ALBERT

Description: Since the early discovery of a black-smoker complex in 1978 on the East Pacific Rise at 21°N, speculations and expectations have been driven about the potential and perspectives of mining seafloor massive sulfide (SMS) deposits in the deep-ocean. With a worldwide accelerating industrialization, emerging markets, increased commodity prices and metal demand, and advance¬ments in deep-water mining and extraction technologies, mining of SMS may become economically feasible in the near future (Kowalczyk, 2008). However, we still know little about the resource potential of SMS deposits, and the development of geophysical methods for an assessment of their spatial extent, composition, and inner structure is crucial to derive a proper assessment of their economic value. Novel geophysical mapping techniques and exploration strategies are required to locate extinct and buried clusters of SMS deposits, away from the active vent fields and of larger economic potential, but are difficult to find and sample by conventional methods. In 2015 the International Seabed Authority (ISA) assigned an exploration license for polymetallic sulfide deposits to the German Federal Institute for Geosciences and Natural Resources (BGR) in a specified area comprising 100 patches, each 10 . 10 km in size, distributed along the Central and Southeastern Indian Ridge. The challenge to acquire high resolution near-surface electromagnetic (EM) data in such geologically and morphologically complex mid-ocean ridge environments has been addressed by our recent development of the deep-sea profiler Golden Eye that utilizes a frequency-domain electromagnetic (FDEM) central loop sensor, of 3.3 m diameter (Müller et al., 2016). This system has been used in 2015 and 2017 to map active and relict hydrothermal vent fields in the SMS licensing areas. Aside from technological developments, this paper discusses new data processing routines and methods to unravel the conductivity-depth-distribution, induced polarization and magnetic susceptibility, and joint interpretation with geochem¬istry as key elements to map and evaluate SMS deposits.

Description: Non-carbonaceous abyssal fine-grained sediments cover vast parts of the North Pacific’s deep oceanic basins and gain increasing interests as glacial carbon traps. They are, however, difficult to date at an orbital-scale temporal resolution and still rarely used for paleoceanographic reconstructions. Here, we show that sedimentary records of past geomagnetic field intensity have high potential to improve reversal-based magnetostratigraphic age models. Five sediment cores from Central North Pacific mid-latitudes (39–47°N) and abyssal water depths ranging from 3,900 to 6,100 m were cube-sampled at 23 mm resolution and analyzed by automated standard paleo- and rock magnetic methods, XRF scanning, and electron microscopy. Relative Paleointensity (RPI) records were determined by comparing natural vs. anhysteretic remanent magnetization losses during alternating field demagnetization using a slope method within optimized coercivity windows. The paleomagnetic record delivered well interpretable geomagnetic reversal sequences back to 3 Ma. This age span covers the climate-induced transition from a biogenic magnetite prevalence in the Late Pliocene and Early Pleistocene to a dust-dominated detrital magnetic mineral assemblage since the Mid-Pleistocene. Volcaniclastic materials from concurrent eruptions and gravitational or contouritic sediment re-deposition along extinct seamount flanks provide a further important source of fine- to coarse-grained magnetic carriers. Surprisingly, higher proportions of biogenic vs. detrital magnetite in the late Pliocene correlate with systematically lowered RPI values, which seems to be a consequence of magnetofossil oxidation rather than reductive depletion. Our abyssal RPI records match the astronomically tuned stack of the mostly bathyal Pacific RPI records. While a stratigraphic correlation of rock magnetic and element ratio logs with standard oxygen isotope records was sporadically possible, the RPI minima allowed to establish further stratigraphic tie points at ∼50 kyr intervals. Thus, this RPI-enhanced magnetostratigraphy appears to be a major step forward to reliably date unaltered abyssal North Pacific sediments close to orbital-scale resolution.