Mixing with seawater

In individual deposits, S S of sulfides generally increases stratigraphically upwards (Fig. 1.42). (Kajiwara, 1971). Based on the sulfur isotope evidence, Kajiwara (1971) deduced that the ore solutions underwent a progressive cooling and oxidation due to mixing with seawater. 

Where fluids discharge from hot springs and mix with seawater, they cool quickly and precipitate clouds of fine-grained minerals. The clouds are commonly black with metal sulfides, giving rise to the term black smokers. Some vents give off clouds of white anhydrite these are known as white smokers. Structures composed of chemical precipitates tend to form at the vents, where the hot fluids discharge into the ocean. The structures can extend upward into the ocean for several meters or more, and are composed largely of anhydrite and, in some cases, sulfide minerals. 

As fluid from the hydrothermal vent mixes with seawater, chemolithotrophic microbes by this process harvest energy from the chemical disequilibrium among redox reactions, forming the base of the ecosystem s food chain. Microbes can... 

Table 22.5. Electron donating and accepting redox couples and limiting reactant species for various anaerobic and aerobic microbial metabolisms favored in fluid from a subsea hydrothermal vent, as it mixes with seawater...

Cation exchange is particularly important in estuaries because particles transported in river flows experience increasingly higher major cation concentrations as the freshwater mixes with seawater. This process is represented by... 

Movement of groundwater into the ocean illustrating that mixing with seawater can occur in marine sediments and soils located landward of the coastline. MSL = mean sea level. 

The formation of these anhydrite walls prevents the hydrothermal fluids flowing through the chimney from mixing with seawater and provides a framework to enable precipitation of sulfide minerals. In some cases, the discharge of fluids is so rapid that the sulfide precipitates are emitted as clouds of black particles moving at a speed of... 

The input of terrestrial DOM via rivers and aeolian transport was discussed in Chapter 23.3. Riverine concentrations of DOC range from 2 to 20mgC/L. In contrast, little or no terrestrial DOM is detectable in seawater, leading to the cmrent consensus that most is removed close to its point of input. In some estuaries, removal is associated with flocculation reactions promoted by the large increase in ionic strength that occms when river water mixes with seawater. In other estuaries, DOC exhibits conservative behavior, leaving marine chemists with a mystery as to how and where DOC is removed. 

Estuary A semi-enclosed coastal body of water (e.g. bays, lagoons, inlets, and fjords), where fresh water (usually from rivers) mixes with seawater. Chesapeake Bay is an estuary. 

This is reflected in the complexation capacity. Usually a high organic matter content of river and estuarine waters will, together with the colloids in the "dissolved" fraction, result in a high CCqu (100 - 500 nM Cu2+), Fig. 7a. Coastal waters (Fig. 7b), as a result of mixing with seawater, have a lower CCcu (60 - 150 nM Cu2+). Open ocean surface waters of the North Atlantic have a CC u of 20 - 70 nM Cu2+, which in case of low in situ biological activity might be well below this value (Fig. 7c). 

Figure 13 Schematic representation of an MOR hydrothermal system and its effects on the overlying water column. Circulation of seawater occurs within the oceanic crust, and so far three types of fluids have been identified and are illustrated here high-temperature vent fluids that have likely reacted at >400 °C high-temperature fluids that have then mixed with seawater close to the seafloor fluids that have reacted at intermediate temperatures, perhaps 150 °C. When the fluids exit the seafloor, either as diffuse flow (where animal communities may live) or as black smokers, the water they emit rises and the hydrothermal plume then spreads out at its appropriate density level. Within the plume, sorption of aqueous oxyanions may occur onto the vent-derived particles (e.g., phosphate, vanadium, arsenic) making the plumes a sink for these elements biogeochemical transformations also occur. These particles eventually rain-out, forming metalliferous sediments on the seafloor. While hydrothermal circulation is known to occur far out onto the flanks of the ridges, little is known about the depth to which it extends or its overall chemical composition because few sites of active ridge-flank venting have yet been identified and sampled (Von Damm, unpublished).

These waters are derived from both pristine and polluted carbonate systems. The analyses are ordered according to their increasing TDS contents. The freshest waters have a prevalent chemical character (PCC) of the calcium-bicarbonate type. As TDS values increase, the waters become relatively more enriched in Na, Cl, and/or SO4. The high Na and Cl content of analysis number 9 in Table 6.7 (TDS = 1269 mg/L) is derived from its pollution by sewage. The even higher Na and Cl concentrations of analysis number 10 reflect the fact that waters in the X-Can well, which is in coastal Yucatan, have been mixed with seawater. 

Differences in the physical properties of creosote and oils may also affect PNA distributions. When creosote was mixed with seawater In the laboratory, three phases were formed one more dense than seawater, one dissolved in seawater, and one less dense than seawater. Analysis of the phase more dense than seawater produced a gas chromatogram indistinguishable from that of Intact creosote. At a spill site, the phase with a density greater than seawater may be rapidly removed to the sediments with only slight alteration. The water-soluble compounds may then be slowly leached into the water column and act as a chronic PNA source. Petroleum,... 

---