CREATING THE CONTEMPORARY PARADIGM OF QUATERNARY CLIMATE/ENVIRONMENTAL CHANGE Deep Ocean Sediment Cores & Ice-Cores

The Pleistocene Glaciations

The Oxygen Isotope Record in Deep Ocean Sediments

Questions: Compare and contrast the information gained from the study of ice cores in Antarctica and Greenland regarding Pleistocene environmental change - Will be something along these lines

The New Paradigm

The Ice-Core Record

Introduction and Background

  • Their records go back >5 million years
  • 1cm of sample = 600 years of elapsed time - i.e. it's a very fine measurement
  • The shells from creatures called foraminifera layered on the ocean floor store Oygen-18 to Oxygen-16 ratios which reveal past climatic conditions
  • 1940/50s:
    • Wrey found radio isotopes in ocean sediment reflect temperature change
    • Investigations began based on Urey's 1947 prediction that as the foraminifera die and their shells fall slowly to the deep ocean-floor to accumulate over time, the gradually thickening layer of remains would preserve a continuous record of changing 16O: 18O ratios of seawater through time
  • 1950/60s:
    • Coring of the deep ocean floors by Emiliani, Imbrie, Shackleton and others revealed the existence of this isotopic record
  • 1970s:
    • Ocean and ice core sampling was strongly developed
    • Cores from the Indian, Atlantic and Pacific Oceans were all obtained and the ratio of oxygen isotopes over time was recorded

Introduction

The Pleistocene Epoch has always been associated with glaciation:

  1. James Hutton (1795):
  • Recognised the evidence pointing towards extensive ice cover
  • Was one of the first to suggest the earth was millions of years old
  • Came up with the idea of Uniformitarianism - Processes we see controlling the landscape today can be used to explain the changes to the landscape over the last millions of years
  1. Charles Lyell:
  • Popularised Hutton's work which was subsequently read by Darwin aboard the Beagle --> understanding of the time frame for evolution
  1. Louis Agassiz (1837):
  • Developed a 'Glacial Theory'
  • Saw the erratics seen in the Alps are identical to the ones seen in Scandinavia and thus ice must have transported rocks across this distance
  • Main contribution: Said earth went through periods of ice age – invoked understanding of earth having cycles – i.e. not being in a constant state
  • Suggested these shifts were responsible for extinctions in biological records
  1. Penck and Bruckner (1909):
  • Research found that generally warm conditions were punctuated by four major cold phases/ice-advances/glaciations

IMPORTANT: Recent work has shown it to be much more complex and dynamic (see below) and to have been characterized by predominantly cold conditions (glacials) punctuated by brief warm interludes (inter-glacials), one of which is occurring at the present time - This is what is meant by the CONTEMPORARY PARADIGM of the Pleistocene

The Contemporary Model:

  • Focuses on repetitive cycles of glacial and interglacial conditions
  • Established what really went on using ocean cores and ice cores
  • NB: It's only DEEP, ABYSAL planes, remote from any land where we can find cores that date back millions of years - Anything under less than 200m of water is subject to processes of sediment supply and erosion due to SL change --> no continuous record

Dating the Past: What do we need?

  • Preserved evidence that can be dated (to a precise age if possible)
  • Need to be able to reconstruct environmental conditions (aka Paleo-Environmental Reconstruction)
  • Need to be able to establish environmental parameters (such as temperature, rainfall etc.)
  • Works best with a continuous sequence

How the Shells Work

  1. When the creatures form their shells, they're made out of sea water and food - the oxygen is in the sea water
  2. This combined to form CaCO2 which is the shell composition
  3. Within the water, there is O-18 and O-16 and whatever ratio is in the water at the time of shell formation is maintained in the solid shell - it doesn't change
  4. The ratio of these two types of oxygen then acts as an environmental proxy

The Oxygen Ratio Riebeek (2005)

  • 16-O:
    • Makes up 99.8% of all oxygen molecules on earth
    • Forms 'ordinary' water which has an atomic mass of 18
  • 18-O:
    • Makes up just 0.2% of all oxygen molecules
    • Forms 'heavier' water which has an atomic mass of 20
  • Also forms 'heavy' water (rare) with an atomic mass of 20 or 22 depending on the oxygen atom involved

How it acts as a proxy: (Dessler, 2012; Mannion, 1999)

  • Due to their heavier mass, 'heavier' water requires MORE ENERGY TO EVAPORATE
  • It therefore doesn't participate so much in the hydrological cycle
  • During cold climates:
    • The return pathway of the hydrological cycle is disrupted due to the development of land ice
    • Less and less ‘heavier water’ is evaporated from the oceans to fall as snow and then become trapped as ice - often condenses over the sea before reaching ice (fractionation)
    • This leads to the increasing accumulation of 16-O in glacial ice-sheets, and thus to the decline in 16-O concentrations in the Oceans
    • The ratio of 16-O:18-0 in the sea water thus decreases
  • During interglacial climates:
    • The return of melt-waters rich in 16-O to the oceans has the effect of ‘diluting’ the oceans once again, so that the proportion of ‘heavier water’ diminishes
  • These ratios are measured using the Uranium Series against a standard (derived from the Pee Dee Formation in North Carolina), and it is the deviation from this standard that is recorded - this process is known as Ocean Isotope Stratigraphy
  • Thus:
    • Lower amounts of 16-O compared to 18-O indicate COLD TEMPERATURES
    • Higher amounts of 16-O compared to 18-O indicate WARM TEMPERATURES

Notes

  • Mannion (1997) also notes that winds carry 16-O enriched water vapour from the oceans to the poles where it eventually becomes trapped in ice, creating a mirror image of ocean sediments
  • Pattern of fluctuations varied slightly from core to core due to variations in deposition rate, sampling frequency and absolute dates, which is not surprising as sediment accumulation rates are about 1cm/600 years
  • This issue is overcome via 'stacking', i.e. taking cores from different locations and matching them together
  • Whilst giving information on water temperatures, it gives us little information on air temperatures
  • As the 16-O:18-O ratio is a proxy for the volume of terrestrial ice, it's therefore also a SL change proxy
  • Much wider coverage than ice cores
  • Clasts (aka glacial drop-stones/ice-rafted debris IRD) in cores tell us about spread of ice-bergs – important identification of Heinrich events
  • Magnesium in cores: Calcium can also be used to get better temperature picture (less magnesium = colder)

What did the Ocean Core records show?

  • Produced a long and clearly defined record of climate change that covers not merely the entire Quaternary, but extending back >5 million years and further (Lisiecki and Raymo, 2005)
  • Revealed a pattern of regular changes:
    • PRESENT-900,000BP: 100,000 year periodicity for the full glacial/inter-glacial cycle
    • 2.75M BP - 900,000BP: 41,000 year periodicity,
    • PRE-2.75M BP: Ruddiman et al (1986) showed small amplitude cycles of 23,000 year periodicity
  • The amplitude of the cycles has been increasing
    • increased once the 41kyr cycle started
    • again when the 100kyr cycle began
  • These cycles are all then superimposed onto a general cooling trend
  • Pre 2.75Myr BP: The cycles indicate a general cooling-warming cycle
  • Post 2.75Myr BP: The onset of interglacial-glacial cycles occurred
    • Around 450Kyr BP, there's a clearly defined change where the amplitude of the cycles increases - this is the Early Mid Bruhnes Event
    • The amplitude is mainly due to increased warmth of the interglacials

Introduction:

  • Records go back around 800Kyrs (possibly 1.5Myrs at some point) (Fisher et al, 2013)
  • Higher resolution than sediment cores - can see seasonal changes in Greenland (1cm sample = 1-1.5 years)
    - NB: Whilst the Deep Ocean Cores GO BACK FURTHER, ice cores show more detail and thus show the SUB-MILANKOVITCH effects
    - NB: The Greenland cores show the effects of the proglacial outburst floods much better than the Antarctic
  • Records of GHGs are contained within air bubbles in ice - don't get this in deep ocean cores
  • Temperature proxy uses the ratios of deuterium in the ice
  • Geographically limited to areas with permanent ice cover
  • Can give detailed information down to the decade or even yearly level
  • Dust contained in air bubbles gives records of volcanic activity
  • NB: Ice age is greater than the gas age stored in the cores: When ice is initially formed, it is not very dense (lack of pressure on top) --> gas can still diffuse through ice until a point where pressure from weight of new ice seals the gas into pockets
  • NOTE: Overlaying the oxygen records from the deep sea sediment cores and the ice cores from antarctica gives remarkably similar patterns given the differences in their accumulation processes (Burroughs, 2007)

Northern Greenland Cores (e.g. GRIP)

Antarctica Cores (e.g. Vostok)

  • A number of cores have been taken - originally to explore the subglacial lakes detected by seismic sensing
  • Vostok was originally rejected due to contamination (2012)
  • Lake Whillans (2013) was the first accepted study and showed 4000 species of microbial life in the lake - trapped beneath ice for millions of years
  • 1991 analysis of the Vostok ice core gave the first continuous record through a full glacial-interglacial - see Drewry et al (1993)
  • The analysis was based on the 2-H:1-H ratio to get the temperature (2-H decrease leads to temperature decrease)
  • NB: The amount of years recorded per metre increase with depth due to the effects of pressure
  • 1999 analysis:
    • Showed a 420,000 year continuous record
    • Subsequently extended to 750,000 years
    • Revealed clear glacial-interglacial cycles with a periodicity of 100,000 years
    • The dust concentrations also are seen to be greatest at the glacial peaks: wind-blown or volcanic, the assumption in both cases is that glacial periods are more arid - there's colder air --> less rainfall --> deserts expand --> increased loess (i.e. wind-blown silt)
  • Further examination of Vostok cores shows each glacial phase is marked by a prolonged period of climatic deterioration with progressive ice accumulation at high latitudes and altitudes (lasting around 85,000 years)
    • In the Northern Hemisphere: This would lead to accumulation of extensive and thick ice sheets on land (Strahler and Strahler (1996)
      • E.g. the Last Glacial Maximum 22-20,000 BP which covered most of Northern Europe in a thick layer of ice
      • The progressive cooling period was NOT constant - it was interrupted by warm phases --> advancement in pulses:
        • Stadial: Glacial Advance
        • Inter-Stadial: Glacial Retreat
    • There is then usually an abrupt retreat after the maximum (5-8000 years), leading to an inter-glacial lasting around 5-10,000 years)
      • This pattern decreases in intensity and complexity towards the tropics where the main effect during glaciations was increased aridity and some loss of forested land - graphs from CLIMAP (1981) show that the least affected areas in glacials are the tropics - stayed free of sea ice during the LGM
      • E.g. Adams et al 1990: Show differences in land cover between the LGM and the Holocene
        • Tropical Forest cover: 11.9 compared to 26.1 (x10^6km^2)
        • Temperate Forest cover: 7.1 compared to 33.7
        • Savanna and Scrub cover: 44.0 compared to 31.1
      • E.g. Goudie (1992) shows maps showing sand dune areas between today and the LGM: The LGM has considerably more in the Sahara, Chile, Australia and central Asia
  • In the Southern Hemisphere: Glacial ice accumulation was limited to Antarctica and the southern Andes, with southern continents experiencing increased aridity

What was the new Paradigm?

  • Work on deep ocean cores (extensive by the late 1970s) built a >3Myr chronology and along with other studies of the Pleistocene, revealed a radically new interpretation of Pleistocene climate change in which ice-sheets repeatedly advanced and retreated on numerous occasions.
  • Ice core analysis in the early 1990s provided much needed detail: the striking similarity between the ice-core and oxygen isotope records confirmed the existence of repetitive, Global climatic change extending back over the past 2.75 million years, superimposed on a progressive cooling of the Earth
  • During the past 900,000 year period, it appears to have taken about 80,000 – 85,000 years to grow each ice-sheet to its full extent and less than 10,000 years for each one to melt, resulting in a brief ‘Warm House’ inter-glacial of up to 10,000 years
    • When this repetitive model was first established in the 1970s (Hays et.al.1976), it was recognised that a new glaciation could occur in the not too distant future, as the present inter-glacial had already lasted for c10,000 years. This partly explains why the recognition of Global Warming as a threat was somewhat delayed

What were the impacts?

  • Each of the recent marked glacial phases resulted in significant reductions in global temperature (by at least 5°C and possibly up to 8°C) and in changed patterns of temperature and precipitation (each cold phase appears characterised by increased aridity, including near the ice-sheet margins). As a consequence, there were repeated major shifts in the climatic belts and sea-water temperatures, so that the deserts expanded and the TRF shrank in extent during “glacials”, with the reverse occurring during “inter-glacials”
  • Global sea-levels also fluctuated greatly, often falling by 120 – 140 metres during glacial phases, so that shorelines were repeatedly abandoned as coastlines shifted great distances and rivers extended across the exposed portions of continental shelves
  • During each deglaciation phase, huge quantities of meltwater formed strings of major temporary lakes (‘Proglacial Lakes’ if dammed by the stagnant masses of ice) in Europe and especially in North America, because former river valleys could not operate because they were either filled with debris or blocked by ice. Every now and then the retreating ice would reveal a new escape route and huge surges of flood water would escape to the oceans.
    • The most dramatic case is ‘Lake Missoula’ which repeatedly formed in the Rockies of NW USA and produced a series of cataclysmic floods in the Columbia River system over the period 18,000 – 14,000BP (Bell and Walker) that resulted in the creation of the heavily eroded ‘scablands’ of Washington State
    • The huge scale of the features led to Baker and Bunker's (1985) interpretation being rejected as ‘unbelievable’ for over a decade
  • The ever-changing conditions of the Pleistocene and especially the increasing scale of the climatic oscillations, placed the biosphere under stress and contributed to the extinction of many species, including megafauna such as the various mammoths (last known found on Rangel Island, dated to 4,000 BP), sabre-toothed cats and the megaloceros
    • Humans may have contributed to their downfall, but only during the last 20,000 years.

What's left to explain?


As there is no known mechanism within the Geosystem capable of producing the regular, large amplitude fluctuations in climate that is recorded by both ice-cores and the oxygen isotope record, scientists were forced to consider the effects of extra-terrestrial mechanisms, including the long known, but generally dismissed, variations in the Orbital Behaviour of the Earth.

Petit et al (1999) Paper

The recent completion of drilling at Vostok station in East Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial–interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.

Glacial–interglacial climate changes are documented by complementary climate records1,2 largely derived from deep sea sediments, continental deposits of flora, fauna and loess, and ice cores. These studies have documented the wide range of climate variability on Earth. They have shown that much of the variability occurs with periodicities corresponding to that of the precession, obliquity and eccentricity of the Earth’s orbit1,3. But understanding how the climate system responds to this initial orbital forcing is still an important issue in palaeoclimatology, in particular for the generally strong 100,000-year (100-kyr) cycle.
Ice cores give access to palaeoclimate series that includes local temperature and precipitation rate, moisture source conditions, wind strength and aerosol fluxes of marine, volcanic, terrestrial, cosmogenic and anthropogenic origin. They are also unique with their entrapped air inclusions in providing direct records of past changes in atmospheric trace-gas composition. The ice-drilling project undertaken in the framework of a long-term collaboration between Russia, the United States and France at the Russian Vostok station in East Antarctica (78 S, 106 E, elevation 3,488 m, mean temperature −55 C) has already provided a wealth of such infor- mation for the past two glacial–interglacial cycles4–13. Glacial periods in Antarctica are characterized by much colder temperatures, reduced precipitation and more vigorous large-scale atmospheric circulation. There is a close correlation between Antarctic temperature and atmospheric concentrations of CO2 and CH4 (refs 5, 9). This discovery suggests that greenhouse gases are important as amplifiers of the initial orbital forcing and may have significantly contributed to the glacial–interglacial changes14–16. The Vostok ice cores were also used to infer an empirical estimate of the sensitivity of global climate to future anthropogenic increases of greenhouse- gas concentrations15.

Two strong amplifiers, greenhouse gases acting first, then degla- ciation and ice-albedo feedback. Our data suggest a significant role of the Southern Ocean in regulating the long-term changes of atmospheric CO2.