March 20, 2006

Funded by the National Science Foundation Office of Polar Programs

Location: Latitude 63° 20.8' S, Longitude 55° 43..3' W

Air Temperature: -0.1°C

How Old Are the Cores?

For all of you avid readers of the SHALDRIL chronicles out there, you may have been wondering how we determine the ages of rocks that we have drilled. In fact, once the cores start to come up from the seafloor, one of the first questions asked is "How old are the cores?" The ages of the sedimentary rocks being recovered by the SHALDRIL project range from the very recent geological past (the last 10,000 years) to far back into "deep time" (millions of years before present). The oldest rocks that we expect in the drilling area are sediments that were deposited roughly 40 million years ago. Since there is no way to determine the age of sediments just by looking at them, the trick is to figure out when during the last 40 million year they were formed. Sands deposited 20 million years ago, for instance, may look very much the same as sand formed 1 million years ago. One would expect very old rocks to be very hard and rigid, but this is often not the case. Geologists, therefore, need to find ways to date the rocks with reasonable precision. This is a very critical aspect to our studies since we are drilling into the seafloor at many different locations, and it's important to know how all of the cores relate to one another in time. If we were to discover an important event or change in the past climate, it's essential to know when it happened!

There are many techniques that geologist use to determine the age of sediment cores. Sometimes it is possible to date rocks using a "radiometric clock," in which the abundance of a certain radioactive element and its daughter products are used to determine the time since a mineral was formed. In sedimentary rocks, this technique can be used on ash layers, but ashes are generally rare in marine sediments. Another dating method relies on the magnetization of sediments. The direction of the Earth's magnetic field has switched direction numerous times over geologic time, and this record of changes in the direction of the Earth's magnetic field is preserved in sediments and can be read much like a barcode. This is a very good method to use with long cores that continuously span many millions of years of time. Another technique, known as biostratigraphy, uses small microscopic fossils that occur in marine sediments. Often these "microfossils" comprise a significant proportion of the sediment, and the occurrence of certain species can be used to determine an age of the sediment. Since this method only requires a microscope, it can easily be used on ship to quickly derive an age estimate.

Several groups of microfossils are used for biostratigraphy, but the use of each group for the age dating of sediments relies on the same set of principles. From other well-dated drill cores, the initial appearances and eventual extinctions of many microfossil species have been documented. These known "ranges" can be then be applied to cores of unknown age, such as those from the SHALDRIL cruise. For example, one microfossil species may have lived during a short interval between 12 and 16 million years ago. If we find this particular species in our samples, then we know that the rock must have been deposited sometime between 12 and 16 million years ago. Going a step further, it is often possible to further narrow down the age by using a combination of different species that are present in a sample. In this case, the age of the rock can be narrowed to the time in geologic history when their ranges all overlap, or when all of the species were living at the same time.

My specialty is with a group of microfossils known as diatoms. Diatoms are single-cell organisms that are photosynthetic and build an ornate skeleton made out of glass. Each diatom cell produces two parts (or valves) that fit together in a way that is similar to a petri dish. Diatoms come in all shapes and sizes, and the skeletons of most species display an elaborate array of perforations and ridges. They are a very impressive example of biological engineering, particularly considering their microscopic size! Typically, diatoms range in size from 5 to 100 microns (there are 1000 microns in a millimeter). Many of the samples that I have examined from the SHALDRIL cores contain diatoms, and I have been able to identify many different species. An image of a diatom found in one the core samples is shown below.

So far we have recovered sediment from several geological epochs, including the Pliocene (~4 million years ago), Miocene (~12 million years ago) and Eocene-Oligocene (~35 million years ago). Each new core provides a short segment of time that will eventually fill in the gaps in our knowledge of Antarctica's climate history. While it's hard even for geologists to grasp the amount of time represented by ancient rocks, it's quite amazing that we are able to decipher climatic changes that took place many millions of years ago.

Steven Bohaty
University of California, Santa Cruz

A diatom from ~35 million year old sediments of SHALDRIL Hole 3C.

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