Water column

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

Net removal of major marine dissolved organic carbon fractions in the subsurface ocean

Abstract: [1] Marine dissolved organic matter is a massive reservoir of carbon holding >200x the ocean biomass inventory. Primarily produced at the ocean surface and then exported to depth with overturn of the water column, this carbon can be sequestered in the ocean interior for centuries. Understanding the loss of dissolved organic carbon (DOC) upon export has been data limited, but recent global ocean surveys are overcoming that problem. Here we characterize three fractions of exported carbon by apparent continuity in removal rates: semi-labile and semi-refractory, summing to 20 PgC, and the balance as refractory DOC. Distinct lifetimes coupled with ocean circulation control where the fractions are exported to depth, and thus the carbon sequestration time scales. Maximum remineralization rates of exported DOC occur in the convergent subtropical gyres, where a range of ∼500 to <1500 mmol C m−2 yr−1 can exceed remineralization of sinking biogenic particles. Regions of high particle export production and highly stratified systems exhibit minimal exported DOC remineralization.

Wave-induced shear stresses, plant nutrients and chlorophyll in seven shallow lakes

Abstract: 1 Sediment resuspension dynamics were investigated in relation to changes in water column nutrients (TP, TN, PO4-P, NO3-N and NH4-N), chlorophyll a and phaeopigment in seven shallow (Zm < 15 m) lakes in South Island, New Zealand, ranging in area from 01 to 180 km2 2 Benthic shear stress, calculated from wind speed, effective fetch and depth, was a considerably better predictor of nutrient and pigment concentrations than wind speed 3 For TP, TN, chlorophyll a and phaeopigment, sixteen of the possible twenty-eight linear correlations with benthic shear stress were significant at P < 005, with 16–87% of the variation being explained by shear stress 4 Wind decreased the ratios of TN : TP, with ratios exponentially approaching those of the sediments as shear stress increased in four of the lakes 5 Relationships of dissolved inorganic nutrients to shear stress were considerably weaker than those for total nutrients and showed no consistent trend over the seven lakes 6 Estimated annual mean TP inclusive of resuspension was over four times higher than that derived from measured calm samples in two lakes 7 The number of nutrient and pigment parameters that were significantly correlated with shear stress and the strengths of the relationships varied widely from lake to lake We could establish no simple relationships between these effects and any single characteristic of the lake, sediment, or water 8 A function is developed to predict the rate of entrainment of TN and TP in response to an applied shear stress, where the independent variables are sediment nutrient content and particle size, and the macrophyte density in the lake

Scavenging of 230Th and 231Pa near the antarctic polar front in the South Atlantic

Abstract: Vertical profiles of dissolved and particulate 230 Th and 231 Pa were obtained across the Antarctic Circumpolar Current (ACC) in the southern Atlantic. North of the Polar Front, dissolved and total 230 Th increase with depth in conformity with published scavenging models. There is no depletion of 230 Th or 231 Pa in the water column south of the Polar Front, thought to be an area of enhanced biological productivity. 230 Th concentrations increase three-fold to the Weddell Sea across the ACC. Dissolved and total 231 Pa concentrations are relatively constant below 500 m depth at about 0.3 dpm m −3 , and change little with depth or latitude. The results from the Weddell Gyre are explained by a mixing-scavenging model that takes into account the input of lower Circumpolar Deep Water through upwelling, which is the main source of water in the Weddell Gyre and is enriched in 230 Th but not in 231 Pa. 230 Th accumulates in the Weddell Gyre as a result of a reduction in the scavenging rate and by ingrowth from 234 U. Ingrowth is more significant for 230 Th than for 231 Pa because the residence time of water in the gyre (about 35 years) is similar to the scavenging residence time of Th in the south Atlantic (29 years) but shorter than that of Pa (120 years). It is argued that changes in 230 Th accumulation in the past may reflect changes in water residence time and in the formation rate of Weddell Sea Deep Water.

Depositional model for shallow-marine manganese deposits around black shale basins

Abstract: Some of the world's largest and most valuable manganese resources formed as shallow-marine chemical sediments. We hypothesize that manganese deposition was on the margins of black shale (and related) facies of stratified seas. Manganese was deposited when deep anoxic water with high manganese solubility mixed with shallower oxygen-bearing water with low manganese solubility. The deposits show little or no evidence of volcanic contribution of manganese from nearby sources. The size of the manganese deposit varies with the vigor and dimensions of mixing and with the dissolved manganese content of anoxic deep water, the dilute ore-forming solution. The purity of the deposit depends on prior removal of dissolved iron from the system, primarily by pyrite precipitation in basinal environments, and on the absence of clastic dilution. Deposits may form one or two members of a zonal spectrum that includes sulfides, phosphorite, and barite. In many deposits there is evidence of great organic productivity, probably a by-product of vigorous mixing across stratification surfaces in the water column. Also common are primary sedimentary pisolites or oolites, glauconite, and biogenic silica.Manganese oxide facies deposition is preserved on oxic substrates in shallow water. Carbonate facies deposition can apparently be either on these oxic substrates or on reduced substrates in slightly deeper water. The reduced carbonate facies apparently forms by replacement of calcareous substrates by anoxic waters saturated with MnCO 3 , just below the water column redox interface. A single zoned deposit may show landward oxide facies and basinward reduced carbonate facies. Depositional regression may preserve the water column redox interface as a contact between overlying oxide facies and underlying carbonate facies manganese deposits.Stratigraphic evidence indicates that the deposits formed most commonly during high sea level stands in narrow time intervals when ocean anoxia was widespread. Eight manganese-precipitating modern environments and seven manganese deposits are described.