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Late Paleozoic Setting
The major continental masses came together during the late Paleozoic to form one supercontinent, Pangea, surrounded by a superocean, Panthalassia
. Sea level was low relative to this supercontinent, in part because plate movements that drove the continents together reduced the global continental area relative to global oceanic area as the continents were sutured together. A small-scale, Cenozoic analogy is the drop in sea level that resulted from the collision of India with Asia to form the Himalayas. The continental area lost during the collision (crumpled into the Himalayas) is roughly the area of modern India. During the collision, the Earth's ocean area increased by roughly the area of modern India, which is an approximately 0.8% increase. Since the average ocean depth is about 3,800 m, increasing the area by 0.8% reduces the depth comparably, resulting in a sea level drop of roughly 30 m. The Paleozoic lowering would have been much greater.
Radiolaria were the only significant producers of biogenic pelagic sediments in the Paleozoic; calcareous producers of pelagic sediments had not yet evolved. Therefore, both deep sea sediments and oceanic biogeochemical cycles were quite different from those of the mid-late Mesozoic and Cenozoic. The relatively few deep-sea sediments preserved as sedimentary rocks were primarily of terrigenous or volcanic origin. Shelf carbonates are common in the early-mid Permian records from the southwestern North America, the Perm region of Russia and elsewhere. Late Permian sequences are dominated by evaporites and redbeds, the latter being evidence for widespread fluvial sedimentation from the eroding uplands.
The paleohistory of massive, non-structured limestone bodies described as reef deposits extends from Cambrian to the present Figure 8.37 . These are preserved, because the environment of deposition was the shallow shelf areas which were part of the continental blocks. The discussion of reef development will be presented for only the Cretaceous to modern reefs.
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The paleogeographic setting for the Triassic was similar to that of the late Paleozoic; the continents were joined into one supercontinent, Pangea, and sea level was low relative to the continental margins. But terrestrial flood basalts interbedded with evaporites and redbeds indicate the onset of rifting of the continents, as heat from the mantle began to build beneath the massive supercontinent.
The continent of Africa provides something of a small-scale modern analogy for Triassic Pangea. Africa lacks extensive continental shelves
and its Great Rift Valley is characterized by flood basalts, redbeds and evaporitic lakes.
In terms of neritic and terrestrial biotas, a great extinction event marks the Paleozoic-Mesozoic boundary, better known as the Permian-Triassic boundary. Approximately 95% of fossilizable late Paleozoic species did not survive into the Triassic. The boundary is characterized by a prolonged hiatus of approximately 8 million years in neritic carbonate deposition. When carbonate deposition resumed in the Tethyan region during the middle Triassic, the sediment-producers were a depauperate biota of cyanobacteria, calcareous sponges and problematic taxa.
Evolutionary events that occurred in the mid to late Triassic forever altered both neritic and pelagic sedimentation and geochemical cycles. The importance of the evolution of coccolithophorids and planktonic foraminifera cannot be overemphasized, for these events made possible the shift of large-scale carbonate sedimentation from shelves and shallow seas to the deep ocean. The series of events that altered shelf carbonate sedimentation included the appearance of Scleractinian corals in the mid Triassic. By the late Triassic, these corals apparently hosted algal symbionts, which allowed them to grow to much larger sizes and produce and trap much larger volumes of carbonate sediments as corals became the dominate reef-building organisms. Wood 1993 attributes the latter event to the evolution of dinoflagellates with the potential for entering into symbiotic relationships not only with corals, but also with planktic and benthic foraminifera and bivalve mollusks.
Global paleoclimate was relatively uniform and relatively mild during the early Mesozoic. Surface circulation in Panthalassa was probably more symmetric between the northern and southern hemisphere than in the modern Pacific. North and south anticyclonic subtropical gyres were separated by an equatorial countercurrent; cyclonic subarctic gyres characterized the high latitudes.
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The Jurassic was the time of change from a supercontinent-superocean global setting to the rapidly separating continents of the Cretaceous. Modern analogies for the Jurassic can be found in modern rifts. In Ethiopia, the north end of the Africa's Great Rift Valley is periodically invaded by marine waters, accumulating thick sequences of evaporites. The Arabian Gulf and Red Sea provide examples of progressively later stages of rifting, the Arabian Gulf being characterized by shallow-water carbonates and evaporites, while the Red Sea is a deep basin connected to the Indian Ocean by a shallow seaway that strongly influences deep-water circulation. These rift settings provide some insight into the depositional environments created by the initial breakup of Pangea as seaways and basins began to form between Laurasia (North America, Europe and Asia) and Gondwana (South America, Africa, India, Australia and Antarctica).
By the early Jurassic, significant basins had begun to open in what is now the Gulf of Mexico. Thick sequences of evaporites were deposited as the deepening basin was alternatively joined to and isolated from oceanic waters. Those salt beds are the reason why salt domes are common around the Gulf of Mexico. Great quantities of recoverable hydrocarbons have been found trapped by these domes. Similar early to mid Jurassic evaporites are also found off eastern North America and west Africa. By approximately 190 million years ago, Laurasia and Gondwana were effectively separated, providing at least a shallow-water opening for initiation of circumtropical circulation through the Tethys seaway Figure 8.39.
Sea level was relatively low in the early Jurassic, fluctuating throughout the period with an overall trend towards substantially higher levels in the Cretaceous. Factors driving sea level rise included relative increase in continental area as the continents were stretched, thinned and broken by rifting, subsidence as the continents moved away from spreading ridges, and accelerating rates of sea floor spreading. Fluctuations in sea level alternatively isolated and reconnected marginal seas, providing optimum conditions for the origin (in isolation) and subsequent dissemination of new taxa. With increasing sea floor spreading rates came increasing partial pressures of CO2 in the atmosphere, further ameliorating global climates.
Pelagic sedimentation patterns are poorly known because most Jurassic seafloor has been subducted. The best known deep sea sediments of late Jurassic age are found in the North Atlantic. Jansa et al. recognized Oxfordian and Kimmeridgian limestones overlain by Tithonian-Hauterivian chalk, the latter representing pelagic oozes produced primarily by planktic foraminifera and coccolithophorids. Along the margins and shallow seas of the Tethys, shallow water carbonates were widespread and diverse. Besides Scleractinian corals, major carbonate-producing organisms included coralline algae, sponges, and bivalves.
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The Cretaceous Period is exceptional for a variety of reasons. On land, the "Age of the Dinosaurs" continued and concluded, while flowering plants (angiosperms) expanded in diversity and ecological importance. The appearance of benthic diatoms was significant, not so much for their influence in the Cretaceous, but for their future in the Cenozoic. The shallow marine realm was characterized by widespread carbonates. The shallowest shelves and epeiric seas of the expanded "Tethys" were dominated by a diverse biota of unique giant clams known as rudists
. Scleractinian corals were common and diverse, particularly in slightly deeper waters along bank margins, but were secondary to the rudists in producing extensive limestone deposits. Most notable in the Cretaceous were the coccolithophorids and planktic foraminifera that produced widespread chalk deposits on the deeper shelves and epieric seas and in the open ocean. The French word "cretacé" means "chalk"; "Terrain Cretacé" (chalk terrains) are widespread in northern France and England, also in the Middle East, Australia and around the Gulf of Mexico. Cretaceous limestones and chalks are among the most common rocks worldwide.
The Cretaceous was a relatively quiet time on the receding continents. The closest modern analogy is Australia, with its low mean elevation and extensive marginal temperate and tropical carbonate margins. It is moving northward away from Antarctica; the plate collision margin is to the north of its shallow northern seas. Compare that with Cretaceous North (or South) America, moving westward followed the breakup of Pangea. Swampy lowlands bordered vast shallow shelves; the Western Interior Seaway separated the continent from the trench-island arc collision margin to the west. The major tectonic action was along the very actively rifting oceanic ridges, the rapidly subducting trenches, and the comparably rapidly accreting island arcs that surrounded the shrinking Pacific Ocean. Sea floor spreading rates of up to 10 cm/yr not only pushed sea level to record highs, but emissions of volcanic gases into the atmosphere from ocean ridges and island-arc volcanoes resulted in atmospheric CO2 concentrations 3-10 times higher than modern levels.
The result of both high sea level and high CO2 concentrations were warm global climates, often called "Greenhouse World" conditions, in which polar regions were ice-free. There were only three major biogeographic regions, the northern boreal (temperate), Tethyan (tropical) and southern boreal provinces. Whether tropical climates were warmer or cooler than present tropics is controversial. Paleotemperature data based on stable oxygen isotopes, as well as some global climate models, indicate tropical ocean temperatures as much as 5o cooler than present (18-23o C), while paleontological interpretations indicate a core "Supertethys" several degrees warmer than the modern tropics. Because water loses as much energy during evaporation as it takes to heat water further, open ocean water temperatures cannot rise above 32o C, thereby limiting global warming.
The globally mild climate had a profound effect on deep ocean circulation and sedimentation. Bottom water formation is thought to have been halothermal (driven primarily by salinity changes and secondarily by temperature changes), rather than the thermohaline mode in modern oceans. A modern analogy for halothermal bottom water formation is in the Mediterranean Sea. Evaporation exceeds freshwater input from rivers, so salinities in the Mediterranean are higher than in the Atlantic. Local winter cooling (to 10-14o C) of this slightly hypersaline water increases density, resulting in sinking of cooled water masses to form Mediterranean bottom water. In the case of the Mediterranean, normal salinity surface water from the Atlantic flows into the Mediterranean, while hypersaline Mediterranean bottom water flows out over the Gibralter sill, contributing Mediterranean intermediate water to subsurface North Atlantic circulation. Similar conditions are believed to be responsible for most bottom water formation during the Cretaceous. Cool, slightly hypersaline deep waters initially carried less oxygen than do near-freezing, normal salinity modern bottom waters. Furthermore, rates of bottom water formation are estimated to have been 1-2 orders of magnitude slower, so rates of deep-water turnover were on the order of 104-105 years, rather than modern rates of 102-103 years. The significance for deep sea sedimentation were profound. The oxygen minimum zone was greatly expanded during much of the Cretaceous, sometimes including entire basins, resulting in widespread deposition of anoxic black shales Figure 8.41 . Even where deep waters were oxygenated, they had much longer to accumulate CO2 and therefore were more corrosive to calcareous sediments, resulting in relatively shallow carbonate compensation depths.
Thus, despite extensive pelagic production of calcareous particles by planktic forams and coccolithophorids, there was strong fractionation of carbonate sediments, with widespread chalks and limestones representing shallow to upper slope depths. Deep sea sediments were predominantly organic-rich clays in the lower to middle Cretaceous and multicolored clays in the late Cretaceous. For example, in the expanding North Atlantic basins, Hauterivian chalks were replaced by Aptian-Cenomanian black bituminous shale, followed by multicolored and red clays during the Cenomanian. Because the western North Atlantic was bordered by higher continental terrain than the eastern North Atlantic, terrigenous sedimentation rates were higher in the west. In fact, much of what is now northwestern Europe was shallow epieric sea, accumulating chalks and rudistid limestones. During the latest Cretaceous (middle Maestrchtian), the CCD deepened to in excess of 5 km in the North Atlantic and deposition of calcareous oozes commenced throughout the North Atlantic.
The opening of the South Atlantic occurred from south to north during the early Cretaceous. The Cape and Argentine Basins opened first, followed later by the Brazil and Angola Basins; the Rio Grande Rise-Walvis Ridge separated the southern basins from those to the north Figure 8.42a . The Brazil and Angola Basins accumulated thick evaporite sequences during the early Cretaceous, while the Cape and Argentine were characterized by terrigenous and black shale deposition. Isolation of the northern basins continued until the late Cretaceous, though sedimentation shifted from evaporites to black shales as the northern basins expanded and deepened. Sedimentation in the southern basins was more similar to that of the North Atlantic, oxygenated terrigenous sediments and clays deposited under a shallow CCD. Eroding land masses on either side of the expanding South Atlantic delivered terrigenous sediments into these basins throughout most of the Cretaceous.
Permanent connection between the North and South Atlantic commenced about 90 Ma, establishing open ocean conditions throughout the Atlantic Figure 8.42b . Nevertheless, the deep sea topography created by the mid-Atlantic Ridge, the Rio Grande Ridge and the Walvis Rise promoted subtle interbasinal differences in the CCD and deep-sea sedimentation, even in modern times.
At about the same time as South America and Africa were separating, India began to rift away from Australia-Antarctica Figure 8.43 . Subsequently, in the Late Cretaceous, India began to separate from Madagascar, as it continued its northward movement toward Asia. Despite the relatively shallow Cretaceous CCD, extensive areas of the newly developing Indian Ocean were sufficiently shallow and ridges common that carbonate sedimentation was extensive. Terrigenous and volcanic sediments have also been recorded, often organic rich, indicating deposition under anoxic conditions.
Deep sea sedimentation in the North Pacific reflects the interplay of plate motion, the CCD and equatorial productivity. DSDP cores from this region characteristically show initial deposition of calcareous ooze on the newly formed seafloor at mid-ocean ridge depths. As the site moved off the ridge and deepened, deposition changed from carbonates to pelagic clays or biogenic silica, depending upon location relative to the equator. Biogenic silica, which lithified to chert, was deposited as the site traversed beneath the higher productivity surface waters of the equatorial upwelling zone. Sedimentation of residual pelagic clays proceeded as the site moved beneath the North Pacific gyre.
Interestingly, even the guyots of the North Pacific reflect this geographic passage through equatorial waters. Scientists on ODP found that flat-topped seamounts, which are capped by sequences of shallow-water carbonates followed by pelagic carbonates, "drowned" as they crossed the equator. That is, shallow-water carbonate sedimentation on the banktops failed to keep pace with subsidence and relative sea-level rise during the transit through equatorial latitudes, so the former oceanic reefs became seamounts. One explanation for this curious phenomenon is the shift of benthic communities in response to elevated nutrient availability, as predicted by Hallock and Schlager. 1986 That is, as the banks moved into the equatorial upwelling zone, benthic communities shifted from predominantly carbonate sediment producing organisms like rudistid bivalves and calcareous algae, to predominantly fleshy algae and bivalves lacking reef-building potential and shallow-water carbonate sedimentation failed to keep pace with subsidence.
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Tethyan biotas, particularly marine organisms with calcareous shells and skeletons, suffered the highest rates of extinctions. Rudistid bivalves were completely eliminated. Scleractinian corals were substantially reduced. Coccolithophorids and planktic foraminifera lost all but a few species, as did larger benthic foraminifera. Both pelagic and neritic carbonate sedimentation ceased briefly, then resumed with depauperate assemblages.
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The continuity of Greenhouse World paleoenvironments through the Paleocene and early Eocene supports the hypothesis that a catastrophic event occurred at the Cretaceous-Tertiary boundary. Corfield suggested that, had there not been a bolide impact, the Mesozoic might have ended 30 million years later, with the terminal Eocene event, when global environments made the first major step in the change from the Greenhouse World to the Icehouse (glacial) World of the late Cenozoic. Although warm-water biotas extended to latitudes as high as 60o at times during the late Paleocene, and particularly during the early Eocene, subantarctic conditions were generally cooling, from 20o C in the early Eocene, to 12-14o by the middle Eocene, to 10o C by the late Eocene.
The paleogeographic changes that perpetrated paleoclimatic changes on Antarctica from ice-free, relatively temperate climates to continental glaciation did not involve any significant movement of Antarctica itself. The continent moved into polar latitudes in the Cretaceous, and has remained there for roughly 100 million years. Rather, it was the breakaway of Australia and South America, combined with events in the Tethys and North Atlantic that generated climatic deterioration and glaciation of Antarctica. Although these paleogeographic changes were fairly gradual, rifting in two high-latitude regions and collision in the eastern Tethys began the gradual changes that would eventually lead to profoundly different global climates and deep ocean circulation (terminal Eocene Event). In the North Atlantic, the Norwegian and Greenland Seas were opening during the Paleocene and Eocene, permitting surface exchange with the Arctic and likely influencing abyssal circulation. The collision of India into Asia commenced in the early Eocene, disrupting circum-equatorial circulation and initiating extensive terrigenous sedimentation in the northern Indian Ocean. Most importantly, in the middle Eocene, Australia began to move northward, altering surface circulation patterns in the high southern latitudes and triggering the onset of cooler deep water formation in the Late Eocene, reflected in cooler bottom water temperatures down to 7o C.
New assemblages of warm-water coccolithophorids and planktic foraminifera evolved from survivors of the K-T extinctions, but relatively shallow CCDs throughout the oceans limited pelagic carbonate sedimentation to depths less than 3,500 m throughout most of the oceans. In both the North and South Atlantic, depositional hiatuses are widespread, so Paleocene sediments are poorly represented. A particularly widespread hiatus occurs at the Paleocene-Eocene boundary. Siliceous diatoms and radiolaria survived the K/T boundary with relatively few extinctions. The pattern of biogenic silica beneath Pacific equatorial waters continued, and, in the Eocene, cherts become more common in the Atlantic as well. A widespread chert horizon is common in Eocene cores from the North Atlantic, and where the chert is missing, there is typically a depositional hiatus, indicating erosive bottom-water movement.
Carbonate sedimentation on shallow shelves and seas was dominated by coralline algae and smaller foraminifera in the early Paleocene, though larger foraminifera quickly rediversified and became major contributors to Paleocene and Eocene shallow-water carbonate buildups. Scleractinian corals survivde but were reduced in diversity to about thirty genera. The Paleocene reefs had corals that were survivors from the Cretaceous. Most reef-building families were present through the Eocene and diversification occurred. With the Eocene, there was a new radiation of zooxanthellate corals.
The terminal Eocene event appears to hve been the onset of deep-sea thermohaline circulation, driven by large-scale sea ice formation in the Ross Sea of Antarctica. Opening of the Tasmanian Seaway, which isolated Antarctica from Australia, is thought to be the trigger for this event. Before the opening, the Ross Sea was warmed by the East Australian current, but formation of the seaway effectively isolated Antarctica from its influence. Bottom water temperatures dropped globally by 4-5oC in as little as 105 years. The terminal Eocene event provides an excellent example of how a relatively simple, regional tectonic change can have profound global influence Figure 8.44.
Both deep-sea benthic and planktic communities were substantially changed. Deep-sea faunas were rather suddenly exposed to colder waters carrying significantly higher concentrations of O2 and lower concentrations of CO2. The CCD abruptly deepened by 1,000 m or more in lower latitudes. "Warm-water" assemblages of planktic foraminifera and coccolithophorids were rapidly replaced by more temperate species, while very low diversity assemblages of planktic foraminifera with simple morphologies replaced temperate species in the high latitudes. Terrestrial biotas were also abruptly changed in mid to high latitudes, as mean annual temperature range increased from a few degrees to as much as 25o C in the Oligocene.
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The Oligocene is perhaps the most enigmatic epoch of the Cenozoic. Apparently contradictory biotic and geochemical data have yet to be completely resolved. The onset of thermohaline deep sea circulation sharply increased the rate of ocean turnover and return of nutrients to surface waters, yet biotic diversity plunged to Cenozoic minima, clearly demonstrating the almost inverse relationship between food supply and biotic diversity. CCD depths dropped, increasing pelagic carbonate sedimentation, and global paleoclimates were relatively cool, yet low latitude corals finally began to construct significant reefs.
Reconstructions of Oligocene global marine circulation patterns superimposed on palinspastic restorations of plate positions and the experimental generation of paleo-oceanic circulation models indicate a number of important relationships. During the Paleogene, with a seaway connecting the northwestern Indian Ocean with the Mediterranean across what is now the Middle East, and with an open Isthmus of Panama, there was a tropical/subtropical westward flowing surface current through the western Tethys, across the central Atlantic, and through the Caribbean into the eastern Pacific Ocean Figure 8.45 . The North American gyre was smaller than at present and positioned far enough north to have relatively little effect in diverting warm westward flowing water up the northeast coast of North America as does the modern Gulf Stream. Thus, there is evidence to contend that migration and dispersal patterns of the cosmopolitan Oligocene reef coral fauna were from east to west.
Though sea level was low during the early Oligocene, the late Oligocene-Early Miocene transgression and global amelioration resulted in latitudinal expansion and diversification of warm-water biotas.
Crucial tectonic events occurred during the Oligocene, altering both tropical and polar surface circulation. The eastern Tethys closed in the Late Oligocene, further restricting circumtropical circulation. The Drake Passage between Antarctica and South America opened in the latest Eocene or earliest Oligocene, and may have been involved in the terminal Eocene cooling. With the Drake Passage open to shallow water, as Australia moved north, Antarctica became progressively isolated. By the early late Oligocene, the South Tasman Rise separated from Victoria Land, Antarctica, initiating the Circum Antarctic Current.
The Oligocene marked a time of world wide maximum abundance and diversity of Tertiary reefs, with their widespread development in both the Caribbean/Gulf of Mexico and the Tethys regions. The zooxanthellate corals that were largely responsible for the construction of these reefs comprise the direct ancestors of the modern Indo-Pacific hermatypic coral fauna. The closing of the Tethys seaway passage into the Mediterranean at the close of the Oligocene Figure 8.46 led to the extinction of major elements of Oligocene reef coral fauna.
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Following the opening of the Drake Passage to deep water in the late Oligocene, major tectonic events shifted from the southern hemisphere to the north. Despite the thermal isolation of Antarctica and active winter sea-ice formation generating the formation of Antarctic Bottom Water, glacial accumulation on Antarctica was apparently limited, not by cold, but by moisture. Events in the northern oceans soon changed that. Three key areas were the Iceland Faeroe Ridge, the Central American Seaway, and the Straits of Gibralter.
First came the submergence of the Iceland-Faeroe Ridge in the middle Miocene, completing the deepwater connection of the Norwegian and Greenland Seas to the North Atlantic Basin. Bottom waters generated by sea ice formation in these northern seas could now flow into the North Atlantic, initiating North Atlantic Deep Water (NADW) formation. NADW is not as cold and not quite as dense as AABW, so after it began to form and flow southward, it began to emerge at the surface in the Antarctic divergence. Surfacing of these slightly warmer waters increased rates of evaporation to the atmosphere, which subsequently slightly increased snowfall on the Antarctic continent, resulting in a significant increase in thickness of the continental glaciers and a lowering of sea level in the late Miocene.
The onset of NADW influenced pelagic sedimentation throughout the Atlantic Ocean and increased sedimentation differences between the Atlantic and the Pacific. NADW carries low concentrations of CO2 through the deep North Atlantic so that, wherever it reached the sea floor, carbonate oozes accumulated. Upwelling NADW in the Antarctic divergence promoted biological productivity and biogenic silica production, establishing high latitude diatom oozes that halo and intermix with glacial sediments rafted from Antarctica.
Appearance of many new Scleractinian genera produced an Early Miocene fauna which is transitional between the cosmopolitan aspect of the Oligocene and endemic fauna of late Miocene-Holocene Caribbean. The zooxanthellate coral assemblages became substantially different from their Indo-Pacific counterparts by Late Miocene. This difference while partially increased by the evolution of endemic corals such as Agaricia, was achieved largely by the progressive regional extinction of corals such as Goniopora, Stylophora, Pocillopora, Pavona, Goniastrea and other long-ranging lineages in the Caribbean. Stephanocoenia and Madracis appear to extend back to late Early Miocene and Agaricia species and Helioseris cucullata appeared by Late Miocene; stock probably derived from a Pavona ancestor. Porites asteroides can be traced into Late Miocene.
The gradual closure of the Central American Seaway influenced Caribbean biotas in the Miocene. The extinctions of a substantial proportion of the reef-building coral fauna and larger foraminifera may have resulted from an increase in nutrient flux to surface waters of the Caribbean. When the Central American Seaway was open, there was active exchange between the Caribbean and the eastern Pacific. The easterly trade winds drove surface currents westward from the Caribbean to the Pacific. However, because evaporation rates are higher in the Atlantic than in the Pacific, sea level was slightly higher in the Pacific, forcing subsurface flow back into the Caribbean. As long as the passages were fairly deep and open, backflow of nutrient rich, eastern tropical Pacific subsurface waters had minimal influence on shallow-water biotas. But as the seaway shoaled and became more complex, topographically induced upwelling of eastern Pacific waters increased delivery of nutrient-rich waters to shallow-water communities.
Pliocene compression of climatic belts and the rise of the Isthmus of Panama restricted reef growth to two distinct regions - the Atlantic-Caribbean and the Indo-Pacific. An episode of faunal turnover affected Caribbean coral during the Plio-Pleistocene. Budd, 1994 The late Miocene and early Pliocene reef coral are distinct from the Pleistocene and modern reef communities. Acropora palmata is only found in later Pleistocene reefs so the time of origin and evolution cannot be determined other than after earliest Pleistocene, but it became the dominant species on Caribbean reefs replacing Pocillopora dominance.
Closure of the Central American Seaway also had global implications. As the passage closed, more and more of the Caribbean Current was diverted northward into a western boundary current via the Florida Loop Current and the Florida Current to join the Gulf Stream. Besides triggering erosion of the Nicaraguan Rise, the Florida Straits and the Blake Plateau, this enhanced western boundary current carried greater volumes of warm water to the far north Atlantic, where enhanced evaporation triggered increased snowfall. The closure of the Isthmus in the late Pliocene is thought to provide a key trigger for northern hemisphere glaciation, pushing global climates towards greater influence by cycles in the Earth's orbit (Milankovitch Cycles).
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The colleagues of Charles Darwin, in the mid-19th century, recognized that vast ice sheets must have covered northern Europe and northern North America at some time in the not too distant past. The first long piston cores recovered by the Albatross Expedition in 1947-48 not only revealed that the Pleistocene sediments in the deep sea recorded alternating climate states, but showed more fluctuations than had been recognized on land. In the deep sea record, these cycles are expressed by a variety of characteristics including changes in supply and distribution of terrigenous sediments, fluctuations in biogenic sediments and in the species making up those sediments, fluctuations in oxygen-18 content in foraminiferal shells, and changes in the relative abundance of carbonate in the sediments.
There were two major factors that brought about the Icehouse World of the Neogene that culminated in the very active, Milankovitch-cycle Glaciation of the Quaternary. Plate movements in key areas, including the far North Atlantic, the Tethys seaway, and particularly those that isolated Antarctica, produced changes in global climate and in surface and deep ocean circulation changes. Falling atmospheric CO2 concentrations, from several times pre-anthropogenic levels, must also have played a role in global cooling. The causes of this drop are not well defined, but certainly involve reduced rates of sea-floor spreading, but may also have involved more efficient CO2 sequestering in sediments by a variety of groups of organisms that arose in the Mesozoic. Radiations of the highly efficient primary producers, the diatom and angiosperm floras, may have sequestered organic matter, especially in terrestrial and shallow-water sediments. The roles of widespread deposition of pelagic CaCO3 by planktic forams and coccolithophorids, in addition to shallow-water carbonate sedimentation in the subtropics and tropics by Scleractinian corals, calcareous algae and larger foraminifera, are unknown. Certainly glacial-interglacial atmospheric CO2 concentrations are significant, and even summer-winter differences indicate the potential for a close feedback between primary production and CO2. While over uptake of CO2 by organisms should cause cooling (reverse greenhouse conditions), progressive cooling shuts down primary production, allowing CO2 levels to catch up. Scientists are only beginning to appreciate range and nature of feedbacks between the hydrosphere, atmosphere, cryosphere and biosphere.
For more discussion of the Quarternary and Holocene, return to Chapter 2, Sea Level Changes
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