − counts remain isolated in the OM-rich matrix.…”

“…Our Rietveld analysis provides the mass balance for the iron budgets within MB 2 tephra. Knowing its bulk composition and the average 1.4 wt.% Fe 2 O 3 t in matrix glass 26,[39][40][41] we estimate that the glass contains only ~ 25% of total Fe, whereas ~ 38% is contributed by titanomagnetite microcrysts containing 71.7 wt.% Fe 2 O 3 t on average (Figs. S3 and S4).…”

“…For instance, typically used Fe-(hydr) oxide proxies in sedimentary records (goethite/hematite ratios 3,50 ) are not applicable under the climatic controls described here. Evidence for the long-term (paleo)environmental importance of this Andosol-sourced micronutrient supply exceeding 4 kyrs is provided by the tephra-sourced Fe-(hydr)oxides preserved in laminae of stalagmite MA1 26 , which grew in a cave underneath these soils (Fig. 6b).…”

“…Burney volcano 20 . Smaller but still significant amounts of volcanic ash have been deposited in the course of other, minor eruptive events during the Holocene [24][25][26] . The studied Dystric Hydric (Epileptic) Vitric Andosols were described in detail by Klaes et al 20 and their observations are briefly reported here: These soils have a sandy texture and are characterized by strong andolization since ~ 2.5 kyrs BP, involving the substantial chemical breakdown of volcanic glass and the simultaneous formation of Fe-/Al-(hydr)oxides.…”