Water column

About: Water column is a research topic. Over the lifetime, 13706 publications have been published within this topic receiving 496626 citations.

Neogloboquadrina pachyderma (sinistral coiling) as paleoceanographic tracers in polar oceans: Evidence from northeast water polynya plankton tows, sediment traps, and surface sediments

Abstract: The abundance and chemistry of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling) have long been used as tools for monitoring polar surface ocean changes and for correlating these changes to atmospheric and thermohaline circulation fluctuations. However, due to its remote habitat, very little is known about how modern N. pachyderma (s.) respond to changing environmental conditions in the polar seas. Modern samples of N. pachyderma (s.) from the Northeast Water Polynya provide a means for studying how environmental conditions affect the vertical distribution and chemistry of this species. Highest abundances of N. pachyderma (s.) were associated with the chlorophyll maximum in the surface 20–80 m, where they are exploiting their primary food source. Evidence suggests that the addition of a calcite crust modifies the calcite tests of some N. pachyderma (s.) between 50 and 200 m, increasing shell density and modifying shell chemistry. The shell mass of encrusted forms is 3–4 times greater than the nonencrusted forms between 50 and 200 m. The oxygen isotope composition of N. pachyderma (s.) shells increase by 1.5‰ in response to local water column gradients. The δ13C values of N. pachyderma (s.) are basically invariant with depth in this region, are consistently 1.0‰ depleted in comparison with the δ13C for equilibrium calcite, and remain basically constant during the shell-thickening process. Mass balance calculations suggest that encrustation occurs at all depths, but abundance counts suggest that the process occurs mostly at the depth of the main pycnocline. Sediment fluxes of N. pachyderma (s.) occur during a 2-week bloom event and decrease to almost zero below complete ice cover. The decoupling of the processes controlling abundances and shell chemistry explain the discrepancies between transfer function and isotopically derived paleotemperature estimates of surface conditions, in some oceanic settings. The ability of δ18O to record surface ocean conditions will depend on vertical water column gradients, as evidenced by the differences in core-top calibrations between the North and South Atlantic.

Sediments : Chemistry and Toxicity of In-Place Pollutants

Abstract: Sediments have only recently been considered as non-point sources of environmental contaminants While most researchers concentrate on a small number of elements including copper, cadmium, lead and mercury, few authors have considered rarer trace elements except in relation to specific sources Examples of trace elements emanating from non-point sources include: boron and fluorine leaching from oil shale; silver from acid mine drainage; arsenic from urban retention basins; and many metals emitted during fossil fuel burning and ore smelting In terms of residence times, the situations in rivers and lakes are very different Following the abatement of chemical pollution into the rivers, elevated concentrations of pollutants continue for a period, depending on a number of physical factors However, in lakes a number of processes cause contaminants to migrate to the sediments and have relatively long residence times Not only are sediments a source of contaminants to the overlying water column, but toxicants associated with sediments can have direct, adverse effects on organisms that live in or near the sediments One approach to studying contaminant releases from sediments is to measure both the species as well as total concentrations of the more common and less common metals in surficial sediments at amore » high enough density in a well-planned sampling grid to allow the construction of maps of the active sediment layers by kriging technique Once contaminant inventories, distribution patterns and relative mobilities have been assessed, the next most critical steps are to determine the average net release rates and potential harmful effects on the biota« less

Diurnal variation of denitrification and nitrification in sediments colonized by benthic microphytes

Abstract: Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a 15N isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and denitrification of nitrate supplied from the water column. Sediments were incubated in a continuous flowthrough system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediments doubled the rate of coupled nitrification-denitrification but reduced the rate of denitrification of nitrate supplied from the water column by -50%. Photosynthesis in the uppermost sediment layers during illumination led to deeper oxygen penetration and a resultant stimulation of nitrification. This stimulation may explain the increased rate of coupled nitrification-denitrification during illumination. The rate of denitrification of nitrate supplied from the water column was reduced during illumination due to a longer diffusion path between water and the anoxic denitrification zone. Denitrification, the bacterial reduction of nitrate to nitrogen gas via nitrite, is an important nitrogen sink in aquatic environments. There are two main sources of nitrate for sediment denitrification: nitrate diffusing into the sediment from the water column and nitrate produced by nitrification in the sediment (Jenkins and Kemp 1984; Seitzinger 1988). The processes of nitrification and denitrification are usually vertically separated within the sediment (Vanderborght and Billen 1975). Nitrification is strictly aerobic and therefore restricted to a thin oxic zone in the upper few millimeters of the sediment. Denitrification takes place in anoxic environments, and in many sediments, most activity is found just below the oxic-anoxic interface (Sweerts and de Beer 1989). The rates of nitrification and denitrification in the active microenvironments are controlled by the availability of substrates, which to a large extent are supplied by diffusion along

Major impacts of climate change on deep-sea benthic ecosystems

Abstract: The deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. Projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep-water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L –1 by 2100. Bathyal depths (200–3000 m) worldwide will undergo the most significant reductions in pH in all oceans by the year 2100 (0.29 to 0.37 pH units). O 2 concentrations will also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century. Unfortunately, how these major changes will affect deep-seafloor ecosystems is, in some cases, very poorly understood. In this paper, we provide a detailed overview of the impacts of these changing environmental parameters on deep-seafloor ecosystems that will most likely be seen by 2100 in continental margin, abyssal and polar settings. We also consider how these changes may combine with other anthropogenic stressors (e.g., fishing, mineral mining, oil and gas extraction) to further impact deep-seafloor ecosystems and discuss the possible societal implications.

Productivity cycles of 200–300 years in the Antarctic Peninsula region: Understanding linkages among the sun, atmosphere, oceans, sea ice, and biota

Abstract: Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.