Hydrothermal sources

The products are thermodynamically favored under the oxic alkaline conditions that are characteristic of most of the ocean. Reaction rates are slow, so metal oxides tend to precipitate onto detritus or preexisting nodules because of the catalytic effect of the surfaces. The Fe and Mn are supplied by both river and hydrothermal sources. For Mn, these two sources are about equal. 

We also know that a considerable enrichment of prebiotic moieties may have come from submarine vents and other hydrothermal sources (see, for example, Miller and Bada, 1988 Holm and Andersson, 1998 Stetter, 1998). Let s start with the 1979 discovery of deep-sea vents with black smokers, which are associated with an extraordinary abundance of the most phylogenetically primitive organisms on Earth. This ecosystem is sulphur based, and is distinct from the more familiar, photosynthetically-based ecosystem that dominates Earth s surface. Corliss et al. (1981) were struck by the richness of the vent biota, based on chemosynthesis, and proposed that these were the origin of life. 

Walker River California—Nevada, USA Weathering of geologic materials, hydrothermal sources 6.25-8.69 <2-65 (0.4 pm filtered) Johannesson et al. (1997), 69... 

An interesting volcanic contribution to lakes and rivers comes from hydrothermal sources (hot water... 

Evans M. J., Derry L. A., Anderson S. P., andFrance-Lanord C. (2001) Hydrothermal source of radiogenic Sr to Himalayan... 

Figure 15 (Fe + Mn + A1)/A1 ratio in marine sediment. Metalliferous sediments have elevated ratios compared to background pelagic sedimentation. The hydrothermal source of Fe and Mn to metalliferous sediments is reflected in the elevated ratios along mid-ocean ridges (source Mills and Elderfield, 1995 after Bostrom et aL, 1969).

The existence in the Archaean of sulphate-reducing bacteria, producing a spread in S isotopes around b S = 0 5%o has long been accepted (e.g. Goodwin et al. 1976 Shen et al. 2001). Sulphate would have come from ambient water, with a component supply of metals and other nutrients from hydrothermal sources, as shown by the REE (Table 1). In modem microbial mats, sulphate-reducing bacteria extract sulphur from sea water, fractionating it by an amount that depends on the efficiency and rate of extraction. Conversely, sulphide-oxidizers reverse the process. 

Many of these reactions are in the direction needed to close the marine mass balances for major ions (Fig. 2.4). The exceptions are that they supply an unnecessary additional siiik for SO4 (CaSO precipitation) and a vast additional source of K+. The additional sink for SO4 does little damage to the marine SO4 mass balance in Fig. 2.4 because its removal affects ordy Ca + and only at the level of about 15% of the Ca + riverine inflow. The hydrothermal source for K+ cannot be rationalized as easily, because there is no adequate sink in the marine environment. Research into the sources and sinks of alkali metals reveals that K+ (and other alkali metals) that are released from basalts at high temperature are reincorporated back into basaltic rock on the sea floor at low temperature. Thus, is recycled in the vicinity of hydrothermal vents. The rates of release and incorporation are uncertain enough to obscure whether the net K+ flux is into or from the ocean in these regions. It is possible that the low-temperature removal of K+ to basalt represents a net sink large enough to accommodate the river inflow. 

An unknown but potentially significant amount of manganese emitted from mid-ocean ridge (MOR) hydrothermal sources is precipitated near black smokers (i.e. immediately removed), so this estimate is not directly comparable to the other inputs. 

According to different estimates, the river runoff to the ocean changes from 0.104 X10 tons S/yr. (Ivanov, 1983) up to 0.162 x 10 tons S/yr. (Dobrovolsky, 1994). A significant amount of various sulfur compounds are input through hydrothermal sources, up to 130 x 10 tons/yr. The volcanic sulfur emission to the continental atmosphere is estimated as 0.001 x 10 tons annually and a similar number applied to for oceanic atmosphere (Fried, 1973). 

Fig. 7.1 Global fluxes of dissolved (Fe j ), highly reactive iron (FeHR) and total iron (FeT) from riverine, glacial, atmospheric and hydrothermal sources. The estuarine mixing zone serves as a major sink for dissolved and highly reactive (dithionite-soluble) iron. Fluxes and concentrations are given according to Poulton and Raiswell (2002). The atmospheric dissolved iron flux was taken from Duce et al. (1991).

FeT), with FCjjjj/FeT = 0.43. In contrast, glaciers discharge sediments which were formed predominantly under physical weathering conditions and reveal a mean FCjjjj/FeT of 0.11. Approximately 40 % of the riverine highly reactive iron fraction is retained in the estuarine and near-coastal zone, which, together with the low FCjjj proportion in glacial sediments leads to a FCjjjj/FeT of 0.26 in marine sediments. The iron inputs from atmospheric and hydrothermal sources are relatively small, i.e. less than 10 % of Fe and FeT as compared to the riverine input (Fig. 7.1). 

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