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Rework your interpretations

Many years ago, when the exploration of the Barents Sea was in its early stages, I was examining microfossils in shallow cores taken from the western margin of the shelf. I was particularly interested in organic-walled microplankton found in the core samples. They are only a few tens of microns (1 ⁄ 1000 mm) in size, but there were thousands of them.

Microfossils are very useful when you work with small samples, such as pieces of core. Organic-walled microfossils are resistant to degradation and are often present when calcareous microfossils are dissolved by chemical processes in the sediments. Among the organic-walled microfossils, dinoflagellate cysts are often superior to other fossils because they were usually short-lived in geological terms. Most dinoflagellate taxa underwent relatively rapid evolutionary changes, resulting in a diversity of species characteristic for short periods of the earth’s history. Because of this, they are good biostratigraphic markers and are very valuable tools for dating marine sedimentary rocks.

Looking through the microscope lenses my eyes swept over numerous speci­mens with beautiful names like Areosphaeridium diktyoplokum, Dracodinium varielongitudum, Eatonicysta ursulae, and so on. My job was to find the age of the cored sediments. And I soon found an answer: an age somewhere around Early to Middle Eocene — 48 to 50 million years old. Sample after sample ap­peared to contain more or less the same assemblage of microfossils. Except for the last one: the sample from the deepest part of the core, and therefore the oldest according to basic principles of stratigraphy. Here I found a few younger Pliocene or Pleistocene species. I thought this was most likely contamination from the laboratory when the sample was processed. No, I was quite sure. Forty samples with Early to Middle Eocene species, one sample with some Pliocene– Pleistocene stuff. I made my report.

A few days later I got a call from the project manager. Prepare for a shock, he told me. Datings based on isotopes of argon from layers of volcanic ash in the core gave an age of 2.4 million years. The few Pliocene–Pleistocene specimens were the ones that were actually in situ. The thousands of Eocene specimens were in fact reworked into the much younger sediments.

Today we have a better understanding of the geological history of the western Barents Shelf. During the past three million years the shelf has been subjected to several phases of uplift and erosion. During several major and minor glaci­ations, large quantities of sediments have been eroded from the shelf in the east and north, transported westwards by the ice and water, and then dumped in huge fans along the shelf edge in the west. In the most active periods of erosion, the amount of sediment transported from east to west was equal to 50 000 dump trucks per day! If you carefully study cores from the huge depositional fans along the western margin, you can actually detect a reversed stratigraphy. The older parts of the fans contain the sediments eroded first, and as the younger glaciations dug deeper into the sedimentary succession on the shelf, an increas­ing proportion of older and older fossils were reworked into the succession.

There are lessons to be learned from this story:

  1. Never for a moment believe that you have the full story covered. Geology, palaeontology, and biostratigraphy are combinations of observations and interpretations. Even the most careful observations will only give small and erratic pieces of what was originally there.
  2. Put maximum effort into careful observations. When there is nothing left that has escaped your eyes, the best possible interpretations will automatic­ally follow.
  3. Check and re-check the sample-processing workflows. Are samples properly handled, are they switched, or have they been contaminated? (In my library I have a wonderful paper written by a Chinese palaeontologist in which he described how he mistakenly described a fragment of burned candlewick as a Precambrian microfossil!)
  4. Do not always trust the geochronologist … they may have been caught out by contaminated samples or erroneous measurements.
  5. Remember the famous words of Galadriel in Lord of the Rings: ‘Even the smallest creature can change the course of the future.’

See the big picture

Pemberton’s laws of stratigraphy, part 1

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