(4) Ocean circulation records- sediment cores and corals

Why is ocean circulation important?

• nutrient circulation (bring deep water to the surface to fuel biological productivity)

• global heat transport

• ocean as a carbon store → CO2 dissolves into ocean and breaks down into different species→ slow ocean circulation accumulates more carbon → faster releases more carbon into the atmosphere

What controls global atmospheric circulation?

• Energy from sun

• Rotation of earth (coriolis force)

Two modes of ocean circulation

1. Surface currents

2. Thermohaline circulation

Surface currents

• surface layer of the ocean

• wind driven

Thermohaline circulation

• involves both deep and surface waters

• related to differences in temperature (thermo) and salinity (haline)

• ^ both affect density and pressure

Unit of measure of salinity

grams of salt per kg of seawater

(~35)

What is the Ekman layer?

• ocean layer (upper few hundred metres) under the influence of the wind

• transport perpendicular to the wind (coriolis effect → right in NH; left in SH)

Ocean gyres

massive, rotating systems of ocean currents driven by wind patterns, the Coriolis effect (Earth's rotation), and landmasses

Surface divergence

• thinner surface layer

• water upwells to fill the space

Surface convergence

piling up of water in the centre→ thicker surface layer → small scale down-welling

Impact of continental boundaries on coastal upwelling

wind blowing along coastline → water surface at coast pushed away from coastline → lower sea surface at the coast + upwelling of deep waters to fill space → supply of colder more nutrient rich waters

Where does surface divergence typically happen?

along equator

Gulf Stream

• Transports 30-150 million cubic metres of water per second (= 30-150 Sverdrups)

• (continental) boundary current

• part of the subtropical gyre

• steep temp gradients → subpolar/ subtropical gyre

Ocean temperature vertical distribution

• Upper 200m (varies): ‘Mixed layer’

• 200m – 1000m (varies): Thermocline = zone of steep temperature gradient

• > 1000m (varies): Deep ocean

Ocean temp distribution at different depths

• 0m → solar heating so strongest at equator + coldest at poles → excursions from patterns due to currents/ wind across coastlines

• 600m → weaker gradients

• 2000m → atlantic a bit warmer than the rest of the deep ocean

Salinity distribution at different depths

• 0m → more evaporation at ocean surface near equator = higher salinity + very fresh water in northern polar regions

• 6000m: some salt in arabian sea + mediterranean outflow waters into Atlantic

• 2000m → uniform

What typically drives dense water formation?

Convection: surface water denser than underlying water → situation unstable → surface water sinks

How does North Atlantic Deep Water form (NADW)?

cooling of salty water + sinking in winter

• in the Labrador and GIN seas

Northwest Atlantic Bottom Water (NWABW)

• LOWER NADW

• coldest and densest

• includes Denmark Strait Overflow Water (DSOW)

Northeast Atlantic Deep Water (NEADW)

• MIDDLE NADW

• more saline and denser

• includes Iceland-Scotland Overflow Water (ISOW)

Labrador Sea Water (LSW)

• UPPER NADW

• warmer and fresher

How does Antarctic Intermediate Water (AAIW) form?

by sinking north of the Polar Front

Polynyas

areas of ice-free water within the winter ice cover

How does Antarctic Bottom Water (AABW) form?

• brine rejection (coastal polynyas)→ brine formation due to sea ice causes water to sink

• convection (open ocean polynas) → heat exchange with atmosphere allows for sinking

• partially through cooling in winter

Salinity through the Atlantic Ocean

• Deep waters are formed in the north and south

• NADW is warmer and saltier

• AABW is cold and fresh

Atlantic Meridional Overturning Circulation

AMOC

Atlantic component of the global overturning circulation

Why does no deep water form in the North Pacific Ocean?

• net export of rainfall from Atlantic to Pacific

• connection to Northern freshwater

Salinity section through the Pacific Ocean

• No deep water formation in the North Pacific

• Only North Pacific Intermediate Water is formed

• Deep waters here are old and low in oxygen

• North Pacific much fresher than the North Atlantic

Antarctic Circumpolar Current (ACC)

strong wind-driven current that connects the ocean basins

Link between North Atlantic and Antarctica bottom water

NADW upwells in the Southern Ocean and is converted to AABW

Equilibrium isotope fractionation

4°C change in temperature produces approx. 1 ‰ change in 18O isotope

What is water 18O affected by?

• Ice volume effect

• Salinity effect

• Local changes

Ice volume effect

• Ice melting causes 18O seawater to decrease globally

• Ice growth causes 18O seawater to increase globally

Salinity Effect

• Changes in advection/ upwelling of water with different 18O

• dependent on latitude of water e.g low latitude river water vs. high latitude ice meltwater

Local changes in 18O seawater

• Evaporation causes local levels to increase

• Precipitation causes local levels to decrease

Foraminiferal 18O during glacial periods

• light oxygen that evaporated from the oceans is trapped in ice sheets

• seawater 18O becomes heavier when there is more continental ice

• foram shells calcify under colder temperatures

• oxygen isotopes in foraminiferal shells record a combination of temperature and ice volume effects

How well do you think it can be used to generate an age model in the deep Pacific Ocean versus the deep Atlantic Ocean?

• Works well as a correlation tool and climate record

Limitations

• circulation signature → NADW not as cold as AABW → temp effect → record in NADW would be lighter in oxygen isotopes

What can be used to constrain 18O water in the past?

• Foraminiferal Mg/Ca ratios are a temperature proxy

• Can extract 18O water to get 18O of ocean water → removal of temp control

Considerations for using forams as a proxy

• How many individuals do I need to analyse to be representative?: Some shells may represent a particular season

• How constant is the proxy – environment link (i.e. modern calibration)?

• What other secondary controls impact the proxy (e.g. T, S, [CO3 2- ])?

What do single vs. bulk foram analyses show?

• bulk record → take all and crush them up→ could reflect ice volume/ climate change

• single measurements → large variability in individual shell for temp and salinity→ driven by local factors e.g La Nina/ El Nino years + in between

How can carbon isotopes trace biological processes (nutrient content)?

• Biological activity removes DIC (dissolved inorganic carbon) and phosphate in the surface ocean

• Photosynthesis preferentially uses 12C in forming organic matter – so the remaining DIC becomes enriched in 13C, giving surface ocean high 13C

• When the 12C-rich organic matter sinks and is broken down in the deep ocean (respiration), the carbon is released to the DIC pool i.e. deep ocean has low 13C

Foraminiferal 13C reconstructions

1. Not much of a temperature effect on 13C of calcite (0.035 ‰/°C)

2. 13C of shells reflects the dissolved inorganic carbonate (DIC) of seawater

3. 13C is not in isotopic equilibrium with seawater, due to kinetic fractionation (~1‰ offset, but can be corrected)

Controls on foraminiferal 13C

Local Changes

• respiration (decrease) + photosynthesis (increase)

• changes in upwelling of water with different 13C

Global Change

• release of carbon from the lithosphere

• growth of terrestrial biosphere causes 13C to increase

Ocean Proxies

• Accumulation rates of CaCO3 and organic C (biological pump, carbonate system)

• Sediment grain size (deep water flow speed)

• Microfossil assemblages (cold/warm species, upwelling, sea ice)

• Coral chemistry (temperature) or radiocarbon (ocean circulation)

• IRD and provenance (ice sheet histories)

Static tracers of deep circulation

fingerprint (origin) water masses but not their fluxes

• Carbon isotopes 13C

• Nd isotopes

Dynamic tracers of deep circulation

reveal fluxes/ flow speed but not water masses origin

• sortable silt (grain size)

• radiocarbon 14C

How can carbon isotopes trace water masses?

regeneration of nutrients along the thermohaline circulation pathway

Properties of Pacific Ocean Water

• “old” water

• poor in O₂

• rich in CO₂

• low in 13C

Properties of Atlantic Ocean Water

• “young” water

• rich in O₂

• poor in CO₂

• high in 13C

Controls other than ocean circulation

Reservoir changes in 3C of whole ocean: affect benthic and planktonic foraminiferal 13C equally

Primary productivity: change surface-to-deep gradient (steeper gradient = more productivity)

Circulation in glacial period

• high 13C in North Atlantic = young, nutrient poor

• but more old, nutrient rich in south

• expansion of AABW = more water in deep ocean coming from Antarctic

Radiogenic isotope tracers

• Mantle melting: Nd is more incompatible so Sm/Nd in melt (~crust) is lower

• Radioactive decay from ¹⁴⁷Sm produces ¹⁴³Nd through time

• Rocks of different ages (and Sm/Nd ratios) have different ¹⁴³Nd/¹⁴⁴Nd ratios

• Small differences, but readily resolvable (epsilon notation)

Epsilon notation (εNd)

• Deviation from ratio of bulk earth (0.512638) in parts per 10,000

• The Nd isotope composition of seawater is typically expressed in the epsilon notation

Application of Nd isotopes to trace ocean currents

• Sea water reflects surrounding continental geology through riverine deposits + exchange of sediments eroded from continental margins

• Surface signal can be carried with deep ocean water as they move around oceans

Archives of Nd isotopes

• deep sea corals

• fish teeth

Why is coral a great archive?

Can be dated to give absolute age as well as Nd isotopes of composition of water

What do the similar patterns of the benthic 13C and Nd isotope records suggest?

• both controlled by ocean circulation patterns

Radiocarbon as a tracer

• Measured as 14C (ratio of 14C/12C in a sample relative to a pre-industrial, pre-nuclear atmospheric standard in ‰)

• High in the North Atlantic (-60 ‰)

• Low in the Southern Ocean (-160 ‰)

• Dynamic tracer → flow speed

• Recorded in corals

• high radiocarbon content (young) in N. Atlantic

• low radiocarbon content in older water as it decays → cannot add more radiocarbon

Sortable silt as a tracer

• SS MGS (mean grain size) = mean size of 10-63 um fraction

• Local flow speed proxy → location specific

• Need to demonstrate that the material is current-sorted in the first place

• Clay/ silt/ fine sands→ terrigenous (not carbonate)

How can you approximate grain size?

• can use chemical tracers → Zr/ Rb ratio

• XRF-scans derive Zr/Rb ratios

• more Rb in fine fractions and more Zr in coarse fractions

Other useful XRF-scan proxies…

• Calcium (Ca): indicator of carbonate content

• Barium (Ba): indicator of productivity

• Titanium (Ti) or Aluminium (Al): indicator of detrital input (e.g. dust, clays)