Is It Possible To Retrodict? Fossils as Environmental Indicators

Lecture Slides

• Paleobiogeochemistry: Stable Isotopes
• Oxygen isotopes include ¹⁶O (light 8P, 8N- 99.756%) and ¹⁸O (heavy 8P, 10N --0.205%)
• Isotopes compared:
• Positive & Negative Excursions
• Ratio expressed as ¹⁸O with positive values enriched in ¹⁸O; negative values enriched in ¹⁶O
• Standard is calcite in belemnites from K Pee Dee Fm (PDB);
• Standard for water is SMOW (standard mean ocean water; ¹⁸O_smow = -1 to 0‰)
• Oxygen Isotopes & Temperature
• Urey & Emiliani found 1‰ change corresponded with 4.5C temperature change.
• Measurements made to ±0.5C because precision is 0.1‰.
• Paleotemperatures can only be derived from unaltered calcite shells
• Depletion & Enrichment
• Complicated because when rain sequestered in ice caps, oceans enriched in ¹⁸O by +1.6‰.
• During glacial intervals, ¹⁸O reflects ice volume because ¹⁸O = -50‰ in polar ice
• During interglacial or non-glacial intervals, ¹⁸O reflects temperature
• Stable Isotopes & Salinity
• Only applies when temperatures assumed to be constant
• When freshwater mixes with normal salinity water, ¹⁸O is depressed by ~ -6‰
• Correlation exists between Sr:Ca ratio and salinity in lacustrine ostracod shells
• Carbon Isotopes
• Most carbon is ¹²C (6P, 6N -- 98.89%);
• Heavier ¹³C (6P, 7N - 1.11%).
• Carbon cycle takes element through atmosphere, ocean, continents, carbonate rocks, organic materials, and atmospheric CO₂.
• Positive or Negative Shifts?
• Marine OM is low in ¹³C and has a negative ¹³C value of -20‰
• When decays, releases ¹²C causing ¹³C to decrease.
• Deep ocean OM is ¹²C-enriched, when upwelling occurs, ¹³C values shift negative
• Changes in ¹³C reflect ocean circulation pattern changes and productivity.
• Does the OMZ Play a Role?
• High productivity sequesters ¹²C and causes positive ¹³C values
• When OM settles into the OMZ, ¹²C is depleted and may be locked into deep anoxic waters
• When OM settles into the OMZ and encounters oxygenated waters below, ¹²C is released resulting in deep water negative ¹³C values
• Carbon Isotopes on Land
• Photosynthesis favors incorporation of ¹²C over ¹³C; negative ¹³C value of -25‰
• Isotope system used is determined by photosynthetic system of plant - C₃, C₄, CAM (crassulacean acid metabolism)
• Interpreting Depth
• Many critical limiting factors are related to depth, may restrict organism to bathymetric range
• Parameters include:
• Nutrient supply,
• Light penetration,
• Substrate mobility,
• Rate of sedimentation,
• Temperature,
• Salinity & dissolved oxygen.
• Depth & Assemblage Character
• % planktic vs benthic organisms; ichnofacies
• Algae are definitive for shallow water.
• Molluscan-dominated inner shelf assemblages vs. bryozoan-brachiopod outer shelf (Paleozoic)
• Pelagic organisms - coccoliths, diatoms, radiolarians, etc. - are rare in shallow waters; abundant in outer shelf and bathyal settings
• Diversity & Taphonomy
• Mapping belts of maximum faunal diversity:
• microfaunal diversity just inside shelf edge
• macrofaunal - just below mean wave base
• Taphonomic signature is depth dependent
• mean wave base vs. mean storm base;
• inshore preservation controlled by hydrographic energy;
• offshore essentially in situ and preserved by benthic productivity
• Paleozoic BA's
• BA 1 - beach and upper shoreface 0-5 m
• BA 2 - shore face to fair weather wave-base 5-15 m
• BA 3 - inner shelf, transition zone 15-30 m
• BA 4 - mid-shelf to effective storm-wave base 30-60 m
• BA5 - outer shelf below effective storm-wave base 60-200 m
• BA 6 - > 200 m
• Terrestrial Temperature
• Leaf Physiognomy
• Assume selection of leaf features that confer maximum functional advantage
• Maximize photosynthesis, minimize water loss
• Long-lived angiosperms have high leaf diversity that can be calibrated with environmental factors
• Plant distribution correlates with climate and, hence, their physiognomic character states
• Leaf Margin Analysis
• Bailey & Sinnott (1917) - correlation between dicot character states and warmth
• Higher % untoothed leaf margin taxa, warmer the climate
• Paleodendrology
• Tree rings provide long-term data record detailing:
• Seasonality,
• Annual growing conditions,
• Water availability,
• Limiting temperatures
• Forest production
• Annual Sensitivity
• Tree ring widths vary from season to season
• Annual sensitivity (variation between neighbors) calculated where:
• x = ring width
• t = year number of ring
• Mean Sensitivity
• Mean variability in ring width over a series of rings is calculated where:
• x = ring width
• t = year number of ring
• n = number of rings in sequence
• Stomatal Density & Index
• Fluctuations in pCO₂ affect gas exchange in leaves
• Enrichment in pCO₂ results in stomatal density reduction : 200-340 ppmv
• Standardize stomatal density to epidermal cell density results in stomatal index:
• Developmental Densities
• Stomatal density varies with changes in:
• Temperature - higher T may lead to higher density through leaf expansion
• Soil Water Supply - effects changes in epidermal size and SI
• Irradiance - sun/shade leaf morphologies
• Calculation of SI should be conducted on same area of leaf (mid-lamina)
• Nearest Living Relative
• Fossil taxa used to infer qualitative paleoclimate based upon tolerances of NLR
• Qualitative Indicator Species approach
• Shortcomings include:
• Assumption that NLR reflects similar climate regime
• Provide only qualitative (subtropical, tropical, temperate) assessment
• Taphonomic biases in any terrestrial deposit
• Quantitative Approaches
• NRL's assumed to have same quantitative climate requirements
• Coexistence approach (CA) uses the parameters that are common to all taxa.

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