[Coral-List] Sargassum season and dust

[Eugene Shinn]

When I posted my rant about African dust and Sargassum on the Coral-List 
I had no idea it would turn into a discussion about illegal mining and 
run off from the Oronoco River. These discussions reminded me of some 
earlier work we conducted in the Florida Keys. Many of you in the 
Florida Keys will know there is a red/brown very dense crust on the 
surface of the limestone. It is thicker (up to 4 inches in the lower 
keys where it overlies Pleistocene oolitic limestone.) It is usually 
thinner in the upper keys where it overlies Pleistocene coral reef 
limestone. It can also be found offshore where it underlies Holocene 
coral reefs.These crusts are composed mainly dense laminated limestone 
consisting of calcite. These crusts also contain iron, which gives them 
the rusty brown color and quartz silt, and yes they come from African 
dust. They are the best evidence of repeated fluctuations of sea level. 
Our coring has revealed at least 5 distinct layers of this crust in the 
first 100 feet of Pleistocene limestone. Each one represents a time of 
lower sea level and exposure to the atmosphere.

What is most interesting is the upper-most crust one sees most anywhere 
in the Florida keys is almost 10,000 years old by C14 dating. The upper 
surface dates essentially to the present day. The crust in the Keys is 
still forming today. What is surprising is they contain Mercury, which 
is also present in African Dust.It should be obvious to anyone that the 
Mercury did not come from the Oronoco river plume. It also did not come 
from electric power plants although today power plants do in fact emit 
mercury. As an employee of a government agency the reader can imagine 
the situation I was in when I found Mercury in laminated dense limestone 
that predated any industry in the Americas. There were lawsuits going on 
between EPA and industry at the time.

I don’t imply there is no Mercury in the Oronoco plume but one thing I 
am sure about, is that the Mercury in these crusts came from the 
atmosphere several thousand years ago and it is still arriving in the 
African dust. Unfortunately we never tested for Mercury in the older 
crusts. This is all described and illustrated in my book, /Geology of 
the Florida Keys/, co authored by Barbara H. Lidz. (University Press of 
Florida, 2018) Gene

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No Rocks, No Water, No Ecosystem (EAS)
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E. A. Shinn, Courtesy Professor
University of South Florida
College of Marine Science Room 221A
140 Seventh Avenue South
St. Petersburg, FL 33701
<eugeneshinn at mail.usf.edu>
Tel 727 553-1158
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