Evaporites ancient

Since the experimental studies of van t Hoff at the turn of the century, geochemists have sought a quantitative basis for describing the chemical evolution of seawater and other complex natural waters, including the minerals that precipitate from them, as they evaporate. The interest has stemmed in large part from a desire to understand the origins of ancient deposits of evaporite minerals, a goal that remains mostly unfulfilled (Hardie, 1991). 

Evaporites are important sources of economic minerals that have been exploited for at least the past 6000y. For example, the evaporite mineral trona (NaHCOj -Na2C03 2H2O) was used by the ancient Egyptians to preserve mummies. Evaporite salts continue to be used for food preservation, construction, road deicing, and in industrial processes. The marine evaporites of Saskatchewan (Canada) are the world s largest source of potash (KCl), which is used as an agricultural fertilizer. In the United States,... 

Dolomites compose a significant portion of ancient evaporites. This mineral is essentially a Mg-rich calcite, having variable Mg content. Its empirical formula is commonly given as MgCa(C03)2. Having no modern examples of dolomite precipitation beyond those of a few salinas and sabkhat, geochemists have been unable to determine conclusively how the ancient deposits must have formed. 

The ancient evaporites of the Phanerozoic eon were deposited at rates as fest as 100 m per lOOOy. These rapid rates are thought to have been caused by a lowering of sea level associated with tectonic activity and glaciation. Some of the largest of the salt giants are the Messinian evaporites that formed in the Mediterranean Sea during the late Miocene epoch, 5.5 to 6.5mybp. 

Cap rock A relatively impermeable rock that retards the migration of petroleum through marine sediments. Many are composed of ancient evaporites, also called diapirs. 

Diapir An ancient evaporite that has become buried in marine sediments. The overlying pressure causes the rock to flow like toothpaste out of a tube, thereby forming vertical pillar and domal structures. 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen - A novel isotope effect and its cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson T, Zipf EC, Erdman PW, van Egmond C (1995) Carbon dioxide and oxygen isotope anomalies in the mesophere and stratosphere. Science 270 969-972 Thode HG, Monster J (1964) The sulfur isotope abundances in evaporites and in ancient oceans. In Vinogradov AP (ed) Proc Geochem Conf Commemorating the Centenary of V I Vernadsku s Birth, vol 2, 630 p... 

Evaporites, and Ancient Seas, in Fluids in Subsurface Environments —A Symposium, Am. Assoc. Petrol. Geol. Memoir 4 (1965) 367. 

EVAPORITE. A sedimentary rock formed hy precipilalion from waters ai the earth s surface. As described by Lnwenstcin I.Science. 1090, September 8. 1989), ancient evaporiles have been used to track the chemistry of ancient surface waters, particularly seawaler. Study ol marine evaporiles has led to Ihe general inol unanimous) conclusion that ihe major elemental chemistry of seawater has not changed significantly during the lust 600 million years,... 

Arakel A. V. and Hongjun T. (1994) Seasonal evaporite sedimentation in desert playa lakes of the Karinga Creek drainage system, central Austraha. In Sedimentology and Geochemistry of Modem and Ancient Saline Lakes (eds. R. W. Renaut and W. M. Last). Society for Sedimentary Geology (SEPM), Tulsa, pp. 91-100. 

Thode H. G. and Monster J. (1965) Sulfur-isotope geochemistry of petroleum, evaporites, and ancient seas. In Fluids in Sub Surface Environments—A Symposium of American Association of Petroleum Geologists, Tulsa, OK, Memoir, pp. 367-371. 

Messinian evaporites from the Mediterranean. Many probable ancient examples exist, although detailed geochemical investigations commonly show at least a minor terrestrial solute contribution (i.e. B1 below)... 

Gomitz, V.M. Schreiber, B.C. (1981) Displacive halite hoppers from the Dead Sea Some implications for ancient evaporite deposits. Journal of Sedimentary Petrology 51, 787-794. 

Lowenstein, T.K., Spencer, R.J. Zhang, P. (1989) Origin of ancient potash evaporites Clues from the modern nonmarine Qaidam basin of western China. 

There is evidence that the major ion composition of seawater has varied over many millions of years, linked to very long term geochemical cycling. Evidence from ancient marine evaporite sequences (Box 6.2) sets limits on the possible extent of that variability. 

The evidence that the salinity and major ionic composition of seawater have remained reasonably constant over at least the last 900 million years comes from ancient marine evaporite deposits. Evaporites are salts that crystallize from evaporating seawater in basins largely cut off from the open ocean. 

The sulphite (SO2-) is subsequently oxidized to SOI-. Sedimentary pyrite, formed as a byproduct of sulphate reduction in marine sediments, is a major sink for seawater SO -. The presence of pyrite in ancient marine sediments shows that SO4- reduction has occurred for hundreds of millions of years. On a geological timescale, removal of SO4- from seawater by sedimentary pyrite formation is thought to be about equal to that removed by evaporite deposition (Section 6.4.2). Compilations of pyrite abundance and accumulation rates are used to calculate modern SO - removal by this mechanism and to derive the estimate in Table 6.2. 

The S S value for modem seawater varies between about 18.5 /oa and 21.0 /oo (Kerridge et al. 1983 Chambers, 1982). Present-day sulphate evaporites are enriched in 5 S by between 1 and 2 Vm relative to seawater, a relationship exploited by Claypool et al. (1980) to determine the value of ancient seawater. Their seawater curve shows marked excursions from the present value (Figure 7.22) with a particular low (+10.5 Vm) in the Pennian and a marked high at the base of the C bmbrian (+31.0 V ). There is a superficial similarity between the... 

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