ENVIRONMENTAL QUALITY AND PRESERVATION— Bedrock Beneath Reefs: the Importance of Geology in Understanding Biological Decline in a Modern Ecosystem

The Importance of a Introduction Healthy Reef Ecosystem

At issue is the health of the entire The importance of a healthy reef eco- By examining the geologic record, coral reef ecosystem that lines the outer system is essential to the quality of human in this case the record of ancient reef shelf off the Keys. In recent life, whether in Florida or in areas world- growth, we can understand what controls years, nearly all biological components wide where other reef systems are also where and why modern reefs grow. Corals have declined markedly. In particular, deteriorating. The hard rock of reef frame- require narrow conditions: relatively shal- loss of coral species that are the building work serves as a protective offshore bar- low depths and stable salinity, clear nutri- blocks of a solid reef framework has rier to catastrophic wave action and storm ent-poor water, and warm temperatures. In significant negative implications for eco- surges generated by tropical storms and tropical areas, the primary control of these nomic vitality of the region. What are the hurricanes. Without this protection, elements (in other words, reef growth) is reasons for this decline? Is it due to natu- homes, marinas, and infrastructure on the sea level, whereas the primary control of ral change, or are human activities (recre- that are designed to capture reef distribution is bedrock topography, ational diving, ship grounding, farmland a lucrative tourism industry and inland that is, the shape of the rock surface under runoff, nutrient influx, air-borne contami- agricultural areas of South Florida that the modern reefs (Plate 1). In Florida, nants, groundwater pollutants) a contrib- feed much of the Nation are more sus- ancient reefs form much of the bedrock uting factor and if so, to what extent? To ceptible to destruction by these forces. surface. Mapping surface contours of the answer these questions, one must be able Moreover, biologic resources of the reefs pre-existing reef rock is the first step to distinguish between natural change and shoreline mangroves that include hab- toward reconstruction of the geologic his- and human impact. Logically, then, the itats for endangered species, productive tory and processes at work. Geophysical place to begin is to establish the geologic marine and wetland nurseries, and eco- data, collected on seismic profiles with framework and evolutionary history of nomic fisheries are also at risk. The irony high-resolution sound-wave instruments the ecosystem, before the advent of is that the very riches of this ecosystem that see through the overlying sediments, man. By so doing, accurate scientific attract the residential, recreational, and provide the bedrock-surface depth (eleva- information becomes available for proper commercial activities that may be contrib- tion) below sea level. authorities and Marine Sanctuary man- uting to its decline. agement to understand natural geologic and biologic change. Thus they are able to assess and lessen potential human bank, shelf, platform upper slope Florida impact more effectively. Keys rock ledge erosional surface fossil & modern outer-shelf 125 ka reefs & backreef trough patch reef 2 submerged fossil reef today lagoon Hawk Channel or margin reef 80 ka Figure 1. Schematic cross section (not to 1 3 lagoonal shelf backreef trough 5 scale) showing the six general bedrock 4 outlier reef provinces along the South Florida shelf and terminology used in text. The 200 ka(?) Pleistocene bedrock erosional terrace 6 surface Pleistocene ranges from ~1.77 Ma to 10 Ages and Positions of Sea Level ka. The Holocene ranges from 10 ka to the present. Sea-level positions and timing Pleistocene Physiographic Provinces shown in years before present. If sea level were to rise from its present position by 1 Shallow, submerged, inland lagoons or embayments 5 Reef ridge underlying Holocene accumulations and 5 to 7 m, it is very likely that framework- forming margin escarpment 2 Emergent Key Largo Limestone (coral facies) building corals would once again become and Limestone (sandbar facies) 6 Upper-slope terrace established on those areas of the Florida (a) seismic facies indicate sediment-covered dune Keys that are basically sediment free. 3 Submerged limestone rock ledge ridges on terrace to northeast 4 Broad trough-like bedrock depression lined (b) seismic facies indicate high-relief outlier reefs Ma = million years with non-coral grainstone and deep troughs on terrace to southwest ka = thousand years m = meters

U.S. Department of the Interior USGS Open-File Report 00–046 Figure 2. Seismic profile showing a “slice” or cross section through geologic features. This cross section displays the shelf margin at left and multiple prominent, parallel outlier reefs off Sand Key Reef, located at the edge of the shelf margin southwest of Key West (Plate 1, insert). The deep 'V'-shaped line called a multiple, at the bottom of the profile, mimics the sediment surface between the two largest outlier reefs. Outlier reefs are so called because they are disconnected from the adjacent shelf margin.

The U.S. Geological Survey (USGS) exist beneath offshore reefs, reveals when believed that sea level at that time was 7.6 Coastal and Marine Geology Program, in shorelines existed farther seaward. The m higher than at present. It has not been cooperation with the National Oceanic and mapped bedrock surface delineates areas as high since then. Atmospheric Administration’s (NOAA) of different elevations (Plate 1) and shows National Marine Sanctuary Program, con- where new coral growth would have The Shelf Margin tinues investigations of factors that may first become established. Because coral The same species of massive head be affecting Florida’s reefs. The USGS larvae generally settle on sites where the corals that constructed the Key Largo studies described here identify location bottom is hard and free of sediment, reefs Limestone also built an immense, younger and degree of development of major geo- began growing on elevated bedrock. Sand, reef that is now the submerged shelf logic features during the last 125,000 which prevents reef establishment, filled margin (Figs. 1-3). Uranium-thorium years (125 ka) and are based primarily on the depressions. The mapped surface also dates from these corals yield an age of 80 seismic geophysical data. shows pathways the water would have ka, marking yet another period when sea taken inland as the sea flooded the shelf level was high but not as high as when Environmental Signatures and thus positions and configurations of the Keys were formed. The massiveness in the Geologic Record ancient shorelines. (up to 30 m of relief) and regional extent of the margin reef indicate that conditions Bedrock Surface The Florida Keys were once optimal for coral growth nearly Analysis of the bedrock surface The upper and middle Florida Keys everywhere in the area. No younger reefs reveals facts on natural processes of past are themselves composed of the emergent can compare in either relief or extent reef growth and demise, rising and falling skeletal remains of a discontinuous coral with the 80 ka shelf-margin reef, denoting sea level, flooding of the bedrock sur- reef, called the Key Largo Limestone, that that environmental factors for reef growth faces, sediment movement and accumula- grew long before the patch reefs and out- since then have never been as good. tion, and changing water depth, circula- er-shelf reefs one sees today (Fig. 1). The tion, water chemistry, and wave energy. lower Keys were once sandbars actively Outlier Reefs Supplemental knowledge obtained from moving back and forth with the tides. Because of the optimal environment samples taken by coring through reefs Uranium-thorium dates from corals in the for reef growth at 80 ka, of particular documents various responses by corals fossil reef give an age of ~125 ka, identi- interest is the evolution of massive linear and reefs to rising sea level, such as coral fying a time when sea level was higher reefs that lie seaward of the margin reef species zonation and landward reef and than today. The dominant coral species off the lower and middle Keys. Called mangrove-shoreline migration. Uranium- comprising the emergent reef are massive outlier reefs, these reefs are located on thorium and carbon-14 (C14) dating of framework-building head corals that live an eroded upper-slope surface (terrace) corals tells us when they began to grow, in low-energy (deeper than 5 m) condi- thought to have formed ~200 ka (Figs. how long they lived, and when they tions (m = meters). They indicate that sea 2, 3). Although the outlier reefs contain died. C14 dating of mangrove peat, which level was too high for a surf zone to have the same massive head-coral species and forms at the shoreline and is known to existed where they grew. It is commonly are the same age as the reef at the shelf

2 In Florida, vitality of the fragile coral reef ecosystem depends on a critical equilibrium between physical processes and landforms. The primary process is rising sea level. Plate 1. Index map showing tip of South Florida and Cape Sable, the Florida Keys, The landform is the bedrock surface and its shape and elevation above and beneath the the in the Gulf of Mexico, and the bedrock surface along the shelf sediments. Coral reefs and associated organisms fl ourished along the outer shelf until and margin. The upper and middle Keys are formed of the 125-ka coral reef; the lower the sea fl ooded shallow inland bedrock depressions. As bay waters circulated seaward, The USGS serves the nation by providing basic, objective, Keys are formed of cemented tidal bars of the same age. Colors on the map represent and informative scientific knowledge on Earth history different bedrock elevations ranging from highest (pale blues) to lower (dark blues) to water quality and chemistry around the reefs changed, forcing a shift in biotic communities. and processes, and on human impact on resources. lowest (shades of green). Contacts between colors can be interpreted as representing Reef-building corals are no longer alive. Understanding the natural connection between Two of the most effective ways are through descriptive positions and configurations of ancient shorelines as sea level rose. Outlier reefs are and interpretive maps and through the USGS marine in shades of brown (light=high relief, dark=low relief or buried). The 80-ka shelf-margin geologic parameters and the well being of biologic resources is an imperative fi rst step homepage on the world wide web at: reef coincides with a dark blue line just landward of the outlier reefs. Note elevated toward resource-wise restoration, preservation and management of the ecosystem. http://marine.usgs.gov. rectangular platform on which the Marquesas Keys are located. Deep water on three sides and a current-swept, sediment-free channel on the east indicate that sediments 81° 30' 81° 80° 30' on the platform are formed in place. In general, the map shows bedrock elevations are several meters higher to the northeast than southwest, meaning the shelf flooded sooner The Shelf and shelf-margin reefs began growth sooner to the southwest. The least healthy modern reefs occur off the middle Keys, where wide, deep tidal passes between the Keys allow Margin turbid bay and cold gulf waters onto the reef tract. Cape Sable 82° 30' 82°

North

Gulf of Mexico Upper Keys Florida Bay Key Largo 25° 25°

Middle Keys Measured in Meters

Ellis Rock Lower Keys Land 24 - 26 New Ground Shoal Paleoshorelines 26 - 28 Buried Reef Flats 28 - 30 Outlier Reefs 30 - 40 1 - 2 40 - 50 2 - 4 50 - 60 Rebecca Shoal 4 - 6 60 - 70 6 - 8 70 - 80 Marquesas Keys Marquesas-Quicksands Ridge 8 - 10 80 - 90 10 - 12 90 - 100 Halfmoon 12 - 14 100 - 140 Shoal Key West 14 - 16 140 - 180 16 - 18 180 - 220 24° 18 - 20 220 - 260 24° 30' 20 - 22 260 - 300 30' 22 - 24 300 - 340 Miles Shelf Scale = 1:650000 Margin 0 5 10 15 20

Sand Key Reef 82° 30' 82° 81° 30' 81° 80° 30'

Open-File Report 00-046 Plate 1

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. N sea level S Figure 3. Interpreted seismic profile shown in 0 Figure 2. Largest outlier reef developed at seaward "satellite" outlier reefs Sand Key Reef core ~83-80 ka Straits of Florida edge of broad erosional surface (Pleistocene terrace). Note terrace surface is not visible beneath the reefs. Coral reefs and reef rubble or debris 15 characteristically obscure sound-wave reflectivity of Pleistocene backreef primary reef ridge trough or outlier reef rock surfaces beneath them. A core drilled into the at shelf basin top of the main outlier recovered 6.09 m of coral margin rock. Uranium-thorium dating yielded ages of 83 to 30 80 ka for corals in the core. Tick marks at bottom of Holocene interpreted profile indicate time (24-hr scale). sediments ~40 m Meters (m) Pleistocene terrace ka = thousand years 45 km = kilometers reef rubble m = meters

Pleistocene bedrock 60 1 km vertical exaggeration 14x 1840 1835 1830 1825

margin, they differ from the shelf-margin Outer-Shelf Reefs occurred. Creation of the bays contributed reef in regional extent, and they exhibit The most recent rise in sea level to demise of the reefs by allowing tidal varied stages of evolution. The outliers began at ~10 ka as a result of warming exchange of turbid lagoonal waters onto are mature off the lower Keys (28 m of temperatures and melting glaciers. C14 the reef tract. The lagoonal bays were relief), moderately well developed off the dates on corals from fossil outer-shelf filled when sea level was about 0.5 m middle Keys (less relief), and appear in reefs landward of the margin reef indicate lower than today. Prior to about 40 years seismic profiles off the upper Keys as that they first began growing at about 7 ago, modern reefs consisted of healthy low-evolution beach-dune features buried to 6 ka. Thus, what is now the modern branching corals. Today, very few of these by lime sand. The diverse evolution of reef tract was land until flooding of the corals are alive and those that are, are in the outlier reefs is believed to have shelf began about that time. The coral very poor condition. resulted in part from differences in sand- species in the outer-shelf reefs (Fig. 1) Cores taken from various locations dune development and the angle at which are different from the massive head corals through the outer-shelf reefs give us prevailing east southeasterly winds and that built the older reefs. Framework- insights into reef growth during rising waves impinged upon the arc-shaped shelf building branching corals that prefer high- sea level. Location of reefs (distribution), margin (Plate 1). Off the upper Keys, energy conditions constructed the outer- types of corals (head or branching), land- onshore tides during a period of lower sea shelf reefs and thus verify presence of ward reef migration (known as backstep- level washed sands over the dunes, bury- the surf zone when the reefs were alive. ping), and zones of changing coral species ing them. Off the lower Keys, longshore Branching corals are known to grow only are some of the corals’ responses to wind and wave energy moved sand paral- in water depths of up to 5 m, which indi- suitable site (elevated-bedrock availabili- lel to the margin, keeping the higher-ele- cates another relative position of sea level. ity), environmental conditions (protected vation incipient reefs sediment free (Fig. The extent and ages of the outer-shelf or open to surf), and rising sea level. 4A-E). Through time and with recurring reefs show they too thrived and lived from An increase in water depth alters cir- rises and falls in sea level, the outlier about 7 to 2 ka. Although discontinuous culation and wave energy, which are reefs off the lower Keys were repeatedly and not as massive as the older Key Largo also controlled by local bedrock topogra- colonized by corals that eventually con- Limestone reef, the shelf-margin reef, and phy. Increasing water depth changes water structed tall rock ridges as wide as 0.5 the outlier reefs, thickness of the outer- chemistry and temperature through tidal km. Today, the outlier reefs remain sepa- shelf reefs relative to thickness of modern flow of murky, nutrient-rich bay water rated from the margin by comparatively reefs indicates that conditions for coral and cold Gulf of Mexico water into the sediment-free backreef troughs (Figs. 2, growth from 7 to 2 ka were nonetheless clear warm water surrounding the reefs. 3). Only the top 6 m of the largest outlier considerably better than they are today. Human influence on these deleterious ele- have been cored and dated. The corals are Much of what is now under water was ments can severely impact an already nat- 80 ka. Although the age of the base of the still land until about 2 ka, when flooding urally weakened reef ecosystem. outliers is not known, it is likely that it is of the large shallow bedrock depressions many tens of thousands of years older. that became Florida and Biscayne Bays

3 Margin Evolution Off Lower Florida Keys Summary 0 E Holocene sands 10 The corals that form the middle and 20 Sand Key Reef ~80 ka upper Florida Keys verify the previous 30 existence of an inshore reef ecosystem. 40 Mangrove peats found beneath offshore sea level at present 50 outer-bank reefs backstep reefs record the locations of earlier, sea- lower-elevation corals on platform die 60 entire platform floods ward, shorelines. Zonation of coral spe- cies within a reef is evidence of increas- 0 D ing water depth and thus changing con- 10 ditions, from quiet water protected from 20 ~83 ka waves to high-energy surf to water too 30 sea level at -10 m deep for a surf zone. Sea level has outlier reefs die at 80 ka 40 risen and fallen many times before, upper 5.5 m of primary outlier ~83-80 ka 50 narrower troughs behind satellite outliers partially filled and the massive limestone outlier reefs outermost outlier reefs and margin build upward 60 were accordingly colonized, killed during exposure, and re-colonized. These signa- 0 C tures in the geologic record indicate that 10 reefs and shorelines have come and gone 20 in the past. Could the fragile modern eco- 30 system be undergoing yet another natural 40 TIME decline, or are human activities respon- 50 sea level at -20 m sible or, at the very least, an added com- patch reefs grow on platform 60 fringing reefs grow on outlier reefs and at paleomargin ponent? We cannot know without first

Meters Below Sea Level knowing what controlled evolution and 0 B 10 development of the ancient reefs, i.e., the bedrock characteristics, the processes 20 that produced them, and the responses 30 Pleistocene 40 the ancient reefs had to environmental infill change. The bedrock is far more impor- 50 Pleistocene sea level at -40 (dotted) and -30 m tant than simply being the surface beneath sediment-flow direction parallels margin and (?)dune ridges 60 fringing reefs grow at edges of (?)dune ridges modern reefs. The environmental signa- 0 tures it contains provide the baseline A position of modern margin escarpment 10 information from which understanding position of paleomargin at ~100 ka decline of the modern reef ecosystem cemented dune ridges(?) 20 ~125 ka(?) 30 must be derived. 40 50 terrace age ~200 ka(?) For more information, 60 Pleistocene sea level below terrace please contact:

Barbara H. Lidz Figure 4. Proposed model for evolution of lower Keys shelf margin (sea level fluctuated many 600 4th Street South times but positions correspond to events shown). (A) Formation and cementation of sand dunes St. Petersburg, FL 33701 on the erosional terrace surface during a sea-level lowstand. (B) Colonization of sediment-free shelf-margin reef and dune ridges by corals. (C,D) Upward building of margin reef and outlier phone: 727-803-8747 x3031 reefs over time and through numerous rises and falls of sea level. (E) Final demise of Pleistocene fax: 727-803-2030 corals on outliers at ~80 ka and later migration of younger (~7- to 2-ka) outer-shelf reefs with [email protected] shelf-wide flooding. Essentially no corals are alive on the outliers today. However, if sea level were to fall by some 10 m or so, it is very likely that framework-building corals would once again become established on the hard rock surface of the outlier reefs. This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards ka = thousand years or with the North American Stratigraphic Code. Reference Holocene = <10 ka therein to any specific commercial product, process, or Pleistocene: service by trade name, trademark, manufacturer, or otherwise outlier-reef age = ~83-80 ka does not necessarily constitute or imply its endorsement, rec- terrace age = ~200 ka(?) ommendation, or favoring by the United States Government m = meters or any agency thereof.

4 USGS Open-File Report 00–046