Reading the Chemistry of Ancient Oceans: New Approaches and Future Directions

The chemistry of the oceans records the balance of weathering, depositional and magmatic process at the Earth’s surface, and acts as a fundamental control on the response of the ocean-atmosphere system to perturbation. Some aspects, such as redox and temperature, have well developed suites of proxies, but those for direct estimation of its major ions (Ca2+, Mg2+, Sr2+, K+, Na+, Cl-, SO42-, alkalinity) and nutrients (NO32-, PO43-) have historically been limited or absent. Much of our direct evidence for changes in the major ion makeup of seawater has come from the analysis of fluid inclusions in halite and yet calculations of absolute concentrations from these data for some ions are based on untested assumptions. These data are also only available at low time resolution and have large age uncertainties. The substitution of Mg into abiotic or biogenic calcium carbonate minerals has long been used to estimate both temperature and the Mg: Ca ratio of seawater. However, other ions such as Na-, SO42- and PO43- are present at similar concentrations in carbonate minerals, and are much less well developed as seawater chemical proxies. I will outline progress to date for some of these more novel approaches, present some new data from work in the Jurassic and assess the future potential for creating high resolution records of seawater chemistry.