Stratigraphy n The endless rain of sediments produces layers. n Stratigraphy is the study of the thickness and composition of sedimentary layers. n Such study provides insight into rates of sedimentation, n sea-floor spreading, n and even climate.

Stratigraphy n If conditions are right, these sediments can eventually be converted into sedimentary rock n The record is retained: n climate n environment of deposition n life forms

Stratigraphy Stratigraphy - the process of describing and categorizing marine (or other) sediments - trying to make some sense of them allow comparisons of sediments from different ages, environments by really understanding any one sequence of sediments, you can look at others and learn by comparing and contrasting the two.

Sediment Overview n Here is how these factors work: n 1. Delivery n 2. Dissolution n 3. Dissemination n 4. Dilution n 5. Diagenetic alteration Atmospheric Biogenic Rivers

Stratigraphy Analogue n 1. Delivery n 2. Dissolution n 3. Dissemination n 4. Dilution n 5. Diagenetic alteration

Types of Stratigraphy n Lithostratigraphy: sediment chemistry, grain size, physical characteristics n Biostratigraphy: fossils n magnetostratigraphy: magnetic field direction n stable isotope stratigraphy: ratio of one isotope to another n chronostratigraphy: absolute age from radiogenic isotopes

Lithostratigraphy Distinguish layers basis of lithology or physical characteristics: Distinguish layers basis of lithology or physical characteristics: grain size, grain size, mineralogy mineralogy mode of deposition etc. mode of deposition etc. Facies = character of the rock or sediment; Facies = character of the rock or sediment; controlled by environment controlled by environment A lithologic unit may be continuous over a large geographic area, but formed at different times in different areas -is diachronous. A lithologic unit may be continuous over a large geographic area, but formed at different times in different areas -is diachronous. diachronous lithologic units = ones which cross time line since lithologic units are based solely on aspect or facies, diachronous lithologic units = ones which cross time line since lithologic units are based solely on aspect or facies, e.g.. Chert beds e.g.. Chert beds

Lithostratigraphy Key correlations between cores from different areas are by lithologic units known as marker or key beds: Key correlations between cores from different areas are by lithologic units known as marker or key beds: volcanic ash layers (tephrochronology) volcanic ash layers (tephrochronology) lithologically distinctive, lithologically distinctive, widespread widespread dateable dateable unique chemistry determined by the source unique chemistry determined by the source Turbidites Turbidites microtektites microtektites These units are often bioturbated (organisms living in the mud can mix lower layers up and vice versa) These units are often bioturbated (organisms living in the mud can mix lower layers up and vice versa)

Biostratigraphy strata are characterized by their fossil content strata are characterized by their fossil content organize into units based on fossil characteristics (assemblage, ranges etc.) organize into units based on fossil characteristics (assemblage, ranges etc.) not large fossils (which are rare) but microfossils not large fossils (which are rare) but microfossils Fossils are extremely common in marine sediments; in fact they are the main constituent in many cases as you'll see in the lab Fossils are extremely common in marine sediments; in fact they are the main constituent in many cases as you'll see in the lab Organisms are sensitive to their environment - Organisms are sensitive to their environment - live where they are comfortable - live where they are comfortable - you don't find polar bears in Florida of alligators in Maine. you don't find polar bears in Florida of alligators in Maine. We can use these relationships to determine something about the environment We can use these relationships to determine something about the environment

Biostratigraphy Biostratigraphy: based on appearances and disappearances of species Biostratigraphy: based on appearances and disappearances of species This assumes that EITHER: This assumes that EITHER: species appeared (evolved) or disappeared (became extinct) globally species appeared (evolved) or disappeared (became extinct) globallyor conditions changed locally causing species to migrate conditions changed locally causing species to migrate Evolutionary changes: Evolutionary changes: global global irreversible irreversible allow determination of chronology allow determination of chronology Environmental changes: Environmental changes: local or global local or global reversible reversible provide environmental information provide environmental information

Chronstratigraphy Chronstratigraphy: based on age relations Chronstratigraphy: based on age relations Useful because: Useful because: we can determine rates (including sediment accumulation rate) we can determine rates (including sediment accumulation rate) another dimension for correlation over long distances helps determine sequence of events another dimension for correlation over long distances helps determine sequence of events Compare samples from different areas which don't overlap in time Compare samples from different areas which don't overlap in time Based on the predictable decay of radioactive isotopes: Based on the predictable decay of radioactive isotopes: Uranium Uranium Thorium Thorium Carbon Carbon etc.. etc..

Chronostratigraphy

Chronostratigraphy n Lots of radioactive isotopes can be used for studies of the ocean n choice depends on chemistry and half-life

Stable Isotope Stratigraphy Not radioactive! (“Stable”) Based on the relative abundances of isotopes of: oxygen (18 ‑ 16) carbon (13 ‑ 12) Hydrogen sulfur Oxygen isotopes The ratio is expressed (as are others) as a '  18 O' 18 O __ 18 O 16 O sample 16 O standard  18 O = × O 16 O standard

Oxygen Isotope Stratigraphy 18 O __ 18 O 16 O sample 16 O standard  18 O = × O 16 O standard standard is PDB (PeeDee belemnite (from Cretaceous belemnite from the PeeDee formation of South Carolina) or SMOW (standard mean ocean water) A  18 O value of 2 would mean that the sample was enriched in 18 O relative to SMOW by 2 parts per mil higher values contain more 18 O than the standard, so we call them “heavy”

Oxygen Isotope Stratigraphy The 18 O/ 16 O ratio of marine carbonate microfossils varies as you look down a core The 18 O/ 16 O ratio of marine carbonate microfossils varies as you look down a core Changes are synchronous throughout the world oceans Changes are synchronous throughout the world oceans Both physical and biological processes can lead to fractionation: Both physical and biological processes can lead to fractionation: Two sources of  's: Two sources of  's: 1) temperature at which organism grows (biological effect) 1) temperature at which organism grows (biological effect) 2) ratio in seawater (physical effects) 2) ratio in seawater (physical effects) Evaporation of water (any water) preferentially vaporizes 16 O, concentration the 18 O in the remaining water Evaporation of water (any water) preferentially vaporizes 16 O, concentration the 18 O in the remaining water Freshwater (rain and snow), which is derived from water vapor which has moved inland from ocean areas is enriched in 16 O. Freshwater (rain and snow), which is derived from water vapor which has moved inland from ocean areas is enriched in 16 O. Saltwater, which is left behind, is enriched in 18 O. Saltwater, which is left behind, is enriched in 18 O. Therefore  18 O is an indicator of salinity at the time the fossil grew. Therefore  18 O is an indicator of salinity at the time the fossil grew.

Oxygen Isotope Stratigraphy  18 O as an indicator of global climate  18 O as an indicator of global climate in periods of cold climate, ice accumulates on the continents in periods of cold climate, ice accumulates on the continents this ice is derived from snow this ice is derived from snow During glacial stages,  18 O rises (seawater is enriched in 18 O) During glacial stages,  18 O rises (seawater is enriched in 18 O) During interglacial stage  18 O lowers ( 16 O returns in meltwater) During interglacial stage  18 O lowers ( 16 O returns in meltwater) The 18 O/ 16 O ratio ( 18 O) can also be used as a measure of paleotemperature: The 18 O/ 16 O ratio ( 18 O) can also be used as a measure of paleotemperature: There is a growth temperature effect when organisms take up molecules containing oxygen: There is a growth temperature effect when organisms take up molecules containing oxygen: as seawater temp lowers, 18 O rises in fossil shells as seawater temp lowers, 18 O rises in fossil shells Note that the ice volume and growth temperature effects are reinforcing Note that the ice volume and growth temperature effects are reinforcing (i.e.. cold temperatures make large ice volumes and both effects result in increased 18 O ("heavier" = more positive number) (i.e.. cold temperatures make large ice volumes and both effects result in increased 18 O ("heavier" = more positive number)

 18 O summary cold climatewarm climate cold climatewarm climate more iceless ice more iceless ice seawater has more 18 O seawater has less 18 O seawater has more 18 O seawater has less 18 O  18 O in seawater  18 O in seawater  18 O in seawater  18 O in seawater is less negative is more negative is less negative is more negative (more positive)(less positive) (more positive)(less positive) seawater is "heavy"seawater is "light” seawater is "heavy"seawater is "light” Thus 18 O curves should quite clearly indicate glacial maxima and minimum in sediment cores, particularly in the Pleistocene. Thus 18 O curves should quite clearly indicate glacial maxima and minimum in sediment cores, particularly in the Pleistocene.

Stratigraphy and Sea Level History n The rest of the story on continental shelf sedimentation n Remember we talked about wave energy…….

Continental Shelf Sedimentation n Water depth increases with distance offshore, so: n The energy at the bottom decreases with increasing distance offshore. n Hence grain size diminishes with increasing distance offshore.

Continental Shelf Sedimentation (Cont.) n In the ideal case, grain size would look like this:

Continental Shelf Sedimentation n All of this assumes a “steady state” condition, but this isn’t true! n Sea level has been rising so rapidly over the last 10,000 years that conditions have not had a chance to equilibrate

Continental Shelf Sedimentation (Cont.) n Local changes varied quite a bit from global ones n local subsidence or uplift n local sedimentation / accumulation n Even so, global changes can be determined if you’re diligent

Sea Level History n Radiocarbon dating is almost always involved n These curves are for a variety of samples

Sea Level History n What causes these changes? n We think it has to do has to do with the earth’s orbital parameters:

Continental Shelf Sedimentation (Cont.) n As sea level rises, the coastline (where water and land meet) retreats n This map shows the changes over the last 15,000 years n since the peak of the last ice age n Will this retreat continue??!!

Sea Level History n Sea Level variations control coastal depositional patterns

Continental Shelf Sedimentation (Cont.) n River channels extended onto the shelf during periods of lowered sea level. n Coarse material was deposited on top of fine material. n Deposition and erosion in the deep sea increased. n Coral reefs exposed by reduced sea level died.

Continental Shelf Sedimentation (Cont.) n Actual shelf sediments therefore: n Are coarse-grain sediments on the outer shelf. n They were deposited at an earlier time under different conditions than the surrounding sediments. n They provide evidence of a previous time of lowered sea level.

Continental Shelf Sedimentation n We call these ancient sediments “Relict sediments” n because they reflect conditions in a previous time

Our local environment: Sand

Our local environment: Grain size

Continental Shelf Sedimentation n During the earth’s history: n the positions of the continents have changed n (we already covered this) n plate boundaries were different n climate was different n sea level was often MUCH higher n much of North America was under water

Sea Level History n Tectonic changes have caused even larger variations n Over the millennia, sea level fluctuations have been the rule rather than the exception n rapid fall and slow recovery n up to 300m higher than present n up to 120m lower than present

Summary n Sediment stratigraphy helps unlock the earth’s history n climate n sea level n circulation n evolution n Sea level has changed during earth’s history n substantial fluctuations n caused in part by climate changes n sea level controls n sedimentation patterns n chemistry of seawater