Coccolithophores as palaeoecological indicators for shifts of the ITCZ in the Cariaco Basin during the late Quaternary

Coccoliths were studied from the ODP Hole 1002C and core PL07‐39PC in the Cariaco Basin. Increases in Emiliania huxleyi are synchronous with decreases of Gephyrocapsa oceanica and vice versa. A new index (GEX) based on the relative abundances of these two taxa is proposed, and correlates with various other proxies. It is shown that GEX can serve as upwelling proxy. This confirms that the Intertropical Convergence Zone shifted north during the Bølling/Allerød, south during the Younger Dryas and back north during the Preboreal. The upwelling proxy shows few discrepancies with the terrigenous record. Copyright © 2008 John Wiley & Sons, Ltd.     

Impacts of storms and evolution of the coastline in western France

The morphological impact of storms on coastal accumulations varies considerably in space along the coast of western France. At some locations, the storm produces erosion, whereas at others, impacts may vary from erosion to accumulation, or no effect. This is explained by the variability of incident waves. Wave amplitude variability, cross-checked with wave direction variability, is presented on a phase diagram and shows that the response of the coast is not linked to the absolute values reached during the storm, but to changes in wave patterns that take place during the event. The morphology of the bottom (skerries, banks) is greatly forcing these changes because of the high tidal range and the complicated and fast-changing patterns of refraction. Locations with a flat seafloor have an almost predictable response to storms. Places with a complex submarine morphology have a response characterized by high variability. The variability of the wave pattern during a half tidal cycle storm (from low tide at 1 m to high tide at 13 m in 6 h) is a microscale equivalent of the variability of wave patterns changing during sea level rise in the Holocene (from −7 to 0 m in 6 ka). This fact explains why, during the late Holocene, the same locations (1) have displayed a very chaotic response to sea level rise and (2) are displaying, today, a highly variable response to storms. In that respect, seafloor morphology is a mesoscale to macroscale control on beach/barrier behaviour.     

Pre-1991 sulfur transfer between mafic injections and dacite magma in the Mt. Pinatubo reservoir

Before the 1991–1992 activity, a large andesite lava dome belonging to the penultimate Pinatubo eruptive period (Buag ∼ 500 BP) formed the volcano summit. Buag porphyritic andesite contains abundant amphibole-bearing microgranular enclaves of basaltic–andesite composition. Buag enclaves have lower K₂O and incompatible trace element (LREE, U, Th) contents than mafic pulses injected in the Pinatubo reservoir during the 1991–1992 eruptive cycle. This study shows that Buag andesite formed by mingling of a hot, water-poor and reduced mafic magma with cold, hydrous and oxidized dacite. Depending on their size, enclaves experienced variable re-equilibration during mixing/mingling. Re-equilibration resulted in hydration, oxidation and transfer of mobile elements (LILE, Cu) from the dacite to the mafic melts and prompted massive amphibole crystallization. In Buag enclaves, S-bearing phases (sulfides, apatite) and melt inclusions in amphibole and plagioclase record the evolution of sulfur partition among melt, crystal and fluid phases during magma cooling and oxidation. At high temperature, sulfur is partitioned between andesitic melt and sulfides (Ni-pyrrhotite). Magma cooling, oxidation and hydration resulted in exsolution of a S–Cl–H₂O vapor phase at the S-solubility minimum near the sulfide–sulfate redox boundary. Primary magmatic sulfide (pyrrhotite) and xenocrystic sulfide grains (pyrite), recycled together with olivines and pyroxenes from old mafic intrusives, were replaced by Cu-rich phases (chalcopyrite, cubanite) and, partially, by Ba–Sr sulfate. Sulfides degassed and transformed into residual spongy magnetite in response to fS₂ drop during final magma ascent and decompression. Our research suggests that a complete evaluation of the sulfur budget at Pinatubo must take into account the en route S assimilation from the country rocks. Moreover, this study shows that the efficiency of sulfur transfer between mafic recharges and injected magmas is controlled by the extent and rate of mingling, hydrous flushing and melt oxidation. Vigorous mixing/mingling and transformation of the magmatic recharge into a spray of small enclaves is required in order to efficiently strip their primary S-content that otherwise remains locked in the sulfides. Hydrous flushing increases the magma oxidation state of the recharges and modifies their primary volatile concentrations that cannot be recovered by the study of late-formed mineral phases and melt inclusions. Conversely, S stored in both late-formed Cu-rich sulfides and interstitial rhyolitic melt represents the pre-eruptive sulfur budget immediately available for release from mafic enclaves during their decompression.