Mantle seawater subduction

Holland, G. and C. J. Ballentine (2006) Seawater subduction controls the heavy noble gas composition of the mantle. Nature 441, 186-191 Holland, G., M. Cassidy and C. I Ballentine (2009) Meteorite Kr in earth s mantle suggests a late accretionary source for the atmosphere. Science 326, 1522-1525 Holland, H. D. (1984) The chemical evolution of the atmosphere and oceans. Princeton Univ. Press, New Jersey (USA), 582 pp. 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

Prior to —2.2 Ga, low seawater sulfate concentrations would have limited precipitation and subduction of sulfate-bearing minerals. This would maintain a lower oxidation state in volcanic gases derived in part from recycled cmst (Holland, 2002). Thus, even while the oxidation state of the mantle has remained constant since —4.0 Ga (Delano, 2001), the crust and volcanic gases derived from subduction of the crust could only... 

Hydrothermal reactions between seawater and young oceanic crust have been proposed as an influence on atmospheric O2 (Walker, 1986 Carpenter and Lohmann, 1999 Hansen and Wallmann, 2002). While specific periods of oceanic anoxia may be associated with accelerated hydrothermal release of mantle sulfide (i.e., the Mid-Cretaceous, see Sinninghe-Damste and Koster, 1998), long-term sulfur and carbon isotope mass balance precludes substantial inputs of mantle sulfur to the Earth s surface of a different net oxidation state and mass flux than what is subducted at convergent margins (Petsch, 1999 Holland, 2002). 

Studies of the sulfide inclusions in diamonds have given some indications of sulfur cycling on the Archean earth. These inclusions often appear to be enriched in which has been interpreted as indicative of photolysis of volcanic SO2 at wavelengths <193 nm. This photolysis yields elemental sulfur enriched in while the oceanic sulfate was depleted in (see Eigure 4). The elemental sulfide becomes incorporated into sulfides and can be subducted, and included in diamonds. By contrast seawater has sulfates depleted in and gave Archean barites that are also depleted in the While such studies offer an indication of the sulfur cycle in the Archean, they can also offer insight into the nature of mantle convection through time (Earquhar et ah, 2002). 

Re-Os studies of mantle nodules demonstrate that the craton keel is Archaean in age and that there is no correlation with depth in the keel. The keel, therefore, did not thicken over time. Many eclogite inclusions in diamond are the same age as depleted peridotite or even slightly older. The age data on eclogite inclusions, coupled with evidence from stable isotope measurements that the eclogites formed from seawater-altered basalts, indicate that subduction processes were probably important in Archaean... 

Sulfur is also returned to the mantle. This may follow one of two routes. The sulfur that is mobilized in the oceanic crust during high-temperature hydrothermal activity is a mix of ocean crust sulfide and seawater sulfate. Some of the seawater sulfate is converted into sulfide and fixed in the ocean crust and subsequently subducted into the mantle. In addition sulfur is removed from the oceans to form pyritized sediments and returned to the mantle during sediment subduction. Estimates of the fluxes are given in Canfield (2004) and are shown in Fig. 5.5. 

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