ALBERT

Description: Semiquantitative data of calcareous nannofossil species abundance were collected for 31 samples from 380.00 to 696.61 m below sea floor (mbsf) in the CIROS-1 core from the western McMurdo Sound (Ross Sea). This core has provided the most complete record of Paleogene glacial history of Antarctica. Only half of the samples yielded calcareous nannofossils and species diversity is generally low. However, several samples contain diverse assemblages and nannofossil zonal markers. Isthmolithus recurvus was found between 406.57 and 681.16 mbsf. This species has an age range of ~35-39 Ma in the mid to high latitudes, as calibrated previously by magnetostratigraphy at a number of DSDP/ODP sites. The interval from 406.57 to 681.16 mbsf can be assigned to the late Eocene-earliest Oligocene (~35-39 Ma), and the sample at 391.85 mbsf, which still contains Reticulofenestra hillae and Reticulofenestra umbilica but does not contain I. recurvus, is identified as early Oligocene age (~33-34 Ma). This represents an age refinement of the previous nannofossil biostratigraphy, where the interval from 385.77 to 690.40 mbsf was dated as middle Eocene-early Oligocene. Comparison of the nannofossil assemblages in the CIROS-1 core with those in a similar glaciomarine sequence recovered in Prydz Bay (East Antarctica) and those at deep-sea ODP Sites 738 and 744, where lowermost Oligocene ice-rafted debris (IRD) were found, suggests that the nannofossils in the CIROS-1 core samples examined are in situ. The semiquantitative nannofossil data also suggest that Isthmolithus recurvus and Reticulofenestra davies¸ are most tolerant of cold waters within the late Eocene-early Oligocene nannofloras.

Description: During Dive 408 of Shinkai 2000 on the flank of the Kaikata Seamount located on the volcanic front of the Izu-Ogasawara (Bonin) Arc, possibly recent hydrothermal manganese deposits were observed on sandy volcanic sediments at water depths of between 800 and 1200 m. The deposits cover almost the entire surface sediment across the manganese belt on the northwestern flank on the seamount on a scale of kilometers. The occurrence of the deposits, seafloor morphology and temperature of the sediments, together with previous shipboard results, lead to a possible model for the formation of the manganese deposits: A low-temperature hydrothermal solution ascends through fractures or faults in the volcanic substrate into permeable volcanic sands and precipitates manganate minerals which cement several centimeters of surface sand as hardpan. More intense and probably intermittent discharge of hydrothermal solution follows and forms many typical ridge-like vents and mounds on the seafloor. Subsequently, pure dense manganate layers developed beneath the semi-consolidated volcanic sandstone and act as a cap to the ascending fluids. The proposed mechanism may be the dominant type of low-temperature hydrothermal mineralization in submarine island-arc volcanoes and rifts. Similar hydrothermal manganese deopsits most probably occur in other active areas of the modern seafloors and in inactive island arcs.