EES-205 Lecture 3: Oceanography PDF

Lecture 3: Oceanography Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research, Bhopal  Marine sediment geochronology: a past-time window Sediment core sampling Sediment core cutting...

Lecture 3: Oceanography Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research, Bhopal  Marine sediment geochronology: a past-time window Sediment core sampling Sediment core cutting Sediment core repository By studying the pristine marine sedimentary records of different fossils, minerals, or bulk materials and their compositions, we learn Past sedimentation controls Past oceanic/atmospheric circulation Well established Past seawater chemistry geochronology Past sea level Past climatic, continental and atmosphere conditions  Primordial nuclide 208Pb 207Pb 204Pb 206Pb (stable) (stable) (stable) (stable) Relative isotopic abundance 24.1 % 52.4 % 22.1 % 1.4 %  210Pb dating xs Supported 210Pb: Sediments contain a background level of 210Pb that is “supported” by the decay of 226Ra (radium is an alkaline earth), which is already present in eroded rock sediments. As fast as this background 210Pb is lost by radioactive decay, new 210Pb is created by the decay of 226Ra. Excess or Unsupported 210Pb: Young sediments also include an excess of “unsupported” 210Pb. Decaying 238U in continental rocks generates 222Rn (radon is a gas) some of which escapes into the atmosphere. This 222Rn decays to 210Pb which is efficiently sequestered out of the atmosphere and incorporated into new sediments. This unsupported 210Pb is not replaced as it decays because the radon that produced it is in the atmosphere.  Activity - Definition In order understand how 210Pb is used to determine sedimentation rates we need to understand the activity of a radionuclide in sample Activity (A) is the number of disintegrations in unit time per unit mass (units are decays per unit time per unit mass. For 210Pb the usual units are dpm/g = decays per minute per gram ). It is related to concentration by the relation Where C be the detection coefficient, a value between 0 and 1 which reflects the fraction of the disintegrations are detected (electrically or photographically). Therefore, equations of radioactivity can be written as or Solving the equation - 1 The equation relating activity to the radioactive decay constant Integrating this with the limits of integration set by two points A relationship between age and activity Solving the equation - 2 Substitute in the relationship between age and depth An expression for the sedimentation rate  Pb-210 activity in sediments AB Pb-210 activity A2 A1 Surface mixed layer - bioturbation t1 Measured Pb-210 activity Region of radioactive decay. Excess or unsupported Age of sediments, t Pb-210 activity t2 (measured minus background) For a constant sedimentation rate, S (cm/yr), we can replace the depth axis with a time axis Background Pb-210 levels from decay of Radon in sediments (“supported” Pb-210)  Pb-210 sedimentation rates Plot depth against natural logarithm of Pb-210 activity ln(A) Ignore data in mixed layer λ= 0.0311 yr-1 Depth, z Ignore data with background levels  Limitations Assumption of uniform sedimentation rates. Cannot use this technique where sedimentation rate varies with time (e.g., turbidites). Assumption of uniform initial and background Pb-210 concentrations (reasonable if composition is constant).  210Pbxs coupled with anthropogenic radioisotope tracers 210Pbxs (T1/2=22 years) 137Cs (T =30 years) 1/2 90Sr (T =29 years) 1/2 Thanks