Competition between Coral and Algae in Tertiary and Quaternary Reefs: Greenhouse to Icehouse Transitions ()
1. Introduction and Settings
In the Early Paleogene Period, warming was the trend. Currently, coral, algae, and sizeable benthic foraminifera are the most important organisms for the framework reefs. Copper (1985) [1] described the phases of the reef, and how the reef organisms respond through geologic time. The Early Paleogene is known for ice-free conditions (Greenhouse) [2] (Ray, Thomas, 2007; [3] Zachos, et al., 1993). Increasing temperature directly affects the distribution of organisms such as algae, coral, and large benthic foraminifera ( [4] Adams et al., 1990; [5] Pearson et al., 2001). During the late Paleocene to early Eocene the highest concentration of CO2 and CH4 was present, which resulted in a decrease in the coral species and an increase in the algae ( [6] Pagani et al., 2006). The large benthic foraminifera, coral, and algae are significant producers of CaCO3, and all these organisms are sensitive to the water quality ( [7] Khameiss et al., 2013; [8] 2017; [9] 2018; [10] Langer, 2008). The framework is dominated by coral species which declined during the early Paleogene and were replaced by an algal reef ( [11] Geel, 2000; [12] Khameiss et al., 2017; [13] 2016). In the late Eocene, coral reefs became widespread ( [14] Fagerstorm, 1987; [7] Khameiss et al., 2013; [15] 2014). Several coral species declined because grazers fed on them such as gastropods and echinoids; also damaging to the corals, algae hampered or prevented coral growth. Changing the framework from coral to algae reef is an obvious indicator of coral mortality. The Oligocene is well-known and studied primarily in Italy [16] (Pfister, 1980). A recent study in the Oligocene reef in eastern Libya described the species in this reef as Antiquastrea sp., and high crustose coralline red algae ( [7] Khameiss et al., 2013; [15] 2014). Both are good indicators for the falling sea level associated with the icehouse effect at this time. Oligo-Miocene reef sections come from the Al Faidiyah Formation limestone unit in the Al Fatayah Cement Quarry with two species of coral and shell fragments. Miocene reef sections are sporadic in eastern Libya. The Benghazi Formation is Upper Miocene in age; the samples came from the Benghazi Cement Quarry. In this section, only one species was found, which is Tarbellastraea sp., ( [15] Khameiss et al., 2014). It is an algal reef formation associated with a greenhouse at this time. The nine locations used in this study are identified on a modern map of the world (Figure 1).
All the Early Tertiary Libyan sections are from the margins of the equatorial Tethys Sea, the precursor to the Late Tertiary development of the Mediterranean Sea. The Paleocene-aged Alabama Salt Mountain reef was in a similar equatorial setting but further west. The New Caledonia section was an island complex in the Pan thalassic Ocean, the precursor to the Pacific Ocean.
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Figure 1. Shows the locations of the study areas on a world map (red arrows)1.
2. Methods and Limitations
Because hand samples, thin sections, and data summaries of coral-to-algae ratios used in this study were assembled in various ways for the many worldwide localities, it is difficult to assure comparability of those ratios. Where possible, counts of species identified in multiple thin sections were used as the most reliable measure of the abundance of algae and corals. However, consistent qualitative results from these localities are clear even though quantitative results across the studied sections may be somewhat suspect. Differences in growth habit geometries between corals and algae are significant. Corals are generally cylindrical or frondose; algae are generally encrusting or tabulate/frondose. In outcrop or hand specimen scales it is difficult to precisely estimate ratios between coral and algae. On the other hand, point counts across multiple thin section fields are more precise, albeit laborious. Ratios calculated from thin section counts are the most precise.
Another limitation of this study is the fact that we have selected reefs from very different geological settings. We have not analyzed reefs based on tectonics, ecology, sedimentation, latitude, or oceanic conditions. We have only looked at coral-to-algae ratios in shallow foralgal reefs during greenhouse and icehouse times. Finally, the rapidity of Anthropocene climate warming and a variety of human-induced chemical and physical changes are substantially different now compared to the past geological record. Therefore, drawing too many predictions from our data sets may not be warranted.
3. Results and Discussion
The competition between algae and coral is one of the fundamental processes that determine the structure and composition of reefs during geological time. The change mechanism between the coral and algae is vital to understanding the phase shift [17] Littler, and Littler D. S, 1984). There is no strong evidence that simply says the coral was replaced by an overgrowth of algae because many coral reefs were killed or damaged by storms, bleaching, and eutrophication [18] [19] (Khameiss et al., 2019; Khameiss, 2020). During geological time, major fluctuations of light, prey, chemistry, temperature, and sediments are the main contributors to the differences between the algal reef and coral reef [18] (Khameiss, 2019). In this paper, it is essential to fill the lack of data, especially on the Early Tertiary reef frameworks. Second, the evidence of the transition from coral to algae or vice-versa needs to be documented. Third, we wish to demonstrate the lack of data describing the interaction between species that have been ignored, such as large benthic foraminifera, algae, and coral. Finally, such analyses can aid in predicting future reef-building, using the past as a key to the present and future.
3.1. The Late Paleocene Salt Mountain Alabama
This coastal reef was constructed atop a salt dome and exhibited a thickness of 16 meters. While the physical outcrop has since vanished, preserved samples from this section find their repository within the Biostratigraphy Laboratory at Ball State University’s Department of Environment, Geology, and Natural Resources. The reef’s structural composition was primarily composed of red coralline algae, coral, sponges, ostracods, and foraminifers. Illustrated in Figure 2 is the gradual replacement of coral by algae over time, marking the decline of the coral reef. The proportion of coral to algae, as derived from thin sections and rock samples, yielded the static outcomes depicted in the figure. The age of the reef was ascertained through foraminifera analysis, pinpointing it to the P4-Late Paleocene epoch (as established by Khameiss et al., 2018; 2019) [9] [18] .
3.2. Middle Eocene IODP-U1376, II Bed, Limestone Unit
A solitary reef stands atop a tectonically elevated platform, encompassed by two layers of igneous rock. The IODP core, sourced from the Louisville Chain within the southern Pacific Ocean, was drilled in 2013 under the aegis of the Integrated Ocean Drilling Program (at coordinates 32˚12.99'S and 171˚52.84'W). This core is replete with algae, coral, echinoids, foraminifera, and mollusks. Extracted from the 15.3-meter foralagal limestone segment, static data from 22 thin sections were employed to compute the proportions of coral and algae (refer to Figure 3). The age of the core was determined using its foraminifera content, revealing it to be of Middle Eocene origin (as established by Khameiss et al., 2017; 2018; 2019, as well as Khameiss and Fluegeman, 2022) [8] [9] [18] [20] .
3.3. Middle Eocene New Caledonia, Utoe’ Limestone
Presented here is a remarkable depiction of a foram-algal reef and its current correlation with volcanic activity, situated at coordinates 21˚48.4'S/166˚03.7'E. All calculations are rooted in thin sections, precisely nine in number, hailing from a combined thickness of 34 meters. These sections reveal an abundance of algae, coral, foraminifera, mollusks, and sponges within the outcrop (Figure 4). Through analysis of foraminifera, the age was determined to be Middle Eocene, as established by [21] Micheal et al., 2011; [22] Fluegeman, and Nicholson, 2017; [9] [18] Khameiss et al., 2018; 2019).
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Figure 2. Shows the framework of the late Paleocene Salt Mountain, Alabama.
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Figure 3. Shows the framework of the middle Eocene IODP-U1376, II Bed, Limestone unit.
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Figure 4. Shows the framework of the middle Eocene New Caledonia.
3.4. West-Darnah Roadcut Section-Darnah Formation, Northeast Libya
The geographical coordinates for this section are 32˚53'00''N, 21˚55'01''E. Within this location, a profusion of large benthic foraminifera, algae, coral, mollusks, and echinoids is evident. The comprehensive analysis, drawn from 15 collected samples during this interval, readily unveils the intricate composition of coral and algae, as depicted in Figure 5. Although two species are referenced within this section, their nomenclature remains pending as further statistical scrutiny is essential for future research endeavors. Among these species, Caulastrea sp., takes precedence over Stylophora sp., due to its more rapid colony growth, as observed in the dominance of the former [13] (Khameiss et al., 2016).
3.5. Daryanah-Alabyar Roadcut Section—Al Bayda Formation Northeast Libya
Situated at a latitude of 32˚37'60''N and a longitude of 20˚36'98''E, this section is positioned approximately 60 km east of Benghazi City. The formation’s aggregate thickness spans 28 meters and is classified as an Algal Limestone member, signifying its Lower Oligocene origin [23] (Abdulsamad, E. O., 1999a). Within this member, a profusion of Nummulites fichteli and N. vascus, algae, bivalves, and echinoid species thrive. Among the coral species documented in this section is Antiquastrea sp., Notably, the lower segment of this formation, constituting the algal limestone member, harbors a cave and karstic feature in Northeast Libya [24] (Khameiss et al., 2012; [7] 2013; [25] El Sakran et al., 2016). The most prevalent coralline red algae featured in this section is Lithothamnion sp., as detailed in the study by Hassan, and Ghosh, 2003 [26] . Figure 6 visually displays the coral-to-algae ratios for the nine samples.
3.6. Oligo-Miocene Al Faidiyah Formation—Al Fatayah Cement Quarry
Located approximately 27 kilometers east of Darnah City, this exposed rock formation can be pinpointed at latitude 32˚35'46''N and longitude 22˚43'21''. Its total thickness measures 10 meters, and the bed’s distinctive feature is the presence of white limestone. The outcrop is characterized by the prevalence of significant benthic foraminifera such as nummulites, algae, echinoids, and bryozoans. Among the coral species observed are Cyphastrea sp., and Aleveopera sp., as documented by [27] El Ebaidi, 2000; [28] [29] Khameiss, 2008; 2007; [7] Khameiss et al., 2013. Utilizing the data extracted from the thin sections, the timeline of coral emergence and the subsequent replacement of algal reefs during the icehouse period can be readily deduced (refer to Figure 7).
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Figure 5. Shows Darnah Formation, West-Darnah Road cut section, Northeast Libya.
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Figure 6. Shows the framework of the Al Bayda Formation—Daryanah-Alabyar roadcut, Northeast Libya.
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Figure 7. Shows the framework of the Al Faidiyah Formation—Al Fatayah cement quarry.
3.7. Late-Early to Middle Miocene-Limestone Unit-Cicuco Field, NW Colombia
This core 20 came from Cicuco Field, NW Colombia southwest of and may be found between latitude 9˚16'N and longitude 74˚39'W. The total thickness is 29 m, and the white limestone in this bed is distinctive. The Formation contains foraminifera, algae, echinoids, bryozoan, and coral, Mollusk, Khameiss et al., 2023 [30] . Based on the static data from the eleven thin sections, it is simple to determine when the coral began to appear and when the algal reefs were replaced during the icehouse period (Figure 8). Utilizing the static data extracted from the 11 thin sections, it becomes straightforward to ascertain the onset of coral emergence and the subsequent replacement of algal reefs throughout the icehouse period.
3.8. Late Miocene Benghazi Formation in Benghazi Cement Quarry
Positioned approximately 18 kilometers southwest of Benghazi City, this exposed rock formation is situated between the coordinates of latitude 75˚31'59''N and longitude 73˚32'00''E. Spanning a total thickness of 35 meters, the bed is characterized by its distinctive white limestone. Originally, these sizeable fossiliferous limestones from the Middle Miocene were designated the “Benghazi limestone” by Gregory, in 1911 [31] . Subsequently, [32] Klein, 1974 bestowed the formal name “Ar Rajmah Formation” upon this limestone. Within this formation, a diverse array of significant benthic foraminifera, including Nummulites, as well as algae, echinoids, bryozoans, and corals such as Cyphastrea sp., and Aleveopera sp., have been documented [27] (El Ebaidi, 2000; [29] Khameiss; 2007; [7] 2013). By harnessing the static data gleaned from the seven thin sections, it becomes uncomplicated to discern the chronological progression of coral emergence and the subsequent replacement of algal reefs during the icehouse period (refer to Figure 9).
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Figure 8. Shows the Framework of the Limestone Unit—Cicuco Field, NW Colombia.
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Figure 9. Shows the framework of the Benghazi Formation in Benghazi cement quarry.
3.9. Quaternary Reefs
The Bahamas occupies the expanse between 72˚ and 80˚ west longitude and 20˚ and 28˚ north latitude. This intricate reef complex materialized during the final stages of the ice age, fashioned by the collaborative efforts of coral, algae, foraminifera, echinoids, and mollusks [33] [34] (Lessos, 1988; Hirschfield et al., 1968). Within this geographical scope, the taxonomy of coral species is delineated by Acropora palmata, A. cervicornis, Acroporid sp., and Pterois sp., as documented by Deleveaux et al., 2001; 2013 [35] [36] , and Precht and Hoyt, 1987 [37] . Amidst this region’s vibrant tapestry, the red coralline algae, Millepora sp., finds itself amidst an assortment of coral species such as Dendrogyra cylinders and Eusimilina fastigiata.
A perceptible transformation has befallen this coral reef during the Anthropocene, an epoch influenced by human activity. Karmer Pr., 2008 [38] ; Karmar, Karmer P. R., 2010 [39] , observe a discernible decline, sparking a direct impact on trophic levels and community systems. The intricate interplay among sessile invertebrates within the reef undergoes alteration in the face of human-induced phenomena, including elevated temperatures, overfishing, heightened nutrient levels, ocean acidification, and oil contamination. As a result, many coral species suffer decline and eventual demise in response to climate change driven by human actions, whereas algae assert their resilience and flourish. Notably, a drop in fish populations has engendered the proliferation of echinoids like Diadema antillarum. The foremost algae taking root is the crustose variety, indicating a habitat situated within shallow marine environs. As exemplified in Figure 10, an evident dominance of corals is exhibited, yet it is foreseeable that algal reefs will emerge as the prevailing norm in the times ahead.
3.10. The Florida Keys Coral Reef
The formation of this reef complex mirrors the Bahamas’ coral structures in both manner and pattern. Among the species present are Montastrea annularis, millepora hydrocorals, Octocorallia sp., along with foraminifera like Amphistegina sp., and various forms of marine life including red coralline algae such as Dictyota pulchella, green algae exemplified by Halimeda sp., worms, and mollusks. Human activities, particularly fishing, have triggered a decline in the coral species populating this region, as evidenced by the research conducted by Jennings and Pluming (1996) [40] . The reduction in coral populations is a primary outcome of these anthropogenic influences on the reef’s ecosystem by Hughes, 1994 [41] . Paralleling the trends observed in the Bahamas, the Florida Keys’ coral decline and subsequent algal proliferation follow an analogous trajectory, as illustrated in Figure 11.
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Figure 10. Shows the framework of the Bahamas.
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Figure 11. Shows the framework of the Florida Keys coral reef.
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Figure 12. Shows the framework of reefs during the Cenozoic Era.
4. Conclusions
The geological and environmental data depicted in Figure 12 reveal the significant changes that have occurred in reef communities and their underlying framework during the Tertiary and Quaternary periods. These changes are closely associated with various factors, including geological and chemical dynamics, and are intricately interwoven with shifts in climate. The prevalence of coral or algae in these reef communities, and the transition between them, are distinctly influenced by these climatic alterations.
The prevalence of coral within this locale stands as a telltale sign of colder conditions, characterized by ice sheets enshrouding high-latitude terrains. Such conditions are accompanied by diminished atmospheric CO2 levels and a reduced incidence of volcanic activity, which collectively result in cooler water temperatures. Conversely, instances of escalated CO2 levels over geological time correspond to heightened algal presence, commonly referred to as an “algal bloom”. Simultaneously, tectonic activities linked to continental adjustments correlate with rising sea levels. The phenomenon of bleaching, a consequence of changing ocean salinity and coral species, portends significant repercussions for future reefs and reef communities. This bleaching process triggers the degeneration of coral cells and precipitates the disintegration of coral reef structures.
Acknowledgements
Dr. Brendan received special thanks for invaluable funding assistance. The entire Kansas Geological Survey was also appreciated for their support throughout the endeavor.
NOTES
1https://gisgeography.com/high-resolution-world-map/?fbclid=IwAR0HLe6DFSz0OFHEbIVqteU4uZnrodjOYDryPURRqgaHkeCHrevD_w7mkOw.