EAOS111 Earth and Ocean Sciences L36: Paleoceanography III Oceans & Climate in Deep Time

Yesterday’s Key Ideas

• The Quaternary ‘ice-age’ period began 2.58 million years ago, marked by permanent ice sheets in both hemispheres.
• The Quaternary period encompasses the current ‘glacial-interglacial’ climate regime.
• Foraminifera ^{18}O/^{16}O records indicate that glacial-interglacial climate change is paced by Earth’s orbital cycles.
• A multi-proxy approach helps to deconvolve how and when different aspects of the ocean-atmosphere-climate system respond to change.
  • This is because all proxy records are derived from the same sediment archives and are on a common timescale.

What is 'Deep Time'?

• Deep Time spans from the formation of the Earth to the present, encompassing major geological and biological events.
• Key events and their approximate timings include:
  • Formation of the Earth: ~4550 Ma (million years ago)
  • Formation of the Moon: ~4527 Ma
  • End of the Late Heavy Bombardment; first life: ~4000 Ma
  • Earliest start of photosynthesis: ~3200 Ma
  • First major increase in atmospheric oxygen levels; first Snowball Earth event (Huronian glaciation): ~2300 Ma
  • Marinoan Glaciation (Snowball Earth event): ~650-635 Ma
  • Sturtian Glaciation (Snowball Earth event): ~716-660 Ma
  • Cambrian explosion: ~540 Ma
  • First vertebrate land animals: ~380 Ma
  • Non-avian dinosaurs: ~230-66 Ma
  • First Hominins: ~2 Ma
• The geological timescale is divided into eons, eras, and periods, including Hadean, Archean, Proterozoic, Paleozoic, Mesozoic, and Cenozoic.
• The Precambrian encompasses most of Earth's history, including the Hadean, Archean, and Proterozoic eons.

Earth's Geological Timescale

• The geological timescale is divided into eons, eras, periods, and epochs.
• Eons: Phanerozoic, Precambrian(Proterozoic, Archean, Hadean)
• Eras: Cenozoic, Mesozoic, Paleozoic
• The Phanerozoic Eon represents only about 12% of Earth's history, while the Precambrian Eons (Hadean, Archean, and Proterozoic) account for the majority.

Today’s Key Ideas

• Which paleoceanographic proxies and dating tools are available to reconstruct the evolution of the ancient oceans, especially through the last 66 million years leading into the Quaternary?
• What do reconstructions of Earth’s ancient oceans tell us about their physical, chemical, and biological state and how ocean regimes evolved through geological time?
• What do reconstructions of Earth’s ancient oceans tell us about interactions between the oceans, atmosphere and climate, and also the evolution of life through geological time?

Phanerozoic Eon – Last 540 Million Years

• The Phanerozoic Eon represents the last 540 million years of Earth's history.
• It is characterized by significant changes in atmospheric composition and ocean chemistry.
• Atmosphere: The percentage of different gases in the atmosphere has varied significantly throughout the Phanerozoic.
  • CO2 levels have fluctuated, with periods of high CO2 concentration.
  • O2 levels have increased, leading to the diversification of species and the rise of complex life.
  • CH4 (methane) levels have also varied.
• Ocean: The ocean's chemical composition has changed over time.
  • Early oceans were anoxic and ferruginous (iron-rich).
  • Later, the oceans became locally oxic or sulfidic.
  • Fully oxygenated oceans developed during the Phanerozoic.

Phanerozoic Eon – Events

• ‘Second Great Oxidation Event’
• ‘Precambrian Explosion’
• Diversification of species
• Rise of complex life
• Fully oxygenated oceans (mostly)

Phanerozoic Eon - Last 540 Million Years

• The arrangement of continents has changed significantly throughout the Phanerozoic Eon.
• Rodinia: A supercontinent that existed in the late Proterozoic.
• Pangaea: A supercontinent that formed in the late Paleozoic and broke apart during the Mesozoic.
• Iapetus Ocean: An ocean that existed between the continents of Laurentia and Baltica during the Paleozoic.
• Atlantic Ocean: Formed as Pangaea broke apart.
• end Permian (~250 Ma)
  • Mass extinction event.

Quaternary Climate Cycles

• There have been significant climate changes throughout the Quaternary period (the last 2.58 million years).
• Warm periods: Characterized by small ice sheets and high sea levels.
• Cold periods: Characterized by large ice sheets and low sea levels.
• Orbital cycles (Milankovitch cycles) influence these climate changes.
  • 23 ky cycles
  • 41 ky cycles
  • 100 ky cycles
• Diagram shows benthic ^{18}O/^{16}O (‰ deviation) and global mean sea level (m) over the last 5 million years.

Late Cenozoic Climate Regimes

• The Late Cenozoic climate has undergone significant changes, transitioning from warm, ice-free conditions to the development of permanent ice sheets and glacial-interglacial cycles.
• Mid-Pliocene Warm Period (~3.3 Ma): Very warm climates with no Northern Hemisphere ice sheets and very high sea levels.
• Cool Climates: Large ice sheets and low sea levels, such as during the ‘Last Glacial Maximum’ (~20 ky ago).
• Warm Climates: Small ice sheets and high sea levels, as seen in the present day and during the ‘Last Interglacial’ (~125 ky ago).

Cenozoic Climate - Foraminifera ^{18}O/^{16}O

• The Cenozoic Era has experienced significant climate transitions, from ice-free 'super-greenhouse' conditions to the development of ice sheets and colder climates.
• Key events and periods include:
  • Early Eocene Climatic Optimum
  • Mid-Eocene Climatic Optimum
  • Mid-Miocene Climatic Optimum
  • PETM (ETM1) and ETM2
• Foraminifera ^{18}O/^{16}O records provide valuable insights into temperature changes throughout this era.
• The x-axis represents time in millions of years ago, while the y-axis shows the ^{18}O/^{16}O deviation and estimated temperature change.

Extreme ‘Super-Greenhouse' or ‘Icehouse’ climates

• The earth has experienced extreme climate variations, including super-greenhouse and icehouse conditions, which are recorded in foraminifera ^{18}O/^{16}O records.

Oxygen Isotope Proxies: A Recap

• Foraminifera ^{18}O/^{16}O ratios are influenced by two main effects:
  • Ice Volume Effect: Growth of an ice sheet equivalent to 100 m sea level fall ≈ 0.1 ‰ increase in seawater ^{18}O/^{16}O.
  • Temperature Effect: 1°C temperature decrease ≈ 0.026 ‰ increase in foraminifera ^{18}O/^{16}O (even when seawater ^{18}O/^{16}O is constant).
• Planktonic foraminifera record warmer and variable temperatures.
• Benthic foraminifera record cooler and more stable temperatures.

EARLY CENOZOIC: NO ICE SHEETS NO ICE VOLUME EFFECT

• During the Early Cenozoic, there were no ice sheets, meaning there was no ice volume effect on foraminifera ^{18}O/^{16}O records.

Early Cenozoic Climate

• The Early Cenozoic was characterized by ice-free conditions and warmer temperatures.

Early Cenozoic Greenhouse Warming

• Jim Zachos is shown holding a sediment core from the seafloor, displaying the red clay layer that marks the ‘Paleocene-Eocene Thermal Maximum’ (PETM).
• This period was characterized by extreme global warming and ocean acidification around 56 million years ago.
• Causes and characteristics of Early Cenozoic Greenhouse Warming
  • More greenhouse gases (e.g., CO2) in the atmosphere due to enhanced volcanic emissions
  • Warmer global temperatures - no permanent ice at the poles
  • ‘hyperthermals’ – extreme global warming (<10 ky to ~100 ky) from massive injection of CO2 into the atmosphere & oceans
  • ‘Palaeocene–Eocene Thermal Maximum’ (PETM): ~56 Ma
  • ‘Eocene Thermal Maximum 2’ (ETM2): ~54 Ma
  • ‘Eocene Climatic Optimum’ (EECO): ~52 Ma

Key Questions on Early Cenozoic Greenhouse Warming

• What is the sequence of events in the oceans & atmosphere when there is intense global warming? Which aspects of the ocean-atmosphere-climate system respond first?
• How do the oceans & atmosphere recover from intense global warming? Which aspects of the ocean-atmosphere-climate system recover first?
• Early Cenozoic ‘hyperthermals’ act as extreme analogues of today’s warming climate
• How will the oceans continue to change over the coming decades & centuries in response to global warming?

^{18}O/^{16}O & Early Cenozoic ‘Hyperthermals’

• Prior to 35 million years ago, the oceans were ice-free, eliminating the ice volume effect on foraminifera ^{18}O/^{16}O records.
• As a result, ^{18}O/^{16}O of planktonic foraminifera primarily records changes in sea surface temperature.

PETM ‘Hyperthermal’ (56 My ago)

• Over 2,000 Gt C as CO2 entered the atmosphere and ocean.
• Atmospheric CO2 levels reached approximately 1,000 parts-per-million.
• The CO2 injection during the PETM is comparable to some future-climate projections.

PETM ‘Hyperthermal’ (56 My ago)

• Abrupt disturbances occurred in:
  • Global temperature, which increased by approximately 10 °C in less than 10,000 years.
  • Ocean acidification and marine ecology.
  • Primary production (e.g., shifts in sedimentary P, Ba, Zn, C content).
  • Carbon cycle (e.g., shifts in C isotope ratios).

PETM (~56 My ago)

• Abrupt disturbances to:
  • Global temperature - increased by >5 °C in <10 ky
  • Ocean acidification & marine ecology
  • Primary production (eg. shifts in sedimentary P, Ba, Zn, C content)
  • C cycle (eg. shifts in C isotope ratios)

^{18}O/^{16}O & Phanerozoic Climate

• The Phanerozoic climate exhibits transitions from foraminifera-based ^{18}O/^{16}O proxies to other proxies.
• The Earth has experienced ‘Super-greenhouse’ climates, some with global-scale mass extinctions, and intermittent ‘ice age’ climates.
• Scotese et al. (2021) - End-Permian Mass Extinction
• Key question: How did the oceans & atmosphere evolve during very warm ‘Super Greenhouse’ climates?

Sediment Archives

• Sediment archives are essential for reconstructing past environmental conditions. They can be either unconsolidated or consolidated.
• Unconsolidated Sediment: Individual sediment components can be isolated from bulk sediment, such as calcareous foraminifera, siliceous diatoms, and organic matter.
• Consolidated Sediment: Individual sediment components cannot be isolated from bulk sediment, such as limestone (carbonate), chert (biogenic silicate), and black shale (organic-rich).

Pre-Phanerozoic Sedimentary Archives

• Examples of Pre-Phanerozoic Sedimentary Archives:
  • limestone-chert-black shale sequence Italy
  • black shale sequence Canada
  • carbonate 650 Ma limestone sequences, Namibia

Oceans & Atmosphere of the Early Earth

• Key Questions:
  • How did the oceans & atmosphere evolve in relation to rising oxygen and the evolution of life?
  • What are the boundary conditions for a habitable planet?
  • How would habitability & life be recognized on a distant exoplanet?

Precambrian Oceans & Atmosphere of the Early Earth

• The Precambrian Eon includes the Hadean, Archean and Proterozoic Eras.

Sedimentary Archives for the Early Oceans

• ‘Great Oxidation Event’ (GOE)
  • Archean to early Paleoproterozoic
  • 2.4 – 2.1 billion years ago
  • The initial rise of oxygen
  • The ‘first mass extinction’ event

Proxies for Oxygen in the Early Ocean

• ‘Banded Iron Formations’ (BIFs)
  • Alternating layers of iron oxides (Fe- rich) & siliceous chert (Fe-poor) on the seafloor
  • Fluctuating O2 levels in the oceans & atmosphere during the ‘GOE’

A Recap: What can be Reconstructed?

• Paleoceanographic proxies can reconstruct the physical, chemical & biological states of the past oceans

A Recap: What can be Reconstructed?

• Paleoceanographic proxies can reconstruct the physical, chemical & biological states of the past oceans
  • Nutrient content
  • Primary productivity
  • Temperature
  • Ocean acidification
  • Ocean circulation of water masses
  • Sea ice cover
  • Sea level
  • CO2 content
  • O2 content
  • Salinity
  • Stratigraphy (provide a timescale)

A Recap: What can be Reconstructed?

• Paleoceanographic proxies can reconstruct the physical, chemical & biological states of the past oceans
  • Temperature
    • Planktonic foraminifera assemblages
    • Planktonic foraminifera Mg/Ca (& others eg. Sr/Ca)
    • Planktonic foraminifera O isotopes (if ice-free, >32 Ma)
  • Ocean circulation of water masses
    • Neodymium isotopes
  • Sea level
    • Benthic foraminifera O isotopes
    • Planktonic foraminifera O isotopes (paired with Mg/Ca to correct for temperature effect)
  • O2 content
    • Banded iron formations (alternating iron oxides & cherts)

Temperature Proxies

• Table 1. The Proxies and the Parameters they can Help Reconstruct.
  • Temperature
    • Physical properties of sediment (+)
    • Mineralogy (clay minerals) (+)
    • Benthic foraminifers (+)
    • Deep corals (& traces/ isotopes) +
    • Planktic foraminifers +
    • Diatoms +
    • Dinocysts +
    • Coccoliths +
    • Alkenones & biomarkers
    • Boron isotopes in sediment
    • ^{13}C & ^{18}O in foraminifers

How is a Timescale Assigned?

• What tools are available to assign a timescale to proxy reconstructions of the past oceans?

Climate Change & Orbital Cycles

• Illustration of interglacial and glacial periods, along with solar radiation variations at 65°N

Timescale Toolbox

• Predictable, orbitally-driven variations in the solar radiation received by Earth provide a timescale for long, continuous ocean & climate records showing orbital frequencies
• Other Dating Tools
  • Magnetostratigraphy
    • Well-dated reversals in Earth’s geomagnetic field
  • Biostratigraphy
    • Known age & distributions of fossil assemblages
  • Carbon-14 dating (last ~60 ky)
    • Radiometric dating tool based on the natural decay of ^{14}C

Timescale Toolbox

• Tools used to assign a timescale to climate reconstructions:
  • Orbital Cycles: Earth’s orbitally-driven solar radiation cycles.
  • Magnetostratigraphy: Dating based on reversals in Earth’s geomagnetic field.
  • Biostratigraphy: Using the known ages and distributions of fossil assemblages.
  • Carbon-14 Dating: Radiometric dating technique for materials up to ~60,000 years old.

Today’s Take-Home Messages

• The oceans became fully oxygenated only ~540 My ago following the second rise in atmospheric O2
• The Phanerozoic Eon (after ~540 My ago) was dominated by warm, greenhouse climates that were ice-free. This includes early Cenozoic until the late Cenozoic (~66 to ~32 My ago)
• The late Cenozoic (last ~32 My) was characterised by global cooling & the growth of polar ice sheets leading up to the Quaternary ‘ice age’ with a glacial-interglacial climate regime
• Climate reconstructions are based on proxies eg. ocean temperature, circulation & oxygen content, and sea level
• Climate reconstructions are assigned a timescale using eg. Earth’s orbitally-driven solar radiation cycles, magnetostratigraphy, biostratigraphy & carbon-14 dating (or other radiometric dating techniques)