Aug 2, 2012
Tracking seawater chemistry changes
Seawater chemistry has shown major changes over the last 130 million years. Fluid inclusion records indicate that these occurred slowly but sulphur-isotope compositions suggest much more rapid fluctuations. Now researchers from Canada and the US have found a mechanism to account for rapid change – large-scale dissolution and precipitation of evaporites that altered marine sulphate levels.
"It is a paradigm shift in the way we think about how seawater salinity changes over time," Adina Paytan of the University of California Santa Cruz, US, told environmentalresearchweb. "It seems like it is not a slow ongoing process, but rather the 'steady state' process is interrupted by pulses or rapid events – gypsum dissolution and precipitation. And these events change ocean chemistry with implications for evolution and climate."
Paytan and colleague Ulrich Wortmann of the University of Toronto, Canada, used a non-steady-state box model of the global sulphur cycle to try and recreate the global δ34S record. This shows two major events 130–120 million years ago and 55–45 million years ago.
"The sulphur-isotope record – specifically the rate of change at certain time intervals – was striking and hard to interpret using the more traditional processes that impact ocean chemistry without invoking some very dramatic changes in volcanism or sedimentation, evidence for which could not be found," said Paytan. "When Uli [Wortmann] introduced the model linking evaporite dissolution/precipitation to pyrite burial we decided to look into this process and see if it can reproduce the major features of the sulphur isotope curve. We were excited to see how well it does."
Paytan and Wortmann believe that the deposition of a basin-scale evaporite between 122 and 120 million years ago due to the opening up of the South Atlantic could have led to the negative shift in the δ34S record by removing sulphate from seawater and so reducing pyrite burial rates. The microbes that reduce sulphates to sulphide, resulting in the formation of pyrite, tend to prefer 32S over 34S.
Similarly, the dissolution between 51 and 47 million years ago of a large amount of evaporite, perhaps part of the massive deposits along the margins of the Paleotethys Ocean, could account for a positive shift in the sulphur-isotope record, with higher oceanic-sulphate conditions leading to more pyrite burial. (Some of these deposits now stretch from Oman into western India via Iran, Afghanistan and Pakistan.)
"Our study combines the palaeo-seawater reconstruction data with some innovative modelling to come up with a process that will explain many of the geological observations we see in the record of the past 50 million years or so," said Paytan. "And it is in agreement with records of temperature changes on Earth, the type of mineral – calcite or aragonite – that organisms precipitate, etc."
Since the ocean is a major source of sulphate aerosols, seawater-sulphate concentrations are likely to affect atmospheric aerosol chemistry. "Sulphate concentrations remained stable over long periods of time but changed rapidly when continental breakup or collision events resulted in the creation or destruction of basin-scale evaporite deposits," the researchers wrote in Science. "This provides an explanation of the existing seawater S-isotope data, as well as exciting linkages between sulphur and other biogeochemical cycles, long-term trends in evolution, ocean fertility and climate."
Now the researchers hope to find support for this process in marine sediments. "Specifically, we want to see if we can reconstruct the pyrite-burial record," said Paytan. "Also we would like to expand this further back in time to see if it holds throughout Earth's history."
About the author
Liz Kalaugher is editor of environmentalresearchweb.