A Comparison of Methods for Determining Significant Wave Heights-Applied to a 3-m Discus Buoy during Hurricane Katrina

In August 2005, the eye of Hurricane Katrina passed 90 km to the west of a 3-m discus buoy deployed in the Mississippi Sound and operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). The buoy motions were measured with a strapped-down, 6 degrees of freedom accelerometer, a three-axis magnetometer, and from the displacement of a GPS antenna measured by postprocessed-kinematic GPS. Recognizing that an accelerometer experiences a large offset due to gravity, the authors investigated four different means of computing wave heights. In the most widely used method for a buoy with a strapped-down, 1D accelerometer, wave heights are overestimated by 26% on average and up to 56% during the peak of the hurricane. In the second method, the component of gravity is removed from the deck relative z-axis accelerations, requiring pitch and roll information. This is most similar to the motion of the GPS antenna and reduces the overestimation to only 5% on average. In the third method, the orientation data are used to obtain a very accurate estimate of the vertical acceleration, reducing the overestimation of wave heights to 1%. The fourth method computes an estimate of the true earth-referenced vertical accelerations using the accelerations from all three axes but not the pitch and roll information. It underestimates the wave heights by 2.5%. The fifth method uses the acceleration from all three axes and the pitch and roll information to obtain the earth-referenced vertical acceleration of the buoy, the most accurate measure of the true wave vertical acceleration. The primary conclusion of this work is that the measured deck relative accelerations from a strapped- down, 1D accelerometer must be tilt corrected in environments of high wave heights

Daily patterns of fluorescence in vivo in the central equatorial Pacific

A daily cycle of fluorescence in vivo was strikingly apparent in surface waters of the central equatorial Pacific between latitudes 4N and 10S, but not in waters to the north or south of this zone. These changes in fluorescence did not represent changes in chlorophyll-a concentration, but rather a photoinhibition of fluorescence by ambient light. Higher nutrient and chlorophyll-a concentrations were found in the region where cycling occurred...

Wave Heights during Hurricane Katrina: An Evaluation of PPP and PPK Measurements of the Vertical Displacement of the GPS Antenna

In August 2005 the eye of Hurricane Katrina passed 49 n mi to the west of a 3-m discus buoy operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). Buoy motions were measured with a strapped-down 6 degrees of freedom accelerometer, a three-axis magnetometer, and a survey-grade GPS receiver. The significant wave heights were computed from the buoy\u27s accelerometer record and from the dual-frequency GPS measurements that were processed in two different ways. The first method was postprocessed kinematic (PPK) GPS, which requires another GPS receiver at a fixed known location, and the other was precise point positioning (PPP) GPS, which is another postprocessed positioning technique that yields absolute rather than differential positions. Unlike inertial measurement units, either GPS technique can be used to obtain both waves and water levels. The purpose of this note is to demonstrate the excellent reliability and accuracy of both methods for determining wave heights and periods from a GPS record. When the motion of the GPS antenna is properly understood as the motion of the buoy deck and not the true vertical motion of the sea surface, the GPS wave heights are as reliable as a strapped-down 1D accelerometer

Hurricane Katrina Winds Measured By a Buoy-Mounted Sonic Anemometer

The eye of Hurricane Katrina passed within 49 n mi of an oceanographic observing system buoy in the Mississippi Bight that is part of the Central Gulf of Mexico Ocean Observing System. Although a mechanical anemometer failed on the buoy during the hurricane, a two-axis sonic anemometer survived and provided a complete record of the hurricane\u27s passage. This is the first reported case of a sonic anemometer surviving a hurricane and reporting validated data, and it demonstrates that this type of anemometer is a viable alternative to the mechanical anemometers traditionally used in marine applications. The buoy pitch and roll record during the storm show the importance of compensating the anemometer records for winds oblique to the horizontal plane of the anemometers. This is made apparent in the comparison between the two wind records from the anemometers during the hurricane

Development, Operation, and Results From the Texas Automated Buoy System

The Texas Automated Buoy System (TABS) is a coastal network of moored buoys that report near-real-time observations about currents and winds along the Texas coast. Established in 1995, the primary mission of TABS is ocean observations in the service of oil spill preparedness and response. The state of Texas funded the system with the intent of improving the data available to oil spill trajectory modelers. In its 12 years of operation, TABS has proven its usefulness during realistic oil spill drills and actual spills. The original capabilities of TABS, i.e., measurement of surface currents and temperatures, have been extended to the marine surface layer, the entire water column, and the sea floor. In addition to observations, a modeling component has been integrated into the TABS program. The goal is to form the core of a complete ocean observing system for Texas waters. As the nation embarks on the development of an integrated ocean observing system, TABS will continue to be an active participant of the Gulf of Mexico Coastal Ocean Observing System (GCOOS) regional association and the primary source of near-surface current measurements in the northwestern Gulf of Mexico. This article describes the origin of TABS, the philosophy behind the operation and development of the system, the resulting modifications to improve the system, the expansion of the system to include new sensors, the development of TABS forecasting models and real-time analysis tools, and how TABS has met many of the societal goals envisioned for GCOOS

Characterization of Subsurface Polycyclic Aromatic Hydrocarbons at the Deepwater Horizon Site

Here, we report the initial observations of distributions of polycyclic aromatic hydrocarbons (PAH) in subsurface waters near the Deepwater Horizon oil well site (also referred to as the Macondo, Mississippi Canyon Block 252 or MC252 well). Profiles of in situ fluorescence and beam attenuation conducted during 9-16 May 2010 were characterized by distinct peaks at depths greater than 1000 m, with highest intensities close to the wellhead and decreasing intensities with increasing distance from the wellhead. Gas chromatography/mass spectrometry (GC/MS) analyses of water samples coinciding with the deep fluorescence and beam attenuation anomalies confirmed the presence of polycyclic aromatic hydrocarbons (PAH) at concentrations reaching 189 mu g L(-1) (ppb). Subsurface exposure to PAH at levels considered to be toxic to marine organisms would have occurred in discrete depth layers between 1000 and 1400 m in the region southwest of the wellhead site and extending at least as far as 13 km. Citation: Diercks, A.-R., et al. (2010), Characterization of subsurface polycyclic aromatic hydrocarbons at the Deepwater Horizon site, Geophys. Res. Lett., 37, L20602, doi: 10.1029/2010GL045046

Ensuring confidence in radionuclide-based sediment chronologies and bioturbation rates

Author Posting. © The Author(s), 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Estuarine, Coastal and Shelf Science 71 (2007): 537-544, doi:10.1016/j.ecss.2006.09.006.Sedimentary records of naturally occurring and fallout-derived radionuclides are widely used as tools for estimating both the ages of recent sediments and rates of sedimentation and bioturbation. Developing these records to the point of data interpretation requires careful sample collection, processing, analysis and data modeling. In this work, we document a number of potential pitfalls that can impact sediment core records and their interpretation. This paper is not intended as an exhaustive treatment of these potential problems. Rather, the emphasis is on potential problems that are not well documented in the literature, as follows: 1) The mere sampling of sediment cores at a resolution that is too coarse can result in an apparent diffusive mixing of the sedimentary record at rates comparable to diffusive bioturbation rates observed in many locations; 2) 210Pb profiles in slowly accumulating sediments can easily be misinterpreted to be driven by sedimentation, when in fact bioturbation is the dominant control. Multiple isotopes of different half lives and/or origin may help to distinguish between these two possible interpretations; 3) Apparent mixing can occur due simply to numerical artifacts inherent in the finite difference approximations of the advection diffusion equation used to model sedimentation and bioturbation. Model users need to be aware of this potential problem. Solutions to each of these potential pitfalls are offered to ensure the best possible sediment age estimates and/or sedimentation and bioturbation rates can be obtained.Thanks to the U.S. Geological Survey Coastal and Marine Geology Program, the Andrew F. Mellon Foundation, the Earth Institute Postdoctoral Fellowship Program at Columbia University, and the National Science Foundation for funding

Organic geochemistry of sediments from chemosynthetic communities, Gulf of Mexico slope

Abstract We used a research submersible to obtain 33 sediment samples from chemosynthetic communities at 541-650 m water depths in the Green Canyon (GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H2 S-oxidizing bacteria at GC 234 contain oil (mean = 5650 ppm) and C1-C» hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C1-C s hydrocarbons (mean = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes 02 in sediments and triggers bacterial sulfate reduction to produce the H 2 S required by the mats. Sediment samples from GC 185 (Bush Hill) contain high concentrations of oil (mean = 24,775 ppm) and C1-C » hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H2S occur at GC 185 where the sea floor has been greatly modified since the Pleistocene by accumulation of oil, therrnogenic gas hydrates, and authigenic carbonate rock. Venting to the water column is suppressed by this sea-floor modification, enhancing bacterial activity in sediments. Sediments from an area with vesicomyid clams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C1-C5 concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associated with the methanotrophic Seep Mytilid I (GC 233) is characterized by low concentration of oil (82 ppm) but biogenic methane (Ct) is present (8829 ppm)

Evidence for an increase in cosmogenic 10Be during a geomagnetic reversal

Reversals in the geomagnetic field, which occur every few hundred thousand years, represent a dramatic change in the Earth's environment. Although there is no satisfactory theory for such reversals, it is generally accepted that the dipole field intensity decreases to <20% of its 'normal' value for a few thousand years during the change in direction. Because the galactic and solar cosmic rays which impinge on the Earth's atmosphere are charged, a significant fraction (about half) of them are deflected by the geomagnetic field. At the time of a reversal, this magnetic shielding is greatly reduced, and it has been suggested that the increased flux of high-energy particles could have effects on evolutionary or climatic processes. For example, the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments could be caused, directly or indirectly, by the decreased geomagnetic intensity during the reversal. We report here evidence in marine sediments for an increase in cosmogenic 10Be production in the Earth's atmosphere during the Brunhes-Matuyama reversal 730,000 yr ago. In addition to confirming an increase in cosmogenic isotope production, the results provide information on the magnitude and duration of the geomagnetic intensity decrease during such an event, and the depth at which remanent magnetism is acquired in marine sediments

Analyses of Water Samples From the Deepwater Horizon Oil Spill: Documentation of the Subsurface Plume

Surface and subsurface water samples were collected in the vicinity of the Deepwater Horizon (DWH) wellhead in the Gulf of Mexico. Samples were extracted with dichloromethane and analyzed for a toxic component, polycyclic aromatic hydrocarbons (PAHs), using total scanning fluorescence (TSF) and by gas chromatography/mass spectrometry (GC/MS). An aliquot of fresh, floating oil from a surface sample was used as a DWH oil reference standard. Twelve of 19 samples collected from 24 May 2010 to 6 June 2010 on the R/V Walton Smith cruise contained TSF maximum intensities above background (0.7 µg L À1 based on 1 L sample size). These 12 samples had total petroleum hydrocarbon (TPH) concentrations as measured by quantitative gas chromatography flame ionization detector (FID) ranging from 2 to 442 µg L À1 . Quantitative GC/MS analysis of these 12 samples resulted in total PAH concentrations ranging from 0.01 to 59 µg L À1 . Low molecular weight, more water-soluble naphthalene and alkylated naphthalene dominated the PAH composition patterns for 11 of the 12 water samples. Sample 12 exhibited substantially reduced concentrations of naphthalenes relative to other PAH compounds. The total PAH concentrations were positively correlated (R 2 = 0.80) with the TSF maximum intensity (MI). TSF is a simple, rapid technique providing an accurate prediction of the amount of PAH present in a sample. TSFderived estimates of the relative contribution of PAH present in the oil provided evidence that PAH represented~10% of the higher molecular weight TPH. The subsurface oil plume was confirmed by the analyses of discrete water samples for TSF, TPH, and PAH