Description: Drilling at Integrated Ocean Drilling Program Site U1381 on the Cocos Ridge offshore Costa Rica recovered 67 primary Miocene (ca. 8 Ma to ca. 16.5 Ma) marine fallout ash layers. Geochemical, volcanological, and geological criteria link these ashes to Plinian eruptions that carried ash to at least 50–450 km from the Galápagos hotspot. These ash layers are the first documentation of highly explosive Miocene Galápagos hotspot volcanism. This volcanism is bimodal with two-thirds of the tephra layers generated by basaltic magmas (glass compositions 〈57 wt% SiO2) and one-third by rhyolitic magmas. The temporal distribution of the tephra layers, inferred from sediment accumulation rates calibrated by 40Ar/39Ar and biostratigraphic ages, reveals a distinct increase in eruption frequency and hence increased volcanic activity of the Galápagos hotspot after 14 Ma which we interpret in the context of dynamic interaction between the Galápagos plume and spreading ridge.

Description: Drill cores recovered during several ODP and IODP Expeditions offshore Central America contain an extensive Early Cenozoic ash layer record. These ash layers have been deposited by plinian eruptions that originated either at the Central American Volcanic Arc (CAVA) or at the Galápagos Hot Spot. While plinian eruptions are well known from the CAVA, volcanism from the Galápagos region is dominantly recorded in effusive and strombolian deposits from subaerial and submarine eruptions although rare large explosive eruptions of evolved trachytic or dacitic compositions did occur in the Pleistocene (e.g., Geist et al., 1994).We have established a tephrostratigraphy from recent through Miocene times from the unique archive of ODP/IODP sites offhore Central America in which we identify tephra source regions by geochemical compositions of the glass shards. Thus we found numerous CAVA-derived tephra layers characterized by typical arc signatures (e.g., Nb-Ta troughs, LILE enrichments), but more surprisingly also an extensive record of tephra layers mostly of Miocene age featuring ocean island geochemical compositions (e.g., low La/Nb and Ba/La ratios, high Nb/Rb ratios). At this geographical setting the only plausible source for these layers is the Galápagos archipelago. Such Miocene ash layers occur in the cores of ODP Sites 1039, 1241, and 1242. At IODP Site U1381, on the Cocos Ridge offshore Costa Rica, 67 primary Miocene (~8 Ma to ~16.5 Ma) fallout ash layers have been recovered. Inferred transport distances of at least 50to 450 km from their vents imply Plinian eruptions, although two-thirds of the ash beds formed from basaltic magmas and only one-third from rhyolitic magmas that are typically associated with plinian eruptions. Our age model for Site U1381 based on sediment accumulation rates, 40Ar/39Ar dating and biostratigraphic ages, reveals a distinct increase in eruption frequency at around 14 Ma. We interpret this as an increase in magma production rates due to changes in interactions between Galápagos plume and spreading ridge.

Description: The last glacial period was far from quiet. During so-called Heinrich events, large armadas of icebergs were shed from the ice sheet that covered much of North America. The tracks of debris left by the melting icebergs can still be seen in sediment cores from the North Atlantic. In their Perspective, Broecker and Hemming report from a recent miniconference that attempted to chart the climatic impacts of these events.

Description: We studied the tephra inventory of 18 deep sea drill sites from six DSDP/ODP legs (Legs 84, 138, 170, 202, 205, 206) and two IODP legs (Legs 334 and 344) offshore the southern Central American Volcanic Arc (CAVA). Eight drill sites are located on the incoming Cocos plate and ten drill sites on the continental slope of the Caribbean plate. In total we examined ∼840 ash-bearing horizons and identified ∼650 of these as primary ash beds of which 430 originated from the CAVA. Correlations of ash beds were established between marine cores and with terrestrial tephra deposits, using major and trace element glass compositions with respect to relative stratigraphic order. As a prerequisite for marine-terrestrial correlations we present a new geochemical data set for significant Neogene and Quaternary Costa Rican tephras. Moreover, new Ar/Ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates. The resulting correlations and provenance analyses build a tephrochronostratigraphic framework for Costa Rica and Nicaragua that covers the last 〉8 Myr. We define 39 correlations of marine ash beds to specific tephra formations in Costa Rica and Nicaragua; from the 4.15 Ma Lower Sandillal Ignimbrite to the 3.5 ka Rincón de la Vieja Tephra from Costa Rica, as well as another 32 widely distributed tephra layers for which their specific region of origin along Costa Rica and Nicaragua can be constrained.

Description: Highlights: • Two new phonolitic tephra units complementing the two previously known. • First radiometric ages of the CGF. • Contemporaneously evolution of the CGF and the Tope de Coroa. • Marine correlations improve tephra volume estimations for CG I and II. Abstract: The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period. Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent. The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~ 10 km3, about twice the previously published estimate. The tephra volume of CG II is ~ 3 km3; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão.