Bacterial and archaeal communities in deep sea waters near the Ninetyeast Ridge in Indian Ocean

Depth-dependent distribution patterns of bacterial and archaeal communities in deep sea water column around the Ninetyeast Ridge in the Indian Ocean were investigated using 16S rRNA gene profiling.Sampling was conducted at the northern Ninetyeast Ridge(1°59.89′N–9°59.70′S,87°58.90′E–88°00.03′E)from September to November 2016 where samples were collected from the bathyal(1000 m)to bathypelagic depths(>4000 m)in four different stations.A total of 1565405 clean data falling into 6712 bacterial OTUs and 1452727 clean data falling into 806 archaeal OTUs based on 97%similarity level were analyzed.Most of the bacterial 16S rRNA gene sequences were affiliated with Gammaproteobacteria,followed by Alphaproteobacteria and Bacteroidia.The archaeal 16S rRNA gene sequences mostly affiliated to Nitrososphaeria(Thaumarchaeota)dominated with relative abundances ranging from 52.68%to 97.2%,followed by Thermoplasmata(Euryarchaeota).Vertical partitioning of bacterial and archaeal communities among different water layers was observed.Canonical correspondence analysis(CCA)and Spearman’s correlations revealed that depth(P=0.003),dissolved oxygen(P=0.019),and nitrite(P=0.033)were the main environmental factors affecting bacterial community structure at genus level in the Ninetyeast Ridge.On the other hand,the first two CCA axes accounted for 74.4%of the explained total variance,it seems that the archaeal communities at genus level were heavily influenced by the environmental variables including depth,dissolved oxygen(DO),nitrite,salinity,phosphate,ammonia,nitrate,and silicate,but none of them exhibited any significant correlation on the structuring(P>0.1).

Tectonics of the Indo-Australian Plate Near the Ninetyeast Ridge Constrained from Marine Gravity and Magnetic Data

Although the Indo-Australian plate near the Ninetyeast Ridge is important for understanding the formation of new plate boundaries, its tectonic problems are complex and most of them are poorly known. This paper made a detailed tectonic analysis based on the data of bathymetry, gravity and magnetics. Bathymetry and gravity maps show morphological features of many folds, which are related to the intraplate deformation of the Indo-Australian plate due to the collision between the Indian and Asian plates. Gravity anomalies show the structure of fracture zones, which are caused by the seafloor spreading and transform faulting. The characteristics of the folds and fracture zones are consistent with the hypothesis that diffuse plate boundaries and redefined plate components would occur within the Indo-Australian plate. In addition, compiled magnetic data demonstrate magnetic lineations, abandoned spreading centers, southward ridge jumps and plate motions. These features provide useful information for rebuilding the tectonic evolution history of the study area. Magnetic anomalies suggest that an additional plate boundary of transform fault type is developing.

Large Active Faults and the Wharton Basin Intraplate Earthquakes in the Eastern Indian Ocean

In recent years,great earthquakes occurred within the Wharton Basin in the eastern Indian Ocean,and they have been associa-ted with active faulting on the ancient oceanic crust.Large seismogenic faults were thought to be the fault reactivation on the ancient oceanic crust,but these phenomena are still unclear and require examination.This study used high-quality multibeam bathymetry and multichannel seismic data collected over the northern Ninetyeast Ridge to investigate detailed fault geometry,structure,and activity.We recognized 12 large linear active faults by integrating bathymetry maps and multichannel seismic reflection profiles.Our results showed that these faults have high angles,and they all displaced the basement and propagated to the seafloor with distinct fault scarps.They trended NWW-SEE with a spacing of 10–40km and were parallel to each other and the nearby subfault of the 2012 great intraplate earthquake,suggesting similar stress fields.These faults are also in agreement with the orientations of magnetic isochrons,implying their formation by seafloor spreading.Furthermore,regarding the strike-slip focal mechanism of 2012 earthquakes,we proposed that these faults were created early by a normal spreading process and then evolved into a strike-slip pattern since the ancient oceanic crust ap-proached the subduction zones.