A study published this week in Frontiers of Ocean Science based on our Ocean Bottom Deposits Collection has shown the value of historical sediment collections to provide a benchmark to assess global changes in the seafloor environment.
Read on to find out how NHM/Southampton PhD student Marina Rillo used the microfossil content of these collections to develop a method to assess their usefulness in providing details of the state of the ocean floor as long as 150 years ago.
Problems with the sediments
Traditionally scientists have been reluctant to use these sediments as many of them were collected using rather crude methods. For example, we have a complete set of sediments from the HMS Challenger Expedition of 1872-1876 that traveled the world sampling the ocean. Collecting records suggest that most of these were collected using a bucket shaped dredge of the ocean floor (see below left).
Unfortunately this historical dredging method cannot guarantee that the samples don’t represent an average over a long period of time; deeper sediments sampled could be from as long ago as previous glacial intervals. Modern coring methods such as those currently employed by the Integrated Ocean Drilling Program are much more precise and we are able to collect the very top slice of core (core tops). However, the modern collecting archive does not go as far back as 150 years ago like the historical sediments in our collections.
The only way to definitively show the age of the sediments is to carbon date individual calcium forming shells in the sediment or to investigate the carbon isotope composition. The isotopic composition can tell us whether the shells were formed in oceanic conditions representing a glacial or non-glacial period. This is expensive and impractical to do for an entire collection.
Tiny microscopic single celled organisms called Foraminifera are floating in the ocean as we speak. They form tiny shells from calcium carbonate in the water column and when they die, the shells float to the bottom of the ocean. These accumulate within the bottom sediments (see diagram to right to dredge image above). Studying the planktonic Foraminifera from these sediment layers can tell us about the age of the layers and the state of the ocean at the time they lived.
Planktonic Foraminifera are ideal as they are very abundant and provide an excellent fossil record. They also have a well established taxonomy so there are relatively few arguments about how to identify modern species. Their isotopic composition can also be used to tell us about past climate conditions.
PhD student Marina studied the planktonic Foraminifera from a carefully selected set of samples from the Ocean Bottom Deposit Collection that covered most of the world’s oceans and major seas.
Comparing large datasets
Fortunately there are some major datasets available that tell us the planktonic foraminiferal assemblages we would expect from those regions based on core top samples using modern methods (ForCenS) and samples from cores that represent the last glacial period (MARGO).
Marina compared these datasets with the assemblages she obtained from the Ocean Bottom Deposit Collection. This was able to tell her that her selected samples were not significantly different in composition from the expected modern day samples. She was also able to show areas where the modern and last glacial assemblages were expected to be very different (red and orange dots on map below).
It’s almost certain that our historical sediment collections will contain some sediments that are an average over a long time period because of the collecting method used. Using the method Marina developed we will be able to focus studies on areas where it is possible to assess this. The Ocean Bottom Deposits Collection is available for studies like these on request.
It should be noted that this collection also contains material from plankton tows derived from the water column so the issues with time averaging are not relevant. This is part of a study in progress by the Museum comparing Challenger plankton tows with modern collected material from the Tara Expedition.
Finally, you can read more about Marina’s work on planktonic Foraminifera from the Ocean Bottom Deposits Collection and see how digitisation of these collections is answering questions on evolution and even helping to train the scientists of the future.