Fill a glass of water from the sea and try to drink it. You gag and your lips pucker. After all, dissolved in that liter of the ocean are around 35 grams of salt. Now, imagine you tried to do this same thing 1 million years ago, 10 million years ago, 100 million years ago, even 500 million years ago. Would you ever be able to drink the water — or would it ever be as salty as the Dead Sea today? These are the questions that investigating paleosalinity helps us answer. We can use a variety of methods — from rough estimates based on our knowledge of the earth system to direct evidence from water droplets preserved in old rocks — to determine how salty the ocean was throughout Earth’s history.
Figuring out how ocean salinity has changed is important not simply because it helps us fill pages in our history book. Changing salinity affects ocean circulation, which in turn has a huge impact on climate. For instance, recent studies have suggested that changes in thermohaline circulation are part of how the Earth cycles between glacial and interglacial periods. In other words, ocean circulation may be the missing link between orbital variations that we know are linked to the cycle of ice ages and the huge swings in CO2 and temperature that are directly responsible for plunging us into glacial periods. In short, tracing paleosalinity helps us understand just how the Earth’s temperature can change so drastically.
This paper is therefore driven by the question: How can paleosalinity help us understand climatic variation, especially that caused by changes in thermohaline circulation? To begin to answer this question, I will present two methods of determining paleosalinity: first, by making an inventory of evaporites; and second, by looking at fluid inclusions. I then turn to the effects that these changes in salinity have on climate, especially by looking at models of how thermohaline circulation would differ and past cases where similar things occurred (for instance, in the Younger Dryas).Continue reading