Last Monday, I briefly discussed the origins of oceanic crust and why it contradicts the young-Earth paradigm. Prior to that, I summarized how multiple ice ages have been recorded in Siberian speleothems, which precludes the idea that a single “post-Flood” ice age could explain glacial geology. So in case you are tired of hearing about the ocean floor and ice ages, then this post is not for you!
For this week’s Monday Minute, though, I couldn’t help but to relay the latest report from Science News, because the timing couldn’t have been better. Several papers were recently published (including Crowley et al., 2015), which describe how glacially driven changes in sea level actually impacted the rate of volcanism at mid-ocean ridges. Fortunately, the mechanism is surprisingly elegant and very easy to grasp:
- During the peak of ice ages, sea level is estimated to have dropped by ~120 meters worldwide (that missing water was transferred to the land in the form of glacial ice).
- Massive hydrostatic pressure at the bottom of the ocean dampens the rate of seafloor volcanism (watch the video in last week’s post to see that volcanism in action). But when sea level dropped, that pressure was relieved, allowing volcanism to proceed at a slightly higher rate.
- Therefore, the ocean floor should have been slightly thicker during past ice ages, when sea level was lower. These intervals could be visible on the ocean floor in the form of minor ridges and hills that run perpendicular to the spreading axis.
It sounds like a great hypothesis, but how could we ever test it, since none of us were actually there? That was the goal of researchers, who sought to confirm that regularly spaced ridges and hills near the Australian-Antarctic ridge were indeed associated with Milankovitch-driven ice ages over the past few million years. These features were mapped out at high resolution several years ago in the southern Indian Ocean. Using those data, the researchers were able to plot the thickness of the ocean floor along a transect leading away from the ridge (i.e. back in time).
Now, we have long known the age of oceanic crust gets older as one moves away from spreading ridges, but advances in radiometric dating techniques allowed geologists to attach real numbers to the entire oceanic crust. In case you are eager to object to the accuracy of those dating techniques, I ask that you bear with me. You see, assuming the accuracy of those radiometric dates, the researchers could assign real ages to anomalously thick zones (where volcanism had been more active in the past). Using advanced statistical techniques, they determined that volcanism increased significantly at regular intervals of 23, 41, and 100 thousand years.
Do those numbers sound familiar? If you’ve ever studied Milankovitch theory and its application to paleoclimatology, then they should. These are precisely the intervals at which the Earth’s orbit varies with respect to the sun, and hence they explain the timing of past ice ages. In other words, the new studies confirm not only that Earth has endured dozens of ice ages over the past millions of years, which impacted seafloor volcanism, but also that radiometric dating of oceanic crust is highly accurate. If it were not, then we’d be hard pressed to explain this incredible coincidence.
On a final note, the association of volcanism with sea level elucidates the process by which the ice ages met their end. Volcanism at mid-ocean ridges injects massive amounts of carbon dioxide into the oceans, which eventually makes its way to the surface. The further sea level dropped, therefore, the more CO2 was added to the atmosphere. More CO2 lead to a warmer atmosphere, which melted the glaciers that caused sea level to rise. As sea level rose, rates seafloor volcanism dropped, and the atmosphere stabilized.
What an incredible planet we live on… Wouldn’t you say?
Featured image: Bathymetry of the East Pacific Rise via GeoMapApp, for illustrative purposes only.