The Cretaceous Geology of Alabama and Modern Sea Level Rise…

Guest “How not to connect the present with the past” by David Middleton

Alabama’s Return to the Sea
A paleontological site shows how life endured in an Alabama that was almost fully submerged—and how it could again as the ocean rises.
by Jack Tamisiea
September 8, 2021

Alabama’s Harrell Station, roughly 75 kilometers west of Montgomery and 250 kilometers inland from the Gulf of Mexico, seems like the last place someone would go to explore the ocean. But crumbling out of the dusty ground, which has been wrinkled into gullies of white chalk, are the remnants of an ancient sea. During the Late Cretaceous some 82 million years ago, high temperatures melted the polar ice caps submerging the world’s coasts. A shallow sea known as the Mississippi Embayment spilled out over the southeastern United States, blanketing much of Alabama. Harrell Station is one of the best places to glimpse this primeval sea.

Located on a belt of Cretaceous-aged rocks known as the Mooreville Chalk, the gullies at Harrell Station are brimming with the immaculately preserved fossilized remains of the reptiles that dominated this ancient sea. Compacted over time, and carved by eons of erosion, the powdery marl is composed of the crushed skeletons of microscopic algae known as coccolithophores combined with clay. As these plankton sank toward the ocean floor, they entombed larger sea creatures in stunning detail.

Since the 1940s, researchers have worked to exhume this fossilized bounty. To this day, they continue to uncover new species. Adiel Klompmaker, the curator of paleontology at the Alabama Museum of Natural History, which has owned and excavated a large swath of Harrell Station since 1991, likens the site to a Cretaceous time capsule. “We know so much about the late part of the Cretaceous because of Harrell Station,” he says.

[…]

This connection between warming water and sea level rise troubles Klompmaker. “Harrell Station gives a warning,” he says. “It reminds us that sea levels can inundate vast amounts of the state of Alabama.”

This process is already playing out. A combination of sinking land and a warming Gulf means that Alabama is one of the states most at risk from sea level rise. The ocean has risen nearly 30 centimeters over the past 50 years, eating away at the state’s barrier islands and coastlines. Some people may like having beachfront property in northern Alabama, Klompmaker says jokingly, “but I think many people would disagree.”

[…]

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This reminds me of In Search of Noah’s Ark (1976). I recall a paleontologist stating that marine fossils have been found on mountaintops all over the world; proving that a great flood once covered the entire surface of the Earth. This is exactly not how to connect the present with the past. It’s exactly not geology. It’s exactly not how the principle of Uniformitarianism is employed. Yet it’s almost exactly what this aspiring young science writer has done.

During the Late Cretaceous some 82 million years ago, high temperatures melted the polar ice caps submerging the world’s coasts.

Jack Tamisiea, 2020 BS Environmental Studies, minored in Narrative Structure, working on Science Writing MS

The “science writer” assumes that since melting polar ice caps have been the primary cause of rising sea levels since the end of the last Pleistocene glacial stage, this must have been how most of present day Alabama was underwater during the Cretaceous Period.

Cretaceous sea level had nothing to do with melting polar ice caps. While, it’s possible that there were some ephemeral ice sheets and glaciers during the Late Jurassic to Early Cretaceous, there were no significant polar ice caps during the Mesozoic Era.

The paleogeography of the Cretaceous Period was totally different than modern day physical geography. This is why sea level was higher, why it was so much warmer and probably why CO2 levels were so much higher than today. Shallow seas covered much of the continental landmass. Oddly, even though Cretaceous sea level was much higher than today, the modern day oceans are much deeper than they were in the Cretaceous. The oceans are currently deeper than any time in the past 250 million years.

The second phase in the breakup of Pangea began in the early Cretaceous, about 140 million years ago. Gondwana continued to fragment as South America separated from Africa opening the South Atlantic, and India together with Madagascar rifted away from Antarctica and the western margin of Australia opeing the Eastern Indian Ocean. The South Atlantic did not open all at once, but rather progressively “unzipped” from south to north. That is why the South Atlantic is wider to the south.

Other important plate tectonic events occurred during the Cretaceous Period. These include: the initiation of rifting between North America and Europe, the counter-clockwise rotation of Iberia from France, The separation of India from Madagascar, the derivation of Cuba and Hispaniola from the Pacific, the uplift of the Rocky mountains, and the arrival of exotic terranes (Wrangellia, Stikinia) along the western margin of North America.

Globally, the climate during the Cretaceous Period, like the Jurassic and Triassic, was much warmer than today. Dinosaurs and palm trees were present north of the Arctic Circle and in Antarctica and southern Australia. Though there may have been some at the poles during the Early Cretaceous, there were no large ice caps at anytime during the Mesozoic Era.

These mild climatic conditions were in part due to the fact shallow seaways covered the continents during the Cretaceous. Warm water from the equatorial regions was also transported northward, warming the polar regions. These seaways also tended to make local climates milder, much like the modern Mediterranean Sea, which has an ameliorating effect on the climate of Europe.

Shallow seaways covered the continents because sea level was 100 – 200 meters higher than today. Higher sea level was due, in part, to the creation of new rifts in the ocean basins that, as discussed previously in this article, displaced water onto the continents. The Cretaceous was also a time of rapid sea-floor spreading. Because of their broad profile, rapidly spreading mid-ocean ridges displace more water than do slow spreading mid-ocean ridges. Consequently, during times of rapid sea-floor spreading, sea level will tend to rise.

Scotese, C.R., 2002, http://www.scotese.com, (PALEOMAP website). Cretaceous.

Most of Alabama and much of the modern Gulf Coast region were in the Gulf of Mexico during the Cretaceous Period.

Dr. William Galloway, of the University of Texas Jackson School of Geosciences, summarized the depositional history of the Gulf Coast/Gulf or Mexico in this must read paper (I must read it, because he sent me a copy of it)…

Depositional history can be generalized in seven phases: (1) Middle-Late Jurassic evaporite and carbonate deposition in a broad, shallow, restricted to open marine basin. (2) Latest Jurassic-Early Cretaceous sand-rich clastic progradation from the northern margins. (3) Late-Early Cretaceous development of a rimmed carbonate shelf. (4) Late Cretaceous mixed clastic and carbonate aggradation of the continental margins. (5) Resurgent Paleogene clastic progradation and filling centered in the NW basin. (6) Miocene progradation and basin filling centered in the central and NE Gulf. (7) Late Neogene climatically and eustatically influenced progradation along the central Gulf margin. In contrast to the broad, progradational sediment wedge of the northern Gulf, the Florida margin is a primarily aggradational carbonate platform.


Galloway, 2008.

Figure 2. Left to right: Generalized cross section along northern GOM region (Galloway et al., 2009), depositional phases are numbered. Relative sea level (Miller et al., 2005), atmospheric CO2 (Berner & Kothavala, 2001) and temperature anomalies (Royer et al., 2004). Click for image.

Figure 2 clearly demonstrates the importance of sea level cycles in the depositional history of the GOM. Also note that all of the source rock formations were deposited when atmospheric CO2 was above 1,000 ppm and the Earth was considerably warmer than it is today. The temperature and CO2 plots have 10 million year resolutions; they are highly smoothed.

A combination of sinking land and a warming Gulf means that Alabama is one of the states most at risk from sea level rise. The ocean has risen nearly 30 centimeters over the past 50 years, eating away at the state’s barrier islands and coastlines.

Some people may like having beachfront property in northern Alabama, Klompmaker says jokingly, “but I think many people would disagree.”

Jack Tamisiea, 2020 BS Environmental Studies, minored in Narrative Structure, working on Science Writing MS

The closest NOAA tide gauge station with a relatively long, continuous record length is Pensacola FL. Sea level has risen by about 20 cm over the past 50 years. The rate of sea level rise is a bit higher in and around Mobile Bay; however those stations, Dauphin Island and Mobile State Docks are discontinuous and only go back to 1966 and 1980, respectively.

Figure 3. Sea Level Trend at Pensacola FL. NOAA.

Dallas County AL, where Harrell Station is located, is about 580′ above sea level. Sea level at Pensacola is rising at a rate of about 0.83 feet per 100 years. At this rate, it will be about 70,000 years before people have a chance to acquire “beachfront property in northern Alabama.”

Alabama’s Cretaceous marine geology and modern coastal processes are not analogous in any manner.

References

Berner, R.A. and Z. Kothavala, 2001. GEOCARB III: A Revised Model of Atmospheric CO2 over Phanerozoic Time, American Journal of Science, v.301, pp.182-204, February 2001.

Galloway, William. (2008). “Chapter 15 Depositional Evolution of the Gulf of Mexico Sedimentary Basin”. Volume 5: Ed. Andrew D. Miall, The Sedimentary Basins of the United States and Canada., ISBN: 978-0-444-50425-8, Elsevier B.V., pp. 505-549. (Special thanks to Dr. Gallloway for sending me a copy of this)

Galloway, William E., et al. “Gulf of Mexico.” GEO ExPro, 2009, https://ift.tt/3zUScW2.

Miller, Kenneth & Kominz, Michelle & V Browning, James & Wright, James & Mountain, Gregory & E Katz, Miriam & J Sugarman, Peter & Cramer, Benjamin & Christie-Blick, Nicholas & Pekar, S. (2005). “The Phanerozoic Record of Global Sea-Level Change”. Science (New York, N.Y.). 310. 1293-8. 10.1126/science.1116412.

Royer, D. L., R. A. Berner, I. P. Montanez, N. J. Tabor and D. J. Beerling. CO2 as a primary driver of Phanerozoic climate.  GSA Today, Vol. 14, No. 3. (2004), pp. 4-10

Scotese, C. R., 2001. Atlas of Earth History, Volume 1, Paleogeography, PALEOMAP Project, Arlington, Texas, 52 pp.

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September 9, 2021 at 04:12PM

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