Abstract
Lacustrine sequences from active volcanic settings usually hold a rich and continuous record of tephra layers, providing a critical source of information to reconstruct a most complete eruptive history of a region. Lake sedimentary records on volcanic islands are particularly useful as the typical small size of these islands and their steep subaerial and submarine slopes lead to a lower preservation of potential erodible pyroclastic deposits. Here we explore the lacustrine sedimentary record of Lagoa da Lomba, a crater lake in the central upland area of Flores Island (Azores), to gain insight into the recent eruptive history of this island. The strategic location of Lagoa da Lomba, half distance between the two clusters of recent volcanic activity of the island, together with its long-lasting record, back to 23.52 cal kyr BP, makes this lake a privileged site to investigate the Holocene volcanic history of Flores. Based on a detailed stratigraphic characterization of sediments from a lake transect of three cores, supported by glass shard geochemistry and radiocarbon dating, we recognized four Holocene eruptive events taking place between 6.28 and 2.36 cal kyr BP, demonstrating that the Holocene volcanic activity at Flores Island may have lasted longer than previously reported. Glass shard geochemistry from the different tephra layers suggests three populations, basaltic to trachybasaltic in composition, where the last eruption is the least evolved endmember. Two of the four eruptive events correlate with subaerially-exposed pyroclastic sequences, in terms of stratigraphy and geochemistry. The most recent event recorded at Lagoa da Lomba was constrained to 3.66 – 2.36 cal kyr BP and linked to an eruption sourced from Lagoa Comprida Volcanic System. The second most recent eruptive event was sourced from Lagoa Funda Volcanic System and dated at 3.66 cal kyr BP. Our observations show that Flores experienced vigorous volcanic activity during the Late Holocene. Therefore, contrary to what is assumed, the possibility of future eruptions should be properly considered, and the volcanic hazard here should not be underestimated. Moreover, we highlight the importance of tephrostratigraphy in recent lake sediments to reconstruct past volcanic activity, especially at small volcanic islands, such as Flores, where exposure is poor due to erosion within the limited subaerial area and the dense vegetation.
Highlights
Lakes are natural traps in drainage basins, and their sedimentary records are potentially continuous on time scales of several millennia
Three sedimentary facies were defined for the sequences of Lagoa da Lomba based on textural, lithological, and biological features (Figure 3A): 1) Organic-rich fine grained lacustrine facies (F1): This facies is composed of a dark-brown organic-rich mud, with high contents of organic matter and abundant bioclasts with a small amount of dispersed lithoclasts and minerals, ranging in grain size from silts to very fine sands (∼20–100 μm)
Based on tephrostratigraphy supported by glass geochemistry and radiocarbon ages, we correlated lacustrine-hosted tephras with subaerially-exposed sequences from key outcrops, resulting in a robust reconstruction of the eruptive events that took place in central part of Flores Island during the Holocene
Summary
Lakes are natural traps in drainage basins, and their sedimentary records are potentially continuous on time scales of several millennia. Depending on the age of the lake basin and the sediments, as well as on sedimentation rates, lakes may host crucial information to reconstruct climatic, ecological, and tectonic histories of volcanic regions (e.g., Björck et al, 2006; Sáez et al, 2007; Giralt et al, 2008; Stockhecke et al, 2014; Hernández et al, 2017; Vázquez-Loureiro et al, 2019) In these regions, lacustrine sequences are usually rich in tephra layers, which constitute one of the best means to investigate the frequency, size, and style of eruptions affecting that region, often with a detail that cannot be achieved using terrestrial deposits alone (e.g., Wulf et al, 2004; Stern 2008; Van Daele et al, 2014; Kutterolf et al, 2016; McNamara et al, 2018). This study employs lacustrine sediment records from one of the Azores islands – Flores (Figure 1B) – to investigate its recent eruptive history and volcanic hazard potential
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