Paleoceanography Notes

Oceanography

• "Oceanography" is the study of the oceans, including its physics, chemistry, biology, and geology.

Paleoceanography

• "Paleo" means old or ancient.
• "Paleoceanography" is the study of the history of the oceans in the geologic past, regarding circulation, chemistry, biology, geology, and patterns of sedimentation and biological productivity.

Connection to Paleoclimatology

• Paleoceanographic research is intimately tied to paleoclimatology because the oceans and atmosphere are connected across the ocean's surface.

Interactions Between Ocean, Atmosphere, and Climate

Present Day Regime

• Oxygenated atmosphere with trace greenhouse gases.
• Oxygenated oceans.
• Balanced distribution of continents and oceans, leading to efficient, whole-ocean circulation.

Present Day Regime with 1^{\circ}C Warming

• Atmosphere and ocean warming, leading to increased uptake of greenhouse gases in oceans.
• Increased ice sheet and sea ice melting, freshening of oceans and potentially changing ocean circulation.
• Intensified rainfall, increased weathering & runoff, leading to increased nutrient supply to the ocean and stimulated marine productivity.

Deep-Time Regimes

• De-oxygenated atmosphere with elevated greenhouse gases.
• Extreme 'Super-Greenhouse' or 'Icehouse' climates.
• Unbalanced distribution of continents and oceans.
• De-oxygenated oceans.

Key Questions Addressed by Paleoceanography

• How do the oceans respond to changing climates & environmental conditions?
• How closely connected are the oceans & atmosphere?
• How did the oceans & atmosphere evolve to support life?
• How will the oceans continue to change over the coming decades & centuries in response to global warming?

Today’s Key Ideas

• Introduction to paleoceanography: Understanding past oceans to predict the future state of the oceans in response to climate change.
• Sedimentary archives in paleoceanography: Types of marine sediments available for investigating the evolution of Earth’s oceans through geologic time.
• Sedimentary proxies in paleoceanography: Sedimentary properties that can be measured to reconstruct the evolution of Earth’s oceans through geologic time.
• Examples of sedimentary proxies.

Uniformitarianism

• The Present is the Key to the Past.
• Physical, chemical, and biologic processes now at work on and within the Earth have operated with general uniformity through immensely long periods of time and are sufficient to account for all geologic change.

Sediments – the Ocean’s Memory

• Seafloor is slowly dusted by sediments with accumulation rates of centimetres/year to millimetres/million years.
• Sedimentary archives respond to (& preserve) past oceanographic changes.
• Types of sediments: lithigenic, biogenic, authigenic, and cosmogenic.

Lithogenic Sediments

• Eroded material from exposed rocks on land, transported to ocean by rivers (main source), wind & icesheets.
• Dust & volcanic ash, blown by wind onto surface ocean.
• Examples: turbidites, abyssal clays, ice-rafted debris, wind-blown dust.

Biogenic Sediments

• Hard-part siliceous or carbonate shells & soft-part organic matter of marine organisms in ocean water column.
• Marine organisms die & their remains sink to the seafloor.
• Examples: siliceous ooze (diatom individuals), calcareous ooze (foraminifera, coccolithophore individuals), corals, organic-rich sediments (plankton individuals) → chert, limestone, black shale.

Authigenic Sediments

• Inorganic minerals precipitate from dissolved chemical elements & compounds in seawater.
• Generally form slowly (precipitation rates of mm/million yrs).
• Concentrated around hydrothermal vents, continental margins.
• Examples: Ferromanganese crusts & nodules, iron-manganese oxide coatings around sediment grains, evaporites.

Cosmogenic Sediments

• Extra-terrestrial debris deposited on land & in the oceans.
• Examples: Interplanetary dust, meteorites & ejecta from rare asteroid & comet impacts.

Collecting Marine Sediments

• Collect marine sediments in outcrop on land.
• Retrieve sediments from below the seafloor using the International Ocean Discovery Programme (IODP).

Sedimentary Proxies

• A paleoceanographic proxy (or ‘stand in’) is a measurable property of a sedimentary archive that responds to changes in the marine environment in clear and predictable ways.
• Proxies indirectly describe ‘target parameters’ that characterise the physical, chemical & biological state of the past oceans because they cannot be measured directly in sediments.
• Proxies must be calibrated in the modern ocean before they can be applied to the reconstruction of past ocean environments.

What can be Reconstructed?

• Paleoceanographic proxies can reconstruct the physical, chemical & biological states of the past oceans.
• Parameters include: Nutrient content, primary productivity, temperature, ocean acidification, ocean circulation of water masses, sea ice cover, sea level, CO2 content, O2 content, salinity, and stratigraphy (provide a timescale).

Foraminifera in Biogenic Sediments

• Foraminifera (‘forams’ for short) are single-celled organisms that live in the ocean water column.
• External shell (‘test’) made of calcium carbonate (CaCO_3).
  • Planktonic foraminifera live in the upper ocean.
  • Benthic foraminifera live on or in the seafloor (epifaunal and infaunal).
• Fossil foraminifera found in calcareous ooze & limestone.
• Assemblages, morphology & chemistry provide proxies (e.g., past ocean productivity, water mass structure & circulation, temperatures & bottom water oxygen).

CASE 1: Foram Assemblages – Planktonic Foraminifera as Tracers of Past Oceanic Environments – SST Proxy

• Distributions of assemblages follow sea-surface temperature (SST) gradients.
• Warmer sea surface temperatures produce more diverse assemblages with larger individuals.
• Fossil planktonic foraminifera assemblages are a proxy for reconstructing past SST.
• ‘Indicator species’ track the temperature history of ocean surface waters on geological timescales.

CASE 2: Foram Mg/Ca Ratios – SST Proxy

• Foraminifera (CaCO_3) tests preferentially take up Mg over Ca from seawater during growth.
• Greater proportions of Mg are taken up by the tests as seawater temperature rises.
• Higher Mg/Ca in warm water & lower Mg/Ca in cool water regardless of Mg/Ca of seawater.
• 1^{\circ}C temperature increase ≈ 9 % increase in foraminifera Mg/Ca (Lea et al., 2000).
• Surface waters offshore from New Zealand have warmed by ~6°C since the ‘Last Glacial Maximum’ (20,000 years ago).

CASE 3: Fe-Mn Crust Chemistry - Water Mass Proxy

• Ferromanganese (Fe-Mn) crusts form in layers on the deep ocean floor.
• Each layer represents a time slice and has slow growth rates of mm/million years.
• Neodymium (Nd) Isotopes - Tracer for ocean circulation
  • Amount of ‘radiogenic’ ^{143}Nd in rocks increases over time.
  • Younger rocks have lower ^{143}Nd/^{144}Nd.
  • Older rocks have higher ^{143}Nd/^{144}Nd.
  • Water masses inherit unique Nd isotope ‘fingerprints’ from nearby rocks on land.
  • Convergence of Nd isotope signatures of Fe-Mn crusts from different areas of Atlantic Ocean → deepening & widening of Atlantic Ocean → onset of ‘whole-ocean’ circulation

CASE 4: Metals in Cosmogenic Sediments

'K-T boundary' (or ‘K-Pg boundary’)

• Normal sea floor: Burrowing organisms & abundant foraminifera & cocoliths.
• 'K-T boundary': Clay enriched in iridium (high Ir/Al).
• ‘Dead zone’: Laminated organic-rich sediments indicating an absence of burrowing organisms & rare plankton microfossils.
• ‘Recovery interval’: Bioturbated sediments, plankton fossils but fractured flinty rock, overlying foraminifera & coccoliths indicating full recovery over time.

Today’s Take-Home Messages

• Paleoceanography is the study of the physical, chemical & biological state of the past oceans.
• Paleoceanography is closely linked to paleoclimatology because the oceans, atmosphere & climate are inter-connected.
• Sediment archives: lithogenic, biogenic, authigenic, cosmogenic.
• Sediment proxies: Measurable properties of an archive that can be used to reconstruct past ocean environments.
• Examples of proxies:
  • Ocean temperature - assemblages & Mg/Ca ratios in forams
  • Ocean circulation - Nd isotopes of Fe-Mn crusts
  • Asteroid ejecta – Ir in clay & organic-rich sediment