Conference Agenda

On geological timescales, Earth’s climate is closely linked to the silicate weathering feedback through the coupled silica-carbon cycles. Weathering of silicate minerals on land sequesters atmospheric CO₂. This process is counteracted by marine authigenic clay formation (also known as reverse weathering), which consumes alkalinity and releases the beforehand sequestered CO₂ back into the ocean and atmosphere. Despite the significance and first description already decades ago, the balance between the terrestrial and marine silicate weathering feedback remains difficult to quantify and is mainly attempted indirectly via elemental and isotopic shifts in fluid composition.

In this talk, I will give an overview of controlling processes on silicate weathering from the Archean to the present day, with special emphasis on the close coupling of the terrestrial and marine weathering regimes. I will show, how silicon isotopes (δ³⁰Si) and Ge/Si ratios can be used to decipher weathering and reverse weathering processes and how early diagenesis impacts originally-inherited signatures, hampering the interpretation of authigenic clay geochemistry in the geological record. As an outlook, I will discuss the approach of enhanced silicate weathering in the marine environment as a tool to mitigate climate change through ocean alkalinisation.

Quantitative paleotemperature estimates from the Precambrian are rare due to the scarcity of well-preserved sediments and uncertainties about the isotope composition of ancient oceans. Carbonate triple oxygen isotope measurements (δ¹⁸O and ∆’¹⁷O), however, can be used to overcome these challenges and estimate climate conditions in deep time.

In this study, we investigated interglacial carbonates from the Cryogenian Oodnaminta Reef Complex in Australia, deposited between the Sturtian and Marinoan Snowball Earth events. According to petrological and sedimentary analyses, dolomitization of originally aragonitic reef components occurred immediately after deposition, along with the precipitation of primary dolomite cements.

Our dolomite data display a negative trend in triple oxygen isotope space that cannot be explained by diagenesis, yet it can be attributed to precipitation from a single fluid at various temperatures. To be able to calculate carbonate precipitation temperatures, we first modelled the oxygen isotope composition of the seawater. Initially, we simulated a range of conceivable seawater compositions using an extended oxygen mass balance model. Subsequently, from the modelled values, we selected those compositions that best fit our samples. The resulting seawaters exhibit δ¹⁸O values of around -5‰ and ∆’¹⁷O values of around -10 ppm. Such seawater compositions are feasible when considering the fluxes of silicification and carbonatization. Finally, we derived seawater temperatures of 10 °C to 50 °C, indicative of a temperate Cryogenian interglacial climate.

The Cenozoic cooling that occurred over the past 50 Ma is accompanied by an increase of Mg/Ca in seawater. How this change in seawater chemistry is linked to climate change is still disputed. Mg isotope ratios of seawater could distinguish several possible causes including dolomitization, authigenic clay formation and changes in rates of silicate- and carbonate weathering. Previous reconstructions of Mg isotope ratios of paleo-seawater were based on foraminifera, corals or carbonate muds, which however yielded conflicting results. Here we assess the suitability of bivalves from genus Glycymeris as archives for paleo-seawater δ²⁶Mg (the standardized ²⁶Mg/²⁴Mg). Their potential advantage over other archives arises from their strong evolutionary conservatism, thick shells and a fossil record dating back to the Lower Cretaceous. We established a new method to analyze δ²⁶Mg for samples with a very low Mg/Ca ratio. We report Mg isotope signatures from ventral margins of shells of three recent Glycymeris species from the Adriatic Sea that show an increasing fractionation with increasing ontogenetic age. Similar isotope signatures were observed across single shells, a property that we are going to exploit in the future for reconstructions of paleo-seawater δ²⁶Mg from specimens of fossil Glycymeris.