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Paleoceanography

A change in ocean circulation makes for long glacial periods through the Mid-Pleistocene Transition

Hasenfratz, A.P., Jaccard, S.L., Martínez-García, A., Sigman, D.M., Hodell, D.A., Vance, D., Bernasconi, S.M., Kleiven, H.K.F., Haumann, F.A. and Haug, G.H., 2019. The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle. Science363(6431), pp.1080-1084.

BACKGROUND

How the ocean moves impacts us as humans from tides, wave action and storm surges, all the way to the movement of oceanic water masses via ocean circulation.  We care about them because they influence our recreational activities, economies, and natural systems like global heat distribution and carbon storage.

The ocean is naturally structured into water masses based on their densities, a function of their temperature and saltiness (salinity).   In a vertical column of the ocean, the ability of  water masses to mix depends on how different they are in density.   Two water masses that are similar in temperature and salinity will easily mix together, whereas two water masses with very different temperatures and salinities will resist mixing, kind of like how oil and water do not mix well.

Ocean circulation is driven by the movement and mixing of its water masses.  Past studies have focused on retracing how the movement and mixing of water masses has evolved through time to make inferences about changes in the amount of carbon stored in the ocean, biological productivity, and the global heat distribution.

In the research highlighted here, Hasenfrazt and an international team of scientists from Switzerland, Germany, Norway, the UK, and the USA investigate how changes in water mass mixing in the Southern Ocean could have contributed to prolonged periods of ice growth through the Mid-Pleistocene Transition (MPT, between 1.25 million and 700,000 years ago).  During the MPT,  the length of time between glacial periods increased from 41,000 years to ~100,000 years.

Although the transition between glacial and non-glacial periods is typically attributed to the position and orientation of Earth relative to the those of the Sun, the celestial forcing does not explain the more than doubling of time between transitions during the MPT.

METHODS:

Researchers used data from the Antarctic Zone (AZ) to interpret how the interaction of its surface and deep-water masses evolved during the MPT.

Google Earth screen shot of ODP 1094 site

Sediment cores were collected from 53. 2 degrees S, and 5.1 degrees E at Ocean Drilling Program (ODP) site 1094 in 2807 meters of water below the AZ (Figure 1).   Foraminifera shells in sediment cores were used to infer the residence time of the upper water mass in the AZ by comparing the oxygen isotope ratios of sea water (d18O_SW) records from deep and shallow water masses. The scientists used opal and barium records as evidence that sediments were collected from a site that remained in the AZ through the MPT.

From the sediment cores, researchers analyzed benthic (bottom dwellers) and planktonic (floaters and drifters) foraminifera shells for their ratios of oxygen isotopes in calcite and magnesium to calcium.   The oxygen isotope ratios in calcite were then corrected by the magnesium and calcium ratios to determine the d18O_SW at the time the shell formed.

The amplitude of d18O_SW can be influenced by a number of factors, including how much sea-ice has formed from it or melted into it, the input of fresh water, and evaporation (for more on how foraminifera record d18O_SW, check out this 3 minute video: How These Sea Shells know the Weather in Greenland).

Researchers suspected that in their records they would be able to detect if less or more deep water was mixing with the surface if the difference between the d18O_SW of surface and deep water grew wider or smaller, respectively.  In the absence of deep-water mixing, the d18O_SW of water at the surface would have been subject to freshwater input via precipitation and land ice melting.  If these effects were greater than evaporation then the surface would have become fresher and the d18O_SW lower.  On the other hand, if a greater amount of deep water had been ascending to the surface, the surface water mass would have experienced an increase in d18O_SW and the foram test records from the surface and deep water would be similar.

RESULTS/DISCUSSION:

During the MPT glacial cycles, the surface water records indicate that d18O_SW was decreasing relative to deep water.  The decrease in surface d18O_SW is attributed to an increased residence time of surface water and less deep-water inflow.   The team of researchers suspect that the decrease in deep water delivery during the MPT may have been caused by less wind driven upwelling or a reduction in convective mixing between the surface and deep layers. It is estimated that the deep-water delivery supply diminished by 50%.

The reduction of deep water mixing with the surface would have been magnified by the influence of salinity changes expected with a prolonged residence time at the surface, i.e. freshwater inputs.   The input of fresh water would have increased the division in salinity between the water masses (aka the density stratification of the water column) and required a greater amount of energy to mix them, creating a positive feedback effect.  In other words, the more time the water mass spent in the surface, the more likely it was to become different from the deeper water mass and thus the less likely it was to mix, so it remained in the surface, where it became even less likely to mix.

Understanding how water mass movement has evolved through time has important implications for atmospheric carbon, nutrient cycling, and heat distribution throughout Earth’s history.  This study by Hasenfratz et al. highlights how dynamic Earth’s systems are by demonstrating that although celestial forcing is a major factor for glacial-interglacial cycling, its impacts could be resisted for 600,000 years from just a shift in ocean circulation.

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