Radioactive Fossil Dating

To determine how old is a fossil scientists use different methods. One of the most precise is  radioactive fossil dating.

As you may see from the name, this method relies on the radioactive atoms that are trapped inside the rocks and break down at a predictable rate, i.e. over a certain period half of the ”parent” atoms decay into ”daughter” atoms.

By knowing how long is a ”half-life” of a particular radioactive element and the number of parent atoms and decayed stable daughter atoms, we could estimate the number of decay cycles and the age of a rock.

Perissodactyla, astragalus fragment. 

This bone fragment was collected in Utah in September 2020 for the Denver Museum of Nature and Science by me and my dear friend and colleague Dr. Paul Murphey.

But with sedimentary rocks where fossils are usually found it’s a different story. Why? Because sedimentary rocks form from erosion of igneous or metamorphic rocks, and the dating will be applied for other sources so the minerals tested could only provide the ages of their parent source rocks.

Is it impossible to use radiometric dating for sedimentary rocks itself? Yes and no. Carbon-14 dating is used for sedimentary rocks aging but carbon (parent) decays into nitrogen extremely fast from a geological point and after 80,000 years all parent matter could decay into a daughter matter which makes it impossible to date carbon-bearing material older than 80,000 years.

What is a workaround or how does a paleontologist date fossils? Sedimentary rock layer could be estimated as a range in dates between underlying and overlying rocks that could be dated: volcanic ash deposits, igneous and metamorphic rocks.

By bracketing the age of fossils or looking for layers of, for example, volcanic ash deposits above and below the sedimentary layer we could find out the range of how young and how old might be the fossil.

Testing database in Wyoming in September 2020. This photo was taken by Dr. Paul Murphey.

Paleomagnetism and the Geomagnetic Polarity Time Scale

As you know, the magnetic poles and geographic poles are not aligned. Today, the geographic North Pole and the magnetic North Pole are very close to one another. This is referred to as Normal Polarity. The magnetic declination is 11 degrees, which means that the magnetic north pole is at an 11-degree angle from true geographic north.

However, many times through geologic history, the magnetic pole was oriented to the geographic South Pole, a condition referred to as Reversed Polarity. Normal and reversed polarity change at irregular intervals, every few million years on average.

Regardless of the geographic position of the Earth’s pole, a magnetic needle in a compass will always point to the magnetic north in times of normal polarity. Likewise, magnetic particles that are contained in rocks record the polarity at the time the rocks were formed and the magnetic particles were oriented. Thus, the magnetic polarity of the earth is recorded in rocks that contain magnetic minerals.

By analyzing rocks around the world using radiometric dating and comparing their magnetic polarities, the first geomagnetic polarity timescale (GPTS) was revealed in 1959 by Allan Cox, Richard Doell, and Brent Dalrymple.

The geomagnetic polarity time scale represents normal polarity with black stripes, and reversed polarity with white stripes. Each interval is referred to as a Chron, counting backwards in time from C1 (the current chron) to the Jurassic.

With multiple drilling, we can take samples and determine the rates and the numbers of reversals, and construct a log of all reversal events and compare it with the rocks of a similar time period around the world and use paleomagnetism as a correlation tool.

Correlation by Index Fossils

As we know, most fossils are found in sedimentary rocks which form in layers called strata.

Sedimentary rocks are arranged in horizontal layers, the oldest layers will be at the bottom and the younger layers at the top. By looking at the position of layers we could tell the sequence of events, i.e. which layer was deposited first, which second, which deposited the last.

Pennsylvanian Madera limestone in New Mexico in October 2020. Invertebrate fossils collected by me and and my dear friend and colleague Dr. Paul Murphey.

The sequence defines the order but how could we assume the age of strata? We could look at the index fossils.

Some animals lived within a very short period of geological time, abundant, easy to identify, and were widely distributed. Paleontologists and geologists use index fossils to date and correlate the strata. Here are examples of index fossils: Cambrian trilobite Paradoxides pinus lived between 513 to 505 Ma; Pensylvanian brachiopod Dictyoclostus americanus lived between 339.4 to 295 Ma; Jurassic ammonite Perisphinctes tiziani, etc.

Any strata that produce those fossils have to be deposited within the lifespan of the fossils after the fossils appeared and before they went extinct. Thus, a short lifespan of an animal is an important criterion to be an index fossil.

This method was used to correlate layers, or strata, long before radiometric dating and even before the modern understanding of evolution. By observing common fossils in sedimentary layers, geologists were able to correlate geographically separated layers and read the history of time.