Lecture 7: The Middle Pleistocene Transition (MPT) and the Intensification of Northern Hemisphere Glaciation (iNHG)

The Middle Pleistocene Transition (MPT) and the Intensification of Northern Hemisphere Glaciation (iNHG)

I. Introduction: Earth's Climate System Evolution

The Earth's climate has undergone significant transformations over the last 2.7 million years, particularly during the Quaternary period. This epoch is instrumental in understanding the cyclical nature of climate changes resulting from the expansion and contraction of large continental ice sheets, predominantly located in the Northern Hemisphere. These fluctuations are fundamental to the concept of glacial-interglacial cycles, which not only define this period but also shape global climate patterns, sea levels, and biogeographical distributions of species.

Key Intervals of Climate Evolution:

• Late Pliocene (approximately 2.7 Ma):

  • Commencement of the Intensification of Northern Hemisphere Glaciation (iNHG), which marked a significant cooling event. This cooling initiated a shift towards more stable and permanent ice sheets in the Northern Hemisphere.

• Early Pleistocene:

  • Introduction of the “41-kyr world,” characterized by glacial cycles averaging approximately 41,000 years, primarily governed by variations in Earth's axial tilt (obliquity). This influenced climate patterns significantly, as the timing of glacial and interglacial periods was closely tied to obliquity changes.

• Middle Pleistocene Transition (MPT):

  • Represents a pivotal shift in the behavior of glacial cycles, including a movement towards longer and more complex cycles, reflecting deeper and more pronounced climatic fluctuations.

• Late Pleistocene:

  • Development of the “100-kyr world,” in which glacial cycles lengthened to approximately 80,000 to 120,000 years, indicating a transition towards more extreme and stable glaciation patterns that extended over substantial portions of the Earth's surface.

II. Intensification of Northern Hemisphere Glaciation (iNHG)

The iNHG, starting around 2.7 million years ago, marks a major transition in Earth's climate characterized by cooler global temperatures and the formation of extensive ice sheets in the Northern Hemisphere. This period also set the stage for profound ecological and geological changes that are critical for understanding contemporary climates.

Characteristics of the iNHG:

• General Cooling Trend: The Earth experienced a notable drop in temperatures during the iNHG period, driving the expansion of ice sheets and influencing global climate patterns. This cooling likely impacted ocean currents, wind patterns, and atmospheric circulation.

• Ice Sheet Dynamics: The iNHG resulted in the establishment of the Laurentide and Fennoscandian ice sheets, which grew in volume and thickness, profoundly modifying local and regional climates. Evidence shows that these ice sheets contributed to lowered sea levels and changed sedimentation patterns in adjacent marine environments.

• Shift in Climate Variability: The shift from a dominant 41-kyr cycle to a quasi-periodicity of 80,000 to 120,000 years denotes a deeper restructuring of climate dynamics. This change indicates that factors beyond simple orbital variations were influencing climate, leading to prolonged cold phases interspersed with shorter interglacials.

Evidence Supporting iNHG:

1. Benthic δ18O Values:

   • Increased δ18O values in benthic foraminifera shells reflect greater global ice volume and cooler oceanic conditions. As more ice forms on land, heavier isotopes are sequestered in ice sheets, leading to a greater proportion of lighter isotopes in ocean water, which correlates with expanded ice coverage.

2. Ice-Rafted Debris (IRD):

   • The prevalence of IRD in North Atlantic sediments is a strong indicator of glacial activity. The debris transported by melting icebergs provides insights into past ice dynamics, showing that significant glacial events occurred around the timing of the iNHG and linking it to regional climatic conditions.

3. Peaks of IRD Deposition:

   • Observations of peak IRD layers align with distinct glacial stages (specifically G6, G4, G2, and MIS 100). These correlations demonstrate the relationship between ice calving events and broader climate shifts, indicating that removals and additions of ice played roles in the climate system’s feedback mechanisms.

Climate State Transition:

The transition represented by the iNHG signifies a shift from a more temperate “Coolhouse” state to an “Icehouse” state, which brought about an increase in the extent and permanence of ice coverage across the Northern Hemisphere. This transition not only altered the physical landscape but also had profound implications for biological ecosystems, as species adapted to the rapidly changing conditions.

III. The "41-kyr World"

The early Pleistocene is characterized as the "41-kyr world", where glacial cycles were predominantly influenced by Earth's obliquity, leading to significant changes in global climate patterns.

Characteristics:

• Glacial Cycle Dynamics: The glacial cycles during this time exhibited a distinct periodicity of approximately 41,000 years. This regularity can be attributed to variations in Earth's axial tilt, affecting the distribution of solar energy and subsequently influencing glaciation.

• Less Intensity and Duration: Glacial periods were generally shorter, less severe, and more symmetrical in their buildup and retreat than those that followed. The gradual nature of ice accumulation and melting led to less dramatic changes in sea level and ecosystem distributions.

• Linear Climate Response: During this time, the climate system displayed a largely linear response to variations in obliquity. This shows that the relationship between Earth's axial tilt and climate changes was straightforward, with direct impacts on the magnitude and duration of glaciations. Ice sheet response to these orbital changes was rapid and predictable.

Impact on Ice Sheets:

• Ice sheets during the "41-kyr world" tended to be smaller and thinner, which made them more susceptible to the cyclical nature of obliquity-related climate changes. This meant that the ice sheets could respond quickly to changes in solar insolation, resulting in more dynamic and actively changing glacial boundaries. These properties shaped the landscape, hydrological systems, and biogeographical distributions of species during this time.

IV. The Middle Pleistocene Transition (MPT)

The Middle Pleistocene Transition is a pivotal phase in Earth’s climatic history characterized by significant changes in the frequency and scale of glacial cycles.

Changes in Glacial Dynamics:

• Lengthening of Glacial Cycles: The MPT resulted in a marked increase in the duration of glacial cycles, which became longer and more complex in their progression. Interglacial periods also expanded, allowing ecosystems to develop and evolve in response to changing climate.

• Increased Ice Volume: A significant increase in ice sheet volume was observed during the MPT, leading to more pronounced cooling trends and extensive ice coverage across the Northern Hemisphere. This expansion influenced global sea levels, resulting in lower sea levels compared to previous periods.

• Asymmetrical Marine Isotopic Stages: The shapes of the marine isotopic stages (mis) transitioned to a saw-tooth pattern, indicating longer, more catastrophic glacial periods followed by slower melting phases. This asymmetry suggests a nonlinear response of the climate system to orbital and internal forcing, reflecting a more intricate interplay of climate feedback mechanisms than in prior epochs.

Implications:

• The MPT represents the transition from the “41-kyr world” to the “100-kyr world,” indicating deeper climatic shifts that reflect the Earth’s evolving climate dynamics. The increased complexity of the climate system indicates a need for refined models to understand these transitions' interactions and consequences.

• The patterns established during the MPT laid the groundwork for the modern climate system, influencing both short-term and long-term climate variability and trends that we continue to observe.

Oversized Ice Sheets:

• The MPT is marked by the growth of oversized ice sheets, particularly in the Northern Hemisphere. This growth is critical because these larger sheets not only act as climate regulators but also significantly alter oceanic currents and global climate systems.

V. Evidence for Oversized Ice Sheets

Understanding the expansion of oversized ice sheets during the MPT requires the synthesis of multiple lines of evidence.

Analytical Techniques:

1. Deconvolution of Benthic δ18O Record:

   • This analysis segmentally separates the δ18O signals into components corresponding to ice volume and temperature. Studies indicate a significant uptick in continental ice volume around 900 ka, providing evidence for substantial climatic changes over the course of the MPT.

2. Heinrich Events:

   • Recognized by layers of IRD in North Atlantic sediments, Heinrich events typically suggest periods of large, unstable ice sheets after the MPT. These events are characterized by major surges of glacial ice, specifically from the Hudson Strait region.

   • Notable characteristics of Heinrich events include:

     • Layers of IRD rich in detrital carbonate indicating substantial iceberg calving and melting.

     • Evidence from geological records supports the notion that these surges are linked to instabilities within the oversized ice sheets.

Notable Heinrich Event:

• The first Heinrich event, occurring around 650 kyrs ago during Marine Isotope Stage 16 (MIS 16), provides concrete evidence for the relationship between ice sheet instability and climate shifts, reinforcing ideas about ice dynamics during the MPT.

VI. Hypotheses for the MPT

Several hypotheses have emerged to explain the mechanisms behind the expansion of ice sheets across the MPT, each offering valuable insights into the processes driving these glacial dynamics.

1. Regolith Hypothesis:

• Proposed by Clark and Pollard (1998), this hypothesis focuses on alterations to the basal boundary conditions of the Laurentide Ice Sheet (LIS).

• Key aspects include:

  • Basal Conditions: Before the MPT, the LIS was underlain by a layer of soft, deformable sediment, known as regolith. This layer reduced the stability of the ice sheet and allowed for easier flow, consequently limiting thickness and resulting in broader coverage.

  • Erosion and Exposure: Over time, glacial erosion removed the regolith, replacing it with the hard bedrock of the Canadian Shield. This exposure led to enhanced stability and the capacity for thicker ice sheets to form and persist over time.

  • Thicker Ice Formation: The transition from a soft to a hard substrate beneath the ice enabled the accumulation of greater ice volumes, contributing to the patterned glacial cycles observed during the MPT.

2. Lowering of Glacial CO2 Concentrations:

• This hypothesis posits that decreasing atmospheric CO2 concentrations during glacial periods could have fostered cooler temperatures conducive to ice sheet development.

  • Interaction with Orbital Forcing: As CO2 levels diminished, this may have amplified the influences of orbital forcing on climate patterns, prompting longer glacial cycles and strengthening the cold phases brought about by orbital changes.

  • Lack of Historical Data: Notably, the ice core record lacks sufficient temporal depth to conclusively determine CO2 fluctuations across the MPT. This introduces uncertainty, necessitating further research to establish definitive connections between CO2 levels and glacial dynamics.

VII. The Role of CO2

The role of CO2 during the MPT is critical in understanding the interplay between greenhouse gases and climate change.

Ice Core Records:

• Currently, ice core records extend back only to about 800 kyrs, which limits our understanding of CO2 concentrations before the MPT. Foundational data reveal significant minima in glacial CO2 concentrations, approximately 190 ppmv during the coldest periods, illustrating how CO2 levels influence glacial conditions.

Scientific Projects:

1. Beyond EPICA Project:

   • This initiative aims to obtain older ice core samples from Antarctica, potentially revealing CO2 levels before the MPT. Preliminary results indicate a continuous climate record extending back at least 1.2 million years, which will be invaluable in establishing historical trends in greenhouse gas levels.

2. COLDEX Project:

   • This project focuses on blue ice areas in Antarctica, potentially containing ice millions of years old. Analysis of this ancient ice can offer snapshots of atmospheric conditions predating the MPT.

VIII. Boron Isotopes as a CO2 Proxy

Boron isotopes have emerged as a vital tool for reconstructing past oceanic and atmospheric conditions, particularly in relation to CO2 levels.

Importance of Boron Isotopes:

• Beyond Direct Measurements: When ice core records are insufficient, boron isotopes provide indirect proxies for assessing historical ocean pH and atmospheric CO2 levels, enhancing our understanding of climatic conditions during the MPT.

Process Overview:

1. Boron in Seawater:

   • In seawater, boron exists as two primary forms: boric acid (B(OH)3) and borate ion (B(OH)4-). The distribution of these forms is influenced by pH levels, reflecting climate-related changes over geologic timescales.

2. Incorporation by Foraminifera:

   • Foraminifera species incorporate borate into their calcium carbonate shells. The resulting boron isotopes in their shells serve as proxies for past pH levels, which can be correlated with atmospheric CO2 concentrations.

3. Paleo-pH Meter:

   • The δ11B of foraminifera shells can reconstruct past oceanic pH. Given that boric acid is enriched in δ11B compared to borate, the isotopic signature reveals significant insights about historical CO2 conditions.

pCO2 Reconstruction:

• Sediment cores analyzed for their equilibrium with atmospheric CO2 provide valuable data for pCO2 reconstruction, facilitating the interpretation of climatic conditions across the MPT.

• Key assumptions underpinning this process include:

  1. Boron Isotope Reflection: The δ11B in foraminifera must reflect the δ11B of borate in seawater.

  2. DIC Parameters: Accurate calculation of CO2 requires knowledge of other dissolved inorganic carbon (DIC) parameters, typically necessitating further measurements of alkalinity or carbonate concentrations.

IX. Numerical Modeling of the MPT

Numerical climate models serve as important tools for simulating the MPT dynamics and advancing our understanding of climate system behavior.

Climate Models Overview:

• Models like CLIMBER-2, which is an Earth system Model of Intermediate Complexity (EMIC), integrate various components representing atmospheric, oceanic, and cryospheric dynamics. This model helps simulate climate processes while minimizing computational expenses.

Forcing Factors in Models:

1. Orbital Forcing:

   • Variations in Earth’s orbital parameters (eccentricity, obliquity, precession) are crucial forcing factors in climatic simulations, affecting insolation patterns.

2. Sediment Masks:

   • Gradual removals of terrestrial sediments by erosion and glacial processes are incorporated into model scenarios, reflecting the regolith hypothesis.

3. CO2 Levels:

   • Models account for a long-term decrease in atmospheric CO2, approximating a 20% reduction over the course of the MPT. Simulations show that varying these factors yields significant agreement with observed benthic δ18O curves, indicating correlation with glacial dynamics.

X. Long-Term Trends and Alternative Views

The MPT is often interpreted as part of a longer trend toward increased glacial intensity; however, alternative views suggest malleable dynamic processes within Earth's climate system.

Gradual Intensification:

• The standard perspective holds that the MPT represents a gradual intensification of glacial activities, observable from 2.7 to 0.65 Ma, during which global climate transitioned increasingly towards colder periods.

Stepped Transition Model:

• An alternative viewpoint emphasizes that this transition to higher glacial intensity occurred in discrete steps, marked by abrupt shifts rather than gradual changes. This model suggests variations in the severity and frequency of interglacial periods over time.

Benthic δ18O Stack Analysis:

• The benthic δ18O stack (LR04) spanning 5.3 million years indicates that much of the Pliocene epoch had δ18O values significantly lower than Holocene values, hinting at warmer conditions and less continental ice volume. The late Pliocene and early Quaternary features a marked increase in δ18O, signaling the intensification of Northern Hemisphere glaciation.

XI. Conclusion

The MPT signifies a complex and profound transition in Earth's climate dynamics, including notable changes in ice sheet behavior, growth patterns, and orbital forcing responses. These developments underscore the intricate interplay of climate feedback mechanisms that characterize Earth's history.

As we continue to explore the significance of the MPT, ongoing research projects like Beyond EPICA and COLDEX, along with advancements in paleoclimate modeling, are crucial for enhancing our understanding of these climate transitions and their long-lasting impacts on global climate patterns. These insights are vital for comprehensively grasping the factors that have shaped, and continue to influence, our ever-changing climate.