Testing the mantle plume hypothesis: An IODP effort to drill into the Kamchatka-Okhotsk Sea system

The great mantle plume debate (GPD) has been going on for ∼15 years (Foulger and Natland, 2003; Anderson, 2004; Niu, 2005; Davies, 2005; Foulger, 2005; Campbell, 2005; Campbell and Davies, 2006), centered on whether mantle plumes exist as a result of Earth’s cooling or whether their existence is purely required for convenience in explaining certain Earth phenomena (Niu, 2005). Despite the mounting evidence that many of the so-called plumes may be localized melting anomalies, the debate is likely to continue. We recognize that the slow progress of the debate results from communication difficulties. Many debaters may not truly appreciate (1) what the mantle plume hypothesis actually is, and (2) none of the petrological, geochemical and geophysical methods widely used can actually provide smoking-gun evidence for or against mantle plume hypothesis. In this short paper, we clarify these issues, and elaborate a geologically effective approach to test the hypothesis. According to the mantle plume hypothesis, a thermal mantle plume must originate from the thermal boundary layer at the core-mantle boundary (CMB), and a large mantle plume head is required to carry the material from the deep mantle to the surface. The plume head product in ocean basins is the oceanic plateau, which is a lithospheric terrane that is large (1000’s km across), thick (>200 km), shallow (2–4 km high above the surrounding seafloors), buoyant (∼1% less dense than the surrounding lithosphere), and thus must be preserved in the surface geology (Niu et al., 2003). The Hawaiian volcanism has been considered as the surface expression of a type mantle plume, but it does not seem to have a (known) plume head product. If this is true, the Hawaiian mantle plume in particular and the mantle plume hypothesis in general must be questioned. Therefore, whether there is an oceanic plateau-like product for the Hawaiian volcanism is key to testing the mantle plume hypothesis, and the Kamchatka-Okhotsk Sea basement is the best candidate to find out if it is indeed the Hawaiian mantle plume head product or not (Niu et al., 2003; Niu, 2004)

Optical mapping of the Mycobacterium avium subspecies paratuberculosis genome

<p>Abstract</p> <p>Background</p> <p>Infection of cattle with <it>Mycobacterium avium </it>subspecies <it>paratuberculosis </it>(<it>M. ap</it>) causes severe economic losses to the dairy industry in the USA and worldwide. In an effort to better examine diversity among <it>M. ap </it>strains, we used optical mapping to profile genomic variations between strains of <it>M. ap </it>K-10 (sequenced strain) and <it>M. ap </it>ATCC 19698 (type strain).</p> <p>Results</p> <p>The assembled physical restriction map of <it>M. ap </it>ATCC 19698 showed a genome size of 4,839 kb compared to the sequenced K-10 genome of 4,830 kb. Interestingly, alignment of the optical map of the <it>M. ap </it>ATCC 19698 genome to the complete <it>M. ap </it>K-10 genome sequence revealed a 648-kb inversion around the origin of replication. However, Southern blotting, PCR amplification and sequencing analyses of the inverted region revealed that the genome of <it>M. ap </it>K-10 differs from the published sequence in the region starting from 4,197,080 bp to 11,150 bp, spanning the origin of replication. Additionally, two new copies of the coding sequences > 99.8% were identified, identical to the MAP0849c and MAP0850c genes located immediately downstream of the MAP3758c gene.</p> <p>Conclusion</p> <p>The optical map of <it>M. ap </it>ATCC 19698 clearly indicated the miss-assembly of the sequenced genome of <it>M. ap </it>K-10. Moreover, it identified 2 new genes in <it>M. ap </it>K-10 genome. This analysis strongly advocates for the utility of physical mapping protocols to complement genome sequencing projects.</p

Local earthquake seismic tomography of the Southernmost Mariana subduction zone

We employed seismic tomography to examine the velocity structure of the upper mantle in the Southernmost Mariana subduction zone. Our study focuses on data collected during a six-month experiment from 15 December 2016 to 12 June 2017, using 11 ocean bottom seismometers. By examining over 3700 local arrival times, we are able to determine the three-dimensional Vp and Vs structure. The subducting slab in this region displays a P- and S-wave velocity 2~6% higher than normal mantle and a lower Vp/Vs, with an average dip of 45° at depths ranging from 50 to 100 km. Additionally, our velocity images also shed new lights to the velocity anomalies of the mantle wedge region on top of the subducting slab, from the trench to the remnant arc. We observed slower velocity anomalies in the mantle wedge beneath the Southwest Mariana Rift, the West Mariana Ridge, and the forearc. In the outer forearc, a low-velocity anomaly is observed at depths shallower than 50 km, indicating mantle serpentinization and the presence of water. Additionally, a melt production region is observed beneath the central part of the forearc block at a depth of 40–60 km suggesting the possibility of melting processes in this region

Depositional and erosional signatures in sedimentary successions on the continental slope and rise off Prydz Bay, East Antarctica– implications for Pliocene paleoclimate

The Prydz Bay region of Antarctica is the immediate recipient of ice and sediments transported by the Lambert Glacier, the single largest outflow from the East Antarctic Ice Sheet. The continental slope and rise provide records covering multiple glacial cycles and containing paleoclimatic information. Marine geological and geophysical data collected from the continental shelf and adjacent slope of Prydz Bay, Antarctica, including seismic reflection data, bathymetry, and core records from ODP drilling sites, reveal the history of glacial sediment transport and deposition since the early Pliocene times. Seismic facies are interpreted in terms of episodes of slope progradation, contourite, turbidite, trough-mouth fan, and mass transport deposition. Two seismic units with estimated age of early to late Pliocene and late Pliocene to recent have been analyzed in detail for the area immediately offshore the Lambert Glacier and Prydz Bay and the adjacent Mac. Robertson margin. The upper slope is dominated by episodic mass transport deposits, many of which accumulated to form a trough mouth fan since Early Pliocene times. The trough mouth fan contrasts with the adjacent steep (4-6 degrees) continental slope of the Mac. Robertson margin, where glacigenic sediments have been transported down slope as high-velocity turbidity currents via submarine channels. The distal region exhibits evidence for contrasting effects of high-energy, traction-dominated versus lower-energy, fallout-dominated suspension flows. The counter-clockwise Coriolis force modifies the erosion and deposition patterns of turbidity currents creating an asymmetric channel-levee architecture. Since the early Pliocene, turbidite sedimentation surpassed the amount of sediment reworked and transported by westward-flowing contour currents along the base of slope. On the continental rise, contourites and sediment waves were deposited in response to enhanced bottom-water formation, which is consistent with climatic cooling since late Pliocene times. This study, based on existing seismic reflection and ODP data, highlights the need for a future scientific ocean drilling proposal on the Prydz Bay continental slope and rise in order to more accurately determine the timing of the important events that have influenced the evolution of this margin

Recurrent slope failure enhancing source rock burial depth and seal unit competence in the Pearl River Mouth Basin, offshore South China Sea

High-quality 3-D seismic data are used to assess the significance of mass-transport deposits (MTDs) to the evolution of the Pearl River Mouth Basin (South China Sea). Basal shear surfaces and lateral margins of seven recurrent MTDs are mapped to reveal a general NE-SW transport direction throughout the Late Miocene-Quaternary. A key result of our analysis is the perceived relationship between the recurrence of slope instability in the study area and the Dongsha Tectonic Event. Using borehole data to constrain the ages of interpreted MTDs, we show that tectonic uplift in the northern South China Sea led to slope oversteepening in the Late Miocene (between 10.5 Ma and 5.5 Ma), preconditioning it to fail recurrently for more than 10 Ma. Interpreted MTDs are shown to enhance burial depths of source and reservoir units, and improve seal competence in lower-slope areas. Conversely, upper slope regions record important erosion and reduced sealing capacity in Late Cenozoic strata. As a result, we postulate that the thickness variations imposed by MTDs on Late Miocene-Quaternary strata have important implications to petroleum plays in the South China Sea

Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K–Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean–continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup

Recurrent failure of the continental slope as a key factor controlling reservoir potential in the South China Sea (Qiongdongnan Basin, South Hainan Island)

High-resolution multi-beam bathymetry, 3D and 2D seismic reflection profiles from the South China Sea are used to investigate the morphology, characteristics, origin and implications to petroleum systems of recurrent slope failure in the Qiongdongnan Basin, northern South China Sea. Seven Late Miocene-Holocene mass-transport deposits (MTDs) and numerous submarine canyons were identified on the continental slope and rise. Providing new insights on the evolution of an enigmatic region of the South China Sea, this paper defends that the interpreted MTDs were caused by a combination of high sedimentation rates and local tectonic uplift. By comparing the stratigraphy of the study area with local sea-level curves, we show that a significant portion of shelf-edge deposits effectively bypassed most of the continental slope during the Miocene-Quaternary to accumulate as large MTDs on its lower part (i.e. on the 'basin-floor'), independently of sea-level changes. Our work has implications to the current sequence stratigraphic models for continental margins, and sheds new light on the reservoir potential of Miocene units in the South China Sea. Hence, regions where base-of-slope fans are expected to occur are, in the study area, occupied by large-scale recurrent MTDs sourced from the shelf edge. Stratigraphically, recurrent slope instability resulted: a) in abrupt episodes of accommodation space creation on the shelf-edge, b) on a moderate reduction in accommodation space at the base of the continental slope, and c) in a complete separation between shelf and distal slope depositional systems, with most stratigraphic markers on 3D seismic data being diachronous across the continental margin. As MTDs also comprise the bulk of channel-fill deposits in large submarine canyons, we propose that the reservoir potential of channel-fill deposits in the South China Sea is closely dependent on the nature of the sediment (i.e. net-to-gross ratio) eroded and transported by these same MTDs

The Baiyun Slide Complex, South China Sea: a modern example of slope instability controlling submarine-channel incision on continental slopes

The Baiyun Slide Complex is one of the largest submarine landslides on the northern margin of the South China Sea. Newly acquired high-resolution bathymetric data, 2D and 3D seismic data permitted the systematic investigation of the Baiyun Slide Complex in terms of its seafloor morphology and associated sedimentary processes. The headwall region of the Baiyun Slide Complex, located at a water depth between 1000 m and 1700 m, is U-shaped and opens towards the east. It was efficiently and almost completely evacuated, generating pronounced headwall and sidewall scarps. Submarine channels, sediment waves, migrating channels, sediment drifts and moats are observed within and around the headwall region, illustrating the effects of both downslope and along-slope sedimentary processes. Submarine channels are 16–37 km-long 800-1500 m-wide, and 20 to 50 m-deep. As a modern example of the interplay between slope instability and subsequent incision, submarine channels were generated after the formation of the Baiyun Slide scar to suggest intensified downslope sedimentary processes after the slope collapsed. The initiation and formation of these submarine channels result from the evacuation of the Baiyun Slide scar, which provided the necessary space of the continental slope to accommodate subsequent turbidity and mass wasting flows. Our results are an important example of how submarine landslides can influence erosional and depositional processes on continental margins