Late Anisian radiolarian assemblages from the Yarlung‐Tsangpo Suture Zone in the Jinlu area,Zedong, southern Tibet: Implications for the evolution of Neotethys

Abundant Triassic radiolarian fossils were obtained from varicolored bedded cherts exposed in the Buruocang section near Jinlu village, Zedong, southern Tibet. The radiolarian‐bearing rocks represent fragmented remnants of the Neotethys oceanic sediments belonging to the mélange complex of the east part of the Yarlung‐Tsangpo Suture Zone. Two new middle Late Anisian radiolarian assemblages recognized from this section named Oertlispongus inaequispinosus and Triassocampe deweveri, respectively, are compared with those known from Europe, Far East Russia, Japan, and Turkey. These Anisian radiolarian fossils are the first reported in southern Tibet and the oldest radiolarian record within the Yarlung‐Tsangpo Suture Zone. They improve time constraints for the evolution of Neotethys in southern Tibet.     

A middle Miocene post‐rift stress regime revealed by dikes and mesoscale faults in the Kakunodate area,NE Japan

Significant advances were made in the last century in the investigations of the Neogene stress history of the NE Japan arc. However, previous studies have failed to fully resolve middle Miocene post‐rift stress conditions owing to their assumption of Andersonian faulting and an inability to determine maximum and intermediate stress axes from dike orientations. We applied the latest methods of paleostress analysis in this study to igneous dikes and mesoscale faults in the Kakunodate area of the NE Japan arc to elucidate post‐rift stress conditions. Stratigraphic constraints and U–Pb dating indicate that the doleritic and dacitic dikes were formed at 16–12 Ma and 15–12 Ma, respectively. Dolerite and dacite dikes yielded NW–SE extensional stresses with intermediate and low stress ratios, respectively. Mesoscale faults in the middle Miocene formations of the studied area indicated similar stresses. We suggest the sluggish deformations resulting in the dike intrusion and faulting in the normal‐faulting stress regime after the termination of intra‐arc rifting at ca. 15 Ma.     

PALEOCEANOGRAPHY AND EVOLUTION OF THE CENO-TETHYS: MICROPALEONTOLOGICAL EVIDENCE FROM PELAGIC SEDIMENTS IN THE YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET

PALEOCEANOGRAPHY AND EVOLUTION OF THE CENO-TETHYS: MICROPALEONTOLOGICAL EVIDENCE FROM PELAGIC SEDIMENTS IN THE YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET     

Silica minerals in cumulate eucrites: Insights into their thermal histories

Some eucrites contain up to 10 vol% silica minerals; however, silica minerals have not been studied in detail so far. We performed a mineralogical study of silica minerals in three cumulate eucrites (Moore County, Moama, and Yamato [Y] 980433). Monoclinic tridymite was common in all three samples. Moama contained orthorhombic tridymite as lamellae within monoclinic tridymite grains. Y 980433 included quartz around an impact melt vein. The presence of orthorhombic tridymite in Moama indicates that Moama cooled more rapidly than the other two samples at low temperatures (<400 °C). This result is different from the slower cooling rates of Moama (?0.0004 °C yr^?1) than that of Moore County (>0.3 °C yr^?1, after the shock event) at high temperatures (>500 °C) estimated from compositional profiles of pyroxene exsolution lamellae. The difference of the cooling rates may reflect their geological settings. Y 980433 cooled slowly at low temperature, as did Moore County. Quartz in Y 980433 could be a local product transformed from monoclinic tridymite by a shock event. We suggest that silica minerals in meteorites record thermal histories at low temperatures and shock events.     

Air-sea gas transfer in a shallow,flowing and coastal environment estimated by dissolved inorganic carbon and dissolved oxygen analyses

We estimated gas exchange rates in Kabira Reef at Ishigaki Island, southwest Japan, using a mass balance calculation with dual “biological” tracers: dissolved inorganic carbon (DIC) and dissolved oxygen (DO). The nighttime results allowed us to obtain reasonable gas transfer velocity k _w values, all of which exceeded those obtained in wind-dominant studies. The difference is likely due to the contribution of turbulence generated by the interaction between the current and bottom topography. The k _w obtained during high tides is consistent with that reported by Raymond and Cole (2001), whereas k _w during low tides is significantly higher, which seems to be caused by enhanced friction with the bottom of the reef and/or bubble-induced gas transfer by wave breaking at the reef crest.     

Phytoplankton community adaptation to changing light levels in the southern Beaufort Sea,Canadian Arctic

The chlorophyll a specific absorption coefficient of phytoplankton, a_φ^∗(λ) is an important parameter to determine for primary production models and for the estimation of phytoplankton physiological condition. Knowledge of this parameter at high latitudes where nutrient rich cold water submitted to low incident light is a common environment is almost nonexistent. To address this issue, we investigated the light absorption properties of phytoplankton as a function of irradiance, temperature, and nutrients using a large data set in the southern Beaufort Sea during the open water to ice cover transition period. The a_φ^∗(λ) tended to increase from autumn when open water still existed to early winter when sea ice cover was formed, resulting from a biological selection of smaller-size phytoplankton more efficient to absorb light. There was no significant correlation between a_φ^∗(λ) and irradiance or temperature for both seasons. However, a_φ^∗(λ) showed a significant positive correlation with NO₃ + NO₂. Implications of the results for phytoplankton community adaptation to changing light levels are discussed.     

Review on North Pacific Subtropical Countercurrents and Subtropical Fronts: role of mode waters in ocean circulation and climate

A Subtropical Countercurrent (STCC) is a narrow eastward jet on the equator side of a subtropical gyre, flowing against the broad westward Sverdrup flow. Together with theories, recent enhanced observations and model simulations have revealed the importance of mode waters in the formation and variability of North Pacific STCCs. There are three distinct STCCs in the North Pacific, maintained by low potential vorticity (PV) that mode waters carry from the north. Model simulations show that changes in mode water ventilation result in interannual to interdecadal variations and long-term changes of STCCs. STCCs affect the atmosphere through their surface thermal effects, inducing anomalous cyclonic wind curl and precipitation along them. Thus, mode waters are not merely passive water masses but have dynamical and climatic effects. For temporal variability, atmospheric forcings are also suggested to be important in addition to the variability of mode waters. STCCs exist in other oceans and they are also flanked by mode waters on their poleward sides, suggesting that they are maintained by similar dynamics.     

Oxygen and carbon isotope records of cultured freshwater pearl mussel <Emphasis Type="Italic">Hyriopsis</Emphasis> sp. shell from Lake Kasumigaura,Japan

We present oxygen and carbon isotope ratios and the morphological structure of the cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) shell and pearl. The number of first-order fluctuations of δ¹⁸O of the outer shell layer along the maximum growth axis was consistent with the number of cultured years. The dominant factor controlling annual δ¹⁸O fluctuations was water temperature with a minor contribution from the variation in δ¹⁸O of ambient water, especially during the rainy season. The δ¹³C values were approximately constant throughout the life of the mussel, suggesting that the contributions of body size to δ¹³C of the shell were minor. We observed nine distinct disturbance rings on the outer surface of the shell. Five rings coincided with the five winter peaks of the δ¹⁸O profile, indicating winter growth cessation below approximately 10°C, probably because of either inactive growth at low water temperatures or reproduction. Summer disturbance rings were not observed in all years. Moreover, some summer rings showed discontinuity in the inner structure. These findings suggest that summer growth cessation may be caused by occasional events such as heavy rains, as the decrease of dissolved oxygen concentration. The δ¹⁸O profile and shell structures indicated that shell aragonite was precipitated at close to equilibrium conditions with respect to the oxygen isotope composition of the ambient water. Hyriopsis sp. shells can potentially be used for reconstruction of past hydrologic conditions. The δ¹⁸O of a pearl indicated that calcification occurred over a temperature range of at least 13–23°C. The optimal temperature for pearl calcification in this species is lower than that for marine pearl calcification.