Lecture 5 Part 1
Evidence from the Past: Four Pillars of Reconstructing Past Life and Environments
The big question: how do we know about past life and environments when there’s no time machine? We rely on proxies and multiple lines of evidence.
Four main categories of evidence discussed: geomorphology, sedimentology, paleoecology (paleontology), and geochemistry.
Why study the past? Curiosity, understanding origins, and improving predictions of future changes. The present distribution of life is not fixed; nature is in flux and past movements help predict future shifts.
Plan of the course segment: this lecture focuses on how we know about past drivers; next week covers actual impacts.
Geomorphology: Landscapes tell climate and process histories
Geomorphology uses landforms and landscapes to infer past processes and climatic conditions.
Evidence you can observe in landscapes: sea level changes, past glacier extents, size of river systems, depths of lakes.
Valley shapes as indicators: v-shaped valleys often indicate non-glaciated river erosion; glaciated landscapes tend to produce more rounded valleys due to ice movement.
Glaciers as powerful landscape engineers: glaciers are extremely effective at reshaping landscapes, acting like very slow-moving rivers under gravity; ice at the bottom scars the surface as it drags rocks along (striations).
Sea level and lake depth indicators onshore: elevated beach platforms along coastlines and lake shores indicate past higher water levels; relict beaches can be found in places like North Australia.
Subsurface signals: geomorphology can reveal subsurface changes such as faulting and uplifting (example: Grand Teton region with a flat-lying block that dropped along a fault, creating a sharp escarpment).
Periglacial geomorphology and block deposits: not direct glaciers, but frost-shattering and freeze–thaw cycles create block deposits and scree slopes; mapping these deposits helps reconstruct climate bounds.
Diagrams you may have seen: modern glacial extent vs last glacial maximum extents and periglacial bands; these show how climate limits glacier behavior and related landforms varied over time.
Example study: Slay and Schilmeister mapped block deposits and scree slopes around Australia to reconstruct where freeze–thaw cycles occurred, informing past climate boundaries.
Practical takeaway: geomorphology provides broader clues about past climates, ice cover, and landscape-change processes that leave durable records in landforms.
Sedimentology: Depositional environments as records of climate and ecology
Sedimentology examines dirt, sediment sizes, textures, and organic content to infer depositional environments and the conditions under which sediments were laid down.
Modern analogs: studying contemporary sediments helps interpret fossil beds and older deposits.
Alberta Dinosaur Museum example: the back-view shows a mix of finer and coarser sediments with varying organic material; these sedimentary packages contain clues about past environments.
Sediment characteristics to interpret past environments: grain size distribution, layering (interbedding), organic content, and sedimentary structures.
Common sedimentary environments and their deposits: each environment leaves characteristic sediment packages; by recognizing these, we infer the past setting.
Coastal shoreface succession example: in a prograding, fine-grained, low-energy coast, sediments successively show an interpretable vertical sequence as the shoreline moves outward in time.
The shore-phase progression (from inner shelf to back shore) in a progradational sequence:
Inner shelf: fine-grained shales; heavily biodegraded sediments.
Occasional sand layers indicating higher-energy events (floods or storms).
Transition to interbedded fine sand and shale with hummocky cross-stratification indicating storm-driven reworking.
Upward in the section: coarser materials as you approach the shore face, indicating higher energy deposition.
Eventually leading to beach deposits, and if shoreline continues to advance, to terrestrial-type deposits.
Interpretation takeaway: a vertical sequence in a single location can record multiple depositional environments over time, revealing past shoreline dynamics and climate-related processes.
Paleontology and Paleoecology: Fossils and their ecology reveal past life and environments
Paleontology vs paleoecology: study of remains (body fossils) and traces (trace fossils) of past life.
Body fossils: remains of organisms themselves (bones, shells, leaves, etc.).
Trace fossils: evidence of organisms’ activities (footprints, burrows, feeding marks).
Important caution: time-frames matter — dinosaurs and humans never coexisted. The cartoon example illustrates a body fossil (a human) and a trace fossil (a footprint) from different taxa and times.
Example of “death march” fossils (special case): a horseshoe crab left a final trackway after dying in an anoxic lake, providing a trace fossil that records behavior just before death.
Fossil-based inferences about environment:
Plant fossils: leaf size, shape, and margin types correlate with climate; leaf morphology and growth strategies reflect moisture, temperature, and seasonality.
Animal adaptations: limb length and tooth morphology track shifts in ecosystem structure (e.g., forest to grassland transitions affect mobility and feeding strategies).
Paleobotany and pollen studies: pollen and plant macrofossils provide qualitative records of vegetation and climate over time.
Example: Pacific Northwest/Eocene pollen sequences from the Okanagan Highlands show shifts among microthermal, mesothermal, and megathermal taxa, indicating climate changes.
Qualitative proxies illustrate relative changes (warmer vs cooler) without giving exact temperatures.
Qualitative versus quantitative proxies:
Qualitative proxies: provide relative changes (e.g., more cold-adapted taxa vs warm-adapted taxa).
Quantitative proxies: require calibration to convert to numerical values (e.g., mean annual temperature estimates) using modern analogs.
Geochemical proxies briefly touched: chemistry of fossils and sediments can provide past conditions (e.g., isotopic evidence for temperature).
Example of pollen-based climate inference: by comparing pollen taxa that prefer different temperature regimes, one can infer relative warmth or coldness at a site through time.
Nearest Living Relative (NLR) analysis: a quantitative method to estimate past climate by matching fossil taxa to their closest living relatives and their modern ecological tolerances.
Process: identify an assemblage of fossil plants, determine the modern climate tolerances of their closest living relatives, and infer a mean annual temperature range for the past site.
Software and methods: many specialized tools exist to model these relationships across assemblages and time slices.
Example outcome: climate inferences often yield a mean annual temperature range with an error band (e.g., within a few degrees Celsius) for the studied site; some taxa can fall outside expected ranges and require careful interpretation.
Limitations and caveats of paleoecology proxies:
Not all past species have living relatives today; past species may have different environmental tolerances.
Some living species today have different past environmental preferences; niche evolution complicates direct back-extrapolation.
The further back in time, the higher the uncertainty; multiple lines of evidence are needed to constrain interpretations.
The need to consider multiple taxa and a robust sample size to reduce ambiguity (see minimum sample considerations below).
Practical considerations for sample size and material:
Not all proxies work on all materials; you need suitable material (e.g., multiple plant types rather than a single leaf or two leaves).
A minimum sample size is often discussed in the literature; in the lecturer’s comments there was reference to a minimum (and a casual mention of eight as a potential threshold).
Real-world example reflections: pollen-based climate reconstructions often rely on a mix of qualitative and quantitative approaches, combining modern ecological tolerances with fossil assemblages to derive past temperatures and seasonal patterns.
Geochemistry: Chemistry as a recorder of environmental histories
Geochemistry focuses on the chemical signatures preserved in fossils and sediments to reflect their histories.
A key example highlighted: oxygen isotopes in carbonates as paleothermometers. These isotopic ratios change with temperature and can be used to estimate ancient temperatures.
Geochemical proxies complement the morphological and ecological signals from geomorphology, sedimentology, and paleontology, helping to triangulate past climates and environments.
Proxies and the Challenge of Reconstructing the Past
We rely on proxies because we cannot observe past climates directly.
Proxies can be biological, physical, or chemical records that relate to environmental variables in the modern world and are used to infer past conditions.
Proxies can be qualitative (relative changes) or quantitative (numerical estimates) depending on calibration and data availability.
The world of proxies is diverse: hundreds of paleoenvironmental and paleoclimatic techniques exist because there is no one-size-fits-all approach.
From Qualitative to Quantitative: Calibrating Proxies with Modern Analogues
Calibration is needed to convert proxy signals into numerical climate values.
Modern analogues:
Non-biotic remnants of past microorganisms can be used to calibrate proxies.
Study of contemporary ecological tolerances and environmental requirements of living organisms informs transfer functions that convert proxy data into numerical climate parameters.
Nearest Living Relative (NLR) analysis:
Uses the environmental characteristics of fossil taxa by linking them to their closest living relatives.
By comparing fossil assemblages to modern tolerances, you can estimate past mean annual temperature (MAT) for a site.
Example: fossil pollen assemblages mapped to modern ranges of their living relatives to derive an MAT within a certain confidence range.
Practical example workflow:
Compile assemblage of plant taxa from fossil site across time slices.
For each taxon, identify the modern closest living relative and its known climate tolerances.
Determine the overlapping climate space that would allow all identified taxa to co-exist today; infer the past MAT from that overlap.
Often results are within a modest temperature range (e.g., within a few degrees), though some taxa may extend beyond that range.
Limitations and caveats of NLR and calibration:
Not all fossil taxa have direct modern relatives with identical ecological tolerances.
NLR inferences can be sensitive to the selection of taxa and the quality of modern tolerance data.
Complex assemblages and time-averaging can blur precise environmental conditions.
Example narrative from the lecturer:
A fossil pollen record showed shifts in taxa with different temperature preferences (microthermal, mesothermal, megathermal), allowing qualitative conclusions about warming or cooling trends.
In another example, a Jurassic-Cretaceous transition pollen assemblage suggested wetter and somewhat warmer conditions than today, inferred from the dominance of fern and warm-wet indicators.
The role of sample size and material quality:
The type of material available strongly constrains which proxies can be used.
More taxa and better-preserved material improve confidence; tiny or homogeneous samples reduce interpretive power.
A practical takeaway is to seek multiple plant types and robust samples to constrain past climates effectively.
Practical Takeaways and Connections
Past life and environments are reconstructed by integrating four main evidence streams (geomorphology, sedimentology, paleoecology, geochemistry) with proxies and calibration.
Proxies enable us to translate physical records into climate and environmental variables, but they require careful interpretation and multiple lines of evidence to reduce uncertainty.
The modern world provides essential calibration data: living relatives, contemporary ecosystem tolerances, and current sedimentary processes help interpret fossils and ancient sediments.
There are ethical and practical implications in paleoclimate research, such as how robust inferences inform our understanding of modern climate change and future predictions.
Practical exam-focus reminders:
Be able to categorize evidence into geomorphology, sedimentology, paleoecology, and geochemistry, with representative examples.
Explain what qualitative and quantitative proxies are, and how modern analogues are used to calibrate proxies.
Describe the concept of nearest living relative (NLR) analysis and its application and limitations.
Recognize common depositional environments and their sedimentary signatures (e.g., shoreface, inner shelf, cross-stratification).
Discuss how periglacial features and block deposits inform past climate bounds.
Quick Reference: Key Terms and Concepts
Proxies: biological, physical, or chemical patterns used to infer past environmental conditions.
Geomorphology: study of landforms and landscapes to understand past processes and climates.
Sedimentology: study of sediment deposition to interpret environments and climate histories.
Paleoecology / Paleontology: study of past life and its ecosystems, including body and trace fossils.
Geochemistry: chemical signatures in fossils/sediments, such as isotopes, used to infer past conditions.
Periglacial geomorphology: frost-related landforms and processes adjacent to glaciers.
Block deposits: frost-shattered rock fragments formed by freeze–thaw cycles.
Hummocky cross-stratification: sedimentary structure associated with storm deposition.
Prograding coast: shoreline moving outward due to sediment supply or sea-level fall.
Nearest Living Relative (NLR) analysis: a quantitative proxy calibration method using modern relatives to estimate past climate.
Mean Annual Temperature (MAT): a key climate metric inferred from proxies.
Oxygen isotopes in carbonates: a geochemical proxy for paleotemperature (paleothermometry).
If you’d like, I can tailor these notes to a specific exam focus (e.g., emphasis on proxies, or a comparison of evidence streams) or expand any section with more examples.