Quaternary Lecture 5- From extra-terrestrial controls to flickering switches Flashcards
What is absolute dating?
when an age estimate is assigned
Obtained using radiometric dating or incremental dating
What is radiometric dating?
Radiometric dating calculates an age by measuring the presence of a short-life radioactive element (e.g., carbon-14) or a long-life radioactive element plus its decay product (e.g., potassium-14/argon-40)
What is incremental dating?
Incremental dating allows the construction of year-by-year annual chronologies, which can be temporally fixed or floating (e.g. ice cores; varves)
What is relative age dating?
Ccan establish the relative order of antiquity of fossils or stratigraphic units. Often based on stratigraphy (principle of stratigraphic
superposition - oldest unit at base, youngest at top) or comparison of a measurable
parameter (e.g. Rock Surface Weathering; Amino Acid Geochronology)
What is geochronology?
Calculation/estimation of the age of rocks, fossils, sediments
Name 4 types of radiometric dating.
Argon-isotope
Uranium-series
Cosmogenic nuclide
Radiocarbon
Name 3 types of radiation exposure dating
Optically stimulated luminescence
Electron spin resonance
Fission track
Name 3 types of incremental dating using annually banded records
Dendrochronology
Varves
Annual layers in glacier ice
Name 5 relative dating methods
Rock surface weathering
Obsidian hydration dating
Pedogenesis
Relative dating of fossil bone
Amino acid geochronology
Name 4 techniques for establishing age equivalence
Oxygen isotope chronostratigraphy
Tephrochronology
Palaeomagnetism
Palaeosols
When has evidence for climate change at a sub-Milankvotich scale been found?
1990s
What are high frequency climate oscillations refered to as?
Climate’s flickering switch
What could be a cause of sub-Milankovitch events (external forcing)?
Solar irradiance
Bolide impact
Discuss solar irradiance.
- Variation in solar output is a major factor
in short-term climate change. - Alternating active & quiescent phases of
solar activity as reflected in growth &
disappearance of sunspots in a cyclical
fashion. - The influence of the sun on climate was
recognized centuries ago. - Astronomers of the Imperial Court were routinely observing
sunspots in China by 28 B.C. Aristotle’s student Theophrastus was
the first to mention sunspots in the West, around 300 B.C
What do sunspot numbers vary on?
Sunspot numbers vary on 11, 22, 80, 200
& 2000 yr cycles (R number)
Owens et al. (2017) - link between sunspots and climate events.
- LIA spans 1440–1920, although not
all of this period was notably cold. - While the MM occurred within the
much longer LIA period, the timing
of the features are not suggestive
of causation and should not, in
isolation, be used as evidence of
significant solar forcing of climate. - Climate model simulations suggest
multiple factors, particularly
volcanic activity, were crucial for
causing the cooler temperatures
Tenuous link
Discuss solar irradiance in the past.
- Maunder Minimum (1645-1715 AD) -
Little Ice Age cooling – decrease in
sunspot activity. - Medieval warm period (1000-1200 AD) -
peak in sunspot activity (+ decrease in
volcanic activity)
What could be a cause of sub-Milankovitch events (internal forcing)?
Ice sheet growth and collapse and ocean thermohaline circulation (THC)
Freshwater forcing of the younger dryas cold episode and the 8.2ka event
Discuss the cessation of iceberg calving as ice sheet retreats from deepwater (sub-Milankovitch)
- increase in salinity
rapid strengthening of THC
rapid rise in North Atlantic temps
‘salt oscillator’ model
Discuss Heinrich events (sub-Milankovitch)
Reduction of SSTs & freshening of North Atlantic by icebergs.
Rapid shutdown of THC & cessation of northward heat transport = cold
temps.
Discuss IRD (Heinrich Layers) and ice sheet binge/purge cycles relating to sub milankovitch forcing.
Ice sheet binge/purge cycles – ice sheet dynamics = main control mechanism
* MacAyeal (1992) suggested ice sheet cyclical growth and decay was important for iceberg and freshwater flux to the ocean ……which subsequently influences thermohaline circulation
More recent work suggests iceberg flux is related to ocean warming and isostatic adjustment
Bassis et al. (2017)
Bassis et al (2017)
Bassis et al. (2017) use an ice sheet model to show
that the magnitude and timing of Heinrich events
can be explained by the same processes that drive
the retreat of modern marine-terminating glaciers
1. ocean warming associated with variations in
the overturning circulation melt along the
calving face, triggering rapid margin retreat
and increased iceberg discharge.
2. On millennial timescales, isostatic adjustment
causes the bed to uplift, isolating the terminus
from subsurface warming and allowing the ice
sheet to advance again until, at its most
advanced position, it is poised for another
Heinrich event.
3. This mechanism not only explains the timing
and magnitude of observed Heinrich events,
but also suggests that ice sheets in contact
with warming oceans may be vulnerable to
catastrophic collapse even with little
atmospheric warming.
When was the younger dryas cold episode?
12.8-11.7ka
Discuss YD.
*Deglaciation punctuated by the Younger Dryas. Severe cold.
* Ended v. abruptly with ~7 °C warming. Worldwide impact
* YD forced by freshwater outburst from
Glacial Lake Agassiz – a proglacial lake of
the Laurentide Ice Sheet
* Meltwater routed down Mississippi River
to Gulf of Mexico ?
* But with ice sheet retreat northward
drainage took place down St Lawrence
River valley & into the Labrador Sea and
possibly to the NW.
* Freshwater input shut down NADW.
* Cooler temps across the North Atlantic
triggered ice sheet regrowth (e.g. see
Lecture 4 for Younger Dryas/Loch Lomond
Stadial)
