MCD (meters composite depth) definition and explanation
The abbreviation "mcd" stands for "meters composite depth". When cores are recovered from the sea floor, a varying quantity of sediment is usually lost between the sections of core recovered. How much depends on the drilling conditions, but it is usually only a fraction of the volume of the sediment recovered, ranging from the equivalent of a few centimeters to a few meters of sediment*.
But for time series analysis etc, we want a complete sequence. So the drillers skillfully attempt to "offset" the core sections drilled, so that the gaps in a sequence in one hole are covered by the sediment intervals in another hole.
Most of this work is done aboard the drill ship at the time the cores are recovered. So the "mcd" represents an idealised composite of the depth below the seabed surface from which samples were recovered, which allows comparisons to be made more easily between the holes. Ideally, for example, 100m mcd in 925A should be equivalent to 100m mcd in 925C, whereas 100m mbsf (the "raw" depth below seabed surface, meters below sea floor) would certainly not be equivalent to the same mbsf for 925C. The development of an "mcd" scale also allows complete sequences of data to be made up from the raw data with gaps in the individual holes. But the offsetting used is quite a crude linear offset that does not allow for differences of accumulation rates between sites or for drilling distortion, so agemodelling can also be important in removing these discrepancies for individual holes.
It is important to note that the equivalence only works for material from the same site, so for example, 100m mcd in 925C is not equivalent to 100m mcd in 926C.
Sometimes an even more refined version of the depth composite, rmcd or "revised composite depth" is created. This is because errors are sometimes detected in the mcd offsets after publication of the shipboard report, but everyone still needs to refer to the published mcd, or else in order to remove more subtle drilling distortions or accumulation rate differences for statistical purposes.
*Just to complicate matters, it can occasionally happen that the same material is re-drilled, so that there is an overlap between successive cores. The end of the drill can be pulled back up through sediment, for example, before continuing down.
Last updated on 02-Dec-08 13:16