DEFINING 'NORMAL': Geologist takes on black sand 'ignorance' in column (PHOTOS)

Within the past 10 days, the Destin Log reported a news story regarding complaints by citizens of the local area regarding the presence of black sand on local beaches. This was followed by several letters to the editor commenting on this issue. In a Destin city council meeting on Monday night, the subject was brought up by a local citizen.

The complaint centers around the belief that sand used for beach renourishment efforts contains more black sand than what is normally found on the beaches of NW Florida. Unfortunately, this entire complaint and discussion stems from ignorance about beach sediment and the mistaken assertion that sand used for renourishment of the local beaches is contaminated, or worse, simply does not match the sand considered as “native” to the beaches of NW Florida.

For more photos from the scene, click here.

In order to understand the nature of black sand, one must understand what it really is.

Black sand is composed of heavy minerals, not quartz grains. The pure white sand would be composed of only quartz grains. 
When these two sediment types are mixed together, the result is the native beach sand of NW Florida that appears white because the predominant mineral in the mix is colorless quartz.

The reason that heavy minerals can be called sand is because of the grain size.  Sand refers to grain size as detailed in the Udden-Wentworth Scale (commonly called the Wentworth Scale) and is limited to grain sizes ranging from 0.0625mm through 2.0mm. 
Anything above that is a gravel and anything below that range is a silt or mud.

Also, heavy minerals found in our beach sand are not all black in color because, as the plural category name implies, there is more than one mineral type in the mix of heavy minerals found on our beaches and each one has its own color, crystalline structure, hardness, and density among other physical characteristics.  In all the cases of the individual types of heavy minerals, these have a larger specific gravity than quartz (2.66 gm/cm3).

As reported by W.E. Harrison in 1973 in the Journal of Sedimentary Petrology, these heavy minerals were found to exist in beach sand on Horn Island southwest of the Mississippi-Alabama border in the Mississippi Sound.  These minerals mirror the set reported by Tanner, et al, in 1961 in the Journal of Economic Geology for heavy mineral concentrations around the coastal confines of the Apalachicola River.

Leucoxene; density 4.25 gm/cm3; blood red; hardness 6-6.5
Tourmaline; density 2.9-3.1 gm/cm3; pink, light blue or green; hardness 7
Staurolite; density 3.59 gm/cm3; brownish yellow, brownish black, yellow brown, dark brown, reddish brown; hardness 7-7.5
Kyanite; density 3.67 gm/cm3; blue, white, gray, green, black; hardness 5.5-7
Magnetite; density  5.2 gm/cm3; black; hardness 5.5-6.5
Sillimanite; density 3.24 gm/cm3; bluish, brownish greenish, colorless, gray, gray green; hardness 6.5-7.5
Hornblende; density 2.9 - 3.4 gm/cm3; black to dark green; hardness 5-6

Additionally, Tanner found sediments containing zircon, rutile, ilmenite, epidote, and monazite were present on the beaches of barrier islands around Cape San Blas.

More important is the linkage between the source for these sample areas.  As reported by Harrison, the provenance for heavy minerals in the northern Gulf of Mexico coastal areas is likely the southwestern limit of the Appalachian Piedmont and the watersheds that drain into our local area from those northern sources.

Tanner acknowledges the upstream provenance of the heavy minerals found in the offshore shoals by proving the upstream abundance of heavy minerals increases as distance from the Gulf of Mexico increases.

Tanner shows that subaerial samples of coastal sand contain less than one percent (by weight) of heavy minerals and the subaqueous samples in offshore shoals were between one and two percent.

Tanner also describes the process of temporary high-energy wave trains that separate relatively fine-grained, pure quartz sands from the coarser grained quartz sands that tend to concentrate the heavy minerals near the base of the redeposited sands in these offshore shoals.

What this means for finding sand in offshore sediment mining zones is that all the sand between Apalachicola and Mobile bays comes from the same source and has been worked by the same coastal processes.

In essence, it is all the same material.

So, why do heavy minerals appear on our beaches, sometimes seemingly out of nowhere?  Winnowing, dune erosion, and coastal lake breaching are three processes that can increase the concentration of heavy minerals on the active beach.

Winnowing is the process by which eolian transportation erodes lighter materials and leaves behind heavier material.

Think of panning for gold, it is the same process except the wind blows the lighter quartz sand away faster than it does the heavy minerals. The result is a thin layer of highly concentrated heavy minerals.  In many cases, these thin layers get covered by another layer of quartz sand and the thin, dark band of heavy minerals are preserved until the next time the sand is disturbed by a storm.

Often, several layers can be preserved on the back beach area as a result of winter storm front passages (see Fig. 1). If these back beach areas are disturbed by renourishment activities, the heavy mineral layers previously sequestered will be released into the active beach environment until the winnowing process can sort out the heavy minerals from the quartz sand.

Dune erosion, especially on the Walton County coastal area west of Inlet Beach, periodically exposes old coastal lagoon deposits.  When these black mounds are exposed  (see Fig. 2) to the erosive forces of the active shoreline (waves and storms), large amounts of heavy minerals are released.

Research done by the USF Coastal Research Lab shows that the heavy mineral concentration of these deposits exceeds the normal range of heavy mineral deposition in active coastal lakes by factors of more than 4 times.

Carbon dating of these deposits place the age range from 27,000 to 43,000 years before present.  This research work is ongoing and will be published soon.

Another example of this phenomenon can be found on Google Earth.  If you look at the satellite imagery for Walton County beaches, you will see long dark streaks of heavy minerals.  These were the minerals released from the buried coastal lake deposits exposed by Hurricane Ivan.

A ground level image can be seen in Figure 3.

A final source for a surge of heavy mineral and organic contamination is a coastal lake breach.  During storms with heavy rains, coastal lakes will burst through their natural sand berms with a resulting surge of water that scours the outflow channel.  Often, the outflow channel is a historic run-off channel with several hundred years of heavy mineral deposition buried in the channel.

Depending on the severity of the outflow scouring action, large quantities of heavy minerals may be released into the sediment budget of the active shoreline.

Anytime heavy minerals are disturbed, the process of winnowing will create a thin layer of “black sand” that will appear and may seem to be abnormal due to its large extent.  If the heavy minerals are being mixed with the quartz sand, as often happens during the beach renourishment operations as a matter of necessity, then the resulting sand will appear grayer until the natural winnowing process has time to separate the quartz grains from the heavy mineral grains.

The coastlines of the northern Gulf of Mexico are all in a sediment starved condition.  Too little sediment flows out of the watersheds to naturally renourish the beaches.  Therefore, in order to maintain the coastlines as they would be if normal sediment deposition were allowed to occur during spring floods, anthropogenic (manmade) renourishment must occur.

The reality of beach renourishment is that it will disturb sediment on the active shoreface. There is no way to avoid this from happening unless beach renourishment is not done.

Without renourishment, the coastline will continue to erode and private property will be destroyed by erosion. The only way to prevent erosion without beach renourishment is to construct seawalls to prevent the erosion.

Eventually, without beach renourishment, the seawalls will become the primary point of contact for the waves to expend energy because there will be no beach seaward of that hardened contact upon which the waves can dissipate energy.

The problem with hardening the coastline is that unless you construct a seawall from inlet to inlet, any remaining soft shoreline will continue to erode.  This would leave headlands of private property surrounded by seawalls jutting out into the Gulf of Mexico.  Beach renourishment is the best way to preserve shoreline environment for all flora and fauna including humans.

In summary, heavy minerals are a naturally occurring material on the beaches of NW Florida.

Depending on weather patterns, the heavy minerals may or may not be present in large quantities on the immediate surface of the beach.
Beach renourishment activities will disturb previously buried layers of heavy minerals and mix those with the renourished sand being added to the beach.  There may or may not be sufficient amounts of disturbed heavy minerals to discolor the sand.

In either case, the source for any heavy minerals found on the beach in the vicinity of beach restoration activity is likely attributable to the process of adding sand and sculpting the beach rather than solely being the new sand itself.  Mixing any new sand with the current sand can create grey sand if the current beach sediment includes large concentrations of buried heavy minerals from past storm events.

The good news is that the natural winnowing process will separate the quartz sand from the heavy minerals and eventually the beach sand color will return to “normal,” if there is such a thing for beach sand to be.

Rip Kirby is a coastal geologist working on his doctorate at the University of South Florida.  His research area is NW Florida with a specialty in hurricane impact and recovery processes of the coastal zone, erosion control of sand dunes on barrier islands, and preservation of habitat for coastal environments.  He lives in Fort Walton Beach.  jkirby@cas.usf.edu.