Paleoceanography II: Quaternary Oceans & Climate

Paleoceanography II: Quaternary Oceans & Climate

Yesterday's Key Ideas

• Paleoceanography studies the physical, chemical, and biological states of past oceans.
• It's closely linked to paleoclimatology because oceans, atmosphere, and climate are interconnected.
• Sediment archives include:
  • Lithogenic (derived from rocks)
  • Biogenic (derived from living organisms)
  • Authigenic (formed in place)
  • Cosmogenic (derived from cosmic sources)
• Sediment proxies are measurable properties of an archive used to reconstruct past ocean environments.
• Examples of proxies include:
  • Ocean temperature: assemblages and Mg/Ca ratios in foraminifera.
  • Ocean circulation: Nd isotopes of Fe-Mn crusts.
  • Asteroid ejecta: Ir in clay and organic-rich sediment.

Today's Key Ideas

• The Quaternary period began 2.58 million years ago.
• It includes our present-day glacial-interglacial climate regime.
• Reconstructions of Quaternary oceans are important for understanding the climate system and projecting future climate responses to global warming.
• Which sedimentary archives and paleoceanographic proxies are available to assist past ocean reconstruction efforts?

Quaternary Period

• The last 2.58 million years constitute less than 0.1% of all geologic time.
• Characterized by glacial-interglacial climate cycles and the rise of hominids.
• Present-day plate tectonic and continental configuration.

Quaternary Climate Regimes

• The Quaternary 'ice-age' began 2.58 million years ago with the formation of permanent ice sheets in the northern hemisphere.
• Cool climates:
  • Large ice sheets
  • Low sea levels
  • Last Glacial Maximum occurred approximately 20,000 years ago.
• Warm climates:
  • Small ice sheets
  • High sea levels
  • The present day is an 'Interglacial' period
  • Last Interglacial approximately 125,000 years ago

Key Questions

• How do the oceans and other aspects of the climate system respond during warm 'interglacial' periods and during times of rapid climate change (modern climate analogues)?
• How closely connected are the oceans and atmosphere on millennial (1,000 yr), centennial (100 yr), and decadal (10 yr) time scales?
• What is the sequence of events when the climate system is perturbed? Which aspects respond first? What are the drivers of change?
• How will the oceans and atmosphere continue to change over the coming decades and centuries in response to global warming?

Proxies using Foraminifera

• Foraminifera mostly have an external shell (test) made of calcium carbonate (CaCO_3).
• Planktonic foraminifera dwell in the upper ocean.
• Benthic foraminifera live on (epifaunal) or in (infaunal) seafloor sediments.
• Planktonic foraminifera assemblages and Mg/Ca ratios are useful for reconstructing past variations in sea-surface temperature (SST).
• Foraminifera are preserved in Quaternary-aged biogenic sediments and can be isolated from the bulk sediment.

Foraminifera Geochemistry

• Foraminifera take up chemical species from seawater into their carbonate or silicate skeletons as they grow. Examples include: $O2$, HCO3, Fe, Cd, Zn, Si, Ba, Mg, Ca, U, P

Proxies for Ocean Temperature, Ice Sheet Volume & Global Sea Level

• Oxygen isotopes in foraminifera are used as proxies.

Oxygen Isotopes in Seawater

• Oxygen has two main isotopes: 'heavy oxygen' (^{18}O) (0.2%) and 'light oxygen' (^{16}O) (99.7%). (^{18}O) has two extra neutrons, giving it extra mass.
• The (^{18}O/^{16}O) ratio is the relative amount of (^{18}O) to (^{16}O) oxygen atoms.
• The (^{18}O/^{16}O) ratio in seawater varies ('fractionates') as climate changes, leading to oxygen isotope fractionation.
• Isotope Fractionation: During some biogeochemical reactions (e.g., marine carbonate formation), heavy & light isotopes are partitioned between two coexisting phases (e.g., seawater & carbonate) with preference.
• Seawater contains both 'heavy water' (H2^{18}O) (0.2%) and 'light water' (H2^{16}O) (99.7%).

Climate Influence on (^{18}O/^{16}O) Ratio

• 'Heavy' (^{18}O) condenses more easily than 'light' (^{16}O).
• Rain in tropical regions has a high (^{18}O/^{16}O) ratio.
• Snow in polar regions is depleted in 'heavy' (^{18}O), resulting in a low (^{18}O/^{16}O) ratio.
• Larger glacial ice sheets are more depleted in (^{18}O) than smaller interglacial ice sheets.
• Ocean water left behind is more enriched in (^{18}O) during glacials than interglacials.
• Ocean water has:
  • Higher (^{18}O/^{16}O) during glacials.
  • Lower (^{18}O/^{16}O) during interglacials.
• Evaporation preferentially removes the lighter isotope, leading to lower (^{18}O/^{16}O) in ice and higher (^{18}O/^{16}O) in seawater.

Ice Volume Effect

• Preferential removal of lighter isotope (^{16}O) during evaporation leads to lower (^{18}O/^{16}O) in ice and higher (^{18}O/^{16}O) in seawater during glacial periods.

Temperature Effect on Oxygen Isotopes in Foraminifera

• Foraminifera (CaCO_3) preferentially takes up 'heavy' (^{18}O) over 'light' (^{16}O) from seawater during growth.
• A larger proportion of heavy (^{18}O) is taken up by tests in colder seawater (Epstein, 1953).
• Foraminifera record:
  • Higher (^{18}O/^{16}O) in cold water.
  • Lower (^{18}O/^{16}O) in warm water.
  • Irrespective of (^{18}O/^{16}O) of seawater.

Foraminifera Oxygen Isotope Proxies

• Two combined (^{18}O/^{16}O) effects recorded by foraminifera:
  • Ice Volume Effect (seawater): Grow an ice sheet equivalent to 100 m sea level fall ≈ 0.1 % increase in seawater (^{18}O/^{16}O).
  • Temperature Effect (foraminifera): 1°C temperature decrease ≈ 0.026 % increase in foraminifera (^{18}O/^{16}O) (even when seawater (^{18}O/^{16}O) is constant).
  • Warmer & variable temperatures.
  • Cooler & stable temperatures.
• Emiliani (1955) produced the first oxygen isotope record of foraminifera.
  • Planktonic foraminifera (^{18}O/^{16}O) records the combined effects of changes in sea surface temperature and ice sheet volume.
  • Benthic foraminifera (^{18}O/^{16}O) mainly records changes in ice sheet volume and thereby changes in global sea level.

Benthic Foram (^{18}O/^{16}O) – Sea Level Proxy

• Small ice sheets correspond to warm periods with low (^{18}O/^{16}O).
• Large ice sheets correspond to cold periods with high (^{18}O/^{16}O).
• Lisiecki & Raymo (2005) data shows changes in benthic (^{18}O/^{16}O) over the Late Pliocene and Quaternary, reflecting changes in global mean sea level.
• Permanent Antarctic Ice sheets.
• Permanent Northern Hemisphere & Antarctic Ice sheets.
• 23 ky cycles
• 41 ky cycles
• 100 ky cycles

Climate Change & Orbital Cycles

• Solar radiation received by Earth varies spatially and temporally due to cyclic changes in Earth's position in space relative to the Sun.
• Cyclic variations in eccentricity, tilt, and precession of Earth’s axis (Milankovitch, 1941).
  • Precession of the equinoxes (period = 23,000 years)
  • Tilt of the axis (period = 41,000 years):
    • 24.5° = maximum tilt
    • 21.5° = minimum tilt
  • Eccentricity (dominant period =100,000 years):
    • High eccentricity (more elliptical)
    • Low eccentricity (more circular)
• Climate reconstructions based on the foraminifera (^{18}O/^{16}O) proxy correlate with solar radiation received at 65° N (epicenter of northern hemisphere ice sheets).
• Glacial-interglacial climate cycles are driven by slow but predictable variations in Earth’s orbit around the Sun.

Foraminifera Oxygen Isotope Proxies

• Planktonic foraminifera: (^{18}O/^{16}O) records a combination of sea surface temperature (SST) and ice volume (i.e., global sea level).
• Benthic foraminifera: (^{18}O/^{16}O) is a proxy for ice volume (i.e., global sea level).

Foraminifera Mg/Ca Proxy

• Planktonic foraminifera: Mg/Ca is a proxy for sea surface temperature (SST).

Combined (^{18}O/^{16}O) & Mg/Ca in Foraminifera

• Coupled measurements of (^{18}O/^{16}O) (temperature, ice sheet volume) and Mg/Ca ratios (temperature) in the same planktonic foraminifera and on the same timescale.
• Remove the temperature effect from the upper ocean \delta^{18}O record.
• Mg/Ca record gives the upper ocean temperature history.
• Temperature-corrected (^{18}O/^{16}O) record gives ice sheet volume changes → global sea level history.
• Changes in temperature may occur before, after, or at the same time as changes in ice sheet volume / global sea level.
• Which aspect of the climate system responds first?
• What drives climate change?
• Lea et al. (2000) studied two sediment cores from the equatorial Pacific Ocean using coupled Mg/Ca & (^{18}O/^{16}O) records for planktonic foraminifera.

Major Findings

• Tropical Pacific sea surface temperatures (SSTs) were 2.8 °C colder than present during the Last Glacial Maximum.
• Glacial-interglacial SST differences up to 5 °C.
• Changes in SST coincide with changes in Antarctic air temperature BUT precede changes in ice sheet volume by ~3 ky.
• Tropical cooling played a major global role in driving ice-age climate.

Today’s Take-Home Messages

• The Quaternary ‘ice-age’ began 2.58 million years ago when permanent ice sheets formed in the northern hemisphere & contains our current ‘glacial-interglacial’ climate regime.
• Reconstructions of the Quaternary oceans are important for understanding how the future climate system is expected respond to ongoing global warming.
• Paleoceanographic proxies based on (^{18}O/^{16}O) & Mg/Ca records of foraminifera allow changes in ice sheet volume (sea level) & ocean temperature to be reconstructed.
• Foraminifera (^{18}O/^{16}O) records show that glacial-interglacial climate change is paced by Earth’s orbital cycles, also providing a timescale for climate reconstructions.
• A multi-proxy approach helps deconvolve how & when different aspects of the ocean-atmosphere-climate system respond to change.