Is holocene storm-generated stratification in Florida Bay a reflection of solar storm cycles?

Palaeogeography, Palaeoclimatology, Palaeoecology, 76 (1989): 169 185 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

169

Is Holocene storm-generated stratification in Florida Bay a reflection of solar storm cycles? GIANNI GALLI 1 Comparative Sedimentology Laboratory, Fisher Island Station, Miami Beach, FL 33139 (USA)

(Received January 23, 1989; revised and accepted July 24, 1989)

Abstract Galli, G., 1989. Is Holocene storm-generated stratification in Florida Bay a reflection of solar storm cycles? Palaeogeogr., Palaeoclimatol., Palaeoecol., 76: 169-185. A descriptive analysis of surficial sediments of Crane Key (Florida Bay) showed that the sediments consist of storm layers (winter storm and hurricane deposits) and algal laminated sediments. Storm layers are riddled with the following types of cavities: gas escape vugs; dissolution vugs; burrows; rootholes and cryptalgal vugs. Structures and sediment types are arranged into a 15 cm thick thickening-upward, storm-generated sequence which formed in approximately 100 yr under a deepening trend. Periodograms of sea-level variations match the frequency distribution of strong intensity storms which occurred in south Florida since the beginning of this century. The calculated recurrence time of strong storms (10±3yr) and the time interval of formation of the sequence (100±25 yr) are probably a response of climatic parameters to short-period (ll-yr) and longer-period (90-110 yr) cycles of solar activity. Comparison with the ancient record shows analogous dm-thick storm-generated sequences probably linked to solar cycles and 102 yr sea-level rises.

Introduction R e c e n t l y Williams a n d S o n e t t (1985) concluded t h a t climatic cycles reflected by r e g u l a r v a r i a t i o n s in the t h i c k n e s s of v a r v e s in the E l a t i n a F o r m a t i o n (late P r e c a m b r i a n , s o u t h A u s t r a l i a ) are a precise r e c o r d of s o l a r periods. P e r i o d i c v a r i a t i o n s in s o l a r a c t i v i t y in the late P r e c a m b r i a n h a d a direct c o n t r o l on climate as a r e s u l t of a r e d u c e d level of ozone in the a t m o s p h e r e (Williams a n d Sonett, 1985). At p r e s e n t solar cycles w o u l d be c a p a b l e of s u b t l y m o d u l a t i n g the E a r t h ' s climate. T h e r e is growing evidence for a s t r o n g c o n n e c t i o n b e t w e e n the s o l a r cycle a n d w e a t h e r (Gribbin, 1973; King, 1973; S o n e t t a n d Suess, 1984). In this

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p a p e r the n o n - r a n d o m v a r i a t i o n in i n t e n s i t y and f r e q u e n c y of t r o p i c a l storms is t a k e n as an evidence t h a t t h e y are c o n t r o l l e d by climatic f a c t o r s driven by s o l a r periods (11- and 100-yr). This c o r r e l a t i o n results from a s e d i m e n t o l o g i c s t u d y of surficial sediments in F l o r i d a Bay c o n d u c t e d t h r o u g h the following steps: (1) D e s c r i p t i o n of v e r t i c a l v a r i a t i o n s in s t r u c t u r e s w h i c h form a dm-thick s t o r m - g e n e r a t e d seq u e n c e by m e a n s of c a l i b r a t i o n of open-space v u g s and s t o r m layers; (2) e v a l u a t i o n of the role of s h o r t - t e r m sea-level trends a n d s t o r m f r e q u e n c y d i s t r i b u t i o n in the d e v e l o p m e n t of the sequence; and (3) r e l a t i o n s of climatic and s e d i m e n t a r y t r e n d s to cycles of solar activity.

Environmental setting F l o r i d a B a y consists of a complex anastom o z i n g n e t w o r k of low relief c a r b o n a t e b a n k s

© 1989 Elsevier Science Publishers B.V.

170

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and islands developed within a shallow lagoon. Based on variability in density of islands and banks (Fig.l), the Bay is divisible into a western and an eastern area. H. R. Wanless (1987, pers. comm.) distinguishes a "constructional" and a " d e s t r u c t i o n a l " area in the west and east respectively. Bosence (1988) considers " o u t e r Bay mounds" in the west and " i n n e r Bay mounds" in the east and relates this distinction mainly to different rates of carbonate productivity. " O u t e r Bay mounds" are characterized by a hi ghe r sediment productivity. Th ey are also strongly bioturbated as a result of their location in an open marine lagoonal area. In the southwest, mud banks are wider and a c c r e t i o n a r y towards SW; coastal levees which fringe the s out her nm os t border of Everglades prograde seaward. In the "destruct i o nal" area the sedimentation r at e is reduced,

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coastal levees do not prograde and the sedim e n t a r y interface is undergoing a deepening. Banks are n a r r o w e r in extent, do not produce enough sediment for mound growth and are characterized by a predominance of physical structures. In the " d e s t r u c t i o n a l " area the influence of storms on sedimentation is dramatically apparent, as is shown by Wanless (1979) and Taggett et al. (1986) who show t hat i nt ernal stratification in mud banks consists of a number of basal skeletal lags overlain by mudstone units. Mud banks and islands are subjected to two types and directions of storms: winter storms moving from NE and hurri canes from SE. Islands intercepted by winter storm wind paths are elongated in plan, normal to the direction of wave approach and have developed a windward and a leeward side. The keys within Florida Bay were deposited during a

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Fig.1. Florida Bay (Ginsburg, 1956; Enos and Perkins, 1979). Crane Key is probably one of the most suitable places for studying fine-grained storm layering within the Bay. Bioturbation is not as strong as in the westward area; physical structures are predominating; the island, intercepted by winter storms and hurricanes, due to its seaward location, grows by lateral migration of sediment u n d e r the influence of storms, as muddy dunes (Gorsline, 1963).

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continuous relative sea-level rise which began in the SW about 4500 yr ago (Scholl et al., 1969; Enos and Perkins, 1979). The stratigraphic record of the Keys represents a transgressive sequence consisting of an asymmetric cycle capped by intertidal and supratidal deposits as is exemplified by the profile of Crane Key (Fig.l). This consists of the following units (Fig.2). (1) Basal coarse mollusc grainstones and packstones (shallow marine bay); (2) intermediate mud bank deposits containing few fossils (mainly molluscs occurring as storm intercalations); and (3) upper island deposits characterized by algal laminated sediments with intertidal-supratidal structures and cmthick storm layers. The material for this study are four box cores (25 cm in length; 20 cm in width) taken along a NW-SE direction, at Crane Key.

G. GALLI

review of their nomenclature and origin is found in Shinn (1983), and Flfigel (1984). These textures were termed bird's eyes (Ham, 1952), fenestrae (Tebbutt et al., 1965), shrinkage pores, loferites (Fischer, 1966), tubular fenestrae (Read, 1975), etc. They form by bubble escape, differential internal shrinkage (Shinn, 1983), burrowing or root penetration (Read, 1985). Shinn (1983) recently proposed to restrict the term "fenestra" to true fenestrae or bird's eyes. The following five types of cavities are recognizable within the recent sediments of Crane Key: (1) gas escape vugs; (2) dissolution vugs; (3) burrows; (4) root holes; and (5) cryptalgal vugs. Types (1) and (2) represent fenestrae, whereas types (3)-(5) are pseudofenestrae. The use of a genetic terminology, suggested by Shinn (1983), was facilitated by the preservation in several vugs of root tubules, remains of algal mats and burrowers.

Petrography The uppermost uncompacted 30 cm of sediments at Crane Key consist of the following units: (1) algal laminated sediments; and (2) storm layers. Storm layers are pervasively riddled with open space vugs. The following petrographic description deals with calibration of storm layers and open space structures. Fenestrae and pseudofenestrae are visible up to a few decimeters below the sedimentary interface but are obliterated by compaction at greater depths. Conversely, algal mats, storm layers and mud-cracks are preserved (cf. Shinn, 1985: Fig.l).

Open-space structures (Figs.3 and 4) The terminology concerning open-space structures is complicated; a comprehensive

(1) Gas escape vugs (Fig.3A) Most of these vugs are characterized by irregular, scalloped boundaries and highly irregular shapes (Fig.4). Cavities are isolated but locally merge into compound cavities. Sizes range from 0.3 to 1 mm. The most typical vug type is given by a roughly rounded outline with a convex-upward, lower boundary and a more irregular upper surface (Fig.4). Some of the cavities are parallel to the bedding; when obliquely oriented, lateral boundaries display symmetrical undulations with respect to the axis of the elongated vug, with the production of systematic enlargements and narrowings. The bottom of vugs in such cases may be nearly flat. Nearly flat lower surfaces suggest a formation while the sediment was still wet; the site of migration of bubbles is represented by the upper, scalloped, jagged surfaces. The pinch-

Fig.3. Open space structures.. A. Gas escape vug. B. Dissolution vug. C, D. Burrow profile and cross section showing rough, saw-toothed edges of vugs. E. Rootholes characterized by simple geometrical or subspherical shapes and rounded outlines. F. Rootholes. b is probably a gas escape vug (compare the sausage-like shape of b with t h a t of cryptalgal vugs where walls m a i n t a i n a parallelism to each other after dissolution). G. Rootholes with typical "smeared fabric" resulting from a random, patchy distribution of algal debris. I. Fragments of algal mat (above) and, "smeared fabric" below, within h u r r i c a n e D o n n a deposit. J - L . Cryptalgal vugs. Algal mats in J are faintly visible due to their partial dissolution and nearly completely dissolved (K, L) 20 cm below the island floor. Their random o r i e n t a t i o n (K, L) originates from a mass transport during winter storms. The similarity of the vug p a t t e r n to "smeared fabric" shown in G and I suggest t h a t vugs represent algal fragments.

IS STORM-GENERATED STRATIFICATION REFLECTION OF SOLAR STORM CYCLES?

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and-swell shape of the vertical voids points to a progressive upward migration of bubbles through the host sediment.

(2) Dissolution vugs (Fig.3B) These vugs typically occur immediately below the upper surface of some of the peloidal grainstones (described below). Some of these vugs are triangular, with the upper planar surface parallel to the upper layer. They generally tend to be irregular with one side characterized by a downward enlargemen t. They are isolated but regularly spaced from each other and parallel to the bedding. Anastomozing patterns with irregular boundaries result from merging of formerly closely spaced, planar vugs, as is shown in Fig.4. These vugs look like miniature caves in that mimic the outline of vertical profiles of caves formed in karstic regions. They may result

from a progressive lateral and downward enlargement of former vugs produced by a leaching and dissolution of calcium carbonate which may have resulted from microbial decay of organic matter responsible for a rising of carbon dioxide and a consequent lowering in the pH values.

(3) Burrows (Fig.3C, D) Burrow-vugs develop vertical shafts. Horizontal galleries are less common. The following diameters were observed: 0.25 mm; 2 cm; 0.5 cm and 2 mm (Fig.4). They can be easily distinguished from other types of cavities by their sizes (2-0.5 cm), smooth, straight and sinuous tracks, absence of branches, regularity of boundaries as seen from longitudinal profiles and sharp, non gradational margins with a general parallelism between walls. Larger burrows are characterized by dark haloes.

IS STORM-GENERATED STRATIFICATION REFLECTION OF SOLAR STORM CYCLES?

Vertical haloes are intercepted as escape traces. Spreiten structures are frequent. Two mm-wide burrows are attributed to Polychaetes, or to shrimps such as Apseudes sp., 10mm-wide burrows could represent traces formed by Nematode worms or also insects. Three mm-wide burrows are formed by eunicid Polychaetes, such as Morphysa sanguinea. The largest burrows are attributed to fiddler crabs which typically excavate galleries in levees and marsh areas. Darker haloes around burrow outlines (Fig.3D) may represent bio-induced micritization and cementation (Evans and Ginsburg, 1986).

(4) Rootholes (Fig.3E-G, I) Root holes are ubiquitous. Holes show a wide range of diameters (0.2-0.1-0.5-1mm) and shapes. The most common types are shown in Fig.4. The following features allow for a distinction from other types of cavities. Some voids, such as those of Fig.3, still contain remains of root tubules and vegetal debris which typically appears as dark, yellow or reddish stains. Perimeters of walls form regular shapes (e.g. spherical, quadrangular, etc.) which suggests a former adjustment of the host sediment to root penetration. By contrast, shapes of vugs described above are more irregular. Holes do not display any preferential development within the sediment (for example, parallel to the layers) and often occur in patches, irrespective of lithology. In other cases such vugs are equidimensional and statistically distributed within the matrix. Downward bifurcations of vugs occur at places. These rootholes may represent roots of grass such as Distichlis spicata and Monanthochloe littoralis which at present colonize the intertidal mangrove swamp.

(5) Cryptalgal vugs (Figs.3H-L) The majority of these very thin and long vugs (Fig.4) shows a trend parallel to the bedding, but there are frequent variations in length and orientations. The outlines vary from a filigree to a pinch-and-swell pattern with scalloped boundaries. In some cases algal

175

laminae are still recognizable, especially close to the top of box cores (Fig.3H). With increasing depth they probably are subject to a progressive dissolution as is testified by yellow colored vegetal remains parallel to the bedding (Fig.3J). Cryptalgal vugs with variable orientations are thought to represent former fragments of algal mats which have undergone a local transport or deformation during storms (Fig.3K, L).

Storm layers With the exception of the uppermost few cm of algal laminated stromatolites, sediments of Crane Key consist of a stacking of mud layers and peloidal grainstones in association with very thin intervening algal mats (Fig.5).

Mud layers Mud layers occur along three levels within the 25 cm of cored sediments (Fig.5). Maximum thicknesses are about 5 cm. They are easily recognizable from polished surfaces of impregnated box cores by a medium-gray color, homogeneous appearance and rarity of open space rugs, mainly rootholes. A sequence of structures, shown in Fig.6 (left) is 3 cm thick and consists of two units. The lower contact is erosional. The lower unit is composed of mud containing mm-thick levels and poorly sorted calcisiltites and fragments of algal laminae which show various degrees of disruption. The unit is sharply transitional to an upper mud-supported unit which contains scattered vegetal debris and rarer fragments of algae aligned parallel to the bedding. Hummocky cross lamination occurs in the upper mud layer where laminae are subparallel to undulatory with respect to the lower surface and display a lateral thickening and fanning into domes.

Peloidal grainstones Peloidal grainstones are the predominating lithology in the uppermost 30 cm of sediment at

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tion, etc.) as shown by Hardie (1977), Perlmutter (1982), Aigner (1985) and Wanless et al. (1988). In the "destructional" area of Florida Bay hurricanes generate mud layers as recorded by Ball et al. (1967), Perkins and Enos (1968) and Warzesky (1976) who detailed erosional and depositional effects of hurricane Donna (the strongest hurricane reported in Florida: uppermost mud layer in Fig.5A-C). The scarcity of open space vugs within mud layers may be related to a post-storm, lag time period before recolonization which would reflect a long recovery time of biological recolonization after the devastation produced by hurricanes. Thicknesses of peloidal grainstones are within the range of sediments deposited by winter storms (4 mm: Ginsburg, pers. comm., 1987). A storm generation of peloidal grainstones is inferred from the erosional contacts of layers above algal laminated sediments,

Crane Key. They average 0.5 cm in thickness (Fig.5A: g) and consist of poorly sorted admixtures of peloids, intraclasts, bioclasts and vegetal debris. They are light-gray to yellowish in color. Burrows, fenestrae and pseudofenestrae are abundant. These layers are either stacked directly on each other (especially in the lower to middle part of box cores) or regularly interbedded with algal laminated sediments (Fig.5D). A sequence is given by a 1-0.5 cm thick unit (Fig.6: right) which sharply overlies algal laminated sediments. It displays some distribution grading. The upper laminaset, locally hummocky cross laminated, is transitional to algal laminated sediments with interspersed variable amounts of carbonate particles and eventually to dense, undisturbed, smooth algal mats. Storm sedimentation is controlled by environmental parameters (topography, vegetaHURRICANE

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Fig.6. Sketches of cm-scale hurricane and winter storm sequences. Individual laminae within the hurricane deposit are related to a rapid deposition of a suspension load probably associated with the bottom shear which took place during the waning phase, or the product of repeated flooding and ebbing of currents developed during the evolution of the hurricane (Ball et al., 1967). Normal grading within the winter storm layer microsequence indicates a gradual decrease in current energy. Dotted arrows indicate gradual transitions; entire arrows: sharp transitions. Fig.5. Box cores from Crane Key. A. Leeward mangrove swamp. (cf. Fig.2). g: peloidal grainstone layers (winter storm layers), als: algal laminated sediments, m: mud layers (locally mud-cracked), rh: root holes, rt: rhyzoturbation (evidenced by dots). Vertical fractures were produced during sample preparation. B. Internal mangrove swamp (cf. Fig.2). rn: mud layers (the upper one deposited by hurricane Donna). al: algal laminae, b: burrow, rh: root holes: s: spreiten structure, hl: hummocky lamination within storm layers, g: escape vugs. C. Leeward storm ridge (cf. Fig.2). Nearly complete homogenization of sediment by organism and plants. The big hole in the middle is probably produced by a land crab. s: spreiten structure activated probably by a sudden deposition of mud layer (rn) which is faintly visible (arrows). rh: rootholes; g: plane-spiraled gastropod. Thick mud layer at the top was deposited by hurricane Donna in 1960. D. Windward storm ridge (cf. Fig.2) showing supratidal brecciation and alternations between storm layers and algal laminated sediments. Mud layer (m) was affected by rhyzoturbation (rt: evidenced by black spots) probably produced by prop roots of mangroves, sf: smeared fabric (cf. Fig.3F). Black levels in the core consist of compressed algal filaments.

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distribution grading which is visible in some layers and the gradual transitions to the fine grained or algal laminated sediments which indicate a gradual decrease in current intensity. Hummocky lamination is also associated with storm-generated sediments. Frequent spreiten structures and vertical burrows evidence for frequent depositional events. These sediments record winter storms which determine a raise of the Bay level and flooding over the whole intertidal or supratidal flat. There is compositional evidence for a local source of sediment. Although more frequent t h a n hurricanes (14-15 winter storm layers versus 2-3 hurricane layers within 30 cm) winter storms are less destructional as is demonstrated by the

thinner layers (4mm vs. 3 cm of sediment deposited by hurricanes: Fig.5 and 6) and abundant fenestrae and pseudofenestrae which evidence for a rapid post-storm biogenic recoIonization.

Thickening-upward sequence (Fig. 7) The two 15-10 cm thick sequences observable in box cores are composed of repeated alternations of peloidal grainstone units and are characterized by the following vertical trends: - - Change from horizontal to vertical burrows; upward decrease in abundance of openspace vugs; -

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IS STORM-GENERATED STRATIFICATION REFLECTION OF SOLAR STORM CYCLES?

relative upward increase in rootholes and cryptalgal vugs with respect to fenestrae: slight upward increase in thickness of algal laminated sediments (from 7mm to 12-20 mm on top: fig.5A, D); and upward increase in thickness of stormgenerated sediments (thickening-upward trend). The sequences end-up with a thicker 2-3 cm thick storm-generated mud layer of presumed hurricane origin. The sequences are considered below in relation to the time interval of formation, storm frequency distribution and short-term sea-level fluctuations. -

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Age of sediments Two different routes were followed in the calculation of the time interval of deposition of each sequence. The first method stems from the calculation of the rate of growth of algal laminated sediments, radiocarbon dates and thickness of box cores. The second method involves a consideration of the storm frequency distribution. Rubin and Suess (1955) give a radiocarbon date of 3300 ± 240 yr for Crane Key.This value, divided by the thickness of the whole Holocene sediments (3 m), gives an average sedimentation rate of l m m / y r . This value can be reproduced both between individual storms and over the thickness from hurricane Donna layer to successive storms and the top of box cores (dated to 1987). The uniform spacing of algal laminated sediments between stormgenerated sediments and their constant thickness suggest t h a t fair-weather periods between storm events were of the same duration (cf. Fig.5D). Ginsburg, et al. (1954) report an average sedimentation rate of algal laminae at Crane Key of 1.5 mm/y. Algal laminated sediments deposited above hurricane Donna layer (dated to 1960) during the last 27 years are 2.7 cm thick: hence, the growth rate of algal laminated sediments intercalated with storm layers is 1 mm/yr. The average thickness of algal laminated sediments observable from box cores is 1-1.5 cm. This value, multiplied by the

179

sedimentation rate of l m m / y r , provides an estimate of the duration of fair-weather periods between storms of about 10-15 yr. The average time period between hurricanes in south Florida is 5-8 yr (Wanless et al., 1988). However, the yearly variation in the number of storms from 1900 to 1960 (Fig.8), together with a consideration of intensity of storms (Tannehill, 1960: tab.29;NOAA National Ocean Survey) indicates a periodicity of the highest frequency of major to extreme intensity storms (winter storms and hurricanes) of 10± 3 years (1906; 1916; 1926; 1935/1936; 1948; 1960: Fig.8). Only the strongest storms and hurricanes are expected to leave a sedimentary record. Hence, 10 years are a reliable approximation of the periodicity of major storms recorded in box cores. Figure 8 reports the occurrence of strong storms in the whole Florida peninsula. On the other hand, south Florida was affected by one extreme intensity storm every 10 years (exceptions are represented by the occurrence of 3 important storms in the years 1950-1948 and 1910-1906, and by their absence in the period 1880-1890: Tannehill, 1960). the number of storm layers observable from box cores (14-15: Fig.7) multiplied by the highest frequency of storms gives an age of about 200 yr. A close value of 220-250 yrs is obtained by multiplying the sedimentation rate of 1 mm/yr by the thickness of box cores. Two thickening-upward sequences can be observed in the uppermost 30-25 cm of sediment recovered by box cores; consequently the time range of formation of each thickeningupward sequence is 100 + 25 yr.

Storm frequency distribution and sea-level rise In an intriguing paper Wanless (1982) illustrated the importance of short-term (10-100-yr) sea-level fluctuations and their possible influence on sedimentation. Figure 8, redrawn from Wanless (1982) indicates t h a t phases of sea-level rise in the last 48 years in south Florida and Bermuda occurred in 1936, 1947, 1960 and 1972. A visual comparison between periodograms

180

O. GALLI

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IS STORM-GENERATED STRATIFICATION REFLECTION OF SOLAR STORM CYCLES?

of storm frequency distribution and relative sea-level changes suggests t h a t periods of highest frequency of tropical cyclones are related to short-term sea level rises (see also the curve of changes in sea level from Scandinavia reported in Fig.8 from Gutenberg, 1941). The two time series analyzed by means of simple Fourier transform (Fig.9) confirm the visual match because they show a peak in the smoothed power spectrum at about 13 yr (sea level data) and 10-14 yr (storm data). Other peaks are less significant; the peak at about 7-5 yr represents a higher harmonic of t h a t fundamental and may correspond to the average recurrence time of tropical storms reported by Wanless et al. (1988). Alternations of storm layers and associated variations in open space vugs are a response to short-term (100-yr) sea-level rises. Upward increase in thickness of storm layers is a result of an increase of storm intensity through time (each storm was stronger than the previous one) which is probably linked to climatic trends. The upward decrease in true fenestrae and the slight increase in thickness of algal laminated sediments along the sequence are both related to a deepening trend (the rate of

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181

deposition of algal laminated sediments is higher in areas more frequently exposed to tidal flooding: Ginsburg et al., 1954). It is u n k n o w n if/how this trend is related to the general transgressive trend of Florida Platform. Perhaps the deepening is evident in Crane Key and nearby islands and banks owing to a reduced sedimentation rate in the "destructional" area of Florida Bay. As a matter of fact, Crane Key at present is not a typical supratidal-intertidal flat, because the floor of the island remains submerged by a thin veneer of water during most of the year.

R e l a t i o n s h i p s w i t h c y c l e s o f solar a c t i v i t y What does the temporal match between short term sea-level rises and storms mean? Is there a common cause? The following discussion suggests that both are controlled by cycles of solar activity. The number of spots on the surface of the Sun varies periodically with time and approximately every 11 yr (9-14 yr according to Cohen and Lindtz, 1974, and others) the sunspot density reaches a maximum value. This cycle may be related to oscillatory variations in the magnetic field below the photosphere (Bakulin et al., 1984). Associated with sunspot cycles are several other phenomena such as sudden releases of energy (flares), variations in flux of solar radiations, etc. In view of this fact the sunspot number is being considered within a more comprehensive "cycle of solar activity". Superimposed on the l l - y r cycle there is a 22-yr cycle (Hale magnetic cycle) which was related to inversions in the polarity of magnetic fields. Longer term estimates of undulations in the solar cycle are statistically less reliable, due to a limited record of complete ll-yr solar cycles. In fact, observations of solar cycles began more or less 300 years ago. Extrapolations and spectral analyses revealed a 160-190 yr cycle (179 yr according to Cohen and Lindtz, 1974; 200 yr according to Winstanley, 1973) which was related to the tidal effect of planet alignments on the Sun. Studies conducted by Williams and Sonett (1985) from a complete

182

late Precambrian sequence containing 1580 varves led to the discovery of a complete spectrum of temporal cycles probably linked to solar activity: 9-14 yr; 22-25 yr; 90-110 yr (this cycle was also obtained by Sonett and Suess, 1984 and Cohen and Lindtz, 1974, and is evident from trends of sunspot density cycles reported in Fig.8); and 157-314 yr (Elatina cycle: Williams and Sonett, 1985). The above authors found also very long periods (up to 9330 yr) of still obscure link with solar cycles. It is known that solar storms are responsible for an increase in ionization, magnetic storms associated with short-term changes in the magnetic field, and aurora. Physical mechanisms by which solar activity and terrestrial climate are linked are obscure. That short-term changes in solar activity have a counterpart in terrestrial climatic changes was demonstrated impressively by several authors. Gribbin (1973) postulated changes in the Earth's spin rate produced by an increase in level o f solar cosmic rays during large solar storms. King (1973) established an increase in temperature in the upper atmosphere and southward shift of climatic zones. This is also apparent from an examination of Fig.8 where most of the peaks of the chart of the global mean temperature (Jones et al., 1988) match the peaks of the l l - y r solar cycle. Geller (1988) showed that the solar flux influences atmospheric temperature. Sonett and Suess (1984), based on tree-ring studies, found a relation between an increase in 14C and variations in cosmic rays. M6rner (1976) hypothesized a close correlation between short-term magnetic intensity fluctuations, change in magnetic declinations, atmospheric changes and 14C changes, based on detailed studies carried out in Scandinavia. King (1973) correlated the ll-yr cycle to variations in length of the growing season (number of days in the year on which the averaged temperature exceeds 5.6°C), as well as to variations in runoff.

Examination of Fig.8 suggests that the frequency of storms and sea-level rises increases during the solar cycle: this is not surprising, if o n e considers mutual correla-

G. GALLI

tions between variations of solar flux and atmosphere temperature (Geller, 1988), the match between peaks in the chart of global surface air temperature (Jones et al., 1988) the storm frequency distribution (Fig.8) and the correlation between sunspot cycles and sealevel variations (Fairbridge and Krebs, 1962). Inverse correlations have been reconstructed between cold atmospheric periods, such as the Pleistocene ice age (Van der Hammen et al., 1981) and frequency and intensity of tropical cyclones (Hobgood and Cerveny, 1988). Markson (1978) proposed a hypothetical mechanism by which solar variations cause changes in the atmospheric ion±sat±on; these in turn would be responsible for general weather modifications such as electrification of winter storm clouds and a consequent increase in strength of winter storms. The 10-11 yr periodicity of strong tropical storms recorded in the South Florida platform and Great Barrier Reef (Fig.8) would be the product of such a mechanism. Periods of sea-level fluctuations recorded by Gutenberg (1941) and Wanless (1982) and highest frequencies of major to extreme intensity storms (Fig.8) from Florida (and Great Barrier Reef) correlate with periods of high density of solar spots (ll-yr cycle) (as is also suggested by an examination of the smoothed power spectra shown in Fig.9). It is tempting to interpret the calculated time span of formation of the thickeningupward sequence (100 ± 25 yr) and the periodicity of strong storms (10±3 yr) as a direct reflection of the ll-yr and 90-110 yr solar cycles. Vertical increases in thickness of storm-deposited layers have a counterpart in the gradual increase in amplitude of the ll-yr cycles (increase in the intensity of solar storms) along 90-110 yr cycles. The strongest storm (Donna 1960) also matches the peak in the 90-110 yr cycle. The location of hurricane layers at the top of the thickening-upward sequences suggests some physical linkage between hurricanes and cycles of solar activity. However, hurricane layers occurring in box cores at Crane Key are too few to allow for such a correlation. This scarcity may be due to

IS STORM-GENERATED STRATIFICATION REFLECTION OF SOLAR STORM CYCLES?

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the randomness of hurricane tracks in south Florida (Tannehill, 1960). The algal-laminated sediments above hurricane Donna deposit (Fig.5) and a lack of important storms following Donna in 1960 may be explained by a deposition during the beginning of another 100-yr cycle (according to Cohen and Lindtz, 1974, this new cycle should be 86 yr long).

Comparison with the ancient record The finding of a thickening-upward alternation of storm deposits gives an opportunity to evaluate similar alternations in ancient shallow water carbonate platforms. Ancient examples of thickening-upward sequences formed in very shallow water carbonate settings, taken from various localities and ages, are shown in Fig.10. Figure 10A, B (Galli, 1988) show a 1.5 m thick thickening-upward sequence which is analogous in composition and vertical facies trends to the stratigraphic cycle of Florida Bay (Fig.2) (are these m-scale cycles the product of long-term climatic oscillations tuned in the band of solar cycles?). Individual din-thick sequences within the mscale cycle resemble the sequence described above. Other ancient examples shown in Fig.10G (Galli, 1984, 1986) and Fig.10F (Furrer, 1985) consist of an upward increase in thickness of storm deposits probably related to the 100-yr sea-level rises.

Conclusions The calibration of open-space cavities has shown that true bird's eyes (Shinn, 1983) (gas escape and dissolution vugs) represent only a small proportion of open space cavities at Crane Key. Most of vugs consists of burrows, rootholes and cryptalgal vugs. An application of these criteria to the ancient may provide more precise informations on sea-level position and short-term paleoclimatic trends. Lithologies consist of two types of storm layers: mud layers (hurricane deposits) and peloidal grainstones (winter storm deposits). Such lithologies are arranged vertically into

O.GALL~

thickening-upward sequences. Each sequence is 10-15 cm thick and represents a time interval of 100 + 25 yr. Vertical trends of structures and sediment types are related to an increase in frequency and intensity of storms concomitant with bursts of relative short-term sea-level rises. The highest frequency of strong storm layers and the time interval of formation of the sequence match closely l l - y r and 90-110 yr solar cycles. The uppermost 25 cm of sediments at Crane Key preserve an ordered hierarchy of periods linked to solar cycles.

Acknowledgments S a m p l e acquisition a n d preparation w a s conducted under the guidance of Dr. R. N. Ginsburg w h o stimulated the study of recent sediments. Dr. H. R. W a n l e s s provided published a n d unpublished data o n storm sedimentation. T h e content of the paper is m y only responsibility. Drs. E. J. Barton, B. v a n H o u t e n a n d an a n o n y m o u s referee reviewed the text.

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Perkins, R. D. and Enos, P., 1968. Hurricane Betsy in the Florida-Bahamas area - - geological effects and comparison with Hurricane Donna. J. Geol., 76: 710-717. Perlmutter, M. A., 1982. The recognition and reconstruction of storm sedimentation. Thesis. Univ. Miami. Pray, L. C., 1966. Hurricane Betsy (1965) and nearshore carbonate sediments of the Florida Keys. In: Geol. Soc. Am. Annu. Meet., San Francisco, 1966, pp. 168-169. Read, J. F., 1975. Tidal-flat facies in carbonate cycles, Pillara Formation (Devonian), Canning Basin, Western Australia. In: R. N. Ginsburg (Editor), Tidal Deposits. Springer, New York, N.Y., pp. 251-256. Rubin, M. and Suess, H. E., 1955. U.S. Geol. Surv. Radiocarbon Dates. Sci., 121, 3145: 481-488. Scholl, D. W., Craighead, F. C., Sr. and Stuiver, M., 1969. Florida submergence curve revised: its relation to coastal sedimentation rates. Science, 163: 562-564. Shinn, E. A., 1983. Birdseyes, fenestrae, shrinkage pores, and loferites: a reevaluation. J. Sediment. Petrol., 53(2): 619-628. Sonett, C. P. and Suess, H. E., 1984. Correlation of bristlecone pine ring widths with atmospheric C variations: a climate-Sun relation. Nature, 307: 141-143. Tagett, M. G., Wanless, H. R. and Cottrell, D. J., 1986. Gradients in carbonate mudbank stratigraphy and dynamics: Florida Bay, south Florida. Abstr. SEPM Annu. Midyear Meet., Raleigh, 3: 108. Tannehill, I. R., 1960. Hurricanes. Princeton Univ. Press. Tebbutt, G. E., Conley, C. D. and Boyd, D. W., 1965. Lithogenesis of a distinctive carbonate rock fabric. Univ. Wyo. Contrib. Geol., 4:1-13 Van der Hammen, T., Barelds, J., De Jong, H. and De Veer, A. A., 1981. Glacial sequence and environmental history in the Sierra Nevada del Cocuy (Colombia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 32: 247-340. Waldmeier, M., 1961. The Sunspot activity in the Years 1610-1960. Technische Hochschule, Zurich. Wanless, H. R., 1979. Role of physical sedimentation in carbonate bank growth. Am. Assoc. Pet. Geol. Bull., 63: 547. Wanless, H. R., 1982. Editorial. Sea level is rising - - so what? J. Sediment. Petrol., 52(4): 1051-1054. Wanless, H. R., Tyrell, K. M., Tedesco, L. P. and Dravis, J. J., 1988. Tidal-flat sedimentation from Hurricane Kate, Caicos platform, British West Indies. J. Sediment. Petrol., 58(4): 724-738. Williams, G. E. and Sonett, C. P., 1985. Solar signature in sedimentary cycles from the late Precambrian Elatina Formation, Australia. Nature, 318: 523-527. Winstanley, D., 1973. Rainfall patterns and general atmosphere circulation. Nature, 245:190 194.