Challenges in ascertaining the late Quaternary tephrostratigraphy of southernmost Chile and Argentina 1 1 2 1 1 3 Stefan M. Lachowycz , Karen Fontijn , Victoria C. Smith , David M. Pyle , Tamsin A. Mather , José A. Naranjo [1] Department of Earth Sciences, University of Oxford, UK [2] Research Laboratory for Archaeology and the History of Art, University of Oxford, UK [3] Servicio Nacional de Geología y Minería, Santiago, Chile [email protected] # ## ! Overview Palaeoenvironmental archives 76°W 74°W 72°W 70°W 68°W 80°W 60°W 40°W Tephra preservation in palaeoenvironmental records Cha1 ! # 10°N ! Cha2 ! ! - The explosive eruption history and tephrostratigraphy in southernmost Chile/Argentina Minchinmávida Mic1 Issues with using tephra in palaeoenvironmental archives in this region to correlate records and constrain eruption parameters: and explosive eruption history ! # 0° ! Chaitén# Cor3 is significant for volcanic hazard assessment and as a tool to correlate and date reliably Corcovado# ! - 19 volcanic centres are thought to have been Cha2008 10°S Environment Physical preservation Chemical preservation Dating and record bias issues the many palaeoenvironmental archives here, but is currently poorly constrained. Yanteles# active in post-glacial times in southernmost # Yan1 Peat - Spatially and temporally variable - Al, Fe, alkali and alkali earth metals - Tephra layers are dispersed by root growth, 20°S 14 6 - We have reviewed the existing late Quaternary tephrostratigraphic record, and here # accumulation rates (F6, F7) ca use depth a nd are all readily leached from glass so their position and hence age is uncertain Chile and Argentina (>42.5 ⁰S), 12 of which 44°S Melimoyu Mel1 ! # 5 ! 30°S ! summarise the challenges in further constraining the record (i.e., in finding and thickness variations in tephra layers - Rapid dissolution of Fe-Mg minerals - Humic acid and humin fractions of peat may have had a total of at least 18 large (>1 km³ of Men1 Argentina 6 15 16 Mentolat# Chile accurately characterising, correlating, and dating tephra and eruptions) in this region. - Tephra layers dispersed by root growth , so in tephra is possible if the pH <4 give different radiocarbon dates tephra) explosive eruptions in the past 20 kyr. ! 40°S ## Ho Southernmost sector of sector Southernmost Maca precise thickness measurement is difficult - Complete alteration of tephra (e.g., - Typically only preserve postglacial tephra Mac1 7 6 - We note that rigorous geochemical, mineralogical, and physical characterisation of the ! 50°S - Many palaeoenvironmental archives (i.e., ice, (SSVZ) Zone Southern Volcanic ! - Cryptotephra layers may be reworked glass to siderite) is possible Coihaique ! !! ! 26 tephra deposits reported in palaeoenvironmental archives is uncommon, and present a ! ! Lake and - Influx of reworked tephra (F8) and/or - Alkali and alkali earth metals can be - Prone to anomalous and/or offset peat, and cave, lacustrine, and marine !! Cerro Hudson# ! H2 2 17 case study that exemplifies some of the errors that can result from its absence. 46°S ! # 46°S marine pumice rafting may cause the thickness a nd leached from glass radiocarbon ages due to a reservoir effect sediment cores or sections) have been sought !! 8 18 ! ! sediment grain size distribution to not reflect fallout - Glass can be altered to clay or deep water ventilation - It is important to understand the limitations of using tephra from palaeoenvironmental in this region, due to its unique location across H1991 - Remobilisation by turbidites is common9 - Fjords and Andean lakes usually only 4 ! the southern westerly wind belt . ! archives to ascertain the eruptive history, and the necessity of detailed tephra H1 - Tephra layer dispersal is possible without preserve postglacial tephra characterisation (particularly glass or mineral chemical analysis) to overcome these. - We have compiled a dataset of all the tephra textural indications (e.g., sediment mixing)10 Caves - Most tephra present in cave sediment is - Relatively well-preserved - Material from which reliable precise dates deposits reported in palaeoenvironmental 48°S 48°S 11 likely to have been remobilised can be obtained is often rare archives, and found that: - Cryptotephra layers have been found in - Bias in tephra preservation from cave ! - The majority of cores are reported to !!! 12 19 Tephra preservation in terrestrial sections # ! stalagmites in proximal open caves orientation and prevailing wind direction contain discrete tephra layers (F5), so there # Ice - Potentially preserved in local ice caps, but - Relatively well-preserved - May be difficult to date precisely in the ! City 13 - East of the Andes, terrestrial is significant potential to use tephra to may be reworked by meltwater or w ind (F9) absence of resolvable annual layers Land boundary sections are rare due to strong wind correlate and date these archives National border - Southern Andean cryptotephra layers have - Possible seasonal bias to preservation in 50°S El Calafate 50°S 1 # ! 13 erosion preventing soil (and tephra ) Aguilera been found in Antarctic ice cores temperate ice - There are several additional moderate-size ! A1 Lake Ice accumulation (F1 and F2). The few ! 1 m eruptions (reported in multiple cores) that # !! ~ Reclus ! Peatland ! sections present comprise fluvio- ! have not previously been widely recognised # Volcano active in !!! Río Gallegos glacial (F3) or aeolian (F4) deposits, !!! ! the late Quaternary - Cryptotephra units have only rarely been ! 10 cm isopach (F3) (F1) ! ! in which discrete tephra layers are ! ! ! # 1 cm isopach 52°S MB1 ! ! 52°S searched for, despite their potential to Vol1 Eruption code Monte Burney# not typically preserved. ! ! expand the tephrostratigraphic record ! R1 ! ! ! ! Marine sediment core ! ! !! ! ! ! ! Austral Volcanic Zone (AVZ) Zone Volcanic Austral ! Lake sediment core bearing - In the Andes, the topography means MB2Punta Arenas tephra - Most of the tephra units found are poorly ! ! Peat bog core layers ! ! that tephra is only sporadically ! ! ! Other sediment (F6) (F7) (F8) (F9) characterised and attributed to an eruption ! !! !! ! ! preserved and is sometimes ! ! using only tenuous constraints. ! Typical bog surface, which Bog core showing temporal Schematic illustration of the size- Tephra-bearing layers in intra- ! ! H1 remobilised, and that sections are 54°S 54°S encourages depth and thickness variation in accumulation rate dependent processes of reworking caldera ice due to seasonal melting ! ! ! 8 difficult to access (especially close (F2) (F4) ! ! ! variations in tephra deposition. and extent of root growth. of volcanic deposits into lakes . concentrating aeolian deposits. ! ! Ushuaia!!! ! # ! ! ! to the AVZ). Moreover, glass is prone Environmental conditions Typical sections east of the Map of southernmost Chile and Argentina, 2 ! to leaching or alteration and only prevalent east of the Andes. Andes: fluvio-glacial (F3) or showing the volcanic centres active in the past 20 3 post-glacial tephras are preserved . Minimal soil or vegetation aeolian (F4) deposits. kyr with approximate isopachs for their largest km 0 100 200 300 400 (F5) 56°S (F1) due to wind erosion (F2). Tephra layers are rare. eruptions, and the location of palaeoenvironmental 56°S archives containing tephra layers. 76°W 74°W 72°W 70°W 68°W 66°W 64°W Chemical characterisation and correlation Case study: Correlating tephra layers in sections and cores near Cerro Hudson References 1) Wilson, T.M., J.W. Cole, C. Stewart, et al. (2011), Ash storms: impacts of wind-remobilised volcanic ash on rural - Accurate chemical analysis of tephra ~100 µm ~50 µm communities and agriculture following the 1991 Hudson eruption, southern ... Bull. Volcanol. 73, 223–239. samples is often difficult: 2) Cerling, T.E., F.H. Brown, J.R. Bowman (1985), Low-temperature alteration of volcanic glass: hydration, Na, K, 18O and Ar mobility. Chem. Geol. 52, 281–293. - Tephra of basaltic to andesitic 3) Watt, S.F.L., D.M. Pyle, T.A. Mather (2013), The volcanic response to deglaciation: evidence from glaciated arcs composition often contains and a reassessment of global eruption records. Earth-Sci. Rev. 122, 77–102. abundant micro-phenocrysts (F13) 4) Kilian, R., F. Lamy (2012), A review of Glacial and Holocene paleoclimate records from southernmost Patagonia (49–55 S). Quat. Sci. Rev. 53, 1–23. - Tephra of dacitic to rhyolitic (F10) (F11) 5) McCulloch, R.D., S.J. Davies (2001), Late-glacial and Holocene palaeoenvironmental change in the central Strait composition is often skeletal, with of Magellan, southern Patagonia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 173, 143–173. ~100 µm ~50 µm 6) Kilian, R., M. Hohner, H. Biester, et al. (2003), Holocene peat and lake sediment tephra record from the glass patches rarely >5 µm (F12) southernmost Chilean Andes (53–55 ⁰S). Rev. Geol. Chile 30, 23–37. - Physical and chemical alteration of 7) Swindles, G.T., J. Galloway, Z. Outram, et al. (2013), Re-deposited cryptotephra layers in Holocene peats linked to tephra is common (F11). anthropogenic activity. Holocene 23, 1493–1501. 8) Van Daele, M., J. Moernaut, G. Silversmit, et al. (2014), The 600 year eruptive history of Villarrica Volcano (Chile) - The major-element glass composition revealed by annually-laminated lake sediments. Geol. Soc. Am. Bull. 126, 481–498. 9) Moernaut, J., M. De Batist, F. Charlet, et al. (2007), Giant earthquakes in South-Central Chile revealed by of some tephras can be almost (F12) (F13) Holocene mass-wasting events in Lake Puyehue. Sediment.