Exam 3 - Geological Oceanography

Methods of Absolute Dating

• Historic events

• Annual layers

• Radiometric Dating

Petrology

study of the chemical composition and mineral associations in rocks

Hyaloclastic

Brecciated in a glassy basalt matric and typically forms when there are landslides of submarine basalt flows

SONAR

Sound Navigation and Ranging (developed during WWII for submarine detection)

Diagenesis

compaction and chemical reactions of sediments and pore fluids that change the chemistry of the water and solids accumulated

cotectic

Describing the conditions of pressure, temperature and composition at which multiple solid phases crystallize at the same time from a single liquid when cooled

Guyots

seamounts with a flat top with evidence of shallow or subaerial exposure that are more than 1000 m in elevation

formation of seamounts

an example of an endogenic process

olivine and nephilene

which two minerals found in basalt do not exist if the silica concentration is too high

200-400 kHz

Using SONAR, typical frequencies for shallow-water marine bathymetry surveys are:

1500 m/s

The speed of sound in water is approximately

siliceous ooze

a sediment that is 63% made up of radiolarian remains is called a _____

young seafloor

no sediment accumulation around Mid-Ocean ridge indicating that the seafloor is spreading

sources of dissolved matter

• Weathering of rocks

  • continental weathering

    • calcium ions released into rivers transported to the ocean when silicate and carbonate rocks weather

  • Acid rain

    • accelerates dissolution of rocks

• Hydrothermal vents

• Biological processes

  • corals, mollusks, and plankton

Annual layers

• Lamination

• couplet

lamination

layers of sediment differing in composition

couplet

pair of adjacent different layers

Varves

sediment laminations found in anoxic basins

• Biogenous-rich sediments in a spring thaw bloom and fall mixing bloom

• Lithogenous-rich sediments in winter

Radioactive decay

process by which an unstable atomic nucleus loses energy by radiation

Parent isotope

decays to daughter isotope and decay particle

234U-230Th Method

• U2+ is abundant and can replace Ca2+ in carbonate

• Th is rare in seawater, so initial carbonate has little

• Uo + Ut +Thf

234U-230Th

• (10-450ka) corals, calcareous oozes, mollusks shells

• Peking Man is an early hominin fossil (<100 ka). The fossils were partly encrusted with calcium carbonate. What Radioisotope is best?

sediment record from south oahu

(>2 Ma), and found a change from pelagic sediments to turbidites

210Pb

(<100 yr) terrigenous sediments

14C

(100a-40Ka) organic matter

40K-40Ar

(<500ky) Rocks containing K (constrain pelaeomag timescale)

Basis of ¹⁴C-Method

• Cosmic Ray Bombardment

• Nuclear reaction with the atmosphere

• rapid chemical reaction with O₂ in the atmosphere

• Equilibrium between atmosphere and biosphere

Cosmic ray bombardment

creates free neutrons

nuclear reaction with atmospheric ¹⁴N(n,p)¹⁴C

produces and average ratio of ¹⁴C/¹²C=10^-12

rapid chemical reaction with O₂ (21%)

in the atmosphere

¹²C + 1/2O₂ → ¹⁴CO + O

¹⁴CO + HO → ¹⁴CO₂ + H

within 6-8 years

equilibrium between atmosphere and biosphere

What principles of Magnetostratigraphy are show here?

• Superposition

• Lateral continuity

• orignal horizontality

1.42 cm/ka

Calculate the sedimentation rate over the last 0.73 Ma.

594

Which core had the fastest sedimentation rate?

588 or 592

Which cores has the slowest sedimentation rate?

590 Cochiti

Which core collected the oldest sediment?

Biostratigraphy

tracking the changes in fossil fauna and flora

• typically planktonic microfossil

• diversity, abundance, rapid evolution, wide distribution

Biostratigraphy useful for

• Age dating

• environmental information (temp, nuts)

Smear slides used

• to determine the composition of sediment recovered during coring

• Quickly determine presence-absence, relative abundance

microfossil

communities that are also isolated to specific time periods

relative dating

the fixing of a geologic structure or event in a chronological sequence relative to other geologic structures or events

fossil succession used

to identify biostratigraphic zones

index fossil

abundant, cosmopolitan, existed for a relatively short span of time

Taxon Range

define a period of time

Concurrent range zone

period of time where two fossil overlap

Partial range zone

one fossil exist for a long period of time where the two other fossils don’t exist in the same time period with a biozone in the middle

successive first appearance zone

period of time where only one fossil exists is the biozone and once the second fossil appears the sediment is younger

successive last appearance zone

period of time where one fossil is the last to die off and another fossil dies off before it

Which zone based on range

• CF1 (Taxon range)

• P1a(2) (Concurrent range)

Successive First appearance

P0

Partial range

P1b

Biozone

CF1

Foraminiferans

• unicellular, heterotrophic plankton with calcite shells (CaCO3)

• Most studied marine microfossil

• Make up 2.5% of all known animal species of the last 550 Ma

• The foundation of marine paleoecology

  • Large (50-400 µm)

  • high diversity

  • occur in all marine environments, at all latitudes, within all oceans

Climate Indicators in sediments

foraminifera

Viscosity and Drag

decrease with warmer water

compensate for high density shell

tropical shells thinner, more porous, exaggerated apertures, spines

Tropical foram

globigerinoides sacculife

polar foram

neogloboquadrina pachyderma

Foram N. Pachy test

Changes with temperature

• left-coiling (sinestral) in cold water

• right-coiling (dextral) in warmer water

changes in species with temperature

water started cold; conditions change with a period of warm water; ended cold

regional biostratigraphic relationships

are developed because of changes in species with temperature

Benthic forams

indicators of water depth

diatoms

• are indicators of primary production

• More diatoms, must have been more nutrients and more production

Diatom crash in Santa Barbara Basin

• Strong upwelling, but Si-limited

• Shifting baseline-reduced accumulation of diatoms after 1980

Distribution of coral reefs

• tropics and subtropics

• skewed to western side of oceans

• no coral reefs in upwelling regions (except equator)

annual layers in coral cores

• Faster, lower-density growth during summer months

• massive corals typically

• Aragonite skeleton records:

  • changes in accessory elements

  • changes in isotopic ratio

changes in accessory elements in coral cores

• Temp: Mg, Sr

• pH: Basic

• Nutrients, salinity: Ba, Li

Changes in isotopic ration

• Growth rate: 13C/12C

• Temperature: 18O/16O

Seafloor imprint of glaciers

• Iceburg-carved ravines on the shelf

• Dropstones (grain size larger than anticipated for the region)

Isotopes of elements

• the chemical characteristic of an element is determined by the number of protons in its nucleus

• 92 naturally occurring elements

• some are stable; some are radioactive

Atomic number

= # Protons = define the chemistry

Atomic weight

• = protons + neutrons = referred to as isotopes

• Different elements can have different numbers of neutrons and thus atomic weights (the sum of protons plus neutrons

All isotopes of a given element

have the same chemical properties

small differences of all isotopes of a given element

due to the fact that heavier isotopes typically form stronger bonds and diffuse slower

examples for H, C, N, and O

Except for 1H, even atomic weights are more abundant than odd

mass spectrometer

isotopes are measured as ratios of two isotopes by various kinds of detectors. Standards are run frequently to correct for instrument stability

Define H = heavy and L = light

• Report stable isotope abundance as ratio to most abundant isotope (e.g. ¹³C/¹²C)

• Isotope ratio of a sample is reported relative to a standard

Standard for H

Standard Mean Ocean Water (SMOW)

Standard for C

Belemnitella americana from the Cretaceous Peedee formation, South Carolina (PDB)

Standard for N

Atmospheric N₂

Standard for O

• Standard Mean Ocean Water (SMOW)

• Belemnitella americana from the Cretaceous Peedee formation, South Carolina (PDB)

Standard for S

Troilite (FeS) from the Canyon Diablo iron meteorite (CD)

Fractionation

variations in isotope abundance are caused by reactions which preferentially use one isotope resulting in a different fraction in the product than in the reactant

equilibrium isotope effects

Partial separation of isotopes between two (or more) substances in equilibrium. Usually inorganic (CaCO₃), not organic

larger isotope

tends to be more stable

• ex) water is enriched in D₂¹⁸O vs. vapor

• HCO₃^- is enriched in ¹³C vs. CO₂

paleothermometer

• Fractionation during ppt. of CaCO₃ as a result of temperature

• As temperature gets colder, ¹⁸O increases

• As temperature gets warmer, ¹⁸O decreases

Raleigh Distillation

• condensed phase is in isotopic equilibrium with the surrounding vapor and immediately removed

• Results in equilibrium fractionation: H₂¹⁶O_(v) + H₂¹⁸O_(v) = H₂¹⁶O_(l) + H₂¹⁸O_(l)

Foram 18O records

• Change in temperature

• Change in ice volume = ~1‰

• Deep benthic forams

glacial effect

a result of the Raleigh Distillation, during ice ages the oceans are enriched in 18O

Local effects of 18O

elevation and storms

Continental effects of 18O

global atmospheric circulation

assumptions of estimating temps in ancient ocean environment

• Organisms ppted CaCO₃ in isotopic equilibrium with dissolved CO₃^2-

• The ¹⁸O of the original water is known

• The ¹⁸O of the shell has remained unchanged

estimation of temps in ancient ocean environments

Planktonic forams measure sea surface temperature and benthic forams measure benthic temperature

Glacial-Interglacial cycles

extend back more than 1Ma

Isotope stages (relative dating technique)

• Odd = warm periods

• Even = cold periods

Cold Periods

Stadials

Warm Periods

Interstadial

Insolation

incoming solar radiation

High Insolation

Earth close to sun, hemisphere pointed towards sun

Low Insolation

Earth far from sun, hemisphere pointed away from sun