Quartenary climate history and dating methods

Question 1

What is meant by the term “climate”?

Answer 1

Climate - Long-term averaged trends in temperature and precipitation.

Question 2

Give three examples of factors that drives climate.

Answer 2

Many factors drive climate, for example the concentration of greenhouse gases in the atmosphere, sun irradiation, clouds/aerosols, tilt of the earth axis and biosphere changes to name a few. Climatic conditions are generally colder nearer the poles and at higher altitudes.

Question 3

How does the climate affect specific species?

Answer 3

All species have individual climate tolerances, which affect which niche they occupy. For mobile organisms, they can move to a new area if the climate changes to one outside of their tolerance window, but for sessile species this is a major problem.

Question 4

If local climate changes, then there are three options, which?

Answer 4

If local climate changes, then there are three options: Move, adapt, or die.

Question 5

What characterizes the climate of the past (paleoclimate)?

Answer 5

The paleoclimate is characterized by clod and warm periods caused by the glacial cycle, this in turn affects the relative sea level a lot, as much water gets bound in glaciers during glacial periods, causing the sea levels to fall a lot. The glaciers also affect the extent of exposed land and can have an impact on oceanic currents that also affect local climate.

Besides the glacial cycle, the concentration of different greenhouse gases have changed a lot in the past, leading to vastly different climates than what we have today.

Question 6

Which are the major greenhouse gases that affect climate?

Answer 6

CH4 (methane), CO2 (carbon dioxide) and S (Sulphur).

Question 7

What four factors affect the concentration of greenhouse gases?

Answer 7

• Volcanism
• Tectonic uplift
• Vegetation
• Release of fossil fuels

Question 8

When was the last glacial maximum (LGM)?

Answer 8

The LGM spanned around 26.5 to 19.0 ka (kilo annum/thousand years ago).

Question 9

How are we able to reconstruct the temperature of the past?

Answer 9

The temperature of the past can be determined by using oxygen-isotope ratios. There are three oxygen isotopes: 16O, 17O, 18O - where the lighter 16O is by far the most abundant ~99.795%. The ratio of 16O to 18O in ice cores can be used as a ’paleo-thermometer’ (up to 10^6 years). The light isotope 16O evaporates more easily, while 18O precipitates more easily and thus precipitate out at lower latitudes (around the equator). 16O on the other hand is enriched in precipitation at higher latitudes. During cold periods overall, glacial ice is enriched in 16O and the ratio of 18O/16O is higher in ocean sediments (especially around the equator). During warm periods, the opposite (low ratio of 18O/16O) occur.

So, in seawater (and ocean sediments): Lower18O/16O = warmer; higher18O/16O = colder. Whereas, in polar ice (and meltwater): Lower 18O/16O = colder; higher18O/16O = warmer.

Question 10

It can be hard to bring up sediment cores from the ocean, is there something else that can be used to look at the O18/O16 rations in the ocean?

Answer 10

Low-latitude fossils that contain phosphate or carbonate over deep-timescales e.g. corals, conodonts, brachiopods can be used to test the 18O/16O ratio at lower latitudes and to determine past sea surface temperatures (SSTs).

High resolution records from foraminifera tests derived from marine sediments have a timespan up to 10^7 years.

Question 11

The lowest temperatures post-cambrian occured in the carboniferous period ~350-320 million years ago, what caused this? What consequences did it have?

Answer 11

During the carboniferous, there were a lot of plants and vegetation that were sucking a lot of the CO2 out of the air, causing a lower greenhouse effect and lowered overall temperatures. A lot of the coal and fossil fuels we burn today are derived from this time, from dying plants that were converted into coal.

After this came a lot of larger animals and the temp got warmer again, together with a lot of continental drift that changed ocean currents which affected local climate a lot. Today the temps are pretty cool considering.

Question 12

Was antarctica always covered in ice?

Answer 12

No! It seems like antarctica was green and thriving for a long while, when it was connected to australia and south america - evidence that marsupials evolved there! Then, when these connection were broken off by continental drift around 50 million years ago (early eocene), this caused the southern ocean to completely surround it. This lead to the formation of a strong circular current around antarctica forming, that still remain. This current prevents any warm water from entering, leading to antarctica becoming ice covered and cold like it is today.

Question 13

Explain briefly how continental drift can lead to higher temperatures globally.

Answer 13

With continental drift comes volcanism, which releases many greenhouse gases into the atmosphere. Additionally, new land masses arising exposes more bedrock, which leads to an increased area that can be withered and the erosion releases CO2 which contribute to a higher greenhouse effect and higher temperatures follow.

Question 14

Within what time frame does the quaternary lie and which periods are included?

Answer 14

The quaternary is the period that started 2,6 million years ago until present day. It includes the periods Pleistocene: 2,580,000 to 11,650 years ago and Holocene: 11,650 years ago to present.

Question 15

Describe the climate during the Quatenary in broad terms.

Answer 15

The climate during the Quaternary is characterised by alternating glacial and interglacial periods.

• In the early Pleistocene (2,6 million to about 800 thousand years ago) the fluctuations were short (about 41 000 year periodicity) and had a fairly low amplitude.
• In the mid Pleistocene (800 to 150 thousand years ago) the glacial periods got a lot longer with short interglacials inbetween and of a much higher amplitude (about 100 000 year periodicity).
• The late Pleistocene started at the transition between the end of the penultimate glacial and the beginning of the last interglacial (~130 000 - 115 000 years ago) and ends where holocene starts at the end of younger dryas after the last glacial maximum. The climate was fluctuating (stadials and interstadials) in between.

Question 16

Name the term that collectively describes the cause to cyclical climate change over time, which processes are included?

Answer 16

The cause to climate cyclicity on earth is caused by Milankovitch cycles, which describes three processes that when combined causes cyclicity in climate.

• Axial tilt (aka obliquity): The axial tilt varies or wobbles between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE, which correlates with the beginning of the Holocene, the current geological epoch. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.
• Precession: a gravity-induced, slow, and continuous change in the orientation of an astronomical body’s rotational axis, making it shift in a circle and completing one circuit in approximately 26,000 years. When the earth is furthest from the sun =aphelion, when the earth is closest to the sun=perihelium. Earth spends less time near perihelion and more time near aphelion, so when the northern hemisphere is tilted away from the sun at aphelion = glacial, when the northern hemisphere is tilted towards the sun at aphelion = interglacial.
• Eccentricity: The earth’s orbit around the sun varies in being more circular (low eccentricity) and elliptical (high eccentricity) in a cycle that is ~100,000 years long. When Earth’s orbit becomes more eccentric, the magnitude of seasonal changes increase and vice versa.

https://www-science-org.ezp.sub.su.se/doi/10.1126/science.adp3491 Good paper on the topic.

Question 17

Why is glacial buildup more apparent in the northern hemisphere?

Answer 17

Glacial buildup more apparent in the northern hemisphere because it contains more landmass. You get more glacial buildup on terrestrial continents.

Question 18

What does MIS stand for? They are usually assigned a number, what does it mean when the number is even/odd?

Answer 18

MIS stands for marine isotope stage, even numbers are assigned to glacials and odd numbers are assigned to integlacials. The higher the number, the further back in time.

Question 19

MIS 1, 2, 4, 5 and 11 are highly relevant, name what they stand for.

Answer 19

• MIS1 is the interglacial we are currently in (Holocene)
• MIS 2 is the last glacial maximum (LGM) ~ 23,000 to 19,000 years ago
• MIS 4: last true ice period (beginning of last ice age), between MIS4 and MIS 2 there was a lot of climatic instability = stadial.
• MIS 5 is the last interglacial (Eemian interglacial 130-115 kya) ~4-8 °C warmer than today
• MIS 11 was a superinterglacial (very long): longest and warmest interglacial of the past 500,000 years.

Question 20

Which MIS are included in the last ice age cycle? What is special about it?

Answer 20

The last Ice Age cycle is represented by MIS 5e to 1 (last 120 000 years), we know the most about this period because we have super high resolution for this time, due to close time proximity. During 5e (~120kya) we have recorded that there was peak warmth, even hotter than it is today!

Question 21

Explain the terms: Stadials, interstadials and Heinrich events.

Answer 21

Stadials = short-lived cold periods during glacials.

Interstadials = short-lived warm events during glacials. Also termed: Dansgaard–Oeschger (D-O) events

Heinrich events = short-lived cold-event spikes during glacials. We only have evidence for the latest ones (last 120 k years), but they probably occurred earlier as well, it’s just that the resolution is too low.

Question 22

What problems does glaciation bring forth for people that want to study them today?

Answer 22

The major problem with glaciers is that new ice sheets destroy everything they encounter, this means that only the biggest and latest glaciers leave evidence behind. The evidence of any that are intermediate gets wiped away. So we can basically only see the evidence from the LGM (MIS2) and the penultimate glaciation (MIS 6) >140kya.

Question 23

At the last glacial maximum (LGM) a lot of landmass was exposed, why? what consequence did this have?

Answer 23

At the LGM, much water was bound in glaciers, which caused the sea level to fall way below what it is today. For example Beringia (land between russia and alaska) was exposed, as well as Sundaland (an extension of the Asian continent including indonesia, borneo, Bali, Java and a lot of small islands.

These exposed land masses functioned as highways between continents that are disconnected today, which explains why some species are present on different continents today. For example humans in the Americas early and why modern islands of Sundaland are home to many Asian mammals including elephants, monkeys, apes, tigers, tapirs, and rhinoceros. An important consequence of sea level changes it that much archaeological evidence is probably submerged :( especially human as we like to live near coasts.

Question 24

Why is Beringia so central in aDNA research?

Answer 24

Beringia was not covered by glaciers at the LGM (aDNA not destroyed by glacier), and because its also located at a high latitude it is good for aDNA preservation. Also central to many different species that crossed/lived there –> dispersal corridor.

Question 25

When exiting the last glacial (MIS2) there are two important periods that follow, which and what are they characterized by?

Answer 25

At the end of MIS2 we entered the Bølling-Allerød (B-A) interstadial ~14,7 - 12,9 kya, this period was characterized by an initial spike in temperature that likely caused much of the megafaunal extinctions and then the temp went down again to fluctuate (climatic instability). Then came Younger dryas (YD) stadial which was most prevalent in Greenland and surrounding area, seems like the melting NA glaciers from the higher temps caused disruption in the currents bringing hot water to the north - cold local climate. YD ended ~11,7 kya and a rapid warming occurred = the Pleistocene-to-Holocene transition. This is why it's important to know where your data is from, could be a local change - need several data points to state that a change is global.

Question 26

Why does "holocene" as a concept exist?

Answer 26

Holocene is considered its own period probably because its close in time and where we are now, so we put (unnecessary) importance on it. Also because we have a disproportionate amount of data from this time because of better preservation the younger the age.

Question 27

Holocene can be divided up in three stages, which and what are they characterized by?

Answer 27

Early holocene: gradual increase in temp. Middle: Holocene Thermal Maximum (HTM): 10-5 ka (hotter than it is today). Late: A slight cooling compared to middle (with a spike today).

Question 28

Give two examples of proxies that can be used to study glaciation/deglaciation.

Answer 28

Plants, for example in lake sediments one can see which genera are present at diff times. After glacier melting there are mainly pioneer species and later climax species. animals: Also in lake sediments, for example in lake that was part of the sea when water levels were higher one can see marine species followed by semiaquatic/freshwater and then terrestrial.

Question 29

Just in the last 2000 years we can see temp in REALLY high resolution, what two key periods are associated with short changes?

Answer 29

Around year 900-1200 there was a warm period, it's possible that the vikings thrived because of this = more resources. At around 1400-1800 there was a "little ice age" that also happens to coincide with the dark ages....

Question 30

Why is the current warming so alarming when we know that there has been warmer periods before?

Answer 30

The reason todays warming of the planet is so alarming is because it's happening SO fast. Natural warmer periods are slow and give species time to adapt (although some might die) and thus the impact is not too big. With todays warming we are looking at something that eerily resembles a mass extinction...

Question 31

Food for thought: In the long-term, is ongoing anthropogenically-induced climate change a good thing for biodiversity?

Answer 31

Good question. Think about it!

Question 32

What is important to keep in mind when it comes to dating ancient samples?

Answer 32

It is very important to note that the inferred age of a sample is a hypothesis and not a fact. Therefore it's good to use several dating methods to get the moste accurate estimate possible.

Question 33

What is a "nuclide"?

Answer 33

Each isotope (atoms with the same number of protons, but a different number of neutrons) of an element is called a nuclide. A radionuclide is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable and thus radioactive. A ’mother’ nuclide transforms into a ’daughter’ nuclide during radioactive decay.

Question 34

Explain the basis of radionuclide dating.

Answer 34

The number of transformations, loss of Alpha (α) particles: 2 protons + 2 neutrons and Beta (β) particles: negatively charged electrons, per unit of time is a decay constant (λ). This means that all decay is exponential, and is best considered in terms of half-life. If the original amount of the unstable nuclide is known, then the measured amount in the sample and known decay rate (half-life) can be used to calculate an age.

Question 35

Explain the basis of radiocarbon dating.

Answer 35

- Cosmic rays collide with 14N and produce carbon nuclides (12C, 13C, 14C). - The carbon nuclides are rapidly oxidized to carbon dioxide (CO2), mixed throughout the atmosphere and absorbed by the oceans and living organisms. - Following death and burial, wood, bone, shells, etc., lose 14C as it changes to stable 14N. So, by measuring the amount of carbon in the sample, one can count backwards using the known half life (5730 years). - Radiocarbon activity in a sample is usually measured by accelerator mass spectrometry (AMS), which determines the isotope ratio of 14C relative to stable carbon isotopes (12C, 13C). This ratio is then compared to a pre-1950 standard (before nuclear bomb testing). - After 8 half lives (~45 000 yr BP), only about 4‰ of the original 14C remains. This is the technical lower age limit for 14C dating (45-50 ka), so anything older than that needs to be dated by another method. NOTE: Atmospheric radiocarbon: varies over time... due to both natural and anthropogenic factors, so It is necessary to make a clear distinction between ”radiocarbon years” and ”calendar years”.

Question 36

Radiocarbon ages need to be calibrated to actual ages, how is this done?

Answer 36

The calibration of 14C dating is based on dendrochronology: Tree rings have different thickness/grow at different rates depending on annual conditions (different ratios of favorable/bad), these rings when cut horizontally for a sort of circular barcode, and overlapping segments of older and older trees can then be overlapped to go further and further back in time and build a record. Dendrochronology (Tree ring chronology) goes back about 14,000 calendar years (impressive!) At ages >14,000 14C years, calibration is done mainly using marine datasets. The calibration makes sure variations in cosmic radiation are taken into account.

Question 37

Radiocarbon dating of marine samples (e.g. molluscs) is done to calibrate for samples with ages >14,000. when radiocarbon dating marine samples, correction must be made for the marine ’reservoir’ effect, what is this? How do we account for it?

Answer 37

Much of the carbon consumed by organisms in the ocean is older than that consumed by organisms on land (old 14C with an already started decay history), so samples from marine life and from organisms that consumed a lot of sea-based foods while alive may appear older when tested than they truly are. Typically, affected radiocarbon dates appear around 400 14C years older than they would if unaffected, e.g. in north Atlantic, but the effect is highly variable in space and time, and can reach 800 to 1200 14C years in Arctic regions. So to take this effect into account, we need to subtract the "older" age from the resulting 14C age.

Question 38

What are varves and how can they be useful?

Answer 38

Varves are lake sediment layers that show annual variation, similar to that of tree rings in dendrochronology. The varves of a sample can be counted and can strengthens age depth models.

Question 39

There are three different modes of age-depth modelling, which and how do they work? which one is used today (best practice)?

Answer 39

There are three types of age depth models: Interpolation, Best fitting and Bayesian. - Interpolation: Dating one sample here on different selected layers, and then just drawing lines in between to generate a "curve". Not great for samples that fall in between, highly dependent on the amount of samples dated. - Best fitting: Same initial approach but using statistic modelling to generate the best fitting curve. Better than interpolation but not the best. - Bayesian: Dating more samples from each year/layer, and use statistical modelling. This allow you to see variation and has the highest accuracy.

Question 40

Freshwater lake sediments also carries biases with radiocarbon dating, which, why and what do we do to go around this?

Answer 40

Fresh-water lake sediments: water plants and molluscs can have a too old age due to incorporation of old carbon (lake reservoir effect), similar to marine samples. This can be due to dissolved limestone (hard-water effect, limestone is only "dead" carbon), humic acids from soil and peat or old ground water seepage into lake. Therefore the best way to date lake sediments is to use terrestrial (plant) macrofossils flooded into the basin for dating, and not bulk sediment or aquatic plant macrofossils (if possible)! Also, its always good to use several independent methods for dating, if all are consistent, you can be much more sure of the validity of the date.

Question 41

Give three examples of other absolute dating methods (not radiocarbon dating).

Answer 41

- Lead-210 (210Pb) dating - Uranium-Thorium series - Luminescence dating: Two types: ThermoLuminescence (TL) and Optically stimulated luminescence (OSL) dating. - Electron Spin Resonance (ESR) dating

Question 42

How does Lead-210 dating work?

Answer 42

Lead-210 (210Pb) dating: 210 pb forms from terrestrial radon (226Ra) in the atmosphere, precipitates in rainfall and accumulate in sediments. Then it subsequently decays into stable 206Pb with a half-life of 21.4 yr. This provides a means of dating up to ~150 years back in time (i.e. 5-7 half-lives) which is useful because even though this is covered withing the 14C dating range, there is a "radiocarbon plateau" between 1700-1950 for this method, so this is a good complement to get accurate dating so close in time.

Question 43

How does Uranium-Thorium series dating work? When is it useful?

Answer 43

238U, 235U and 232Th all decay to stable lead (Pb) isotopes through a complex decay series of intermediate nuclides, with different (and very long) half-lives. Daughter deficiency dating works when only the parent isotope is initially present, with no daughter isotope. Over time, the parent decays into the daughter until they reach equilibrium. By measuring how much daughter isotope has formed, scientists can estimate the age—assuming no isotopes have entered or left the sample since formation. For example, in corals or mollusks, which only absorb uranium while alive, protactinium-231 (the daughter of uranium-235) starts accumulating after death. Measuring the amount of 231Pa reveals how long decay has been occurring, and thus the sample’s age.

Question 44

There are two types of luminescence dating, which and how do they work?

Answer 44

Two types of luminescence dating: - ThermoLuminescence (TL) dating: Any sediment that contains uranium (U), thorium (Th), or potassium (K) is continuously bombarded by α, β and γ radiation after burial. This causes ionization in the sediment and the ’trapping’ of metastable electrons within mineral grains (especially quartz and feldspar), which steadily accumulate over time. Such electrons can be experimentally freed by heating to give a characteristic emission of light (electrons escaping from their ’traps’). - Optically stimulated luminescence (OSL): OSL dating measures the last time mineral grains (like quartz or feldspar) were exposed to sunlight or heat. When these grains are buried, natural radiation from the environment causes electrons to become trapped in the crystal structure. Later, in a lab, shining light on the sample releases these trapped electrons, emitting a measurable luminescence signal. The amount of light released tells scientists how long it has been since the grains were last exposed to sunlight—essentially dating when the sediment was last moved or buried.

Question 45

What are the main problems with luminescence dating?

Answer 45

Main: bleaching of sediment grains causes zeroing of the geological clock, so they cannot be exposed to sunlight when sampling or exposed due to natural forces during the time since deposition, so for example glacier proximate sediments could have been altered and thus subject to bleaching later than it's deposition.

Question 46

How does Electron Spin Resonance (ESR) dating work?

Answer 46

ESR dating measures trapped electrons in materials like tooth enamel, shells, or quartz that have been exposed to natural radiation over time. These trapped electrons accumulate in defects within the crystal structure. Using a magnetic field and microwaves, scientists detect the number of trapped electrons based on their "spin" behavior. The more trapped electrons there are, the older the sample is—assuming the radiation dose rate is known. This method is especially useful for dating samples that are too old for radiocarbon dating (up to ~2 million years but practical upper limit is about 500 000 years).

Question 47

Give three examples of correlation based dating methods and explain them briefly.

Answer 47

- Palaeomagnetism: The poles of the earth is moving (magnetic motion) and can "switch" so that the north pole becomes the south and vice versa. Combining this knowledge when looking at alignment of minerals from volcanism for example can give you an idea about the date of the sample. - Biostratigraphy: Using "index taxa": usually small fossils that we know for which time they were present, usually fairly short ranges to narrow down dating. For example if a specific tree with pollen was only present between 60-25 kya, and your sample contains this pollen, you know that the date of the sample lies within that range. - Tephrochronology: a method of age determination that makes use of layers of volcanic ash (tephra), that later turn to glass. Every volcanic eruption spews out tephra that is unique in its composition, and since many major volcanic eruptions have been dated to a very narrow range that make up an established chronology, and different tephra layers can be used to date layers surrounding them. Since the date of the tephra is well known it's best to sample close to it.

Question 48

Explain the terms: Chrons, subchrons and superchrons.

Answer 48

- Major periods of ‘normal’ or ‘reverse’ polarity are called chrons - Subchrons are shorter periods of reversal - ‘Superchrons’ are longer periods of reversal, can last 30-40 million years There have been >1000 magnetic reversals over the past 540 million years!

Question 49

How does molecular tip dating work? When is it useful?

Answer 49

Molecular tip dating is based on the faxt that when something dies, it stops evolving. Comparing an ancient DNA sample with a later or modern counterpart allows us to see how much they differ, and taking mutation rate into consideration, we are able to count backwards to date when the organism lived. Another way to explain it would be to look at the phylogenetic branch of the sample and look at "what's missing" in terms of accumulation of mutations compared to today. In order to perform molecular tip dating, you need a lot of good reference material - so easier for some species (extensively studied) than others. This is a good method to use in absence of other available methods. Denisovans were dated this way - totally out of stratigraphic context!

Question 50

Why is combining different dating methods the best?

Answer 50

When combining methods you can narrow down the time frame for when something is from initially, and also provide more strength to your claim if several independent methods are used.