Tatsuo Nozaki

Post-drilling changes in seabed landscape and megabenthos in a deep-sea hydrothermal system, the Iheya North field, Okinawa Trough.

There has been an increasing interest in seafloor exploitation such as mineral mining in deep-sea hydrothermal fields, but the environmental impact of anthropogenic disturbance to the seafloor is poorly known. In this study, the effect of such anthropogenic disturbance by scientific drilling operations (IODP Expedition 331) on seabed landscape and megafaunal habitation was surveyed for over 3 years using remotely operated vehicle video observation in a deep-sea hydrothermal field, the Iheya North field, in the Okinawa Trough. We focused on observations from a particular drilling site (Site C0014) where the most dynamic change of landscape and megafaunal habitation was observed among the drilling sites of IODP Exp. 331. No visible hydrothermal fluid discharge had been observed at the sedimentary seafloor at Site C0014, where Calyptogena clam colonies were known for more than 10 years, before the drilling event. After drilling commenced, the original Calyptogena colonies were completely buried by the drilling deposits. Several months after the drilling, diffusing high-temperature hydrothermal fluid began to discharge from the sedimentary subseafloor in the area of over 20 m from the drill holes, ‘artificially’ creating a new hydrothermal vent habitat. Widespread microbial mats developed on the seafloor with the diffusing hydrothermal fluids and the galatheid crab Shinkaia crosnieri endemic to vents dominated the new vent community. The previously soft, sedimentary seafloor was hardened probably due to barite/gypsum mineralization or silicification, becoming rough and undulated with many fissures after the drilling operation. Although the effects of the drilling operation on seabed landscape and megafaunal composition are probably confined to an area of maximally 30 m from the drill holes, the newly established hydrothermal vent ecosystem has already lasted 2 years and is like to continue to exist until the fluid discharge ceases and thus the ecosystem in the area has been altered for long-term.

Late Jurassic ocean anoxic event: evidence from voluminous sulphide deposition and preservation in the Panthalassa

The historically productive copper-bearing Besshi-type sulphide deposits in the Japanese accretionary complex were formed as volcanogenic massive sulphide deposits on the deep-sea floor of the Panthalassa Ocean. Here we report that eleven typical Besshi-type deposits yielded Re-Os isochron ages around 150 Ma (148.4 ± 1.4 Ma from the composite isochron) in Late Jurassic time. This date coincides with the lowest marine 87Sr/86Sr ratio and highest atmospheric CO2 concentration of the past 300 million years. We infer that intense mid-ocean ridge hydrothermal and volcanic activity in the Late Jurassic produced huge sulphide deposits and large emissions of CO2 gas, leading to global warming and a stratified Panthalassa Ocean with anoxic deep seas that favored preservation of sulphides in the pelagic environment. The emergence of ocean anoxia triggered by seafloor volcanism is also consistent with a positive δ13C excursion and widespread deposition of petroleum source rocks and black shales.

Osmium isotope evidence for a large Late Triassic impact event

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (187Os/188Osi) of ∼0.477 to unradiogenic values of ∼0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous–Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3–7.8 km in diameter) produced a global decrease in seawater 187Os/188Os ratios in the Late Triassic.

Rapid growth of mineral deposits at artificial seafloor hydrothermal vents

Seafloor massive sulphide deposits are potential resources for base and precious metals (Cu-Pb-Zn ± Ag ± Au), but difficulties in estimating precise reserves and assessing environmental impacts hinder exploration and commercial mining. Here, we report petrological and geochemical properties of sulphide chimneys less than 2 years old that formed where scientific boreholes vented hydrothermal fluids in the Iheya-North field, Okinawa Trough, in East China Sea. One of these infant chimneys, dominated by Cu-Pb-Zn-rich sulphide minerals, grew a height of 15 m within 25 months. Portions of infant chimneys are dominated by sulphate minerals. Some infant chimneys are sulphide-rich similar to high-grade Cu-Pb-Zn bodies on land, albeit with relatively low As and Sb concentrations. The high growth rate reaching the 15 m height within 25 months is attributed to the large hydrothermal vent more than 50 cm in diameter created by the borehole, which induced slow mixing with the ambient seawater and enhanced efficiency of sulphide deposition. These observations suggest the possibility of cultivating seafloor sulphide deposits and even controlling their growth and grades through manipulations of how to mix and quench hydrothermal fluids with the ambient seawater.

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

Ultrafine-grained black fault rocks (BFRs) in the Pasagshak Point Thrust of the Kodiak accretionary complex are examples of fault rocks that have recorded seismicity along an ancient subduction plate boundary. Trace element concentrations and 87Sr/86Sr ratios of BFRs and surrounding foliated/non-foliated cataclasites were measured to explore the nature of fluid-rock interactions along a subduction thrust. Foliated and non-foliated cataclasites do not show significant geochemical anomalies, suggesting that they were formed by slowly distributed shear. BFRs are characterized by Li and Sr enrichment, Rb and Cs depletion, and a low 87Sr/86Sr ratio. These geochemical signatures can be explained by fluid-rock interactions at >350°C, which result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Sr concentrations and low 87Sr/86Sr ratios. Geochemical anomalies recorded by the BFRs indicate both frictional heating and external fluid influx into the subduction thrust.

Miocene to Pleistocene osmium isotopic records of the Mediterranean sediments

In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375–376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show 187Os/188Os values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have 187Os/188Os values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite 187Os/188Os is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterraneans drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower 187Os/188Os values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).

Precise determination of Os isotope ratios in the 15-4000 pg range using a sparging method using enhanced-sensitivity multiple Faraday collector- inductively coupled plasma-mass spectrometry

We have developed a protocol for Os isotope analysis employing a sparging method coupled with an enhanced-sensitivity multiple Faraday collector-inductively coupled plasma-mass spectrometry (MFC-ICP-MS) technique. The enhanced-sensitivity ICP interface with 1012 Ω high-gain amplifiers allowed for the stable and precise isotopic ratio analysis of Os by sparging in a very wide concentration range of 15–4000 pg. The analytical reproducibilities of Johnson Matthey Chemical (JMC) Os standards at 50, 100, 200, 400, and 2000 pg Os were 0.8, 0.5, 0.2, 0.1, and 0.02% within two standard deviations (2SD), respectively. The low Os (50–200 pg) results compared with those obtained by sparging multiple-ion counter (MIC)-ICP-MS and high Os (400–2000 pg) results rivalled those of desolvating nebulisation MFC-ICP-MS and negative thermal ionisation mass spectrometry (N-TIMS). The analysed geological standards consisting of JCh-1 (chert; ∼15 pg, n = 3), JMS-2 (marine sediment; ∼150 pg, n = 5), UB-N (lherzoritic peridotite; ∼4 ng, n = 4), and JP-1 (harzburgitic peridotite; ∼3 ng, n = 5) showed 187Os/188Os = 0.657 ± 0.065, 0.842 ± 0.053, 0.12752 ± 0.00016, and 0.12071 ± 0.00069 (errors are in 2SD), respectively; these results are comparable with those obtained by MIC-ICP-MS and N-TIMS. The results showed that the sparging method coupled with enhanced-sensitivity MFC-ICP-MS is a strong tool for determining Os isotope ratios in natural samples over a wide range of Os concentrations. Simple sample digestion and low procedural blanks using Carius tube digestion alone without any further element separation provide an additional advantage for Os isotope analysis by the method.

The tremendous potential of deep-sea mud as a source of rare-earth elements

Potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea mud containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. This REY-rich mud has great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. Here, we estimated the resource amount in REY-rich mud with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value. The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km2 × 0–10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.

Depth profiles of resistivity and spectral IP for active modern submarine hydrothermal deposits: a case study from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan

Submarine hydrothermal deposits are one of the promising seafloor mineral resources, because they can store a large amount of metallic minerals as sulfides. The present study focuses on the electrical properties of active modern submarine hydrothermal deposits, in order to provide constraints on the interpretation of electrical structures obtained from marine electromagnetic surveys. Measurements of resistivity and spectral induced polarization (IP) were made using drillcore samples taken from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan. These hydrothermal sediments are dominantly composed of disseminated sulfides, with minor amounts of massive sulfide rocks. The depth profiles of resistivity and spectral IP properties were successfully revealed to correspond well to layer-by-layer lithological features. Comparison with other physical properties and occurrence of constituent minerals showed that resistivity is essentially sensitive to the connectivity of interstitial fluids, rather than by sulfide and clay content. This suggests that, in active modern submarine hydrothermal systems, not only typical massive sulfide rocks but also high-temperature hydrothermal fluids could be imaged as low-resistivity anomalies in seabed surveys. The spectral IP signature was shown to be sensitive to the presence or absence of sulfide minerals, and total chargeability is positively correlated with sulfide mineral abundance. In addition, the massive sulfide rock exhibits the distinctive IP feature that the phase steadily increases with a decrease of frequency. These results show the effective usage of IP for developing and improving marine IP exploration techniques.Graphical abstract.

New ways for research and development using a deep-sea hydrothermal vent system in the Okinawa Trough

JAMSTEC has an international patent for the environmentally-friendly cultivation system of offshore hydrothermal chimneys as the accessory product of scientific drilling program in the Okinawa Trough at deep water of 1,100 mbsl. This cultivation system was inspired by rapid growth of precipitated minerals (sulfide chimney) on the drilled hole. We introduce in this paper, the achievement from the riserlesss drilling campaign in Iheya North field in 2010 as IODP Exp. 331 and in 2014 as SIP Okinawa. After the IODP Exp. 331, the major findings were the massive continuous hydrothermal fluid supply from the drilled hole and the rapid growth of the precipitated minerals (sulfide chimney). The flow rate estimated from the installed producer is around 300 - 500 liter per minute through 3.5 inch pipe in diameter. The Cu-Pb-Zn-rich sulfide chimney having average concentrations of 4.5 wt% Cu, 6.9 wt% Pb, 30.3 wt% Zn and 8.7 wt% Fe at the outlet of the drilled hole was rapidly grown to be from 2 m to 8 m in 6 months. In Exp. 331 two temporary wells are and 5-1/2” tubing pipe were installed and those are producing the hydrothermal fluid. JAMSTEC are planning to install two new monitoring wells in Feb 2016 at Iheya North field and at Noho field. Those wells are sensing and logging flow rate, temperature and pressure. It can capture the precipitated minerals from hydrothermal fluid and those data by retrieving monitoring cell by ROV. The system was built and tested in March 2015. LWD operation was performed in 2014 as a part of SIP program. 7 wells are drilled with LWD for the depth of 200 - 350 mbsf in only 9 days in riserless drilling throughout the risk of 350 deg.C thermal fluid by applying continuous circulation system. The continuous circulation system successfully suppresses the hydrothermal fluid pressure by the circulation back pressure and the maximum downhole temperature recorded at LWD was only 85 deg.C in a short period. At the last our future plan is discussed focusing on our present concerns.