Florida Bay Mudbanks: Relatively New Piles of Mostly Old Sediment

By: Robert B. Halley, Chuck W. Holmes, and Ellen J. Prager

The study of Florida Bay mudbanks provides two different, but important sources of information for ecosystem restoration. First, they are the primary physiographic (bathymetric) features of Florida Bay. The banks act as partial barriers to circulation and influence water quality, in particular salinity and turbidity. Second, mudbanks are localized accumulations of biologically produced debris resting on a relatively flat, much older, limestone floor. They are the primary repository for sediment formed within the Bay and they contain sedimentological, paleontological, and geochemical records of past conditions in the Bay.

Mudbanks are barriers. Surveys of bank-top elevation, together with recent surveys of ENP tide gauges, allow records of bank exposure and water depth to be calculated from tide data. During a year, these records provide a means of correlating bank-top exposure with climatic events such as still, hot summer days or windy, winter cold fronts. Over several years, tide records reveal extensive inter-annual variation of water depth over the banks, suggesting significant inter-annual variation in circulation throughout the Bay. Understanding long-term (decades to centuries) influence of the mudbanks on circulation requires knowledge of sea-level change as well as changes in mudbank elevation.

Sediment transport and accumulation result in mudbanks with extremely flat tops, probably the result of a balance between physical processes (waves, currents, periodic exposure, sediment properties) and biological influences (sediment trapping by seagrass and bioturbation). Mudbanks in the eastern bay are typically eroding on exposed margins and accumulating on protected margins, resulting in net migration. Most of the transported sediment is carbonate mud, silt, and fine sand. Coarse sand and shell material form erosional lags on eroding margins and may accumulate as ridges, particularly where wave-damping by grass has been eliminated due to seagrass mortality. Coarse lag deposits also occur on some basin floors, the result of repeated winnowing of fine sediment. Fine sediment accumulates in the mudbanks, and on protected margins covered by seagrass. Regions of erosion and deposition are best defined by bottom mapping and wave modeling, both projects currently underway in the USGS and summarized by Prager elsewhere in this symposium.

Lead-210 dating of sediment cores from mudbanks in eastern Florida Bay yields carbonate sedimentation rates on the order of a meter per century. If this rate had been realized for the entire 3,000- year history of the Bay, mudbanks would be 30 m thick. In fact, they are 2 m thick in the eastern Bay, a thickness limited by sea level. The observed sedimentation rates are controlled by transport and redeposition of old sediment. C-14 dating of bulk sediment confirms this interpretation, yielding apparent surface dates many hundreds to thousands of years old.

The importance of seagrass in trapping and binding sediment during the formation of mudbanks has been long-debated. The recognition that much of the sediment in mudbanks is transported to the site of accumulation does not diminish the importance of seagrass trapping as a mechanism for bank construction. The absence of seagrass facies in cores has previously been interpreted to suggest that seagrass is of minor importance in bank construction. However, seagrass has a dampening effect on waves beyond the immediate vicinity of the grass bed, similar to the effect a snowfence has on drifting snow. Grassless deposits may be considered a part of a grass-bed deposit if it is downcurrent from a seagrass bed. Open mud areas on Florida Bay mudbanks, termed "blowouts" appear to be the result of this snowfence-like behavior of grass beds. Elsewhere, blowouts are the result of erosion during storms, but blowouts on mudbanks are bare mud areas shielded by a fence of seagrass and accumulating sediments. These areas accumulate only transported sediment, in contrast to the grass beds around them that accumulate an additional component of skeletal material from organisms living in and on the grass.

Mudbanks are record keepers. At first glance it may seem that because the bulk of mudbank sediments are transported, there would be little opportunity for preservation of a sedimentological record of the ecosystem. But in protected areas, the local contribution of sediment from grass includes a fraction that is more coarse-grained than transported sediment. The local fraction that has accumulated during the last 35 years can be identified because it incorporates excess C-14 produced by atmospheric nuclear bomb testing in the 1960's, an excess still present in shallow surface waters of the Bay.

As bomb-produced C-14 accumulated in the surface water of the oceans, it was also incorporated into the skeletons of annually-banded corals. Analyses of individual annual bands from corals in the Florida Keys, provides a detailed local record of the rapid accumulation and subsequent slow decrease of radiocarbon in sea water. Coral and mudbank chronologies are independent, the former derived from counting bands back from the living surface of the coral, and the latter chronology established from lead-210 dates. This local coral C-14 history can be used as an ideal scale for comparison with the C-14 record from the coarse, in-situ portion of mudbank sediments. The comparison shows good agreement and indicates that 1) a portion of the sediment in mudbanks originates locally in grass beds (as originally proposed by Ginsburg in the 1950's) and 2) there is relatively little mixing in the coarse fraction. The composition of the coarse fraction reflects the ecosystem at the time of growth and deposition.

Seagrass beds in protected areas accumulate both transported (allochthonous) sediment and sediment from the organisms that live there (autochthonous sediment). The transported sediments are generally fine-grained because the energy of these areas is insufficient to move coarse-grained material. For sediments deposited since the late 1960's, allochthonous material has much less C-14 than autochthonous sediment which contains bomb produced radiocarbon. C-14 analyses of size fractions indicated that sediment coarser than about 250 mm is not transported, sediment between 250 and 62 mm is partially transported, and sediment less than 62 mm is almost entirely transported and includes much older material. This data indicates coarse fractions of Florida Bay cores reflect local conditions near the core site. The fine fraction represents conditions over a much broader region of the bay and is diluted by large contributions of transported older material.

The recognition that the bulk of Florida Bay sediment is old (more than 1,000 years) and has been deposited in mudbanks relatively recently (past few hundreds of years) attests to the importance of sediment transport processes in the Bay. Mudbank formation requires sediment transport, and in turn requires erosion, suspension, turbidity, and redeposition. The very existence of mudbanks is evidence that periodic turbidity is a long-standing characteristic of the estuary.

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