paleosalinity

How Do Ice Ages Happen? Exploring Paleosalinity and Thermohaline Circulation

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).

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How Salty Has The Sea Been Over the Past 541 Million Years?

Take a bottle 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 salts (mostly sodium chloride). 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 (that is, throughout the Phanerozoic eon). Would you ever be able to drink the water? Alternatively, would the sea ever have been so salty that today’s ocean creatures would not have survived? A 2006 article by Hay et al. helps answer precisely these questions. The authors tracked variable chloride levels to demonstrate how salinity has changed throughout the Phanerozoic, noting a significant overall decline. These changes have had important effects on ocean circulation and on plankton levels — and possibly contributed to the explosion of complex life in the Cambrian, 541–520 million years ago. Continue reading