tidal rhythmites and the ancient orbit of the moon
Fueled by an interest in both geology and astrophysics, Dr. Ojakangas has long been
fascinated by a class of sedimentary rocks that hold clues to the ancient orbit of the
moon. Under certain conditions, sediments deposited by tidal currents can record
systematic variations in the heights of successive tides. These rocks, known as tidal
rhythmites, record tidal cycles, such as the twice-monthly neap-spring cycle as well as
others. This occurs because as the tides came in and out, successive deposited layers varied in thickness due to the changes in the
magnitudes of the tides and hence by the tidal currents that supplied them. Working with his
father, Emeritus professor of geology Dr. Richard Ojakangas at the University of Minnesota Duluth,
Ojakangas had the good fortune of discovering 1.85 billion–year-old tidal rhythmites
in Northern Minnesota. Studying these sediments with high-resolution images of
slabbed specimens, Ojakangas was able to extract information on the distance
between the earth and the moon 1.85 billion years ago. The brackets on the image on the left delineate pairs of layers that alternate in thickness, interpreted as evidence of the diurnal inequality. This discovery constitutes
some of the earth’s oldest known tidal rhythmite deposits. Their research led to this
publication.
In the image on the left, brackets delineate (interpreted) tidal neap-spring cycles, a phenomenon well-known to students of the ocean. This variation in tide height occurs as the tides due to the moon and the sun alternately add constructively (at full moon and new moon) and destructively (at first and third quarter moons).
Sequences of layer thicknesses were carefully measured from digital images of the slabbed specimens, resulting in the dataset on the right (top). Spectral analysis yielded the data in the lower right, which were used to put constraints on the ancient lunar orbit, through the application of some simple astrophysical calculations. On the left (bottom) is a relationship between the number of days in a month and the distance from the moon to the earth. Through the use of this relationship, it was determined that the moon was no closer than 30 earth radii from the earth, 1.85 billion years ago. This is a weak constraint, because it is already believed on theoretical grounds that the moon was considerably farther than 30 earth radii at that time.
