Spear, N., H.D. Holland, J. Garcia-Veigas, T.K. Lowenstein, R. Giegengack, and H. Peters (2014) Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry. GEOLOGY, v. 42(2) p. 103-106. doi: 10.1130/G34913.1
When you think of seawater you probably think simply of ‘salt water’. Seawater is more than just salt in its simplest form of NaCl. Although sodium (Na+) and chloride (Cl–) are two major ions in seawater there is also potassium (K+), calcium (Ca+), magnesium (Mg+), sulfate (SO42-), and bicarbonate (HCO3–). The ratio of the major ions in seawater is not constant or homogenous spatially or temporally; differences in seawater chemistry can be driven by biological, geological, chemical, and physical processes. For example, seawater measured from the shore line of a salt wedge estuary is going to differ from seawater in the Pacific gyres which will differ from seawater from the Mediterranean Sea. Similarly, if you were to measure two sea water samples from the shoreline of an estuary, one from before a rain event and one from after, you would expect to measure a difference in the chemistry as a result of run off, for example.
With this type of reasoning in mind it is plausible to expect that seawater chemistry also varies on much longer time scale, like 800 million years. Some of the places scientists can observe snap shots of sea water chemistry through time is in marine evaporite fluid inclusions, foraminifera tests, oolite deposits, and reefs. Prior to investigations by Spear et al. the oldest reported seawater chemistry measurement was from a halite (marine evaporate) fluid inclusion dating back 544 million year ago during the end of the Neoproterozoic. The goal the Spear et al. research was to extend the seawater chemistry record back another 300 million years to the Mid-Neoproterozoic (830 million years ago) by measuring halite fluid inclusions in the Browne Formation, Officer Basin, Western Australia (Figure 1).
The Officer Basin covers 300,000 km2 of Western Australia. Previous published works have determined the Brown Formation is mid-Neoproterozoic with a combination of lithology, biostratigraphy, radiometric dating, and stable isotope stratigraphy. Stratigraphic, petrographic (minerals), and X-ray diffraction analyses in previous work established that the Browne formation has basic seawater precipitation. The basin is unique because it contains halite crystals that have never experienced recrystallization, leading to the inference that the fluid trapped between them is preserved from the time the crystals formed. These characterizations make the Browne Formation ideal for Spear et al. investigation.
Spear et al. analyzed halite fluid inclusions in two cores (Lancer 1 and Empress 1A, Figure 2) of tectonically stable Browne Formation, Officer Basin. A total of 42 fluid inclusion samples were chemically characterized (elemental analysis) by cryogenic scanning electron microcopy-energy dispersive X-ray spectrometry (cryo-SEM-EDS). Then, a brine equilibrium model was used to estimate the amount of salt evaporated and how it influenced the fluid inclusion with time, the goal being to reconstruct the chemistry of seawater prior to evaporation.
Analysis suggests that the relative concentration of calcium and sulfate to each other was significantly different 830 million years ago than it is today. Rather than observing a greater concentration of sulfate than calcite the opposite relationship is true. Similar observations were made in the Gillen Member, Amadeus Basin, Western Australia (Figure 1). The link suggests that mid-Neoproterozoic seawater was alike across this region. There is also evidence that sulfate concentrations in general were much lower during the mid-Neoproterozoic than in present day and increased towards modern values towards the end of the Neoproterozoic. This last finding agrees with other publications and is possibly linked to increases in atmospheric and oceanic oxygen concentrations. However, disagreements with sulfur isotope records that suggest low sulfate and oxygen concentrations during the end of the Neoproterozoic will need to be investigated further. Currently the most probably explanation for the discrepancy is the history of seawater varies from region to region.
The importance of research like this is that results can expand available knowledge of earth history before anyone was around to directly observe it. Information about how the earth changed in the past is a helpful guide to what changes earth may expect in the future.