Relationship of bacterial richness to organic degradation rate and sediment age in subseafloor sediment

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Applied and Environmental Microbiology 82 (2016): 4994-4999, doi:10.1128/AEM.00809-16.Subseafloor sediment hosts a large, taxonomically rich and metabolically diverse microbial ecosystem. However, the factors that control microbial diversity in subseafloor sediment have rarely been explored. Here we show that bacterial richness varies with organic degradation rate and sediment age. At three open-ocean sites (in the Bering Sea and equatorial Pacific) and one continental margin site (Indian Ocean), richness decreases exponentially with increasing sediment depth. The rate of decrease in richness with depth varies from site to site. The vertical succession of predominant terminal electron acceptors correlates to abundance-weighted community composition, but does not drive the vertical decrease in richness. Vertical patterns of richness at the open-ocean sites closely match organic degradation rates; both properties are highest near the seafloor and decline together as sediment depth increases. This relationship suggests that (i) total catabolic activity and/or electron donor diversity exerts a primary influence on bacterial richness in marine sediment, and (ii) many bacterial taxa that are poorly adapted for subseafloor sedimentary conditions are degraded in the geologically young sediment where respiration rates are high. Richness consistently takes a few hundred thousand years to decline from near-seafloor values to much lower values in deep anoxic subseafloor sediment, regardless of sedimentation rate, predominant terminal electron acceptor, or oceanographic context.This work, including the efforts of Mitchell L. Sogin and Steven D’Hondt, was funded by Sloan Foundation (Census of Deep Life). This work, including the efforts of Steven D’Hondt, was funded by U.S. Science Support Program for IODP. This work, including the efforts of Steven D’Hondt, was funded by National Science Foundation (NSF) (OCE- 0752336 and OCE-0939564). The work of E. A. Walsh, J. B. Kirkpatrick, R. Pockalny, and J. Sauvage was funded by the grants to S. D’Hondt

Fine-scale seismic structure of the shallow volcanic crust on the East Pacific Rise at 9°50′N

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B12104, doi:10.1029/2004JB003152.We use a combination of body wave and interface wave observations from an on-bottom seismic refraction survey to constrain the fine-scale seismic structure of the upper crust in a ∼3 × 3 km field area centered on the East Pacific Rise at 9°50′N. We detonated 18 explosive shots (18 sources) in a circular pattern (1.5 km radius) on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers (eight receivers). We observed 30–40 Hz compressional body waves from all shots (144 P waves) and 1–3 Hz Stoneley (interface) waves on a subset of source-receiver pairs (58 interface waves). Using a station correction inversion, we find that roughly half of the variance in the P wave first-arrival times results from lateral variations in the thickness of the surface low-velocity layer (SLVL), a layer of extremely porous lava and basalt breccia with an average P wave velocity of 2.2 km s−1. The SLVL thickness increases from <20 m along the axial summit trough (AST) to ∼120 m at near-axis lava depocenters, which are not symmetric about the rise axis. Depocenters are located ∼0.5 km to the west and ∼1.5 km to the east of the rise axis. Tomographic inversion of the Stoneley wave first arrivals reveals that shear velocities in the SLVL covary with the layer thickness, exhibiting a similar asymmetric pattern, with shear velocities increasing from ∼320 m s−1 near the AST to ∼520 m s−1 at the near-axis depocenters. Our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes. The thickness and velocity of the SLVL are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited. We find that azimuthal anisotropy exerts only a weak influence on our P wave first-arrival times, which we model as weak (4%) seismic azimuthal anisotropy in the upper dikes with a fast axis oriented N23°–32°W. We find no evidence for seismic azimuthal anisotropy in the extrusive layer

Glacial-interglacial changes in central tropical Pacific surface seawater property gradients

Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).This is the publisher’s final pdf. The article is copyrighted by American Geophysical Union and published by John Wiley & Sons Ltd. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186/Keywords: tropical Pacific, oxygen isotopes, foraminifer

Spatial and temporal variability in flow at the mouth of Narragansett Bay

Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5 - 15 cm s-1) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay

Mesoscale Convective System Surface Pressure Anomalies Responsible for Meteotsunamis Along the U.S. East Coast on June 13th, 2013

Two destructive high-frequency sea level oscillation events occurred on June 13th, 2013 along the U.S. East Coast. Seafloor processes can be dismissed as the sources, as no concurrent offshore earthquakes or landslides were detected. Here, we present evidence that these tsunami-like events were generated by atmospheric mesoscale convective systems (MCSs) propagating from inland to offshore. The USArray Transportable Array inland and NOAA tide gauges along the coast recorded the pressure anomalies associated with the MCSs. Once offshore, the pressure anomalies generated shallow water waves, which were amplified by the resonance between the water column and atmospheric forcing. Analysis of the tidal data reveals that these waves reflected off the continental shelf break and reached the coast, where bathymetry and coastal geometry contributed to their hazard potential. This study demonstrates that monitoring MCS pressure anomalies in the interior of the U.S. provides important observations for early warnings of MCS-generated tsunamis

Multiple melt source origin of the Line Islands (Pacific Ocean)

The Line Islands volcanic chain in the central Pacific Ocean exhibits many characteristics of a hotspot-generated seamount chain; however, the lack of a predictable age progression has stymied previous models for the origin of this feature. We combined plate-tectonic reconstructions with seamount age dates and available geochemistry to develop a new model that involves multiple melt regions and multiple melt delivery styles to explain the spatial and temporal history of the Line Islands system. Our model identifies a new melt source region (Larson melt region at ~17°S, ~125°W) that contributed to the formation of the Line Islands, as well as the Mid-Pacific Mountains and possibly the Pukapuka Ridg

HarrisRobertCEOASCobaltBasedAgeTableS1.xls

Dating pelagic clay can be a challenge due to its slow sedimentation rate, post-depositional alteration, and lack of biogenic deposition. Co-based dating techniques have the potential to create age models in pelagic clay under the assumption that the flux of non-detrital Co to the seafloor is spatially and temporally constant, resulting in the non-detrital Co concentrations being inversely proportional to sedimentation rate. We apply a Co-based method to the pelagic clay sequences from Sites U1365, U1366, U1369, and U1370 drilled during Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre. We distinguished non-detrital Co from detrital Co using multivariate statistical partitioning techniques. We found that the non-detrital flux of Co at Site U1370 is approximately twice as high as that at the other sites, implying that the non-detrital Co flux is not regionally constant. This regional variation reflects the heterogeneous distribution of Co in the water column, as is observed in the present day. We present an improved approach to Co-based age modeling throughout the South Pacific Gyre and determine that the Co-based method can effectively date oxygenated pelagic clay deposited in the distal open ocean, but is less reliable for deposition closer to continents. When extending the method to geologically old sediment, it is important to consider the paleolocation of a given site to ensure these conditions are met.Keywords: sedimentary geochronology, processes and transport, cobalt age models, sedimentary geochemistry, marine sediments, pelagic clay, South Pacific Gyr