Summary of field activity 2018-602-FA

Location Map:


Location: Rio Grande Rise, SW Atlantic

Dates: 2018-01-26 to 2018-02-24

Port Locations: Port Santos, Brazil, Port Santos, Brazil

Principal Investigator(s): Kira Mizell, James Hein

Data Type(s): Biological Field Study: Surveys, Electro-Magnetic: Magnetics, Environmental Data: Chlorophyll, Environmental Data: Conductivity, Environmental Data: CTD, Environmental Data: Current, Environmental Data: Density, Environmental Data: Depth, Environmental Data: Dissolved Oxygen, Environmental Data: Fluorescence, Environmental Data: Nutrients, Environmental Data: pH, Environmental Data: Sediment Properties, Environmental Data: Temperature, Geochemical: Surveys, Location-Elevation: Navigation, Sampling: Biology, Sampling: Chemistry, Sampling: Geology, Seismics: Sub Bottom Profiler, Sonar: Multibeam, Sonar: Sidescan, Sonar: Sound Velocity, Time Series: Mooring (physical oceanography)

Information to be Derived: sample analysis of marine minerals to understand processes of mineralization

Summary of Activity and Data Gathered: Jim Hein and his PhD student and USGS intern, Kira Mizell, PCMSC, participated in a research cruise from 28 January to 21 February to the Rio Grande Rise in the SW Atlantic Ocean aboard the University of São Paulo (Brazil) ship R/V Alpha Crucis (formally University of Hawaii’s ship, Moana Wave). The goals of the cruise were to better understand this enigmatic plateau and to collect samples to determine the genesis of the mineral deposits and processes of mineralization that have occurred on this flat-topped plateau. We had some ideas about the origin of the mineral deposits, ferromanganese (FeMn) crusts and phosphorite, based on huge slabs Jim saw in a warehouse some years ago collected by CPRM (Geological Survey of Brazil). Unfortunately, the Alpha Crucis did not have the capacity to collect such large samples, but using a variety of 1-meter wide dredges, we were able to collect abundant phosphorite and FeMn crust samples, mostly from the upper parts of the plateau, 600-900 m water depths. Another goal of the cruise was to train Kira, as well as the University of São Paulo researchers and ship’s crew, on dredging logistics and techniques, which they had not done previously, and to provide guidance on the techniques of study and current understanding of the geneses of these types of deep-ocean mineral deposits. Of the 22 days aboard ship, 15 days were packed full with various operations, rock and sediment sampling, multibeam and back-scatter mapping, water-column hydrography and sampling, and laboratory activities, during which Kira and Jim assisted in the completion of 17 dredges that recovered several hundred kilograms of rocks, of which more than 250 rock sample were cut on a diamond saw and described in detail. The rock samples proved very interesting with an amazing variety of phosphorites, FeMn crusts, volcanic rocks, limestones, and others, as well as some very curious rocks, such as granitic gneiss, which should not occur on this plateau; we guess that these rounded pebbles were ice rafted from perhaps Patagonia during the last ice age. The FeMn crusts are particularly unique in that they appear to be heavily phosphatized throughout, and their surfaces are smoothed and polished, indicating that they are not presently growing and that current activity has been vigorous in this region, which eroded the non-phosphatized parts of FeMn crusts found on the flat top of the plateau. In addition, very few of the volcanic rocks were broken from outcrop, which indicates that they were talus debris; follow-up studies using a ROV, cameras, and larger, heavier dredges will be important. This cruise was indispensable as a teaching tool for how to interpret the in situ context of rocks that are recovered by dredge alone and how to proceed with additional studies.

Equipment: Magnetometer, Gravity Core, Other, Other, water bottle, borrowed equipment, samples, CTDmeter, boxcore, multibeam, dredge, temperature, pressuregauge, watersamples, acousticbackscatter, flourometer, Sensors - pressure, hydrometer, salinity sensor, GPS, current meter, ADVF, magnetometer, corer, oxygenprofiler, acousticdoppler, computer, 512 chirp-source, Nutrient sampler