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Temporal and Spatial Distribution of Whitings on Great Bahama Bank and a New Lime Mud Budget

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Temporal and Spatial Distribution of Whitings on Great Bahama Bank and a New Lime Mud Budget Temporal and spatial distribution of whitings on Great Bahama Bank and a new lime mud budget L. L. Robbins* Department of Geology, University of South Florida, Tampa, Florida 33620 Y. Tao C. A. Evans Lockheed, Engineering and Science Program, 2400 NASA Road 1, P.O. Box 58561, Houston, Texas 77258 ABSTRACT Yates and Robbins, 1995; Robbins et al.,1996) or Analysis of 69 shuttle and satellite images of the northern Bahamas taken from 1965 to unicellular green algae (Yates, 1996). 1993 allowed us to calculate the regional and temporal distribution and longevity of 888 whit- If a significant fraction of the carbonate sedi- ings on the Great Bahama Bank. At any time, whitings cover between 35 and 200 km2 in the area ment within whitings is precipitated within the wa- centered at lat 25°N and 78°50′W. The highest occurrences of whitings are in April and October, ter column during a whiting event (and the refer- suggesting a seasonal component. Using a measured average of 10.6 mg/L suspended sediment in ences cited above suggest that to be the case), then typical whitings, we calculated that 1.35 × 106 metric tons of lime mud are suspended every year. the size and frequency distribution of whitings If the suspended carbonate is precipitated in the water column, these phenomena account for provides a measure of carbonate mud production. 280% of all of the Holocene accumulated bank top mud and more than 40% of the total bank We analyzed photographs of the northern Ba- top mud and periplatform mud that have accumulated on the west side of Great Bahama Bank. hamas taken by NASA astronauts from low Earth orbit to determine areal coverage of whitings and INTRODUCTION 1984; Shinn et al., 1989; Robbins and Black- to calculate the amount of mud produced. Some Photographs taken from low Earth orbit pro- welder, 1992; Boss and Neumann, 1993) and it is aerial surveying work was undertaken to docu- vide a unique opportunity to document spatial not our intent to present new evidence here (see ment the distribution of whitings on Great Bahama and seasonal components of the Earth’s surface instead Yates and Robbins, 1995; Yates, 1996; Bank (F. Morris in Morse et al., 1984; Shinn et al., features (Andreae, 1993), including the highly Robbins et al., 1996; and Thompson et al., 1997). 1989; Boss and Neumann, 1993), although no sys- debated drifting patches of lime mud found in Field and laboratory experiments (Robbins et al., tematic survey of the geographical extent, season- subtropical seawater, called whitings (Tao, 1994). 1996; Yates, 1996) have documented that certain ality of whitings, size, and temporal magnitude The phenomena have fascinated scientists for at whitings on Great Bahama Bank originate had been determined until now. These recent data least 30 years, and data have been provided through epicellular precipitation of calcium car- provide a means for calculating a lime mud budget mainly through field-based studies (Cloud, 1962; bonate induced by photosynthesis in blooms of model as well as documenting a seasonal compo- Broecker and Takahashi, 1966; Morse et al., picoplankton (Robbins and Blackwelder, 1992; nent of these phenomena. 1984; Shinn et al., 1989; Robbins and Black- welder, 1992; Robbins et al., 1996). The use of photographs from space provides a distinctive approach in determining the location and transport of suspended lime mud on Great Bahama Bank. These data can then be used to calculate a sediment budget of this “carbonate machine.” Estimating the production of neritic carbonates today and for the past is important in understanding the role of carbonates within the oceanic and atmospheric carbon reservoirs (Mil- liman and Droxler, 1995). We believe that a significant fraction of the Figure 1. Low-orbit photo- carbonate mud in whitings is precipitated within graph of Andros Island, the water column in association with cyano- Bahama (top is north), bacteria based on (1) calcite and aragonite crystal showing group of whitings size, shape, and geochemistry (Loreau, 1982; to the northwest in upside- down V shape. These whit- Milliman et al., 1993); (2) the intimate associa- ings were tracked for at tion of carbonate crystals and cyanobacteria least seven days and were cells; (3) cell counts along transects of whitings, still discrete entities on where concentrations of planktonic cyanobacteria last day of documentation. Group of whitings can are double and as much as 10 times higher inside also been seen to south. than outside whitings (Robbins et al., 1996; Thompson et al., 1997); and (4) the amino acid content of the organic fraction of the whitings (Robbins and Blackwelder, 1992). The origin of whitings and lime mud remains controversial (Broecker and Takahashi, 1966; Morse et al., *E-mail: [email protected]. Geology; October 1997; v. 25; no. 10; p. 947–950; 4 figures; 1 table. 947 METHODS fraction on the bank was produced by inorganic trast, the lower frequency index values occur in Whitings on Great Bahama Bank are easily precipitation. We used this number (75%) in our January, March, May, June, July, August, and observed from low Earth orbit (Fig. 1). A total of calculations. September, with about 30 km2 of whitings per 888 individual whitings on Great Bahama Bank day. Our temporal data suggest that during the acquired from photographs were selected, RESULTS AND DISCUSSION 28 yr period studied, there were more whitings mapped, and registered (Tao, 1994). The photo- Whitings Spatial Distribution during the month of April and during the fall and graphs were taken from the NASA manned Figure 2 is a map of spatial distributions of winter months of October, November, and De- spacecraft program (1965–1993) and catalogued whitings documented during 1965–1993. The cember than at other times of the year. Spring and by the NASA Space Shuttle Earth Observation whitings distribution showed a clear nonrandom fall high production months also have been ob- Project. The Space Shuttle Earth observations pattern, and a large concentration around lat 25°N served in lake whitings where seasonal blooms of photograph data base can be accessed on the and 78°50′W, where 75%–85% of the whitings cyanobacteria have been cited (Strong and Eadie, World Wide Web at http://ersaf.jsc.nasa.gov. occurred. Smaller, more geographically dispersed 1978; Thompson and Ferris, 1990). Although the Interpretation of the selected images was based whitings were also often found in the southern resuspension of bottom sediment occurs more on a priori knowledge of Bahama Bank geogra- central part of the bank. Earlier studies (Morse et frequently in the winter, the January through phy. Photographs were rectified and registered to al., 1984; Shinn et al., 1989; Boss and Neumann, March (excluding February, for which there were 1:1 000 000 scale Operational Navigation Charts 1993) noted similar distributions of whitings for no data) frequency index values were the lowest. using a Bausch and Lomb zoom transfer scope. shorter time frames, concentrating primarily over Furthermore, whiting features analyzed had mor- The whitings size and location were then com- the northern part of the bank; our comprehensive phologically discrete shapes and distinct edges as puted and recorded and a frequency index was data set takes this one step further and rigorously compared with widespread resuspension events determined using the following equation: defines the whiting occurrence on the entire bank caused by stormy weather, which often showed sum of whitings area/month over a 28 yr period. Although some occurrences as large bank suspensions with indistinct edges. frequency index = of whitings were within the oolitic zone, most mission days whitings occurred within the mud facies in the Time Series Whitings Observations where mission days are the total number of days central part of the bank and the pellet-mud facies Shinn et al. (1989) documented the longevity in orbit for each month from the data set consid- in the east-central part of the bank (Fig. 2). of a single whiting for at least 48 hours. This type ered. The frequency index allowed normalization of observation was also documented by space of the data to eliminate bias introduced by the un- Seasonal Variation shuttle photographs, which showed that some even distribution of the mission days (Andreae, Whitings were observed over the entire calen- whitings existed for days to weeks and did not 1993; Tao, 1994). These values represent the dar year on Great Bahama Bank, but April and settle out in the time frame expected if they were monthly variation of whitings. An additional test October show highest frequency index values of simply stirred up bottom sediment (Tao, 1994). to verify the validity of the normalized data was 75 and 72 km2/day, respectively (Fig. 3). In con- For example, a distinct group of whitings per- performed by randomly choosing two photo- graphs from each month and calculating the whit- ing frequency index, represented by the sum of the whiting size from these two photographs (Tao, 1994). Another potential bias, that 85% of the photographs only partially covered the study area (the 3300 km2 of Great Bahama Bank), was ad- dressed by selecting 10 whole bank coverage photographs and calculating the average daily whitings area. The calculated averages were con- sistent with observations covering the entire bank. For lime-mud budget calculations, we show that the amount of mud found on the periplat- Figure 2. Spatial distri- form can be easily produced by whitings. The bution of 888 whitings mapped from 69 NASA average water column depth used was 5 m, and photographs. Whitings the specific gravity of wet aragonite mud was (black circles) demon- 1.150 × 106 g/m3. We used 10.6 g/m3 as the aver- strate dense distribution age whitings concentration (Shinn et al., 1989), in northwestern part of corroborating our own field data.
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