摘要:

This special section of the Journal of Geophysical Research reports a multi‐investigator study of a number of sunlight‐initiated photoprocesses taking place in the varied biogeochemical and oceanographic environment found in the tropical Eastern Caribbean and Orinoco River delta in the spring and fall of 1988. Principal conceptual themes that were addressed by the program included (1) the characterization of the role of dissolved organic matter as the main chromophore initiating photoprocesses in surface seawater, (2) the determination of the fluxes and pathways of reactants and transient species involved in oxygen photoredox chemistry, and (3) the continuing development of chemical mapping strategies, including observing and modelling reactive phototransient distribution in terms of their sources, mixing, and fates. Ancillary supporting studies included observation of water mass tracers, dissolved trace gases, atmospheric components, nutrients and the geochemistry of estuarine mixing processes in an important continental margin. The observational and mechanistic investigations reported here feature a number of novel or improved methods allied with some advanced underway sampling, sensing and computing facilities that were implemented aboard the R/VColumbus Iselin. Results from the study showed large‐scale (∼1000 km) seasonal variations in surface water photoreactivity, optical and biooptical characteristics over much of the Caribbean basin. These changes resulted from seasonally varying riverine inputs of organic chromophores, nutrients and suspended material. Smaller scale (10–100 km) studies carried out in the Orinoco delta and the Gulf of Paria showed that estuarine mixing processes did not affect major net removal of dissolved organic matter, consistent with the hypothesis that riverine chromophore input plays a dominant role in open‐water pho

ISSN:0148-0227    

DOI:10.1029/92JC02759

年代:1993

数据来源: WILEY

摘要:

Radium isotopes provide a means of identifying the source of freshened waters in the ocean and determining the time elapsed since these waters were in the estuary. We present evidence that during April, waters from the Amazon mixing zone pass within 50 km of the mouth of the Orinoco River. These Amazon waters are characterized by a lower228Ra/226Ra activity ratio (AR) than are waters from the Orinoco at similar salinities. During autumn, the increased discharge of the Orinoco displaces the freshened Amazon waters seaward, yet the two can be distinguished clearly. Within the Caribbean Sea, waters of Orinoco origin carry a characteristic radium signature including excess activities of224Ra. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on224Ra dating ranged from 15 to 33 cm/s during April. The radium isotopes also provide an assessment of sediment mixing in the estuary. During low discharge (April), considerable mixing of older sediment by physical or biological processes or dredging maintained high activities of228Ra in the estuary and produced the highest228Ra/226Ra AR's yet measured in any estuary. During high discharge (September), a large fraction of the228Ra was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of224Ra were high in the estuary during both high and low discharge, indicating that considerable mixing of recently introduced sediment must occur during each period. The high224Ra/228Ra AR's measured in September imply that the same sediments were being resuspended during high discharge, and that there was little exposure of older sediments which had been stored in the estuary for months to years. During April,224Ra and228Ra activities in the water were about equal, indicating that most of the sediment being resuspended had been stored in the estuary long enough to reestablish radioactive equilibrium in the232Th decay series (i.e., 20 years).

ISSN:0148-0227    

DOI:10.1029/92JC02760

年代:1993

数据来源: WILEY

摘要:

Nitrogenous nutrients, dissolved silicate, and salinity were measured in surface waters and shallow hydrocasts along similar cruise tracks during the spring (dry season) and fall (wet season) of 1988. Both cruises transected the eastern Caribbean, transited the Gulf of Paria, ran parallel to the Orinoco Delta and into the main channel of the Orinoco River. Trends in primary productivity were also measured by daily carbon 14 incubations. In both seasons, samples covered the range from highly oligotrophic and transparent to highly productive and rich in biogenic and abiogenic particulate matter. Most of the Orinoco outflow appears to turn N to NW and remains in shallow waters off Venezuela and surrounding Trinidad, permitting benthic regeneration of river‐borne nutrients. However, the role of the Orinoco and associated low‐salinity coastal waters in fertilizing large areas of the eastern Caribbean basin, as suggested by satellite imagery, can be approximated crudely from the nutrient composition at Boca de Dragon, which is representative of the nutrient status of these waters as they flow into deeper Caribbean waters. Additional nutrients may be supplied to the area primarily from Amazon‐derived water entering the Caribbean Basin further north, with some coastal upwelling along the continental shelf in the dry s

ISSN:0148-0227    

DOI:10.1029/92JC02761

年代:1993

数据来源: WILEY

摘要:

Seasonal measurements of photosynthetic pigments were performed in the Caribbean Sea to investigate the potential effects of Orinoco River discharge on the vertical distributions of the major phytoplankton groups. Samples were collected during periods of low (April 1988) and high (September 1988) riverine input. The chlorophyllaconcentration of the subsurface chlorophyll maximum (SCM) sampled at inshore and offshore stations displays only minor seasonal variations. However, the depth and pigment composition of the SCM during periods of low and high outflow are markedly different. A “two‐layer” phytoplankton distribution pattern is observed for all stations occupied during the spring and those stations not influenced by Orinoco River discharge in the fall: an upper “light‐adapted” population dominated by prokaryotic phytoplankton (cyanobacteria and possibly prochlorophyte‐like phytoplankton) and a deeper “shade‐adapted” population comprised of chromophytes and “green algae.” The depth of the SCM at these stations averaged 77 ± 12 m. By contrast, stations influenced by Orinoco River discharge during the fall where characterized by near‐surface diatom‐dominated communities and shallower SCM depths (39 ± 16 m). Nutrient loading, elevation of absorption and attenuation coefficients, and transport of coastal “seed” phytoplankton populations are possible mechanisms by which the Orinoco River modifies the composition and distribution of p

ISSN:0148-0227    

DOI:10.1029/92JC02762

年代:1993

数据来源: WILEY

摘要:

An extensive series of optical absorption spectra were recorded for waters in the eastern Caribbean Sea, the Gulf of Paria, and the Orinoco River Estuary during the high‐flow period of the Orinoco River in the Fall of 1988. Evidence for high levels of dissolved, colored organic matter (COM) was found throughout the eastern Caribbean, with these levels increasing substantially at stations nearer the Gulf of Paria and the Orinoco River. The dependence of the absorption at 300 nm (a300) on salinity exhibited conservative mixing behavior for transects through the Gulf of Paria. In contrast, transects through the Orinoco Estuary revealed thata300remained approximately constant or slightly increased with increasing salinity up to 27‰, after which it decreased linearly with increasing salinity. This nonconservative behavior was not produced by COM release from suspended particulate matter at higher salinity, nor did it appear to originate from the sediment. Below 30‰, the slopes of the log‐linearized absorption spectra were independent of salinity (0.0140 nm−1). At higher salinities, the slopes apparently increased, suggesting that the COM is modified. Based on specific absorption coefficients measured for COM isolated from this area, COM input by the Orinoco River is estimated to be ∼ 2.5 × 1012g C/yr, which represents about 1% of the total global transport of dissolved organic carbon to the ocean. This input is likely to influence substantially the carbon cycle, photochemical properties, and optical characteristics of the waters in

ISSN:0148-0227    

DOI:10.1029/92JC02763

年代:1993

数据来源: WILEY

摘要:

In the spring and fall of 1988, two cruises were conducted in the Caribbean to examine the effects of the Orinoco River on the photochemistry of the eastern Caribbean. Detailed irradiance measurements were made at 29 stations using two scanning spectrophotometers; one measuring surface irradiance, the second measuring the underwater light field. The underwater light field data are presented and examined in this paper. The increased Orinoco River input in the rainy season produced significant changes in the underwater irradiance. This influence included a shift to longer wavelength in the light that penetrated deepest in the water column and an overall reduction in the total irradiance at any depth by approximately a factor of 2. Data will also be presented indicating the importance of accurate depth measurements collected in concert with the spectral irradiance data.

ISSN:0148-0227    

DOI:10.1029/92JC02764

年代:1993

数据来源: WILEY

摘要:

Two cruises were used to investigate the effect of the Orinoco River on the photochemistry of the eastern Caribbean in 1988. Hydrogen peroxide distribution was used as an indicator of the extent of modification and persistence of photochemical activity, with comparisons being made between high (fall) and low (spring) river input. During the spring cruise, H2O2profiles were typical of oligotrophic waters, with accumulation rates and steady state concentrations similar to those reported for open ocean. Results from the fall cruise were characterized by increased surface concentrations, sharper gradients, and shallower penetration of the photochemically generated H2O2. The impact of the river water can be traced in the eastern Caribbean by increased dissolved organic matter fluorescence, lower salinity, and faster hydrogen peroxide accumulation rates in surface samples (up to 20 times greater for some stations). It is apparent that peroxide distribution throughout the eastern Caribbean was significantly affected by the Orinoco river input during the fall.

ISSN:0148-0227    

DOI:10.1029/92JC02767

年代:1993

数据来源: WILEY

摘要:

Superoxide ion‐radical, O2‐, the one‐electron reduction product of molecular oxygen, is a long‐suspected first intermediate in chemical reactions using oxygen as the ultimate electron acceptor. We have detected photochemical production of O2‐in a variety of eastern Caribbean waters studied in Spring and Fall by using a chemically direct O2‐trapping reaction combined with15N and/or18O isotopic labelling of reactants to provide sensitivity and specificity. Photochemical superoxide production rates ranged from 0.1–6 nmol L−1min−1of full‐sun irradiation in Spring and from 0.2–8 nmol L−1min−1in Fall. These superoxide fluxes correlate well with independently measured total (NO‐scavengeable) radical fluxes in both seasons and account for ∼ 1/3 of the total radical production. Hence we confirm that O2‐is a quantitatively important, perhaps dominant, reactive transient species in the photochemical radical array. Semiquantitative estimates suggest that O2‐dismutation to HOOH and O2can account for most or all of the photochemical HOOH production in surface waters. O2‐production rates also

ISSN:0148-0227    

DOI:10.1029/92JC02766

年代:1993

数据来源: WILEY

摘要:

The photochemical decomposition of hydrogen peroxide has been studied in surface waters collected on a transect from the Atlantic Ocean north of Puerto Rico through the eastern Caribbean Sea to the mouth of the Orinoco River. Absolute rates of photochemical decomposition were measured by adding18O labelled H2O2to the water samples and irradiating with a shipboard solar simulator. Photochemical decomposition was measurable at all station locations. Decomposition rates increased along the transect towards the Orinoco and were directly proportional to photochemical production rates, which also increased. Photodecomposition rates were also linearly related to the intensity of absorption of samples at 300 nm. Decomposition rates were generally ∼5% of formation rates, indicating that photochemical decomposition is probably a minor sink for H2O2in the mixed layer compared to biological decomposition. In all seawater samples, photodecomposition led exclusively to18O2, indicating that reaction with photochemically produced oxidants was the dominant reaction mechanism. The oxidizing species are not the best known photooxidants in seawater, singlet oxygen and OH and O2radicals. Unidentified oxidants with formation rates ∼5% of the H2O2formation flux are required. In contrast, H218O2decomposition in unfiltered samples of Orinoco River water led solely to H218O2, indicating reaction with a photochemically produced reductant was the dominant mechanism. Further experiments indicated that the reductant was probably Fe(II) produced by photoreduction of particulate Fe(III), a known process. This pathway would produce an equivalent flux of OH radical: Fe(II) + HOOH → Fe(III) + OH

ISSN:0148-0227    

DOI:10.1029/92JC02768

年代:1993

数据来源: WILEY

摘要:

We have developed a vertical‐flux model of undersea irradiance (300–500 nm) using measured absorption, scattering, midday spectral irradiance, and midday vertical attenuation (Kd) for stations in the eastern Caribbean in 1988. The model varies the pathlength and geometry of undersea light with solar zenith angle according to scattering in the water column, and according to the ratio of direct sunlight to diffuse skylight. Modeled UV and visibleKdhave diel variations up to 67%, which are greatest above 400 nm. Using an experimentally determined photoproduction quantum yield (normalized to total dissolved organic matter absorption) which decreases exponentially as wavelength increases, modeled H2O2photoproduction is primarily due to 320–360 nm irradiance. A significant fraction of total production occurs above 400 nm, especially at depth and at smaller zenith angles. The diel distribution of H2O2photoproduction is notably deeper at midday than would be predicted by UV excitation in a simple nonscattering

ISSN:0148-0227    

DOI:10.1029/92JC02933

年代:1993

数据来源: WILEY