# Facies Characterization and Volcanic Source Assignment of Marine Tephra Deposits Around São Miguel Island, Azores Archipelago

J C Schindlbeck-belo, M Andrade, J Kling, S Kutterolf, A Pimentel, E Lebas, A Freundt, K Pank, T H Hansteen

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► To cite this version:

J C Schindlbeck-belo, M Andrade, J Kling, S Kutterolf, A Pimentel, et al. Facies Characterization and Volcanic Source Assignment of Marine Tephra Deposits Around São Miguel Island, Azores Archipelago. Geochemistry, Geophysics, Geosystems, 2025, 26 (12), pp.e2025GC012619. 10.1029/2025gc012619. hal-05446600

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Geochemistry, Geophysics,
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RESEARCH ARTICLE

10.1029/2025GC012619

**Key Points:**

* Marine tephra inventory around the eastern Azores

* Machine learning approach in tephrostratigraphy

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

J. C. Schindlbeck-Belo,
jbelo@geomar.de

**Citation:**

Schindlbeck-Belo, J. C., Andrade, M., Kling, J., Kutterolf, S., Pimentel, A., Lebas, E., et al. (2025). Facies characterization and volcanic source assignment of marine tephra deposits around São Miguel Island, Azores Archipelago. *Geochemistry, Geophysics, Geosystems*, 26, e2025GC012619. https://doi.org/10.1029/2025GC012619

Received 4 SEP 2025

Accepted 23 NOV 2025

**Author Contributions:**

**Conceptualization:** J. C. Schindlbeck-Belo, M. Andrade

**Data curation:** J. C. Schindlbeck-Belo, J. Kling

**Formal analysis:** J. Kling

**Funding acquisition:** J. C. Schindlbeck-Belo, S. Kutterolf, A. Freundt, T. H. Hansteen

**Investigation:** J. C. Schindlbeck-Belo, A. Pimentel, E. Lebas, A. Freundt

**Methodology:** J. C. Schindlbeck-Belo, S. Kutterolf, E. Lebas, A. Freundt, K. Pank

**Project administration:**

J. C. Schindlbeck-Belo, A. Freundt, T. H. Hansteen

**Resources:** S. Kutterolf, A. Pimentel, T. H. Hansteen

**Software:** J. Kling

**Visualization:** J. C. Schindlbeck-Belo, M. Andrade, J. Kling

© 2025 The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Facies Characterization and Volcanic Source Assignment of Marine Tephra Deposits Around São Miguel Island, Azores Archipelago

<figure><img src="image_3.png" /></figure>

J. C. Schindlbeck-Belo¹ ID, M. Andrade¹,² ID, J. Kling¹ ID, S. Kutterolf¹ ID, A. Pimentel³ ID, E. Lebas⁴ ID, A. Freundt¹ ID, K. Pank¹ ID, and T. H. Hansteen¹ ID

¹GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, ²Faculdade de Ciências, IDL—Instituto Dom Luiz, Universidade de Lisboa, Lisboa, Portugal, ³IVAR—Instituto de Investigação em Vulcanologia e Avaliação de Riscos, Universidade dos Açores, Ponta Delgada, Portugal, ⁴Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, Clermont-Ferrand, France

**Abstract** Long-term tephrostratigraphies of volcanic islands such as the Azores are often limited to young and incomplete subaerial records. Here, we present a Pleistocene-Holocene marine tephra archive around the eastern islands of the Azores based on 22 marine gravity cores. We provide a new extensive data set comprising major element glass compositions of 350 marine tephra layers and 26 tephra samples collected from subaerial outcrops on São Miguel Island. The marine tephra record is dominated by trachytic glass compositions with lesser amounts of trachy-basaltic/basaltic materials. We categorize the marine deposits into four facies: primary fall (F1), primary flow (F2), eruption-related secondary flow (F3), and non-eruption-related secondary flow deposits (F4). Cores collected close to the islands contain a high proportion (>50%) of volcaniclastic material deposited during secondary processes. Using machine-learning algorithms, we assign potential volcanic sources to individual tephra layers. Furthermore, our spatial characterization of primary/secondary deposits demonstrates the importance of carefully selecting core locations during sea-going campaigns, as the preservation and distribution of primary and reworked tephras considerably differ depending on several factors (e.g., on paleo-wind directions, island topography, seafloor bathymetry, and distance from source volcanoes). We also report the occurrence of mafic primary fall tephras tens to hundreds kilometers away from the closest island, indicating a potential hazard from mafic Plinian eruptions in the Azores. This study aims to improve the estimation of the eruptive frequency of the archipelago and opens new pathways for more detailed studies concerning long-term volcanic cyclicity in the region.

**Plain Language Summary** Ocean islands are predominantly generated by effusive and explosive volcanism. The surrounding marine environment may retain extensive evidence of the site's volcanic history, which contributes to a better assessment of potential future hazards. The Azores archipelago, located in the North Atlantic, represents a number of islands where this is applicable but hitherto the long-term tephrostratigraphies of their explosive eruptions are limited mostly to on shore occurrence and are therefore incomplete and often limited to the youngest deposits on the islands. We study the marine tephra inventory of 22 marine gravity cores from a German research expedition to unravel the depositional history around the islands.

1. Introduction

Highly explosive volcanic eruptions, producing tephra fall and flow deposits, rank among the most violent events on Earth. Eruption history and frequency are often relatively well known for historical times, but detailed information on the long-term behavior of volcanic systems is usually missing, which is essential for a robust assessment of volcanic hazards. This is especially important for ocean islands, where (highly explosive) volcanic eruptions—often associated with earthquakes, landslides, and flank collapses, sometimes with generation of tsunamis—are major threats that repeatedly destroy local infrastructure and cause fatalities (e.g., Cole et al., 1995; Eisele et al., 2015a, 2015b; Medeiros et al., 2021; Storey, 1982; Walker and Croasdale, 1971).

The Azores Archipelago, located in the North Atlantic, is composed of nine volcanic islands, most of which contain active volcanic systems (Figure 1). Historical eruptions (i.e., since the Portuguese settlement in the mid-15th century) have been described on five of these islands—Faial, Pico, São Jorge, Terceira, and São Miguel (Figure 1)—but also offshore (Gaspar, Queiroz, et al., 2015; Madeira and Brum da Silveira, 2003). Additionally,
**Writing – original draft:**

J. C. Schindlbeck-Belo, M. Andrade

**Writing – review & editing:** M. Andrade,

J. Kling, S. Kutterolf, A. Pimentel,

E. Lebas, A. Freundt, K. Pank,

T. H. Hansteen

<figure><img src="image_4.png" /></figure>

**Figure 1.** (a) Bathymetric and topographic maps showing the working area in the eastern sector of the Azores Archipelago. White circles mark the locations of investigated marine cores (M141-) collected during R/V Meteor cruise M141/1. Blue circles mark the location of cores SU-92-17 and-18 of Lehman et al. (1996), and orange circles mark the location of cores 1230 and 1219 studied by Chang, Mitchell, Hansteen, et al. (2023); Chang, Mitchell, Schindlbeck-Belo, et al. (2023). Three transects are marked: Transect 1 (dotted line) from west to east, transect 2 (solid line) from west to east, and Transect 3 (dashed line) from north to south. GF, Gloria Fault; TR, Terceira Rift; EAFZ, East Azores Fracture Zone; MAR, Mid-Atlantic Ridge. Map created using QGIS and bathymetry from GEBCO. Inset map (b) is an overview of the tectonic setting at the triple junction of the North American (NA), Nubian (Nu) and Eurasian (Eu) lithospheric plates. The red square marks the location of the close-up map shown in (a).

the Azorean islands preserve an extensive terrestrial record of highly explosive eruptions during the Holocene and upper Pleistocene (e.g., Croasdale et al., 1978; Kueppers et al., 2019; Pimentel et al., 2015; Queiroz et al., 2015; Wallenstein et al., 2015). However, their tephrostratigraphies are often limited or incomplete due to erosion, which is exacerbated by intense rainfall, or due to limitations in accessing outcrops either as a result of the overlapping of younger deposits or dense vegetation (Andrade et al., 2021; Kueppers et al., 2019; Pimentel et al., 2021).

Moreover, the relatively small size of the islands and/or the proximity of eruption vents to the shoreline cause large (unknown) tephra volumes to be deposited on the ocean floor (Kueppers et al., 2019; Pimentel et al., 2015, 2021). Thus, tephra layers in offshore marine sediments provide a valuable stratigraphic archive of long-term explosive volcanic activity.

Following this premise, we investigate 22 marine gravity cores forming three transects around São Miguel Island to report, for the first time, on the Pleistocene to Holocene marine tephra record of the eastern Azores region (Figure 1). Apart from its extensive record of explosive volcanism over the past millennia, São Miguel is the main and most populated island of the archipelago, with ~137,000 inhabitants, and therefore is particularly vulnerable to volcanic hazards. Assessing long-term tephra records in the eastern Azores region is thus crucial to constrain the frequencies of primary volcanic events and secondary deposition to better assess potential volcanic hazard.

## 2. Geological Setting

The Azores Archipelago is formed by nine volcanic islands and several smaller islets located in the mid-North Atlantic (Figure 1). The islands rise from the Azores Plateau, which is expressed by a triangular-shaped bathymetric anomaly along the triple junction of the North American, Eurasian, and Nubian lithospheric plates (Figure 1b; Miranda et al., 2018; Ramalho et al., 2017). Magmatism in the Azores region has traditionally been
explained by a mantle melting anomaly, often referred to as the Azores mantle plume, and its interplay with the plate triple junction, including hotspot-ridge interaction (Cannat et al., 1999; Gente et al., 2003; Tripanera et al., 2014). However, alternative models exist that attribute magmatic activity to a so-called “wet-spot” or to volatile-induced melting (Bonatti, 1994; Métrich et al., 2014; Schilling, 1975). The Azores islands cluster in three groups: (a) the western group includes Flores and Corvo, which are the only islands located to the west of the Mid-Atlantic Ridge on the North American Plate (Figure 1), (b) the central group which includes Terceira, Graciosa, Faial, São Jorge and Pico islands, and (c) the eastern group which is formed by São Miguel and Santa Maria islands (Figure 1; Table 1). The oldest dated subaerial products of the archipelago are of late Miocene age (6.01 Ma; Ramalho et al., 2017) and have been dated at Santa Maria Island where, such as Corvo, volcanism is currently considered extinct. All the remaining islands have active volcanic systems that erupted during the Holocene and upper Pleistocene (Table 1; e.g., Andrade et al., 2022, 2023; Ferreira et al., 2015; Gaspar, 1996; Gertisser et al., 2010; Nunes, 1999; Pacheco et al., 2001; Queiroz et al., 2015; Self, 1976; Wallenstein et al., 2015).

Exposed subaerial sequences across the islands show that highly explosive trachytic eruptions, sub-Plinian to Plinian in style, have been frequent at Faial, Graciosa, Terceira, and São Miguel during recent geological times (Table 1; Booth et al., 1978; Gertisser et al., 2010; Guest et al., 2015; Pacheco et al., 2001; Queiroz et al., 2015; Self, 1976; Wallenstein et al., 2015), making these islands potential sources of marine tephra deposits across the eastern Azores region. Critically, the wind regime in the Azores has been dominated by the westerlies (Booth et al., 1978; Gaspar, Guest, et al., 2015; McLean et al., 2024; Sáez et al., 2025), facilitating the transport of tephra from the central to the eastern Azores and beyond toward Europe and North Africa. Basaltic eruptions are common to all islands, varying in style from effusive/low-explosive events to violent Strombolian eruptions, occasionally experiencing phreatomagmatic phases (e.g., Andrade et al., 2022; Nunes, 1999; Zanon and Viveiros, 2019). As the oceanographic campaign was conducted around São Miguel, we first introduced this island and continued with the other islands in a counterclockwise direction. Corvo is not presented as its stratigraphy is poorly constrained and available ages are ambiguous (Genske et al., 2012). Furthermore, its small size (6 × 4 km) and distance to the coring sites (>350 km) make it a highly unlikely source of the studied marine tephras. Table 1 summarizes key information for each island.

## 2.1. São Miguel

São Miguel is the largest of the Azorean islands with an area of 744 km² (Figures 1 and 2a). The island is built by three active central volcanoes with summit calderas—Sete Cidades, Fogo (also known as Água de Pau), and Furnas (Figure 2)—separated by the Picos and Congro basaltic fissure zones, and two older volcanic centers—Povoação and Nordeste—located in the eastern part that are deemed extinct. The stratigraphy of São Miguel records several large Plinian eruptions that are associated with caldera-forming events at the Sete Cidades, Fogo, and Furnas volcanoes (Guest et al., 2015; Queiroz et al., 2015; Wallenstein et al., 2015). Using the well-studied Fogo A deposit (4.6 ka) as a stratigraphic marker, at least 73 eruptions are known within the last 5 ka, which yields an eruptive recurrence rate of 68.5 years (Gaspar, Guest, et al., 2015).

### 2.1.1. Sete Cidades

Sete Cidades is the westernmost of the three active central volcanoes of São Miguel (Figure 2a). Its volcanosstratigraphy is subdivided into two groups. The Inferior Group (210–36 ka) is mostly formed by basaltic and trachytic lavas with some minor pyroclastic deposits. The Superior Group (<36 ka) is formed by the products of low magnitude trachytic and basaltic eruptions intercalated with thick trachytic pyroclastic deposits of highly explosive eruptions, related to paroxysmal caldera-forming events, namely the Risco (36 ka), Bretanha (29 ka), and Santa Bárbara (16 ka) Formations (Queiroz, 1997; Queiroz et al., 2015). Post-caldera volcanism is represented by the products of explosive and small dome-forming trachytic eruptions and basaltic events. At least 17 trachytic sub-Plinian intracaldera eruptions (Pepom Member P1 to P17 deposits; Figure 2b) and 12 basaltic flank eruptions occurred over the last 5 ka (Queiroz et al., 2008). Although there is no record of subaerial historical eruptions, three submarine eruptions took place offshore Sete Cidades in 1638 AD and twice in 1811 AD, with the last event forming a short-lived islet named Sabrina (Queiroz et al., 2015).
<table><caption>**Table 1**
Key Stratigraphic Information for Each of the Volcanic Islands With a Focus on Explosive Volcanic Activity</caption><thead><tr><th>Island</th><th>Volcanic system</th><th>Major groups and formations</th><th>Approximate eruption ages</th><th>Brief eruption history</th></tr></thead><tbody><tr><td rowspan="2">São Miguel</td><td rowspan="2">Sete Cidades</td><td>Superior Group</td><td>&lt;5 ka</td><td>17 intracaldera trachytic sub-Plinian eruptions (Pepom Member, P1 to P17 deposits; Figure 2b) and 12 basaltic flank eruptions in the last 5 ka. No historical eruptions; three submarine eruptions offshore Sete Cidades in 1638 AD and 1811 AD (Queiroz et al., 2008, 2015)</td></tr><tr><td>Inferior Group</td><td>36–5 ka</td><td>Numerous low magnitude trachytic and basaltic eruptions intercalated with thick trachytic pyroclastic deposits from highly explosive eruptions, related to paroxysmal caldera-forming events, namely the Risco (36 ka), Bretanha (29 ka), and Santa Bárbara (16 ka) formations (Queiroz et al., 2015).</td></tr><tr><td rowspan="2">Fogo</td><td rowspan="2">Upper Group</td><td>210–36 ka</td><td>Basaltic and trachytic lavas and some minor pyroclastic deposits (Queiroz et al., 2015)</td></tr><tr><td>&lt;40 ka</td><td>Numerous trachytic pyroclastic (fall and flow) deposits of multiple sub-Plinian and Plinian eruptions, including the major Ribeira Chã (~12–8 ka) and Fogo A (4.6 ka) formations. Post-Fogo A volcanism is recorded by three sub-Plinian trachytic eruptions (Fogo B to Fogo D), a few scoria cones and lavas (including a historical eruption in 1563 AD), and the historical intra-caldera eruptions (1563 and 1564 AD) (Wallenstein et al., 2015)</td></tr><tr><td rowspan="3">Furnas</td><td>Lower Group</td><td>280–40 ka</td><td>Basaltic products interbedded with trachytic pyroclastic products and lavas, and undifferentiated volcaniclastic deposits (Wallenstein et al., 2015)</td></tr><tr><td>Upper Group</td><td>&lt;5 ka</td><td>10 intracaldera trachytic sub-Plinian eruptions (Furnas A to Furnas I + Furnas 1630 AD pumice fall deposits), some of them forming trachytic domes in the final stages, the last two were historical (Guest et al., 1999, 2015)</td></tr><tr><td>Middle Group</td><td>30–5 ka</td><td>Numerous pumice fall deposits, ignimbrites, and trachytic lavas, including the major Tufo (~27 ka) and the Ponta da Graça Ignimbrite (17 ka) formations (Guest et al., 1999, 2015)</td></tr><tr><td rowspan="2">Picos Fissure System</td><td>Lower Group</td><td>138–30 ka</td><td>Trachytic pyroclastic deposits, debris flow deposits, and trachytic lavas, including the Povoação Ignimbrite Formation (~30 ka; Figure 2c) (Guest et al., 1999, 2015)</td></tr><tr><td></td><td></td><td>Hawaiian-Strombolian monogenetic basaltic eruptions. At least 30 eruptions over the last 5 kyr (Ferreira et al., 2015)</td></tr><tr><td>Congro Fissure System</td><td></td><td>&gt;4 ka</td><td>Hawaiian-Strombolian monogenetic basaltic eruptions. One single eruption over the last 5 kyr (Ferreira et al., 2015)</td></tr><tr><td>Santa Maria</td><td></td><td>Volcanic activity terminated around 2.8 Ma ago</td><td>Exclusively basaltic, Strombolian to Surtseyan in style, from monogenetic to polygenetic shield volcanism (Ramalho et al., 2017)</td></tr><tr><td rowspan="2">Terceira</td><td rowspan="2">Santa Bárbara</td><td>Upper Terceira Group</td><td>&lt;23 ka</td><td>Several basaltic and trachytic effusive to sub-Plinian eruptions, mainly at the flank of the volcano which produced several fall deposits, lava domes and coulées. Historical eruption in 1761 AD (Self, 1976)</td></tr><tr><td>Lower Terceira Group</td><td>65–23 ka</td><td>Predominantly basaltic lavas and minor scoria deposits, with minor peralkaline silicic lava domes and coulées</td></tr><tr><td rowspan="2">Pico Alto</td><td rowspan="2">Upper Terceira Group</td><td>&lt;23 ka</td><td>Predominantly peralkaline silicic lava domes and coulées, and related sub-Plinian eruptions (Self, 1976). Includes the Lajes–Angra Ignimbrite Formation (23–20 ka), which forms a widespread lithostratigraphic horizon across much of the island (Pimentel et al., 2021).</td></tr><tr><td>141–23 ka</td><td>Numerous pumice fall deposits and trachytic lavas, as well as major ignimbrite-bearing formations such as the Pedras Negras Ignimbrite Formation, Ignimbrite-i Formation (86 ka), Grota do Vale Ignimbrite Formation, Caldeira Castelinho Ignimbrite Formation (83–71 ka), Vila Nova-Fanal Ignimbrite Formation (58–39 ka), and Linhares-Matela Ignimbrite Formation (37–34 ka) (Gertisser et al., 2010)</td></tr></tbody></table>
<table><caption>**Table 1**
*Continued*</caption><thead><tr><th>Island</th><th>Volcanic system</th><th>Major groups and formations</th><th>Approximate eruption ages</th><th>Brief eruption history</th></tr></thead><tbody><tr><td rowspan="5">Graciosa</td><td>Fissure System</td><td></td><td>&gt;43 ka</td><td>Basaltic Hawaiian and Strombolian eruptions. Historical eruption in 1761 AD (Self, 1976).</td></tr><tr><td>Guilherme Moniz</td><td>Lower Terceira Group</td><td>&gt;270–111 ka (extinct)</td><td>Domes, coulées, and possibly some ignimbrite-forming eruptions (Gertisser et al., 2010)</td></tr><tr><td>Vitória-Vulcão Central</td><td>Vulcão Central</td><td>&lt;125–11 ka</td><td>Basaltic and trachytic products (A to V) that record a succession of Strombolian eruptions, with minor Surtseyan events, and sub-Plinian and Plinian eruptions. Caldera forming eruption (subunit S) at 12 ka (Gaspar, 1996)</td></tr><tr><td>Vitória Unit</td><td></td><td>&lt;96</td><td>Monogenetic basaltic volcanism. Few eruptions during the Holocene (Gaspar, 1996)</td></tr><tr><td>Serra Branca</td><td></td><td>430–260 ka (extinct)</td><td>Trachytic pyroclastic fall and flow deposits (ignimbrites and surges) and lahars resulting from explosive volcanism (Gaspar, 1996)</td></tr><tr><td>São Jorge</td><td>Manadas Fissure System</td><td></td><td>Holocene age</td><td>Hawaiian-Strombolian and Surtseyan fissural volcanism. Two historical eruptions on land in 1580 AD and 1808 AD, and a probable submarine eruption in 1964 AD (Madeira and Brum da Silveira, 2003)</td></tr><tr><td rowspan="5">Faial</td><td>Caldeira</td><td>Cedros Volcanic Complex, Upper Group</td><td>&lt;16 ka</td><td>12 trachytic sub-Plinian eruptions (C1 to C12), and two minor intra-caldera eruptions (C13 and C14) (Pacheco et al., 2001)</td></tr><tr><td></td><td>Cedros Volcanic Complex, Lower Group</td><td>&gt;440–16 ka</td><td>Predominantly basaltic lavas and minor scoria deposits, one trachytic lava dome (Pacheco et al., 2001)</td></tr><tr><td>Capelo Peninsula Fissure System</td><td>Capelo Formation</td><td>&lt;8 ka</td><td>Monogenetic basaltic volcanism with Hawaiian-Strombolian and surtseyan eruptions. Historical eruption in 1672/73 AD and 1957/58 AD (Di Chiara et al., 2014; Madeira, 1998)</td></tr><tr><td>Horta Platform Fissure System</td><td></td><td>30–11 ka</td><td>Monogenetic basaltic volcanism forming lava fields, scoria and tuff cones (Madeira, 1998; Pacheco et al., 2001).</td></tr><tr><td>Ribeirinha</td><td></td><td>850–580 ka (extinct)</td><td>Basaltic polygenetic volcanism (Madeira, 1998)</td></tr><tr><td rowspan="2">Pico</td><td>Pico Mountain</td><td></td><td>~53 ka</td><td>Polygenetic basaltic volcanism with eruptions from the summit crater and Hawaiian-Strombolian eruptions on the flanks. Historical eruptions in 1718 AD and 1720 AD (Nunes, 1999; Madeira and Brum da Silveira, 2003)</td></tr><tr><td>Planalto da Achada Fissure System</td><td></td><td>&lt;270 ka?</td><td>Fissural basaltic volcanism, Hawaiian-Strombolian eruptions. Historical eruption in 1562/64 AD and a submarine eruption in 1963 AD (Nunes, 1999; Madeira and Brum da Silveira, 2003).</td></tr><tr><td>Flores</td><td>Upper Complex</td><td>Upper Unit</td><td>&lt;200?–3 ka</td><td>Monogenetic basaltic volcanism with Hawaiian, Strombolian and phreatomagmatic eruptions. Last eruption occurred ~3 ka (Azevedo &amp; Portugal Ferreira, 2006; Andrade et al., 2022, 2023).</td></tr></tbody></table>

Note. More references are provided in the main text.

### 2.1.2. Fogo

Fogo is the largest of the three active volcanoes and is located right at the center of São Miguel Island (Figure 2a). The stratigraphy of Fogo Volcano comprises two main groups. The Lower Group (280–40 ka) includes basaltic products interbedded with trachytic pyroclastic products, lavas, and undifferentiated volcaniclastic deposits. Despite the difficulty of correlating eruptive sequences of the northern and southern flanks, the stratigraphy of the Upper Group (<40 ka) comprises numerous trachytic pyroclastic (fall and flow) deposits of multiple sub-Plinian and Plinian eruptions, including the major Ribeira Chã (~12–8 ka) and Fogo A (4.6 ka) Formations. Post-Fogo A volcanism is recorded by four trachytic fall deposits (Fogo B to Fogo 1563 AD) and a few scoria cones and lava flows (Wallenstein et al., 2015). Two eruptions took place at Fogo in 1563 AD in just 4 days (a sub-Plinian
<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Map of São Miguel Island showing outcrop locations. Map created using GeoMapApp https://www.geomapapp.org (Ryan et al., 2009). (b) Outcrop S3 in which some brown paleosol horizons separate light and dark gray wavy layers and light gray to white pumice lapilli layers (Pepom Member, Sete Cidades Volcano), scale is 1 m. (c) Outcrop Sm3 showing the welded facies of Povoação Ignimbrite (Furnas Volcano) with dark fiamme at the Povoação beach, hammer for scale.</figcaption></figure>

intracaldera eruption and a Hawaiian flank eruption). In 1564 AD, a phreatic eruption occurred inside the caldera (Wallenstein et al., 2015).

### 2.1.3. Furnas

Furnas Volcano is located in the eastern third of the island (Figure 2) and is the youngest of the three central volcanoes. Its stratigraphy is divided into three groups. The Lower Group (~138–30 ka) consists of trachytic pyroclastic deposits, debris flow deposits, and trachytic lavas, including the Povoação Ignimbrite Formation (~30 ka; Figure 2c), which records the first paroxysmal caldera-forming eruption of Furnas. The Middle Group
(30–5 ka) is also composed of numerous pumice fall deposits, ignimbrites, and trachytic lavas, including the major Tufo (~27 ka; 45 m thick at type locality; Guest et al., 1999) and the Ponta da Graça Ignimbrite (17 ka; >15 m total thickness; Guest et al., 1999) Formations. The Upper Group (<5 ka) comprises the pumice fall deposits of 10 intracaldera trachytic sub-Plinian eruptions (Furnas A to Furnas 1630 AD), some of them forming trachytic domes in the final stages (Guest et al., 1999, 2015). Two historical eruptions were recorded at Furnas in 1439–43 AD and 1630 AD (Guest et al., 1999, 2015).

## 2.2. Santa Maria

Santa Maria Island, situated some 80 km south of São Miguel (Figure 1), is the oldest island of the archipelago (6.01 Ma; Ramalho et al., 2017). Eruptions on Santa Maria were exclusively basaltic, Strombolian to Surtseyan in style, from monogenetic to polygenetic shield volcanism. Although the onshore record implies that the volcanic activity terminated around 2.8 Ma ago (Ramalho et al., 2017; Sibrant et al., 2015a), recent high-resolution bathymetric data reveal a seemingly younger (upper Pleistocene) submerged volcanic edifice located offshore north of the island. It has been interpreted as a partially eroded tuff ring that probably erupted just prior to a rapid sea-level drop approximately 43 ka ago and then submerged during the sea-level rise of the Last Glacial Termination (Ramalho et al., 2020).

## 2.3. Terceira

Terceira Island is located some 140 km northwest of São Miguel (Figure 1). It comprises three active volcanic systems—Santa Bárbara and Pico Alto central volcanoes and the fissure zone—and two older central volcanoes considered extinct—Guilherme Moniz and Cinco Picos (Pimentel et al., 2021; Self, 1976). The stratigraphy of Terceira is divided into two main groups. The Lower Terceira Group (~401–23 ka) comprises basaltic to trachytic/rhyolitic lavas, pumice and scoria falls, as well as several major ignimbrite-bearing formations. These include the Ignimbrite-i (86 ka), the Grota do Vale Ignimbrite, the Pedras Negras Ignimbrite, the Caldeira-Castelinho Ignimbrite (~83–71 ka), the Vila Nova-Fanal Ignimbrite (~58–39 ka), and the Linhares-Matela Ignimbrite (37–34 ka) Formations, which erupted from Pico Alto and possibly Guilherme Moniz volcanoes (Gertisser et al., 2010; Jeffery et al., 2017). The Upper Terceira Group (<23 ka) includes the Lajes-Angra Ignimbrite Formation (23–20 ka), which erupted from the Pico Alto Volcano and forms a widespread lithosphatigraphic horizon across much of the island (Pimentel et al., 2021). It has been correlated with a thick (>1 m) tephra deposit in a marine sequence ~40 km north of Terceira (core M141-1230; Chang, Mitchell, Hansteen, et al., 2023; Chang, Mitchell, Schindlbeck-Belo, et al., 2023; Figure 1). In addition, over the last 20 ka, Pico Alto and Santa Bárbara were the source of at least nine trachytic sub-Plinian eruptions, producing pumice fall deposits (A to I), and numerous lava domes and coulées, which alternated with basaltic eruptions in the fissure zone (Self, 1976). Historical eruptions on Terceira took place in 1761 AD (forming basaltic scoria cones and lavas on the fissure zone and trachytic domes on the east flank of Santa Bárbara; Pimentel et al., 2016), and offshore on the Serreta Submarine Ridge in 1867 AD and 1998–2001 AD (Casas et al., 2018; Gaspar, Queiroz, et al., 2015).

## 2.4. Graciosa

Graciosa is the most septentrional island of the central group (Figure 1), located approximately 225 km northwest of São Miguel. The island comprises three volcanic complexes—Serra das Fontes, Serra Branca, and Vitória-Vulcão Central (Gaspar, 1996). The Serra das Fontes Complex (~620 ka) corresponds to an extinct shield volcano formed by basaltic volcanism. The Serra Branca Complex (~430–260 ka) is an extinct central volcano composed mostly of trachytic pyroclastic fall and flow deposits (ignimbrites and surges) and lahars resulting from explosive volcanism (Gaspar, 1996). The Vitória-Vulcão Central Complex is considered active and is divided into two units. The Vitória Unit is composed of basaltic products resulting from monogenetic activity. The stratigraphy of Vulcão Central includes several subunits of basaltic and trachytic products (A to V) that record a succession of Strombolian eruptions, with minor Surtseyan events, and sub-Plinian and Plinian eruptions. An important highly explosive event (recorded by the Upper Hydromagmatic Sequence—subunit S) formed the caldera of the Vulcão Central ~12 ka ago. Post-caldera volcanism was mainly effusive, forming basaltic lava flows and trachytic lava domes (Gaspar, 1996).
## 2.5. Faial

Faial is the westernmost island of the central group, situated some 250 km west-northwest of São Miguel (Figure 1). The island hosts two central volcanoes—Ribeirinha and Caldeira—and two fissure systems—Horta Platform and Capelo Peninsula (Pacheco et al., 2001). Ribeirinha Volcano (~850 ka) is an extinct shield edifice formed by basaltic volcanism. The Horta Platform and Capelo Peninsula are characterized by monogenetic basaltic activity (Strombolian to Surtseyan style). The stratigraphy of Caldeira Volcano is divided into two groups. The Lower Group (~440–16 ka), which is mostly formed by basaltic lavas, and the Upper Group (<16 ka), which comprises a series of trachytic pyroclastic deposits (C1 to C12), record sub-Plinian style eruptions (Pacheco et al., 2001). Among these, the C11 deposit is associated with the formation of the summit caldera ~1,000 years BP (Pimentel et al., 2015). It was one of the last major explosive events recorded in the Azores and produced trachytic ash, lapilli fall deposits and an extensive ignimbrite, which are preserved in marine sequences 20 km away from the island (core M141-1219; Chang, Mitchell, Hansteen, et al., 2023; Chang, Mitchell, Schindlbeck-Belo, et al., 2023; Figure 1). Historical eruptions took place at the Capelo Peninsula fissure system in 1672/73 AD and 1957/58 AD, with Strombolian and Surtseyan events, respectively (Pacheco et al., 2001; Pimentel et al., 2015).

## 2.6. São Jorge, Pico, and Flores

São Jorge and Pico islands, located 180 and 200 km, respectively, west-northwest of São Miguel (Figure 1), are built exclusively by basaltic volcanism. São Jorge corresponds to a long, narrow volcanic ridge composed of numerous alignments of monogenetic cones, which resulted from Strombolian and, to a lesser extent, Surtseyan eruptions. Its age ranges from 1.85 Ma to the present, including two historical eruptions on land in 1580 AD and 1808 AD, and a probable submarine eruption in 1964 AD (Madeira and Brum da Silveira, 2003; Marques et al., 2018). Pico is the youngest island of the archipelago (~270 ka) and is formed by monogenetic and polygenetic basaltic volcanism. The western part of the island is marked by the prominent Pico Mountain Volcano (2,351 m above sea level), while the eastern part comprises a volcanic ridge. Historical eruptions took place on land in 1562/64 AD, 1718 AD, 1720 AD, and a probable submarine eruption in 1963 AD (Madeira and Brum da Silveira, 2003; Nunes, 1999). Flores Island, located 500 km west-northwest of São Miguel (Figure 1), is 2.15 Ma old (Azevedo et al., 1991) and its history is marked by different periods of volcanic activity separated by long periods of quiescence, with the last volcanic event occurring ~3 kyr ago. Although formed by overlapping polygenetic volcanoes, its recent volcanic activity (late Pleistocene-Holocene) was characterized by monogenetic basaltic volcanism. Holocene activity produced Strombolian and violent Strombolian eruptions with important phreatomagmatic phases, which dispersed tephras toward the east (Andrade et al., 2022, 2023), but it is unlikely that the products of these eruptions reached the eastern islands of the archipelago some 500 km away.

## 2.7. Submarine Eruptions

At least 252 cones formed by explosive submarine eruptions were identified around São Miguel Island. They occur predominantly on the submarine flanks of Fogo and Sete Cidades volcanoes, as lineaments of cones along tectonic structures or as superimposed cones in a volcanic cluster SE of São Miguel (Weiß et al., 2015). During historical times, approximately 13 submarine eruptions have been reported in the Azores, exhibiting styles ranging from Surtseyan to effusive submarine activity (Gaspar, Queiroz, et al., 2015). The most recent event occurred between 1998 and 2001 AD on the Serreta Ridge 10 km west of Ponta da Serreta (Terceira Island) and was characterized by the intermittent emission of lava balloons (Casas et al., 2018; Gaspar, Queiroz, et al., 2015).

# 3. Methods

## 3.1. Description and Sampling of Marine Cores

A total of 44 gravity cores were collected in the Azores region in 2017 during the R/V Meteor cruise M141/1 (Hansteen, 2017) in order to unravel the long-term tephrostratigraphy and distribution of highly explosive eruptions around the central and eastern islands of the Azores. The cores include distal and more proximal records, with distances to the coastline of São Miguel ranging from 25 to 250 km (Figure 1). The recovered cores comprise hemipelagic to pelagic background sediments and epiclastic turbidite beds intercalated with ash and pumice lapilli layers. Grain sizes of the deposits, especially the volcanics, are generally coarser in proximity to the islands and decrease with increasing distance. Based on core length, abundance of tephra layers, and spatial distribution around São Miguel, three transects with a total of 22 cores were carefully selected as the most
<caption>**Table 2**</caption>

List of Core Locations (Coordinates), Water Depths, and Sediment Recovery Lengths

<table><thead><tr><th>Station #</th><th>Latitude (N)</th><th>Longitude (W)</th><th>Water depth (m)</th><th>Recovery (m)</th></tr></thead><tbody><tr><td>M141-1136</td><td>37°00,215'</td><td>022°27,615'</td><td>4,820</td><td>6.92</td></tr><tr><td>M141-1138</td><td>36°47,734'</td><td>024°03,634'</td><td>4,295</td><td>3.77</td></tr><tr><td>M141-1140</td><td>36°22,340'</td><td>025°32,835'</td><td>4,049</td><td>1.76</td></tr><tr><td>M141-1144</td><td>37°08,044'</td><td>025°28,268'</td><td>2,277</td><td>4.45</td></tr><tr><td>M141-1146</td><td>35°53,977'</td><td>025°41,182'</td><td>3,943</td><td>7.93</td></tr><tr><td>M141-1148</td><td>36°22,243'</td><td>025°32,764'</td><td>3,999</td><td>6.66</td></tr><tr><td>M141-1150</td><td>36°53,887'</td><td>024°45,743'</td><td>2,571</td><td>5.19</td></tr><tr><td>M141-1152</td><td>37°34,226'</td><td>023°55,242'</td><td>2,900</td><td>7.25</td></tr><tr><td>M141-1154</td><td>37°52,712'</td><td>024°25,121'</td><td>2,881</td><td>7.98</td></tr><tr><td>M141-1156</td><td>37°31,421'</td><td>024°47,544'</td><td>2,900</td><td>4.1</td></tr><tr><td>M141-1160</td><td>37°20,421'</td><td>025°27,565'</td><td>2,100</td><td>4.33</td></tr><tr><td>M141-1162</td><td>37°29,603'</td><td>026°04,801'</td><td>2,345</td><td>9</td></tr><tr><td>M141-1164</td><td>37°10,097'</td><td>025°56,361'</td><td>2,534</td><td>9</td></tr><tr><td>M141-1168</td><td>37°11,561'</td><td>026°24,875'</td><td>2,608</td><td>9.6</td></tr><tr><td>M141-1176</td><td>37°41,989'</td><td>027°14,272'</td><td>2,270</td><td>11.75</td></tr><tr><td>M141-1184</td><td>37°37,419'</td><td>027°29,615'</td><td>2,229</td><td>5.48</td></tr><tr><td>M141-1236</td><td>38°15,295'</td><td>025°40,955'</td><td>2,477</td><td>7.58</td></tr><tr><td>M141-1238</td><td>38°04,800'</td><td>025°32,522'</td><td>2,010</td><td>4.2</td></tr><tr><td>M141-1240</td><td>38°16,056'</td><td>024°53,237'</td><td>3,262</td><td>3.65</td></tr><tr><td>M141-1245</td><td>37°58,092'</td><td>026°05,427'</td><td>3,213</td><td>3.72</td></tr><tr><td>M141-1247</td><td>36°52,020'</td><td>025°28,737'</td><td>2,374</td><td>5.36</td></tr><tr><td>M141-1249</td><td>37°27,616'</td><td>026°12,150'</td><td>2,364</td><td>11.15</td></tr></tbody><tfoot><tr><td colspan="5">Total core length: 140.83</td></tr></tfoot></table>

representative to gain insights on the volcanic history and the depositional marine environment around the islands of the eastern group of the archipelago (Figure 1). The selected cores were taken at water depths of 2,010–4,820 m, with sediment recovery varying from 1.76 to 11.75 m, yielding a total of 140.83 m of investigated sediments (Table 2). Variations in core length are attributed to (a) differences in the length of the deployed coring equipment, (b) heterogeneity in sediment thickness, and (c) the occurrence of coarse or consolidated layers that limited penetration depth. The quality of the cores is very good with only limited marginal shearing and internal structures (e.g., bedding, laminations) are mostly well preserved in ash layers thicker than 2 cm, while thinner layers are often overprinted by bioturbation. The uppermost centimeters of each core are often soupy and disturbed due to mixing with seawater.

Cores were cut onboard into 1-m-long sections, split longitudinally into halves, and subsequently logged for structure and lithological composition by visual core description (see Table S1). The cores were photographed and each volcaniclastic horizon was sampled directly on board (Figure S3 in Supporting Information S1). Thicker (e.g., several decimeter) or stratified layers were sub-sampled at multiple levels (e.g., top, middle, bottom, or at visually distinct horizons) to account for potential intra-layer geochemical variability. In total, 454 samples from 350 ash- or pumice/scoria/lapilli-bearing horizons were examined. For a more detailed characterization of the fine-grained horizons, smear slides have been prepared and described (see Table S1).

## 3.2. Sampling of Subaerial Pyroclastic Deposits

Subaerial pyroclastic deposits were sampled on São Miguel Island during two field campaigns in 2003 and 2020, as it is the closest island to the coring sites and the most volcanically active in the archipelago. Representative samples of deposits from the three active central volcanoes were collected to obtain an extended major element
<table><caption>**Table 3**
List of Locations of Outcrops and Descriptions of Samples From Each Volcano</caption><thead><tr><th>Outcrop #/Volcano</th><th>Sample #</th><th>Location/Coordinates</th><th>Description</th></tr></thead><tbody><tr><td>Sm1/Fogo Volcano</td><td>Sm03-1 to Sm03-6</td><td>Vila Franca do Campo western border</td><td>white and gray pumice lapilli (lower, middle, and upper parts of Fogo A)</td></tr><tr><td>Sm2/Fogo Volcano</td><td>Sm03-11</td><td>Ribeira Chã coastal section</td><td>welded ignimbrite and pumice lapilli (Ribeira Chã Formation)</td></tr><tr><td>Sm3/Furnas Volcano</td><td>Sm03-12</td><td>Povoação beach</td><td>welded ignimbrite and pumice fall (Povoação Ignimbrite Formation)</td></tr><tr><td>Sm4/Furnas Volcano</td><td>Sm03-9</td><td>Furnas caldera southern rim</td><td>pumice lapilli and blocks (lower part of Furnas 1630 AD)</td></tr><tr><td rowspan="3">S2/Sete Cidades Volcano</td><td>S2-1 to S2-7</td><td>Candelária cliff 37°49'12.9" N 025°49'33.0" W</td><td>white to light gray pumice lapilli and brown ash layers (Pepom Member)</td></tr><tr><td>S2-1 to S2-5</td><td></td><td>0–140 cm above the grass: light brown and white to light gray layers, light gray pumice lapilli</td></tr><tr><td>S2-6</td><td></td><td>145–175 cm above the grass: beige ash</td></tr><tr><td>S2-7</td><td></td><td></td><td>140–175 cm below the grass: brown horizon with beige pumice lapilli and dark lithics</td></tr><tr><td>S3/Sete Cidades Volcano</td><td>S3-1 to S3-4</td><td>Candelária village 37°49'53.0" N 025°49'17.7" W</td><td>light and dark gray wavy layers, with some brown soil layers light gray to white pumice lapilli layers (Pepom Member)</td></tr><tr><td>S4/Sete Cidades Volcano</td><td>S4-2 to S4-4</td><td>Mosteiros cliff 37°53'17.7" N 025°48'54.9" W</td><td>scoria layers, dark gray to black scoria lapilli (Pico de Mafra?)</td></tr></tbody></table>

glass data set for the explosive eruptions of São Miguel. During the 2003 campaign, samples from Fogo Volcano were collected along its southern flank at Vila Franca do Campo (pumice lapilli of Fogo A Formation; outcrop Sm1; Figure 2a) and at Ribeira Chã (welded ignimbrite and pumice lapilli of Ribeira Chã Formation; outcrop Sm2; Figure 2a). Samples from Furnas Volcano were collected at the southern rim of the caldera (pumice lapilli of the Furnas 1630 AD deposit; outcrop Sm4, Figure 2a) and at the Povoação beach (Povoação Ignimbrite Formation; outcrop Sm3, Figures 2a and 2c). In 2020, additional samples were collected from Sete Cidades Volcano on its western sector, at Candelária coastal cliff and village (two sequences of pumice lapilli and ash of Pepom Member; outcrops S2 and S3, Figures 2a and 2b) and at the Mosteiros area (scoria lapilli possibly from Pico de Mafra cone; outcrop S4, Figure 2a) to extend the data set. A total of 26 ash, pumice and scoria lapilli samples of primary volcanic deposits were collected and geochemically analyzed to gather volcanic glass compositional data for marine-subaerial tephra correlations. The list of outcrops from each volcano and sample descriptions are available in Table 3, and glass geochemical compositions are provided in Table S2.

### 3.3. Sample Preparation

Marine and subaerial lapilli samples were cleaned in an ultrasonic bath and then crushed. All ash samples and the previously crushed lapilli samples were separately wet-sieved into grain size fractions of >250 µm, 250–125 µm, 125–63 µm, and 63–32 µm. The 125–63 µm fraction was embedded with epoxy in pre-drilled acrylic mounts and polished for geochemical microanalyses (for more details on sample preparation see also Kutterolf et al., 2014, 2016, 2021; Schindlbeck et al., 2016, 2018).

### 3.4. Electron Microprobe (EMP)

Major and minor element compositions of glass shards for all samples were determined using a JEOL JXA 8200 wavelength dispersive EMP at GEOMAR Helmholtz-Center for Ocean Research, Kiel, Germany. The accelerating voltage was 15 kV and we used a defocused electron beam of 10 µm with a beam current of 6 nA for all elements. Counting times were 20–40 s for most elements except for Na (5s) and 10–20 s for backgrounds. With a larger beam diameter and the short counting time for Na we avoid Na-loss during analyses reproducing well the compositions of the international, natural and synthetic standards used to test and calibrate the analytical settings. Every 60 individual glass shard analyses were bracketed by a set of four standard analyses (two times Lipari rhyolite (Kuehn et al., 2011) and two times Smithsonian basaltic standard VGA-99 basalt from Makaopuhi Lava
Lake (Jarosewich, 2002; Jarosewich et al., 1980)). We analyzed 15–25 single glass shards per sample (~7,000 analyses in total). Outliers with totals below 92 wt.% as well as accidental mineral analyses were removed from the data set. All analyses were normalized to 100 wt.% anhydrous composition to eliminate the effects of post-depositional hydration. The complete glass data set, including the standard analyses, is available in Table S2 as well as in Belo (2025a, 2025b) on PANGAEA.

### 3.5. Machine Learning

Machine learning algorithms can be extremely valuable for analyzing highly multidimensional data. Bolton et al. (2020) compared the performance of different algorithms for assigning tephra layers to source volcanoes, provided insights into the procedure for performing the classification, and concluded which algorithms are best suited for this application. We compiled an extensive training data set by combining published geochemical glass compositions from São Miguel, Terceira, and Faial islands (Ellis et al., 2022; Horn et al., 2025; Jeffery et al., 2016, 2017; Johansson et al., 2017; Kueppers et al., 2019; Pimentel et al., 2015; Wastegård et al., 2020), and our own major element glass data of subaerial pyroclastic samples from Sete Cidades, Fogo, and Furnas volcanoes. In total, we used 2,799 individual analyses: 1701 from São Miguel, 1018 from Terceira, and 80 from Faial, as the most plausible sources for the marine tephra deposits. Since most coring sites are in the vicinity of São Miguel, we further distinguish the main volcanic centers: 1128 analyses from Sete Cidades Volcano, 151 from Fogo, and 278 from Furnas. Moreover, as the deposits of the Ribeira Chã Ignimbrite from Fogo (43 analyses) and the Povoação Ignimbrite Formation from Furnas (101 analyses) are geochemically distinct, we decided to treat them individually.

Four different models were selected for the classification task: (a) k-nearest neighbors (KNN), (b) neural network (NN), (c) random forest (RF), and (d) a combination of the last two models, which averages their output (AVG). The “k-nearest neighbors” is often used as a baseline for comparison with more complex alternatives, and has the advantage of being a simple model. Although the results for all four models are presented in Tables S3, we have worked with the results obtained by the AVG model (d), since it has an optimal balance between performance and complexity as proposed by Bolton et al. (2020).

All four classifiers were tuned and fitted using a cross-validation strategy on 80% of the training data set (own data and data published in Pimentel et al. (2015), Jeffery et al. (2016, 2017), Johansson et al. (2017), Kueppers et al. (2019), Wastegård et al. (2020), Ellis et al. (2022), Horn et al. (2025)). Since the number of samples from each source volcano is unbalanced, ranging from 1128 analyses from Sete Cidades to only 43 analyses from the Ribeira Chã Ignimbrite, we follow Bolton et al. (2020) and use Cohen's kappa coefficient as the selection criterion (Figure S1 in Supporting Information S1). In contrast to accuracy of predictions, the kappa coefficient takes into account the probability that a prediction is correct by chance, therefore, indirectly taking into account the frequency of each class. The predictions were then validated with the ground truth on the remaining 20% of the data. All models exhibit very high accuracy and kappa coefficient values (Figure S1 in Supporting Information S1) on the validation set. Model AVG (iv) has the best performance with an accuracy of 97.6% and kappa coefficient of 0.969. Implementation details for each model, training and validation procedures, and results are provided in Supporting Information S1, and Table S3, and the code in Supporting Information S2.

It is important to stress that although these results show that the classifiers applied can differentiate among the sources present in the data, the marine tephras exhibit larger compositional variability (Figure S2 in Supporting Information S1). Therefore, the accuracies, when applying these models, are expected to be lower.

Furthermore, we draw attention to the fact that our reference data for evolved tephras become sparse with age and that we presently only have incomplete mafic tephra compositions from most islands.

### 3.6. Radiocarbon Dating

To obtain additional direct ages and to be able to calculate sedimentation rates (indirect ages) also for the more proximal cores, three samples of planktonic foraminifera (>315 µm) were picked and sent to the Radiochronology Laboratory of the Center d'Études Nordiques, University of Laval (Canada) for AMS $^{14}$C dating. The radiocarbon ages were calibrated using the radiocarbon calibration program Calib Rev8.2 (Stuiver and Reimer, 1993) with the calibration data set marine20.14c of Heaton et al. (2020). Sample M141-1238, 96 cm yielded an age of 22,443 years cal. BP, sample M141-1238, 193 cm an age of 43,081 years cal. BC, and sample M141-1240, 51 cm an age of 13,001 years cal. BP. The results of the age calibration are provided in Supporting Information S1.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** (a) Core images showing marine tephra layers representative of the four different facies. (b) Stratigraphic columns for all investigated marine gravity cores sorted from west to east for Transect 1 and Transect 2 and from north to south for Transect 3. Each investigated layer is color coded according to its allocated facies. Question marks indicate layers that are not unambiguously allocated to one facies. CC—core catcher. Red stars mark the location of the samples collected for AMS ¹⁴C dating. Dashed rectangles mark tephra layers that were correlated with an ~166 kyr old tephra from Vlag et al. (2004). The map in the lower right corner shows the core locations along the three transects around São Miguel Island.</figcaption></figure>

# 4. Results

## 4.1. Characterization of Marine Tephra Layers

A total of 350 tephra bearing horizons were identified and described both macroscopically and microscopically for their depositional characteristics (Table S1). Their thickness varies from 0.5 cm to several dm, and grain sizes range from fine ash to medium lapilli (Figure 3a). The contact between each tephra layer and the underlying hemipelagic and pelagic sediment is generally sharp while there is mostly a diffuse transition into background sediment at the top, often disturbed by bioturbation (Figure 3a). However, some tephra layers have sharp erosional upper surfaces. Colors are commonly light gray, beige to dark gray and dark brown (Figure 3a). The detailed descriptions of all investigated samples are provided in Table S1, which includes the macroscopical and microscopical core descriptions, microscopical descriptions, information about the geochemical compositions, and a facies assignment.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Map with pie diagrams showing the respective proportions of the different facies in each core (M141-). Facies F1: primary fall deposits. Facies F2: primary density current deposits. Facies F3: secondary or syn-eruptively reworked density current deposits that might be related to a single volcanic eruption. Facies F4: volcaniclastic, reworked density current deposits/turbidites that are not related to a volcanic eruption. Map source QGIS, bathymetry from GEBCO.</figcaption></figure>

Based on textural, componentry, and compositional features, tephra layers were grouped into four different facies (F1 to F4; Figure 3a), interpreted to reflect different emplacement processes:

F1. Massive or (normal, inversely) graded, well sorted deposits, >75 vol.% of juvenile volcanic components (glass shards, minerals) with homogenous and/or bimodal geochemical composition of glass: primary fall deposits.

F2. Massive or (density, inversely) graded, moderately to poorly sorted deposits, erosive basal contact, >75 vol. %, of juvenile volcanic components (glass shards, minerals) but also incorporated bioclasts with homogenous and/or bimodal geochemical composition of glass: primary density current deposits.

F3. Massive or graded deposits, often with erosive basal contacts, <75 vol.% of juvenile volcanic components (glass shards, minerals) and considerable amounts of lithic and biogenic components, with homogeneous geochemical composition of glass: secondary or syn-eruptively reworked density current deposits that might be related to a single volcanic eruption.

F4. Massive or graded deposits, often with erosive basal contacts, <75 vol.% of juvenile volcanic components (glass shards, minerals) and considerable amounts of lithic and biogenic components, with heterogeneous geochemical composition of glass: volcaniclastic, reworked density current deposits/turbidites that are not related to a volcanic eruption.

It should be noted that core disturbances and bioturbation can modify the internal structures of marine tephra layers and complicate or hinder the facies assignment but this is rarely the case in our cores, and, although not a general rule, tephras thicker than 1 cm normally provide internal structures if available. This is, however, especially relevant when facies assignment cannot be supported by distinct geochemical compositions of glass shards, as it happens between facies F1-F2 and F2-F3, since all three facies comprise homogeneous or bimodal compositions. When a clear distinction of facies was not possible, both options were considered (Table S1, Figure 4).
From the 350 investigated tephra layers, 23% were attributed to facies F1, 22% to facies F2, 28% to facies F3, and 27% to facies F4 based on microscopical and macroscopical observations and geochemical compositions. Thus, we interpret ~45% of the tephra layers as primary (F1 and F2) and ~55% (F3 and F4) as emplaced by secondary processes. The relative facies distribution varies between cores (Figure 4), and depends on the core location with respect to distance from islands, and on bathymetric setting.

Cores on the northern submarine slope of São Miguel (M141-1236, -1238, -1240) have a high percentage of secondary deposits, whereas more distal cores in the east and southeast of São Miguel (M141-1144, -1247, -1148, -1140, -1138, 1136) contain more primary fall deposits (Figure 4). To the west and southwest on the Azores Plateau, increasing amounts of facies F3 and facies F4 were identified, with the latter being more abundant at the most western core sites (M141-1184 and -1176).

## 4.2. Glass Geochemistry of Marine and Subaerial Samples

### 4.2.1. Marine Tephra Layers

The vast majority of all 350 investigated marine tephra layers have trachytic glass compositions with alkali contents between 12 and 14.5 wt.% and silica contents >60 wt.% (TAS diagram, Figure 5a), with only a few samples tending toward a more trachy-andesitic composition (e.g., core M141-1160; Figure 5a). Mafic layers with trachy-basaltic to basaltic glass compositions with silica contents <50 wt.% (Figure 5a) are present in cores M141-1160, -1168, -1176, and -1245. Furthermore, there is a subset of tephra layers with a bimodal trachytic and trachy-basaltic to basaltic composition in cores M141-1144, -1154, -1160, -1176, -1184, -1238, -1245 (Figure 5a, Table S2), which are predominantly located west of São Miguel.

### 4.2.2. Subaerial Pyroclastic Deposits

Major element compositions of glasses from the felsic subaerial pyroclastic deposits (pumice lapilli and ash falls, and ignimbrites) of São Miguel are relatively homogeneous, plotting in the trachyte field of the TAS diagram, with 62–67.5 wt.% of SiO₂ and 12–14.5 wt.% of Na₂O + K₂O (Figure 6b). Regarding samples from Fogo Volcano, the Fogo A Formation (outcrop Sm1, Figure 6a) has lower total alkali contents (<13 wt.% Na₂O + K₂O), higher CaO contents (>0.8 wt.%), and higher TiO₂ contents at given FeOt concentrations (Figure 6d). The Ribeira Chã Formation (outcrop Sm2, Figure 6a) has high CaO/TiO₂ but lower TiO₂ contents (<0.5 wt.%) at given FeOt contents (>3.5 wt.%) (Figure 6d).

On what concerns Furnas Volcano, the samples of the Furnas 1630 AD deposit (outcrop Sm4, Figure 6a) are characterized by high alkali (>14 wt.%) and high FeOt contents (>3 wt.%) (Figures 6b–6d). The Povoação Ignimbrite Formation (Sm3, Figure 6a) can be distinguished by low FeOt contents (<2.5 wt.%) and high TiO₂ contents (Figure 6d).

Since the Ribeira Chã Ignimbrite (Fogo) and the Povoação Ignimbrite (Furnas) show distinct geochemical characteristics (Figures 6c and 6d), we opted to present and discuss these data individually in the following sections instead of grouping them with the remaining data for Fogo and Furnas volcanoes (Figures 6–8).

Felsic samples from the Pepom Member of Sete Cidades Volcano (outcrops S2 and S3; Figure 2a) show the greatest variability, with Al₂O₃ varying from 15.5 to 18.5 wt.%, FeOt from 1 to 3.6 wt.%, and TiO₂ from 0.1 to 0.9 wt.% (Figures 6c and 6d).

Mafic samples from Sete Cidades, scoria lapilli and ash layers, possibly of Pico de Mafra cone (outcrop S4, Figures 2a and 6) show relatively homogenous basaltic/trachy-basaltic glass compositions characterized by 45–47.5 wt.% of SiO₂ and 4–6 wt.% of Na₂O + K₂O (Figure 6b).

## 4.3. Volcanic Source Assignment

Reference geochemical fields were built using a compilation of published geochemical glass compositions from São Miguel, Terceira, and Faial islands (oxide concentrations and ratios; Ellis et al., 2022; Horn et al., 2025; Jeffery et al., 2016, 2017; Johansson et al., 2017; Kueppers et al., 2019; Pimentel et al., 2015; Wastegård et al., 2020), and our own major element glass data of subaerial pyroclastic samples from Fogo, Furnas, and Sete Cidades volcanoes to graphically correlate the primary marine tephra layers (facies F1 and F2) with possible onshore volcanic sources (Figure 5) (glass compositions for Graciosa Island are not yet available for correlation;
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (a) Total alkali versus silica (TAS) diagram following Le Bas et al. (1986) for classification of primary marine tephra samples (facies F1 and F2). (b) CaO/TiO₂ versus Al₂O₃ wt.% showing the large overlap of the marine samples but also slight compositional differences most likely associated with the volcanic source. In both panels the symbols show average glass shard compositions of ash beds in the cores listed at right, with standard deviations shown by gray bars. Correlation fields for the different volcanic sources are color coded following the legend shown as pie diagram. Reference fields are based on glass data from pyroclastic material analyzed by electron microprobe from Kueppers et al. (2019), Johansson et al. (2017), Jeffery et al. (2016, 2017), Wastegård et al. (2020), Pimentel (2015), Ellis et al. (2022), Horn et al. (2025), and new data from this study. We exclude Graciosa Island for which glass compositions are not yet available, Santa Maria and Corvo, which did not produce eruptions over the time frame considered in the study, and Flores, which is too far away to contribute ash to the study area.</figcaption></figure>

Santa Maria did not experience subaerial volcanism over the time frame considered in our study; eruptions on Flores and Corvo were likely not large enough to disperse tephra across the study area). The São Miguel major element glass compositions can be reasonably well distinguished from those of Terceira and Faial, although there is some overlap. We thus observe that the majority of the marine tephra layers are derived from São Miguel, although some distal ash layers show good correlation with deposits from Terceira (Figure 5, light blue) and possibly with the Faial Islands (Figure 5, dark blue). The glass chemical distinction between volcanic centers on São Miguel and between single eruption products is difficult due to considerable overlap. Therefore, standard tools for visualization of major element geochemical data proved insufficient in discriminating individual tephra layers and/or volcanoes/volcanic centers, apart from a few distinctive cases (e.g., some tephra layers in cores M141-1160, or -1245; Figure 5). Machine learning classification algorithms proved, in turn, to be a powerful and
<figure><img src="image_9.png" /><figcaption>**Figure 6.** (a) Map showing outcrop locations on São Miguel of new samples obtained for this study. Map created using GeoMapApp https://www.geomapapp.org (Ryan et al., 2009). (b–d) Symbols show major element glass compositions of these samples. Symbols represent single shard analyses and refer to sampling location in panel (a). Sample descriptions are provided in Table 3. Total alkali versus silica diagram in (b) following Le Bas et al. (1986).</figcaption></figure>

indispensable tool to overcome the compositional similarities and complex distribution of our large and multi-
dimensional database. This is particularly true when trace element contents are not available, allowing the
assignment of probable sources to the majority of the primary and secondary marine tephra layers. Exceptions are
some marine tephras with mafic composition (e.g., <60 wt.% SiO₂ and >4 wt.% CaO), probably due to a lack of
onshore equivalents in our data set, which only contains mafic compositional data from Sete Cidades (São
Miguel, dark green in Figure 5) and Faial (dark blue in Figure 5). Figure 7 illustrates the proportion of the different
attributed volcanic sources of tephra layers in each marine core, both within the primary facies F1 and F2
(Figure 7a), and the secondary facies F3 and F4 (Figure 7b).

Transect 1, from west to east along the north of São Miguel, comprises the cores M141-1245, -1236, -1238, -1240, -1154, -1152, and -1136 (Figures 3, 4 and 7). The tephra inventory of core M141-1245 in the vicinity of Sete Cidades Volcano is dominated by primary deposits with mafic and bimodal compositions (Figure 3b) that are primarily sourced from eruptions at Sete Cidades volcano (Figure 7a). The cores on the northern slope of the island (M141-1236, -1238, -1240) are dominated by secondary deposits (facies F3 and F4; Figure 4), which show similarities with compositions that are known for eruptions from Sete Cidades, Fogo and Furnas volcanoes on São Miguel, and compositions that are known from Terceira Island (Figure 7b). Primary tephra layers in core M141-1236 have similarities to deposits known from Fogo volcano, Sete Cidades volcano, and eruptions on Terceira, while primary layers in core M141-1238 show a wider range of allocated sources (Figure 7a). In contrast, primary deposits in core M141-1240, which is located further away (75 km from São Miguel coastline), are
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Maps with pie diagrams each indicating the allocated source predictions (AVG model) for (a) primary facies F1 and F2 and (b) for secondary facies F3 and F4. Core images on the left side show exemplary layers of each facies. Maps created using QGIS and bathymetry from GEBCO. The number of tephra beds per coring site is illustrated in Figure 3.</figcaption></figure>

probably exclusively sourced from Terceira (Figure 7a). The three cores located at the eastern margin (M141-1152, -1154, -1156) as well as the distal core M141-1136 also contain a high proportion of secondary deposits (Figures 3b and 4), which derive from various sources (Figure 7b). In core M141-1156, these secondary deposits predominantly derive from Furnas volcano, whereas the sources at cores M141-1154 and -1152 are more diverse. Primary layers in these cores are mostly comparable with Fogo, Sete Cidades, and Furnas volcanoes but also with Terceira (Figure 7a). The sources for primary deposits in the distal core M141-1136 are dominated by the Furnas Volcano (Figure 7).

Transect 2 starts in the southwest of São Miguel with distal cores M141-1184 and -1176, continues with cores M141-1168, -1249, -1164, -1162, -1160, and -1144 south of São Miguel and west of Santa Maria, and ends in the east with distal cores M141-1150, -1138, and -1136. All cores, beside M141-1164, -1160, -1144, -1138, and -1136 are dominated by secondary deposits (Figure 4). Secondary deposits in cores M141-1184 and -1176 are sourced from Terceira Island, Fogo Volcano, Furnas Volcano, and to a minor proportion from Sete Cidades volcano (Figure 7b). Most of the primary layers likely derive from Terceira and, to a minor extent, from Furnas, Sete Cidades, and Faial (Figure 7a). The primary tephra layers in core M141-1168, -1249, and -1162 are dominated by compositions known from Sete Cidades. Overall, the sources for primary and secondary
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Stratigraphic columns for the investigated marine gravity cores (M141-) showing the primary tephra deposits (facies F1 and F2). Each layer is color coded according to its predicted source. Cores are sorted from West to East for Transect 1 and Transect 2, and from North to South for Transect 3. Tephra layers that are two-colored are either bimodal in composition or the source prediction of different samples from the same layer is ambiguous. The map in the lower right corner shows the core locations along the three transects around São Miguel Island.</figcaption></figure>

tephra layers are more diverse in these cores. The primary and secondary tephra layers in cores M141-1164, -1144, and -1160 are all dominated by tephra layers with glass compositions that can be assigned to eruptions known from Fogo and Furnas volcanoes as well as to mafic eruptions from Sete Cidades (M141-1160). Tephra layers in core M141-1144 are dominated by compositions similar to the Povoação Ignimbrite Formation from Furnas Volcano.

The fraction of tephra layers that are assigned to Terceira decreases from west to east (Figures 7a and 7b) and tephra layers in the eastern cores M141-1150, -1138, and -1136 can be further assigned to Sete Cidades, Fogo, and Furnas volcanoes (Figure 7).

*Transect 3* extends southward from core M141-1160 (~50 km South of Fogo Volcano) to core -1146 (~250 km away). Most of the cores, except M141-1146, are dominated by primary tephra deposits (Figures 4 and 7a). The main sources of the deposits (primary and secondary) are Fogo, and Furnas Volcano, and with increasing distance Sete Cidades Volcano (e.g., core M141-1146; Figure 7). Cores M141-1160, -1144, and -1247 all contain primary mafic layers that show similarities to mafic compositions known from either the Sete Cidades Volcano on São Miguel or Faial.

# 5. Discussion

## 5.1. Spatial Distribution of Facies

The type and proportion (45% primary to 55% reworked) of tephra facies across the eastern Azores region reflect the balance between the constructive (e.g., volcanism) and destructive (e.g., erosion, landslides, caldera or flank
collapses) episodes that have been shaping the geological and morphological evolution of the region, and in particular of the islands.

Primary fall deposits (facies F1) from volcanic eruptions are found up to ~230 km away from their source island (e.g., M141-1146, -1136), and are generally well distributed across the working area (Figure 4). Their higher proportion in the eastern and southeastern sectors of São Miguel agrees well with the dominant westerly wind patterns of the Azores (Gaspar, Queiroz, et al., 2015; Gaspar, Guest, et al., 2015; McLean et al., 2024; Sáez et al., 2025) as well as with paleo-wind directions reconstructed from tephra dispersal on São Miguel over the last 5,000 years (Booth et al., 1978). Simulated moderate-sized Plinian eruptions (15 km plume height) from Sete Cidades using Ash3D generally show that the results strongly depend on the simulated run date and time; however, the main ash dispersal was generally directed toward the east and northeast (McLean et al., 2024).

Although eastward ash dispersal may explain the relatively higher proportions of secondary facies found in the west and southwest (e.g., M141-1184, -1176, -1168, -1249), other factors may contribute to the asymmetry in facies proportions along the northern and southern sectors of the island. While the submarine southern flank of São Miguel is characterized by a constant downslope sediment transport, resulting in a low-relief morphology with cyclic steps and chutes-and-pool structures, the northern submarine flank is characterized by large undulated lobe deposits and a well-developed net of channels (Figures 1 and 4), which have been interpreted as the result of turbidity currents during North Atlantic storm swells (Weiß et al., 2016). This morphology has allowed turbidity currents to pass the shelf break through retrogressive canyons/gullies and follow the widespread system of channels toward the deep sea (Weiß et al., 2016), which is also reflected by the higher proportions of secondary (reworked; facies F3 and F4) deposits in cores M141-1236, -1238, -1240 and -1154 (Figure 4). In cases where gullies are not connected to channel systems or turbidity currents overspill the channel walls, stacked lobe deposits are formed (Weiß et al., 2016). More details about the submarine sedimentation processes in the eastern Azores are discussed in Weiß et al. (2016).

## 5.2. Age Constraints for the Marine Tephra Archive

The spatial distribution of the different sedimentary facies offers valuable insights into past environmental conditions, including paleo-wind directions, seafloor morphology, and the topography of nearby islands. However, a more comprehensive interpretation emerges when temporal constraints are integrated into the analysis. The incorporation of a robust chronological framework allows for the reconstruction of the timing, duration, and sequence of volcanic, climatic, and other geological events, thereby significantly enhancing the understanding of the processes that shape the depositional environment. Creating age models for the different marine cores is, however, a major challenge due to the variable depositional environments around the islands, which may lead to sedimentation rates varying from core to core.

Previous studies in the area estimated sedimentation rates of 2.4 cm/kyr for a core ~90 km northeast of Terceira (SU-92-17) and 3.5 cm/kyr for a core ~100 km south of Terceira (SU-92-18) (Lehman et al., 1996), the latter in close proximity to our core M141-1176 (~10 km) (Figure 1).

For core SU-92-18, oxygen isotope stratigraphy has been established spanning the last ~270 ka (Lehman et al., 1996), complemented with rock magnetism analysis, paleointensity estimates and XRF (X-ray fluorescence) measurements by Kruiver et al. (1999) and Vlag et al. (2004). Although no geochemical glass data is available, we were able to correlate core SU-92-18 with our cores M141-1176 and M141-1168 using a prominent dark, normally graded tephra layer dated at ~166 ka, which is described in Kruiver et al. (1999) and Vlag et al. (2004) between 582 and 605 cm depth, and which we found as primary deposit at 528–545 cm depth in core M141-1176 and between 520 and 546 cm depth in core M141-1168 (Figure 3). Extrapolating the sedimentation rate of 3.5 cm/kyr (assuming a constant sedimentation rate) to the entire length of our cores results in an age of ~323 ka at a maximum depth of 1175 cm bsf (below sea floor; core M141-1176). For core M141-1168, this correlation suggests a slightly lower sedimentation rate of 2.8 cm/kyr, with a basal age of ~254 ka (960 cm bsf). Note that these rates refer to hemipelagic sedimentation rates, thus excluding the thicknesses of sudden event deposits (turbidites, tephra layers) in the calculations (Table 4; Table S1).

To better estimate the sedimentation rates in core locations that are more proximal to the islands, we measured three radiocarbon ages for intervals M141-1238, 96 and 193 cm, and M141-1240, 51 cm (Figure 3b). The ages are 22,443 years cal. BP, 43,081 years cal. BC, and 13,001 years cal. BP, respectively, corresponding to average
<table><caption>**Table 4**
Core Lengths, Estimated Sedimentation Rates, and Estimates of the Maximum Age for Each Core</caption><thead><tr><th>Station #</th><th>Distance to closest island (km)</th><th>Recovery (m)</th><th>Background sediment, event layers subtracted (m)</th><th>Age estimate (ka)</th></tr></thead><tbody><tr><td>M141-1136</td><td>230</td><td>6.92</td><td>6.7</td><td>191</td></tr><tr><td>M141-1138</td><td>90</td><td>3.77</td><td>3.51</td><td>100</td></tr><tr><td>M141-1140</td><td>75</td><td>1.76</td><td>1.4</td><td>40</td></tr><tr><td>M141-1144</td><td>30</td><td>4.45</td><td>2.92</td><td>83</td></tr><tr><td>M141-1146</td><td>130</td><td>7.93</td><td>7.46</td><td>213</td></tr><tr><td>M141-1148</td><td>75</td><td>6.66</td><td>5.14</td><td>146</td></tr><tr><td>M141-1150</td><td>30</td><td>5.19</td><td>4.51</td><td>128</td></tr><tr><td>M141-1152</td><td>110</td><td>7.25</td><td>5.7</td><td>162</td></tr><tr><td>M141-1154</td><td>60</td><td>7.98</td><td>6.44</td><td>184</td></tr><tr><td>M141-1156</td><td>37</td><td>4.1</td><td>2.91</td><td>83</td></tr><tr><td>M141-1160</td><td>37</td><td>4.33</td><td>2.75</td><td>79</td></tr><tr><td>M141-1162</td><td>42</td><td>9</td><td>7.35</td><td>210</td></tr><tr><td>M141-1164</td><td>70</td><td>9</td><td>8.17</td><td>233</td></tr><tr><td>M141-1168</td><td>85</td><td>9.6</td><td>5.79</td><td>254</td></tr><tr><td>M141-1176</td><td>105</td><td>11.75</td><td>11.33</td><td>323</td></tr><tr><td>M141-1184</td><td>100</td><td>5.48</td><td>4.2</td><td>120</td></tr><tr><td>M141-1236</td><td>40</td><td>7.58</td><td>6.5</td><td>185</td></tr><tr><td>M141-1238</td><td>25</td><td>4.2</td><td>2.71</td><td>77</td></tr><tr><td>M141-1240</td><td>52</td><td>3.65</td><td>2.01</td><td>53</td></tr><tr><td>M141-1245</td><td>25</td><td>3.72</td><td>1.3</td><td>37</td></tr><tr><td>M141-1247</td><td>26</td><td>5.36</td><td>4.4</td><td>125</td></tr><tr><td>M141-1249</td><td>50</td><td>11.15</td><td>10</td><td>285</td></tr></tbody></table>

sedimentation rates between 3.4 and 3.8 cm/kyr (in Supporting Information S1), fitting well with previously published sedimentation rates for the region (Lehman et al., 1996).

Therefore, we calculated rough age estimates for the remaining cores using 3.5 cm/kyr as an average sedimentation rate, consistent with core SU-92-18. Considering this sedimentation rate, the M141/1 marine cores cover time intervals from ~35 ka for shorter cores close to São Miguel (e.g., M141-1245), and up to ~320 ka for the longest cores in the more distal region (e.g., M141-1176; Table 4).

Although based on average sedimentation rates and ignoring possible local and temporal variations in these rates, our age models allow the investigation of long-term variations within cores and along the different transects. However, to create more robust age models and elaborate temporal changes of eruption processes and volcanic activity in greater detail, additional age dating and tuning are needed.

### 5.3. Source Variations Over Time

Competition between volcanic construction and erosive destruction on volcanic islands varies in space and time as island edifices evolve. Volcanism commonly migrates, leaving the extinct areas exposed to destruction processes and adding land to the new active zones. Despite its east-west elongated shape, São Miguel Island does not show a clear directional migration of volcanic activity. With the exception of the eastern part of the island, which is composed of the older and extinct Povoação and Nordeste volcanic centers (Duncan et al., 2015), the other volcanic systems are active and have erupted contemporaneously during the last millennia. Although equally active, current age constraints show different timings for the onset of volcanism on its three active central volcanoes Fogo (~280 ka; Muecke et al., 1974), Sete Cidades (~210 ka; Moore, 1983), and Furnas (~138 ka; Sibrant et al., 2015b). The ~280 ka age attributed to Fogo Volcano has been obtained from submarine lavas sampled from a drill core on the northern flank (McGraw et al., 1976; Muecke et al., 1974), which are considered to correspond
to the initial stages of formation of the volcanic edifice (Wallenstein et al., 2015). This is consistent with the oldest age for a subaerial lava flow of ~270 ka (Sibrant et al., 2015b). To investigate possible variations in the source of explosive volcanism over time, we show how predicted tephra sources for primary deposits are distributed in the core profiles along the three transects around São Miguel (Figure 8). As outlined in Section 5.2, the presented ages for the tephra layers result from rough age-depth models, and therefore should be seen as general references and not as absolute ages. Therefore, we also refrain from including these first age estimates in Figure 8.

Tephra layers assigned to Fogo (orange in Figures 7 and 8), Sete Cidades (light and dark green in Figures 7 and 8), and Furnas (red and light pink in Figures 7 and 8) dominate the marine tephra record.

Tephra layers from Fogo are generally present over the entire marine record, with its oldest recorded eruption dating at ~228 ka (M141-1146, 764–767 cm) and the youngest at ~17–19 ka (M141-1148, 66–80 cm and M141-1144, 61–63 cm) (Figures 3b and 8). Our results expand the time period for subaerial explosive volcanism at Fogo Volcano to ~228 ka, which agrees with the oldest age ~270 ka obtained by Sibrant et al. (2015b) for subaerial products.

It is worth noting that Fogo tephras are common in the long-term record of explosive eruptions in the marine cores (Figure 8), whilst they are subordinate in the well constrained <5 ka tephra record on land, in comparison to Sete Cidades or Furnas tephras (Booth et al., 1978; Gaspar, Guest, et al., 2015). Interestingly, the M141/1 sediment cores also do not contain any Fogo tephra layers younger than 5 ka. This suggests that Fogo Volcano may have become less active over the last 5 kyr. This is in agreement with the lower eruption frequency of Fogo in the last 5 ka, when compared to the Sete Cidades and Furnas volcanoes (Gaspar, Queiroz, et al., 2015; Gaspar, Guest, et al., 2015). However, it has to be noted that unconsolidated sediments near the seafloor can be disturbed or removed by gravity-corer impact, which generally complicates the reconstruction of the most recent tephra records.

Tephra layers correlated to the Sete Cidades Volcano (light and dark green in Figure 8) are distributed along the temporal record of our marine sequences with the oldest tephra layer yielding an age of ~274 ka (M141-1249, 1,069–1,071 cm) and the younger being as recent as the present time/last centuries (M141-1245, 0–10 cm), ages that generally fit within the age range of the volcanic system (see Queiroz et al., 2008, 2015). Most of these tephra layers occur in the upper part of the cores, over the last 100 kyr (Figure 8). For the period between ~200 and ~274 ka, our marine record shows several tephras from explosive eruptions that have been assigned to Sete Cidades (e.g., M141-1249, 968–970 cm and M141-1249, 1,069–1,071 cm). This time period corresponds to the early history of the volcano from the shield-building stage to the subaerial construction of the central edifice of Sete Cidades, which was predominantly formed by the effusion of basaltic and trachytic lavas, although some pyroclastic deposits (e.g., ignimbrites, surges, and pumice fall layers) also occur (Queiroz et al., 2015).

Tephra layers assigned to the Furnas Volcano (light and dark red in Figure 8) mainly occur in the upper part of the cores over the last 100 kyr. This is in agreement with the known ages of less than ~138 ka for subaerial volcanic products (Guest et al., 1999; Sibrant et al., 2015b). A few tephra layers that may be linked to Furnas occur in the lower part of cores M141-1168, -1164, -1162, and -1146) at depths with estimated ages of >100 ka. This is consistent with the oldest age obtained by Sibrant et al. (2015b) for subaerial trachytic pyroclastic products of Furnas Volcano, and may imply that Furnas is older than previously thought, although the uncertainties in source prediction preclude any robust interpretations.

## 5.4. Correlation to Individual Eruptions

Although the correlation between marine tephra layers and individual eruptions falls out of the scope of this study, our new geochemical data from the subaerial deposits suggests a direct correlation between marine tephra layers in cores M141-1140, -1144, -1146, -1148, -1152, -1154, -1162, -1164, -1168, -1238, -1247, and -1249 and subaerial deposits from the Povoação Ignimbrite Formation (Figures 8 and 9), which records the largest explosive trachytic eruption sourced from Furnas Volcano, responsible for the formation of its main caldera (Duncan et al., 1999; Guest et al., 2015). Based on our age estimations, several tephra layers assigned to the Povoação Ignimbrite Formation occur at depths equivalent to ~39–25 ka (e.g., cores M141-1140, -1144, -1146, -1152, -1247), which is close to the ~30 ka age attributed to this eruption (Duncan et al., 1999). In several cores, there are multiple tephra layers associated to the Povoação Ignimbrite Formation (e.g., cores M141-1144; Figure 8), which might be the products of older eruptions. Charcoal fragments in soils below the Povoação Ignimbrite yielded ages
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Glass compositions of primary tephra layers (facies F1 and F2) that are geochemically similar to the Ribeira Chã and Povoação Ignimbrite Formation. (a) K₂O versus FeO and (b) Al₂O₃ versus CaO/TiO₂ illustrating how the marine tephra samples match the compositional fields derived from land data. Both panels show average glass shard compositions with standard deviations shown by gray bars. Reference fields are based on glass data from pyroclastic material analyzed by electron microprobe from Horn et al. (2025), and new data from this study.</figcaption></figure>

around 35 to 30 ka (Guest et al., 1999). Only proximal exposures are preserved on land; therefore, most of the pyroclastic material resulting from this eruption might have been deposited offshore (Duncan et al., 1999). In fact, our marine tephra data traces the presence of Povoação Ignimbrite ash in all three transects and up to 200 km south of Furnas.

There are three primary marine tephra layers with similar geochemical compositions to the Plinian Ribeira Chã eruption (Figures 8 and 9), which is assumed to have taken place between 12 and 8 ka at Fogo Volcano (see Wallenstein et al., 2015). However, the estimated ages between 51 and 44 ka for these marine tephra layers are too old (M141-1160, 215–261 cm; 51 ka and M141-1164 (163–176 cm and 180–187 cm; 44 and 45 ka, respectively). So either the calculated ages for these cores are too old, because the sedimentation rate used was too slow, or there have been older eruptions with similar geochemical signatures.

Although the majority of source predictions are related to volcanoes on São Miguel, a considerable number of tephras along Transect 1 and the western sector of Transect 2 have been assigned to Terceira Island. These include both fall and flow deposits. The majority occurred in the time interval between ~100 and ~20 ka (Table S1), the period during which Pico Alto Volcano was intensively active, with several Plinian and sub-Plinian eruptions, including several ignimbrite-forming eruptions (Gertisser et al., 2010; Jeffery et al., 2017).

Chang, Mitchell, Hansteen, et al. (2023) and Chang, Mitchell, Schindlbeck-Belo, et al. (2023) correlated a marine tephra deposit in a core ~40 km north of Terceira (core M141-1230; Figure 1) to the Lajes-Angra Ignimbrite Formation. However, future trace element analyses and additional age dating will help to substantiate this and other possible correlations. Nevertheless, these data show that highly explosive eruptions at Terceira were frequent until ~20 ka. This is in agreement with on-land stratigraphy, which shows that the Lajes-Angra Ignimbrite Formation (23–20 ka) records the last major explosive event(s) on Terceira (Pimentel et al., 2021). Some of these highly explosive eruptions dispersed tephra up to >300 km away from the source. The lack of marine tephra layers over the last 20 ka, in turn, suggests a decrease in the explosiveness of Terceira's eruptions in recent times, which is compatible with the on-land record. Recent eruptions (<20 ka) on Pico Alto and Santa Bárbara volcanoes were dominated by the effusion of lava domes and coulées (see Self, 1976).

## 5.5. Widespread Explosive Basaltic Volcanism at the Azores

Highly explosive volcanic eruptions on the Azores, such as the Fogo A eruption at São Miguel Island or the Lajes-Angra ignimbrite-forming eruption(s) at Terceira, which spread tephra over large areas, are commonly trachytic in composition (e.g., McLean et al., 2024; Pimentel et al., 2021). However, explosive basaltic eruptions, such as the 1957/58 AD Capelinhos Surtseyan eruption on Faial (Cole et al., 2001), have also occurred across the archipelago. Their products are, however, mostly deposited proximally to the vent or in the vicinity of the islands. Highly explosive basaltic eruptions generating Plinian-like ash dispersals are not known so far in the Azores.

The studied cores contain a subset of marine tephra layers of basaltic (s.l.) or bimodal compositions (Figures 5 and 10). Although most of these layers derive from secondary depositional processes (Table S1), 19 have been
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Average glass shard compositions per sample of primary mafic layers (standard deviation shown as gray bars). (a) SiO₂ vs. Na₂O + K₂O, and (b) FeO vs. TiO₂. Reference fields are based on data from Johansson et al. (2017), Jeffery et al. (2017), and data from this study. (c) Exemplary core image of interval M141-1176, ~528-545 cm, showing a well sorted dark ash bed with sharp curved basal contact, a thinly laminated lower 3 cm section, and an upper part mixed with background sediment. (d) Exemplary core image of interval M141-1160, 382-390 cm, which is a massive to vaguely laminated dark coarse ash layer with reworked top and a sharp but irregular basal contact. The wavy shape of the bed was possibly formed by disturbance during coring.</figcaption></figure>

classified as primary tephra layers, eight of which having homogeneous mafic glass compositions, and the remaining 11 comprising bimodal compositions with both mafic and felsic glass shard populations (Figure 10, Table S1). Such deposits are widely distributed around São Miguel and are preserved in cores ~70 km east of Furnas (M141-1154), ~30 km northeast of Sete Cidades (M141-1238), ~30 km west of Sete Cidades (M141-1245), between ~45 and 100 km south of Fogo (M141-1160, -1144, -1247), as well as in core M141-1176 located between the eastern and central group of islands (Figures 1 and 7). With such wide dispersals, these tephra layers may have indeed resulted from highly explosive mafic eruptions.

Our machine learning approach suggests the Sete Cidades Volcano as a possible source for the majority of these mafic tephra layers, and sedimentation rate-based age estimates are compatible with the time period of the Sete Cidades Superior Group (<36 ka), except for one layer (M141-1154, 710-740 cm) with an estimated age of ~159 ka. However, it should be pointed out that uncertainties in the source prediction by the machine learning algorithms cannot be overlooked due to the lack of glass geochemistry data from other volcanic systems with recent mafic eruptions (i.e., Picos and Congro fissure zones, and Fogo Volcano).

A minority of mafic ash layers show geochemical signatures that differ from Sete Cidades and reveal similarities to known deposits from Faial Island (Capelinhos 1957/58 AD), although other sources should be considered. These tephra layers occur in cores M141-1247 (513-524 cm; ~121 ka), and M141-1176 (484-488 cm and 528-545 cm; ~129 and ~166 ka), which roughly correspond to a NW-SE trajectory south of São Miguel (Figure 1). It should be noted that our data set for correlation only contains mafic glass compositions from Faial and Sete Cidades; therefore, we do not account for other sources of the mafic marine tephras, which may include other islands (e.g., Pico and São Jorge) or seamounts known to have produced basaltic explosive eruptions (e.g., Ferreira et al., 2015; Nunes, 1999; Self, 1976; Weiß et al., 2015). Further studies are needed to determine the provenance of these mafic marine layers. However, regardless of the source, our data show that highly explosive basaltic eruptions capable of dispersing ash over great distances have repeatedly occurred in the Azores region in the past.

## 5.6. Why Core Locations Matter

Volcanic islands result from a complex interplay between constructive and destructive processes which are often preserved on the seafloor by event layers. The access to these event layers and/or to the background marine sediments allows the reconstruction of the geological, tectonic, climatic, or environmental evolution of a region (e.g., Ausín et al., 2020; Schindlbeck et al., 2018; Triantaphyllou et al., 2021). Accordingly, a thorough selection of coring sites has to be done depending on the scientific questions being investigated. Targeting the best coring locations can, however, be a complex task, especially when core acquisition is limited either by funding or time constraints, which may reduce the number of sampling sites. The 22 investigated marine cores presented here
provide an opportunity to assess the selection of coring sites and to provide guidance for future coring around
oceanic islands.

Our three transects show a general increase in sediment recovery but also a decrease in the number and overall grain size of tephra layers per depth interval as we move away from the islands (e.g., core M141-1245 vs. M141-1136 on Transect 1; Figure 8). Therefore, more distal cores might be more suitable for long-term climatic and environmental reconstructions, as they preserve comparatively long and continuous marine records. However, they provide limited information regarding the volcanic or mass-wasting events taking place on land. Although more complete, proximal sequences tend to preserve coarser sediments, making core penetration more challenging, ultimately resulting in the recovery of shorter cores, and subsequently acquisition of shorter time records (e.g., core M141-1245; Figure 8). Obtaining an optimal balance between proximal and distal coring locations often requires a thorough analysis of the bathymetry and sedimentation processes around the islands.

For the investigated M141/1 marine cores, the distribution of the primary tephra fall deposits (facies F1; Figures 3 and 4) in combination with the source allocation clearly reflect the prevailing westerly winds, including their relation with periods of positive and negative phases of the North Atlantic Oscillation (NAO), and latitudinal shifts in the trajectory of the westerlies and jet stream current over time (Sáez et al., 2025). Thus, marine sequences in the eastern sectors of the volcanoes are crucial to reconstruct eruptive histories and eruption source parameters (e.g., tephra volume, column height, eruption duration), since they likely preserve a higher number of marine tephra layers. Volcanic plumes from highly explosive eruptions can, however, show complex behaviors due to wind stratification as shown by the 2011 Grímsvötn eruption in Iceland (Prata et al., 2017) or the Laacher See eruption in the Eifel (van den Bogaard & Schmincke, 1985), which were characterized by a dual wind ash dispersion. When investigating individual Plinian eruptions, for example, for real volume estimations of the deposits or tephra dispersal reconstructions, limiting the sampling campaign to a single area may preclude access to crucial information. Accordingly, for tephrostratigraphic studies, information regarding current and/or past wind directions, depending on the timing of the eruptions being investigated and their estimated eruptive column height, may help delimit areas of interest for coring.

While fallout deposition is generally uniform across the sea floor, deposition of density currents (either primary or secondary) is generally more complex and depends on the conjugation of different factors including sea floor morphology (e.g., slope angles and presence of topographic highs, basins or submarine channels) and the internal characteristics of the flow (e.g., internal energy, and particle concentration and density), resulting in great lateral changes of the deposits facies (Sumner et al., 2012). Similar to other ocean island settings (e.g., Kwasnitschka et al., 2024), the submarine domain of the Azores is dominated by several bathymetric highs of volcanic origin, including lava terraces, lava flows, seamounts (up to 3,000 m in diameter and 25° of slope angle), and crater and collapse structures, which intercalate with relatively deep basins (Weiß et al., 2015). Moving sediments tend to pond in basins, resulting in a sudden decrease in particle concentration and energy that prevents the flow to progress further. When encountering a topographic high (e.g., fault scarps or seamounts), low energy flows will tend to be blocked and deposit sediments at the foot of such obstacles. High energy flows, on the other hand, may be able to partially ascend along the slopes of the topographic highs or redistribute the currents around them, causing the deposition of drift bodies. Little sedimentation occurs along the slopes, which are also subject to high remobilization rates and thus should be avoided as core locations, giving preference either to bathymetric highs or lows.

Bathymetric lows typically have higher sedimentation rates and usually higher recovery, with the possibility to better distinguish between individual layers. However, the chance of sampling remobilized sediments (e.g., from creeping or sliding) and small turbidites originating from the slopes is higher, as shown by core M141-1162 (Figures 3 and 4). Moreover, if relatively proximal to the foot of bathymetric heights, such reworked deposits may be thick and/or coarse enough to stop core penetration, as shown by the sediments found in the core catcher of cores M141-1156 and -1247 (CC, Figure 3).

At bathymetric highs, sequences are usually more condensed and less rich in flow deposits, which may result in the acquisition of longer time records. At the same time, however, the distinction of single events might become more challenging. Moreover, the occurrence of highly compacted sediment layers may hinder gravity cores from penetrating deep into the sediments.
Depending on the goal of the study, it might also be necessary to account for potential factors controlling deposit preservation, such as slope angles, bottom currents (especially in shallower areas), or intense bioturbation, which can lead to total or partial erosion, and remobilization of the deposits. Studies around Faial and São Miguel islands have shown that offshore directed bottom currents, sometimes as a result of intense onshore winds, may be capable of mobilizing and transporting the sediments to the shelf edges of the islands (Chang et al., 2021; Weiß et al., 2016). Channelized regions, such as the northern submarine slope of São Miguel, constitute erosional/non-depositional environments, and their surroundings often contain stacked deposits from overspilled turbidity currents. Therefore, slope settings and regions within or proximal to submarine channels should be avoided when aiming for primary volcanic deposits.

After selecting areas of potential interest, that is, the topographic lows or highs where the intended deposits may be preserved, bathymetric and/or echosounder surveys are usually required to choose the coring locations, which based on the identification of layering (opposed to massive chaotic sediment successions) provide some control on the possible depth to be achieved with gravity coring.

## 6. Conclusions

A total of 350 tephra horizons were identified in the 22 studied gravity cores around the eastern sector of the Azores Archipelago. Each tephra horizon has been characterized in detail in terms of componentry and major element glass geochemistry and, based on its origin (primary/secondary) and emplacement mechanism (fallout/flow), classified in one of four different depositional facies. The obtained percentages of primary and reworked deposits (~45% and ~55%, respectively) reflect a balance between constructive and destructive processes in the submarine realm. Wind and oceanographic conditions, together with bathymetry, seem to play a major role in the deposition of different facies across the eastern Azores. Wind direction not only controls the aerial dispersion of tephras and subsequent areas of deposition of fall deposits but may also influence the deposition and remobilization of turbidite deposits through the creation of downwelling offshore currents near the seabed.

Enhanced through machine learning algorithms, we assign possible volcanic sources for the marine tephra layers. The majority correlates with compositions from subaerial pyroclastic deposits from São Miguel, although similarities with deposits from Terceira, and possibly Faial, have also been found. Explosive eruptions sourced from the Sete Cidades Volcano are dominant over eruptions sourced from the Fogo and Furnas volcanoes. The marine sediments in the eastern Azores record an intense period of explosive volcanism at Terceira, between ~20 and ~100 ka, which is likely associated with major ignimbrite-forming eruptions produced by the Pico Alto Volcano.

The machine learning approach proved to be a very powerful tool for volcanic source assignment of tephra layers. However, its performance is limited by the input data (i.e., by the availability of geochemical data from subaerial deposits), which is far from being complete in the Azores. We therefore highlight the need for future glass geochemical studies on major and trace element compositions across the archipelago in order to make the method more robust.

Although the majority of primary tephra layers can be associated with trachytic (s.l.) explosive eruptions, our study demonstrates that several highly explosive basaltic (s.l.) eruptions capable of dispersing tephras for >100 km have occurred in the Azores over the last 125 kyr. Basaltic eruptions are usually expected to be of Hawaiian or Strombolian nature and therefore much less hazardous than the more commonly highly explosive trachytic eruptions. Therefore, our findings on widely distributed mafic tephra layers have direct implications on the hazard assessment of the region, with possible impacts for local populations and air traffic across the North Atlantic.

Based on our core collection, which provides a representative distribution of the different marine tephra facies in the eastern Azores region, we ultimately discuss the main factors controlling the deposition and preservation of deposits in the vicinity of volcanic islands, hoping to deliver insights for future coring campaigns around oceanic islands.

Being the first large-scale and long-term marine tephrostratigraphic study conducted for this region, our work constitutes the first step for the future establishment of a more complete eruptive record for São Miguel Island, which is crucial to robustly assess volcanic hazard. Moreover, it paves the way for in-depth research into possible long-term volcanic and mass-wasting cyclicities in the Azores.
## Acknowledgments

We appreciate all the work of the scientific party and crew members of R/V Meteor during cruise M141/1. Cruise M141/1 and initial core sampling were funded by the German Research Foundation. This work is part of the VolCLimA project funded by the German Research foundation (DFG Grant 509166013). AP is supported by a CEEC Institutional contract CEECINST/00024/2021/CP2780/CT0003 funded by the Fundação para a Ciência e a Tecnologia (https://doi.org/10.54499/CEECINST/00024/2021/CP2780/CT0003). We thank Volker Liebetrau and Mark Schmidt for their help with sampling on São Miguel in 2021. We thank Mathilde Bablon and Alastair Hodgetts for their encouraging and valuable comments, and appreciate the time they dedicated to our work. Open Access funding enabled and organized by Projekt DEAL.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

The marine cores of M141/1 are stored in the core repository of the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany and are accessible upon request. The code for model selection, training, and prediction is available in the Supporting Information S1. All geochemical data are provided in the supplement and in the PANGAEA database Belo (2025a, 2025b) https://doi.pangaea.de/10.1594/PANGAEA.986779 and https://doi.pangaea.de/10.1594/PANGAEA.986777.

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