Robert F. Chen

Modifications of the local environment by natural marine hydrocarbon seeps

The bubble gas partial pressures, dissolved gas and oil, and fluid motions within the rising bubble streams of three shallow ( 108times) than atmospheric equilibrium values; at two of the seeps it was slightly supersaturated with respect to the bubbles partial pressure. This indicates that the rate limiting step for methane, CH4, transfer into the water column was not bubble gas transfer, but rather turbulence transfer of water out of the saturated bubble stream to the bulk ocean. Strong upwelling flows were observed, as well as elevated fluorescence indicative of dissolved oils. At the bottom, bubbles are circa 90% CH4, but at the surface they were circa 60% CH4, 30% air, and 10% higher hydrocarbons.

Effects of cross-flow filtration on the absorption and fluorescence properties of seawater

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. Colloid spectroscopic properties, contamination artifacts, and instrumental variability were evaluated during a cross-flow filtration (CFFr intercomparison exercise. Analyses were based on absorbance (at 337 nmr, humic and protein fluorescence (337/420 and 270/320 nm, respectivelyr, absorption and fluorescence spectra, log-linearized and seawater-normalized absorbance spectra, and fluorescence-to-absorbance ratios. Permeates and retentates (1000 daltons; 1 kDr from five types of CFF systems and two types of seawater samples, coastal surface water (WHOIr and open-ocean deep water (off Hawaii; s600 m deepr, were examined. Retention of absorbance and humic fluorescence (337/420 nmr varied by a factor of s3 within system types and by a factor of s5 between system types. Despite these variations, good absorbance and humic fluorescence balances were obtained by most systems for both samples, although Hawaiian seawater appeared to be more prone to contamination effects. Only two of the 5 CFF system types (Amicon and Desalr showed significant colloid retention. Based on those two systems, major differences in absorbance and fluorescence properties were found for the two water types. For the coastal WHOI samples, s40% of the original dissolved organic matter (DOMr absorbance signal was retained as colloid, in agreement with organic carbon (OCr results. However, for the deep Hawaiian samples, only s15% of the absorbance was retained as colloid, in contrast to an average >40% for OC. These results indicate that a greater percentage of the total pool of absorbing DOM is lower molecular weight (<1 kDr in the deep Hawaiian samples compared to the coastal samples. Humic fluorescence displayed the opposite trend. The absorbance spectra of the retained colloids for both water types were significantly different from those of the unfractionated seawaters. Thus, the qualitative nature of the colloid fraction changed as function of both concentration factor during CFF and sample type, a result not obtainable from simple OC mass balances. These results indicate that organic matter (OMr concentrated by CFF is different from OM in unfractionated seawater in terms of chemical composition and spectroscopic properties, i.e. chromophoric DOM is not uniformly distributed over the total OM pool in seawater. Major, unexpected differences were also found for the fluorescence efficiencies and the slopes (Sr of the log-linearized absorption spectra for the two seawater types prior to CFF.

Estimation of chromophoric dissolved organic matter in the Mississippi and Atchafalaya river plume regions using above‐surface hyperspectral remote sensing

[1] A method for the inversion of hyperspectral remote sensing was developed to determine the absorption coefficient for chromophoric dissolved organic matter (CDOM) in the Mississippi and Atchafalaya river plume regions and the northern Gulf of Mexico, where water types vary from Case 1 to turbid Case 2. Above‐surface hyperspectral remote sensing data were measured by a ship‐mounted spectroradiometer and then used to estimate CDOM. Simultaneously, water absorption and attenuation coefficients, CDOM and chlorophyll fluorescence, turbidities, and other related water properties were also measured at very high resolution (0.5–2 m) using in situ, underwater, and flow‐through (shipboard, pumped) optical sensors. We separate ag, the absorption coefficient a of CDOM, from adg (a of CDOM and nonalgal particles) based on two absorption‐ backscattering relationships. The first is between ad (a of nonalgal particles) and bbp (total particulate backscattering coefficient), and the second is between ap (a of total particles) and bbp. These two relationships are referred as ad‐based and ap‐based methods, respectively. Consequently, based on Lee’s quasi‐analytical algorithm (QAA), we developed the so‐called Extended Quasi‐Analytical Algorithm (QAA‐E) to decompose adg, using both ad‐based and ap‐based methods. The absorption‐backscattering relationships and the QAA‐E were tested using synthetic and in situ data from the International Ocean‐Colour Coordinating Group (IOCCG) as well as our own field data. The results indicate the ad‐based method is relatively better than the ap‐based method. The accuracy of CDOM estimation is significantly improved by separating ag from adg (R 2 = 0.81 and 0.65 for synthetic and in situ data, respectively). The sensitivities of the newly introduced coefficients were also analyzed to ensure QAA‐E is robust.

Dissolved organic carbon in the Mid-Atlantic Bight

The Mid-Atlantic Bight (MAB) is a highly productive continental shelf region on the eastern North American coast and a potential sight for the export of organic carbon to the open ocean. The dynamics of dissolved organic carbon (DOC) in the MAB were studied during three cruises in April 1994, March 1996 and August 1996 in order to characterize DOC distributions on the shelf, estimate inventories of DOC on the shelf and quantify DOC transport. There were clear spatial and temporal trends in DOC distributions. Concentrations of DOC were greater inshore than offshore and increased southward along the shelf. The total DOC inventory on the shelf during the March and April cruises was estimated at B5.88 � 10 12 g C and this increased by 0.4 � 10 12 g C (7%) in August. A simple mass balance of DOC input and export in the MAB resulted in total export of 18.7–19.6 � 10 12 gC yr � 1 . Up to 67% of this carbon is recycled oceanic DOC that is refractory with respect to MAB seawater residence times, B4% is introduced through estuaries, sediments and rainwater and at least 29% of the exported shelf DOC (4.1–7.4 � 10 12 gC yr � 1 ) is produced on the shelf. Net ecosystem production of DOC in the MAB is estimated to be between 10% and 18% of MAB primary productivity estimates. Our data supports the hypothesis that the MAB as a whole is net autotrophic for the periods of study and that DOC is an important component in ocean margin carbon budgets. r 2002 Elsevier Science Ltd. All rights reserved.

In situ fluorescence measurements in coastal waters

Blue fluorescence of seawater resulting from ultraviolet excitation has been used to study various aspects of the marine carbon cycle. The optical properties of dissolved organic matter affect ocean color and control the quantity and quality of sunlight that penetrates seawater. While most fluorescence measurements involve discrete sampling, this paper presents some instrumental developments that allow real-time in situ measurements with high temporal and spatial resolution in coastal waters. Fluorescence is used to study benthic fluxes of organic compounds, to estimate fluxes of dissolved organic matter (DOM) off continental margins, and to identify sources and sinks of DOM in coastal waters.

Laser-induced fluorescence of pyrene and other polycyclic aromatic hydrocarbons (PAH) in seawater

Polycyclic aromatic hydrocarbons (PAH) in the marine environment are currently of great concern due to their potential carcinogenicity. The standard methods of detection and quantification of PAH in seawater and sediments are costly, time-consuming and do not account for the heterogeneous nature of their distribution and sources. Laser-induced, time-resolved fluorescence spectroscopy may help to overcome these limitations. Several PAH have relatively long-lived stimulated fluorescence emissions, which allow them to be detected among a background of more intense but shorter-lived chromophores. Using time-delayed techniques we have shown an ability to detect PAH, principally pyrene, at environmental levels (ng l(-1)) both in the laboratory and in situ in Boston Harbor and other study areas. Further development may lead to the rapid determination of several PAH in typical near-shore marine environments.

Dissolved organic carbon on Georges Bank

Dissolved organic carbon (DOC) in sea water from Georges Bank was measured by a high temperature combustion/direct injection (HTC/DI) technique during the spring bloom period in April 1993. Concentrations in surface waters (72–85 μM) and deep waters (54–56 μM) were similar to DOC concentrations measured in the oligotrophic north-west Atlantic Ocean by a number of other investigators by various techniques. Although surface values for Chl-a concentrations ranged from 2 to 5 μg l−1, NO3− ranged from 3 to 9 μM and particulate organic carbon (POC) concentrations ranged from 1.0 to 10μM, DOC concentrations only varied by <18‰ DOC was slightly higher (∼5 μM) in the highly productive central bank region than in surrounding stratified surface waters. Relatively constant stable carbon isotopes (DO13C = −22.0 ± 0.5 could not be used to identify a source for these small variations. Slight decreases of DOC (5–12μM) and dissolved organic nitrogen (DON) (0.5–1.0 μM) in filtered sea water kept in the dark was observed over six months, suggesting the presence of a small, labile pool of dissolved organic matter in addition to a large, more refractory reservoir of DOC (∼70 μM) during the spring bloom period on Georges Bank.

The fluorescence of dissolved organic matter in the Mid-Atlantic Bight

Abstract The fluorescence of dissolved organic matter has been measured on three cruises covering the Mid-Atlantic Bight in the spring of 1994 and 1996 and the summer of 1996. An attempt at a “mass” balance of fluorescent dissolved organic matter (FDOM) on the shelf suggests that more FDOM is present during the spring than during the summer and that deficits during the summer are due to photodegradation. However, subsurface production of FDOM was also prevalent during the summer. These processes were observed due to high shelf productivity and a stable water column that isolates a “cold pool” below sunlit surface waters. Rivers contributed about 10% of the total FDOM to the shelf with the majority of FDOM being advected onto the shelf from slope waters. Net production of FDOM was observed in the spring while net degradation of FDOM was observed in the summer. The fluorescence of dissolved organic matter appears to be a useful measurement for better understanding dissolved organic carbon dynamics as well as ocean optics in ocean margins.

Contribution of “Old” carbon from natural marine hydrocarbon seeps to sedimentary and dissolved organic carbon pools in the Gulf of Mexico

Natural radiocarbon (14C) abundances and stable carbon isotope (δ13C) compositions were measured for sediment total organic carbon (TOC), and total lipid fractions of sediments, bottom water, and hydrate-water collected from two hydrocarbon seepage sites in Green Canyon, Northern Gulf of Mexico to determine the contribution of “old” carbon from seeps to sediment TOC and dissolved organic carbon (DOC) pools. Our results indicate that 40–60% of the organic carbon preserved in the sediments and 30% of the DOC in the deep water above the seeps were seep-derived 14C-depleted organic carbon. This new evidence along with our previous studies suggest that natural marine hydrocarbon seepage could be a significant source contributing “old” carbon to the marine environment. Our findings suggest that the global importance and the long-term impact of this contribution to biogeochemical carbon cycling in the ocean need to be more thoroughly investigated.

Adaptive decentralized control of underwater sensor networks for modeling underwater phenomena

Understanding the dynamics of bodies of water and their impact on the global environment requires sensing information over the full volume of water. We develop a gradient-based decentralized controller that dynamically adjusts the depth of a network of underwater sensors to optimize sensing for computing maximally detailed volumetric models. We prove that the controller converges to a local minimum. We implement the controller on an underwater sensor network capable of adjusting their depths. Through simulations and experiments, we verify the functionality and performance of the system and algorithm.