Satellite-Derived Estimates of Suspended CaCO₃ Mud Concentrations From the West Florida Shelf Induced by Hurricane Ian

Hurricane Ian was a Category 5 system which developed in the Caribbean Sea in late September 2022. Ian achieved tropical storm strength on September 24, and reached hurricane strength on the next day. The storm moved north over western Cuba, and on September 27, at Category 5 strength, passed over the West Florida Shelf before making a September 28 landfall on the west coast of Florida near Sanibel Island. Ian subsequently crossed the Florida peninsula and into the Atlantic Ocean, gained strength, and made a second landfall on the South Carolina coast.

Following the passage of Hurricane Ian over the West Florida Shelf, true color images from the Moderate Resolution Imaging Spectroradiometers (MODIS) on the NASA Terra and Aqua satellites showed that nearly all of the southern West Florida Shelf was highly reflective, with peak backscatter at ~480 nm. This wavelength corresponds to the color "Maya Blue" (RGB - 115, 194, 251) and is indicative of surface suspension of fine carbonate (CaCO₃) sediments (mud) in tropical seas throughout the global ocean. Because of Ian's windspeed and strike properties it is likely that the water column was fully mixed to a depth of at least 60 m.

In the following days, a large plume of Maya Blue slurry was observed extending from west of the Dry Tortugas and curving to the east into the Straits of Florida. This discreet target offered a unique opportunity to quantify the slurry concentration.

Estimating the concentration of sediment in a plume of suspended carbonate by satellite sensor observations has been stymied up to now owing to a lack of in situ suspended sediment measurements during events such as Ian. “Sea truth” data for such events is difficult to acquire. However, the Particulate Inorganic Carbon (PIC) standard product provided by the Ocean Biology Distributed Active Archive Center (OBDAAC) is based on MODIS observations of a plume of coccolith chalk released from a ship in the “Chalk-Ex” experiment. Due to the similarities (particle size, mineralogy, and reflectance properties) of the suspended chalk features and the Ian-induced slurry, we utilized this data product to make initial estimates of the concentration of suspended sediment in the plume. Our results are in accord with historical measurements of storm-suspended sediment and show a range of 0.6-3.0 g/m3. These findings highlight a stark ignorance of process sedimentology as it unfolds during storms, and show the need for fast-response sampling of suspended sediment during events.

Most importantly, these estimates are an initial, unprecedented step for the broad application of satellite-derived suspension estimates during, and immediately subsequent to, sediment-mobilizing storms. As such, they are providing answers to basic questions in process sedimentology which have gone unanswered for over 80 years. With improved imagery and in-water sampling it will be possible to calculate total mass transport of carbonate from banks, platforms, and shelves worldwide.