How do Calcium Carbonate Veins become a new tool to estimate past seawater cation ratios?

The ratio of cations in seawater is balanced between supply and removal in the ocean, and this ratio can control important geochemical processes. For example, high ratio of Mg / Ca in the precipitation of carbonate rocks favor the formation of aragonite, whereas low ratio of Mg / Ca favor calcite. Therefore, it is necessary to predict the cation ratio of past seawater. In the past, mass-balance modeling and fluid capture in marine cement, fossil, and rock salt have been analyzed to predict this. However, marine sedimentary carbonates are sensitive to the diagenesis and reactions during the formation of halite may change the ratio of cation in seawater. Thus, the authors want to predict the seawater Mg / Ca and Sr / Ca ratios through composition of carbonate vapors (CCVs) obtained by ocean drilling.

CCVs are created by the flow of seawater through the upper ocean crust around the central ridge and react with basalt. In the basement lava, precipitation of calcite and aragonite occurs by this fluid. Based on this, it is possible to determine the temperature at which the CCVs was generated through the cation composition of the carbonates. The temperature dependence of element partitioning between fluid and mineral has already been calculated. If CCVs form at a low temperature (<6 ° C), the reaction between seawater and basalt is minimized and the age of the carbonate is calculated from 87Sr / 86Sr of well-established seawater. However, if CCVs form at temperatures (<60 ° C), sedimentation occurs after substantial seawater-basalt exchange.

The authors calculated 87Sr / 86Sr, Sr / Ca, and Mg / Ca of the pore fluid and CCVs through the samples at the base of Juan de Fuca Ridge (JdFR) between 1.6 to 3.6 million years ago. The values decreased gradually and fluid-rock interaction increased with increasing temperature (Fig. 1).

In the Atlantic and Pacific oceans, the values in the CCVs formed in 1.6 to 170 million years old upper ocean crust also show patterns similar to those observed in JdFR. In the past, the molar ratio of Mg / Ca in seawater was about 1.5 to 2.5 between 170 and 24 million years ago. Basalt-hosted CCVs that were preicitated during the Cretaceous and Paleogene are mainly calcitic, whereas aragonite veins are common in young basalts that were the result of sedimentation at high temperature by a fluid with a Mg / Ca ratio caused by fluid rock exchange.

It can be seen that the molar ratio of Sr / Ca in the past is also about 30% smaller than the modern value. However, the ratio of Sr / Ca is much lower than estimates based on benthic foraminifera and macrofossil calcite. The authors attribute this discrepancy to the uncertainties in biogenic carbonate-Sr partitioning. To reduce this uncertainty, they used the partition coefficients calculated from modern biogenic carbonate. For example, Sr / Ca ratios of the Cretaceous and Jurassic seawater have been estimated from analyses of bivalve and belemnites using the average partition coefficients for modern brachiopods and bivalves.

In order to investigate the causes of Mg / Ca and Sr / Ca ratios in the seawater after the Oligocene, the authors considered the sources of major oceans, sinks of these elements, discharge of major rivers, burials of sediments and hydrothermal exchange. Changes in past composition of the world's rivers are difficult to quantitatively calculate, but most of them are in the range of modern rivers. The majority of modern rivers have a lower Mg / Ca ratio than the seawater since 170 million years ago, but show a wider range of Sr / Ca ratios (Fig. 2). Based on this, a decrease in global river discharge in the Neogene would have increased the Mg / Ca ratio of seawater but not the Sr / Ca. The change in the flow rate of the stream partially explains the change in seawater composition.

In terms of mineralogy, they can explain the sedimentation effect on Sr / Ca and Mg / Ca ratio of seawater. The post burial alteration of carbonates affects the composition of seawater. The exchange between Mg and Ca during the formation of dolomite reduces the ratio of seawater and its effect depends on the time of reaction. The reactions of the black smoker type remove the Mg during hydrothermal circulation and reduce the Sr / Ca ratio of the fluid. The hydrothermal fluids should always have lower Sr/Ca and Mg/Ca ratios than contemporaneous sea water. The major decrease (>30%) in ocean crust production rates from the Cretaceous to the Tertiary and consequent lower black smoker fluid volumes would have increased seawater Sr/Ca and Mg/Ca ratios. Presently, only ~ 55% of the ocean floor is less than 65 million years old and shows hydrothermal circulation of the mid ocean ridge flank, which is ~ 85% when compared with the late Cretaceous. The fluid-rock exchange which decreases in the Tertiary explain the observed increase in Mg / Ca and Sr / Ca ratios of seawater since the Oligocene.