ALBERT

Description: Younger Sand layers (YSLs) have been identified in drill cores of a Late Glacial to Early Holocene muddy contourite succession from the Gulf of Cádiz. In this study, we evaluate the grain size characteristics of the YSL, which display bi-gradational grading, with inverse grading (from silt to fine- or medium-grained sand) followed by normal grading (from fine- or medium-grained sand to silt). Radiocarbon dating shows that the YSL formed at three distinct times: 1) the Bølling-Allerød (sites U1387 and U1386); 2) the Younger Dryas (site U1390), and; 3) the beginning of the Boreal (site U1389). Ichnological analyses and radiocarbon dating of cores from drill site U1389C indicate that the YSLs consist of reworked materials deposited at a high sedimentation rate. The results of sedimentological analyses, ichnological treatment, spatial distributions, and radiocarbon dating of the YSLs suggest that the possibility of an origin of the YSLs is tsunami-related but later reworked by bottom current. However, the source areas of the sandy sediments remain unclear, although our results show them to be variable and area dependent. Some sandy deposits are transported by the Mediterranean Outflow Water (MOW) from the proximal sector of the Gulf of Cádiz Contourite Depositional System (CDS), which is close to the Strait of Gibraltar. However, in other cases it is more probable that local gravity flows, which are the result of instability on adjacent margins, provide the sandy material.

Description: Petrographic and geochemical analyses of basaltic rocks dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction have been completed in order to investigate water-rock interaction processes during mid-ocean ridge (MOR) hydrothermal alteration in the Indian Ocean. In the study area, we have successfully recovered a serial section of upper oceanic crust exposed along a steep rift valley wall which was uplifted and emplaced along a low angle normal fault. On the basis of microscopic observation, dredged samples are classified into three types: fresh lavas, low-temperature altered rocks, and high-temperature altered rocks. The fresh lavas have essentially the same chemical composition as typical N-MORB, although LILE and Nb are slightly enriched and depleted, respectively. Low temperature alteration brought about the enrichment of K2O, Rb, and U due to the presence of K-rich celadonite and U-adsorption onto Fe-oxyhydroxide and clay minerals. On the other hand, chloritization, albitization, and addition of base metals by high temperature hydrothermal alteration result in enrichments of MnO, MgO, Na2O, Cu, and Zn and depletions of CaO, K2O, Cr, Co, Ni, Rb, Sr, and Ba. In addition, U-enrichment is also observable in the high temperature altered rocks probably due to the decrease of uranite solubility in the reducing high-temperature hydrothermal solution. These petrological and geochemical features are comparable to those of the volcanic zone to transition zone rocks in the DSDP/ODP Hole 504B, indicating that our samples were recovered from the upper ~1000 m section of the oceanic crust. Only the alteration minerals related to off-axis alteration are absent in our samples dredged from near the spreading axis. The similarity of alteration between our samples from the Indian Ocean and the Hole 504B rocks from the Pacific Ocean suggests that MOR hydrothermal systems are probably similar across all world oceans.