Oxygen, isotope composition depositional environment

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

In this paper, we will examine the physiochemical processes that act in concert to determine the isotopic composition of speleothem calcite and, based upon a worldwide data compilation, argue that it is important to base any speleothem paleoclimatology study upon as thorough an understanding of the modern cave depositional environment as is possible. The focus of the discussion will be on oxygen isotopes, but we also will discuss carbon isotopes to a limited extent. Our observations and comments are based upon a comprehensive isotopic database compiled for precipitation, cave seepage water. 

The catalytically active, Wyoming-type montmorillonites are inferred to have developed their distinctive, high-Na smectite-rich compositions following deposition of granitic volcanic ash into a shallow, saline ocean during the Late Cretaceous. Oxygen isotopic data from Hadean zircons have been interpreted as indicating that oceans may have been present on the Earth by as early as 4,300-4,400 million years ago (62-63). While current evidence provides no information on the composition of the hypothesized Hadean seawater, the existence of oceans on the Hadean Earth is a necessary environment on the ancient Earth if catalytically active montmorillonites formed. 

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