Ash plumes of numerous plinian, phreatoplinian and ignimbrite-forming eruptions from calderas and stratocones of the central and eastern Aegean Volcanic Arc dispersed ash mostly eastward across the Aegean and Mediterranean seas (e.g., Keller et al., 1978). The resulting marine ash and lapilli layers represent the major fraction of the erupted tephra volumes because the relatively small volcanic islands in the Aegean Sea provide very limited onshore depositional areas (Figure 1). Such marine tephras have previously been sampled by sediment coring across the central and eastern Mediterranean at great distances from the source volcanoes. During RV Poseidon

This review focusses on the recognition of volcanic ash occurrences in marine sediment cores and on using their appearance and properties to deduce their origin. Widespread marine tephra layers are important marker horizons for both volcanological as well as general geological investigations. We describe ash detection by visual inspection and logging of sediment cores. Ash layer structure and texture, particle morphologies and lithological compositions of primary volcanic deposits are summarized and processes modifying them are discussed, both natural processes acting on and in the seafloor, i.e., erosion and bioturbation, and man-made modifications during drilling/coring and core preparation. We discuss primary emplacement processes of marine fall and flow tephra deposits derived from either subaerial or submarine sources in order to identify distinguishing properties. We also elaborate on processes generating secondary, resedimented volcaniclastic layers such as submarine landslides and shelf erosion as well as fluvial input and ice-rafting, and how they can be distinguished from primary volcaniclastic deposits, which is essential in tephrostratigraphy. Finally, methods of tephra correlation between cores and on-land deposits/volcanoes are illustrated because they allow us to extend the 1-D information from single cores to 3-D distribution and facies changes of tephras and to bridge the land-sea gap.

Felsic magmas from the Izu‐Bonin rear arc have compositions that resemble average continental crust in some respects. In order to understand their origin, we studied 1.1–4.4 Ma tephras in a rear‐arc drill core from International Ocean Discovery Program Expedition 350, Site U1437. They provide a well‐dated record of changing magmatic compositions during the early stages of the most recent episode of Izu‐Bonin arc rifting. Based on our comprehensive recontextualization of published analyses of <7 Ma regional dredged rocks across the arc, basalts to rhyolites are shown to vary in coherent chronological and spatial trends and can be classified into three series: light rare earth element‐depleted volcanic front series; a rift‐related series with nearly flat rare earth element patterns; and light rare earth element‐enriched rear‐arc seamount chain‐type (RASC‐type) series, which are abundant in the studied Site U1437 tephra record. All three series erupted simultaneously between 4.4 and 1.1 Ma, including the RASC‐type rhyolites which erupted until 1.1 Ma in significant quantities. Remarkably, trace element and radiogenic isotope ratios are similar between rhyolites and basalts from the same region. This recontextualization of rhyolite affinity represents a significant departure from existing frameworks. Geochemical modeling shows that fractional crystallization can largely explain <4.4 Ma RASC‐type rhyolites with some additional open system processing evident in rhyolites with >73% SiO2. However, trace element and Hf isotope ratios preclude rear‐arc rhyolite derivation by partial melting of the Oligocene‐Eocene arc basement. Thus, we favor a model where fractional crystallization is more important than crustal melting in producing intra‐oceanic arc rhyolites in this region.

The climactic Los Chocoyos (LCY) eruption from Atitlán caldera (Guatemala) is a key chronostratigraphic marker for the Quaternary period given the extensive distribution of its deposits that reached both the Pacific and Atlantic Oceans. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. Using zircon (U-Th)/He geochronology, we present the first radioisotopically determined eruption age for the LCY of 75 ± 2 ka. Additionally, the youngest zircon crystallization 238 U-230 Th rim ages in their respective samples constrain eruption age maxima for two other tephra units that erupted from Atitlán caldera, W-Fall (130 +16 / −14 ka) and I-Fall eruptions (56 +8.2 / −7.7 ka), which under-and overlie LCY tephra, respectively. Moreover, rim and interior zircon dating and glass chemistry suggest that before eruption silicic magma was stored for >80 kyr, with magma accumulation peaking within ca. 35 kyr before the LCY eruption during which the system may have developed into a vertically zoned magma chamber. Based on an updated distribution of LCY pyroclastic deposits, a new conservatively estimated volume of~1220 ± 150 km 3 is obtained (volcanic explosivity index VEI > 8), which confirms the LCY eruption as the first-ever recognized supereruption in Central America.