Lecture 23 - Lexington Polar Forests and Plate Tectonics Study Guide

The Race to the South Pole: - In the early 20th century, two primary teams competed to reach the South Pole: - Team Amundsen (1911): Led by Roald Amundsen. - Team Scott (1912): Led by Robert Falcon Scott (Terra Nova Expedition). - During these expeditions, fossils such as Glossopteris indica were discovered, providing early evidence of past polar vegetation.
The Significance of Glossopteris: - These are Permian glossopterids, a group of seed ferns that dominated southern Gondwana from the early to late Permian. - Physical Characteristics: - These were deciduous trees with wood anatomy similar to modern conifers. - They possessed tongue-shaped leaves characterized by net-like venation. - Reproductive structures were attached to highly modified leaves. - Fossil patterns across continents served as the foundation for the Snider-Pellegrini–Wegener fossil map, illustrating how these plants linked separate landmasses.

Continental Drift (1915): - Proposed by Alfred Wegener (1880-1930). - The theory suggests that continents were once joined and have since drifted apart.
Mechanisms of Plate Tectonics: - Oceanic Crust Formation (A): Formed continuously at mid-oceanic ridges. - Continental Displacement (B): Continental crust is pushed around due to seafloor spreading. - Subduction (C): Oceanic crust disappears elsewhere through subduction.
Mantle Convection: - Material is heated deep in the mantle, rises, and subsequently cools at the surface, driving the movement of tectonic plates.

Geological Periods and Climate: - From the Late Permian to the mid-Paleogene (comprising the Paleocene and Eocene), the Earth experienced a greenhouse climate phase. - This was followed by a subsequent long-term cooling trend.
Equator-to-Pole Temperature Gradients: - Mesozoic to Mid-Eocene (Warm Earth): - Equator temperatures were $2-6^{\circ}\text{C}$ higher than present. - Polar temperatures were $20-60^{\circ}\text{C}$ higher than present. - This period had low equator-to-pole gradients. - Cenozoic Cooling (Transition to Cold Earth): - Characterized by stepwise lower temperatures and decreased $[CO_2]$ . - The planet became more arid with steep equator-to-pole gradients. - Biological Impact: - During warm periods, polar forests were prevalent, and there were fewer biomes globally. - As the planet cooled, grasslands rose, global biomes increased, and polar forests declined.

Cretaceous Biomes ( $70\text{ MA}$ ago): - Alexander Island, Antarctica (mid-Cretaceous): Fossils of angiosperms (flowering plants) have been identified here. - Reconstruction of Alexander Island Flora: Includes cycads, ginkgos, conifers, ferns, and flowering plants.
West Antarctic Upper Cretaceous Rainforest: - Seafloor drilling near the South Pole (approx. $900\text{\,km}$ from the Pole) discovered ancient root networks. - Findings suggest surprisingly high temperatures in the Antarctic during the Upper Cretaceous. - Information was inferred via palynological, geochemical, sedimentological, and organic biomarker data.
Eocene Biomes ( $60-50\text{ MA}$ ago): - Axel Heiberg Flora: Notable for its Eocene Metasequoia (Dawn Redwood) forest.

Antarctica's Last Holdouts: - Today, only two vascular plant species remain in Antarctica: - Colobanthus quitensis (Antarctic pearlwort). - Deschampsia antarctica (Antarctic hair grass).
Arctic's Last Holdouts: - Polar forests no longer exist in the Arctic; the remaining vegetation include species like Salix arctica (Rock Willow).

Light Seasonality: - High latitudes involve extreme seasonality in daylight. - Modern analog: The Emperor Penguin lives almost exclusively between $66^{\circ}$ and $77^{\circ}$ south latitude, dealing with this seasonality.
Deciduousness as Adaptation: - Typically an adaptation to harsh conditions including seasonality in temperature, precipitation (dry-season deciduous), and light.
The Respiration Problem (Ralph Chaney, 1940s): - Warm polar winters presented a metabolic problem for evergreen trees. - High winter temperatures promote high rates of respiration during the dark months when no photosynthesis can occur.

Respiration: - Equation: $Glucose + O_2 \rightarrow CO_2 + H_2O + \text{energy}$ - Gas Exchange: $O_2$ in, $CO_2 + H_2O$ out. - Time: Occurs at all times, day and night.
Photosynthesis: - Equation: $CO_2 + H_2O + \text{sunlight} \rightarrow glucose + O_2$ - Gas Exchange: $CO_2$ in, $O_2$ out. - Time: Requires sunlight; daytime only.

Researchers use specific indicators to determine if fossil plants were deciduous or evergreen: - Fossil Deposits: Leaf mats (suggesting deciduous leaf drop) vs. continuous deposits. - Leaf/Cuticle Thickness: Primarily useful for flowering plants. - Leaf Traces: Examining leaf vascular bundles running through multiple growth rings. - Wood Anatomy: Measuring the percentage of late wood (e.g., Larix is deciduous; Cedrus is evergreen for 3-6 years).
Regional Trends: - Antarctic forests were mainly evergreen. - Arctic forests were a mix of evergreen and deciduous.

Royer et al. (2003) Studies: - Simulated polar climates in growth chambers using species such as Nothofagus cunninghamii, Sequoia sempervirens, Metasequoia glyptostroboides, Taxodium distichum, and Ginkgo biloba. - Variables: - Different photoperiods (e.g., $700$ hours vs. $450$ hours of light). - Carbon dioxide levels at $400\text{\,ppm}$ (colored bars) and $800\text{\,ppm}$ (black bars). - Findings on Nutrient Environments: - Evergreens (Nothofagus, Sequoia): Lived in nutrient-poor environments; did not need to invest heavily in new biomass annually. - Deciduous (Metasequoia, Ginkgo): Lived in nutrient-rich environments; used long polar days to rebuild their entire canopies quickly.

Polar forests existed from the late Paleozoic to the mid-Cenozoic.
They contained both deciduous and evergreen components.
Evergreens were optimized for nutrient-poor sediments and avoided high biomass reinvestment.
Deciduous trees capitalized on high-nutrient sediments and the intense light of the long polar summer to regenerate foliage lost during the dark winters.