A recent study by graduate student Leonardo Tedeschi (Wolfson 2013) has caused a diverse group of experts, including oil industry geologists, palaeoceanographers and biogeochemists, to rethink long-standing ideas about the exact age and significance of Cretaceous salt deposition in the South Atlantic.
In a paper published in Geology, co-authored by Oxford Earth Sciences Professor Hugh Jenkyns (Emeritus Tutorial Fellow at St Edmund Hall) and Associate Professor Stuart Robinson (St Edmund Hall 1995, now Tutorial Fellow at St Anne’s), alongside researchers from Petrobras, Tedeschi reveals that assumptions about the age of the salt are wrong, particularly in the large basins of offshore Brazil. Correcting this misconception offers a significant gain to the oil industry in the modelling of sub-salt hydrocarbon reserves and has revealed an interesting coincidence between the age of salt deposition, an interval of widespread oceanic anoxia and changes in the sulphate concentration of seawater.
Tedeschi, who recently completed his DPhil in the Oxford Earth Sciences Department, was funded by the Brazilian Petroleum Company Petrobras. Industrial links such as these provide university researchers with access to data and samples that would be otherwise denied to them, whilst also providing the companies with new insights. For this study, core samples were taken from Petrobras wells in the Campos and Santos Basins, where potential source rocks are now known to be found below thick (~2 km) salts (evaporites).
Researchers from Petrobras determined the identity of tiny planktonic fossils known as foraminifera at different depths in the cores, a discipline known as biostratigraphy, which helps give an age to the samples. In the laboratory in Oxford, geochemical analysis on material from the same cores revealed distinctive patterns in the ratio of stable carbon isotopes that reinforced the biostratigraphy. These results were then compared to other better known data-sets, with well-constrained ages, to arrive at an age of the evaporites themselves.
The evaporites have generally been believed to be between 116–111 million years old, based on past analysis of poorly described igneous rocks. However, Tedeschi’s results show that the evaporites must have formed during a much earlier time interval, and are likely around 125–120 million years old.
“Determining the age of salt deposition is crucial to understanding how seawater entered between Africa and South America, when the continents were beginning to split apart and the ocean could dry up,” explains Tedeschi. “In addition, the salt seals some of the largest hydrocarbons discoveries in the western world.”
The desiccation of the South Atlantic early in its history and formation of the evaporites is now known to correspond with a so-called Oceanic Anoxic Event, a phenomenon characterized by widespread deposition of organic matter on the sea floor, a process that can ultimately lead to the formation of petroleum source rocks. Formation of evaporites would have removed large quantities of the sulphate ion from global seawater to form the mineral gypsum, significantly changing the chemical composition of Early Cretaceous seawater. But because the sulphate ion reacts with and destroys organic matter, the preservation of such material would have been uniquely favoured by the changing composition of marine waters, hence promoting the formation of petroleum source rocks in many parts of the world during the Oceanic Anoxic Event.
Paper: New age constraints on Aptian evaporites and carbonates from the South Atlantic: Implications for Oceanic Anoxic Event 1a, by L. Tedeschi, H. C Jenkyns, S.A Robinson, A.E.S.Sanjines, M.C.Viviers, C.M.S.P. Quintaes and J.C. Vazquez, was published in Geology doi: 10.1130/G38886.
Image: Tedeschi in the lab in Oxford, with the Prism isotope ratio mass spectrometer used for his research. For more information on this facility, take a look at the Stable Isotope lab website.