1,652 research outputs found
Quantifying photosynthetic rates of microphytobenthos using the triple isotope composition of dissolved oxygen
Author Posting. © Association for the Sciences of Limnology and Oceanography, 2013. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography: Methods 11 (2013): 360-373, doi:10.4319/lom.2013.11.360.Microphytobenthos are important mediators of nutrient and carbon fluxes in coastal environments. However, quantifying production rates by microphytobenthos is difficult, and existing methods necessitate perhaps erroneous assumptions that dark respiration equals light respiration. Here we present a new method for quantifying photosynthetic rates of microphytobenthos, i.e., gross primary production, by using the triple isotope composition of dissolved oxygen in benthic flux chambers. Because the triple oxygen isotope signature is sensitive to photosynthesis, but not to respiration, this method allows quantification of gross photosynthetic oxygen fluxes by microphytobenthos without assumptions about respiration. We present results from field experiments in Waquoit Bay, Massachusetts, that illustrate the method.We gratefully
acknowledge funding for this work by the Coastal Ocean Institute of
Woods Hole Oceanographic Institution and the National Science
Foundation (OCE-82964400). EH was supported by a National Defense
Science and Engineering Graduate Fellowship award
Ecosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy
at the
Massachusetts Institute of Technology
and the
Woods Hole Oceanographic Institution February 2017Salt marshes are physically, chemically, and biologically dynamic environments found globally at
temperate latitudes. Tidal creeks and marshtop ponds may expand at the expense of productive grass-covered
marsh platform. It is therefore important to understand the present magnitude and drivers of
production and respiration in these submerged environments in order to evaluate the future role of salt
marshes as a carbon sink. This thesis describes new methods to apply the triple oxygen isotope tracer of
photosynthetic production in a salt marsh. Additionally, noble gases are applied to constrain air-water
exchange processes which affect metabolism tracers. These stable, natural abundance tracers complement
traditional techniques for measuring metabolism. In particular, they highlight the potential importance of
daytime oxygen sinks besides aerobic respiration, such as rising bubbles. In tidal creeks, increasing
nutrients may increase both production and respiration, without any apparent change in the net
metabolism. In ponds, daytime production and respiration are also tightly coupled, but there is high
background respiration regardless of changes in daytime production. Both tidal creeks and ponds have
higher respiration rates and lower production rates than the marsh platform, suggesting that expansion of
these submerged environments could limit the ability of salt marshes to sequester carbon.Financial support for my doctoral research was provided by the United States Department of Defense
through the National Defense Science and Engineering Graduate Fellowship Program, the National
Science Foundation under grant OCE-1233678, and the Woods Hole Oceanographic Institution (WHOI)
under grants from the WHOI Coastal Ocean Institute, Ocean and Climate Change Institute, and Ocean
Life Institute. WHOI Academic Programs Office also provided funding support for research, through the
Ocean Ventures Fund, and for my stipend, as graduate research assistantships including an assistantship
from the United States Geological Survey administered by WHOI
Three Super-Earths Orbiting HD 7924
We report the discovery of two super-Earth mass planets orbiting the nearby
K0.5 dwarf HD 7924 which was previously known to host one small planet. The new
companions have masses of 7.9 and 6.4 M, and orbital periods of 15.3
and 24.5 days. We perform a joint analysis of high-precision radial velocity
data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to
robustly detect three total planets in the system. We refine the ephemeris of
the previously known planet using five years of new Keck data and high-cadence
observations over the last 1.3 years with the APF. With this new ephemeris, we
show that a previous transit search for the inner-most planet would have
covered 70% of the predicted ingress or egress times. Photometric data
collected over the last eight years using the Automated Photometric Telescope
shows no evidence for transits of any of the planets, which would be detectable
if the planets transit and their compositions are hydrogen-dominated. We detect
a long-period signal that we interpret as the stellar magnetic activity cycle
since it is strongly correlated with the Ca II H and K activity index. We also
detect two additional short-period signals that we attribute to
rotationally-modulated starspots and a one month alias. The high-cadence APF
data help to distinguish between the true orbital periods and aliases caused by
the window function of the Keck data. The planets orbiting HD 7924 are a local
example of the compact, multi-planet systems that the Kepler Mission found in
great abundance.Comment: Accepted to ApJ on 4/7/201
The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets
Probing the connection between a star's metallicity and the presence and
properties of any associated planets offers an observational link between
conditions during the epoch of planet formation and mature planetary systems.
We explore this connection by analyzing the metallicities of Kepler target
stars and the subset of stars found to host transiting planets. After
correcting for survey incompleteness, we measure planet occurrence: the number
of planets per 100 stars with a given metallicity . Planet occurrence
correlates with metallicity for some, but not all, planet sizes and orbital
periods. For warm super-Earths having days and , planet occurrence is nearly constant over metallicities spanning
0.4 dex to +0.4 dex. We find 20 warm super-Earths per 100 stars, regardless
of metallicity. In contrast, the occurrence of warm sub-Neptunes () doubles over that same metallicity interval, from 20 to 40
planets per 100 stars. We model the distribution of planets as , where characterizes the strength of any metallicity
correlation. This correlation steepens with decreasing orbital period and
increasing planet size. For warm super-Earths ,
while for hot Jupiters . High metallicities in
protoplanetary disks may increase the mass of the largest rocky cores or the
speed at which they are assembled, enhancing the production of planets larger
than 1.7 . The association between high metallicity and short-period
planets may reflect disk density profiles that facilitate the inward migration
of solids or higher rates of planet-planet scattering.Comment: 32 pages, 15 figures, 9 tables, accepted for publication in The
Astronomical Journa
Recommended from our members
Low-Volume and High-Volume Readers of Neurological and Musculoskeletal MRI: Achieving Subspecialization in Radiology.
ObjectiveDifferentiate high- versus low-volume radiologists who interpret neurological (Neuro) MRI or musculoskeletal (MSK) MRI and measure the proportion of Neuro and MSK MRIs read by low-volume radiologists.MethodsWe queried the 2015 Medicare Physician and Other Supplier Public Use File for radiologists who submitted claims for Neuro or MSK MRIs. Radiologists were classified as high-volume versus low-volume based on their work relative value units (wRVUs) focus or volume of studies interpreted using three different methodologies: Method 1, percentage of wRVUs in Neuro or MSK MRI; Method 2, absolute number of Neuro or MSK MRIs interpreted; and Method 3, both percentage and absolute number. Multiple thresholds with each methodology were tested, and the percentage of Neuro or MSK MRIs interpreted by low-volume radiologists was calculated for each threshold.ResultsWith Method 1, 33% of Neuro MRI and 50% of MSK MRI studies were interpreted by a radiologist whose wRVUs in Neuro or MSK MRI were less than 20% (Method 1). With Method 2, 22% of Neuro MRIs and 37% of MSK MRIs were interpreted by radiologists who read fewer than the mean number of Neuro or MSK MRIs interpreted by an "average full-time radiologist" whose wRVUs in Neuro or MSK MRI were approximately 20%. With Method 3, 38% of Neuro MRIs and 57% of MSK MRIs were interpreted by "low-volume" radiologists. If instead a 50% wRVU threshold is used for Methods One, Two, and Three, then 70%, 58%, and 77% of Neuro MRIs and 86%, 80%, and 90% of MSK MRIs are read by low-volume radiologists.DiscussionA large number of radiologists read a low volume of Neuro or MSK MRIs; these low-volume Neuro or MSK MRI radiologists read a substantial portion of Neuro or MSK MRIs. It is unknown which of the methods for distinguishing low-volume radiologists, combined with which threshold, may best correlate with high-performing or low-performing radiologists
The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
The size of a planet is an observable property directly connected to the
physics of its formation and evolution. We used precise radius measurements
from the California-Kepler Survey (CKS) to study the size distribution of 2025
planets in fine detail. We detect a factor of 2 deficit
in the occurrence rate distribution at 1.5-2.0 R. This gap splits
the population of close-in ( < 100 d) small planets into two size regimes:
R < 1.5 R and R = 2.0-3.0 R, with few planets in
between. Planets in these two regimes have nearly the same intrinsic frequency
based on occurrence measurements that account for planet detection
efficiencies. The paucity of planets between 1.5 and 2.0 R supports
the emerging picture that close-in planets smaller than Neptune are composed of
rocky cores measuring 1.5 R or smaller with varying amounts of
low-density gas that determine their total sizes.Comment: Paper III in the California-Kepler Survey series, accepted to the
Astronomical Journa
Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star
Discoveries from the prime Kepler mission demonstrated that small planets (<
3 Earth-radii) are common outcomes of planet formation. While Kepler detected
many such planets, all but a handful orbit faint, distant stars and are not
amenable to precise follow up measurements. Here, we report the discovery of
two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located
656 pc from Earth. We detected the transiting planets in photometry
collected during Campaign 3 of NASA's K2 mission. Analysis of transit light
curves reveals that the planets have small radii compared to their host star,
2.60 0.14% and 3.15 0.20%, respectively. We obtained follow up NIR
spectroscopy of K2-21 to constrain host star properties, which imply planet
sizes of 1.59 0.43 Earth-radii and 1.92 0.53 Earth-radii,
respectively, straddling the boundary between high-density, rocky planets and
low-density planets with thick gaseous envelopes. The planets have orbital
periods of 9.32414 days and 15.50120 days, respectively, and have a period
ratio of 1.6624, very near to the 5:3 mean motion resonance, which may be a
record of the system's formation history. Transit timing variations (TTVs) due
to gravitational interactions between the planets may be detectable using
ground-based telescopes. Finally, this system offers a convenient laboratory
for studying the bulk composition and atmospheric properties of small planets
with low equilibrium temperatures.Comment: Updated to ApJ accepted version; photometry available alongside LaTeX
source; 10 pages, 7 figure
Biological production, export efficiency, and phytoplankton communities across 8000 km of the South Atlantic
Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 31 (2017): 1066–1088, doi:10.1002/2016GB005488.In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March–May 2013. We used observations of picophytoplankton, nanophytoplankton, and microphytoplankton to evaluate community structure and diversity and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m−2 d−1, gross primary production (GPP) between 40 and 100 mmol O2 m−2 d−1, and NCP/GPP, a measure of export efficiency, ranging from 0.1 to 0.2 (0.05–0.1 in carbon units). However, notable exceptions to this basin-scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of dissolved organic carbon export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.National Science Foundation (NSF) Grant Number: OCE 1029676;
Gordon and Betty Moore Foundation Grant Grant Number: 537.01;
Woods Hole Oceanographic Institution (WHOI);
WHOI Devonshire Postdoctoral Scholarship;
National Defense Science and Engineering Graduate Fellowship;
WHOI Ocean Life Institute;
Woods Hole Oceanographic Institution (WHOI) Ocean and Climate Change Institute
NSF Grant Numbers: OCE 1029676, OCE 11543202018-01-1
Planet Candidates from K2 Campaigns 5-8 and Follow-Up Optical Spectroscopy
We present 151 planet candidates orbiting 141 stars from K2 campaigns 5-8
(C5-C8), identified through a systematic search of K2 photometry. In addition,
we identify 16 targets as likely eclipsing binaries, based on their light curve
morphology. We obtained follow-up optical spectra of 105/141 candidate host
stars and 8/16 eclipsing binaries to improve stellar properties and to identify
spectroscopic binaries. Importantly, spectroscopy enables measurements of host
star radii with 10% precision, compared to 40% precision when
only broadband photometry is available. The improved stellar radii enable
improved planet radii. Our curated catalog of planet candidates provides a
starting point for future efforts to confirm and characterize K2 discoveries.Comment: Accepted for publication in the Astronomical Journal; 17 pages, 8
figures, 2 tables, download source for full table
The California-Kepler Survey. I. High Resolution Spectroscopy of 1305 Stars Hosting Kepler Transiting Planets
The California-Kepler Survey (CKS) is an observational program to improve our
knowledge of the properties of stars found to host transiting planets by NASA's
Kepler Mission. The improvement stems from new high-resolution optical spectra
obtained using HIRES at the W. M. Keck Observatory. The CKS stellar sample
comprises 1305 stars classified as Kepler Objects of Interest, hosting a total
of 2075 transiting planets. The primary sample is magnitude-limited (Kp < 14.2)
and contains 960 stars with 1385 planets. The sample was extended to include
some fainter stars that host multiple planets, ultra short period planets, or
habitable zone planets. The spectroscopic parameters were determined with two
different codes, one based on template matching and the other on direct
spectral synthesis using radiative transfer. We demonstrate a precision of 60 K
in effective temperature, 0.10 dex in surface gravity, 0.04 dex in [Fe/H], and
1.0 km/s in projected rotational velocity. In this paper we describe the CKS
project and present a uniform catalog of spectroscopic parameters. Subsequent
papers in this series present catalogs of derived stellar properties such as
mass, radius and age; revised planet properties; and statistical explorations
of the ensemble. CKS is the largest survey to determine the properties of
Kepler stars using a uniform set of high-resolution, high signal-to-noise ratio
spectra. The HIRES spectra are available to the community for independent
analyses.Comment: 20 pages, 19 figures, accepted for publication in AJ; a full version
of Table 5 is included as tab_cks.csv and tab_cks.te
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