The Oceans by Eelco J. Rohling

Author:Eelco J. Rohling
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2017-06-15T00:00:00+00:00

CHOKING OCEANS

In a warmer world, like the Cretaceous and the Mesozoic in general, ocean density changes are less sensitive to changes in the surface freshwater budget (that is, the balance of freshwater in versus freshwater out) than in a colder world, like today. In addition, Mesozoic temperature contrasts over the globe were much weaker than today, and this is especially important when considering the equator-to-pole gradient. With much less cooling going on, surface-water density contrasts were much smaller across the Mesozoic oceans than today.

Density contrasts are needed to make new deep waters sink effectively. Deep-sea temperatures reflect surface temperatures in the regions where the new deep waters are sinking, because temperature inside the ocean changes only with mixing. Therefore, deep-sea temperatures of 10°C to 12°C indicate that surface waters in the Mesozoic regions of new deep-water formation did not cool much below these levels. This immediately indicates that there was no sea-ice formation to help drive up the water density. The prevailing moderate temperature contrasts would drive only moderate density increases, relative to what happens in a cold world like today. In addition, any salinity increase would have been less effective at driving up density in a warm world, relative to the impact of a similar salinity change in a cold world. In consequence, we might expect deep-water circulation to be weaker in a warm world compared with a cold world, although recent computer modeling suggests that this was not necessarily the case.

Oxygen is more soluble in water at low temperatures than at high temperatures; from 0°C to 30°C, the oxygen solubility drops by almost 50%. This means that, for a given atmospheric oxygen concentration, cold water can hold more oxygen than warm water. When Mesozoic deep waters were formed at 10°C to 12°C, they started out with 25% to 30% less oxygen than modern deep waters that form at about 2°C. In consequence, deep-sea oxygenation in a warm world may have taken a double hit from both weakened deep-water formation and circulation, and lower initial oxygen concentrations within the new deep waters. And then the hit may have been triple: for every 10°C warming, the rate of life processes (metabolism) doubles, so that warmer deep waters would have caused increased oxygen utilization during respiration/decomposition. To see if this triple hit indeed took place, we first need to specify our expectations about Mesozoic ocean circulation and oxygenation in more detail, so that we can then compare these with the available evidence.

During the greenhouse times of the Mesozoic, culminating through the Jurassic in the Middle Cretaceous, the equator-to-pole sea-surface temperature gradient appears to have become less than 10°C, compared with today’s gradient of 30°C. This reduced gradient mainly resulted from much warmer poles. Middle Cretaceous high-latitude sea-surface temperatures as high as 25°C to 30°C have been reconstructed, but these may represent the growing season at those latitudes for the algae and zooplankton whose remains are used to determine the temperatures. That growing season likely was the summer half year, given that high latitudes receive little to no sunlight for primary production during the winter half year.

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