L2: Isotopes and climate change

What are isotopes?

Forms of an element with different numbers of neutrons but the same number of protons and electrons. Isotopes are heavier/lighter depending on their mass number. Stability can also vary.

How can oxygen isotope records act as proxies for climate change?

• Variations in the ratio of 16O and 18O in isotope records indicates the changing of isotopic composition of ocean waters between glacials and inter-glacials.

• 16O/18O ratio in seawater is controlled by fluctuations in land-ice volume.

How do oxygen isotope compositions in ocean water change during glacials/inter-glacials?

Glacials → 16O is evaporated from water more easily. Ocean waters have higher 18O and ice sheets have higher 16O.

Inter-glacials → melting ice sheets return more 16O to the oceans.

What are water, ice and snow samples compared to in order to express changes in 18O?

They are compared to Standard Mean Ocean Water (SMOW).

What are marine isotope stages (MIS)?

Glacial and interglacial periods in geological time determined by the analysis of marine oxygen isotopes. Cold stages are even numbers, and warm stages are odd numbers.

Which MIS stage do we currently reside in?

Stage one. Termination one (T1) and termination two (T2) mark the end of the previous two glacial periods.

What is the best preserved example of the palaeoclimate and why?

The deep sea record. This is due to low erosion and sedimentation.

What three proxies can the deep sea record be used to analyse?

• Biogenic sediments

• Inorganic material

• Ice cores

How can biogenic sediments be used as a climate proxy?

• Show the isotopic balance of the water they inhabited.

• Abundance of different species.

How can inorganic material be used as a climate proxy?

• Sediment and ice-rafted debris can enter oceans.

• Provide a deep-sea record of ice sheet fluctuations.

How can ice cores be used as a climate proxy?

• Show annual layers that depict the O2 record, atmospheric gas content, and dust levels.

• This lets the temp (from O18), atmospheric composition, and volcanic activity levels at certain time periods be estimated.

• Correlation between long-term climate change and greenhouse gas composition established.

What are ‘sub milankovitch’ events?

• High frequency climate oscillations that present as short interstadials/stadials within glacial/inter-glacial cycles.

• Can appear cyclical.

• Temp changes between 6-8 degrees during warm/cold stages, with an abrupt start and gradual decline.

What are the three types of sub-Milankovitch event?

• Heinrich events → large ice berg discharges into the ocean with a cyclic occurence.

• Bond cycles → long-term cooling cycles 1,000 - 15,000 years long during the last cold stage. Long cooling cycles and abrupt warming.

• Dansgaard-Oeschger cycles (D-O events) → rapid climate fluctuations during last cold stage. Periodicity between 1,000 - 2,000 years.

What factors are thought to drive sub-milankovitch events?

• Thermohaline circulation

• Atmospheric circulation

• Ice sheet growth and collapse

How can changes to the thermohaline circulation influence sub-milankovitch forcing?

• If it slows down/stops, patterns of cooling/warming in the N Atlantic can be influenced due to ice sheet growth/flux.

• NADW overturn impacted.

How do Heinrich layers (ice-rafted debris) and ice sheet binge/purge cycles affect sub-milankovitch forcing?

• Ice sheet cyclical growth and decay is important for iceberg and freshwater flux to the ocean.

• This influences thermohaline circulation.

What can influence ice sheet binge/purge cycles?

• Internal ice sheet dynamics

• Ocean warming