Determining TDI or PMI from Skeletal Remains

• Postmortem Interval

  TDI: Time-Death Interval, PMI: Postmortem Interval =Length of time between death and discovery

  Applications of PMI in Forensic Investigations:

  •Aid in identification

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  •Aid in intelligence

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  •Aid in investigation
• Methods

  -Insects

  -Carbon dating

  -Decomposition/ADD/total body score

  -Others: differing depending on the circumstances (e.g fresh versus skeletonised)
• OUTLINE

  -Fresh

  -Bloat

  -Active decay

  -Advanced decay, and

  -Dry/remains

  The general stages of decomposition are coupled with two stages of chemical decomposition: autolysis and putrefaction.

  
• Methods for determining PMI in early decomposition stages

  Early period after death– methods employed by the forensic pathologist

  •Vitreous humour of the eye (potassium concentrations rise predictably after death)

  •Algor mortis (body cooling-most useful in 24hours after death - based on 'normal' initial body temp)

  •Livor mortice (settling of blood due to gravity)

  •Rigor mortis (stiffening of the muscles depleted of adenosine triphosphate (ATP) - commenced, stays, passes off, )

  •Last meal (light meal= 1~3 hrs)

  •Entomology – tree/root rings - forensic marine contexts

  
• Methods for estimating PMI decomposing and skeletal remains

  •Radiocarbon dating

  •Decomposition/ADD/total body score

  •Insects (more in Entomology lecture)

  •Pollen analysis

  •Botany

  •Biochemical methods

  
• Pollen, Botany, Biochemicals methods…

  Biochemical methods: changes in body chemistry after death

  •Blood pH ↓due to accumulation metabolites (early post-mortem period)

  •Electrolytes (concentration of sodium, potassium, calcium, etc over time)

  •Degradation of RNA 

  
• Radiocarbon dating

  •Radiocarbon dating or C-14 dating used PMI for living organisms absorb carbon-14

  •C-14 is unstable radioactive isotope constantly formed in the upper atmosphere by interaction of cosmic radiation with nitrogen 14 atoms

  •C-14 is rapidly oxidised in air to form Carbon dioxide CO2 in the atmosphere

  •Plants and animals take up C-14 from the atmosphere through CO2 throughout lifetime

  
• Radiocarbon dating

  •While living…the organism is in equilibrium with the C-14 levels in their environment (and C12, C13…)

  •After death, plant or animal stop assimilating C-14 into its being

  •C-14 quantity progressively decays back into nitrogen-14 at a known rate, called a half-life….

  
• Carbon 14 decay

  •Decay is constant it can be used as a “clock” to measure elapsed time – compared to the calibration curve (built up over time using C-14 measurements collected from materials of known age and other resources such as tree-ring data)

  •Decays to Nitrogen 14 –half life 5,730 years (the time taken for an amount of a radioactive isotope to decay to half its original value)

  •  Can be used to date approx. up to 50,000 years old

  •Archaeological or forensic significance (Estimate year of death)

  
• Forensic Carbon Dating: Bomb Peak and Curve

  •1955 -1963 saw a double of C-14 in the atmosphere due to nuclear testing but THEN stopped by the nuclear test ban treaty

  •  Past 60 years shows an increase and then decrease of C14 level

  BOMB PULSE DATING to determine archaeological or forensic remains:

  •THUS, if the C-14 level from human remains is above environmental levels seen in 1950, it can be estimated that the person died after 1955 when the nuclear testing commenced -> forensic significance!

  
• Forensic Carbon Dating: Bomb Peak and Curve

  •Relies on the fact that with C14 levels dropping, the amount of radiocarbon in people depends when their tissues were formed.

  -Requires knowledge of tissue formation rates.

  •  Can be used to tell year of birth from tooth enamel (for those born after 1965).

  •  Soft tissue needed to predict year of death, while trabecular bone (they absorbs C-14 up until death).

  
• Some facts to consider…

  •Cost $$$

  •  Levels now dropping as C-14 soaked up by oceans, etc…

  •Will only be useful for next 10/20 years while levels are still returning to normal.

  -Tissues like hair/nails more accurate as shorter turnover

  -Collagen on bone can be used (consider inaccurate estimations within 3 years to several decades) due to the slow and slightly unpredictable turnover rate of bone

  
• Decomposition and ADD

  •Looks at the state of decomposition of the body and assigns scores to it resulting in a total decomposition/body score (TBS)

  •ADD-accumulated degree days (T° is key)

  •TBS with the ADD are used to give a time since death interval (TSDI/PMI)

  
• Decomposition stages

  Gross morphological Stages of Decomposition

  •Fresh

  •Initial Decay - Bloating

  •Putrefaction-Black putrefaction

  •Butyric fermentation

  •Dry Decay

  Megyesi et al. (2005) Stages of Decomposition- visual analysis of BROAD changes

  A.Fresh

  B.Early decomposition

  C.Advanced Decomposition

  D.Skeletonization
• Total Body Score

  Not all stages of decomposition apply equally to all parts of the Body – Differential decomposition

  •Take each of these

  decompositional changes

  •Place them in order

  •Assign scores to them

  •Score different areas differently due to the decompositional changes that they undergo

  
• Megyesi et al. (2005) Criteria for Scoring Human Decomposition

  
• ADD-Accumulated degree days

  Then, to correlate the condition of the remains in terms of soft tissue lost to decomposition to ADDs

  •ADD ----Originally a measurement of thermal units required for growth and development of insects: it is based on 24 periods of time (days) BUT can be calculated as ADH(hours)

  •Insects require a certain ambient temperature to develop -> If temp too high or too low, development stops

  This principle also applied to body cells and thus to the stages of decomposition
• ADD-Accumulated degree days

  •Calculation of ADD= Take max and minimum temp for day, add together and divide by 2 = average Temp for each day (DD)

  •For decomposition ADD-the threshold is the minimum temperature is considered to be 0 degrees (though no one has proved that this is correct)

  •One degree day is accumulated for each 24-hour period where the temperature is just ~1 degree above the ‘threshold’
• Accumulating an ADD of 100 can be done in the following ways:

  
• Relationship between ADD and TBS

  TDI will be calculated taking the local average daily temperature, then degree-days would then be added together, working backwards in time from the day of discovery of the remains until the accumulated sum equaled the obtained ADD

  The date of death for the individual would be the day that y ADD is reached

  Decomposition is best modelled as dependent on accumulated temperature, not just time.