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Climate Change

The Not So Stable Holocene

Article:

Elmore, A.C., Wright, J.D., Southon, J., 2015, Continued meltwater influence on North Atlantic Deep Water instabilities during the early Holocene, Marine Geology 360, 17-24. doi:10.1016/j.margeo.2014.11.015 

 

Background:

The Holocene epoch, a period of time spanning form 11,700 years ago to present (11.7 ka) has traditionally been considered warm and climatically stable especially when compared to the last glacial period. At 11.7 ka, the world was coming out of the Younger Dryas cold period, a short-lived (~1000 years) return to glacial conditions. Although anatomically modern humans were in existence for nearly 200,000 years, the Holocene encompasses major cultural advancements such as writing, religion and major civilizations.

Recently, researchers have challenged the idea of Holocene climate stability with evidence pointing to abrupt climate changes occurring as recently as 8.2 ka. In particular, an abrupt change called the “8.2 ka event” was a rapid cooling event due to a large amount of freshwater being introduced into the North Atlantic Ocean. Freshwater is less dense than the salty ocean, and thus “floats” at the surface. The North Atlantic is an area where water masses typically sink to form deep water masses. This circulation can be greatly impacted by the introduction of freshwater, as it tends to remain at the surface. The source of this freshwater came from glacial meltwater that collected in lakes. The lakes progressively filled up until a path to the ocean allowed them to drain. Elmore and her coauthors investigate freshening of surface water in the North Atlantic and how it altered deep water circulation during the early Holocene (11.7 – 8.2 ka).

 

Methods:

This study relied on a sediment core taken from a body of sediment located south of Iceland in the northern North Atlantic Ocean called the Gardar Drift. The sediment is composed in part of the calcium carbonate (CaCO3) shells of small protists called foraminifera. Since components of the shells are derived from seawater, the shells of these small creatures keep a record of the composition of the ocean during the time that the shells were made. The researchers used this to their advantage, employing proxies to understand how the physical properties of the ocean changed during the early Holocene.

For example, oxygen is a component of the CaCO3 shells of forams. Oxygen typically has 8 protons and 8 neutrons (16O), but can also exist as a heavier isotope containing 8 protons and 10 neutrons (18O). Seawater contains both 18O and 16O,but the ratio of these isotopes varies over geologic time. This ratio is more simply written as δ18O. The lighter isotope, 16O, more readily evaporates into the atmosphere, leaving a greater fraction of 18O in seawater in places where there is a lot of evaporation (equatorial regions) and a greater fraction of 16O in colder places (polar regions) (Fig 1). For this reason, glacial ice in polar regions has very low δ18O values compared to seawater. During glacial periods, the volume of glacial ice greatly increases, stealing more and more of the light isotope 16O from seawater. This leaves seawater with a high δ18O signature, compared to non-glacial (interglacial) periods. When researchers measure the δ18O from the shells of creatures that live in seawater, they can use this as a proxy for global ice volume. Besides the primary contributor of δ18O (ice volume), it is also affected by ocean temperature. For this reason, it is important to know how deep in the ocean the foram lived, because the shell is also used as a proxy for ocean temperature, and surface temperatures can differ greatly from deep ocean temperatures. The researchers in this study used several proxies (Fig 2):

Figure 1. Latitudinal gradient in δ18O  The light isotope 16O is preferentially evaporated and the heavy isotope 18O is preferentially precipitated out of the atmosphere.  With net transport of water vapor away from the equator and towards the poles, the δ18O of seawater becomes more and more negative towards the poles.

Figure 1. Latitudinal gradient in δ18O
The light isotope 16O is preferentially evaporated and the heavy isotope 18O is preferentially precipitated out of the atmosphere. With net transport of water vapor away from the equator and towards the poles, the δ18O of seawater becomes more and more negative towards the poles.

δ18O – Tracer of surface water temperature, bottom water temperature.

High δ18O = colder temperatures, higher ice volume

Low δ18O = warmer temperatures, lower ice volume

Fraction of lithics (small rocks) in sediment – Tracer of the amount of freshwater input to the surface ocean.

High fraction of lithics = higher amounts of freshwater input

Low fraction of lithics = lower amounts of freshwater input

δ13C – Tracer of deep ocean circulation.

High δ13C = high rates of circulation

Low δ13C = low rates of circulation

Fraction of silt sized sediment – Tracer of deep water flow speeds.

High fraction of silt = slower deep water flow

Low fraction of silt = faster deep water flow

Figure 2. Proxy records. A. Global sea level rise after the last glacial period.  Note the 30 meters of rise during the early Holocene (11.7 – 8.2 ka) representing the collapse of northern hemisphere ice sheets.  B. GISP2 ice core δ18O values.  Note there is an inverse relationship between δ18O of ice cores and δ18O of seawater (derived from foram shells).  C. δ18O values from planktonic (surface water) forams.  Generally δ18O decreases (note inverted y-scale) through the early Holocene, suggesting decreasing ice sheet volume and increasing surface water temperatures, with large abrupt variability.  D. Fraction of lithics shows a steady decrease from high values (note y-scale) during the early Holocene, representing reduction in freshwater input to the North Atlantic.  E. δ13C values from benthic (deep water) forams.  Red arrows indicate increased southern source water, which implies reduced northern deep water formation resulting from more freshwater input to the surface waters.  Variability in deep water formation can be observed through the early Holocene.

Figure 2. Proxy records.
A. Global sea level rise after the last glacial period. Note the 30 meters of rise during the early Holocene (11.7 – 8.2 ka) representing the collapse of northern hemisphere ice sheets. B. GISP2 ice core δ18O values. Note there is an inverse relationship between δ18O of ice cores and δ18O of seawater (derived from foram shells). C. δ18O values from planktonic (surface water) forams. Generally δ18O decreases (note inverted y-scale) through the early Holocene, suggesting decreasing ice sheet volume and increasing surface water temperatures, with large abrupt variability. D. Fraction of lithics shows a steady decrease from high values (note y-scale) during the early Holocene, representing reduction in freshwater input to the North Atlantic. E. δ13C values from benthic (deep water) forams. Red arrows indicate increased southern source water, which implies reduced northern deep water formation resulting from more freshwater input to the surface waters. Variability in deep water formation can be observed through the early Holocene.

 

Results:

The early Holocene (11.7 – 8.2 ka) marks the beginning of a warming trend (Fig 2c). There is also a general decreasing trend in surface freshwater, though it is punctuated by episodic increases. Proxy data also supports abrupt deep circulation changes (Fig 2e). The timing of circulation changes coincides with the episodic increases in surface freshwater. These findings support the final melting of northern hemisphere ice sheets during the early Holocene, which contributed to approximately 30 meters of sea level rise during this time period (Fig 2a). The variability as recorded by proxies suggests that the early Holocene was not nearly as stable as previously thought and that a full interglacial period was not reached until approximately 9,000 years ago.

The middle Holocene to present (8.2 – 0 ka) sees continued sea surface temperature warming (Fig 2c). The Holocene Thermal Maximum occurred in the middle Holocene, which is characterized by very warm Northern Hemisphere conditions due to the configuration of the Earth’s orbit (and increased solar energy). There appears to be little to no freshwater input to the northern North Atlantic during this time period, thus deepwater flow and circulation were no longer impacted by freshwater inputs and remained continuously high (Fig 2e).

 

Significance:

This study provides strong evidence that the early Holocene was not nearly as quiescent as previously thought. A direct link between surface water freshening and changes in deep water circulation was made through the use of proxy data. It is also clear that the delivery route and rate of freshwater input to the North Atlantic Ocean play an important role in deep water circulation. Though episodic freshwater input was prevalent in the early Holocene, there was very little response of the climate, with the notable exception being the 8.2 ka event, which resulted in abrupt and short lived northern hemisphere cooling.

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