Supplementary information for 2005_03_02845B: Methods
Backstripping and Sediment Budgets
Seismic sections from the Pakistan margin, collected by the German Geological Survey
(BGR) and analyzed by Clift et al. (1,2) were supplemented by newly released industrial data.
The seismic sections were interpreted, and then converted from time to depth scales, using
the stacking velocities from processing. Each profile was then “decompacted”, a method that
removes each dated layer in turn from youngest to oldest, allowing the underlying sediments
to be restored to their original thickness prior to burial and compaction (3,4). In doing so, the
original volumes of sediment deposited between dated intervals can be estimated. The weight
of eroded rock deposited with a single dated interval can be determined by correcting for the
porosity of each unit so that weight of eroded rock can be determined for each sedimentary
package. In this study we used the porosity-depth model of Sclater & Christie (4) and the
lithologies described from Indus Marine A-1 to calculate sedimentary volumes. Uncertainties
in the sediment budgets are estimated to be <20%, largely caused by lateral variations in the
seismic velocity of sedimentary layers, thus making a more accurate time-depth conversion
difficult. Dates are derived by correlation from industrial borehole Indus Marine A-1, based
on cuttings (5). As a result the age picks could be in error by ~10 m, which is insignificant
compared to the total thicknesses being decompacted. A mass accumulation budget for the
entire Indus Fan is then made by adding together the budgets for the individual seismic
profiles to produce a regional budget, when can then be normalized. By using the estimated
volume of the whole Indus Fan we then calculate total accumulation rates in the Pakistan
offshore region.
Nd isotope analysis
Nd isotopes were measured from powdered whole sediment samples. After dissolution, Nd
was concentrated using standard column extraction techniques, and isotopic compositions
were determined by Finnigan “Neptune” multi-collector inductively coupled plasma mass
spectrometer (MC-ICP-MS) at Woods Hole Oceanographic Institution. All samples were
corrected against La Jolla Nd standard 143Nd/144Nd=0.511847. We calculate the parameter
Nd (6) using 143Nd/144Nd value of 0.512638 for the Chondritic Uniform Reservoir (CHUR
(7)).
References cited
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and the history of sedimentation in the Arabian Sea. Mar. Geophys. Res. 23, 223–245 (2002).
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Lambiase, J.J.) 33–56 (Geological Society of London Special Publication 80, 1995).
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Geophys. Res. Letts. 3, 249–252 (1976).
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Archean metasediments and metavolcanics from west Greenland and their implication for the
earth’s early history. Earth Planet. Sci. Letts. 62, 263–272 (1983).