Subsurface Patches Research Articles

Dynamics of passive admixture in surface and subsurface external deformation field above an ocean guyot

Within the framework of a three-layer quasi-geostrophic model in the f-plane approximation, a numerical study of the effect of an isolated submarine obstacle on the movement of surface and subsurface patches of a neutral admixture transfered by a vertically homogeneous zonal current was carried out. The possibility of partial capture of impurities by quasi-stationary topographic eddy in the vicinity of the submarine obstacle is shown. The role of the initial location (both horizontally and vertically) on the resulting pollution distribution pattern is revealed.

An equivalent parameter geometric shape representation using independent coordinates of cubic Bézier control points

Bézier surface has been commonly applied to represent the complex geometric shape. Generally, all control points are dealt with by the same blending functions, regardless of the effect of independent coordinate. It causes to lack the modeling flexibility. Therefore, this paper proposes an equivalent parameter geometric shape representation method using the independent coordinates of control points. Since the coordinate components of control points are independent, the geometric modeling becomes more flexible. Firstly, a general Bézier curve is described in detail. Related expression is brought out in the form of independent coordinates by introducing two parameters. Then, their geometric meanings are analyzed in detail. Since both parameters are independent to parametric variables u and v, Bézier curve possess the same interval in the discrete parametric space, namely equivalent parameter. Next, a bicubic Bézier subsurface patch representation is discussed, including regular and non-regular subsurface patch. A general surface expression is given out in the form of independent coordinates, as well as the parameter structure and the geometric transformations. Finally, an example of ‘Bézier tree branch’ is constructed by using the proposed method. Results shows that the proposed method is feasible and reasonable.

The silent carbon pool: Cryoturbic enriched organic matter in Canadian High Arctic semi-deserts

High Arctic soil organic carbon (SOC) is an important component in the global C cycle, yet there is considerable uncertainty in the estimates for the polar deserts and semi-deserts that dominate these regions. Some of this uncertainty in SOC estimates arises from the cryoturbic processes including diapirism that structure polar desert soils. Diapirism occurs when the top, viscous layer of permafrost is deformed during freezing and ejected upwards into the soil profile forming a distinct diapiric soil patch or diapir. The diapiric is often nutrient rich relative to the surrounding soil; plants seek out and forage for nutrients in these patches creating a localized mixture of old carbon ejected from the permafrost and new carbon. Here we investigate how the subsurface SOC-rich patches in frost boils resulting from diapirism contribute to overall SOC storage in these environments. We quantify the rates of diapirism and fine-scale distribution of SOC in 560 frost boils at two Canadian high Arctic polar desert sites differing in parent material (dolomite versus granitic) with strikingly different plant surface communities. Though total soil organic carbon content did not differ between the dolomite and granite polar semi-deserts, SOC was being stored differently. The dolomitic site had greater SOC content below 10 cm reflecting the more common occurrence of subsurface SOC patches (46% of frost boils) compared to the granitic site (30%). When a subsurface patch of SOCC (SOC expressed on a m−2 basis) was present in a frost boil the boil contained nearly double the SOCC compared to frost boils without subsurface patches (11 ± 6.3 kg SOC m−2 compared to 6.4 ± 3.6 kg SOC m−2). Diapirism occurs in only 35% of all frost boils, but these diapiric patches represent an important, yet heterogenous, pool of SOC in polar semi-deserts. We upscale from these data to generate an improved estimate of SOC stored in the active layer of High Arctic polar semi-deserts of 8.14 ± 0.45 Pg SOC.

Substantial Sub-Surface Chlorophyll Patch Sustained by Vertical Nutrient Fluxes in Fram Strait Observed With an Autonomous Underwater Vehicle

We present results from a coordinated frontal survey in Fram Strait in summer 2016 using an autonomous underwater vehicle (AUV) combined with shipboard and zodiac-based hydrographic measurements. Based on satellite information, we identified a front between warm Atlantic Water and cold Polar Water. The AUV, equipped with oceanographic and biogeochemical sensors, profiled the upper 50 m along a 10 km-long cross-front oriented transect resulting in a high-resolution snapshot of the upper ocean. The transect was dominated by a 6 km-wide, 10 m-thick subsurface patch of high chlorophyll, located near the euphotic depth within a band of cold water. Nitrate was depleted in the surface, but abundant below the pycnocline. Potential vorticity and Richardson number estimates indicate conditions favorable for vertical mixing, which indicates that the high chlorophyll patch may have been sustained by upward nitrate fluxes. Our observations underline the complex hydrographic and biogeochemical structure in a region featuring fronts and meanders, and further underline the patchy and small-scale nature of subsurface phytoplankton blooms potentially fueled by submesoscale dynamics, which are easily missed by traditional surveys and satellite missions.

Phytoplankton assemblage patterns in the southern Mid-Atlantic Bight

Abstract As part of the Wallops Coastal Oceans Observing Laboratory (Wa-COOL) Project, we sampled a time-series transect in the southern Mid-Atlantic Bight (MAB) biweekly. Our 2-year time-series data included physical parameters, nutrient concentrations, and chlorophyll a concentrations. A detailed phytoplankton assemblage structure was examined in the second year. During the 2-year study, chlorophyll a concentration (and ocean color satellite imagery) indicated that phytoplankton blooms occurred in January/February during mixing conditions and in early autumn under stratified conditions. The chlorophyll a concentrations ranged from 0.25 μg l-1 to 15.49 μg l-1 during the 2-year period. We were able to discriminate approximately 116 different species under phase contrast microscopy. Dominant phytoplankton included Skeletonema costatum, Rhizosolenia spp., and Pseudo-nitzschia pungens. In an attempt to determine phytoplankton species competition/succession within the assemblage, we calculated a Shannon Weaver diversity index for our diatom microscopy data. Diatom diversity was greatest during the winter and minimal during the spring. Diatom diversity was also greater at nearshore stations than at offshore stations. Individual genera appeared patchy, with surface and subsurface patches appearing abruptly and persisting for only 1–2 months at a time. The distribution of individual species differed significantly from bulk variables of the assemblage (chlorophyll a) and total phytoplankton assemblage (cells), which indicates that phytoplankton species may be limited in growth in ways that differ from those of the total assemblage. Our study demonstrated a highly diverse phytoplankton assemblage throughout the year, with opportunistic species dominating during spring and fall in response to seasonal changes in temperature and nutrients in the southern MAB.

Physical and biological processes underlying the sudden surface appearance of a red tide in the nearshore

The sudden appearance at the surface of an alongshore‐parallel band of red tide near Huntington Beach, California, is described in high spatial and temporal resolution using novel instrumentation including a global positioning system–tracked jet‐ski. The scale of the surface chlorophyll a (Chl a) band was small (~ 200 m cross‐shore) and ephemeral (3 h) compared with the subsurface extent of the red tide (~ 2 km, > 7 d). The red tide was dominated by the regionally common dinoflagellate Lingulodinium polyedrum (F. Stein) and had developed as a subsurface Chl a layer during the 7 d prior to the surface appearance. A few hours before the surface appearance, a subsurface patch of elevated Chl a (Chl a > 30 μg L−1) was observed in 13‐m total depth in the trough of a shoreward‐propagating internal wave, consistent with dinoflagellate vertical swimming interacting with the internal wave‐driven convergence. Internal wave‐breaking–induced vertical mixing in ~ 8‐m water depth vertically spread the Chl a patch to the surface, creating the alongshore surface band ~ 500 m from shore. Both the subsurface Chl a patch and the surface Chl a band were prevented from entering the surf‐zone by a density barrier of warm water adjacent to the beach. These high‐resolution observations emphasize the role of nearshore physical dynamics in controlling the duration and intensity of red tide exposure to coastal habitats.

Concentrated perchlorate at the Mars Phoenix landing site: Evidence for thin film liquid water on Mars

NASA's Phoenix mission, which landed on the northern plains of Mars in 2008, returned evidence of the perchlorate anion distributed evenly throughout the soil column at the landing site. Here, we use spectral data from Phoenix's Surface Stereo Imager to map the distribution of perchlorate salts at the Phoenix landing site, and find that perchlorate salt has been locally concentrated into subsurface patches, similar to salt patches that result from aqueous dissolution and redistribution on Earth. We propose that thin films of liquid water are responsible for translocating perchlorate from the surface to the subsurface, and for concentrating it in patches. The thin films are interpreted to result from melting of minor ice covers related to seasonal and long‐term obliquity cycles.

Towed vehicle observations of thin layer structure and a low-salinity intrusion in Northern Monterey Bay, CA

Thin phytoplankton layers are common features in the coastal environment; however sampling these fine-scale optical features across broad horizontal scales remains a challenge. To investigate the horizontal spatial structure of thin phytoplankton layers, we performed an overnight survey in northern Monterey Bay, CA, USA using a SeaSciences Acrobat towed-vehicle. Physical and optical measurements were collected between the surface and near-bottom-depths along four parallel, across-shore transects. Three coherent chlorophyll features were observed: (1) a broad, sub-surface patch at the offshore end, (2) a near-surface patch at the nearshore end, and (3) a deep patch located between the nearshore and offshore patches. The offshore and nearshore patch were separated by a change in seafloor slope and a region of compressed, shoaling isopycnals. Both the offshore and nearshore features were located at the pycnocline, had similar optical properties, and were co-located with a low-salinity intrusion. The deep chlorophyll patch had associated physical and optical properties that were distinct from the patches at the pycnocline. The results from this study further underscore the heterogeneous horizontal spatial structure of thin layers and also add to the growing evidence suggesting that low-salinity intrusions may be strongly linked to the formation of thin phytoplankton layers over the northern shelf of Monterey Bay.

Multiscale effects of surface–subsurface exchange on stream water nutrient concentrations

AbstractStream–riparian ecosystems are landscapes composed of dynamic interacting terrestrial and aquatic patches. Patch composition and configuration affects both the form of transported materials and the amount of nutrient retention and export. We describe spatial patterns of nutrients in the surface water of an arid-land stream using surveys conducted at 3 different scales, ranging from 30 m to 10 km in extent and from 1 m to 25 m in grain. We then relate these patterns to connections with subsurface patches at channel subunit, channel unit, and reach scales. Our objectives were to compare spatial variation in nutrients across scales, to determine the causes of downstream changes in nutrient concentration in terms of intervening patches, and to investigate whether subsurface patches at different scales behaved similarly in terms of net nutrient processing.Nutrients varied spatially at all scales sampled. The highest variation was observed in nitrate-N (NO3-N) in the survey with the smallest grain (CV =...

Phytoplankton patches at fronts: A model of formation and response to wind events

A subsurface patch of chlorophyll is a feature common to many fronts. The dynamics underlying the patch formation are not well understood; however, there appears to be a strong link between the biological and physical dynamics. Here we test the hypothesis that patch formation is a result of wind-forced motions of the front. Frontal responses to transient wind events include acceleration of the surface layers, changes in mixed-layer depth, and excitation of nonlinear oscillations at the front. Such dynamics can affect nutrient fluxes across the pycnocline and the coupling between trophic levels. To test this hypothesis, we developed a two-dimensional coupled mixed-layer/primitive-equation/ecosystem model, which we forced with transient wind events. We explored a range of initial conditions, including the cross-frontal nutrient gradient, the sinking of phytoplankton, the depth of the euphotic zone, and the depth of the front. The model showed the subsurface chlorophyll patch to be dependent on all these factors. The cross-frontal scale of the patch was found to be a historical artifact of past wind events, and so was not strongly related to the scale of the front (given by the Rossby radius of deformation). Winds aligned with the frontal jet weakened the cross-frontal density gradient, causing a flux of nutrients into the warmer waters of the front, and eroding the chlorophyll patch through deep vertical mixing. Winds aligned against the frontal jet enhanced the cross-frontal density gradient and isolated the subsurface patch from vertical mixing. The transient forcing of the fronts led to relatively high f-ratios in the subsurface patches; this enhanced biomass would be invisible to most satellite sensors.

Aquatic humic substances; influence on fate and treatment of pollutants

High Arctic soil organic carbon (SOC) is an important component in the global C cycle, yet there is considerable uncertainty in the estimates for the polar deserts and semi-deserts that dominate these regions. Some of this uncertainty in SOC estimates arises from the cryoturbic processes including diapirism that structure polar desert soils. Diapirism occurs when the top, viscous layer of permafrost is deformed during freezing and ejected upwards into the soil profile forming a distinct diapiric soil patch or diapir. The diapiric is often nutrient rich relative to the surrounding soil; plants seek out and forage for nutrients in these patches creating a localized mixture of old carbon ejected from the permafrost and new carbon. Here we investigate how the subsurface SOC-rich patches in frost boils resulting from diapirism contribute to overall SOC storage in these environments. We quantify the rates of diapirism and fine-scale distribution of SOC in 560 frost boils at two Canadian high Arctic polar desert sites differing in parent material (dolomite versus granitic) with strikingly different plant surface communities. Though total soil organic carbon content did not differ between the dolomite and granite polar semi-deserts, SOC was being stored differently. The dolomitic site had greater SOC content below 10 cm reflecting the more common occurrence of subsurface SOC patches (46% of frost boils) compared to the granitic site (30%). When a subsurface patch of SOCC (SOC expressed on a m−2 basis) was present in a frost boil the boil contained nearly double the SOCC compared to frost boils without subsurface patches (11 ± 6.3 kg SOC m−2 compared to 6.4 ± 3.6 kg SOC m−2). Diapirism occurs in only 35% of all frost boils, but these diapiric patches represent an important, yet heterogenous, pool of SOC in polar semi-deserts. We upscale from these data to generate an improved estimate of SOC stored in the active layer of High Arctic polar semi-deserts of 8.14 ± 0.45 Pg SOC.

Biological studies in the vicinity of a shallow-sea tidal mixing front. I. Physical and chemical background

A study has been made of the distribution and activities of bacteria and zooplankton as they varied seasonally in 1980 and 1981 in the vicinity of a shallow-sea tidal mixing front in the western Irish Sea (approximate position 53° 20' N, 5° 45' W to 53° 50' N, 5° 0' W ). This paper presents the physical and chemical background to these studies as shown by the variations in temperature and salinity and concentrations of chlorophyll a , phaeopigments, cellular adenosine triphosphate (ATP), nitrate, nitrite and ammonium nitrogen, in sections normal to the front. Observations at drogue stations were made to establish the extent of diurnal variations in these properties but these appeared to be small relative to other variations. As the front developed, higher chlorophyll a concentrations appeared in the surface stratified water, in contrast to the bottom stratified water and mixed water, with highest concentrations at the surface at the stratified side of the front and in subsurface patches in the vicinity of the pycnocline. As the phytoplankton populations increased nitrate became depleted in the surface stratified water but nitrite and ammonium nitrogen concentrations remained at about the same levels. Cellular ATP concentration did not appear to be a useful measure of total biomass but indicated high biological activity in the surface stratified water.

Subsurface patch of a dinoflagellate (Ceratium tripos) off Southern California: Patch length, growth rate, associated vertically migrating species

Ceratium tripos dominated a multi-species dinoflagellate patch in the sub-surface chlorophyll maximum in August 1978 on the Southern California shelf. The specific growth rate (μ) ofC. tripos averaged 0.25 d-1. Patch length was about 45 km along the shelf. Several members of the subsurface dinoflagellate assemblage were also present in surface samples, but only during the daytime. These apparent vertical migrators includedProrocentrum micans, C. furca, Gonyaulax polyedra and other less common forms. The growth ofC. tripos in the California patch is compared with that in aC. tripos patch off New York in 1976.